Abortive products as initiating nucleotides during transcription by T7 RNA polymerase

The kinetics of formation of abortive initiation products during transcription of a synthetic template (encoding the transcript GAUGGC) by T7 RNA polymerase have been determined. This study revealed that while total RNA was formed in the reaction as expected, the levels of the dinucleoside tetraphosphate guanylyl-3',5'-adenosine-5'-triphosphate (pppGpA) and trinucleoside pentaphosphate guanylyl-3',5'-adenosine-3',5'-uridine-5'-triphosphate (pppGpApU) formed by premature termination of transcription reached a maximum after 10 min, and then decreased. Transcription of the same template, in the presence of either [gamma-32P]GTP and ATP, or GTP and [alpha-32P]ATP, gave the 32P-labeled dinucleotides *pppGpA and pppG*pA. Incorporation of each of these substrates into longer RNA transcripts in the same enzyme-template system was demonstrated. The incorporation was shown to require the presence of template in the reaction mixture. The requirement for base complementarity restricts the position of incorporation to that of initiating (5') nucleotide. Transcription of a second template, which encodes an RNA transcript having the partial sequence GpA at two internal positions, in the presence of each of the labeled dinucleoside tetraphosphates, failed to bring about the synthesis of significant yields of any longer radiolabeled transcripts. It is concluded that dinucleoside tetraphosphate (and perhaps trinucleoside pentaphosphate) can function as initiating nucleotides when complementary to the nucleotide sequence at promoter regions. However, a dinucleotide is not used as substrate for subsequent chain elongation in T7 RNA polymerase catalyzed transcription reactions.