In vitro reconstitution of yeast splicing with U4 snRNA reveals multiple roles for the 3′ stem–loop

Model of functional domains in the 3′ portion of U4 and the spliceosome assembly steps at which they function. The region of U4 that promotes di-snRNP formation is indicated in black, the region that increases tri-snRNP formation is in dark gray, the region that decreases di-snRNP levels is boxed in white, and the region that both decreases di-snRNP and increases tri-snRNP is boxed in light gray. In the lower panel, the portions of U4 tested in this work that we propose to be functionally important are shown in bold, with arrows indicating the points in the splicing cycle at which they act.