SACCHAROMYCES CEREVISIAE STRAIN WITH HIGH YIELD OF ETHYL BUTYRATE AND CONSTRUCTION METHOD AND APPLICATION OF SACCHAROMYCES CEREVISIAE STRAIN

Abstract

A Saccharomyces cerevisiae strain with high yield of ethyl butyrate and a construction method and an application thereof are provided. The strain is obtained by over-expressing in the starting strain acetyl coenzyme A acyl transferase gene Erg10, 3-hydroxybutyryl coenzyme A dehydrogenase gene Hbd, 3-hydroxybutyryl coenzyme A dehydratase gene Crt, trans-2-enoyl coenzyme A reductase gene Ter, and alcohol acyl transferase gene AAT. Compared to the starting bacteria not producing ethyl butyrate, the yield of ethyl butyrate of the constructed strain reaches 77.33±3.79 mg/L, the yield of the ethyl butyrate of the strain with double copy expression of gene Ter and gene AAT reaches 99.65±7.32 mg/L, increased by 28.9% compared with the EST strain, and 40.93±3.18 mg/L of ethyl crotonate is unexpectedly produced.

Claims

1. A genetically engineered Saccharomyces cerevisiae strain with a high yield of ethyl butyrate, wherein the genetically engineered Saccharomyces cerevisiae strain is obtained by using Saccharomyces cerevisiae as an original strain and over-expressing acetyl-CoA C-acetyltransferase gene Erg10, 3-hydroxybutyryl-CoA dehydrogenase gene Hbd, 3-hydroxybutyryl-CoA dehydratase gene Crt, trans-2-enoyl-CoA reductase gene Ter and alcohol acyltransferase gene AAT.

2. The genetically engineered Saccharomyces cerevisiae strain according to claim 1, wherein at least one of the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT is dual-copy expressed.

3. The genetically engineered Saccharomyces cerevisiae strain according to claim 1, wherein a nucleotide sequence of the acetyl-CoA C-acetyltransferase gene Erg10 is as shown by SEQ ID NO:1; a nucleotide sequence of the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd is as shown by SEQ ID NO:2; a nucleotide sequence of the 3-hydroxybutyryl-CoA dehydratase gene Crt is as shown by SEQ ID NO:3; a nucleotide sequence of the trans-2-enoyl-CoA reductase gene Ter is as shown by SEQ ID NO:4; and a nucleotide sequence of the alcohol acyltransferase gene AAT is as shown by SEQ ID NO:5.

4. The genetically engineered Saccharomyces cerevisiae strain according to claim 1, wherein the original strain is Saccharomyces cerevisiae CICC32315.

5. The genetically engineered Saccharomyces cerevisiae strain according to claim 1, wherein the acetyl-CoA C-acetyltransferase gene Erg10 is overexpressed by a strong promoter; the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are arranged in tandem to replace gene GAL80 (shown as SEQ ID NO:51) together and the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are respectively overexpressed by the strong promoter; and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are arranged in tandem to replace gene HXT16 (shown as SEQ ID NO:52) together and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are respectively overexpressed by the strong promoter.

6. The genetically engineered Saccharomyces cerevisiae strain according to claim 5, wherein at least one of the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT is dual-copy expressed; the trans-2-enoyl-CoA reductase gene Ter is dual-copy expressed by replacing gene LPP1 (shown as SEQ ID NO:54) and overexpressing by the strong promoter; and the alcohol acyltransferase gene AAT is dual-copy expressed by replacing gene KU70 (shown as SEQ ID NO:53) and overexpressing by the strong promoter.

7. The genetically engineered Saccharomyces cerevisiae strain according to claim 5, wherein the strong promoter is PGK1.sub.P.

8. A method of using the genetically engineered Saccharomyces cerevisiae strain according to claim 1, comprising using the genetically engineered Saccharomyces cerevisiae strain in fields of fermented brewing, fermented foods and flavors and fragrances.

9. The method according to claim 8, comprising inoculating a seed solution with a 8-12% inoculation amount to a fermentation medium after a two-stage activation of the genetically engineered Saccharomyces cerevisiae strain, and standing for a fermentation for 80-86 h at 28-30° C.; wherein components of the fermentation medium comprise 300-320 g/L of corn flour, (2-5)×10.sup.4 U/L of heat-resisting α-amylase, 90-100 U/L of glucoamylase, 10-20 U/L of an acid proteinase, 5.5-5.6 mL/L of a nutritive salt solution and water as balance; and the nutritive salt solution comprises 140-160 g/L of MgSO.sub.4, 70-80 g/L of KH.sub.2PO.sub.4, 80-85 g/L of carbamide and water as balance.

10. The genetically engineered Saccharomyces cerevisiae strain according to claim 2, wherein the acetyl-CoA C-acetyltransferase gene Erg10 is overexpressed by a strong promoter; the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are arranged in tandem to replace gene GAL80 together and the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are respectively overexpressed by the strong promoter; and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are arranged in tandem to replace gene HXT16 together and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are respectively overexpressed by the strong promoter.

11. The genetically engineered Saccharomyces cerevisiae strain according to claim 3, wherein the acetyl-CoA C-acetyltransferase gene Erg10 is overexpressed by a strong promoter; the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are arranged in tandem to replace gene GAL80 together and the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are respectively overexpressed by the strong promoter; and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are arranged in tandem to replace gene HXT16 together and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are respectively overexpressed by the strong promoter.

12. The genetically engineered Saccharomyces cerevisiae strain according to claim 4, wherein the acetyl-CoA C-acetyltransferase gene Erg10 is overexpressed by a strong promoter; the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are arranged in tandem to replace gene GAL80 together and the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are respectively overexpressed by the strong promoter; and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are arranged in tandem to replace gene HXT16 together and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are respectively overexpressed by the strong promoter.

13. The method according to claim 8, wherein at least one of the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT is dual-copy expressed.

14. The method according to claim 8, wherein a nucleotide sequence of the acetyl-CoA C-acetyltransferase gene Erg10 is as shown by SEQ ID NO:1; a nucleotide sequence of the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd is as shown by SEQ ID NO:2; a nucleotide sequence of the 3-hydroxybutyryl-CoA dehydratase gene Crt is as shown by SEQ ID NO:3; a nucleotide sequence of the trans-2-enoyl-CoA reductase gene Ter is as shown by SEQ ID NO:4; and a nucleotide sequence of the alcohol acyltransferase gene AAT is as shown by SEQ ID NO:5.

15. The method according to claim 8, wherein the original strain is Saccharomyces cerevisiae CICC32315.

16. The method according to claim 8, wherein the acetyl-CoA C-acetyltransferase gene Erg10 is overexpressed by a strong promoter; the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are arranged in tandem to replace gene GAL80 together and the 3-hydroxybutyryl-CoA dehydrogenase gene Hbd and the 3-hydroxybutyryl-CoA dehydratase gene Crt are respectively overexpressed by the strong promoter; and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are arranged in tandem to replace gene HXT16 together and the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT are respectively overexpressed by the strong promoter.

17. The method according to claim 17, wherein at least one of the trans-2-enoyl-CoA reductase gene Ter and the alcohol acyltransferase gene AAT is dual-copy expressed; the trans-2-enoyl-CoA reductase gene Ter is dual-copy expressed by replacing gene LPP1 and overexpressing by the strong promoter; and the alcohol acyltransferase gene AAT is dual-copy expressed by replacing gene KU70 and overexpressing by the strong promoter.

18. The method according to claim 17, wherein the strong promoter is PGK1.sub.P.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] FIG. 1 is a metabolic diagram of an ethyl butyrate synthesis pathway constructed by Saccharomyces cerevisiae;

[0059] FIG. 2 is a schematic diagram of a construction process of recombinant plasmid Yep352-PE/PH/PC/PT/PA;

[0060] FIG. 3A is a verification electrophoretogram of construction of recombinant plasmid Yep352-PE, wherein lane 1 is a Yep352-P plasmid, lane 2 is Yep352-PE, lane 3 is an Erg10 gene segment, and lane 4 is a 10000 bp DNA Ladder Marker;

[0061] FIG. 3B is a verification electrophoretogram of construction of recombinant plasmid Yep352-PA, wherein lane 1 is a Yep352-P plasmid, lane 2 is Yep352-PA, lane 3 is an AAT gene segment, and lane 4 is a 10000 bp DNA Ladder Marker;

[0062] FIG. 3C is a verification electrophoretogram of construction of recombinant plasmid Yep352-PT, wherein lane 1 is a Yep352-P plasmid, lane 2 is Yep352-PT, lane 3 is a Ter gene segment, and lane 4 is a 10000 bp DNA Ladder Marker;

[0063] FIG. 3D is a verification electrophoretogram of construction of recombinant plasmid Yep352-PH, wherein lane 1 is a Yep352-P plasmid, lane 2 is Yep352-PH, lane 3 is a Hbd gene segment, and lane 4 is a 10000 bp DNA Ladder Marker;

[0064] FIG. 3E is a verification electrophoretogram of construction of recombinant plasmid Yep352-PC, wherein lane 1 is a Yep352-P plasmid, lane 2 is Yep352-PC, lane 3 is a Crt gene segment, and lane 4 is a 10000 bp DNA Ladder Marker;

[0065] FIG. 4 is a schematic diagram of construction of a recombinant Saccharomyces cerevisiae strain overexpressing a gene Erg10 at Erg10;

[0066] FIG. 5 is a schematic diagram of construction of a recombinant Saccharomyces cerevisiae strain overexpressing a gene Hbd and a gene Crt at GAL80;

[0067] FIG. 6 is a schematic diagram of construction of a recombinant Saccharomyces cerevisiae strain overexpressing a gene Ter and a gene AAT at HXT16;

[0068] FIG. 7 is a schematic diagram of construction of a recombinant Saccharomyces cerevisiae strain performing double-copy on a gene Ter at LPP1;

[0069] FIG. 8 is a schematic diagram of construction of a recombinant Saccharomyces cerevisiae strain performing double-copy on a gene AAT at KU70;

[0070] FIG. 9A is a verification electrophoretogram of construction of a strain, wherein lane 1 is a 5000 bp DNA Ladder Marker, lane 2 is a verification segment taking a recombinant strain genome as a template and Erg10-FA-U/Erg10-D as a primer, lane 3 is a verification segment taking a recombinant strain genome as a template and Erg10-U/KAN-D as a primer, and lane 4 is a verification segment taking a recombinant strain genome as a template and KAN-U/Erg10-FB-D as a primer;

[0071] FIG. 9B is a verification electrophoretogram of construction of a strain, wherein lane 1 is a 5000 bp DNA Ladder Marker, lane 2 is a verification segment taking a recombinant strain genome as a template and HXT16-FA-U/Ter-D as a primer, lane 3 is a verification segment taking a recombinant strain genome as a template and Ter-U/KAN-D as a primer, lane 4 is a verification segment taking a recombinant strain genome as a template and KAN-U/AAT-D as a primer, and lane 5 is a verification segment taking a recombinant strain genome as a template and AAT-U/HXT16-FB-D as a primer;

[0072] FIG. 9C is a verification electrophoretogram of construction of a strain, wherein lane 1 is a 5000 bp DNA Ladder Marker, lane 2 is a verification segment taking a recombinant strain genome as a template and GAL80-FA-U/Hbd-D as a primer, lane 3 is a verification segment taking a recombinant strain genome as a template and Hbd-U/KAN-D as a primer, lane 4 is a verification segment taking a recombinant strain genome as a template and KAN-U/Crt-D as a primer, and lane 5 is a verification segment taking a recombinant strain genome as a template and Crt-U/GAL80-FB-D as a primer;

[0073] FIG. 9D lane 1 is a 5000 bp DNA Ladder Marker, lane 2 is a verification segment taking a recombinant strain genome as a template and LPP1-FA-U/Ter-D as a primer, lane 3 is a verification segment taking a recombinant strain genome as a template and Ter-U/KAN-D as a primer, and lane 4 is a verification segment taking a recombinant strain genome as a template and KAN-U/LPP1-FB-D as a primer;

[0074] FIG. 9E is a verification electrophoretogram of construction of a strain, wherein lane 1 is a 5000 bp DNA Ladder Marker, lane 2 is a verification segment taking a recombinant strain genome as a template and KU70-FA-U/AAT-D as a primer, lane 3 is a verification segment taking a recombinant strain genome as a template and AAT-U/KAN-D as a primer, and lane 4 is a verification segment taking a recombinant strain genome as a template and KAN-U/KU70-FB-D as a primer;

[0075] FIG. 9F is a verification electrophoretogram of construction of a strain, wherein lane 1 is a 5000bpDNA Ladder Marker, lane 2 is a verification segment taking a recombinant strain genome as a template and KU70-FA-U/AAT-D as a primer, lane 3 is a verification segment taking a recombinant strain genome as a template and AAT-U/KAN-D as a primer, and lane 4 is a verification segment taking a recombinant strain genome as a template and KAN-U/KU70-FB-D as a primer; and

[0076] FIG. 10 is a diagram showing experimental results of yield of ethyl butyrate and ethyl crotonate of a parent strain and a Saccharomyces cerevisiae modified strain in each stage.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0077] The present disclosure is described below through specific embodiments. Unless otherwise specified, the technical means used in the present disclosure are all methods known to those skilled in the art. In addition, the embodiments should be understood as illustrative, rather than limiting the scope of the disclosure, which is only limited by the scope of the claims. For those skilled in the art, without departing from the spirit and scope of the present disclosure, various changes or modifications to the material composition and amount used in these embodiments also belong to the protection scope of the present disclosure.

[0078] The Saccharomyces cerevisiae used in the present invention may adopt Saccharomyces cerevisiae strains from any source. The Saccharomyces cerevisiae strains used in the following embodiments are all α haploids (AY14-α) of Saccharomyces cerevisiae CICC32315.

[0079] Firstly, the gene Erg10 of Saccharomyces was overexpressed (referring to FIG. 1), and 3-hydroxybutyryl-CoA dehydrogenase gene (Hbd) and 3-hydroxybutyryl-CoA dehydratase gene (Crt) derived from Clostridium acetobutylicum as well as trans-2-enoyl-CoA reductase gene (Ter) derived from treponema were subjected to heterologous expression in the Saccharomyces cerevisiae to construct a Saccharomyces cerevisiae strain ET with a butyryl coenzyme A producing pathway; secondly, a Fragaria×ananassa alcohol acyltransferase AAT was subjected to heterologous integration strong expression in the strain ET to obtain an ethyl butyrate-producing Saccharomyces strain EST; thirdly, the gene Ter and the gene AAT were subjected to double-copy expression respectively based on the strain EST to obtain a strain EDT capable of singly double-copying (dual-copying) the gene Ter and a strain EDS capable of singly double-copying the gene AAT. Finally, the AAT was subjected to double copy on the basis of the strain EDT capable of singly double-copying the gene Ter to obtain a strain EDST capable of double-copying the genes Ter and AAT simultaneously.

Embodiment 1: Construction of a Saccharomyces cerevisiae Strain Capable of Producing Ethyl Butyrate

[0080] The embodiment adopts a starting strain CICC32315. The Escherichia coli DH5a is purchased from Takara company. The YPD culture medium is a universal complete culture medium, and the solid culture medium contains 2% (mass percentage) of imported agar powder.

[0081] According to each gene sequence and integration plasmid sequence in NCBI Genebank, the following primer is designed, as shown in Table 1.

TABLE-US-00001 TABLE 1  Primers Name of primer Sequence (5′.fwdarw.3′) SEQ ID NO: P-Erg10-U AAGATCGGAATTCCAGATCTCATGTCTCAGAACGTTTACATTG 7 P-Erg10-D GATCTATCGCAGATCCCTCGAGTCATATCTTTTCAATGACAATAG 8 P-Hbd-U AAGATCGGAATTCCAGATCTCATGAAAAAGGTATGTGTTATAGG 9 P-Hbd-D GATCTATCGCAGATCCCTCGAGTTATTTTGAATAATCGTAGAAACC 10 P-Crt-U AAGATCGGAATTCCAGATCTCATGGAACTAAACAATGTCATCC 11 P-Crt-D GATCTATCGCAGATCCCTCGAGCTATCTATTTTTGAAGCCTTC 12 P-Ter-U AAGATCGGAATTCCAGATCTCATGATTGTTAAGCCAATGGTTAG 13 P-Ter-D GATCTATCGCAGATCCCTCGAGTTATATTCTATCAAATCTTTC 14 P-AAT-U AAGATCGGAATTCCAGATCTCATGGAAAAAATTGAAGTCTC 15 P-AAT-D GATCTATCGCAGATCCCTCGAGTTAAATCAATGTCTTTGGTGAAGC 16 Erg10-FA-U GAAGAATCCTTACGCACATAAGC 17 Erg10-FA-D CAGTTTTGGATAGATCAGTTAGACTGAGACATTTTGAGTACGTC 18 Erg10-FB-U GATCCACTAGTGGCCTATGCGAAGGAGGTAAGATCGGTGTTG 19 Erg10-FB-D GGAACAGGTGCTTAACACTCAC 20 Erg10-U CTATCCTCCAAGACAGCAGTG 21 Erg10-D GTGTAACAACCACTCTAGCACC 22 KAN-U CAGCTGAAGCTTCGTACGCTG 23 KAN-D GCATAGGCCACTAGTGGATC 24 GAL80-FA-U CCATAGAGAGAAGGAGCAAGC 25 GAL80-FA-D CAGTTTTGGATAGATCAGTTAGACGGTTGAGACCGAAGATCTCTTG 26 GAL80-FB-U GATCCACTAGTGGCCTATGCCCGTTAGCAATATCTCGCATTATAG 27 GAL80-FB-D CATGCTACCTTCCATGGTTGAG 28 Hbd-U GGAATTGCTCAGGCATTTGCAG 29 Hbd-D GTGGTCTATACTTAGAATCTCCAG 30 Crt-U GTAGCAGGAGCAGATATTTCTG 31 Crt-D CTATGAAAGCTGTCATTGCATCC 32 HXT16-FA-U GATGTGCCTATGAATATGCAGC 33 HXT16-FA-D CAGTTTTGGATAGATCAGTTAGACTGGTGAGGACTGTTCGCTTG 34 HXT16-FB-U GATCCACTAGTGGCCTATGCCCAAGGAGAGGAGCTTCTTCC 35 HXT16-FB-D GGAATGGTACAGTGTTACGTTCC 36 Ter-U CGTATTACAGCTGAAGTCAAGGC 37 Ter-D CTGTGTGCAGTTGCCTCCAAG 38 AAT-U GGATCAGTTAACTCCACCAGC 39 AAT-D GCCTCAATACCAGAACCGCAC 40 LPP1-FA-U GCTGTGTATGAAGAATTAGTTCACG 41 LPP1-FA-D CAGTTTTGGATAGATCAGTTAGACCATGACAGAGATCATCCTTGG 42 LPP1-FB-U GATCCACTAGTGGCCTATGCGAGACATACTTCCTTCACCGG 43 LPP1-FB-D CCTTGAGCGATATCTGGAGATTG 44 KU70-FA-U GCCTTGATCAACAATGCAATCC 45 KU70-FA-D CAGTTTTGGATAGATCAGTTAGAGTGACTGAGCGCATAATATTCC 46 KU70-FB-U GATCCACTAGTGGCCTATGCCTGAGAAGTCAGAAGATCCAATC 47 KU70-FB-D GCAGGTCTTGATAATGATAGAGG 48 PGK1p-U TCTAACTGATCTATCCAAAACTG 49 PGK1t-D CAGCGTACGAAGCTTCAGCTGTAACGAACGCAGAATTTTCGAG 50

[0082] The PCR amplification system used in the embodiment is shown in Table 2.

TABLE-US-00002 TABLE 2 PCR Amplification System Reaction System Sample Adding Quantity ddH.sub.2O Supplemented to 50 μL 10× PCR Buffer 5.0 μL dNTP (0.2 m mol/L)   4 μL Upstream and downstream primers 1.5 μL for each one (10 m mol/L) Template: Saccharomyces total DNA 1.0 μL Taq DNA polymerase 0.5 μL

[0083] The main construction process of the strain is as follows:

[0084] (1) Construction of Yep352-PE/PH/PC/PT/PA Plasmid

[0085] Recombinant plasmids carrying genes Erg10, Hbd, Crt, Ter and AAT were constructed by taking Yep352-P as a basic plasmid, short for recombinant plasmids Yep352-PE, Yep352-PH, Yep352-PC, Yep352-PT and Yep352-PA (short for Yep352-PE/PH/PC/PT/PA). The construction process is shown in FIG. 2, and the verification electrophoretogram is shown in FIG. 3A-E. By taking a haploid genome of the Saccharomyces cerevisiae CICC32315 as a template, PCR amplification was conducted by primer pairs P-Erg10-U (SEQ ID NO:7) and P-Erg10-D (SEQ ID NO:8) to obtain a 1197 bp Erg10 segment; PCR amplification was conducted by primer pairs P-Hbd-U (SEQ ID NO:9) and P-Hbd-D (SEQ ID NO:10) to obtain a 849 bp Hbd segment; PCR amplification was conducted by primer pairs P-Crt-U (SEQ ID NO:11) and P-Crt-D (SEQ ID NO:12) to obtain a 786 bp Crt segment; PCR amplification was conducted by primer pairs P-Ter-U (SEQ ID NO:13) and P-Ter-D (SEQ ID NO:14) to obtain a 1194 bp Ter segment; PCR amplification was conducted by primer pairs P-AAT-U (SEQ ID NO:15) and P-AAT-D (SEQ ID NO:16) to obtain a 1359 bp AAT segment; Yep352-P was subjected to enzyme cutting by restriction endonuclease Xho I, and the plasmid after enzyme cutting was respectively in recombinant connection to the five gene segments obtained by PCR by a lightening cloning kit to obtain recombinant plasmids Yep352-PE, Yep352-PH, Yep352-PC, Yep352-PT and Yep352-PA respectively.

[0086] The plasmid Yep352-P and the construction method thereof are derived from the patent SACCHAROMYCES CEREVISIAE STRAIN CAPABLE OF PRODUCING HIGH-YIELD FLAVOR ETHYL ESTER AND CONSTRUCTION METHOD THEREOF with publication number CN105586282A. The Yep352-P plasmid is obtained by taking a plasmid pPGK1 as a template, amplifying a strong promoter segment PGK1p-PGK1t on the plasmid pPGK1 and connecting onto an expression vector Yep352.

[0087] (2) construction of a Saccharomyces strain capable of producing ethyl butyrate

[0088] {circle around (1)} By taking the genome of CICC32315 Saccharomyces a haploid (hereinafter referred to as AY14-α) as a template, PCR amplification was conducted by primer pairs Erg10-FA-U (SEQ ID NO:17) and Erg10-FA-D (SEQ ID NO:18) to obtain an upper homologous arm Erg10-FA of an Erg10 site; PCR amplification was conducted by primer pairs Erg10-FB-U (SEQ ID NO:19) and Erg10-FB-D (SEQ ID NO:20) to obtain a lower homologous arm Erg10-FB of the Erg10 site; by taking a recombinant plasmid Yep352-PE as a template, PCR amplification was conducted by primer pairs PGK1p-U (SEQ ID NO:49) and PGK1t-D (SEQ ID NO:50) to obtain a PGK1p-Erg10-PGK1t segment with a strong promoter and a terminator; and by taking a pUG6 plasmid as a template, PCR amplification was conducted by primers KAN-U (SEQ ID NO:23) and KAN-D (SEQ ID NO:24) to obtain a selection marker KanMX

[0089] By taking a Saccharomyces cerevisiae strain AY14-α as a starting strain and Erg10 as an integration site, the four segments Erg10-FA, Erg10-FB, PGK1p-Erg10-PGK1t and KanMX obtained by PCR were transformed into the a haploid obtained by raw spore separation of the Saccharomyces cerevisiae CICC32315 simultaneously by a lithium acetate transformation method, and were sequentially connected to and inserted into the integration site, and intracellular integration was conducted to obtain a Saccharomyces cerevisiae recombinant strain 1 after homologous recombination. The homologous recombination process diagram is shown in FIG. 4.

[0090] Three groups of upstream and downstream primers were designed respectively according to the gene sequences at the two ends of the recombination site of the Saccharomyces cerevisiae CICC32315, and PCR amplification was conducted by taking a haploid transformant genome growing well as a template to verify a recombinant. The primer pairs Erg10-FA-U (SEQ ID NO:17) and Erg10-D (SEQ ID NO:22) served as upstream verification primers; the primer pairs Erg10-U (SEQ ID NO:21) and KAN-D (SEQ ID NO:24) served as midstream verification primers; and the primer pairs KAN-U (SEQ ID NO:23) and Erg10-FB-D (SEQ ID NO:20) served as downstream verification primers. A transformant verification agar gel electrophoretogram is shown in FIG. 9A, wherein lane 2 is an upstream verification band with a band size being about 3100 bp and consistent with the expectation, lane 3 is a midstream verification band with a band size being about 3300 bp and consistent with the expectation, and lane 4 is a downstream verification band with a band size being about 2100 bp and consistent with the expectation.

[0091] The plasmid pGAPza with Cre recombinant enzyme was transformed into the recombinant strain 1 by the lithium acetate transformation method to obtain a transformant; a monoclonal antibody was picked and induced in a galactose medium for 4 h to 5 h, dilution and coating was conducted, and a single colony was picked out onto a YPD plate and was photocopied on a G418 resistance plate; a strain capable of growing on the YPD plate and not growing on the G418 resistance plate was picked out and a genome was extracted to conduct PCR verification. The band of about 1600 bp cannot be obtained by amplifying the segment KanMX by taking the genome of the recombinant strain 1 as control, and the recombinant strain 1 can be amplified to obtain the segment, thus obtaining a recombinant strain 2 losing the selection marker KanMX. The recombinant strain 2 was inoculated into a YPD liquid culture medium for subculture and was transferred for once every 12 h, and the plasmid pGAPza may be lost after several generations, thus obtaining a recombinant strain 3 not containing the plasmid pGAPza.

[0092] {circle around (2)} By taking the genome of the AY14-α as a template, PCR amplification was conducted by primer pairs AY14-α (SEQ ID NO:25) and GAL80-FA-D (SEQ ID NO:26) to obtain an upper homologous arm GAL80-FA of a GAL80 site; PCR amplification was conducted by primer pairs GAL80-FB-U (SEQ ID NO:27) and GAL80-FB-D (SEQ ID NO:28) to obtain a lower homologous arm GAL80-FB of the GAL80 site; by taking recombinant plasmids Yep352-PH and Yep352-PC as templates, PCR amplification was conducted respectively by primer pairs PGK1p-U (SEQ ID NO:49) and PGK1t-D (SEQ ID NO:50) to obtain PGK1p-Hbd-PGK1t and PGK1p-Crt-PGK1t segments with a strong promoter and a terminator; and by taking a pUG6 plasmid as a template, PCR amplification was conducted by primers KAN-U (SEQ ID NO:23) and KAN-D (SEQ ID NO:24) to obtain a selection marker KanMX.

[0093] By taking the gene GAL80 as an integration site, the five segments GAL80-FA, PGK1p-Hbd-PGK1t, PGK1p-Crt-PGK1t, KanMX and GAL80-FB obtained by PCR were transformed into the recombinant strain 3 simultaneously by the lithium acetate transformation method, and were sequentially connected to and inserted into the integration site, and intracellular integration was conducted to obtain a Saccharomyces cerevisiae recombinant strain 4 after homologous recombination. The homologous recombination process diagram is shown in FIG. 5.

[0094] Four groups of verification were designed respectively according to the gene sequences at the two ends of the recombination site of the Saccharomyces cerevisiae CICC32315 and the inserted homologous recombination sequence, and PCR amplification was conducted by taking a haploid transformant genome growing well as a template to verify a recombinant. The primer pairs GAL80-FA-U (SEQ ID NO:25) and Hbd-D (SEQ ID NO:30) served as upstream verification primers; the primer pairs Hbd-U (SEQ ID NO:29) and KAN-D (SEQ ID NO:24) served as midstream verification primers; the primer pairs KAN-U (SEQ ID NO:23) and Crt-D (SEQ ID NO:32) served as midstream verification primers; and the primer pairs Crt-U (SEQ ID NO:31) and GAL80-FB-D (SEQ ID NO:28) served as downstream verification primers. A transformant verification agar gel electrophoretogram is shown in FIG. 9C, wherein lane 2 is an upstream verification band with a band size being about 2700 bp and consistent with the expectation, lane 3 is a midstream verification band with a band size being about 3000 bp and consistent with the expectation, lane 4 is a midstream verification band with a band size being about 5000 bp and consistent with the expectation, and lane 5 is a midstream verification band with a band size being about 1500 bp and consistent with the expectation.

[0095] The plasmid pGAPza with Cre recombinant enzyme was transformed into the recombinant strain 4 by the lithium acetate transformation method to obtain a transformant; a monoclonal antibody was picked and induced in a galactose medium for 4 h to 5 h, dilution and coating was conducted, and a single colony was picked out onto a YPD plate and was photocopied on a G418 resistance plate; a strain capable of growing on the YPD plate and not growing on the G418 resistance plate was picked out and a genome was extracted to conduct PCR verification. The band of about 1600 bp cannot be obtained by amplifying the segment KanMX by taking the genome of the recombinant strain 4 as control, and the recombinant strain 4 can be amplified to obtain the segment, thus obtaining a recombinant strain 5 losing the selection marker KanMX. The recombinant strain 5 was inoculated into a YPD liquid culture medium for subculture and was transferred for once every 12 h, and the plasmid pGAPza may be lost after several generations, thus obtaining a recombinant strain 6 not containing the plasmid pGAPza.

[0096] {circle around (3)} By taking the genome of the AY14-α as a template, PCR amplification was conducted by primer pairs HXT16-FA-U (SEQ ID NO:33) and HXT16-FA-D (SEQ ID NO:34) to obtain an upper homologous arm HXT16-FA of a HXT16 site; PCR amplification was conducted by primer pairs HXT16-FB-U (SEQ ID NO:35) and HXT16-FB-D (SEQ ID NO:36) to obtain a lower homologous arm HXT16-FB of the HXT16 site; by taking recombinant plasmids Yep352-PT and Yep352-PA as templates, PCR amplification was conducted respectively by primer pairs PGK1p-U (SEQ ID NO:49) and PGK1t-D (SEQ ID NO:50) to obtain PGK1p-Ter-PGK1t and PGK1p-AAT-PGK1t segments with a strong promoter and a terminator; and by taking a pUG6 plasmid as a template, PCR amplification was conducted by primers KAN-U (SEQ ID NO:23) and KAN-D (SEQ ID NO:24) to obtain a selection marker KanMX.

[0097] By taking the gene HXT16 as an integration site, the five segments HXT16-FA, PGK1p-Ter-PGK1t, PGK1p-AAT-PGK1t, KanMX and HXT16-FB obtained by PCR were transformed into the recombinant strain 6 simultaneously by the lithium acetate transformation method, and were sequentially connected to and inserted into the integration site, and intracellular integration was conducted to obtain a Saccharomyces cerevisiae recombinant strain 7 after homologous recombination. The homologous recombination process diagram is shown in FIG. 6.

[0098] Four groups of verification were designed respectively according to the gene sequences at the two ends of the recombination site of the Saccharomyces cerevisiae CICC32315 and the inserted homologous recombination sequence, and PCR amplification was conducted by taking a haploid transformant genome growing well as a template to verify a recombinant. The primer pairs HXT16-FA-U (SEQ ID NO:33) and Ter-D (SEQ ID NO:38) served as upstream verification primers; the primer pairs Ter-U (SEQ ID NO:37) and KAN-D (SEQ ID NO:24) served as midstream verification primers; the primer pairs KAN-U (SEQ ID NO:23) and AAT-D (SEQ ID NO:40) served as midstream verification primers; and the primer pairs AAT-U (SEQ ID NO:39) and HXT16-FB-D (SEQ ID NO:36) served as downstream verification primers. A transformant verification agar gel electrophoretogram is shown in FIG. 9B, wherein lane 2 is an upstream verification band with a band size being about 2700 bp and consistent with the expectation, lane 3 is a midstream verification band with a band size being about 3000 bp and consistent with the expectation, lane 4 is a midstream verification band with a band size being about 5100 bp and consistent with the expectation, and lane 5 is a midstream verification band with a band size being about 2100 bp and consistent with the expectation.

[0099] The plasmid pGAPza with Cre recombinant enzyme was transformed into the recombinant strain 7 by the lithium acetate transformation method to obtain a transformant; a monoclonal antibody was picked and induced in a galactose medium for 4 h to 5 h, dilution and coating was conducted, and a single colony was picked out onto a YPD plate and was photocopied on a G418 resistance plate; a strain capable of growing on the YPD plate and not growing on the G418 resistance plate was picked out and a genome was extracted to conduct PCR verification. The band of about 1600 bp cannot be obtained by amplifying the segment KanMX by taking the genome of the recombinant strain 7 as control, and the recombinant strain 7 can be amplified to obtain the segment, thus obtaining a recombinant strain 8 losing the selection marker KanMX. The recombinant strain 8 was inoculated into the YPD liquid culture medium for subculture and was transferred for once every 12 h, and the plasmid pGAPza may be lost after several generations, thus obtaining a recombinant strain 9 not containing the plasmid pGAPza (that is, obtaining a Saccharomyces strain EST).

[0100] (3) Singly Double-Copying the Gene Ter

[0101] By taking the genome of the AY14-α as a template, PCR amplification was conducted by primer pairs LPP1-FA-U (SEQ ID NO:41) and LPP1-FA-D (SEQ ID NO:42) to obtain an upper homologous arm LPP1-FA of a LPP1 site; PCR amplification was conducted by primer pairs LPP1-FB-U (SEQ ID NO:43) and LPP1-FB-D (SEQ ID NO:44) to obtain a lower homologous arm LPP1-FB of the LPP1 site; by taking the recombinant plasmid Yep352-PE as a template, PCR amplification was conducted by primer pairs PGK1p-U (SEQ ID NO:49) and PGK1t-D (SEQ ID NO:50) to obtain a PGK1p-Ter-PGK1t segment with a strong promoter and a terminator; and by taking a pUG6 plasmid as a template, PCR amplification was conducted by primers KAN-U (SEQ ID NO:23) and KAN-D (SEQ ID NO:24) to obtain a selection marker KanMX.

[0102] By taking the gene LPP1 as an integration site, the four segments LPP1-FA, PGK1p-Ter-PGK1t, KanMX and LPP1-FB obtained by PCR were transformed into the recombinant strain 9 simultaneously by the lithium acetate transformation method, and were sequentially connected to and inserted into the integration site, and intracellular integration was conducted to obtain a Saccharomyces cerevisiae recombinant strain 10 after homologous recombination. The homologous recombination process diagram is shown in FIG. 7.

[0103] Three groups of verification were designed respectively according to the gene sequences at the two ends of the recombination site of the Saccharomyces cerevisiae CICC32315 and the inserted homologous recombination sequence, and PCR amplification was conducted by taking a haploid transformant genome growing well as a template to verify a recombinant. The primer pairs LPP1-FA-U (SEQ ID NO:41) and Ter-D (SEQ ID NO:38) served as upstream verification primers; the primer pairs Ter-U (SEQ ID NO:37) and KAN-D (SEQ ID NO:24) served as midstream verification primers; and the primer pairs KAN-U (SEQ ID NO:23) and LPP1-FB-D (SEQ ID NO:44) served as downstream verification primers. A transformant verification agar gel electrophoretogram is shown in FIG. 9D, wherein lane 2 is an upstream verification band with a band size being about 3000 bp and consistent with the expectation, lane 3 is a midstream verification band with a band size being about 3100 bp and consistent with the expectation, and lane 4 is a downstream verification band with a band size being about 2100 bp and consistent with the expectation.

[0104] The plasmid pGAPza with Cre recombinant enzyme was transformed into the recombinant strain 10 by the lithium acetate transformation method to obtain a transformant; a monoclonal antibody was picked and induced in a galactose medium for 4 h to 5 h, dilution and coating was conducted, and a single colony was picked out onto a YPD plate and was photocopied on a G418 resistance plate; a strain capable of growing on the YPD plate and not growing on the G418 resistance plate was picked out and a genome was extracted to conduct PCR verification. The band of about 1600 bp cannot be obtained by amplifying the segment KanMX by taking the genome of the recombinant strain 10 as control, and the recombinant strain 10 can be amplified to obtain the segment, thus obtaining a recombinant strain 11 losing the selection marker KanMX. The recombinant strain 11 was inoculated into the YPD liquid culture medium for subculture and was transferred for once every 12 h, and the plasmid pGAPza may be lost after several generations, thus obtaining a recombinant strain 12 not containing the plasmid pGAPza. (That is, obtaining a Saccharomyces strain EDT)

[0105] (4) Singly Double-Copying the Gene AAT

[0106] By taking the genome of the AY14-α as a template, PCR amplification was conducted by primer pairs KU70-FA-U (SEQ ID NO:45) and KU70-FA-D (SEQ ID NO:46) to obtain an upper homologous arm KU70-FA of a KU70 site; PCR amplification was conducted by primer pairs KU70-FB-U (SEQ ID NO:47) and KU70-FB-D (SEQ ID NO:48) to obtain a lower homologous arm KU70-FB of the KU70 site; by taking the recombinant plasmid Yep352-PA as a template, PCR amplification was conducted by primer pairs PGK1p-U (SEQ ID NO:49) and PGK1t-D (SEQ ID NO:50) to obtain a PGK1p-AAT-PGK1t segment with a strong promoter and a terminator; and by taking a pUG6 plasmid as a template, PCR amplification was conducted by primers KAN-U (SEQ ID NO:23) and KAN-D (SEQ ID NO:24) to obtain a selection marker KanMX.

[0107] By taking the gene KU70 as an integration site, the four segments KU70-FA, PGK1p-AAT-PGK1t, KanMX and KU70-FB obtained by PCR were transformed into the recombinant strain 9 simultaneously by the lithium acetate transformation method, and were sequentially connected to and inserted into the integration site, and intracellular integration was conducted to obtain a Saccharomyces cerevisiae recombinant strain 13 after homologous recombination. The homologous recombination process diagram is shown in FIG. 8.

[0108] Four groups of verification were designed respectively according to the gene sequences at the two ends of the recombination site of the Saccharomyces cerevisiae CICC32315 and the inserted homologous recombination sequence, and PCR amplification was conducted by taking a haploid transformant genome growing well as a template to verify a recombinant. The primer pairs KU70-FA-U (SEQ ID NO:45) and AAT-D (SEQ ID NO:40) served as upstream verification primers; the primer pairs AAT-U (SEQ ID NO:39) and KAN-D (SEQ ID NO:24) served as midstream verification primers; and the primer pairs KAN-U (SEQ ID NO:23) and KU70-FB-D (SEQ ID NO:48) served as downstream verification primers. A transformant verification agar gel electrophoretogram is shown in FIG. 9E, wherein lane 2 is an upstream verification band with a band size being about 3300 bp and consistent with the expectation, lane 3 is a midstream verification band with a band size being about 3100 bp and consistent with the expectation, and lane 4 is a downstream verification band with a band size being about 2100 bp and consistent with the expectation. (That is, obtaining a strain EDS)

[0109] (5) Double-Copying the Gene Ter and the Gene AAT Simultaneously

[0110] By taking the gene KU70 as an insertion site, the four segments KU70-FA, PGK1p-AAT-PGK1t, KanMX and KU70-FB (the segment obtaining method is as same as the method of singly double-copying the gene AAT in (4)) obtained by PCR were transformed into the recombinant strain 12 (the strain prepared by (3) singly double-copying the gene Ter) simultaneously, and were sequentially connected to and inserted into the integration site, and intracellular integration was conducted to obtain a Saccharomyces cerevisiae recombinant strain 14 after homologous recombination. The homologous recombination process diagram is shown in FIG. 8.

[0111] Four groups of verification were designed respectively according to the gene sequences at the two ends of the recombination site of the Saccharomyces cerevisiae CICC32315 and the inserted homologous recombination sequence, and PCR amplification was conducted by taking a haploid transformant genome growing well as a template to verify a recombinant. The primer pairs KU70-FA-U (SEQ ID NO:45) and AAT-D (SEQ ID NO:40) served as upstream verification primers; the primer pairs AAT-U (SEQ ID NO:39) and KAN-D (SEQ ID NO:24) served as midstream verification primers; and the primer pairs KAN-U (SEQ ID NO:23) and KU70-FB-D (SEQ ID NO:48) served as downstream verification primers. A transformant verification agar gel electrophoretogram is shown in FIG. 9F, wherein lane 2 is an upstream verification band with a band size being about 3300 bp and consistent with the expectation, lane 3 is a midstream verification band with a band size being about 3100 bp and consistent with the expectation, and lane 4 is a downstream verification band with a band size being about 2100 bp and consistent with the expectation. That is, obtaining a strain EDS.

Embodiment 2: Corn Raw Material Thick Mash Fermentation Experiment of Starting Strain and Modified Strain

[0112] (1) Corn Raw Material Thick Mash Fermentation Experiment of the Recombinant Strains EST, EDT, EDS and EDST, and the Parent Strain (AY14-α)

[0113] The parent strain AY14-α, and the recombinant strains EST, EDT, EDS and EDST were subjected to corn raw material thick mash fermentation experiments respectively, the fermentation process route diagram: corn flour.fwdarw.soaking.fwdarw.liquification.fwdarw.saccharification.fwdarw.cooling.fwdarw.inoculation.fwdarw.fermentation.fwdarw.win e steaming.fwdarw.index measurement; and

[0114] one ring of saccharomyces cells were picked respectively, inoculated into a test tube filled with 5 mL of primary seed culture medium respectively for static culture at 30° C. for 24 h, inoculated into a 150 mL triangular flask filled with 45 mL of secondary seed culture medium according to 10% of inoculation amount for static culture at 30° C. for 16 h until the later stage of a logarithmic phase, and inoculated into a fermentation medium according to 10% of inoculation amount for static culture at 30° C. Weighing was conducted for one time every 12 h, and fermentation ended when the weight loss of two times was less than 1 g, that is, fermentation ended after 84 h fermentation. After fermentation, 100 mL of water was added to 100 mL of mash to steam 100 mL of wine sample. The fermentation performance indexes such as CO.sub.2 cumulative emission, alcoholic strength, residual reducing sugar and the like were measured. The result is shown in Table 3.

[0115] The primary seed culture medium consists of corn flour of 82 g/L, high-temperature resistant α-amylase with an adding amount being about 1.0×10.sup.4 U/L, saccharifying enzyme with enzyme activity being about −35 U/L and the balance of water, the sugar degree being 8° BX.

[0116] The primary seed culture medium consists of corn flour of 125 g/L, high-temperature resistant α-amylase with an adding amount being about 1.5×10.sup.4 U/L, saccharifying enzyme with enzyme activity being about 50 U/L and the balance of water, the sugar degree being 12° BX.

[0117] The fermentation medium consists of corn flour of 315 g/L, high-temperature resistant α-amylase of 3.5×10.sup.4 U/L, saccharifying enzyme of 95 U/L, acid proteinase of 15 U/L, nutritive salt solution of 5.5-5.6 mL/L and the balance of water, wherein the nutritive salt solution consists of MgSO.sub.4 of 150 g/L, KH.sub.2PO.sub.4 of 75 g/L, urea of 81 g/L and the balance of water, and is stored at 4° C. after being filtered.

[0118] The treatment process condition of the fermentation medium is as follows:

[0119] soaking condition: soaking the corn flour at 60° C. to 70° C. for 20 min; liquefying condition: at 85° C. to 90° C., adding the high-temperature resistant α-amylase according to the above proportion and liquefying for 90 min; and saccharifying condition: at 55° C. to 60° C., adding the saccharifying enzyme, saccharifying for 20 min, adding the nutritive salt solution and the acid proteinase, reacting at 30° C. for 20 min to obtain the fermentation medium.

TABLE-US-00003 TABLE 3 Comparison of Fermentation Performance between Parent Strain and Recombinant Strain 20° C. Standard Alcohol Strength Residual Sugar CO.sub.2 Weight Strain (% vol) (g/100 mL) Loss (g) AY14-α 16.23 ± 0.41 0.23 ± 0.01 23.49 ± 0.89 EST 15.94 ± 0.37 0.40 ± 0.03 23.61 ± 0.94 EDT 15.70 ± 0.44 0.35 ± 0.01 23.81 ± 0.49 EDS 16.37 ± 0.51 0.23 ± 0.03 23.58 ± 1.13 EDST 16.53 ± 0.45 0.21 ± 0.01 23.97 ± 0.76 Note: the data shown is the average value of three parallel experimental results.

[0120] Referring to Table 3 and FIG. 10, it can be seen that the alcohol content and the residual sugar content of the fermented recombinant strains EST, EDT, EDS and EDST are not significantly different from those of the starting strain AY14-α, which indicates that the growth and fermentation performance of the modified strain do not change significantly.

[0121] (2) Yield of Ester Measured by GC-MS

[0122] For the 100 mL of wine sample finally obtained from the corn raw material thick mash fermentation experiment of the recombinant strains EST, EDT, EDS and EDST and the parent strain (AY14-α) mentioned in (1), the yield of the ethyl butyrate and the yield of the ethyl crotonate were measured.

[0123] Measurement method: setting the GC condition of a gaschromatograph: chromatographic column HP-5MS, and 60 m×0.32 mm×0.25 μm quartz capillary column; the temperature of a sample inlet is 250° C.; the carrier gas is high-purity helium with a flow speed of 1 mL/min; the column temperature starts at 40° C. for 3 min, rises to 116° C. at 9/min ° C. for 4 min, then rises to 260° C. at 9/min ° C. for 5 min; and splitless sample injection. The mass spectrometer condition: the ion source is an EI source, the temperature of the ion source is 230° C., electronic energy is 70 eV, the temperature of a quadrupole rod is 150° C., the interface temperature is 280° C., the voltage of an electron multiplier is 1280 V, and the scanning range m/z is 40 u to 450 u.

[0124] The measured yields of the ethyl butyrate and the ethyl crotonate of the recombinant strains EST, EDT, EDS and EDST and the parent strain (AY14-α) are shown in Table 4.

TABLE-US-00004 TABLE 4 Ester Yield of Parent Strain and Recombinant Strains (unit: mg/L) AY14- Strain α EST EDT EDS EDST Ethyl NF 77.33 ± 3.79 86.6 ± 2.03 78.16 ± 5.31 99.65 ± 7.32 butyrate (mg/L) Ethyl NF  1.49 ± 0.37 8.38 ± 1.04 68.01 ± 1.01 40.93 ± 3.18 crotonate (mg/L) Note: the data shown is the average value of three parallel experimental results.

[0125] In Table 3 and Table 4, the AY14-α is the original strain, the EST is the strains of the overexpressed genes Erg10, Hbd, Crt, Ter and AAT, the EDS is the strain only double-copying AAT based on the EST, the EDT is the strain only double-copying Ter based on the EST, and the EDST is the strain double-copying AAT and Ter based on the EST.