Isolation and Characterisation of E. coli xerC, a Gene Required for cer Site-Specific Recombination

The formation of plasmid multimers by homologous recombination can greatly reduce the heritable stability of the natural multicopy plasmid ColE1. ColE1 carries a 250 bp site (cer) which, when present in direct repeat (as in a ColE1 dimer) efficiently resolves plasmid multimers to momomers. cer-mediated monomerisation is strongly correlated with an increase in plasmid stability. Current evidence suggests that ColE1 segregates randomly at cell division, and that cer-dependent monomerisation ensures stability by increasing the number of independently segregating plasmid copies (Sherratt et a 7, 1984; Summers and Sherratt, 1984)