title: A dominant mutation in mediator of paramutation2, one of three second-largest subunits of a plant-specific RNA polymerase, disrupts multiple siRNA silencing processes.
abstract: Paramutation involves homologous sequence communication that leads to meiotically heritable transcriptional silencing. We demonstrate that mop2 (mediator of paramutation2), which alters paramutation at multiple loci, encodes a gene similar to Arabidopsis NRPD2/E2, the second-largest subunit of plant-specific RNA polymerases IV and V. In Arabidopsis, Pol-IV and Pol-V play major roles in RNA-mediated silencing and a single second-largest subunit is shared between Pol-IV and Pol-V. Maize encodes three second-largest subunit genes: all three genes potentially encode full length proteins with highly conserved polymerase domains, and each are expressed in multiple overlapping tissues. The isolation of a recessive paramutation mutation in mop2 from a forward genetic screen suggests limited or no functional redundancy of these three genes. Potential alternative Pol-IV/Pol-V-like complexes could provide maize with a greater diversification of RNA-mediated transcriptional silencing machinery relative to Arabidopsis. Mop2-1 disrupts paramutation at multiple loci when heterozygous, whereas previously silenced alleles are only up-regulated when Mop2-1 is homozygous. The dramatic reduction in b1 tandem repeat siRNAs, but no disruption of silencing in Mop2-1 heterozygotes, suggests the major role for tandem repeat siRNAs is not to maintain silencing. Instead, we hypothesize the tandem repeat siRNAs mediate the establishment of the heritable silent state-a process fully disrupted in Mop2-1 heterozygotes. The dominant Mop2-1 mutation, which has a single nucleotide change in a domain highly conserved among all polymerases (E. coli to eukaryotes), disrupts both siRNA biogenesis (Pol-IV-like) and potentially processes downstream (Pol-V-like). These results suggest either the wild-type protein is a subunit in both complexes or the dominant mutant protein disrupts both complexes. Dominant mutations in the same domain in E. coli RNA polymerase suggest a model for Mop2-1 dominance: complexes containing Mop2-1 subunits are non-functional and compete with wild-type complexes.
authors: Department of Plant Sciences, University of Arizona, Tucson, Arizona, United States of America.
authors: Lyudmila
authors: L
authors: Sidorenko
authors: Jane E
authors: JE
authors: Dorweiler
authors: A Mark
authors: AM
authors: Cigan
authors: Mario
authors: M
authors: Arteaga-Vazquez
authors: Meenal
authors: M
authors: Vyas
authors: Jerry
authors: J
authors: Kermicle
authors: Diane
authors: D
authors: Jurcin
authors: Jan
authors: J
authors: Brzeski
authors: Yu
authors: Y
authors: Cai
authors: Vicki L
authors: VL
authors: Chandler
mesh_terms: D000483:Alleles; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D020029:Base Pairing; D017124:Conserved Sequence; D012321:DNA-Directed RNA Polymerases / Q000737:chemistry / Q000235:genetics*; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D005799:Genes, Dominant / Q000235:genetics*; D017343:Genes, Plant; D056426:Genetic Loci / Q000235:genetics; D006579:Heterozygote; D006720:Homozygote; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics*; D010641:Phenotype; D011401:Promoter Regions, Genetic / Q000235:genetics; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000378:metabolism*; D013045:Species Specificity; D020080:Tandem Repeat Sequences / Q000235:genetics; D014158:Transcription, Genetic; D019076:Transgenes / Q000235:genetics; D003313:Zea mays / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development
chemical_list: D021122:Protein Subunits; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D012321:DNA-Directed RNA Polymerases