Evolutionary fates and origins of U12-type introns.

U2-type and U12-type introns are spliced by distinct spliceosomes in eukaryotic nuclei. A classification method was devised to distinguish these two types of introns based on splice site sequence properties and was used to identify 56 different genes containing U12-type introns in available genomic sequences. U12-type introns occur with consistently low frequency in diverse eukaryotic taxa but have almost certainly been lost from C. elegans. Comparisons with available homologous sequences demonstrate subtype switching of U12 introns between termini of AT-AC and GT-AG as well as conversion of introns from U12-type to U2-type and provide evidence for a fission/fusion model in which the two splicing systems evolved in separate lineages that were fused in a eukaryotic progenitor.