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

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 o f the T-even single-stranded DNA binding protein gp32 by the product o f the nonessential P2 tin gene. Synthesis of Tin protein from a gene c loned in a multicopy plasmid is necessary and sufficient to completely prevent de novo DNA replication and growth of wild-type T2 or T4 phag e. We isolated more than 20 independent mutants that render T-even pha ges resistant to poisoning by the P2 Tin protein. In all of these muta nts, which we call asp, Asp codon 163 of gene 32 is changed to a Gly o r Asn codon. The mutant alleles are recessive; i.e., when wild-type an d asp mutants coinfect the same host cells, most DNA replication is po isoned 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 gene 32 protein on single-stranded DNA Thereb y Tin protein inhibits either assembly or function, or both, of the T4 replisome. The inhibition of late gene expression by P2 Tin protein m ay be an indirect consequence of inhibition of DNA replication. (C) 19 97 Academic Press.