Microbial Sulfur Metabolism and Cysteine Fermentation

Cysteine (cystine) is an important amino acid in pharmaceutical, food, and cosmetic industry. However, cysteine is toxic to cells even at low concentrations; thus, its intracellular levels are stringently controlled through several modes of regulation, which hinders the achievement of cysteine fermentative production. These regulatory modes include the feedback inhibition of key metabolic enzymes, degradation, efflux pumps, and the transcriptional regulation of biosynthetic genes. Accordingly, shedding light on how to manipulate these complex regulations are crucial for optimizing metabolic flux toward cysteine over production. Searching and engineering high capacity efflux pump of cysteine is also crucial for both Down-regulating cysteine intracellular level to avoid toxic effects, and secreting it into the fermentation media to facilitate further isolation process. Recently, we have succeeded in establishing the technology for fermentative production of cysteine and launched a commercial basis production. In this presentation we discuss the microbial sulfur metabolism and metabolic engineering of the cysteine-producing strains.