Translational polarity of a mutation in the initiator AUG codon of the lambda cI gene.

A PRM-cI-lacZ fusion inserted into the b2 region of bacteriophage lambda was used to isolate mutations affecting expression of both the lambda cI gene and the lacZ gene. One such mutation, a change in the cI initiator codon from AUG to AUA, reduces immunity of a lambda prophage to superinfection, and causes a 60-70% reduction in beta-galactosidase synthesis, even when repressor is supplied in trans. The effect of the mutation on lacZ gene expression is eliminated in a rho- bacterial strain, and the mutation has no effect on transcription initiated at PRM in vitro. Therefore, the effects of the mutation are due to premature p-dependent termination of transcription in the absence of translation of the cI gene, as if the mutation were a nonsense polar mutation.