Interactions between satellite bacteriophage P4 and its helpers.

The helper dependence of satellite phage P4 superimposes an additional set of regulatory interactions on those required for the independent maintenance of P4 or its helpers. These interactions allow P4 to exploit a helper phage under a variety of circumstances and can affect expression of the immunity functions and late genes of both phages. The phage P2 lysis/lysogeny decision involves two competing repressors regulating mutually exclusive promoters in the early control region. In the absence of a helper phage, the P4 immunity function plays a role in the choice between lysogeny or the multicopy plasmid state. No evidence exists for a P4-encoded immunity repressor; in P4-lysogenic cells, expression of the P4 DNA replication gene alpha appears to be prevented by premature termination of transcription. Immunity-independent expression of alpha in the multicopy plasmid state involves initiation of transcription at an alternative upstream promoter that is positively regulated by P4 delta protein; the same promoter is activated by P2 Cox protein during derepression of P4 by P2. The mechanism of derepression of P2 by P4 remains to be determined, and the relationship between the P4 immunity and derepression functions and the mutations that allow P4 to grow with a P3 prophage helper is an intriguing area for further exploration. Expression of P2 and P4 late genes is regulated by phage-encoded, zinc-binding transcriptional activators that appear to interact directly with the alpha subunit of RNA polymerase of E. coli. Stimulation of P2 late transcription by P2 Ogr protein depends upon phage DNA replication, whereas activation of transcription from the same promoters by the related P4 delta gene product is replication-independent. Elucidation of the mechanisms underlying these interactions promises to provide new insights into strategies for control of gene expression.