Transcription activation by Myc and Max: flanking sequences target activation to a subset of CACGTG motifs in vivo

The Myc oncoprotein has been implicated in control of cell growth, division and differentiation. Although Myc contains a bHLH-LZ motif, it fails to bind DNA alone but can do so by forming heterodimers with an unrelated bHLH-LZ protein, Max. Max homodimers and Myc-Max heterodimers share the ability to bind CACGTG or CATGTG elements. Current models, based on experimentally induced overexpression of Myc and Max in mammalian cells, propose that Max-Max homodimers repress while Myc-Max heterodimers activate transcription through CACGTG binding sites. The interpretation of the results using mammalian cells is complicated by the presence of numerous unrelated CACGTG binding transcription activators and the existence of two alternative Max dimerization partners, Mad and Mxi-1. Thus, the mechanism whereby overexpression of Max leads to transcriptional repression remains to be established. Using a yeast system we show that Max homodimers have the potential to activate transcription through CACGTG motifs. Activation by Max requires DNA binding and amino acids outside the bHLH-LZ domain but is reduced compared with activation by Myc-Max heterodimers. Moreover, transcriptional activation by Myc-Max heterodimers, but not Max-Max homodimers, is strongly inhibited in vivo by specific sequences flanking the core CACGTG binding motif, presumably reflecting reduced DNA binding affinity. These results suggest a mechanism for directing the Myc-Max complex to a specific subset of CACGTG-containing target genes.