Overproduction of threonine by Saccharomyces cerevisiae mutants resistant to hydroxynorvaline.

In this work, we isolated and characterized mutants that overproduce threonine from Saccharomyces cerevisiae. The mutants were selected for resistance to the threonine analog alpha-amino-beta-hydroxynorvalerate (hydroxynorvaline), and, of these, the ones able to excrete threonine to the medium were chosen. The mutant strains produce between 15 and 30 times more threonine than the wild type does, and, to a lesser degree, they also accumulate isoleucine. Genetic and biochemical studies have revealed that the threonine overproduction is, in all cases studied, associated with the presence in the strain of a HOM3 allele coding for a mutant aspartate kinase that is totally or partially insensitive to feedback inhibition by threonine. This enzyme seems, therefore, to be crucial in the regulation of threonine biosynthesis in S. cerevisiae. The results obtained suggest that this strategy could be efficiently applied to the isolation of threonine-overproducing strains of yeasts other than S. cerevisiae, even those used industrially.