Cysteine degradation gene yhaM, encoding cysteine desulfidase, serves as a genetic engineering target to improve cysteine production in Escherichia coli

Gen Nonaka, Kazuhiro Takumi

Cysteine is an important amino acid for various industries; however, there is no efficient microbial fermentation-based production method available. Owing to its cytotoxicity, bacterial intracellular levels of cysteine are stringently controlled via several modes of regulation, including cysteine degradation by cysteine desulfhydrases and cysteine desulfidases. In Escherichia coli, several metabolic enzymes are known to exhibit cysteine degradative activities, however, their specificity and physiological significance for cysteine detoxification via degradation are unclear. Relaxing the strict regulation of cysteine is crucial for its overproduction; therefore, identifying and modulating the major degradative activity could facilitate the genetic engineering of a cysteine-producing strain. In the present study, we used genetic screening to identify genes that confer cysteine resistance in E. coli and we identified yhaM, which encodes cysteine desulfidase and decomposes cysteine into hydrogen sulfide, pyruvate, and ammonium. Phenotypic characterization of a yhaM mutant via growth under toxic concentrations of cysteine followed by transcriptional analysis of its response to cysteine showed that yhaM is cysteine-inducible, and its physiological role is associated with resisting the deleterious effects of cysteine in E. coli. In addition, we confirmed the effects of this gene on the fermentative production of cysteine using E. coli-based cysteine-producing strains. We propose that yhaM encodes the major cysteine-degrading enzyme and it has the most significant role in cysteine detoxification among the numerous enzymes reported in E. coli, thereby providing a core target for genetic engineering to improve cysteine production in this bacterium.