Lysis gene of bacteriophage MS2 is activated by translation termination at the overlapping coat gene

The 3' boundary of the coat gene of the RNA bacteriophage MS2 lies 46 nucleotides downstream from the beginning of the lysis (L) cistron. The translation of both reading frames is coupled; the synthesis of the lysis protein does not occur unless translation of the overlapping coat gene takes place. In the preceding paper we showed that de novo initiation at the L gene is prevented by a hairpin structure that sequesters the ribosomal binding site. Here we examine how translation of the coat gene activates the L gene start site. The experiments show that the movement of ribosomes through the hairpin is in itself not sufficient to expose the lysis gene. Rather, the endpoint of translation is important. Termination at the natural end of the coat gene triggers the lysis response, but further downstream terminations do not. Activation of the L gene is suppressed when the stability of the lysis initiator hairpin is increased by mutations that create additional base-pairs. We assume that the ribosome, terminating at the coat reading frame, covers part of the lysis hairpin, thereby destabilizing the secondary structure. This may be sufficient to promote the binding of a vacant ribosome to the L gene start. Alternatively, the terminated but not yet released ribosome may reach the L gene start by random lateral movements along the mRNA and reinitiate there. The present findings are also discussed in relation to an earlier proposal for L gene activation.