Molecular cloning and characterization of an unusually large intergenic spacer from the Nor-B2 locus of hexaploid wheat.

An allelic rDNA variant from the Nor-B2 locus of 'Bezostaya' wheat that forms an especially active nucleolus was cloned and characterized. It carries an unusually large intergenic spacer compared with rDNA units in most other wheat genotypes. The additional intergenic length is in the array of 135-bp A repeats and not in other internal repeats. These A repeats have sequences nearly identical to other A repeats described for other alleles. It is suggested therefore that the more active Nor-B2 locus of 'Bezostaya' may be due to the constituent rDNA units possessing a larger array of A repeats.