The genome of the P1 phage is maintained as a low copy number plasmid in the bacterium. The relatively large size of the plasmid requires it to keep a low copy number lest it become too large a metabolic burden while it is a lysogen. As there is usually only one copy of the plasmid per bacterial genome, the plasmid stands a high chance of not being passed to both daughter cells. The P1 plasmid combats this by several methods:
* The plasmid replication is tightly regulated by a RepA protein dependent mechanism. This is similar to the mechanism used by several other plasmids. It ensure that the plasmid divides in step with the host genome.
* Interlocked plasmids are quickly unlinked by Cre-lox recombination
* The plasmid encodes a plasmid addiction system that kills daughter cells that lose the plasmid. It consists of a stable protein toxin and an antitoxin that reversibly binds to and neutralizes it. Cells that lose the plasmid get killed as the antitoxin gets degraded faster than the toxin.