RNA Polymerase I-Specific Subunit CAST/hPAF49 Has aRole in the Activation of Transcription by UpstreamBinding Factor

Abstract

Eukaryotic RNA polymerases are large complexes, 12 subunits of which are structurally or functionally homologous across the three polymerase classes. Each class has a set of specific subunits, likely targets of their cognate transcription factors. We have identified and characterized a human RNA polymerase I (Pol I)-specific subunit, previously identified as ASE-1 (antisense of ERCC1) and as CD3ε-associated signal transducer (CAST), and here termed CAST or human Pol I-associated factor of 49 kDa (hPAF49), after mouse orthologue PAF49. We provide evidence for growth-regulated Tyr phosphorylation of CAST/hPAF49, specifically in initiation-competent Pol Iβ complexes in HeLa cells, at a conserved residue also known to be important for signaling during T-cell activation. CAST/hPAF49 can interact with activator upstream binding factor (UBF) and, weakly, with selectivity factor 1 (SL1) at the rDNA (ribosomal DNA repeat sequence encoding the 18S, 5.8S, and 28S rRNA genes) promoter. CAST/hPAF49-specific antibodies and excess CAST/hPAF49 protein, which have no effect on basal Pol I transcription, inhibit UBF-activated transcription following functional SL1-Pol I-rDNA complex assembly and disrupt the interaction of UBF with CAST/hPAF49, suggesting that interaction of this Pol I-specific subunit with UBF is crucial for activation. Drawing on parallels between mammalian and Saccharomyces cerevisiae Pol I transcription machineries, we advance one model for CAST/hPAF49 function in which the network of interactions of Pol I-specific subunits with UBF facilitates conformational changes of the polymerase, leading to stabilization of the Pol I-template complex and, thereby, activation of transcription.

We thank P. Schultz (Ecole Superieure de Biotechnologie de Strasbourg, France) for the yeast Pol I image, G. Kular for PTPase (MRC Protein phosphorylation Unit, Dundee), D. Lamont and K. Beattie in the Proteomics Facility (School of Life Sciences, University of Dundee) for peptide mass fingerprinting by matrix-assisted laser desorption ionization-time of flight mass spectrometry, and the National Cell Culture Center (Minneapolis, MN) for supplying HeLa cell nuclei. We thank our colleagues in the Zomerdijk laboratory for advice and critical reading of the manuscript.

T.B.P. received a BBSRC Ph.D. studentship. J.C.B.M.Z. is a Wellcome Trust Senior Research Fellow in the Basic Biomedical Sciences.