ABSTRACT
Regulation of rDNA transcription depends on the formation and dissociation of a functional complex between RNA polymerase I (pol I) and transcription initiation factor Rrn3p. We analyzed whether phosphorylation is involved in this molecular switch. Rrn3p is a phosphoprotein that is predominantly phosphorylated in vivo when it is not bound to pol I. In vitro, Rrn3p is able both to associate with pol I and to enter the transcription cycle in its nonphosphorylated form. By contrast, phosphorylation of pol I is required to form a stable pol I-Rrn3p complex for efficient transcription initiation. Furthermore, association of pol I with Rrn3p correlates with a change in the phosphorylation state of pol I in vivo. We suggest that phosphorylation at specific sites of pol I is a prerequisite for proper transcription initiation and that phosphorylation/dephosphorylation of pol I is one possibility to modulate cellular rDNA transcription activity.
Subject(s)
Pol1 Transcription Initiation Complex Proteins , RNA Polymerase I/biosynthesis , Saccharomyces cerevisiae Proteins , Escherichia coli/genetics , Macromolecular Substances , Phosphorylation , RNA Polymerase I/chemistry , RNA Polymerase I/genetics , Recombinant Proteins/chemistry , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Saccharomyces cerevisiae/enzymology , Saccharomyces cerevisiae/genetics , Transcription Factors/chemistry , Transcription Factors/genetics , Transcription Factors/metabolism , Transcription, GeneticABSTRACT
RNA polymerase I (Pol I) is dedicated to transcription of the large ribosomal DNA (rDNA). The mechanism of Pol I recruitment onto rDNA promoters is poorly understood. Here we present evidence that subunit A43 of Pol I interacts with transcription factor Rrn3: conditional mutations in A43 were found to disrupt the transcriptionally competent Pol I-Rrn3 complex, the two proteins formed a stable complex when co-expressed in Escherichia coli, overexpression of Rrn3 suppressed the mutant phenotype, and A43 and Rrn3 mutants showed synthetic lethality. Consistently, immunoelectron microscopy data showed that A43 and Rrn3 co-localize within the Pol I-Rrn3 complex. Rrn3 has several protein partners: a two-hybrid screen identified the C-terminus of subunit Rrn6 of the core factor as a Rrn3 contact, an interaction supported in vitro by affinity chromatography. Our results suggest that Rrn3 plays a central role in Pol I recruitment to rDNA promoters by bridging the enzyme to the core factor. The existence of mammalian orthologues of A43 and Rrn3 suggests evolutionary conservation of the molecular mechanisms underlying rDNA transcription in eukaryotes.
Subject(s)
DNA, Fungal/metabolism , DNA, Ribosomal/metabolism , Pol1 Transcription Initiation Complex Proteins , RNA Polymerase I/chemistry , RNA Polymerase I/metabolism , Saccharomyces cerevisiae Proteins , Saccharomyces cerevisiae/enzymology , Transcription Factors/metabolism , Amino Acid Sequence , Binding Sites , DNA, Fungal/genetics , DNA, Ribosomal/genetics , Epistasis, Genetic , Fungal Proteins/genetics , Fungal Proteins/metabolism , Gene Expression Regulation, Fungal , Image Processing, Computer-Assisted , Macromolecular Substances , Microscopy, Electron , Models, Molecular , Molecular Sequence Data , Mutation/genetics , Promoter Regions, Genetic , Protein Binding , Protein Subunits , RNA Polymerase I/genetics , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Sequence Alignment , Transcription Factors/genetics , Transcription, Genetic , Two-Hybrid System TechniquesABSTRACT
There is limited information on how eukaryotic RNA polymerases (Pol) recognize their cognate preinitiation complex. We have characterized a polypeptide copurifying with yeast Pol III. This protein, C17, was found to be homologous to a mammalian protein described as a hormone receptor. Deletion of the corresponding gene, RPC17, was lethal and its regulated extinction caused a selective defect in transcription of class III genes in vivo. Two-hybrid and coimmunoprecipitation experiments indicated that C17 interacts with two Pol III subunits, one of which, C31, is important for the initiation reaction. C17 also interacted with TFIIIB70, the TFIIB-related component of TFIIIB. The interaction domain was found to be in the N-terminal, TFIIB-like half of TFIIIB70, downstream of the zinc ribbon and first imperfect repeat. Although Pol II similarly interacts with TFIIB, it is notable that C17 has no similarity to any Pol II subunit. The data indicate that C17 is a novel specific subunit of Pol III which participates together with C34 in the recruitment of Pol III by the preinitiation complex.
