Cooperativity in long-range gene regulation by the λ CI repressor

Effective repression of cI transcription from PRM by the bacteriophage lambda CI repressor requires binding sites (OL) located 2.4 kb from the promoter. A CI tetramer bound to OL1.OL2 interacts with a tetramer bound near PRM (OR1.OR2), looping the intervening DNA. We previously proposed that in this CI octamer:DNA complex, the distant OL3 operator and the weak OR3 operator overlapping PRM are juxtaposed so that a CI dimer at OL3 can cooperate with a CI dimer binding to OR3. Here we show that OL3 is necessary for effective repression of PRM and that the repressor at OL3 appears to interact specifically with the repressor at OR3. The OL3-CI-OR3 interaction involves the same CI interface used for short-range dimer-dimer interactions and does not occur without the other four operators. The long-range interactions were incorporated into a physicochemical model, allowing estimation of the long-range interaction energies and showing the lysogenic state to be ideally poised for CI negative autoregulation. The results establish the lambda system as a powerful tool for examining long-range gene regulatory interactions in vivo.