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Journal of Bacteriology, January 2008, p. 602-612, Vol. 190, No. 2
0021-9193/08/$08.00+0     doi:10.1128/JB.01388-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

The Leucine-Responsive Regulatory Protein, Lrp, Activates Transcription of the fim Operon in Salmonella enterica Serovar Typhimurium via the fimZ Regulatory Gene{triangledown}

Kirsty A. McFarland,1 Sacha Lucchini,2 Jay C. D. Hinton,2 and Charles J. Dorman1*

Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Dublin 2, Ireland,1 Molecular Microbiology Group, Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, United Kingdom2

Received 27 August 2007/ Accepted 24 October 2007

The fim operon of Salmonella enterica serovar Typhimurium encodes type 1 fimbriae. The expression of fim is controlled in response to environmental signals through a complex regulatory cascade involving the proteins FimW, FimY, and FimZ and a genetic locus, fimU, that encodes a rare arginine tRNA. We discovered that a knockout mutation in lrp, the gene that codes for the leucine-responsive regulatory protein (Lrp), inhibited fim transcription. The loss of fim gene expression was accompanied by a corresponding loss of the mannose-sensitive hemagglutination that is a characteristic of type 1 fimbriae. Normal type 1 fimbrial expression was restored following the introduction into the knockout mutant of a plasmid carrying a functional copy of the lrp gene. Electrophoretic mobility shift analysis revealed no interactions between purified Lrp protein and the regulatory region of the fimA, fimU, or fimW gene. Instead, Lrp produced protein-DNA complexes with the regulatory region of the fimZ gene, and the nature of these complexes was leucine sensitive. DNase I footprinting showed that Lrp binds within a region between –65 and –170 with respect to the fimZ transcription start site, consistent with the binding and wrapping of the DNA in this upstream region. Ectopic expression of the fimZ gene from an inducible promoter caused Lrp-independent type 1 fimbriation in serovar Typhimurium. These data show that Lrp makes a positive contribution to fim gene expression through direct interaction with the fimZ promoter region, possibly by antagonizing the binding of the H-NS global repressor protein.

* Corresponding author. Mailing address: Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Dublin 2, Ireland. Phone: 353 1 896 2013. Fax: 353 1 679 9294. E-mail: cjdorman{at}tcd.ie

{triangledown} Published ahead of print on 2 November 2007.

Journal of Bacteriology, January 2008, p. 602-612, Vol. 190, No. 2
0021-9193/08/$08.00+0     doi:10.1128/JB.01388-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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