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Identification and characterization of a yeast homolog of U1 snRNP‐specific protein C

U1C is one of the three human U1 small nuclear ribonucleoprotein (snRNP)‐specific proteins and is important for efficient complex formation between U1 snRNP and the pre‐mRNA 5′ splice site. We identified a hypothetical open reading frame in Saccharomyces cerevisiae as the yeast homolog of the human U1C protein. The gene is essential, and its product, YU1C, is associated with U1 snRNP. YU1C depletion gives rise to normal levels of U1 snRNP and does not have any detectable effect on U1 snRNP assembly. YU1C depletion and YU1C ts mutants affect pre‐mRNA splicing in vivo, and extracts from these strains form low levels of commitment 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 mutants, indicating that U1C contributes to a proper 5′ arm structure prior to its base pairing interaction with the pre‐mRNA 5′ splice site.