Polarity in segments of the Escherichia coli trp operon with deleted intraoperonic translational initiation signals.

The effect of deletion of the operator-distal genes of the trp operon, including the trpE-trpD intercistronic punctuation point, on the degree of transcriptional polarity (in this case the effect of a nonsense mutation on the level of mRNA from the distal part of the very gene where the mutation is located) was investigated. Double mutants which contain a nonsense mutation and a deletion in trpE were constructed, and the degree of transcriptional polarity was estimated by the decrease in messenger RNA for the operator-distal trpE beyond the nonsense mutation, as well as by the production of truncated messenger RNA for the region of trpE proximal to the nonsense mutation. The content of mRNA of operator-distal trpE and the size of the mRNA of operator-proximal trpE of the double mutants show that transcriptional polarity is not relaxed as a function of distance of the nonsense mutation from the operator-distal end of the trpE segment (at which the subsequent high efficiency translational initiation signal has been deleted). These findings are consistent with the conclusion that the degree of polarity depends on the distance of the nonsense mutation from the subsequent translation initiation signal, but not on its distance from the operator-distal end, including possible translational or transcriptional termination signals.