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 Mspl and Hpall 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.
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