Deletion of MUD2, the yeast homolog of U2AF65, can bypass the requirement for sub2, an essential spliceosomal ATPase.

Mammalian U2AF65 and UAP56 are required for prespliceosome (PS) formation. We tested the predictions that the yeast UAP56 homolog, SUB2, is required for the same step and functions collaboratively with MUD2, the yeast homolog of U2AF65. Unexpectedly, sub2-1 extracts accumulate PS-like complexes. Moreover, deletion of MUD2 exacerbates the cs phenotype of sub2 alleles yet suppresses both the ts sub2-1 and the lethal Deltasub2 phenotypes. We propose that Sub2 functionally interacts with Mud2 both before and after PS formation. In the absence of Mud2, Sub2 function becomes dispensable.