Fig. 4: Three contact points facilitate the recruitment of U1 snRNP to the elongating Pol II.

a AlphaFold3 prediction of the SPT6^CTR-U1A interaction, superimposed onto the U1 snRNP structure¹³. The predicted aligned error plot is shown in the lower panel, with the interface highlighted with red boxes. b Mutations that disrupt the predicted SPT6^CTR-U1A interaction strongly reduced the amount of U1A bound to MBP-tagged SPT6. The SPT6 C-terminal helix was replaced with GSA linkers that can span the length of the helix (SPT6^Δhelix), or large aromatic and hydrophobic residues in the SPT6 C-terminal helix were mutated to alanines (SPT6^3A: W1674A, W1681A, L1682A). This pulldown was repeated in triplicate. Source data are provided as a Source Data file. c Cartoon schematic showing the three contact points between U1 snRNP and the transcription elongation complex: U1-70K^RRM with RPB2 and RPB12, U1-70K^RRM with the phosphorylated Pol II CTD, and U1A with SPT6^CTR. Arrows indicate the direction of transcription.