ADDICTION PROTEIN PHD OF PLASMID PROPHAGE P1 IS A SUBSTRATE OF THE CLPXP SERINE-PROTEASE OF ESCHERICHIA-COLI

Plasmid-encoded addiction genes augment the apparent stability of vari ous low copy number bacterial plasmids by selectively killing plasmid- free (cured) segregants or their progeny. The addiction module of plas mid prophage P1 consists of a pair of genes called phd and doc. Phd se rves to prevent host death when the prophage is retained and, should r etention mechanisms fail, Doc causes death on curing. Doc acts as a ce ll toxin to which Phd is an antidote. In this study we show that host mutants with defects in either subunit of the ClpXP protease survive t he loss of a plasmid that contains a P1 addiction module. The small an tidote protein Phd is fully stable in these two mutant hosts, whereas it is labile in a wild-type host. We conclude that the role of ClpXP i n the addiction mechanism of P1 is to degrade the Phd protein. This co nclusion situates P1 among plasmids that elicit severe withdrawal symp toms and are able to do so because they encode both a cell toxin and a n actively degraded macromolecule that blocks the synthesis or functio n of the toxin.