3D Cryo-EM Structure of an Active Step I Spliceosome and Localization of Its Catalytic Core

► 3D cryo-EM structure of the catalytically active spliceosomal C complex ► 3D structure of its salt-stable RNP core and of the postspliceosomal 35S U5 RNP ► Immunolocalization of the 5′ exon of the pre-mRNA in the C complex ► Structural organization of the C complex and location of its catalytic center