CsrA controls ToxR levels by increasing the stability and translation of mRNA.

Intestinal colonization and virulence factor production in response to environmental cues is mediated through several regulatory factors in , including the highly conserved RNA-binding global regulatory protein CsrA. We have shown previously that CsrA increases synthesis of the virulence-associated transcription factor ToxR in response to specific amino acids (NRES) and is required for the virulence of in the infant mouse model of cholera. In this study, we mapped the 5' untranslated region (5' UTR) of and showed that CsrA can bind directly to an RNA sequence encompassing the 5' UTR, indicating that the regulation of ToxR levels by CsrA is direct. Consistent with this observation, the 5' UTR of contains multiple putative CsrA binding sequences (GGA motifs), and mutating these motifs disrupted the CsrA-mediated increase in ToxR. Optimal binding of CsrA to a defined RNA oligonucleotide required the bridging of two GGA motifs within a single RNA strand. To determine the mechanism of regulation by CsrA, we assayed transcript levels, stability, and efficiency of translation. Both the amount of mRNA in NRES and the stability of the transcript were increased by CsrA. Using an translation assay, we further showed that synthesis of ToxR was greatly enhanced in the presence of purified CsrA, suggesting a direct role for CsrA in the translation of mRNA. We propose a model in which CsrA binding to the 5' UTR of the transcript promotes ribosomal access while precluding interactions with RNA-degrading enzymes.IMPORTANCE is uniquely adapted to marine environments as well as the human intestinal tract. Global regulators, such as CsrA, which help translate environmental cues into an appropriate cellular response, are critical for switching between these distinct environments. Understanding the pathways involved in relaying environmental signals is essential for understanding both the environmental persistence and the intestinal pathogenesis of this devastating human pathogen. In this study, we demonstrate that CsrA directly regulates the synthesis of ToxR, a key virulence factor of . Under conditions favoring high levels of active CsrA in the cell, such as in the presence of particular amino acids, CsrA increases ToxR protein levels by binding to the transcript and enhancing both its stability and translation. By responding to nutrient availability, CsrA is perfectly poised to activate the virulence gene regulatory cascade at the preferred site of colonization in the human host, the nutrient-rich small intestinal mucosa.