CLONING AND FUNCTIONAL-CHARACTERIZATION OF PTRF, A NOVEL PROTEIN WHICH INDUCES DISSOCIATION OF PAUSED TERNARY TRANSCRIPTION COMPLEXES

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, pa using of elongation complexes is mediated by the transcription termina tion factor TTF-I bound to the 'Sal box' terminator downstream of the rDNA transcription unit, Dissociation of paused ternary complexes requ ires a cellular factor, termed PTRF for Pol I and transcript release f actor. Here we describe the molecular cloning of a cDNA corresponding to murine PTRF, Recombinant PTRF is capable of dissociating ternary Po l I transcription complexes in vitro as revealed by release of both Po l I and nascent transcripts from the template. Consistent with its fun ction in transcription termination, PTRF interacts with both TTF-I and Pol I. Moreover, we demonstrate specific binding of PTRF to transcrip ts containing the 3' end of pre-rRNA, Substitution of 3'-terminal urid ylates 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 transcript ion.