trachealess encodes a bHLH-PAS protein that is an inducer of tracheal cell fates in Drosophila.

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Abstract
The embryonic tracheal system in Drosophila develops from placodes of precursor cells on the ectoderm. A transcription factor of the basic helix-loop-helix (bHLH)-PAS family, which is expressed in the nuclei of the tracheal cells throughout development, was identified. The protein shows the highest degree of homology to the Single-minded (Sim) protein. The transcript represents the previously identified trachealess (trh) locus, essential for tracheal development. Ectopic expression of trh leads to generation of extra tracheal pits and branches and to the expression of tracheal markers by patches of ectodermal cells. The expression of trh is consistent with a biphasic mode of transcriptional regulation. Expression is first induced by exogenous cues and is subsequently autoregulated. trh is also expressed and required in the posterior spiracles and the salivary gland ducts. The role of Trachealess in the formation of several tubular tissues in the embryo suggests that it may induce a general fate of branched tubular structures of epithelial origin.
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