Chitin disaccharide (GlcNAc)2 induces natural competence in Vibrio cholerae through transcriptional and translational activation of a positive regulatory gene tfoXVC

A pathogenic marine bacterium Vibrio cholerae shows natural competence for genetic transformation in the presence of chitin, a polymer of N-acetylglucosamine (GlcNAc). In this study, we extensively analyzed the regulatory mechanisms of tfoXVC, encoding an activator protein for the chitin-induced competence. Using a chromosomal tfoXVC-lacZ reporter system, we showed that a disaccharide of chitin, (GlcNAc)2, at least was needed to activate both the transcription and translation of tfoXVC. This activation was moderate at the transcriptional level but was strong at the translational level. We also identified two sequence elements, one for transcription and another for translation. The transcriptional control element (TCE) included a 34-bp potential transcriptional operator overlapped by the tfoXVC promoter, while the translational control element (TLE) consisted of a 42-bp sequence located within the 5′-untranslated region. Deletion of either TCE or TLE still resulted in (GlcNAc)2-dependent competence for exogenous DNA. However, the deletion in both elements induced competence for transformation at high efficiency regardless of the presence or absence of (GlcNAc)2. These results suggested the dual activation of tfoXVC expression to be essential to induce competence. The highly transformable strain created here should aid the study of natural competence in V. cholerae.