Conjugation-independent, site-specific recombination at the oriT of the IncW plasmid R388 mediated by TrwC.

Plasmids containing a direct repeat of plasmid R388 oriT are capable of site-specific recombination, which results in deletion of the intervening DNA. This reaction occurs in the presence, but not in the absence, of the region of R388 implicated in DNA processing during conjugation. This region contains three genes, trwA, trwB, and trwC. By using mutants of each of the three genes, it was demonstrated that only trwC is required for the oriT-specific recombination. Further analysis showed that the N-terminal 272 amino acids of the protein are sufficient to catalyze recombination. TrwC is also capable of promoting intermolecular recombination between two plasmids containing oriT, suggesting that double-strand breaks in both plasmid DNAs are involved in the process. Additionally, intramolecular recombination between R388 oriT and R46 oriT did not occur in the presence of both nickases. This suggests that the half-reactions at each oriT are not productive if they occur separately; therefore, an interaction between the recombination complexes formed at each recombining site is required. This is the first report in which a nicking-closing enzyme involved in conjugal DNA transfer promotes oriT-specific recombination of double-stranded DNA in the absence of conjugation.