Pre-mRNA splicing is executed by the spliceosome. Structural characterization of the catalytically activated complex (B*) is pivotal for mechanistic understanding of catalysis of the branching reaction by the spliceosome. In this study, we assembled the B* complex on two different pre-mRNAs from Saccharomyces cerevisiae and determined the cryo-EM structures of four distinct B* complexes at overall resolutions of 2.9-3.8 Å. The duplex between U2 snRNA and the branch point sequence (BPS) is located 13-20 Å away from the 5′-splice site (5′SS) in the B* complexes that are devoid of the step I splicing factors Yju2 and Cwc25. Recruitment of Yju2 into the active site brings the U2/BPS duplex into the vicinity of 5′SS, ready for branching. In the absence of Cwc25, the nucleophile from BPS is positioned about 4 Å away from, and remains to be activated by, the catalytic metal M2. This analysis reveals the functional mechanism of Yju2 and Cwc25 in branching. These four structures constitute compelling evidence for substrate-specific conformations of the spliceosome in a major functional state.
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