Transcription termination at the thr attenuator. Evidence that the adenine residues upstream of the stem and loop structure are not required for termination.

The Escherichia coli thr operon attenuator has a structure similar to other Rho-independent terminators. The DNA sequence immediately 5' to the termination site is dG+dC-rich and contains a region of dyad symmetry that, when transcribed into RNA, encodes a hairpin structure in the transcript. It also contains a stretch of 9 consecutive dA-dT residues immediately distal to the region of dyad symmetry which encode uridine residues at the 3' end of the terminated transcript. In addition, the thr attenuator has a stretch of 6 dA-dT residues immediately upstream of the region of dyad symmetry which encode 6 adenines. These adenines could potentially pair with the distal uridines to form a hairpin structure extended by as much as 6 A-U base pairs. In this report we have examined the role of the upstream adenines in transcription termination. We used templates that specify mismatches or create new base pairs in the potential A-U secondary structure of the transcript as well as templates that delete segments of the A residues upstream of the hairpin. We conclude that A-U pairing is not required for efficient transcription termination at the thr attenuator. This conclusion is likely to apply to other Rho-independent terminators that contain hairpin-proximal dA-dT residues.