High concentration of RNA polymerase I is responsible for the high rate of nucleolar transcription.

When isolated rat liver nuclei and nucleoli are compared for RNA synthesis in vitro, the rate of nucleolar RNA synthesis is found to be more than 10 times higher. In order to understand this high rate of nucleolar transcription, DNA from both nuclear and nucleolar fractions was isolated and compared for the ability to direct RNA synthesis with homologous RNA polymerases. No difference between these two templates is evident. On the other hand, when the total nuclear and nucleolar RNA polymerases are isolated and compared on a per-unit-weight-of-DNA basis, it becomes clear that the nucleolus has a 10-fold higher RNA polymerase concentration than the nucleus. This result suggests that RNA polymerase I concentration rather than the nucleolar DNA template efficiency is responsible for the observed high rate of nucleolar transcription under the normal steady-state condition.