Non-templated addition of nucleotides to the 3 ' end of nascent RNA duringRNA editing in Physarum

RNAs in Physarum mitochondria contain extra nucleotides that are! not encod ed by the mitochondrial genome, at least in the traditional sense, While it is known that insertion of non-encoded nucleotides is linked to RNA synthe sis, the exact nature of this relationship remains unclear. Here we demonst rate that the efficiency of editing Is sensitive not only to the concentrat ion of the nucleotide that is inserted, but also to the concentration of th e nucleotide templated just downstream of an editing site. These data stron gly support a co-transcriptional mechanism of Physarum RNA editing in which non-encoded nucleotides are added to the 3' end of nascent RNAs, These res ults also suggest that transcription elongation and nucleotide insertion ar e competing processes and that recognition of editing sites most likely inv olves transient pausing by the Physarum mitochondrial RNA polymerase. In ad dition, the pattern of nucleotide concentration effects, the context of edi ting sites and the accuracy of the mitochondrial RNA polymerase argue that the mechanism of Physarum editing is distinct from that of other co-transcr iptional editing systems.