THE PRUNED GENE ENCODES THE DROSOPHILA SERUM RESPONSE FACTOR AND REGULATES CYTOPLASMIC OUTGROWTH DURING TERMINAL BRANCHING OF THE TRACHEAL SYSTEM

We identified a Drosophila gene, pruned, that regulates formation of t he terminal branches of the tracheal (respiratory) system, These branc hes arise by extension of long cytoplasmic processes from terminal tra cheal cells towards oxygen-starved tissues, followed by formation of a lumen within the processes, The pruned gene is expressed in terminal cells throughout the period of terminal branching. pruned encodes the Drosophila homologue of serum response factor (SRF), which functions w ith an ETS domain ternary complex factor as a growth-factor-activated transcription complex in mammalian cells. In pruned loss of function m utants, terminal cells fail to extend cytoplasmic projections, A const itutively activated SRF drives formation of extra projections that gro w out in an unregulated fashion. An activated ternary complex factor h as a similar effect, We propose that the Drosophila SRF functions like mammalian SRF in an inducible transcription complex, and that activat ion of this complex by signals from target tissues induces expression of genes involved in cytoplasmic outgrowth.