This invention is intended to improve glutathione productivity by fermentation using microorganisms. This invention relates to a microbial strain capable of overexpression of γ-glutamylcysteine, bis-γ-glutamylcystine, γ-glutamylcystine, reduced glutathione, and/or oxidized glutathione in which the expression level of a gene encoding serine-O-acetyltransferase (EC:2.3.1.30) is enhanced and a method involving the use of such microbial strain.

One or more embodiments of the present invention provide a microbial strain with improved productivity of ?-glutamylcysteine, bis-?-glutamylcystine, ?-glutamylcystine, reduced glutathione, and/or oxidized glutathione. Such microbial strain has disruption of [1] a gene encoding ?-glutamyltransferase and [2] a gene encoding phosphoglycerate mutase and enhanced expression of [3] a gene encoding glutamate-cysteine ligase and/or a gene encoding glutathione synthetase or [4] a gene encoding bifunctional glutathione synthetase. This invention also discloses a method for producing the substances mentioned above via culture of the microbial strain.