Vibrio cholerae LexA Coordinates CTX Prophage Gene Expression

Abstract

The filamentous bacteriophage CTX⌽ transmits the cholera toxin genes by infecting and lysogenizing its host, Vibrio cholerae. CTX⌽ genes required for virion production initiate transcription from the strong P A promoter, which is dually repressed in lysogens by the phage-encoded repressor RstR and the host-encoded SOS repressor LexA. Here we identify the neighboring divergent rstR promoter, P R , and show that RstR both positively and negatively autoregulates its own expression from this promoter. LexA is absolutely required for RstR-mediated activation of P R transcription. RstR autoactivation occurs when RstR is bound to an operator site centered 60 bp upstream of the start of transcription, and the coactivator LexA is bound to a 16-bp SOS box centered at position ؊23.5, within the P R spacer region. Our results indicate that LexA, when bound to its single site in the CTX⌽ prophage, both represses transcription from P A and coactivates transcription from the divergent P R . We propose that LexA coordinates P A and P R prophage transcription in a gene regulatory circuit. This circuit is predicted to display transient switch behavior upon induction of CTX⌽ lysogens.The horizontal transfer of CTX⌽, the lysogenic filamentous phage that carries the genes encoding cholera toxin, contributes to the emergence of new toxigenic strains of Vibrio cholerae, the cause of epidemic cholera (6, 31). For stable CTX⌽ transfer to occur, the infecting single-stranded DNA CTX⌽ genome must integrate site-specifically into the V. cholerae genome and establish a lysogenic program that involves the transcriptional repression of CTX⌽ genes involved in bacteriophage replication and morphogenesis (17). The CTX⌽ prophage expresses a 14-kDa repressor, RstR, that represses the expression of CTX⌽ structural genes and provides immunity to secondary CTX⌽ infection (12, 32). The CTX⌽ prophage can be induced with DNA-damaging agents such as mitomycin C and UV radiation, a process that is mediated by a second repressor, the host SOS repressor LexA (25). Prophage induction does not kill the host bacterium and results in only a modest increase in CTX⌽ titers, unlike the massive phage growth and host lysis that occurs upon induction of phage lambda.The CTX⌽ replication and morphogenesis genes initiate transcription from the strong P A promoter located in ig-2, an intergenic region separating the rstA replication gene from the divergently transcribed rstR gene encoding the CTX⌽ repressor (12) (Fig. 1B). RstR strongly represses P A transcription in CTX⌽ lysogens by binding to a set of three operator sites that surround P A (13). The host SOS repressor LexA also represses P A by binding to a single SOS box centered Ϫ48.5 bp from the start of P A transcription (25) (Fig. 1B). LexA, together with RecA, regulates a network of more than 30 host genes that monitors and responds to DNA damage (7). Upon DNA damage or replication fork arrest, RecA, together with singlestranded DNA and ATP, stimulates the autoproteolysis of LexA (14), leading to the transient d...

Phage-Borne Factors and Host LexA Regulate the Lytic Switch in Phage GIL01

“…A unique example where LexA exerts a dual function is provided by the Vibrio cholerae filamentous phage CTX. The phage-encoded repressor RstR acts together with host LexA to activate and repress gene transcription (36,54). Such a mechanism allows the phage to respond to small variations in RstR/LexA levels and switch from a low-particle-formation mode to higher yields and vice versa.…”

“…The repressors of coliphages 186 and N15, for example, are not RecA sensitive but are instead antagonized by phage-borne antirepressors that are under LexA control (47,61). In CTX⌽, the filamentous phage that encodes cholera toxin in Vibrio cholerae, host LexA and the phage repressor function as both activators and repressors of gene expression to ensure the permanent production and secretion of CTX⌽ (36).…”

Effect of LexA on Chromosomal Integration of CTXϕ in Vibrio cholerae

“…To gain insight into mechanisms by which LexA contributes to CTX integration, we focused on the three abundant ssDNA binding proteins, RecA, RstB, and Ssb, in toxigenic cholera strains. LexA tightly or moderately represses the expression of all three ssDNA binding proteins during normal physiological conditions (7,27). First, we compared CTX integration in RecA-positive and RecA-deficient V. cholerae cells ( Table 7).…”

A New Twist on a Classic Paradigm: Illumination of a Genetic Switch in Vibrio cholerae Phage CTXΦ

“…The study by Kimsey and Waldor provides important insight regarding the control of the CTX⌽ lysogenic switch and raises a number of further questions (5). First, precisely how do the features of the CTX⌽ switch identified in their study enable the switch to exhibit transient, reversible kinetics?…”

“…Proper functioning of the switch requires that the intracellular levels of CI be precisely controlled, which is accomplished through the ability of CI to regulate expression of its own gene through an auto-feedback loop. In this issue of the Journal of Bacteriology, Kimsey and Waldor illuminate features of a regulatory circuit that controls production of the filamentous bacteriophage CTX⌽ in Vibrio cholerae (5), which bears similarities to the genetic switch of . The CTX⌽ switch is governed by two transcription factors, the host-encoded SOS response regulator LexA and the phage-encoded repressor RstR.…”