Synthesis of trp mRNA in vitro directed by plasmid DNA carrying the entire trp operon was studied using crude protein extracts (S-100) from Escherichia coli strains carrying the nusA or nusB mutation or both. It was found that the levels of trp mRNA transcribed from the promoter-distal genes (trpCBA) relative to that from the promoter-proximal genes (trpED) was markedly lower with extracts from the nus- mutants than that from the nus+ strain. Kinetic experiments suggest that termination of RNA transcripts at intragenic transcriptional barriers is prevented by the nus gene products from allowing efficient expression of the operon.