Stable packaging of phage PRD1 DNA requires adsorption protein P2, which binds to the IncP plasmid-encoded conjugative transfer complex

The double-stranded DNA bacteriophage PRD1 uses an IncP plasmid-encoded con jugal transfer complex as a receptor. Plasmid functions in the PRD1 life cy cle are restricted to phage adsorption and DNA entry. A single phage struct ural protein, P2, located at the fivefold capsid vertices, is responsible f or PRD1 attachment to its host. The purified recombinant adsorption protein was judged to be monomeric by gel filtration, rate zonal centrifugation, a nalytical ultracentrifugation, and chemical cross-linking. It binds to its receptor with an apparent K-d of 0.20 nM, and this binding prevents phage a dsorption. P2-deficient particles are unstable and spontaneously release th e DNA with concomitant formation of the tail-like structure originating fro m the phage membrane. We envisage the DNA to be packaged through one vertex , but the presence of P2 on the other vertices suggests a mechanism whereby the injection vertex is determined by P2 binding to the receptor.