GROUP I and II introns are mobile elements that propagate by insertion into different genes1. Some introns of both types self-splice in vitro by transesterification reactions catalysed by the intron RNA2. These transesterifications are reversible3–5, and it has been suggested that reverse splicing followed by reverse transcription and recombination with genomic DNA may be a mechanism for intron transposition6,7. In vivo the splicing of many, if not all, group I and II introns requires protein factors, which may facilitate correct folding of the intron RNAs8. Here we show that the Neurospora mitochondrial large rRNA intron, a group I intron that is not self-splicing in vitro9, undergoes reverse splicing in a reaction promoted by the CYT-18 protein, the Neurospora mitochondrial tyrosyl-tRNA synthetase, which is required for splicing the intron in vivo10. In contrast to known RNA-catalysed reverse splicing reactions, this protein-assisted reverse splicing is sufficiently rapid to compete with forward splicing at low RNA concentrations under physiologically relevant conditions, including high GTP and low Mg2+ concentrations. Our results indicate that proteins that promote splicing could contribute to intron mobility by promoting reverse splicing in vivo.
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