title: Allelic inactivation of rDNA loci.
abstract: Human cells contain several hundred ribosomal genes (rDNA) that are clustered into nucleolar organizer regions (NORs) on the short arms of five different acrocentric chromosomes. Only approximately 50% of the gene copies are actually expressed in somatic cells. Here, we used a new cytological technique to demonstrate that rDNA is regulated allelically in a regional manner, with one parental copy of each NOR being repressed in any individual cell. This process is similar to that of X-chromosome inactivation in females. Early in development, one copy of each NOR becomes late-replicating, thus probably marking it for inactivation and subsequent targeted de novo methylation at rDNA promoter regions. Once established, this multichromosomal allelic pattern is then maintained clonally in somatic cells. This pathway may serve as an epigenetic mechanism for controlling the number of available rDNA copies during development.
authors.affiliation: Department of Cellular Biochemistry and Experimental Medicine, Hebrew University Medical School, Ein Kerem, Jerusalem 91120, Israel.
authors.forename: Sharon
authors.initials: S
authors.lastname: Schlesinger
authors.forename: Sara
authors.initials: S
authors.lastname: Selig
authors.forename: Yehudit
authors.initials: Y
authors.lastname: Bergman
authors.forename: Howard
authors.initials: H
authors.lastname: Cedar
mesh_terms: D000483:Alleles; D000818:Animals; D002460:Cell Line; D019175:DNA Methylation; D042522:DNA Replication Timing; D004275:DNA, Ribosomal / Q000235:genetics*; D018507:Gene Expression Regulation, Developmental; D020868:Gene Silencing; D020459:Genes, rRNA / Q000235:genetics; D006657:Histones / Q000378:metabolism; D006801:Humans; D051379:Mice; D009693:Nucleic Acid Hybridization
chemical_list: D004275:DNA, Ribosomal; D006657:Histones