Bacteriophage λ RexA and RexB functions assist the transition from lysogeny to lytic growth

The CI and Cro repressors of bacteriophage λ create a bistable switch between lysogenic and lytic growth. In λ lysogens, CI repressor expressed from the P RM promoter blocks expression of the lytic promoters P L and P R to allow stable maintenance of the lysogenic state. When lysogens are induced, CI repressor is inactivated and Cro repressor is expressed from the lytic P R promoter. Cro repressor blocks P RM transcription and CI repressor synthesis to ensure that the lytic state proceeds. RexA and RexB proteins, like CI, are expressed from the P RM promoter in λ lysogens; RexB is also expressed from a second promoter, P LIT , embedded in rexA . 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 the control of the lytic P L and P R promoters. 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 lytic P L and P R promoters. RexB modulates the effect of RexA and may also help establish phage DNA replication as lytic growth ensues.