Translational Polarity of a Mutation in the Initiator AUG Codon of the cI Gene

Gary N. Gussin ¹, Susan Brown ¹, and Karen Matz ¹

¹ Department of Biology, University of Iowa, Iowa City, Iowa 52242

A P_RM-cI-lacZ fusion inserted into the b2 region of bacteriophage was used to isolate mutations affecting expression of both the cI gene and the lacZ gene. One such mutation, a change in the cI initiator codon from AUG to AUA, reduces immunity of a prophage to superinfection, and causes a 60-70% reduction in ß-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 P_RM in vitro. Therefore, the effects of the mutation are due to premature -dependent termination of transcription in the absence of translation of the cI gene, as if the mutation were a nonsense polar mutation.