INTEGRATION OF A GROUP-I INTRON INTO A RIBOSOMAL-RNA SEQUENCE PROMOTED BY A TYROSYL-TRANSFER RNA-SYNTHETASE

GROUP I and II introns are mobile elements that propagate by insertion into different genes 1. Some introns of both types self-splice in vitro by transesterification reactions catalysed by the intron RNA 2. These transesterifications are reversible 3-5, and it has been suggested that reverse splicing followed by reverse transcription and recombination with genomic DNA may be a mechanism for intron transposition 6-7. 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 RNAs 8. Here we show that the Neurospora mitochondrial large rRNA intron, a group I intron that is not self-splicing in vitro 9, 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 vivo 10. 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.