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

Abstract

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 (PL and PR) and lysogenic (PRM) promoters, referred to as the Genetic Switch. Other less well‐characterized elements in the phage immunity region include the PLIT promoter and the immunity terminator, TIMM. The PLIT 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 PLIT promoter and the lytic PL and PR promoters are synchronously activated. Proper RexA and RexB protein levels are critical for the switch from lysogeny to lytic growth. Mutation of PLIT 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, TIMM, is a Rho‐independent, intrinsic terminator. Inactivation of TIMM has minimal effect on λ lysogenization or prophage induction.

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

“…However, this postulated membrane association is not the tight membrane tethering of the λ genome observed by Hallick and Echols (1973). Our two-hybrid data also show the interaction between RexB and RexA (Thomason et al, 2019). Bottom Panel: It is possible that CI, RexB, and RexA colocalize at the periphery of the inner membrane to form a complex that includes all three proteins.…”

“…Thus, the CI-RexB interaction likely occurs via the RexB central cytoplasmic loop (Figure 13). We previously showed that RexA and RexB physically interact in vivo (Thomason et al, 2019). The strength of the RexA-RexB interaction (108 β-galactosidase units), however, is slightly lower than that of RexA with CI ( the molecular level, we suspect that the Rex system functions as a rheostat that modulates phage repressor protein interactions to fine-tune the bistable switch, with RexA and RexB each having independent and complementary effects as the phage switches from lysogenic development to lytic growth (see Figure 12).…”

“…The rexA and rexB genes are downstream of cI. The P LIT promoter is embedded in the terminal coding sequence of rexA, with the consequence that rexB is transcribed from two promoters, P RM and P LIT , whereas cI and rexA are transcribed only from P RM (Thomason et al, 2019). Black arrows represent the beginning of the various promoter transcripts shown.…”

“…We previously used this system to show that RexA and RexB interact with themselves and each other (Thomason et al, 2019). For that study, we generated eight plasmids that express the rexA and rexB genes fused in frame at either the N-or the C-terminus to each of two different cyclase domains.…”

“…A second promoter, P LIT , is located within the distal end of rexA and transcribes just rexB (Hayes et al, 1997;Landsmann et al, 1982). Thus, rexB can be transcribed from either P RM or P LIT (Hayes & Szybalski, 1973;Liu et al, 2013;Thomason et al, 2019). A λ lysogen can switch to the lytic pathway, produce progeny phage, and lyse its host, releasing phage.…”