DISCRIMINATION BETWEEN DIFFERENT E-BOX-BINDING PROTEINS AT AN ENDOGENOUS TARGET GENE OF C-MYC

c-myc plays a key role in regulating mammalian cell proliferation and apoptosis. The gene codes for a transcription factor, Myc, that belong s to the helix-loop-helix/leucine zipper (HLH/LZ) family of proteins. Myc heterodimerizes with a partner protein termed Max; the heterodimer ic complex binds to CAC(G/A)TG (E-box) sequences and activates transcr iption from these sites. However, several other HLH/LZ proteins, inclu ding USF and TFE-3, bind to and trans-activate from the same element, yet have no documented effect on cell proliferation or apoptosis. Ther efore, it is likely that mechanisms exist that discriminate between th ese proteins for activation of natural target genes of Myc. We now sho w that trans-activation from the E-box in the rat prothymosin-alpha in tron enhancer is indeed specific for Myc, and identify both the distan ce from the start site of transcription and a second E-box element adj acent to that recognized by Myc as critical determinants of specificit y. Surprisingly, transcription activation domains required for Myc to activate from this distal enhancer position differ from previously map ped domains and closely correlate with those domains essential for tra nsformation. As observed in transformation assays, Myc and Max strongl y synergize in activation from a distal enhancer position. Our data su ggest that trans-activation from the prothymosin intron enhancer is a faithful reflection of the transforming properties of the Myc protein.