Yet another way that phage λ manipulates its Escherichia coli host: λrexB is involved in the lysogenic-lytic switch

The life cycle of phage λ has been studied extensively. Of particular interest has been the process leading to the decision of the phage to switch from lysogenic to lytic cycle. The principal participant in this process is the λcI repressor, which is cleaved under conditions of DNA damage. Cleaved λcI no longer acts as a repressor, allowing phage λ to switch from its lysogenic to lytic cycle. The well-known mechanism responsible for λcI cleavage is the SOS response. We have recently reported that the Escherichia coli toxin-antitoxin mazEF pathway inhibits the SOS response; in fact, the SOS response is permitted only in E.coli strains deficient in the expression of the mazEF pathway. Moreover, in strains lysogenic for prophage λ, the SOS response is enabled by the presence of λrexB. λRexB had previously been found to inhibit the degradation of the antitoxin MazE, thereby preventing the toxic action of MazF. Thus, phage λ rexB gene not only safeguards the prophage state by preventing death of its E.coli host but is also indirectly involved in the lysogenic-lytic switch. E. coli phage λ exists in two developmental pathways: a) the lysogenic; when DNA stays resident within the host chromosome, and b) lytic; when it replicates as a virulent phage. Under DNA damage, the phage is transferred from its lysogenic to its lytic pathway by a mechanism called SOS response. Here we describe a novel pathway in which λrexB is indirectly involved in the lysogenic-lytic swicth.