U2 small nuclear ribonucleoprotein particle (snRNP) auxiliary factor of 65 kDa, U2AF65, can promote U1 snRNP recruitment to 5′ splice sites

The splicing factor U2AF⁶⁵, U2 small nuclear ribonucleoprotein particle (snRNP) auxillary factor of 65 kDa, binds to pyrimidine-rich sequences at 3′ splice sites to recruit U2 snRNP to pre-mRNAs. We report that U2AF⁶⁵ can also promote the recruitment of U1 snRNP to weak 5′ splice sites that are followed by uridine-rich sequences. The arginine- and serine-rich domain of U2AF⁶⁵ is critical for U1 recruitment, and we discuss the role of its RNA–RNA annealing activity in this novel function of U2AF⁶⁵.