Structural insights into a regulatory mechanism of FIR RRM1-FUSE interaction

FUBP-interacting repressor (FIR) is a suppressor of transcription of the proto-oncogene MYC. FIR binds to the far upstream element (FUSE) of the MYC promoter. Competition of FIR with FUSE-binding protein 1 (FUBP1) is a key mechanism of MYC transcriptional regulation. To gain insights into the structural mechanisms regulating FIR DNA interaction, we determined the crystal structure of two FIR RRM domains (RRM1-2) with single-stranded FUSE DNA sequences. These structures revealed an ability of the RRM domain to recognize diverse FUSE regions through distinct intermolecular interactions and binding modes. Comparative structural analyses against available RRM-ssDNA/RNA complexes showed that the nucleotide configurations in FIR were similar to those in other RRMs that harbour a tyrosine at the conserved aromatic position in the RNP2 motif (Y-type RRM), but not those with a phenylalanine (F-type RRM). Site-directed mutagenesis experiments demonstrated that a single substitution, Y115F, altered the binding affinities of oligonucleotides to FIR RRM, suggesting an important role of this conserved aromatic residue in ssDNA/RNA interactions. Our study provides the structural basis for further mechanistic studies on this important protein-DNA interaction.