The RNA Chaperone Hfq Independently Coordinates Expression of the VirB Type IV Secretion System and the LuxR-Type Regulator

The type IV secretion system encoded by the virB operon is required for full virulence of Brucella sp., and the present study links the RNA chaperone Hfq to wild-type expression of virB in Brucella abortus 2308. Studies employing virB-lacZ fusions, quantita-tive reverse transcription-PCR, and immunoblot analysis showed that both transcription and translation of virB are decreased in an isogenic hfqmutant compared to those in the parental strain. These results led to the hypothesis that Hfq regulation of virB is mediated through an intermediate transcriptional regulator. Subsequent experiments determined that expression of the gene encoding the putative Brucella quorum-sensing regulator BabR (also known as BlxR), a known virB regulator, is also controlled by Hfq at the posttranscriptional level, and a cis-acting element in the 5 = untranslated region of the babR transcript responsible for this regulation was identified. Consistent with its role as a virB regulator, recombinant Brucella BabR binds to the virB pro-moter region in electrophoretic mobility shift assays. However, experiments employing a babRmutant strain determined that BabR is a repressor, not an activator, of virB transcription. These findings suggest that Hfq regulates virB expression through both BabR-dependent and BabR-independent pathways. The Brucella spp. are part of the 2 subclass of proteobacteria(28), and these bacteria infect wild and domestic animals, such as cattle, goats, and sheep, causing abortions in pregnant females and sterility in males. The Brucella spp. can also infect humans, and brucellosis is described as the most common zoonosis world