Cell cycle-dependent regulation of RNA polymerase I transcription: The nucleolar transcription factor UBF is inactive in mitosis and early G1

Cell cycle-dependent regulation of RNA polymerase I transcription: The nucleolar transcription factor UBF is inactive in mitosis and early G₁

Joachim Klein and
Ingrid Grummt*

Division of Molecular Biology of the Cell II, German Cancer Research Center, D-69120 Heidelberg, Germany

Edited by Robert G. Roeder, The Rockefeller University, New York, NY, and approved March 30, 1999 (received for review January 25, 1999)

Abstract

Transcription of ribosomal RNA genes by RNA polymerase (pol) I oscillates during the cell cycle, being maximal in S and G₂ phase, repressed during mitosis, and gradually recovering during G₁ progression. We have shown that transcription initiation factor (TIF)-IB/SL1 is inactivated during mitosis by cdc2/cyclin B-directed phosphorylation of TAF_I110. In this study, we have monitored reactivation of transcription after exit from mitosis. We demonstrate that the pol I factor UBF is also inactivated by phosphorylation but recovers with different kinetics than TIF-IB/SL1. Whereas TIF-IB/SL1 activity is rapidly regained on entry into G₁, UBF is reactivated later in G₁, concomitant with the onset of pol I transcription. Repression of pol I transcription in mitosis and early G₁ can be reproduced with either extracts from cells synchronized in M or G₁ phase or with purified TIF-IB/SL1 and UBF isolated in the presence of phosphatase inhibitors. The results suggest that two basal transcription factors, e.g., TIF-IB/SL1 and UBF, are inactivated at mitosis and reactivated by dephosphorylation at the exit from mitosis and during G₁ progression, respectively.

Footnotes

↵ * To whom reprint requests should be addressed. e-mail: I.Grummt{at}DKFZ-Heidelberg.de.
This paper was submitted directly (Track II) to the Proceedings Office.
ABBREVIATIONS:

pol,

RNA polymerase;

TBP,

TATA box-binding protein;

TAFI,

pol I-specific TBP-associated factor;

UBF,

upstream binding factor;

TIF,

transcription initiation factor;

PP,

protein phosphatase;

cdk,

cyclin-dependent kinase;

TF,

transcription factor;

rDNA,

rRNA-encoding DNA;

FACS,

fluorescence-activated cell sorting;

AMP-PNP,

adenosine 5′-[β,γ-imido]triphosphate;

GMP-PNP,

guanosine 5′-[β,γ-imido]triphosphate