A ribosome binding site sequence is necessary for efficient expression of the distal gene of a translationally-coupled gene pair.

Expression of trpB and trpA of the Escherichia coli tryptophan operon is shown to be "translationally coupled", i.e., efficient translation of the trpA coding region is dependent on prior translation of the trpB coding region and termination of translation at the trpB stop codon. To examine the dependence of trpA expression on the ribosome binding site sequence in the distal segment of trpB, deletions were produced that replaced this trpB sequence. Analysis of trpA expression in these deletion mutants established that the ribosome binding site sequence is required for efficient translation of the trpA segment of trp mRNA. A modest effect of translation over the trpA ribosome binding site on independent initiation at that site was also observed.