We also find that the transcription terminator, T IMM, is a Rho-independent, intrinsic terminator. The RexA and RexB proteins interact with themselves and each other in a bacterial two-hybrid system. Mutation of P LIT reduces RexB levels relative to RexA, compromising cellular energetics and causing a 10-fold reduction in lytic phage yield. Proper RexA and RexB protein levels are critical for the switch from lysogeny to lytic growth. LexA repressor, like the λ CI repressor, is inactivated during the SOS response to DNA damage, and this regulation ensures that the P LIT promoter and the lytic P L and P R promoters are synchronously activated. The P LIT promoter is repressed by the bacterial LexA protein in λ lysogens. Other less well-characterized elements in the phage immunity region include the P LIT promoter and the immunity terminator, T IMM.

There is a bistable switch between lysogenic and lytic growth that is orchestrated by the CI and Cro repressors acting on the lytic ( P L and P R) and lysogenic ( P RM) promoters, referred to as the Genetic Switch. Genetic elements in the bacteriophage λ immunity region contribute to stable maintenance and synchronous induction of the integrated Escherichia coli prophage.

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