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Journal of Bacteriology, November 2008, p. 7406-7413, Vol. 190, No. 22
0021-9193/08/$08.00+0     doi:10.1128/JB.01136-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Regulation of the Salmonella enterica std Fimbrial Operon by DNA Adenine Methylation, SeqA, and HdfR{triangledown}

Marcello Jakomin,1 Daniela Chessa,2 Andreas J. Bäumler,2 and Josep Casadesús1*

Departamento de Genética, Universidad de Sevilla, Apartado 1095, Seville 41080, Spain,1 Department of Medical Microbiology and Immunology, School of Medicine, University of California at Davis, One Shields Avenue, Davis, California 956126-86452

Received 13 August 2008/ Accepted 30 August 2008

DNA adenine methylase (dam) mutants of Salmonella enterica serovar Typhimurium grown under laboratory conditions express the std fimbrial operon, which is tightly repressed in the wild type. Here, we show that uncontrolled production of Std fimbriae in S. enterica serovar Typhimurium dam mutants contributes to attenuation in mice, as indicated by the observation that an stdA dam strain is more competitive than a dam strain upon oral infection. Dam methylation appears to regulate std transcription, rather than std mRNA stability or turnover. A genetic screen for std regulators showed that the GATC-binding protein SeqA directly or indirectly represses std expression, while the poorly characterized yifA gene product serves as an std activator. YifA encodes a putative LysR-like protein and has been renamed HdfR, like its Escherichia coli homolog. Activation of std expression by HdfR is observed only in dam and seqA backgrounds. These data suggest that HdfR directly or indirectly activates std transcription. Since SeqA is unable to bind nonmethylated DNA, it is possible that std operon derepression in dam and seqA mutants may result from unconstrained HdfR-mediated activation of std transcription. Derepression of std in dam and seqA mutants of S. enterica occurs in only a fraction of the bacterial population, suggesting the occurrence of either bistable expression or phase variation.

* Corresponding author. Mailing address: Departamento de Genética, Facultad de Biología, Universidad de Sevilla, Apartado 1095, E-41080 Sevilla, Spain. Phone: 34 95 455 7105. Fax: 34 95 455 7104. E-mail: casadesus{at}us.es

{triangledown} Published ahead of print on 19 September 2008.

Journal of Bacteriology, November 2008, p. 7406-7413, Vol. 190, No. 22
0021-9193/08/$08.00+0     doi:10.1128/JB.01136-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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