Molecular characterization of a new variant of toxin-coregulated pilus protein (TcpA) in a toxigenic non-O1/non-O139 strain of Vibrio cholerae

A toxigenic non-O1/non-O139 strain of Vibrio cholerae (10259) was found to contain a new variant of the toxin-coregulated pilus (TCP) protein gene (tc pA) as determined by PCR and Southern hybridization experiments. Nucleotide sequence analysis data of the new tcpA gene in strain 10259 (O53) showed i t to be about 74 and 72% identical to those of O1 classical and Fl Tor biot ype strains, respectively. The predicted amino acid sequence of the 10259 T cpA protein shared about 81 and 78% identity with the corresponding sequenc es of classical and Fl Tor TcpA strains, respectively. An antiserum raised against the TCP of a classical strain, O395, although it recognized the Tcp A protein of strain 10259 in an immunoblotting experiment, exhibited consid erably less protection against 10259 challenge compared to that observed ag ainst the parent strain. Incidentally, the tcpA sequences of two other toxi genic non-O1/non-O139 strains (V2 and S7, both belonging to the serogroup O 37) were determined to be almost identical to that of classical tcpA, Furth er, tcpA of another toxigenic non-O1/non-O139 strain V315-1 (O nontypeable) was closely related to that of El Tor tcpA. Analysis of these results with those already available in the literature suggests that there are at least four major variants of the tcpA gene in V. cholerae which probably evolved in parallel from a common ancestral gene. Existence of highly conserved as well as hypervariable regions within the sequence of the TcpA protein woul d also predict that such evolution is under the control of considerable sel ection pressure.