Opposite orientations of DNA bending by c-Myc and Max.

The control of gene transcription requires specific protein-protein and protein-DNA interactions. c-Myc, the protein product of the c-myc protooncogene, is a member of the basic helix-loop-helix leucine-zipper class of transcription factors. Although c-Myc is able to bind to a specific core hexanucleotide DNA sequence (CACGTG), its precise function in modulating transcription remains unclear. The recent discovery of Max, a basic helix-loop-helix leucine-zipper partner protein for c-Myc, suggests that the ability of c-Myc to regulate transcription is modulated by the presence of Max. By taking advantage of the altered mobility of protein-bound DNA in the mobility-shift assay, we demonstrate the homo- and heterodimeric complexes of c-Myc and Max are able to cause increased DNA flexure as measured by the circular permutation assay. Based on phasing analysis, c-Myc and Max homodimers bend DNA in opposite orientations, whereas c-Myc-Max heterodimers cause a smaller bend, in an orientation similar to that induced by Max homodimers. To address the possibility that the apparent opposite orientation of bending was the result of DNA unwinding by one of the proteins, we measured the ability of c-Myc and Max homodimers to affect DNA unwinding; we were unable to show any specific unwinding caused by c-Myc or Max. In addition to demonstrating that members of the basic helix-loop-helix leucine-zipper class of transcription factors are able to induce DNA bending, these results suggest that different transcription factor dimers are able to bind to identical DNA sequences and yet have distinct structural effects.