The ImmC and ImmI regions of the closely related, but heteroimmune, phages P1 and P7 code for regulatory proteins involved in directing the phage toward lytic or lysogenic growth. Within ImmC, c1 codes for a repressor of lytic gene expression, while coi codes for an antagonist of c1 repressor activity. The c4 repressor, encoded within ImmI, prevents expression of ant, a closely linked gene whose product interferes with c1-mediated repression. Differences in c4 specificity are responsible for the heteroimmune character of P1 and P7. Regulation of gene expression within ImmC and ImmI will be investigated by monitoring the expression of lacZ from transcriptional fusions in the presence of compatible plasmids coding for phage regulatory proteins. The regulatory role played by multiple c1 operator sites within ImmC will be investigated by in vitro mutagenesis and DNase protection studies. Mutations in c1 that suppress the virulent phenotype caused by Coi overproduction will also be sought. The contribution to heteroimmune specificity of sequence differences within P1ImmI and P7 ImmI will be investigated by assaying the effect of c4 and ant expression on transcription within these regions, and within the downstream region containing the lytic replicon.//
