STUDY OF THE SULPHUR METABOLISM OF METHIONINE-RICH YEASTS

Abstract

Mutants were produced by mild mutagenic agents such as DV-radiation and nitrite treat- ment, from yeast strains belonging to different genera. The primary selection has been based on an assumed higher sulphate requirement. Norleucine has been used as methionine antagonist and the resistance of the mutants was compared to that of the parent strains. Our assays showed that the mutants selected were more sensitive to norleucine as compared to the untreated strains. Thus, the higher methionine content cannot be attributed to homoserine-0-transsuccinase derepression. Simultaneously with the increased sulphate requirement, the mutants responded to a higher methyl donor concentration also by an enhanced growth. In methionine enriched mutants also the lipoic acid concentration of the yeast in- creased in parallel with the increase in the sulphurous amino acid concentration. The concentrations of both components react very sensitively to the aeration intensity applied during fermentation and these begin to fall at the value of 200 mmol O2h-11-1. Sulphur present in sulphate form has been found to be a much better sulphur source than methionine. Applying the latter, a slower cell growth can be observed due to sulphur limitation. Our experimental results indicate that with the mutants produced, the sulphate reduction step became more efficient and provided an increased methionine content.