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

Abstract

Effective repression of cI transcription from P RM by the bacteriophage λ CI repressor requires binding sites ( O L ) located 2.4 kb from the promoter. A CI tetramer bound to O L 1 .O L 2 interacts with a tetramer bound near P RM ( O R 1 .O R 2), looping the intervening DNA. We previously proposed that in this CI octamer:DNA complex, the distant O L 3 operator and the weak O R 3 operator overlapping P RM are juxtaposed so that a CI dimer at O L 3 can cooperate with a CI dimer binding to O R 3. Here we show that O L 3 is necessary for effective repression of P RM and that the repressor at O L 3 appears to interact specifically with the repressor at O R 3. The O L 3-CI- O R 3 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 λ system as a powerful tool for examining long-range gene regulatory interactions in vivo.