Assembly of the U1 snRNP involves interactions with the backbone of the terminal stem of U1 snRNA

Summary of site-specific interferences in the in vitro assembly of U1 snRNA–protein complexes. The U1 stem regions are numbered with Roman numerals. The bold “A”s in the loop closing stem II are primary points of interaction for U1A protein binding. The open and gray nucleotides represent different degrees of phosphorothioate interference (kappa values) with core protein assembly. The gray box highlights the Sm binding site sequence.