FUNCTION OF THE DROSOPHILA POU DOMAIN TRANSCRIPTION FACTOR DRIFTER ASAN UPSTREAM REGULATOR OF BREATHLESS RECEPTOR TYROSINE KINASE EXPRESSION IN DEVELOPING TRACHEA

Organogenesis of the Drosophila tracheal system involves extensive dir ected cell migrations leading to a stereotypic series of interconnecte d tubules. Although numerous gene products have been shown to be essen tial for tracheal morphogenesis, direct functional relationships betwe en participants have not been previously established, Both the breathl ess gene, encoding a Drosophila fibroblast growth factor receptor tyro sine kinase homologue, and the POU-domain transcription factor gene, d rifter, are expressed in all tracheal cells and are essential for dire cted cell migrations. We demonstrate here that ubiquitously expressed Breathless protein under control of a heterologous heat-shock promoter is able to rescue the severely disrupted tracheal phenotype associate d with drifter loss-of-function mutations, In the absence of Drifter f unction, breathless expression is initiated normally but transcript le vels fall drastically to undetectable levels as tracheal differentiati on proceeds. In addition, breathless regulatory DNA contains seven hig h affinity Drifter binding sites similar to previously identified Drif ter recognition elements, These results suggest that the Drifter prote in, which maintains its own expression through a tracheal-specific aut oregulatory enhancer, is not necessary for initiation of breathless ex pression but functions as a direct transcriptional regulator necessary for maintenance of breathless transcripts at high levels during trach eal cell migration. This example of a mechanism for maintenance of a c ommitted cell fate offers a model for understanding how essential gene activities can be maintained throughout organogenesis.