Bacteriophage λ RexA and RexB Functions Assist the Transition from Lysogeny to Lytic Growth

SummaryThe CI and Cro repressors of bacteriophage λ create a bistable switch between lysogenic and lytic growth. In λ lysogens, CI repressor expressed from thePRMpromoter blocks expression of the lytic promotersPLandPRto allow stable maintenance of the lysogenic state. When lysogens are induced, CI repressor is inactivated and Cro repressor is expressed from the lyticPRpromoter. Cro repressor blocksPRMtranscription and CI repressor synthesis to ensure that the lytic state proceeds. RexA and RexB proteins, like CI, are expressed from thePRMpromoter in λ lysogens; RexB is also expressed from a second promoter,PLIT, embedded inrexA.Here we show that RexA binds CI repressor and assists the transition from lysogenic to lytic growth, using both intact lysogens and defective prophages with reporter genes under control of the lyticPLandPRpromoters. Once lytic growth begins, if the bistable switch does return to the immune state, RexA expression lessens the probability that it will remain there, thus stabilizing the lytic state and activation of the lyticPLandPRpromoters. RexB modulates the effect of RexA and may also help establish phage DNA replication as lytic growth ensues.