The Fox-1 protein regulates alternative splicing of tissue-specific exons by binding to GCAUG elements. Here, we report the soln. structure of the Fox-1 RNA binding domain (RBD) in complex with UGCAUGU. The last three nucleotides, UGU, are recognized in a canonical way by the four-stranded b-sheet of the RBD. In contrast, the first four nucleotides, UGCA, are bound by two loops of the protein in an unprecedented manner. Nucleotides U1, G2, and C3 are wrapped around a single phenylalanine, while G2 and A4 form a base-pair. This novel RNA binding site is independent from the b-sheet binding interface. Surface plasmon resonance analyses were used to quantify the energetic contributions of electrostatic and hydrogen bond interactions to complex formation and support our structural findings. These results demonstrate the unusual mol. mechanism of sequence-specific RNA recognition by Fox-1, which is exceptional in its high affinity for a defined but short sequence element. [on SciFinder (R)]