Abstract
Ribosomal RNA genes are organised in tandem arrays at complex loci called nucleolus organisers. The structure of a locus and of a wheat rRNA gene is described in detail. Active or potentially active genes are in the nucleolus during interphase, while inactive genes are excluded from the nucleolus. Genetic variation exists within a species for the number of the rRNA genes at a locus and also for the structure of the intergenic, regulatory DNA. This variation can affect the activity of a locus, relative to that of another in the same cell and the proportion of the rRNA genes at a locus included in the nucleolus. The active loci are enriched with genes that are not methylated at specific CpG residues in the intergenic regulatory DNA and that are in a chromatin conformation accessible to DNase I. A model is presented that attempts to relate the structural variation between genes to the differential expression and nucleolar organisation of the rRNA genes at different loci. The model is based on the affinity of proteins, in limiting concentration, to specific regulatory DNA sequences. These sequences are subject to change as a result of mechanisms that can spread mutations through a locus. The resulting variation, which may not be eliminated by selection unless it is very deleterious and accounts for a large part of the total rDNA, may be one reason why plants maintain a large excess of ribosomal RNA genes.Key words: nucleolus, ribosomal RNA gene, methylation.
Publisher
Canadian Science Publishing
Subject
Genetics,Molecular Biology,General Medicine,Biotechnology
Cited by
43 articles.
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