Ribosomal RNA gene expression and localization in cereals

Abstract

The rRNA genes are the most accessible genes for the study of gene expression at a molecular, structural, and ultrastructural level. This paper describes the occurrence and distribution of rRNA genes in wheat, barley and rye. These cereal species are closely related and intercrossable but the rRNA genes show only a very loose conservation to chromosomes of the same homoeologous group which is surprising considering the close co-linearity of single copy markers. The cereals have an apparent excess in the number of genes required to sustain ribosome synthesis. Some of the “redundant” genes may occur in inactive rDNA loci that are not associated with a nucleolus. Factors affecting locus suppression are reviewed. Loci involved in rDNA transcription include regions that contain actively transcribing genes and regions that contain inactive genes. The active genes are probably associated with the dense fibrillar component of the nucleolus, while the inactive genes remain condensed. In rye, the condensed, inactive genes occur in perinucleolar chromatin and the active genes are decondensed at the distal end of the rDNA locus. In wheat condensed genes occur inside and outside the nucleolus with each active locus showing several sites with decondensed genes separated by condensed, intranucleolar presumably inactive genes. In rye and wheat active rDNA loci extend through the nucleolus and active rDNA sequences do not loop out from perinucleolar chromatin.