Abstract
The mechanism by which pre-mRNAs are initially recognized by the splicing machinery is not well understood. In the yeast system, commitment complexes are the earliest identified splicing complexes. They contain pre-mRNA, U1 snRNP, and the splicing factor Mud2p and probably correspond to the mammalian E complexes, which contain pre-mRNA, U1 snRNP, and the splicing factor U2AF. To identify other yeast commitment complex components, we have characterized mutant strains that are synthetic lethal with viable U1 snRNA mutations. We report here that MUD13 is a nonessential gene that encodes the yeast homolog of CBP20, the small subunit of the vertebrate nuclear cap-binding complex (CBC). Characterization of splicing in the delta-MUD13 strain and extract indicates that Mud13p is a yeast splicing factor and is the second identified non-snRNP commitment complex component. The observations also suggest that CBC interacts with other commitment complex components as well as with the substrate cap. Taken together with the accompanying results for a mammalian system, our data indicate that cap-binding proteins as well as the pre-mRNA cap contribute to early steps in spliceosome assembly.