Salmonella typhimurium leucine-rich repeat proteins are targeted to the SPI1 and SPI2 type III secretion systems

Salmonellae encode two virulence-associated type III secretion systems (TTS S) within Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2). Two Sal monella typhimurium genes, sspH1 and sspH2, that encode proteins similar to the Shigella flexneri and Yersinia species TTSS substrates, IpaH and YopM, were identified. SspH1 and SspH2 are proteins containing leucine-rich repe ats that are differentially targeted to the SPI1 and SPI2 TTSS. sspH2 trans cription was induced within RAW264.7 macrophages, and was dependent upon th e SPI2-encoded regulator ssrA/ssrB. In contrast, sspH1 transcription is ind ependent of SPI2, and is not induced after bacterial phagocytosis by eukary otic cells. Infection of eukaryotic cells with strains expressing a SspH2-C yaA fusion protein resulted in SPI2 TTSS-dependent cAMP increases. In contr ast, SspH1-CyaA-mediated cAMP increases were both SPI1 and SPI2 TTSS depend ent. sspH2-like sequences were found in most Salmonella serotypes examined, whereas sspH1 was detected in only one S. typhimurium isolate, indicating that the copy number of sspH genes can be variable within Salmonella seroty pes. S. typhimurium deleted for both sspH1 and sspH2 was not able to cause a lethal infection in calves, indicating that these genes participate in S. typhimurium virulence for animals.