Structural basis for recognition of intron branchpoint RNA by yeast Msl5 and selective effects of interfacial mutations on splicing of yeast pre-mRNAs

Yeast Msl5 and Sub2 in early spliceosome assembly. (A) Bridging interactions between U1 snRNP at the 5′SS and Msl5·Mud2 at the branchpoint prepare a scaffold for subsequent recruitment of the U2 snRNP to the branchpoint. Exchange of U2 snRNP for Msl5·Mud2 is mediated by the DExD-box ATPase Sub2. (B) Mutations of Msl5 RNA-binding residues bypass the essentiality of Sub2. Yeast SUB2 mud2Δ and sub2Δ mud2Δ cells and sub2Δ msl5Δ cells harboring the indicated MSL5 allele on a CEN HIS3 plasmid were grown in liquid cultures to mid-log phase. The cultures were adjusted to A₆₀₀ of 0.1 and aliquots of serial dilutions were spotted to YPD agar and incubated at 37°C, 34°C, or 30°C as indicated.