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Mol. Cell. Biol., May 1995, 2420-2428, Vol 15, No. 5
H Conrad-Webb and RA Butow
Transcription of ribosomal DNA by RNA polymerase I is believed to be the
sole source of the 25S, 18S, and 5.8S rRNAs in wild-type cells of
Saccharomyces cerevisiae. Here we present evidence for a switch from RNA
polymerase I to RNA polymerase II in the synthesis of a substantial
fraction of those rRNAs in respiratory-deficient (petite) cells. The
templates for the RNA polymerase II transcripts are largely, if not
exclusively, episomal copies of ribosomal DNA arising from homologous
recombination events within the ribosomal DNA repeat on chromosome XII.
Ribosomal DNA contains a cryptic RNA polymerase II promoter that is
activated in petites; it overlaps the RNA polymerase I promoter and
produces a transcript equivalent to the 35S precursor rRNA made by RNA
polymerase I. Yeast cells that lack RNA polymerase I activity, because of a
disruption of the RPA135 gene that encodes subunit II of the enzyme, can
survive by using the RNA polymerase II promoter in ribosomal DNA to direct
the synthesis of the 35S rRNA precursor. This polymerase switch could
provide cells with a mechanism to synthesize rRNA independent of the
controls of RNA polymerase I transcription.
Copyright © 1995, American Society for Microbiology
A polymerase switch in the synthesis of rRNA in Saccharomyces cerevisiae
Department of Biochemistry, University of Texas Southwestern Medical Center, Dallas 75235.
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