ArgR and PepA, accessory proteins for XerCD-mediated resolution of ColE1 dimers, are also required for stable maintenance of the P1 prophage.

Dimers of low copy number plasmids must be resolved to monomers to prevent interference with active partition. For the P1 prophage this is achieved by the Cre site-specific recombinase acting at lox. Multimerisation of multicopy plasmids threatens stability via copy number depression, and multimers of ColE1 are resolved by XerCD-mediated recombination at cer. Xer-cer is constrained to multimer resolution by accessory proteins ArgR and PepA. Recently, it has been shown that ArgR and PepA influence Cre-mediated recombination at a cer-lox hybrid site in vitro, defining the structure of the synaptic complex. We show here that both ArgR and PepA are required for stable maintenance of the P1 prophage. It is extremely difficult to establish P1 in a strain lacking PepA and the prophage was lost rapidly once selection was removed. ArgR plays a less crucial role although its absence significantly increased prophage loss. The effect of the accessory proteins is seen only at physiological concentrations of Cre; when the recombinase is expressed from a multicopy plasmid, the prophage is unstable even in the presence of ArgR and PepA. We propose that ArgR and PepA are involved in Cre-lox recombination in vivo, probably by constraining the system to resolution of prophage dimers.