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. 2021 Jul 1;13(8):559–579. doi: 10.1007/s13238-021-00856-5

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© The Author(s) 2021

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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U2-dependent (major) spliceosome versus U12-dependent (minor) spliceosome. (A) The main determinants for distinguishing U2- and U12-type introns are the 5′ splice site (5′SS) and branch point sequence (BPS). U12-type introns are subdivided into AT-AC or GT-AG minor introns based on their terminal dinucleotides. (B) U2- and U12-type introns are recognized differently by their respective spliceosomes. U2-type introns are initially recognized by the U1 snRNP binding to the 5′SS, splicing factor 1 (SF1) binding to the BPS and U2 auxiliary factor (U2AF2/1) heterodimer binding to the polypyrimidine tract (PPT) and 3′SS, respectively. Subsequently, the U2 snRNA associates with the BPS and displaces SF1, converting the E complex to A complex. In contrast to the U2-type introns, the 5′SS and BPS of U12-type introns are recognized cooperatively by the U11 and U12 snRNAs of the di-snRNP, respectively, thereby forming the U12-type A complex. The following steps in the splicing process are similar between the U2- and U12-dependent pathway, and lead to similar catalytic structures and catalytic reactions of splicing