Faculty Bibliography

SigD and SigE (Salmonella invasion gene) are proteins needed for optimal invasion of Salmonella typhimurium into eukaryotic cells in vitro. SigD is a secreted protein and SigE is a putative chaperone required for SigD stability and/or secretion. SigD is secreted by a type III secretion apparatus encoded within a pathogenicity island on the Salmonella chromosome known as Salmonella pathogenicity island 1 (SPI1). The expression of sigDE, which is not linked to SPI1, is co-ordinately regulated with the SPI1 genes and is dependent on the transcriptional regulators SirA, HilA and InvF. These three proteins alone are unable to activate transcription from the sigD promoter in Escherichia coli, therefore it is likely that other factors are needed for expression. A screen for genes required for the expression of a sigD-lacZYA reporter fusion found a mutant with a transposon insertion in spaS, an SPI1 gene which encodes a putative inner-membrane component of the type III secretion system. The expression of a SPI1 operon encoding a putative chaperone (SicA) and several secreted proteins (Sips B, C, D and A) was also reduced in this mutant. The regulation defect of the spaS mutant was complemented by sicA and not by spaS. Because sicA is encoded immediately downstream of spaS, the mutation in spaS was likely to be polar on the expression of sicA. In addition, a sicA disruption mutant was as defective as an invF deletion mutant for the expression of sigD, sicA and sipC reporter fusions. The introduction of plasmids encoding invF and sicA into a non-pathogenic E. coli K-12 strain stimulated the transcription of both a sicA- and a sigD-lacZYA promoter fusion. This result suggests that InvF and SicA are sufficient for the expression of these genes. This is the first demonstration of a positive regulatory role for a putative type III secretion system chaperone in the expression of virulence genes

The expression of genes encoding proteins secreted by the SPI1 (Salmonella pathogenicity island) type III secretion apparatus is known to require the transcriptional activators SirA and HilA. However, neither SirA nor HilA is believed to directly activate the promoters of these genes. invF, the first gene of the inv-spa gene cluster, is predicted to encode an AraC-type transcriptional activator and is required for invasion into cultured epithelial cells. However, the genes which are regulated by InvF have not been identified. In this work, an in-frame deletion in invF was constructed and tested for the expression of Phi(sigD-lacZYA), sipC::Tn5lacZY, and a plasmid-encoded Phi(sicA-lacZYA). SigD (Salmonella invasion gene) is a secreted protein required for the efficient invasion of Salmonella typhimurium into cultured eucaryotic cells. sicA (Salmonella invasion chaperone) is the first gene of a putative operon encoding the Sip/Ssp (Salmonella invasion/Salmonella secreted proteins) invasion proteins secreted by the SPI1 type III export apparatus. invF was required for the expression of the sigD, sicA, and sipC fusions. This is the first demonstration that there is a functional promoter in the intergenic sequence between spaS and sicA. In addition, several proteins were either absent from or found in reduced amounts in the culture supernatants of the invF mutant. Therefore, invF is required for the optimal expression of several genes encoding SPI1-secreted proteins. Genetic evidence is also presented suggesting there is HilA-dependent readthrough transcription from the invF promoter at least through sipC

Salmonella is one of the most extensively characterized bacterial pathogens and is a leading cause of bacterial gastroenteritis. Despite this, we are only just beginning to understand at a molecular level how Salmonella interacts with its mammalian hosts to cause disease. Studies during the past decade on the genetic basis of virulence of Salmonella have significantly advanced our understanding of the molecular basis of the host-pathogen interaction, yet many questions remain. In this review, we focus on the interaction of enterocolitis-causing salmonellae with the intestinal mucosa, since this is the initiating step for most infections caused by Salmonella. Animal and in vitro cell culture models for the interaction of these bacteria with the intestinal epithelium are reviewed, along with the bacterial genes that are thought to affect this interaction. Lastly, recent studies on the response of epithelial cells to Salmonella infection and how this might promote diarrhea are discussed