Transcription activates repressed domains in the Drosophila bithorax complex.

A series of mutations have been recovered in the bithorax complex of D. melanogaster that transform the first segment of the abdomen into a copy of the second or third abdominal segment. These dominant Ultraabdominal alleles are all associated with P element insertions which are transcribed in the first abdominal segment. The transcripts proceed past the end of the P element for up to 50 kb, extending through the regulatory regions for the second and third abdominal segments. Blocking transcription from the P element promoter reverts the mutant phenotype. Previously identified Ultraabdominal alleles, not associated with P elements, also show abnormal transcription of the same region.