1. Department of Biochemistry and Molecular Biology, Dalhousie University, 5850 College Street, Halifax, Nova Scotia B3H 1X5, Canada
2. These authors contributed equally to this work.

Correspondence to: Michael W. Gray¹ The A.castellanii U12 snRNA sequence and the additional gene copy of AcaORF1 are deposited in Genbank (DQ888370 and DQ888371, respectively). Correspondence and requests for materials should be addressed to M.W.G (Email: m.w.gray@dal.ca).

The minor spliceosome is a ribonucleoprotein complex that catalyses the removal of an atypical class of spliceosomal introns (U12-type) from eukaryotic messenger RNAs^{1, 2}. It was first identified and characterized in animals, where it was found to contain several unique RNA constituents that share structural similarity with and seem to be functionally analogous to the small nuclear RNAs (snRNAs) contained in the major spliceosome^{3, 4, 5, 6, 7, 8}. Subsequently, minor spliceosomal components and U12-type introns have been found in plants^{9, 10, 11, 12} but not in fungi. Unlike that of the major spliceosome, which arose early in the eukaryotic lineage^{13, 14}, the evolutionary history of the minor spliceosome is unclear because there is evidence of it in so few organisms. Here we report the identification of homologues of minor-spliceosome-specific proteins and snRNAs, and U12-type introns, in distantly related eukaryotic microbes (protists) and in a fungus (Rhizopus oryzae). Cumulatively, our results indicate that the minor spliceosome had an early origin: several of its characteristic constituents are present in representative organisms from all eukaryotic supergroups for which there is any substantial genome sequence information. In addition, our results reveal marked evolutionary conservation of functionally important sequence elements contained within U12-type introns and snRNAs.

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