title: Cell cycle and growth stimuli regulate different steps of RNA polymerase I transcription.
abstract: Transcription of the ribosomal RNA genes (rDNA) by RNA polymerase I (Pol I) is a major control step for ribosome synthesis and is tightly linked to cellular growth. However, the question of whether this process is modulated primarily at the level of transcription initiation or elongation is controversial. Studies in markedly different cell types have identified either initiation or elongation as the major control point. In this study, we have re-examined this question in NIH3T3 fibroblasts using a combination of metabolic labeling of the 47S rRNA, chromatin immunoprecipitation analysis of Pol I and overexpression of the transcription initiation factor Rrn3. Acute manipulation of growth factor levels altered rRNA synthesis rates over 8-fold without changing Pol I loading onto the rDNA. In fact, robust changes in Pol I loading were only observed under conditions where inhibition of rDNA transcription was associated with chronic serum starvation or cell cycle arrest. Overexpression of the transcription initiation factor Rrn3 increased loading of Pol I on the rDNA but failed to enhance rRNA synthesis in either serum starved, serum treated or G0/G1 arrested cells. Together these data suggest that transcription elongation is rate limiting for rRNA synthesis. We propose that transcription initiation is required for rDNA transcription in response to cell cycle cues, whereas elongation controls the dynamic range of rRNA synthesis output in response to acute growth factor modulation.
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Sandy S
authors.initials: SS
authors.lastname: Hung
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Analia
authors.initials: A
authors.lastname: Lesmana
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Abigail
authors.initials: A
authors.lastname: Peck
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Rachel
authors.initials: R
authors.lastname: Lee
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Elly
authors.initials: E
authors.lastname: Tchoubrieva
authors.affiliation: The John Curtin School of Medical Research, The Australian National University, Acton 2601, ACT, Australia; Department of Biochemistry and Molecular Biology, The University of Melbourne, Victoria 3010, Australia.
authors.forename: Katherine M
authors.initials: KM
authors.lastname: Hannan
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Jane
authors.initials: J
authors.lastname: Lin
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia; Department of Biochemistry and Molecular Biology, The University of Melbourne, Victoria 3010, Australia; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Victoria 3010, Australia.
authors.forename: Karen E
authors.initials: KE
authors.lastname: Sheppard
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Jastrzebski
authors.affiliation: Department of Anatomy and Cell Biology, The University of Melbourne, Victoria 3010, Australia.
authors.forename: Leonie M
authors.initials: LM
authors.lastname: Quinn
authors.affiliation: Department of Cell Biology, University of Oklahoma College of Medicine, Oklahoma City, OK 73104, United States.
authors.forename: Lawrence I
authors.initials: LI
authors.lastname: Rothblum
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia; Department of Biochemistry and Molecular Biology, The University of Melbourne, Victoria 3010, Australia; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Victoria 3010, Australia. Electronic address: rick.pearson@petermac.org.
authors.forename: Richard B
authors.initials: RB
authors.lastname: Pearson
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia; The John Curtin School of Medical Research, The Australian National University, Acton 2601, ACT, Australia; Department of Biochemistry and Molecular Biology, The University of Melbourne, Victoria 3010, Australia; Sir Peter MacCallum Department of Oncology, The University of Melbourne, Victoria 3010, Australia; School of Biomedical Sciences, The University of Queensland, Queensland 4072, Australia. Electronic address: ross.hannan@anu.edu.au.
authors.forename: Ross D
authors.initials: RD
authors.lastname: Hannan
authors.affiliation: Research Division, Peter MacCallum Cancer Centre, Melbourne, Victoria 3000, Australia; The Department of Pathology, The University of Melbourne, Victoria 3010, Australia. Electronic address: elaine.sanij@petermac.org.
authors.forename: Elaine
authors.initials: E
authors.lastname: Sanij
mesh_terms: D000818:Animals; D002453:Cell Cycle; D002455:Cell Division; D051379:Mice; D041681:NIH 3T3 Cells; D012318:RNA Polymerase I / Q000235:genetics*; D014158:Transcription, Genetic
chemical_list: D012318:RNA Polymerase I
keywords: RNA polymerase I; Rrn3; rDNA