Abstract
U1 small nuclear ribonucleoprotein (snRNP) is important for pre-mRNA splicing both in yeast (
Saccharomyces cerevisiae) and mammalian systems. The RNA component of U1 snRNP, U1 snRNA, interacts by base pairing with pre-mRNA 5′ splice sites. This article examines recent evidence suggesting that U1 snRNP is important for an early step in spliceosome assembly rather than a late step that contributes to the specificity of 5′ splice-site cleavage.