Genetic and physical interactions involving the yeast nuclear cap-binding complex

Yeast strains lacking the yeast nuclear cap-binding complex (yCBC) are viab le, although impaired in growth. We have taken advantage of this observatio n to carry out a genetic screen for components that show synthetic lethalit y (SL) with a cbp20-Delta cbp80-Delta double mutation. One set of SL intera ctions was due to mutations that were complemented by components of U1 smal l nuclear RNP (snRNP) and the yeast splicing commitment complex. These inte ractions confirm the role of yCBC in commitment complex formation. Physical interaction of yCBC with the commitment complex components Mud10p and Mud2 p, which may directly mediate yCBC function, was demonstrated. Unexpectedly , we identified multiple SL mutations that were complemented by Cb5p and No p58p. These are components of the two major classes of yeast small nucleola r RNPs, which function in the maturation of rRNA precursors. Mutants lackin g yCBC were found to be defective in rRNA processing. Analysis of the yCBC deletion phenotype suggests that this is likely to be due to a defect in th e splicing of a subset of ribosomal protein mRNA precursors.