A POLYMERASE SWITCH IN THE SYNTHESIS OF RIBOSOMAL-RNA IN SACCHAROMYCES-CEREVISIAE

Transcription of ribosomal DNA by RNA polymerase I is believed to be t he sole source of the 25S, 18S, and 5.88 rRNAs in wild-type cells of S accharomyces cerevisiae. Here we present evidence for a switch from RN A polymerase I to RNA polymerase II in the synthesis of a substantial fraction of those rRNAs in respiratory-deficient (petite) cells The te mplates for the RNA polymerase II transcripts are largely, if not excl usively, episomal copies of ribosomal DNA arising from homologous reco mbination events within the ribosomal DNA repeat on chromosome XII. Ri bosomal DNA contains a cryptic RNA polymerase II promoter that is acti vated in petites; it overlaps the RNA polymerase I promoter and produc es a transcript equivalent to the 35S precursor rRNA made by RNA polym erase I. Yeast cells that lack RNA polymerase I activity because of a disruption of the RPA135 gene that encodes subunit II of the enzyme, c an survive by using the RNA polymerase II promoter in ribosomal DNA to direct the synthesis of the 35S rRNA precursor. This polymerase switc h could provide cells with a mechanism to synthesize rRNA independent of the controls of RNA polymerase I transcription.