Sequence-specific transcriptional activation by Myc and repression by Max.

The c-Myc oncoprotein, which is required for cellular proliferation, resembles in its structure a growing number of transcription factors. However, the mechanism of its action in vivo is not yet clear. The discovery of the specific cognate DNA-binding site for Myc and its specific heterodimerization partner, Max, enabled the use of direct experiments to elucidate how Myc functions in vivo and how this function is modulated by Max. Here we demonstrate that exogenously expressed Myc is capable of activating transcription in vivo through its specific DNA-binding site. Moreover, transcriptional activation by Myc is dependent on the basic region, the integrity of the helix-loop-helix and leucine zipper dimerization motifs located in the carboxy-terminal portion of the protein, and the regions in the amino terminus conserved among Myc family proteins. In contrast to Myc, exogenously expressed Max elicited transcriptional repression and blocked transcriptional activation by Myc through the same DNA-binding site. Our results suggest a functional antagonism between Myc and Max which is mediated by their relative levels in the cells. A model for the activity of Myc and Max in vivo is presented.