Ligand-dependent activation of breathless FGF receptor gene in Drosophila developing trachea.

Spatially and temporally regulated activity of Branchless/Breathless signaling is essential for trachea development in Drosophila. Early ubiquitous breathless (btl) expression is controlled by binding of Trachealess/Tango heterodimers to the btl minimum enhancer. Branchless/Breathless signaling includes a Sprouty-dependent negative feedback loop. We show that late btl expression is a target of Branchless/Breathless signaling and hence, Branchless/Breathless signaling contains a positive feedback loop, which may guarantee a continuous supply of fresh receptors to membranes of growing tracheal branch cells. Branchless/Breathless signaling activates MAP-kinase, which in turn, activates late btl expression and destabilizes Anterior-open, a repressor for late btl expression. Biochemical and genetic analysis indicated that the minimum btl enhancer includes binding sites of Anterior-open.