Rat-liver nucleoli (10–15 pg DNA) were digested with either 0.6 or 3 units of DNase I for various times (up to 1 h). RNA synthesis was then measured in the absence or presence ol 3 units of Escherichia coli RNA polymerase. It was found that the nucleolar chromatin supporting the endogenous engaged RNA polymerase I transcription was compl-etely destroyed in 3 min with either concentration of DNase I. The nucleolar chromatin template transcribed by E. coli RNA polymerase retained 50% of its original capacity even 60 min alter 3 units of DNase I digestion. When hybridization experiments were conducted, it was found that the DNAs derived from both levels of DNase-Idigested nucleoli were incapable of forming hybrids with the labelled nucleolar RNA synthesized by the engaged RNA polymerase I from the untreated nucleoli. Since the engaged RNA polymerase I transcribes only the physiologically active genes of the nucleolar chromatin, and the RNA transcripts represent active gene product, these data suggest that DNase I digestion has completely destroyed the active genes of the nucleolar chromatin, and E. coli RNA polymerase is able to transcribe the inactive nucleolar chromatin template.

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