[Cis-active function of plasmid R 57 resolving co-integrates formed during transposition of ISI-elements]

The mechanism of pBR322 plasmid mobilization in the cells of Escherichia coli K-12 recA by conjugative factor R57 was studied. It was shown that mobilization of pB322 is achieved by formation of unstable IS1-mediated cointegrates with R57. In the rec+ E. coli strains cointegrates resolved with formation of pBR322:IS1 plasmids. In the recA bacteria the cointegrates dissociated to pBR:IS1, as well as to other insertion derivatives of pBR322. Some of the latter contain Tn9-like sequence, i.e. a transposon flanked by direct repeats of IS1. The subsequent transposition of IS1 from pBR-. IS1 to pBR3.1 plasmid (Aps deletion derivative of RP1) leads to formation of stable cointegrates incapable of dissociating even in the presence of coresident plasmid R57. It is suggested that R57 encodes the cys-acting function providing recA-independent recombination between direct repeats of IS1.