Envelope Stress and Regulation of the Salmonella Pathogenicity Island 1 Type III Secretion System

Salmonella enterica serovar Typhimurium uses a type three secretion system (T3SS) encoded on the Salmonella pathogenicity island 1 (SPI1) to invade intestinal epithelial cells and induce inflammatory diarrhea. The SPI1 T3SS is regulated by numerous environmental and physiological signals, integrated to either activate or repress invasion. Transcription of hilA, encoding the transcriptional activator of the SPI1 structural genes, is activated by three AraC-like regulators, HilD, HilC, and RtsA, that act in a complex feed-forward loop. Deletion of bamB, encoding a component of the β-barrel assembly machinery, causes a dramatic repression of SPI1, but the mechanism was unknown. Here, we show that partially defective β-barrel assembly activates the RcsCDB regulon, leading to decreased hilA transcription. This regulation is independent of RpoE activation. Though Rcs has been previously shown to repress SPI1 when disulfide bond formation is impaired, we show that activation of Rcs in a bamB background is dependent on the sensor protein RcsF, whereas disulfide bond status is sensed independently. Rcs decreases transcription of the flagellar regulon, including fliZ, the product of which indirectly activates HilD protein activity. Rcs also represses hilD, hilC, and rtsA promoters by an unknown mechanism. Both dsbA and bamB mutants have motility defects, though this is simply regulatory in a bamB background; motility is restored in the absence of Rcs. Effector secretion assays show that repression of SPI1 in a bamB background is also regulatory; if expressed, the SPI1 T3SS is functional in a bamB background. This emphasizes the sensitivity of SPI1 regulation to overall envelope homeostasis.IMPORTANCESalmonella causes worldwide foodborne illness, leading to massive disease burden and an estimated 600,000 deaths per year. Salmonella infects orally and invades intestinal epithelial cells using a type 3 secretion system that directly injects effector proteins into host cells. This first step in invasion is tightly regulated by a variety of inputs. In this work, we demonstrate that Salmonella senses the functionality of outer membrane assembly in determining regulation of invasion machinery, and we show that Salmonella uses distinct mechanisms to detect specific perturbations in envelope assembly.