RNA polymerase I remains intact without subunit exchange through multiple rounds of transcription in Saccharomyces cerevisiae.
Previous experiments using mammalian cells suggested that after each round of transcription, RNA polymerase I (Pol I) dissociates into subunits that leave and reenter the nucleolus as individual subunits, before formation of a new initiation complex. In this study, we show that the size and subunit composition of Pol I did not change significantly when Pol I was not engaged in rRNA transcription, brought about by either the absence of Pol I-specific rDNA template or specific inhibition of the transcription initiation step that requires Rrn3p. In fact, Pol I purified from cells completely lacking rDNA repeats was more active than when purified from wild-type cells in an in vitro transcription system designed to assay active Pol I-Rrn3p complexes. Furthermore, measurements of the exchange of A135 and A190 subunits between preexistent Pol I and newly synthesized Pol I showed that these two largest subunits of Pol I do not disassociate through many rounds of transcription in vivo. Thus, Pol I is not a dynamic protein complex but rather a stable enzyme.
Department of Biological Chemistry, University of California, Irvine, CA 92697, USA.
David A
DA
Schneider
Masayasu
M
Nomura
D004271:DNA, Fungal / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics; D008954:Models, Biological; D009154:Mutation; D010957:Plasmids / Q000235:genetics; D035463:Pol1 Transcription Initiation Complex Proteins / Q000235:genetics / Q000378:metabolism; D021122:Protein Subunits; D012318:RNA Polymerase I / Q000737:chemistry / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
D004271:DNA, Fungal; D004275:DNA, Ribosomal; D035463:Pol1 Transcription Initiation Complex Proteins; D021122:Protein Subunits; C101161:RRN3 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D012318:RNA Polymerase I