Elements in the λ immunity region regulate phage development: beyond the ‘Genetic Switch’

Summary Genetic elements in the bacteriophage λ immunity region contribute to stable maintenance and synchronous induction of the integrated Escherichia coli prophage . 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. Other less well‐characterized elements in the phage immunity region include the P LIT promoter and the immunity terminator, T IMM . The P LIT promoter is repressed by the bacterial LexA protein in λ lysogens. 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. Proper RexA and RexB protein levels are critical for the switch from lysogeny to lytic growth. Mutation of P LIT reduces RexB levels relative to RexA, compromising cellular energetics and causing a 10‐fold reduction in lytic phage yield. The RexA and RexB proteins interact with themselves and each other in a bacterial two‐hybrid system. We also find that the transcription terminator, T IMM , is a Rho‐independent, intrinsic terminator. Inactivation of T IMM has minimal effect on λ lysogenization or prophage induction.