Functional Study of a Drosophila Hox Gene Enhancer Essential for Segment-Specific Sense Organ Patterning

The conserved Hox transcription factors (TFs) are well known for their functions in specifying segmental identity along the anterior/posterior (A/P) axis of developing animal embryos. However, Hox TFs also function post-embryonically to pattern limbs and other organs. During development, spatially modulated expression of the Drosophila Hox gene, Sex combs reduced (Scr), in legs of the first thoracic segment (T1 legs) is required for generation of segment-specific patterning of mechanosensory microchaete sense organs. Scr expression is specifically elevated in the primordia of a cluster of T1 microchaetes, called the transverse bristle rows (TBRs), and promotes their development by altering the leg microchaete patterning pathway. We have previously shown that an Scr enhancer, ScrE, controls Scr expression in the TBR microchaete primordia and is essential for the development of these bristles. In this study, we investigated the functions of the proximal/distal (P/D) patterning genes Distalless (Dll), dachshund (dac) and bric-a- brac1/2 (bab1/2) in regulating Scr expression through the ScrE enhancer. The homeodomain (HD) TF, Dll, activates Scr expression through several sites in the ScrE enhancer. Dac is a conserved transcriptional co-factor that is required for upregulated expression of both Scr and ScrE-GFP. Previous studies suggested that two paralogous genes, bab1 and bab2 of the bab locus, which encode structurally related TFs, regulate modulated Scr expression in T1 legs, but the individual functions of bab1 and bab2 in the process were not known. Our observations suggest that bab1 and bab2 function coordinately to control TBR patterning and Scr distal repression in T1 legs. In addition, ScrE is responsive to repression by Bab1/2, and potential Bab1/2-response sequences have been mapped to a 25 bp conserved region within ScrE. An in vivo function analysis of ScrE suggests this minimal Scr enhancer is sufficient to promote development of these sense organs in vivo and that specific ScrE sequences have functional relevance in vivo. This investigation provides insights into the regulatory mechanisms underlying modulated Hox gene expression and generation of morphological diversity among Drosophila limbs and understanding the in vivo function of Hox gene enhancers.