The Leucine-Responsive Regulatory Protein, Lrp, Activates Transcription of thefimOperon inSalmonella entericaSerovar Typhimurium via thefimZRegulatory Gene

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Abstract
Thefimoperon ofSalmonella entericaserovar Typhimurium encodes type 1 fimbriae. The expression offimis 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 inlrp, the gene that codes for the leucine-responsive regulatory protein (Lrp), inhibitedfimtranscription. The loss offimgene 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 thelrpgene. Electrophoretic mobility shift analysis revealed no interactions between purified Lrp protein and the regulatory region of thefimA,fimU, orfimWgene. Instead, Lrp produced protein-DNA complexes with the regulatory region of thefimZgene, 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 thefimZtranscription start site, consistent with the binding and wrapping of the DNA in this upstream region. Ectopic expression of thefimZgene from an inducible promoter caused Lrp-independent type 1 fimbriation in serovar Typhimurium. These data show that Lrp makes a positive contribution tofimgene expression through direct interaction with thefimZpromoter region, possibly by antagonizing the binding of the H-NS global repressor protein.