Octamerization of λ CI repressor is needed for effective repression of P-RM and efficient switching from lysogeny

Abstract

The CI repressor of bacteriophage λ is a model for the role of cooperativity in the efficient functioning of genetic switches. Pairs of CI dimers interact to cooperatively occupy adjacent operator sites at OR and at OL. These CI tetramers repress the lytic promoters and activate transcription of the cI gene from PRM. CI is also able to octamerize, forming a large DNA loop between OR and OL, but the physiological role of this is unclear. Another puzzle is that, although a dimer of CI is able to repress PRM by binding to the third operator at OR, OR3, this binding seems too weak to affect CI production in the lysogenic state. Here we show that repression of PRM at lysogenic CI concentrations is absolutely dependent on OL, in this case 3.8 kb away. A mutant defective in this CI negative autoregulation forms a lysogen with elevated CI levels that cannot efficiently switch from lysogeny to lytic development. Our results invalidate previous evidence that Cro binding to OR3 is important in prophage induction. We propose the octameric CI:OR–OL complex increases the affinity of CI for OR3 by allowing a CI tetramer to link OR3 and the third operator at OL, OL3