title: Histone methyltransferases regulating rRNA gene dose and dosage control in Arabidopsis.
abstract: Eukaryotes have hundreds of nearly identical 45S ribosomal RNA (rRNA) genes, each encoding the 18S, 5.8S, and 25S catalytic rRNAs. Because cellular demands for ribosomes and protein synthesis vary during development, the number of active rRNA genes is subject to dosage control. In genetic hybrids, one manifestation of dosage control is nucleolar dominance, an epigenetic phenomenon in which the rRNA genes of one progenitor are repressed. For instance, in Arabidopsis suecica, the allotetraploid hybrid of Arabidopsis thaliana and Arabidopsis arenosa, the A. thaliana-derived rRNA genes are selectively silenced. An analogous phenomenon occurs in nonhybrid A. thaliana, in which specific classes of rRNA gene variants are inactivated. An RNA-mediated knockdown screen identified SUVR4 {SUPPRESSOR OF VARIEGATION 3-9 [SU(VAR)3-9]-RELATED 4} as a histone H3 Lys 9 (H3K9) methyltransferase required for nucleolar dominance in A. suecica. H3K9 methyltransferases are also required for variant-specific silencing in A. thaliana, but SUVH5 [SU(VAR)3-9 HOMOLOG 5] and SUVH6, rather than SUVR4, are the key activities in this genomic context. Mutations disrupting the H3K27 methyltransferases ATXR5 or ATXR6 affect which rRNA gene variants are expressed or silenced, and in atxr5 atxr6 double mutants, dominance relationships among variants are reversed relative to wild type. Interestingly, these changes in gene expression are accompanied by changes in the relative abundance of the rRNA gene variants at the DNA level, including overreplication of the normally silenced class and decreased abundance of the normally dominant class. Collectively, our results indicate that histone methylation can affect both the doses of different variants and their differential silencing through the choice mechanisms that achieve dosage control.
authors.affiliation: Department of Biology, Indiana University, Bloomington, Indiana 47405, USA.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Pontvianne
authors.forename: Todd
authors.initials: T
authors.lastname: Blevins
authors.forename: Chinmayi
authors.initials: C
authors.lastname: Chandrasekhara
authors.forename: Wei
authors.initials: W
authors.lastname: Feng
authors.forename: Hume
authors.initials: H
authors.lastname: Stroud
authors.forename: Steven E
authors.initials: SE
authors.lastname: Jacobsen
authors.forename: Scott D
authors.initials: SD
authors.lastname: Michaels
authors.forename: Craig S
authors.initials: CS
authors.lastname: Pikaard
mesh_terms: D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002466:Cell Nucleolus / Q000201:enzymology; D019175:DNA Methylation; D018628:Gene Dosage; D018506:Gene Expression Regulation, Plant; D020459:Genes, rRNA; D011495:Histone-Lysine N-Methyltransferase / Q000235:genetics / Q000378:metabolism*; D006657:Histones / Q000378:metabolism; D008780:Methyltransferases / Q000235:genetics / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D006657:Histones; C542364:ATXR5 protein, Arabidopsis; C542365:ATXR6 protein, Arabidopsis; D008780:Methyltransferases; C512047:SUVH5 protein, Arabidopsis; D011495:Histone-Lysine N-Methyltransferase; C506468:SUVH6 protein, Arabidopsis; C515570:SUVR4 protein, Arabidopsis