GENE-EXPRESSION IN A YOUNG MULTIGENE FAMILY - TISSUE-SPECIFIC DIFFERENCES IN THE EXPRESSION OF THE HUMAN ALCOHOL-DEHYDROGENASE GENES ADH1, ADH2, AND ADH3

Three human alcohol dehydrogenase genes, ADH1, ADH2, and ADH3, were fo rmed by tandem duplications and have since diverged in their tissue-sp ecific and developmental expression, Their proximal promoters remain 8 0-84% identical in sequence, approximately the same degree of identity as at synonymous sites in the coding regions of these three genes, To understand the evolution of tissue specificity, gene expression must be studied in many different cells and tissues, A systematic compariso n of their promoters reveals the effects of subtle sequence difference s on the binding of nuclear proteins to their cis-acting elements, The re are differences in the affinity with which some proteins are bound to altered sites including C/EBP sites, USF/MLTF sites, and the G3T si te (which binds Spl), There are also differences in the sites that are occupied, e.g., CTF/NF-I-related sites, These sequence differences ar e reflected in differences in gene expression in three cell lines, In H4IIE-C3 hepatoma cells, the ADH1 promoter was more active than the AD H2 promoter, and the ADH3 promoter was nearly nonfunctional, In HeLa c ells, both ADH1 and ADH2 promoters directed expression; again the ADH3 promoter was extremely weak, None of the three promoters had much act ivity in CV-1 cells, Coexpression of C/EBP alpha greatly stimulated ex pression of the ADH1 promoter in HeLa cells and in CV-1 cells, but onl y weakly stimulated expression in H4IIE-C3 cells, The stimulation of t he ADH1 promoter by C/EBP alpha was comparable to that of ADH2, despit e the weaker binding to the C/EBP sites that flank the TATA box in ADH 1, The ADH3 promoter was not greatly stimulated by C/EBP alpha; despit e good binding of C/EBP alpha. These results demonstrate that small di fferences in the cis-acting elements affect affinity of binding by tra nscription factors and the pattern of gene expression.