Conserved Sequence/genetics; Nucleic Acid Conformation; RNA-Binding Proteins/chemistry; RNA/chemistry; RNA/metabolism; Ribonucleoprotein, U1 Small Nuclear/chemistry; Ribonucleoprotein, U1 Small Nuclear/metabolism; Amino Acid Motifs; Base Sequence; Binding Sites; Crystallography, X-Ray; Hydrogen Bonding; Kinetics; Models, Molecular; Mutation/genetics; Nucleic Acid Denaturation; Peptide Fragments/chemistry; Peptide Fragments/genetics; Peptide Fragments/metabolism; Phenylalanine/genetics; Phenylalanine/metabolism; Protein Binding; Protein Conformation; RNA/genetics; RNA-Binding Proteins/genetics; RNA-Binding Proteins/metabolism; Ribonucleoprotein, U1 Small Nuclear/genetics; Substrate Specificity; Thermodynamics; Untranslated Regions/chemistry; Untranslated Regions/genetics; Untranslated Regions/metabolism