The large upstream control region of the Drosophila homeotic gene Ultrabithorax

Ultrabithorax (Ubx) is a Drosophila homeotic gene that determines the segmental identities of parts of the thorax and abdomen. Appropriate Ubx transcription requires a long upstream control region (UCR) that is defined genetically by the bithoraxoid (bxd) and postbithorax (pbx) subfunction mutations. We have directly analyzed UCR functions by the examination of β-galactosidase expression in flies containing Ubx-lacZ fusion genes. 35 hb of UCR DNA confers upon β-galactosidase an expression pattern that closely parallels normal Ubx expression throughout development. In contrast, 22 kb of UCR DNA confers fewer features of normal Ubx expression, and with 5 kb of UCR DNA the expression pattern has no resemblance to Ubx expression except in the visceral mesoderm. We have also shown that bxd chromosome breakpoint mutants form a comparable 5' deletion series in which the severity of the effect on Ubx expression correlates with the amount of upstream DNA remaining in the mutant. In Ubx-lacZ fusions containing 22kb of UCR DNA, and in comparable bxd mutants, there is a persistent pair-rule pattern of metameric expression in early development, demonstrating that there are distinct mechanisms with different sequence requirements for the initial activation of Ubx in different metameres. The correction of this pair-rule pattern later in embryogenesis shows that there are also distinct mechanisms for the activation of Ubx at different times during development.