ENHANCER SHARING IN A PLASMID MODEL CONTAINING THE ALPHA-FETOPROTEIN AND ALBUMIN PROMOTERS

It has been proposed that the alpha-fetoprotein (AFP) enhancers also r egulate the adjacent albumin gene, since the -10 kbp albumin enhancer is inactive in a number of cell lines that express albumin. In transfe ction experiments, the AFP enhancers strongly stimulate the albumin pr omoter in cells that silence the AFP promoter. These observations led us to develop a plasmid model of AFP-albumin gene switching, in which the albumin and AFP promoters would compete for the three AFP enhancer s. However, when AFPCAT + ALBgal genes were combined with the AFP enha ncers in one plasmid, both genes were driven at full activity. There w as no change in the relative promoter expression over a wide range of transfected DNA concentrations, demonstrating that relative promoter a ctivity was independent of DNA concentration and of promoter. concentr ation, and that neither promoter was limiting the expression of the ot her. In contrast, a control plasmid containing two albumin promoters s howed mutual inhibition, indicating the expected promoter competition. The albumin and AFP promoters noncompetitively shared the three enhan cers on this plasmid, resulting in high levels of transcription from b oth promoters.