Control of gene expression in the P2-related temperate coliphages. V. The use of sequence analysis of 186 Vir mutants to indicate presumptive repressor binding sites.

The prophage of coliphage 186 produces a repressor protein that is required for maintenance of lysogeny and that renders lysogenic cells immune to superinfection by 186. The repressor is likely to be a DNA-binding protein that prevents transcription of the 186 early-lytic genes from promoter pR. To identify the binding site of the repressor, we have isolated virulent mutants that are able to form plaques in the presence of repressor and determined their DNA sequences around pR. The mutants all have mutations in an inverted repeat within pR, and we predict that this repeat is the primary binding site of the repressor. Many of the mutants have second mutations near pR, which allow them to form plaques in the presence of higher concentrations of repressor. The sequences containing these "secondary" mutations show no homology with the putative repressor-binding site, and the role of these mutations in virulence is not clear.