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

Abstract

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 overproduce threonine in a controlled way, 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 ,P CYC1-HSE2 , and P GPH1 . The amino acid contents of strains bearing the different con- structs were analyzed both under repression and induction conditions. Although some differences in overall threonine production were found, a maximum of around 400 nmol/mg (dry weight) was observed. Other factors, such as excretion to the medium and activity of the catabolic threonine/serine deaminase, also affect threonine accumulation. Thus, improvement of threonine productivity by yeast cells would probably require manipula- tion of these and other factors.