Temporal regulation of late expression of Bar homeobox genes during Drosophila leg development by Spineless, a homolog of the mammalian dioxin receptor

The spatial and temporal regulation of genes encoding transcription factors is essential for the proper development of multicellular organisms. In Drosophila leg development, the distal-most tarsus (ta5) is specified by the strong expression of a pair of Bar homeobox genes in late third instar. This expression is regulated under the control of the ta5 enhancer activated by Bar. No activation of the ta5 enhancer, however, occurs in early third instar when considerable Bar is produced. The ta5 enhancer was comprised of a basal enhancer required for driving Bar expression and a negative regulatory motif serving as a binding site for the heterodimer of Spineless and Tango, homologs of the mammalian dioxin receptor and aryl hydrocarbon nuclear translocator, respectively. The spineless and tango were essential for suppressing the basal enhancer activation in early third instar. The spineless was transiently expressed in early third instar in the Bar expression domain. ta5 Bar expression may thus be temporally regulated through transient inhibition of premature activation of the basal enhancer via specific binding of the Spineless/Tango heterodimer to the negative regulatory motif in early third instar and subsequent release from the inhibition due to the disappearance of spineless expression at later stages.