A conserved amino acid sequence directing intracellular type III secretionby Salmonella typhimurium

Type III secretion systems (TTSS) are important virulence factors that Gram -negative bacteria use to translocate proteins into the cytoplasm of eukary otic host cells. Salmonellae encode two virulence-associated TTSS, The Salm onella pathogenicity island 1 (SPI1)-encoded TTSS is active on contact with host cells, whereas the Salmonella pathogenicity island 2 (SPI2)-encoded T TSS is expressed after phagocytosis of bacteria by host cells. Previously, no consensus signal sequence for translocation has been identified among TT SS effector proteins. In this work, seven proteins, termed Salmonella-trans located effecters (STE), are described that contain conserved amino acid se quences that direct translocation by TTSS in Salmonella typhimurium. STE th at are coordinately regulated with SPI2 gene expression contain translocati on signals that are recognized by the SPI2 but not by the SPI1 TTSS. STE th at are constitutively expressed contain signals that direct translocation t hrough both SPI1 and SPI2 TTSS, Of the seven STE examined, SspH1 and SspH2 have been previously shown to be translocated and involved in virulence; Sl rP and SifA were identified as virulence factors, but were not previously k nown to be associated with TTSS; and SseI, SseJ, and SifB were previously u nidentified. Three STE genes (sspH1, sspH2, and sseI) are located within te mperate bacteriophages. suggesting a common mechanism for the dissemination of more recently evolved STE.