Function and misfunction of the two promoters of the Drosophila Antennapedia gene.

In the Antennapedia (Antp) gene of Drosophila melanogaster, structurally distinct RNAs arise from different transcription initiation sites. When the two sites are separated by a chromosome inversion, transcripts are produced from each fragment of the split Antp locus, and these RNAs initiate at the same nucleotide as in wild-type animals. Thus, the initiation sites are regulated by independent promoters. We show by in situ hybridization that transcripts from each promoter accumulate in spatially distinct patterns in a subset of wild-type imaginal discs. Importantly, these patterns are generally maintained in the inversion mutant. We conclude that the promoters possess independent and dissimilar regulatory elements for spatial activation. Finally, we have looked at transcription in seven different dominant Antp mutants, all of which show a transformation of head tissue to thoracic tissue. In each mutant, the second promoter is improperly activated in the eye-antennal imaginal disc. Because all but one of these mutations have inversion breakpoints distantly upstream of the activated promoter, they probably act via long-range euchromatic position effects. Our studies define how the dual promoters and chromatin structure of the Antp gene contribute to the generation of a complex pattern of transcription.