XerCD-mediated recombination at converts multimers of plasmid ColE1 to monomers, maximizing the number of independently segregating molecules and minimizing the frequency of plasmid loss. In addition to XerCD, recombination requires the accessory factors ArgR and PepA. The promoter P, located centrally within , is also required for stable plasmid maintenance. P is active in plasmid multimers and directs transcription of a short RNA, Rcd, which appears to inhibit cell division. It has been proposed that Rcd is part of a checkpoint which ensures that multimer resolution is complete before the cell divides. This study has shown that ArgR does not act as a transcriptional repressor of P in plasmid monomers. P is unusual in that the −35 and −10 hexamers are separated by only 15 bp and this study has demonstrated that increasing this to a more conventional spacing results in elevated activity. An increase to 17 bp resulted in a 10- to 20-fold increase in activity, while smaller effects were seen when the spacer was increased to 16 bp or 18 bp. These observations are consistent with the hypothesis that P activation involves realignment of the −35 and −10 sequences within a recombinational synaptic complex. This predicts that a 17 bp spacer promoter derivative should be down-regulated by plasmid multimerization, and this is confirmed experimentally.


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