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Nucleolar U3 RNA (8 μM) from rat Novikoff hepatoma cells, m₃^(2,2,7)G (20 mM), and m^(7)Gppp-Am (120 μM) increased rat liver nucleolar RNA transcription in vitro by 640%, 233%, and 218% respectively during 15 min of incubation at 37 ℃. Kinetic anaysis of effects of U3 RNA at a concentration of 4 μM showed marked increase in rate (200%) and net transcription (400%) of nucleolar RNA in isolated nucleoli of rat liver. These results indicate that U3 RNA may function in the initiation as well as in the elongation reaction of nucleolar RNA transcription. However, kinetic analysis of effects of m₃^(2,2,7)G nucleoside increased the initial transcription rate (150%) but not the net transcrption of nucleolar RNA. Also, m₃^(2,2,7)G prevented RNA polymerase inactivation in preincubation period. In agreement with theses results, 5´ terminal labeling with [γ- ^(32)P]ATP or [γ- ^(32)P]GTP did not change in the presence of m₃^(2,2,7)G nucleoside. These results show that m₃^(2,2,7)G does not increase new initiation of RNA transcription but stabilizes the existing transcription complex and increases the rate of transcription as well as preventing loss of complex during preincubation. These results suggest that trimethylguanosine capped U3 RNA appears to be essential for optimal nucleolar RNA transcription.
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