Transcription analyses with heteroduplex trp attenuator templates indicate that the transcript stem and loop structure serves as the termination signal.

DNA sequences that control transcriptional termination by prokaryotic RNA polymerases normally contain an inverted repeat, or self-complementary sequence, about 10 base pairs upstream from the site of RNA chain termination. Point mutations that interrupt this self-complementarity can reduce or eliminate RNA chain termination. We have constructed heteroduplex DNA templates using wild type and mutant attenuators for the Escherichia coli trp operon to probe the relative contributions of the two DNA strands in the termination process. Transcription analyses show that only the sequences in the transcribed DNA strand determine whether or not a heteroduplex terminator can function. This result strongly supports a model in which the formation of a stem and loop structure in the nascent RNA transcript is the signal for transcriptional termination.