A highly conserved sequence in the 3 '-untranslated region of the Drosophila Adh gene plays a functional role in Adh expression

Phylogenetic analysis identified a highly conserved eight-base sequence (AA GGCTGA) within the 3'-untranslated region (UTR) of the Drosophila alcohol d ehydrogenase gene, Adh. To examine the functional significance of this cons erved motif, we performed in vitro deletion mutagenesis on the D. melanogas ter Adh gene followed by P-element-mediated germline transformation. Deleti on of all or part of the eight-base sequence leads to a twofold increase in in vivo ADH enzymatic activity. The increase in activity is temporally and spatially general and is the result of an underlying increase in Adh trans cript. These results indicate that the conserved 3'-UTR motif plays a funct ional role in the negative regulation of Adh gene expression. The evolution ary; significance of our results may be understood in the context of the am ino acid change that produces the ADH-F allele and also leads to a twofold increase in ADH activity. While there is compelling evidence that the amino acid replacement has been a target of positive selection, the conservation of the 3'-UTR sequence suggests that it is under strong purifying selectio n. The selective difference between these two sequence changes, which have similar effects on,ADH activity, may be explained by different metabolic co sts associated with the increase in activity.