A Novel Mechanism of Virus–Virus Interactions: Bacteriophage P2 Tin Protein Inhibits Phage T4 DNA Synthesis by Poisoning the T4 Single-Stranded DNA Binding Protein, gp32

Abstract

P2 prophages have been known to inhibit DNA replication and growth of T-even phages. We show here that this inhibition is due to poisoning of the T-even single-stranded DNA binding protein gp32 by the product of the nonessential P2tingene. Synthesis of Tin protein from a gene cloned in a multicopy plasmid is necessary and sufficient to completely preventde novoDNA replication and growth of wild-type T2 or T4 phage. We isolated more than 20 independent mutants that render T-even phages resistant to poisoning by the P2 Tin protein. In all of these mutants, which we callasp,Asp codon 163 of gene32is changed to a Gly or Asn codon. The mutant alleles are recessive; i.e., when wild-type andaspmutants coinfect the same host cells, most DNA replication is poisoned by P2 Tin protein. To explain our results, we propose that the P2 Tin protein interacts with T-even gp32 at position 163 and distorts the helical filament of gene32protein on single-stranded DNA. Thereby Tin protein inhibits either assembly or function, or both, of the T4 replisome. The inhibition of late gene expression by P2 Tin protein may be an indirect consequence of inhibition of DNA replication.