Cnr Protein, the Negative Regulator of Bacteriophage P4 Replication, Stimulates Specific DNA Binding of Its Initiator Protein a

Bacteriophage P4 DNA replication depends upon the phage-encoded a protein, which has DNA helicase and DNA primase activity and can specifically bind to the replication origin (ori) and to the cis replicating region (crr). The P4 Cnr protein functions as a negative regulator of P4 replication, and P4 does not replicate in cells that overexpress cnr. We searched for P4 mutants that suppressed this phenotype (Cnr resistant [acr]). Eight independent mutants that grew in the presence of high levels of Cnr were obtained. None of these can establish the plasmid state. Each of these mutations lies in the DNA binding domain of gpa that occupies the C terminus of the protein. Five different sequence changes were found: T675M, G732V (three times), G732W (twice), L733V, and L737V. A TrxA-Cnr fusion protein does not bind DNA by itself but stimulates the ori and crr binding abilities of a protein in vitro. The acr mutant proteins were still able to bind specifically to ori or crr, but specific DNA binding was less stimulated by the TrxA-Cnr protein. We present evidence that Cnr protein interacts with the gpa domain that binds specifically to DNA and that gpacr mutations impair this interaction. We hypothesize that gpa-Cnr interaction is essential for the control of P4 DNA replication.