IAI Accepts, published online ahead of print on 26 October 2009

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Infect. Immun. doi:10.1128/IAI.00931-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Systematic analysis of the SsrAB virulon of Salmonella enterica

Xin Xu and Michael Hensel^*

Mikrobiologisches Institut, Universitätsklinikum Erlangen, Erlangen, Germany

^* To whom correspondence should be addressed. Email: Michael.Hensel{at}biologie.uni-osnabrueck.de.

Intracellular Salmonella enterica serovar Typhimurium deploy the Salmonella Pathogenicity Island 2 (SPI2)-encoded Type III Secretion System (T3SS) to modify host cell functions and accomplish intracellular replication. This virulence function is controlled by the two-component system SsrAB that regulates expression of several operons in SPI2 and, in addition, a large number of genes for non-SPI2 encoded effector proteins. Here, we analyzed the relative expression levels of members of the SsrAB virulon. We used a novel reporter fusion strategy for single copy chromosomal fusions, all done in an identical manner in order to enable direct quantitative comparison. We observed very high expression levels for sseJ and sifA, high expression levels for ssaG, steC, sseL and sopD2, moderate expression levels for ssaB, sseA, sseG, sifB, pipB2 and sspH1 and low expression levels for sspH2, sseI, slrP, sseK1, sseK2, pipB, and gogB. The expression of the SsrAB virulon was highly dependent on the function of SsrB, but also required EnvR/OmpZ. A deletion of PhoP, part of the global regulatory system PhoPQ, resulted in a highly delayed expression of the SsrAB virulon under in vitro conditions, however, maximal expression was similar to that in wild-type background. The expression levels of SsrAB-dependent genes in intracellular bacteria were in good agreement to in vitro analyses. We provide here a comprehensive and fully comparable analysis of expression of genes in the SsrAB virulon. This information will be of interest for the selection of in vivo-activated promoters, for example for rational design of recombinant vaccines.