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 PRM promoter blocks expression of the lytic promoters PL and PR to allow stable maintenance of the lysogenic state. When lysogens are induced, CI repressor is inactivated and Cro repressor is expressed from the lytic PR promoter. Cro repressor blocks PRM transcription and CI repressor synthesis to ensure that the lytic state proceeds. RexA and RexB proteins, like CI, are expressed from the PRM promoter in λ lysogens; RexB is also expressed from a second promoter, PLIT, 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 PL and PR 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 PL and PR promoters. RexB modulates the effect of RexA and may also help establish phage DNA replication as lytic growth ensues. Bacteriophage λ reactivation from lysogeny to lytic growth is controlled by two phage repressors, CI and Cro, that bind to operator sites flanking the phage lytic promoters and regulate gene expression. The phage RexA and RexB proteins modulate this switch. RexA assists the transition from lysogenic to lytic growth and RexB antagonizes the effect of RexA. The Rex functions refine the ability of the virus to switch between developmental states.