Disulfide bond formation in periplasmic proteins is catalysed by the DsbA/DsbB system in most Gram-negative bacteria. Salmonella enterica serovar Typhimurium also encodes a paralogous pair of proteins to DsbA and DsbB, DsbL and DsbI, respectively, downstream of a periplasmic arylsulfate sulfotransferase (ASST). We show that DsbL and DsbI function as a redox pair contributing to periplasmic disulfide bond formation and, as such, affect transcription of the Salmonella pathogenicity island 1 (SPI1) type three secretion system genes and activation of the RcsCDB system, as well as ASST activity. In contrast to DsbA/DsbB, however, the DsbL/DsbI system cannot catalyse the disulfide bond formation required for flagellar assembly. Phylogenic analysis suggests that the assT dsbL dsbI genes are ancestral in the Enterobacteriaceae, but have been lost in many lineages. Deletion of assT confers no virulence defect during acute Salmonella infection of mice.
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