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-sporulatingbrewing strain of Saccharomyces carlsbergensis after treatment with N-methyl-N'-nitro-Nnitrosoguanidine. One closer studied mutant, C77-T70 had a threonine deaminase activity which was less sensitive to L-isoleucine than that of the parent strain. The inhibition with varying concentrations of L-isoleucine showed that part of the activity was as sensitive as that of the parent strain, suggesting that the mutant isheterozygous for a dominant change in the structural gene for threonine deaminase. Of twelve mutants selected on 25 mM-thiaisoleucine all are believed to have undergone a change similar to that of C77-T70, since their threonine deaminase activity was partly resistant to 1 mM-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 theparent strain, while the syntheses in the mutants of 3-methyl-l-butanol (isoamyl alcohol) and 2-methyl-lpropanol(isobutanol) were similar or slightly reduced compared to the level found in the parent strain.