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Journal of Bacteriology, December 2003, p. 7257-7265, Vol. 185, No. 24
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.24.7257-7265.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Pathways Leading from BarA/SirA to Motility and Virulence Gene Expression in Salmonella

Max Teplitski, Robert I. Goodier,{dagger} and Brian M. M. Ahmer*

Department of Microbiology, The Ohio State University, Columbus, Ohio 43210-1292

Received 31 July 2003/ Accepted 17 September 2003

The barA and sirA genes of Salmonella enterica serovar Typhimurium encode a two-component sensor kinase and a response regulator, respectively. This system increases the expression of virulence genes and decreases the expression of motility genes. In this study, we examined the pathways by which SirA affects these genes. We found that the master regulator of flagellar genes, flhDC, had a positive regulatory effect on the primary regulator of intestinal virulence determinants, hilA, but that hilA had no effect on flhDC. SirA was able to repress flhDC in a hilA mutant and activate hilA in an flhDC mutant. Therefore, although the flhDC and hilA regulatory cascades interact, sirA affects each of them independently. A form of BarA lacking the two N-terminal membrane-spanning domains, BarA198, autophosphorylates in the presence of ATP and transfers the phosphate to purified SirA. Phosphorylated SirA was found to directly bind the hilA and hilC promoters in gel mobility shift assays but not the flhD, fliA, hilD, and invF promoters. Given that the CsrA/csrB system is known to directly affect flagellar gene expression, we tested the hypothesis that SirA affects flagellar gene expression indirectly by regulating csrA or csrB. The sirA gene did not regulate csrA but did activate csrB expression. Consistent with these results, phosphorylated SirA was found to directly bind the csrB promoter but not the csrA promoter. We propose a model in which SirA directly activates virulence expression via hilA and hilC while repressing the flagellar regulon indirectly via csrB.

* Corresponding author. Mailing address: Department of Microbiology, The Ohio State University, 484 West 12th Ave., Columbus, OH 43210. Phone: (614) 292-1919. Fax: (614) 292-8120. E-mail: ahmer.1{at}osu.edu.

{dagger} Present address: Q-One Biotech Ltd., Todd Campus, West of Scotland Science Park, Glasgow G20 0XA, Scotland.

Journal of Bacteriology, December 2003, p. 7257-7265, Vol. 185, No. 24
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.24.7257-7265.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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