Targeted melting and binding of a DNA regulatory element by a transactivator of c-myc.

A far upstream element (FUSE) of c-myc stimulates promoter activity when bound by a newly identified trans-acting protein, which is expressed in cycling cells. Since FUSE binding protein (FBP) binds only the noncoding strand (NCS) of its regulatory element in a sequence-specific manner, and not double-stranded (ds) DNA, formation of the protein DNA complex in vivo first requires unwinding of the DNA helix. In this report, we show evidence that FBP forces strand separation of short stretches of linear dsDNA. Because FUSE is contained within a region of helical instability that is partially unwound in negatively supercoiled DNA, it is a target for more extensive duplex strand separation by FBP, which first exposes and then selectively binds its NCS cognate sequence. In contrast, other single-stranded DNA binding proteins (SSBs) do not demonstrate this FUSE targeting activity. The novel linkage of regional dsDNA melting with cis-element binding by a transcriptional activator has broad implications in the regulation of eukaryotic gene expression.