Properties of in vitro transcription by isolated Xenopus oocyte nucleoli

Some properties of in vitro transcription by isolated Xenopus oocyte nucleoli were described. When incubated with labeled RNA precursors, Xenopus oocyte nucleoli exhibited prolonged incorporation of radioactivity into RNA. The synthetic activity was exclusively due to type I RNA polymerase as revealed by its insensitivity to low and high doses of alpha-amanitin. The size of the in vitro transcript was mostly larger than 28S at 10 minute incubation and became smaller as incubation proceeded. When [gamma-32P]ATP was included in the reaction mixture, 32P radioactivity was incorporated into RNA suggesting the possible initiation of transcription in this system. However, analysis of the terminal nucleotide of the transcript revealed that the incorporation of radioactivity from [gamma-32P]ATP was not due to the initiation of transcription but due to polynucleotide kinase activity in the nucleolar preparation. These results demonstrate that the incorporation of radioactivity from [gamma-32P] labeled nucleoside triphosphates cannot necessarily be regarded as an index of the initiation of transcription.