RecA independent, site-specific recombination between ColE1 or ColK and a miniplasmid they complement for mobilization and relaxation: Implications for the mechanism of DNA transfer during mobilization

Abstract

We report a novel type of recA independent recombination between plasmids ColE1 or ColK and a naturally occurring miniplasmid (pLG500). This miniplasmid can be complemented for mobilization and relaxation in the presence of ColE1 or ColK. Recombination between ColE1 and pLG500, or ColK and pLG500, was site-specific, and was only detected following the mobilization of these plasmids. The composite plasmids thus formed were stable, but recombination (resulting in dissociation of their component replicons) was again detected following mobilization. For ColE1, the site at which cointegration with pLG500 occurred was mapped to within 47 base pairs of the relaxation nicking site; for ColK, the recombination site was localized to the same region as its genetically defined transfer origin. The generation of these cointegrate plasmids is consistent with the hypothesis that mobilization entails relaxation nicking, transfer of the nicked single strand of DNA, and recircularization of the transferred single strand by ligation of 3′ and 5′ termini by the relaxation protein bound to the 5′ nick terminus. Since both plasmids are mobilized by the same proteins, their cointegration can be explained as a consequence of the ligation of the 5′ end of one plasmid to the 3′ end of the other, and vice versa.