More than 70% of the RNA synthesized by T7 RNA polymerase during run-off transcription in vitro can be incorrect products, up to twice as long as the expected transcripts. Transcriptions with model templates indicate that false transcription is mainly observed when the correct product cannot form stable secondary structures at the 3`-end. Therefore, the following hypothesis is tested: after leaving the DNA template, the polymerase can bind a transcript to the template site and the 3`-end of the transcript to the product site and extend it, if the 3`-end is not part of a stable secondary structure. Indeed, incubation of purified transcripts with the polymerase in transcription conditions triggers a 3`-end prolongation of the RNA. When two RNAs of different lengths are added to the transcription mix, both generate distinct and specific patterns of prolonged RNA products without any interference, demonstrating the self-coding nature of the prolongation process. Furthermore, sequencing of the high molecular weight transcripts demonstrates that their 5`-ends are precisely defined in sequence, whereas the 3`-ends contain size-variable extensions which show complementarity to the correct transcript. Surprisingly, a reduction of the UTP concentration to 0.2-1.0 mM in the presence of 3.5-4.0 mM of the other NTPs leads to faithful transcription and good yields, irrespective of the nucleotide composition of the template.

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