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E. coli growth is impaired in the presence of micromolar amounts of D-cysteine. Soutourina et al. found that E. coli contain a D-cysteine desulfhydrase which can convert D-cysteine into pyruvate, H2S, and NH3. Overexpression of yedO (the E. coli gene encoding D-cysteine desulfydrase activity) protected E. coli against D-cysteine, whereas its inactivation rendered E. coli hypersensitive. With yedO intact, E. coli growth was improved by addition of D-cysteine as the sole sulfur source. Similar to L-cysteine, D-cysteine exerts toxicity through inhibition of threonine deaminase, a key enzyme of the isoleucine, leucine, and valine biosynthesis pathway. Growth protection against D-cysteine in minimal medium was conferred by simultaneous addition of isoleucine, leucine and valine. L-aspartate was also observed to exert a protective effect against D-cysteine toxicity.