AraC/XylS family members, HilD and HilC, directly activate virulence gene expression independently of HilA in Salmonella typhimurium

Salmonella typhimurium is a Gram-negative enteric pathogen that can infect intestinal epithelial cells and induce inflammation of the intestinal mucosa. These processes are mediated by a type III secretion system (TTSS), which is encoded on Salmonella pathogenicity island 1 (SPI1). Previous studies showed that four SPI1-encoded transcriptional regulators, HiID, HiIC, HiIA and InvF, act in an ordered fashion to coordinately activate expression of the SPI1 TTSS. HiID and HiIC derepress hilA transcription. HiIA activates invF as well as SPI1 genes that encode components of the TTS apparatus. InvF then activates genes that encode proteins secreted by the SPI1 TTS apparatus. In this scheme, HiID and HiIC indirectly activate expression of the SPI1 TTS apparatus and its secreted substrates by affecting hilA expression. Here, we report that HiID and HilC can also activate expression of a subset of SPI1 genes independently of HiIA. Our studies show that MiID and HiIC activate transcription of invF from a promoter that is far upstream of its HiIA-dependent promoter. This activation is most probably through direct binding of HiID and HiIC to sequences upstream and downstream of this alternative HiIA-independent promoter. We conclude that MiID and HiIC have a second role in. SPI1 gene regulation that is separate from their role in co-ordinating expression of the SPI1 TTSS through hiIA.