Selective nucleolus organizer inactivation in Arabidopsis is a chromosome position-effect phenomenon.
Nucleolus organizer regions (NORs) are chromosomal loci where hundreds of rRNA genes are clustered. Despite being nearly identical in sequence, specific rRNA genes are selected for silencing during development via choice mechanism(s) that remain unclear. In Arabidopsis thaliana, rRNA gene subtypes that are silenced during development were recently mapped to the NOR on chromosome 2, NOR2, whereas active rRNA genes map to NOR4, on chromosome 4. In a mutant line deficient for ATXR5 or ATXR6-dependent histone H3 lysine 27 (H3K27) monomethylation, we show that millions of base pairs of chromosome 4, including the telomere, TEL4N, and much of NOR4, have been converted to the corresponding sequences of chromosome 2. This genomic change places rRNA genes of NOR2, which are normally silenced, at the position on chromosome 4 where active rRNA genes are normally located. At their new location, NOR2-derived rRNA genes escape silencing, independent of the atxr mutations, indicating that selective rRNA gene silencing is chromosome 2-specific. The chromosome 2 position effect is not explained by the NOR2-associated telomere, TEL2N, which remains linked to the translocated NOR, implicating centromere-proximal sequences in silencing.
Department of Biology, Indiana University, Bloomington, IN 47405.|Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, IN 47405.
Gireesha
G
Mohannath
Laboratoire Genome et Developpement des Plantes, UMR 5096, Université de Perpignan Via Domitia, 66860 Perpignan, France.
Frederic
F
Pontvianne
Department of Biology, Indiana University, Bloomington, IN 47405; cpikaard@indiana.edu.|Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, IN 47405.|Howard Hughes Medical Institute, Indiana University, Bloomington, IN 47405.
Craig S
0000-0001-8204-7459
CS
Pikaard
D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D055012:Chromosomal Position Effects / Q000235:genetics*; D032461:Chromosomes, Plant / Q000235:genetics*; D003433:Crosses, Genetic; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D017343:Genes, Plant; D005838:Genotype; D006824:Hybridization, Genetic; D008957:Models, Genetic; D009697:Nucleolus Organizer Region / Q000235:genetics*; D012335:RNA, Ribosomal / Q000235:genetics; D016615:Telomere / Q000235:genetics
D029681:Arabidopsis Proteins; D012335:RNA, Ribosomal
chromosome translocation; epigenetic regulation; gene silencing; nucleolar dominance; ribosomal RNA