TRACHEALESS ENCODES A BHLH-PAS PROTEIN THAT IS AN INDUCER OF TRACHEALCELL FATES IN DROSOPHILA

The embryonic tracheal system in Drosophila develops from placodes of precursor cells on the ectoderm. A transcription factor of the basic h elix-loop-helix (bHLH)-PAS family, which is expressed in the nuclei of the tracheal cells throughout development, was identified. The protei n shows the highest degree of homology to the Single-minded (Sim) prot ein. 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 ex pression of trh is consistent with a biphasic mode of transcriptional regulation. Expression is first induced by exogenous cues and is subse quently autoregulated. trh is also expressed and required in the poste rior spiracles and the salivary gland ducts. The role of Trachealess i n the formation of several tubular tissues in the embryo suggests that it may induce a general fate of branched tubular structures of epithe lial origin.