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J. Biol. Chem., Vol. 281, Issue 40, 30112-30121, October 6, 2006

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The Bacterial Signal Molecule, ppGpp, Mediates the Environmental Regulation of Both the Invasion and Intracellular Virulence Gene Programs of Salmonella^*

Arthur Thompson¹, Matthew D. Rolfe, Sacha Lucchini, Peter Schwerk, Jay C. D. Hinton, and Karsten Tedin²

From the Molecular Microbiology Group, Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, United Kingdom and the Institut für Mikrobiologie und Tierseuchen, Freie Universität Berlin, Philippstrasse 13, D-10115 Berlin, Germany

During infection of mammalian hosts, facultative intracellular pathogens have to adjust rapidly to different environmental conditions encountered during passage through the gastroin-testinal tract and following uptake into epithelial cells and macrophages. Successful establishment within the host therefore requires the coordinated expression of a large number of virulence genes necessary for the adaptation between the extracellular and intracellular phases of infection. In this study we show that the bacterial signal molecule, ppGpp, plays a major role in mediating the environmental signals involved in the regulation of both the extracellular and intracellular virulence gene programs. Under oxygen limiting conditions, we observed a strong ppGpp dependence for invasion gene expression, the result of severe reductions in expression of the Salmonella pathogenicity island (SPI) 1 transcriptional regulator genes hilA, C, and D and invF. Overexpression of the non-SPI1-encoded regulator RtsA restored hilA expression in the absence of ppGpp. SPI2-encoded genes, required for intracellular proliferation in macrophages, were activated in the wild type strain under aerobic, late log phase growth conditions. The expression of SPI2 genes was also shown to be ppGpp-dependent under these conditions. The results from this study suggest a mechanism for the alternate regulation of the opposing extracellular and intracellular virulence gene programs and indicate a remarkable specificity for ppGpp in the regulation of genes involved in virulence compared with the rest of the genome. This is the first demonstration that this highly conserved regulatory system is involved in bacterial virulence gene expression on a global scale.

Received for publication, June 12, 2006 , and in revised form, August 4, 2006.

^* This work was funded by a Biotechnology and Biological Sciences Research Council core strategic grant (to J. H.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1-S6 and supplemental Tables S1-S5.

² Supported by a grant from the Fondation pour la Recherche Médicale and the Deutsche Forschungsgemeinschaft.

¹ To whom correspondence should be addressed: Molecular Microbiology Group, Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, UK. Tel.: 44-1603-255181; Fax: 44-1603-507723; E-mail: Arthur.Thompson{at}bbsrc.ac.uk.

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