Spliceosomal introns interrupt nuclear genes and are removed from RNA transcripts ("spliced") by machinery called spliceosomes. Although the vast majority of spliceosomal introns are removed by the so-called major (or "U2") spliceosome, diverse eukaryotes also contain a rare second form, the minor ("U12") spliceosome, and associated ("U12-type") introns.1-3 In all characterized species, U12-type introns are distinguished by several features, including being rare in the genome (∼0.5% of all introns),4-6 containing extended evolutionarily conserved splicing motifs,4,5,7,8 being generally ancient,9,10 and being inefficiently spliced.11-13 Here, we report a remarkable exception in the slime mold Physarum polycephalum. The P. polycephalum genome contains >20,000 U12-type introns-25 times more than any other species-enriched in a diversity of non-canonical splice boundaries as well as transformed splicing signals that appear to have co-evolved with the spliceosome due to massive gain of efficiently spliced U12-type introns. These results reveal an unappreciated dynamism of minor spliceosomal introns and spliceosomal introns in general.

Department of Biology and Ecology Faculty of Science University of Ostrava Ostrava Czech Republic

Department of Molecular and Cell Biology University of California Merced Merced CA 95343 USA

Department of Molecular and Cell Biology University of California Merced Merced CA 95343 USA; Department of Biology San Francisco State University San Francisco CA 94132 USA

Curr Biol. 2021 Jul 26;31(14):R912-R914. doi: 10.1016/j.cub.2021.06.008. PubMed

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