Compartmentalization of phage phi 29 DNA replication: interaction between the primer terminal protein and the membrane-associated protein p1

The bacteriophage phi 29 replication protein p1 (85 amino acids) is membran e associated in Bacillus subtilis-infected cells, The C-terminal 52 amino a cid residues of pi are sufficient for assembly into protofilament sheet str uctures. Using chemical cross-linking experiments, we demonstrate here that p1 Delta C43, a C-terminally truncated pi protein that neither associates with membranes in vivo nor self-interacts in vitro, can interact with the p rimer terminal protein (TP) in vitro. Like protein p1, plasmid-encoded prot ein p1 Delta C43 reduces the rate of phi 29 DNA replication ill who in a do sage-dependent manner. We also show that truncated pi proteins that retain the N-terminal 42 amino acids, when present in excess, interfere with the i n vitro formation of the TP-dAMP initiation complex in a reaction that depe nds on the efficient formation of a primer TP-phi 29 DNA polymerase heterod imer, This interference is suppressed by increasing the concentration of ei ther primer TP or phi 29 DNA polymerase, We propose a model for initiation of in vivo phi 29 DNA replication in which the viral replisome attaches to a membrane-associated p1-based structure.