Elements in the λ immunity region regulate phage development: beyond the 'Genetic Switch'.

Genetic elements in the bacteriophage lambda immunity region contribute to stable maintenance and synchronous induction of the integratedEscherichia coliprophage.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)andP(R)) and lysogenic (P-RM) promoters, referred to as the Genetic Switch. Other less well-characterized elements in the phage immunity region include theP(LIT)promoter and the immunity terminator,T-IMM. TheP(LIT)promoter is repressed by the bacterial LexA protein in lambda lysogens. LexA repressor, like the lambda CI repressor, is inactivated during the SOS response to DNA damage, and this regulation ensures that theP(LIT)promoter and the lyticP(L)andP(R)promoters are synchronously activated. Proper RexA and RexB protein levels are critical for the switch from lysogeny to lytic growth. Mutation ofP(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 ofT(IMM)has minimal effect on lambda lysogenization or prophage induction.