Identification and genomic organization of gene loci negatively controlled by the virulence regulatory BvgAS two-component system in Bordetella bronchiseptica.

The two-component system BvgAS positively controls transcription of the virulence genes of Bordetella pertussis and B. bronchiseptica, which include several genes for toxins and adhesins. On the other hand, the BvgAS system negatively controls the expression of a poorly characterized set of genes, the so-called virulence repressed ( vrg) genes. To investigate the function of this group of genes and their relationship to virulence, we identified several novel vrg genes of B. bronchiseptica via the generation of transcriptional fusions with gfp (the ORF encoding Green Fluorescent Protein) by transposon mutagenesis. Expression of all of the vrg genes was enhanced under phenotype-modulating growth conditions and in phase variants, demonstrating that their transcription is indeed controlled by the BvgAS system. In addition, transcription of most of these new vrg genes was found to be affected by the growth phase of the bacteria, with maximal expression being observed in the late logarithmic or stationary phase. The majority of these genes encode putative metabolic functions involved in redox reactions and amino acid transport. Interestingly, several vrg genes of B. bronchiseptica are not expressed or have been lost in B. pertussis, indicating that, possibly as a consequence of its adaptation to a single host organism, many vrg genes of B. pertussis are gradually decaying.