CYTOSINE METHYLATION AND NUCLEOLAR DOMINANCE IN CEREAL HYBRIDS

In wheat-rye hybrids the nucleolus organizer regions (NORs), the sites of ribosomal RNA genes, from rye are suppressed. Wheat and wheat-rye hybrid genetic stocks containing different numbers of wheat and rye nu cleolus organizers, as well as addition lines and rye-barley hybrids, were used in Southern hybridization experiments to determine the cause of nucleolar dominance and suppression in cereal hybrids. Based on th e use of restriction endonucleases that cleave near the ends of the sp acer unit and an additional, methylation-sensitive enzyme, HpaII, whic h does not recognize the CCGG restriction site if the internal C is me thylated, an indirect method of assaying NOR expression was establishe d. The results indicated that cleavage by the HpaII enzyme of the rye NOR sequences; is reduced when major NORs from other cereals were pres ent. The reduction in the number of rye rRNA genes containing an unmet hylated CCGG site in the promoter was associated with the suppression of the rye nucleolus. These results are consistent with a model in whi ch promoter and upstream regulatory repeats of ribosomal RNA genes com pete for limited concentrations of regulatory proteins, and genes that are methylated at key binding sites fail to engage these regulatory p roteins and thus remain inactive.