Regulatory DNA of Ribosomal RNA Genes and Control of Nucleolus Organizer Activity in Wheat

Abstract

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.

Gene Expression and Nucleolar Dominance in Hexaploid Triticale and T. Aestivum

“…There are a number of models for the control of rDNA expression (18,19,26,29). One model for the control of nucleolar dominance including the effect of the unlinked regulatory locus revealed by Neves et al (27) shows the simple interaction between gene(s) on chromosome 2 and the NOR on chromosome 1 in rye.…”

Physical mapping of 5S and 18S–25S rDNA and repetitive DNA sequences in Aegilops umbellulata

“…The 5 s and 18s-25s rRNA genes are highly repeated in the genome and, in the Triticeae, occur in many thousands of copies, most of which are not expressed (see Flavell et al 1993). Because the sequence variation between loci is often as great as that within loci, and because of the potentially large number of loci, the genes are difficult to map by RFLP or genetic methods.…”

Wheat Storage Proteins: Glutenin Diversity in Sphaerococcum Mutant Forms of Tetraploid Wheats

“…Similarly, the tetraploid durum wheats with the A and B genomes contain four HMW genes, of which no more than three are active. Results reported so far have shown that the non-expression of the 1Ay gene can be caused by nucleotide changes in the pro-motor region (3,4) or by the presence of a transposon-like insertion in the encoding region (9). Flavell et al(3) established in Glu -1y allel of 6x-wheats an insertion of 8 kb., called WIS-2, belonging to transposon elements type "retrosposon".…”