Efficient integration of an intron RNA into double-stranded DNA by reverse splicing

SOME group II introns are mobile elements as well as catalytic RNAs 1,2 . Introns aI1 and aI2 found in the gene COX1 in yeast mitochondria encode reverse transcriptases which promote site-specific insertion of the intron into intronless alleles ('homing') 3–6 . For aI2 this predominantly occurs by reverse transcription of unspliced precursor RNA at a break in double-strand DNA made by an endonuclease encoded by the intron 7 . The aI2 endonuclease involves both the excised intron RNA, which cleaves the DNA's sense strand by partial reverse splicing; and the intron-encoded reverse transcriptase which cleaves the antisense strand 8 . Here we show that aI1 encodes an analogous endonuclease specific for a different target site compatible with the different exon-binding sequences of the intron RNA. Over half of aI1 undergoes complete reverse splicing in vitro , thus integrating linear intron RNA directly into the DNA. This unprecedented reaction has implications for both intron mobility and evolution, and potential genetic engineering applications.