Differential regulation of Salmonella typhimurium type III secreted proteins by pathogenicity island 1 (SPI-1)-encoded transcriptional activators InvF and HilA

Salmonella enterica encodes a type III protein secretion system within a pa thogenicity island (SPI-1) that is located at centisome 63 of its chromosom e. This system is required for the ability of these bacteria to stimulate c ellular responses that are essential for their pathogenicity. Expression of components and substrates of this system is subject to complex regulatory mechanisms. These mechanisms involve the function of HilA and InvF, two tra nscriptional regulatory proteins encoded within SPI-1. In this study, we ex amined the functional relationship between these two regulatory proteins. W e found that strains carrying loss-of-function mutations in either hilA or invF differ in their ability to stimulate cellular responses. An S. typhimu rium hilA mutant strain retained considerable signaling capacity that resul ted in significant levels of internalization into host cells. In contrast, introduction of a nonpolar loss-of-function mutation in invF rendered S. ty phimurium significantly impaired in its ability to enter host cells. Consis tent with these different phenotypes, we found that HilA and InvF control t he expression of different genes. HilA regulates the expression of componen ts of the type III secretion machinery, whereas InvF controls the expressio n of type III secreted proteins encoded outside of SPI-1. We also found tha t the expression of secreted proteins encoded within SPI-1 are under the co ntrol of both HilA and InvF. Our results therefore indicate that InvF and H ilA differentially control the expression of components and substrates of t he invasion-associated type In secretion system.