Prothoracic transformation and functional structure of the Ultrabithorax gene of Drosophila.

The activity of the Ultrabithorax (Ubx+) gene is necessary for the characteristic development of a particular anatomical domain of the body of Drosophila. Mutant alleles at the abx, bx, bxd, and pbx loci eliminate specific functions of Ubx+ since their phenotype is part of that of Ubx mutants. We have characterized several abx and bx alleles and found that their effect extends to the same anatomical subdomain. This suggests that they inactivate the same genetic subunit within Ultrabithorax. Also, their wild-type activity is required for two distinct functions: postprothorax, acting early in the embryonic period, and bithorax, acting through embryonic and larval periods. Our results suggest that the Ultrabithorax gene contains two genetic subunits and that each subunit may include two separate functions.