Threonine overproduction in yeast strains carrying the HOM3-R2 mutant allele under the control of different inducible promoters

The HOM3 gene of Saccharomyces cerevisiae codes for aspartate kinase, which plays a crucial role in the regulation of the metabolic flux that leads to threonine biosynthesis. With the aim of obtaining yeast strains able to ov erproduce threonine in a controlled may, we have placed the HOM3-R2 mutant allele, which causes expression of a feedback-insensitive enzyme, under the control of four distinctive regulatable yeast promoters, namely, P-GAL1, P -CHA1, PCYC1-HSE2, and P-GPH1, The amino acid contents of strains bearing t he different constructs were analyzed both under repression and induction c onditions. Although some differences in overall threonine production were f ound, a maximum of around 400 nmol/mg (dry weight) was observed, Other fact ors, such as excretion to the medium and activity of the catabolic threonin e/serine deaminase, also affect threonine accumulation. Thus, improvement o f threonine productivity by yeast cells would probably require manipulation of these and other factors.