Site-specific integration of the conjugal Vibrio cholerae SXT element intoprfC

Vibrio cholerae O139, the first non-O1 serogroup of V. cholerae to give ris e to epidemic cholera, is characteristically resistant to the antibiotics s ulphamethoxazole, trimethoprim, chloramphenicol and streptomycin. Resistanc es to these antibiotics are encoded by a 62 kb self-transmissible, conjugat ive, chromosomally integrating element designated the 'SXT element'. We fou nd that the SXT element integrates site specifically into both V. cholerae and Escherichia coil K-12 into the 5 ' end of prfC, the gene encoding pepti de chain release factor 3, Integration of the SXT element interrupts the ch romosomal prfC gene, but the element encodes a new 5 ' end of prfC that res tores the reading frame of this gene. The recombinant prfC allele created u pon element integration is functional. The integration and excision mechani sm of the SXT element shares many features with site-specific recombination found in lambdoid phages. First, like lambda, the SXT element forms a circ ular extrachromosomal intermediate through specific recombination of the le ft and right ends of the integrated element, Second, chromosomal integratio n of the element occurs via site-specific recombination in a 17bp sequence found in the circular form of the SXT element and a similar 17bp sequence i n prfC. Third, both chromosomal integration and excision of the SXT element were found to require an element-encoded int gene with strong similarities to the lambda integrase family. Based on the properties of the SXT element , we propose to classify this element as a CONSTIN, an acronym for a conjug ative, self-transmissible, integrating element.