title: Functional consequences of subunit diversity in RNA polymerases II and V.
abstract: Multisubunit RNA polymerases IV and V (Pol IV and Pol V) evolved as specialized forms of Pol II that mediate RNA-directed DNA methylation (RdDM) and transcriptional silencing of transposons, viruses, and endogenous repeats in plants. Among the subunits common to Arabidopsis thaliana Pols II, IV, and V are 93% identical alternative ninth subunits, NRP(B/D/E)9a and NRP(B/D/E)9b. The 9a and 9b subunit variants are incompletely redundant with respect to Pol II; whereas double mutants are embryo lethal, single mutants are viable, yet phenotypically distinct. Likewise, 9a or 9b can associate with Pols IV or V but RNA-directed DNA methylation is impaired only in 9b mutants. Based on genetic and molecular tests, we attribute the defect in RdDM to impaired Pol V function. Collectively, our results reveal a role for the ninth subunit in RNA silencing and demonstrate that subunit diversity generates functionally distinct subtypes of RNA polymerases II and V.
authors.affiliation: Department of Biology, Indiana University, Bloomington, IN 47405, USA.
authors.forename: Ek Han
authors.initials: EH
authors.lastname: Tan
authors.forename: Todd
authors.initials: T
authors.lastname: Blevins
authors.forename: Thomas S
authors.initials: TS
authors.lastname: Ream
authors.forename: Craig S
authors.initials: CS
authors.lastname: Pikaard
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003433:Crosses, Genetic; D019175:DNA Methylation / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D012321:DNA-Directed RNA Polymerases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D010802:Phylogeny; D030821:Plants, Genetically Modified; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012319:RNA Polymerase II / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D019076:Transgenes
chemical_list: D029681:Arabidopsis Proteins; D004269:DNA, Bacterial; D021122:Protein Subunits; D012333:RNA, Messenger; C036483:T-DNA; D012319:RNA Polymerase II; C499181:RNA polymerase IV, Arabidopsis; C538841:RNA polymerase V, Arabidopsis; D012321:DNA-Directed RNA Polymerases