An RNA Splicing System that Excises DNA Transposons from Animal mRNAs

All genomes harbor mobile genetic parasites called transposable elements (TEs). Here we describe a system, which we term SOS splicing, that protects C. elegans and human genes from DNA transposon-mediated disruption by excising these TEs from host mRNAs. SOS splicing, which operates independently of the spliceosome, is a pattern recognition system triggered by base- pairing of inverted terminal repeat elements, which are a defining feature of the DNA transposons. We identify three factors required for SOS splicing in both C. elegans and human cells; AKAP17A, which binds TE-containing mRNAs; the RNA ligase RTCB; and CAAP1, which bridges RTCB and AKAP17A, allowing RTCB to ligate mRNA fragments generated by TE excision. We propose that SOS splicing is a novel, conserved, and RNA structure-directed mode of mRNA splicing and that one function of SOS splicing is to genetically buffer animals from the deleterious effects of TE- mediated gene perturbation.