COLE1 MULTIMER FORMATION TRIGGERS INHIBITION OF ESCHERICHIA-COLI CELL-DIVISION

Multimer formation and consequent copy number depression are acknowled ged causes of multicopy plasmid instability. Multimer resolution sites (among which ColE1 cer is best-characterized) have been identified in a variety of plasmids. They participate in the conversion of multimer s to monomers, maximizing the number of independently segregating mole cules and minimizing the frequency of plasmid loss. We show that multi mer resolution alone is insufficient to ensure stable maintenance of C olE1-like plasmids in a recombination-proficient host. The expression of Rcd, a transcript encoded within cer and expressed in multimer-cont aining cells, is also required. The appearance of Rcd correlates with the inhibition of division of multimer-containing cells, presumably al lowing time for the conversion of multimers to monomers by site-specif ic recombination.