Mutational analysis of conserved sequence elements in intron 1 of the Drosophila melanogaster Adh gene. Y. Chen , W. Stephan. Dept Biol, Univ Rochester, Rochester, NY.

   Mutations occurring in non-coding regions, such as introns, upstream and downstream flanking regions, 5' and 3' UTRs, as well as synonymous substitutions are usually thought to be effectively neutral since they do not lead to amino acid changes. However, possible scenarios of selection acting on silent and non-coding sites have been proposed, such as selection on silent and non-coding sites to maintain higher order structures of pre-mRNA. Evidence for the maintenance of a hairpin structure in intron 1 of the D. melanogaster Adh gene, which could be involved in intron processing, is found through phylogenetic comparisons of Drosophila Adh sequences. Results from mutational analysis in D. melanogaster indicate that elimination of the stem-loop structure while maintaining the conserved sequence elements affect the mRNA level as would be expected under a compensatory evolution model: mutations that disrupt the stem-loop structure cause a decrease in the mRNA level, while compensatory mutations that restore the stem-loop structure have a comparable mRNA level to that of the wild type. We also investigate whether the overall stability of the secondary structure in this intron has an effect on mRNA expression.