The RNA Polymerase-associated Factor 1 Complex (Paf1C) Directly Increases the Elongation Rate of RNA Polymerase I and Is Required for Efficient Regulation of rRNA Synthesis

Abstract

The rate of ribosome synthesis is proportional to the rate of cell proliferation; thus, transcription of rRNA by RNA polymerase I (Pol I) is an important target for the regulation of this process. Most previous investigations into mechanisms that regulate the rate of ribosome synthesis have focused on the initiation step of transcription by Pol I; however, recent studies in yeast and mammals have identified factors that influence transcription elongation by Pol I. The RNA polymerase-associated factor 1 complex (Paf1C) is a transcription elongation factor with known roles in Pol II transcription. We previously identified a role for Paf1C in transcription elongation by Pol I. In this study, genetic interactions between genes for Paf1C and Pol I subunits confirm this conclusion. In vitro studies demonstrate that purified Paf1C directly increases the rate of transcription elongation by Pol I. Finally, we show that Paf1C function is required for efficient control of Pol I transcription in response to target of rapamycin (TOR) signaling or amino acid limitation. These studies demonstrate that Paf1C plays an important direct role in cellular control of rRNA expression.

The Transcription Elongation Factor Spt5 Influences Transcription by RNA Polymerase I Positively and Negatively

“…Five ml of culture was then pulselabeled with 25 Ci/ml [methyl- 3 H]methionine (PerkinElmer Life Sciences) for 5 min and then treated with excess cold methionine (500 g/ml) for an additional 5 min. RNA was then extracted and analyzed as described previously (11).…”

“…The activity of transcription initiation factors Rrn3 (TIF-1A in mouse) (3,5,6) and SL1 (12,13) is modified in response to demands for protein synthesis. Several studies have validated these factors as targets for the control of Pol I transcription initiation; however, recent data demonstrate that other steps in transcription by Pol I are also targets of regulation (10,11).…”

“…Transcription by Pol I is controlled at the initiation step (3)(4)(5)(6)(7), by alteration of the fraction of actively transcribed rDNA repeats (8,9), or at the elongation step (10,11). The transcription initiation step is the best characterized target for regulation.…”

DNA Binding by the Ribosomal DNA Transcription Factor Rrn3 Is Essential for Ribosomal DNA Transcription

“…The biogenesis of the basic protein synthetic machinery is an energetically costly process that is tightly regulated (1)(2)(3)(4)(5)(6). A significant fraction of these processes is involved in the synthesis and processing of the RNA backbones of the 40 S and 60 S ribosomal subunits, the 18 S and 5.8 S and 28 S rRNAs, respectively.…”

“…A significant fraction of these processes is involved in the synthesis and processing of the RNA backbones of the 40 S and 60 S ribosomal subunits, the 18 S and 5.8 S and 28 S rRNAs, respectively. The synthesis of rRNA is regulated at both the transcription initiation and elongation steps (1,2,5).…”