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Two domains in U6 RNA were identified, each of which is required for in vitro splicing. Single nucleotide substitutions in these two domains block splicing either at the first or the second step. Invariably, U6 RNA mutants that block the first step of splicing do not enter the spliceosome. On the other hand, those that block the second step of splicing form a spliceosome but block cleavage at the 3\u2032 splice site of the intron. In both domains, the positions of base changes that block the second step of splicing correspond exactly to the site of insertion of pre-mRNA-type introns into the U6 gene of two yeast species, providing a possible explanation for the mechanism of how these introns originated and adding further evidence for the proposed catalytic role of U6 RNA.<\/jats:p>","DOI":"10.1126\/science.2145630","type":"journal-article","created":{"date-parts":[[2006,10,5]],"date-time":"2006-10-05T22:10:14Z","timestamp":1160086214000},"page":"404-409","source":"Crossref","is-referenced-by-count":118,"title":["Two Domains of Yeast U6 Small Nuclear RNA Required for Both Steps of Nuclear Precursor Messenger RNA Splicing"],"prefix":"10.1126","volume":"250","author":[{"given":"Patrizia","family":"Fabrizio","sequence":"first","affiliation":[{"name":"California Institute of Technology, Division of Biology, Pasadena, CA 91125."}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"John","family":"Abelson","sequence":"additional","affiliation":[{"name":"California Institute of Technology, Division of Biology, Pasadena, CA 91125."}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"221","reference":[{"key":"e_1_2_1_1_1","doi-asserted-by":"crossref","first-page":"175","DOI":"10.1128\/mr.44.2.175-205.1980","volume":"44","year":"1980","unstructured":"BANERJEE, A.K., 5'-TERMINAL CAP STRUCTURE IN EUKARYOTIC MESSENGER RIBONUCLEIC-ACIDS, MICROBIOLOGICAL REVIEWS 44: 175 (1980).","journal-title":"MICROBIOLOGICAL REVIEWS"},{"key":"e_1_2_1_2_1","doi-asserted-by":"crossref","first-page":"2415","DOI":"10.1002\/j.1460-2075.1987.tb02520.x","volume":"6","year":"1987","unstructured":"BINDEREIF, A, AN ORDERED PATHWAY OF SNRNP BINDING DURING MAMMALIAN PRE-MESSENGER-RNA SPLICING COMPLEX ASSEMBLY, EMBO JOURNAL 6: 2415 (1987).","journal-title":"EMBO JOURNAL"},{"key":"e_1_2_1_3_1","doi-asserted-by":"crossref","first-page":"531","DOI":"10.1016\/0092-8674(89)90036-6","volume":"59","year":"1989","unstructured":"BLENCOWE, B.J., ANTISENSE PROBING OF THE HUMAN U4\/U6 SNRNP WITH BIOTINYLATED 2'-OME RNA OLIGONUCLEOTIDES, CELL 59: 531 (1989).","journal-title":"CELL"},{"key":"e_1_2_1_4_1","doi-asserted-by":"crossref","first-page":"14","DOI":"10.1038\/337014a0","volume":"337","year":"1989","unstructured":"BROW, D.A., RNA PROCESSING - 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