Synthesis of γ-glutamylcysteine as a major low-molecular-weight thiol in lactic acid bacteria Leuconostoc spp.

Some members of lactic acid bacteria are known to synthesize glutathione (GSH) or to import it from growth medium, whereas others are not. Analysis of the genome sequences of several Leuconostoc spp. indicate the presence of the gene gshA that encodes γ-glutamylcysteine synthetase, but not the gene gshB encoding glutathione synthetase. We report here that, in cells of Leuconostoc kimchii and Leuconostoc mesenteroides, γ-glutamylcysteine (γ-GC) is present in large amount, whereas GSH is not detectable. The level of γ-GC was higher at the stationary phase than at the exponential phase. Expression of the gshA gene in Leuconostoc spp. analyzed by S1 mapping showed the increased mRNA level upon hydrogen peroxide treatment. From high-resolution S1 mapping, the transcriptional start site was mapped and the putative promoter elements were suggested. This work suggests that γ-GC has a significant role in Leuconostoc spp. as the major low-molecular-weight thiol.