Transcription factor UAF, expansion and contraction of ribosomal DNA (rDNA) repeats, and RNA polymerase switch in transcription of yeast rDNA.
Strains of the yeast Saccharomyces cerevisiae defective in transcription factor UAF give rise to variants able to grow by transcribing endogenous ribosomal DNA (rDNA) by RNA polymerase II (Pol II). We have demonstrated that the switch to growth using the Pol II system consists of two steps: a mutational alteration in UAF and an expansion of chromosomal rDNA repeats. The first step, a single mutation in UAF, is sufficient to allow Pol II transcription of rDNA. In contrast to UAF mutations, mutations in Pol I or other Pol I transcription factors can not independently lead to Pol II transcription of rDNA, suggesting a specific role of UAF in preventing polymerase switch. The second step, expansion of chromosomal rDNA repeats to levels severalfold higher than the wild type, is required for efficient cell growth. Mutations in genes that affect recombination within the rDNA repeats, fob1 and sir2, decrease and increase, respectively, the frequency of switching to growth using Pol II, indicating that increased rDNA copy number is a cause rather than a consequence of the switch. Finally, we show that the switch to the Pol II system is accompanied by a striking alteration in the localization and morphology of the nucleolus. The altered state that uses Pol II for rDNA transcription is semistable and heritable through mitosis and meiosis. We discuss the significance of these observations in relation to the plasticity of rDNA tandem repeats and nucleolar structures as well as evolution of the Pol I machinery.
Oakes
M
M
Department of Biological Chemistry, University of California, Irvine, Irvine, California 92697-1700, USA.
Siddiqi
I
I
Vu
L
L
Aris
J
J
Nomura
M
M
D000222:Adaptation, Physiological; D002466:Cell Nucleolus / Q000378:metabolism; D004271:DNA, Fungal / Q000378:metabolism*; D004275:DNA, Ribosomal / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology; D005656:Fungal Proteins / Q000235:genetics / Q000502:physiology; D006655:Histone Deacetylases; D008540:Meiosis; D016296:Mutagenesis; D035463:Pol1 Transcription Initiation Complex Proteins; D012318:RNA Polymerase I / Q000235:genetics / Q000378:metabolism*; D012319:RNA Polymerase II / Q000235:genetics / Q000378:metabolism*; D012322:RNA Precursors / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D056565:Sirtuin 2; D037761:Sirtuins; D015534:Trans-Activators / Q000235:genetics / Q000502:physiology; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism* / Q000502:physiology
D004271:DNA, Fungal; D004275:DNA, Ribosomal; D004268:DNA-Binding Proteins; C105579:FOB1 protein, S cerevisiae; D005656:Fungal Proteins; D035463:Pol1 Transcription Initiation Complex Proteins; D012322:RNA Precursors; C098777:RRN5 protein, S cerevisiae; C090311:RRN6 protein, S cerevisiae; C098778:RRN9 protein, S cerevisiae; C084122:SIR3 protein, S cerevisiae; C061573:SIR4 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D015534:Trans-Activators; D014157:Transcription Factors; C402483:upstream activation factor, S cerevisiae; D012319:RNA Polymerase II; D012318:RNA Polymerase I; C087778:SIR2 protein, S cerevisiae; D056565:Sirtuin 2; D037761:Sirtuins; D006655:Histone Deacetylases