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Abstract: During tissue patterning, developing fields may be subdivided into several non-overlapping domains by region-specific expression of transcription factors. In Drosophila leg development, the most distal segments, the pretarsus and tarsal segment 5 (ta5), are precisely specified by interactions between tarsus homeobox genes (BarH1 and BarH2) and pretarsus homeobox genes (aristaless, clawless, and Lim1). Here, we demonstrate that trachealess and tango, both encoding bHLH–PAS proteins that are required for the formation of the embryonic tracheal system, are essential for forming two adjacent distal segments of the leg. trachealess is expressed in the pretarsus and ta5, and the concerted action of trachealess and tango seems to modulate the activity of homeobox gene regulatory loops by repressing Bar in the pretarsus and activating Bar in ta5. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The subdivision of the developing field by region-specific expression of genes encoding transcription factors is an essential step during appendage development in arthropod and vertebrates. In Drosophila leg development, the distal-most region (pretarsus) is specified by the expression of homeobox genes, aristaless and Lim1, and its immediate neighbor (distal tarsus) is specified by the expression of a pair of Bar homeobox genes. Here, we show that one additional gene, clawless, which is a homolog of vertebrate Hox11/tlx homeobox gene family and formerly known as C15, is specifically expressed in the pretarsus and cooperatively acts with aristaless to repress Bar and possibly to activate Lim1. Similar to aristaless, the maximal expression of clawless requires Lim1 and its co-factor, Chip. Bar attenuates aristaless and clawless expression through Lim1 repression. Aristaless and Clawless proteins form a complex capable of binding to specific DNA targets, which cannot be well recognized solely by Aristaless or Clawless. [Copyright &y& Elsevier]