We report the first evidence for a novel reaction mediated by the self-splicing yeast mitochondrial group II intron bl1; the site-specific recombination of RNA molecules in vitro. Upon incubation of the intron lariat with two different RNAs, each harbouring a short sequence complementary to exon binding site 1 (EBS1) of the intron, novel recombined RNAs are formed. As a result of this intron-mediated shuffling of gene segments, the 5' part of RNA1 is ligated to the 3' part of RNA2 and, reciprocally, the 5' part of RNA2 to the 3' part of RNA1. Sequence analysis of the recombinant junction shows that the site of recombination is precisely located 3' to intron binding site 1 (IBS1). The hypothesized mechanism of recombination involves exchange of RNA 5' parts after the first step of a reverse splicing reaction. The possible role of this mechanism in vivo and during prebiotic evolution is discussed.
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