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A molecular genetic analysis of wheat (Triticum aestivum L.) ribosomal RNA (rRNA) gene locus expression is described. Locus activity was estimated by cytological assays of nucleolus organiser volume. Gene structure was determined from DNA sequencing and restriction endonuclease mapping of isolated genes. Relative nucleolus organiser activity is correlated with the number of 135 base pair (bp) repeats that lie upstream of the gene promoter. A model is proposed in which gene and locus‐relative activity is determined by the ability of the 135 bp and other promoter sequences of the genes in a locus to compete for regulatory proteins in limiting concentration in the nucleus. Active loci are enriched with genes which have unmethylated cytosines at specific sites in the promoter and the 135 bp repeats and in which chromatin structure is perturbed. It is concluded that the processes that produce different numbers of regulatory DNA sequences between rRNA genes are responsible for the differential expression of nucleolus organisers in hexaploid wheat and in interspecies hybrids.

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