A transcriptomic analysis of genes consistently upregulated in high producer clones were each evaluated for their ability to increase the production of a protein of interest. The products of these genes (metabolism influencing products (MIP)), such as actin, Erp27, Erp57, Foxa1, PPAR, Ca3, and Tagap, could be sub-categorized into different functional categories such as signaling, protein folding, cytoskeleton organization and cell survival.

It is an object of the present invention to provide a practical means of producing cysteine from glutathione that is also suitable for use in the field of foods. Cysteine is produced from glutathione by a first step of producing cysteinylglycine by the action of a γ-glutamyl peptidase derived from a microorganism on reduced glutathione; and a second step of producing cysteine by the action of an acid protease derived from a microorganism on the cysteinylglycine.

It is an object of the present invention to provide a practical means for generating a cysteine from a protein, which is suitable for utilization in the field of food products (food product use). According to the present invention, there is provided a method for producing a cysteine-containing proteolytic product, comprising; allowing a -glutamyl peptide hydrolase, a filamentous fungi-derived protease, and a bacterial protease to act on a protein material.