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The capsids of ssRNA phages comprise a single copy of an ~45 kDa maturation protein that serves to recognize the conjugative pilus as receptor, to protect the ends of the viral RNA and also to escort the genomic RNA into the host cytoplasm. In the Alloleviviridae, represented by the canonical phage Qβ, the maturation protein A(2) also causes lysis. This is achieved by inhibiting the activity of MurA, which catalyses the first committed step of murein biosynthesis. Previously, it was shown that Qβ virions, with a single copy of A(2), inhibit MurA activity. This led to a model for lysis timing in which, during phage infection, A(2) is not active as a MurA inhibitor until assembled into virion particles, thus preventing premature lysis before a sufficient yield of viable progeny has accumulated. Here we report that MurA inactivates purified Qβ particles, casting doubt on the notion that A(2) must assemble into particles prior to MurA inhibition. Furthermore, quantification of A(2) protein induced from a plasmid indicated that lysis is entrained when the amount of the lysis protein is approximately equimolar to that of cellular MurA. Qβ por mutants, isolated as suppressors that overcome a murA(rat) mutation that reduces the affinity of MurA for A(2), were shown to be missense mutations in A(2) that increase the translation of the maturation protein. Because of the increased production of A(2), the por mutants have an attenuated infection cycle and reduced burst size, indicating that a delicate balance between assembled and unassembled A(2) levels regulates lysis timing.

A2 expression and assembly regulates lysis in Qβ infections