Ares, M.; Igel, A. H.; Katzman, S.; Donohue, J. P.

Rare, full length circular intron RNAs distinct from lariats have been reported in several species, but their biogenesis is not understood. We envision and test a hypothesis for their formation using Saccharomyces cerevisiae, documenting full length and novel processed circular RNAs from multiple introns. Evidence implicates a previously undescribed catalytic activity of the intron-lariat spliceosome (ILS) in which the 3\'-OH of the lariat tail (with optional trimming and adenylation by the nuclear 3\' processing machinery) attacks the branch, joining the intron 3\' end to the 5\' splice site in a 3\'-5\' linked circle. Human U2 and U12 spliceosomes produce analogous full length and processed circles. Post-splicing catalytic activity of the spliceosome may promote intron transposition during eukaryotic genome evolution.