DNA-BINDING BY THE COLIPHAGE-186 REPRESSOR PROTEIN CI

The cI gene of coliphage 186 maintains lysogeny and confers immunity t o 186 infection by repressing the major early promoter, p(R), and the promoter for the late transcription activator gene, p(B). Gel mobility shift and DNase I footprinting show that CI protein binds to the DNA at p(R) and p(B) and also to sites similar to 300 base pairs upstream and downstream of p(R), called FL and FR. Mutations which cause virule nce reduce CI binding to p(R). The biochemical and genetic data identi fy three CI operators at p(R), two at p(B), and single operators at FL and FR. The operators at the p(B), FL, FR, and central p(R) sites are inverted repeat sequences, separated by 5 base pairs (Type A) or, in the case of p(R), by 4 base pairs (Type A'). A different inverted repe at operator sequence (Type B) is proposed for the binding sites on eac h side of the central site at p(R). Thus, CI appears to recognize two distinct DNA sequences. CI binds cooperatively to adjacent operators, and binding at p(R) is strongly dependent on these cooperative interac tions. A high order CI multimer appears to be the active DNA binding s pecies, even at single operators.