CydDC functions as a cytoplasmic cystine reductase to sensitize to oxidative stress and aminoglycosides.

l-cysteine is the source of all bacterial sulfurous biomolecules. However, the cytoplasmic level of l-cysteine must be tightly regulated due to its propensity to reduce iron and drive damaging Fenton chemistry. It has been proposed that in the component of cytochrome -I terminal oxidase, the CydDC complex, shuttles excessive l-cysteine from the cytoplasm to the periplasm, thereby maintaining redox homeostasis. Here, we provide evidence for an alternative function of CydDC by demonstrating that the phenotype, unlike that of the bona fide l-cysteine exporter , parallels that of the l-cystine importer Chromosomal induction of , but not of , from a strong pLtetO-1 promoter (P) leads to the increased level of extracellular l-cysteine, whereas induction of or causes the accumulation of cytoplasmic l-cysteine. Congruently, inactivation of renders cells resistant to hydrogen peroxide and to aminoglycoside antibiotics. In contrast, induction of sensitizes cells to oxidative stress and aminoglycosides, which can be suppressed by overexpression. Furthermore, inactivation of the ferric uptake regulator ( in P- or P- cells results in dramatic loss of survival, whereas catalase () overexpression suppresses the hypersensitivity of both strains to HO These results establish CydDC as a reducer of cytoplasmic cystine, as opposed to an l-cysteine exporter, and further elucidate a link between oxidative stress, antibiotic resistance, and sulfur metabolism.