A sequence-specific, single-strand binding protein activates the far upstream element of c-myc and defines a new DNA-binding motif.

The far upstream element (FUSE) of the human c-myc proto-oncogene stimulates expression in undifferentiated cells. A FUSE-binding protein (FBP) is present in undifferentiated but not differentiated cells. Peptide sequences from the purified protein allowed cloning of cDNAs encoding FBP. Expression of FBP mRNA declined upon differentiation, suggesting transcriptional regulation of FBP. Features in the FBP cDNA suggest that FBP is also regulated by RNA processing, translation, and post-translational mechanisms. Both cellular and recombinant FBP form sequence-specific complexes with a single strand of FUSE. Transfection of FBP into human leukemia cells stimulated c-myc-promoter-driven expression from a reporter plasmid in a FUSE-dependent manner. Deletion and insertion mutagenesis of FBP defined a novel single-strand DNA-binding domain. Analysis of the primary and predicted secondary structure of the amino acid sequence reveals four copies of a reiterated unit comprised of a 30-residue direct repeat and an amphipathic alpha-helix separated by an 18- to 21-residue spacer. The third and fourth copies of this repeat-helix unit constitute the minimum single-stranded DNA-binding domain. To determine whether the FUSE site, in vivo, possesses single-strand conformation, and therefore could be bound by FBP, cells were treated with potassium permanganate (KMnO4) to modify unpaired bases. Modification of genomic DNA in vivo revealed hyperreactivity associated with single-stranded DNA in the FUSE sequence and protection on the strand that binds FBP in vitro. The role of single-stranded DNA and single-strand binding proteins in c-myc regulation is discussed.