Correlation between the size of the intergenic regulatory region, the status of cytosine methylation of rRNA genes and nucleolar expression in wheat.

A large number of wheat rRNA genes are methylated at all the CCGG sites that are present in the intergenic regions. A smaller number of rRNA genes are not methylated at one or more CCGG sites. A subset of genes was found unmethylated at a specific CCGG site just downstream of the array of 135 bp A repeats in the intergenic region. In all the genotypes studied, the rDNA loci with larger intergenic regions between their genes also possess a greater number of rRNA genes that are unmethylated at one or more CCGG sites in the intergenic regions than do the loci with shorter intergenic regions. In four genotypes (for which data were available), rDNA loci with longer intergenic regions had larger secondary constrictions on metaphase chromosomes, a measure of relative locus activity, than the loci with shorter intergenic regions. The results have been integrated into a model for the control of rDNA expression based on correlations between cytosine methylation patterns and the number of upstream 135 bp repeats in intergenic regions. According to this model the 135 bp repeats play a part in the control of gene activity by binding a protein(s) that is in limiting supply, thereby predisposing the neighbouring gene to become active preferentially.