IDENTIFICATION OF A TRANSCRIPT RELEASE ACTIVITY ACTING ON TERNARY TRANSCRIPTION COMPLEXES CONTAINING MURINE RNA-POLYMERASE-I

Termination of mammalian ribosomal gene transcription by RNA polymeras e I (Pol I) requires binding of the nucleolar factor TTF-I (transcript ion termination factor for Pol I) to specific rDNA terminator elements . We have used recombinant murine TTF-I in an immobilized tailed templ ate assay to analyze individual steps of the termination reaction. We demonstrate that, besides the TTF-I-DNA complex which stops elongating Pol I, an additional activity is required to release both the nascent transcript and Pol I from the template. Moreover, transcript release, but not TTF-I-directed pausing, depends on upstream sequences directl y flanking the terminator element. Together, complete termination of P ol I transcription requires TTF-I bound to the terminator DNA, a stret ch of thymidine residues upstream of the TTF-I-mediated pause site and an activity which releases the RNA transcript and Pol I from the DNA template.