Sequences flanking the E-box contribute to cooperative binding by c-Myc/Max heterodimers to adjacent binding sites.

Previously, we have shown that c-Myc/Max heterodimers, bind cooperatively to the two adjacent, canonical E-boxes (CACGTG) located in the rat ornithine decarboxylase (ODC) gene. In order to study this in more detail, we changed the length of the linker that separates the two E-boxes, as well as their flanking sequences. We found that high affinity, cooperative binding requires a minimal linker length of 1-4 bp and that the binding affinity is influenced by E-box flanking sequences. Binding to the c-Myc responsive element of prothymosin alpha, containing both a canonical and a noncanonical E-box (CAAGTG) was also studied. As shown by DNAseI footprinting analysis, only the canonical E-box is bound by c-Myc/Max and c-Max/Max dimers. Replacing the noncanonical site with a canonical E-box only partially restored high affinity, cooperative binding. By making hybrid fragments between ODC and prothymosin alpha, we found that nucleotides in the linker between the E-boxes influence the affinity of c-Myc/Max heterodimers. Taken together, our results show that E-box sequences and sequences in the linker separating both E-boxes influence cooperative, high affinity binding by c-Myc/Max dimers.