Sequence changes preceding a Shine-Dalgarno region influence trpE mRNA translation and decay.

In studies with a trpE promoter-strength measuring system we observed that constructs containing the Escherichia coli trp promoter and its adjacent transcribed region yielded lower levels of trpE protein than were expected. To analyze this observation we introduced mutational changes in the nucleotide sequence preceding the trpE Shine-Dalgarno region and examined their effects on trpE mRNA synthesis, translation and decay. We found that certain deletion, insertion and substitution mutations in the pre-Shine-Dalgarno region caused a two- to fivefold increase in trpE enzyme activity. These increases were accompanied by increases in steady-state levels of trpE mRNA. Pulse-chase analyses of trpE mRNA degradation revealed that the observed steady-state trpE mRNA levels correlated with changes in trpE mRNA stability. These findings are interpreted in terms of alternative models in which the primary effect of mutational changes that elevate trpE expression is to increase trpE mRNA translation, versus increasing trpE mRNA stability.