Fig. 1.

Three mechanisms of transcription termination. (A) Intrinsic termination is driven by formation of an RNA hairpin in the emerging transcript, the base of which occurs 8–9 nt from the site of release. Release also requires a uridine-rich segment downstream of the hairpin, particularly in the region immediately adjacent to the G/C-rich end of the stem. Although not illustrated, we suggest (as described in the text) that the DNA bubble is partly rewound and that the RNA/DNA hybrid is partly unwound when the hairpin is fully formed. (B) The termination factor Rho is a hexameric RNA translocase that binds ≈60 nt of emerging transcript, moving along it in a 5′–3′ direction in an ATP-dependent reaction. This movement is believed to extract the transcript. (C) Mfd is a DNA translocase that binds duplex DNA upstream of the transcription bubble and RNAP in a region near the site of DNA rewinding. The activity of the translocase causes dissociation of the complex in conditions that do not allow the RNA chain to advance through NTP polymerization.