Identification of a Chitin-Induced Small RNA That Regulates Translation of the tfoX Gene, Encoding a Positive Regulator of Natural Competence in Vibrio cholerae

The tfoX (also called sxy) gene product is the central regulator of DNA uptake in the naturally competent bacteria Haemophilus influenzae and Vibrio cholerae. However, the mechanisms regulating tfoX gene expression in both organisms are poorly understood. Our previous studies revealed that in V. cholerae, chitin disaccharide (GlcNAc)(2) is needed to activate the transcription and translation of V. cholerae tfoX (tfoX(VC)) to induce natural competence. In this study, we screened a multicopy library of V. cholerae DNA fragments necessary for translational regulation of tfoX(VC). A clone carrying the VC2078-VC2079 intergenic region, designated tfoR, increased the expression of a tfoX(VC)::lacZ translational fusion constructed in Escherichia coli. Using a tfoX(VC)::lacZ reporter system in V. cholerae, we confirmed that tfoR positively regulated tfoX(VC) expression at the translational level. Deletion of tfoR abolished competence for exogenous DNA even when (GlcNAc)(2) was provided. The introduction of a plasmid clone carrying the tfoR(+) gene into the tfoR deletion mutant complemented the competence deficiency. We also found that the tfoR gene encodes a 102-nucleotide small RNA (sRNA), which was transcriptionally activated in the presence of (GlcNAc)(2). Finally, we showed that this sRNA activated translation from tfoX(VC) mRNA in a highly purified in vitro translation system. Taking these results together, we propose that in the presence of (GlcNAc)(2), TfoR sRNA is expressed to activate the translation of tfoX(VC), which leads to the induction of natural competence.