The structure and expression of a hybrid homeotic gene.

We describe the structure and expression of an exceptional bithorax complex mutation, C1. This deletion mutation removes major portions of both the Ultrabithorax (Ubx) and abdominal-A (abd-A) protein coding regions, yet retains many of the functions normally executed by these genes. We show that the ends of the C1 deletion map to analogous positions within the Ubx and abd-A transcription units, and that the deletion chromosome gives rise to a set of novel fusion transcripts that may encode hybrid abd-A/Ubx proteins. These fusion products are transcribed from the abd-A promoter, but exhibit a novel spatial pattern of expression that combines aspects of the normal Ubx and abd-A distributions. This pattern provides evidence for the existence of regulatory elements located in the 3' region of the Ubx transcription unit that can act on the abd-A promoter at a distance of at least 20 kb.