Dinucleotide priming of transcription mediated by RNA polymerase II

Mammalian RNA polymerase II was shown to utilize dinucleoside monophosphates for priming of promoter specific RNAs. In a reconstituted system containing purified polymerase and HeLa cell fractions, dinucleotides were incorporated by complementarity with template sequences at the in vivo cap sites of the adenovirus major late and adenovirus early region IV promoters. Incorporation was shown by label transfer experiments and by determining the size of 5'-terminal RNase T1-resistant oligonucleotides. All 16 dinucleotides were tested for priming of RNA chains at the major late promoter. RNA polymerase II initiated with various primers over a contiguous region of 9 bases, centered around the in vivo initiation site. We suggest that the polymerase drifts or oscillates over this region. Using a dinucleotide challenge protocol, the rate of initiation at the major late promoter was measured following preincubation of the template DNA with RNA polymerase II and factors. Initiation with ATP was 90% complete within the 1st min after addition of nucleotide triphosphates. Stimulation of transcription by dinucleotides was not observed, due to this rapid initiation. The 5'-hydroxyl terminus of dinucleotide-primed RNAs remained unmodified. Although transcripts initiated with ATP were rapidly capped in whole cell extracts, ATP-primed RNA synthesized in the reconstituted system retained free 5'-terminal phosphates. Thus, capping was not essential for synthesis of long runoff RNAs.