Mutants in the biosynthesis of isoleucine in a non-mating, non-sporulating brewing strain of Saccharomyces carlsbergensis

Mutants resistant to 2-amino-3-(methylthio)butanoic acid (thiaisoleucine) were selected from a non-mating, non-sporulating brewing strain of Saccharomyces carlsbergensis after treatment with N-methyl-N′-nitro-N-nitrosoguanidine. One closer studied mutant, C77-T70 had a threonine deaminase activity which was less sensitive tol-isoleucine than that of the parent strain.

The inhibition with varying concentrations ofl-isoleucine showed that part of the activity was as sensitive as that of the parent strain, suggesting that the mutant is heterozygous for a dominant change in the structural gene for threonine deaminase. Of twelve mutants selected on 25mm-thiaisoleucine all are believed to have undergone a change similar to that of C77-T70, since their threonine deaminase activity was partly resistant to 1mm-l-isoleucine like that of C77-T70.

Four such mutants were compared to the parent strain for the production of fusel alcohols during aerobic growth in minimal medium. The synthesis of 2-methyl-1-butanol (d-amyl alcohol) was 2–5 times higher in the mutants than in the parent strain, while the syntheses in the mutants of 3-methyl-1-butanol (isoamyl alcohol) and 2-methyl-1-propanol (isobutanol) were similar or slightly reduced compared to the level found in the parent strain.