A GENETIC-ANALYSIS OF BX BXD CIS DOUBLE MUTANTS IN THE DROSOPHILA ULTRABITHORAX GENE

The Ultrabithorax (Ubx) gene of Drosophila melanogaster includes two f unctionally distinguishable regions. One is the Ubx transcription unit , which gives rise by alternative splicing to a family of morphogeneti c Ubx proteins (UBX). The other is its upstream bithoraxoid (bxd) regi on. On the basis of genetic and molecular studies, it is generally ass umed that the Ubx transcription unit contains internal positively acti ng cis-regulatory elements controlling UBX expression in the T3a compa rtment of the body of Drosophila, while the bxd region contains positi ve cis-regulatory elements controlling UBX expression in the T3p and A 1a compartments. We have performed a genetic analysis of bx bxd cis do uble mutant chromosomes containing one mutation (bx alleles) affecting the Ubx unit, and a second (bxd alleles) affecting the bxd region of the Ubx gene. Our study of different bx bxd/bx combinations shows that bxd alleles partially rescue the adult mutant phenotypes of bs allele s, which suggests that the bxd region contains a negative cis-regulato ry element involved in the control of the activity of the Ubx gene in the T3a compartment.