The CII protein of bacteriophage 186 establishes lysogeny by activating a promoter upstream of the lysogenic promoter.

We have shown previously that the cII gene product of the non-lambdoid temperate bacteriophage 186 is required for the establishment of lysogeny. We show here that CII, a potential helix-turn-helix DNA-binding protein, establishes lysogeny by activating a promoter (PE) which spans the apl/cII intergenic region, upstream of the lysogenic promoter, PL. The start site of the PE transcript (+1) has been mapped by primer extension and we have identified the CII binding determinants at PE by DNase I footprinting. CII binds to inverted repeat sequences separated by two turns of the helix, with binding half-sites centred at the 38 and -58 positions of PE. Oligomerisation studies with purified CII protein indicate that a CII tetramer may be the species that binds to this site. We also show that PE is subject to direct negative feedback by the CI repressor.