THE DROSOPHILA HOMEOTIC MUTATION NASOBEMIA (ANTP(NS)) AND ITS REVERTANTS - AN ANALYSIS OF MUTATIONAL REVERSION

The homeotic gene Antennapedia (Antp) controls determination of many d ifferent cell types in the thorax and abdomen of Drosophila melanogast er. The spontaneous mutant allele Nasobemia (Antp(Ns)) and its reverta nts have been widely used to infer normal Antp gene function but have not themselves been thoroughly characterized. Our analysis reveals tha t Antp(Ns) consists of an internal 25-kb partial duplication of the An tp gene as well as a complex insertion of >40 kb of new DNA including two roo transposons. The duplication gives the mutant gene three Antp promoters, and transcripts from each of these are correctly processed to yield functional ANTP proteins. At least two of the promoters are e ctopically active in the eye-antenna imaginal discs, leading to homeot ic transformation of the adult head. A molecular and genetic descripti on of several Antp(Ns) revertants shows them to be diverse in structur e and activity, including a restoration of the wild type, rearrangemen ts separating two of the Antp(Ns) promoters from the coding sequences, and protein nulls and hypomorphs affecting expression from all three of the promoters. Finally, one revertant has a suppressing lesion in t he osa locus far away from Antp. These features explain the unusual ho mozygous viable nature of Antp(Ns), suggest a mechanism by which its h omeotic transformation occurs, and exemplify the diversity of ways in which mutational reversion can take place.