Abstract
In most bacteria, inorganic sulfur is assimilated into cysteine, which provides sulfur for methionine biosynthesis via transsulfurylation. Here, cysteine is transferred to the terminal carbon of homoserine via its sulfhydryl group to form cystathionine, which is cleaved to yield homocysteine. In the enteric bacteria Escherichia coli and Salmonella enterica, these reactions are catalyzed by irreversible cystathionine-gamma-synthase and cystathionine-beta-lyase enzymes. Alternatively, yeast and some bacteria assimilate sulfur into homocysteine, which serves as a sulfhydryl group donor in the synthesis of cysteine by reverse transsulfurylation with a cystathionine-beta-synthase and cystathionine-gamma-lyase. Herein we report that the related enteric bacterium Klebsiella pneumoniae encodes genes for both transsulfurylation pathways; genetic and biochemical analyses show that they are coordinately regulated to prevent futile cycling. Klebsiella uses reverse transsulfurylation to recycle methionine to cysteine during periods of sulfate starvation. This methionine-to-cysteine (mtc) transsulfurylation pathway is activated by cysteine starvation via the CysB protein, by adenosyl-phosphosulfate starvation via the Cbl protein, and by methionine excess via the MetJ protein. While mtc mutants cannot use methionine as a sulfur source on solid medium, they will utilize methionine in liquid medium via a sulfide intermediate, suggesting that an additional nontranssulfurylation methionine-to-cysteine recycling pathway(s) operates under these conditions.