Human NORs show correlation between transcriptional activity, DNase I sensitivity, and hypomethylation

Active human ribosomal gene clusters (NORs) are distinguishable from inactive ones by silver staining. By sequentially applying deoxyribonuclease I (DNase I)-directed in situ nick-translation and silver staining to fixed chromosome preparations, we found that active NORs are more sensitive to DNase I than inactive ones. Use of the two restriction isoschizomeres MspI and HpaII to modify the nick-translation technique showed that active NORs are significantly less methylated than inactive ones. Taken as a whole, our results indicate that ribosomal gene activity, DNase I sensitivity, and DNA methylation are closely interrelated.