UGA nonsense mutation in the alcohol dehydrogenase gene of Drosophila melanogaster

Abstract

A mutant gene, which we have designated Adh nB , codes for a defective form of the enzyme alcohol dehydrogenase in Drosophila melanogaster. We show that the polypeptide encoded by Adh nB is approximately 2000 M r smaller than the protein synthesized under the direction of the wild-type alcohol dehydrogenase gene. In contrast, the alcohol dehydrogenase mRNA produced by both genes is the same size. We cloned and sequenced a portion of the protein-coding region of Adh nB and compared it to the same region in the wild-type gene. We found a single base substitution: a change of the TGG tryptophan codon at amino acid 235 to a TGA termination codon. This nonsense mutation accounts for the observed reduction in size of the alcohol dehydrogenase polypeptide. In further studies, we found that the steady-state levels of alcohol dehydrogenase mRNA in flies carrying the Adh nB gene and the wild-type alcohol dehydrogenase gene were indistinguishable. However, the steady-state level of alcohol dehydrogenase polypeptide was reduced to 1% of wild-type levels in flies with the Adh nB gene. Moreover, the rate of alcohol dehydrogenase synthesis in mutant flies was reduced to 50% of that found in wild type. The aberration in Adh nB thus affects both the rate of synthesis and the rate of degradation of the alcohol dehydrogenase peptide. Adh nB is the first reported nonsense mutant in Drosophila.