IDENTIFICATION AND CHARACTERIZATION OF A YEAST HOMOLOG OF U1 SNRNP-SPECIFIC PROTEIN-C

U1C is one of the three human U1 small nuclear ribonucleoprotein (snRN P)-specific proteins and is important for efficient complex formation between U1 snRNP and the pre-mRNA 5' splice site. We identified a hypo thetical open reading frame in Saccharomyces cerevisiae as the yeast h omolog of the human U1C protein, The gene is essential, and its produc t, YU1C, is associated with U1 snRNP, YU1C depletion gives rise to nor mal levels of U1 snRNP and does not have any detectable effect on U1 s nRNP assembly. YU1C depletion and YU1C ts mutants affect pre-mRNA spli cing in vivo, and extracts from these strains form low levels of commi tment complexes and spliceosomes in vitro. These experiments indicate a role for YU1C in snRNP function. Structure probing with RNases shows that only the U1 snRNA 5' arm is hypersensitive to RNase I digestion when YU1C is depleted. Similar results were obtained with YU1C ts muta nts, indicating that U1C contributes to a proper 5' arm structure prio r to its base pairing interaction with the pre-mRNA 5' splice site.