Cytochrome b mRNAs, which contain a single editing site near their 5' ends, are initially transcribed unedited at that site. In the absence of transcription, UTP is incorporated into transcripts known to undergo editing, and the degree of UTP incorporation is consistent with the degree of editing occurring in these transcripts. ATP is incorporated into the 3' CCA of tRNAs and into mitochondrial messenger RNAs due to polyadenylation. CTP is incorporated into tRNAs as a result of synthesis or turnover of their 3' CCA sequences. Post-transcriptional CTP and ATP incorporation can be ascribed to known enzymatic activities. GTP is incorporated into RNA only as a result of the transcription process. Trypanosome mitochondria incorporate CTP, ATP, and UTP into RNA in the absence of transcription. Utilizing an isolated organelle system we have determined that addition of uridines to edited RNAs does not require ongoing transcription. We have examined the post-transcriptional addition of nucleotides into the mitochondrial RNA of Trypanosoma brucei. RNA editing is a novel RNA processing event of unknown mechanism that results in the introduction of nucleotides not encoded in the DNA into specific RNA molecules.
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