Overproduction of bacteriophage Qβ maturation (A2) protein leads to cell lysis

Double-stranded cDNA from the maturation (or A₂) protein gene of the RNA bacteriophage Qβ has been cloned such that its expression is regulated by the E. coli lac promoter/operator. Induction of the A₂ clone is lethal to the host. The basis of this lethality is cell lysis, which is correlated with synthesis of the A₂ protein. No other major proteins appear to be made from the A₂ gene when inducer is added. Plasmid-derived A₂ protein specifically complements infecting Qβ A₂ amber mutants. Lysis activity is abolished in clones that synthesize truncated or internally deleted A₂ polypeptides ranging from 10% to 95% of the length of wild-type protein. We conclude that host lysis is promoted by the maturation protein itself, rather than by a separate lysis protein. The A₂ protein probably allows for the release of progeny Qβ phage particles following normal infection.