Characterization of TarA, a ToxT-activated Small Regulatory RNA Produced by Vibrio cholerae.

Abstract

Vibrio cholerae produces several virulence factors to establish infection, most of which are regulated by the ToxR regulatory pathway. The direct activator of virulence gene transcription is the protein ToxT. TarA is a ToxT-activated small, regulatory RNA (sRNA) that is conserved throughout toxigenic V. cholerae strains. It lacks open reading frames and has a putative binding site for the bacterial regulatory protein Hfq, which is implicated in the function of several sRNAs. Based on these observations, we hypothesized that TarA has a regulatory function. Microarray analysis of a tarA mutant and wild type V. cholerae revealed increased RNA for ptsG, which encodes the major glucose transporter. This result was confirmed, and we further demonstrated decreased ptsG transcript levels in response to TarA induction. Additionally, V. cholerae overexpressing TarA take up less glucose and grow poorly in glucose-containing media. A working hypothesis is that TarA is stabilized by the protein Hfq and regulates ptsG by base-pairing with the 5ʼ untranslated region of the ptsG transcript, which targets the transcript for degradation by RNase E. Analysis by electrophoretic mobility shift assays demonstrated that TarA binds the ptsG leader sequence in vitro. Additionally, a V. cholerae mutant lacking hfq has lower steady state levels of TarA, presumably due to more rapid turnover in the absence of its interaction partner. Mutational analysis suggests that TarA possesses at least two redundant Hfq binding sites. TarA appears to regulate several metabolic genes in addition to ptsG, including the sialic acid catabolism genes found at the nan locus. In the absence of TarA, nan transcripts are diminished, suggesting that TarA activates these genes. TarA may work in conjunction with the repressor protein NanR to precisely control expression of these genes. These data lead to the hypothesis that TarA regulates V. cholerae adaptation to the changing nutrient conditions it encounters, from aqueous reservoirs in the environment to the intestine of the host.