AN E-BOX ELEMENT LOCALIZED IN THE FIRST INTRON MEDIATES REGULATION OFTHE PROTHYMOSIN-ALPHA GENE BY C-MYC

In RAT1A fibroblasts, expression of the prothymosin alpha gene is unde r the transcriptional control of the c-myc proto-oncogene. We have now cloned the rat gene encoding prothymosin a and show that the cloned g ene is regulated by c-myc in vivo. We find that regulation by c-myc is mediated by sequences do,vnstream of the transcriptional start site, whereas the promoter is constitutive and not regulated by c-myc. We ha ve identified an enhancer element within the first intron that is suff icient to mediate a response to Myc and Max in transient transfection assays and to activation of estrogen receptor-Myc chimeras in vivo. We find that this element contains a consensus Myc-binding site (CACGTG) . Disruption of this site abolishes the response to Myc and Max in bot h transient and stable assays. Mutants of either Myc or Max that are d eficient for heterodimerization fail to regulate the prothymosin alpha gene, suggesting that a heterodimer between Myc and Max activates the prothymosin alpha gene. Our data define the prothymosin alpha gene as a bona fide target gene for c-myc.