Cloning and characterization of a Legionella pneumophila-specific gene encoding a member of the LysR family of transcriptional regulators

Flagellin gene regulation in Legionella pneumophila is modulated by various environmental factors. The expression of the virulent phenotype seems to b e linked genetically to flagellum expression. To better understand the mech anisms of flagellin gene expression in L. pneumophila (Lp), we screened a p ool of plasmids from a L. pneumophila Corby genomic library for the ability to prevent or reduce luciferase activity in the Escherichia coli strain YK 410, which harbours a Lp-pflA-luxAB fusion. We cloned a DNA fragment encodi ng the N-terminal part of a protein with significant similarity to members of the LysR family of transcriptional regulators (LTTRs). The entire gene, closed by inverse PCR. was named flaR. It encodes a protein of 302 amino ac ids. and computer-assisted analysis of the amino acid sequence revealed a h elix-turn-helix motif located near the N-terminus of the protein. The FlaR protein exhibits 21-31% identity to various LTTRs. Furthermore. gel retarda tion experiments indicate that the FlaR protein is able to bind to its own promoter region and, to a lesser extent, to the flaA promoter of L. pneumop hila. The flaR promoter region contains putative LysR binding motifs and tw o putative Fur boxes. Taken together, these results indicate that FlaR is a DNA-binding protein which belongs to the LTTR Family. Southern analysis wi th a L. pneumophila Corby-specific flaR probe revealed homologous genes in various L. pneumophila strains, but not in the 12 non-pneumophila strains t ested so far.