Cloning and functional characterization of PTRF, a novel protein which induces dissociation of paused ternary transcription complexes

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Abstract
Termination of transcription by RNA polymerase I (Pol I) is a two‐step process which involves pausing of elongating transcription complexes and release of both pre‐rRNA and Pol I from the template. In mouse, pausing of elongation complexes is mediated by the transcription termination factor TTF‐I bound to the ‘Sal box’ terminator downstream of the rDNA transcription unit. Dissociation of paused ternary complexes requires a cellular factor, termed PTRF for Pol I and transcript release factor. Here we describe the molecular cloning of a cDNA corresponding to murine PTRF. Recombinant PTRF is capable of dissociating ternary Pol I transcription complexes in vitro as revealed by release of both Pol I and nascent transcripts from the template. Consistent with its function in transcription termination, PTRF interacts with both TTF‐I and Pol I. Moreover, we demonstrate specific binding of PTRF to transcripts containing the 3′ end of pre‐rRNA. Substitution of 3′‐terminal uridylates by guanine residues abolishes PTRF binding and impairs release activity. The results reveal a network of protein–protein and protein–nucleic acid interactions that governs termination of Pol I transcription.