Regulation of ssrAB and ssaH in SPI2 encoding salmonella type III secretion system

Abstract

The Salmonella enterica serovar Typhimurium contains five pathogenicity islands (SPIs). Two independent type III secretion systems (TTSS) for translocation of virulence proteins are encoded by SPI1 and 2. SPI1 is required for the penetration of the epithelial barrier of the intestine. SPI2 is important for the subsequent proliferation of bacteria in deeper tissues such as liver or spleens. Signals sensed by Salmonella in the intracellular compartment activate SPI2 gene expression through the two-component regulatory system SsrAB in SPI2. We examined the effects of environmental signals and regulatory factors on expression of ssrA and ssaH gene. SsrABdependent activation of ssaH expression was detected when cells were grown at low temperature (30¡É) or in minimal medium (M9). The level of ssrA transcription from cells grown under anaerobic conditions was higher than those detected in aerobic growth condition, whereas the induction of ssaH transcription was reduced in anaerobic growth condition. We have found that CRP¡¤cAMP and nucleoid-associated proteins such as Fis and IHF were also involved in the regulation of SPI2 genes. Interestingly, the ssaH gene could be expressed through SsrAB-independent way in an invF mutant strain. The differential activation of ssaH expression may suggest that the expression of SPI2-TTSS can be influenced by SPI1 regulator, InvF, under certain physiological condition especially in anaerobic growth within an infected host. In addition, ppGpp and unphosphorylated SsrB action may participate in SsrAB-independent ssaH expression too.