WRS-85D: A Tryptophanyl-tRNA Synthetase Expressed to High Levels in the DevelopingDrosophilaSalivary Gland

Abstract

In a screen for genes expressed in the Drosophilaembryonic salivary gland, we identified a tryptophanyl-tRNA synthetase gene that maps to cytological position 85D (WRS-85D).WRS-85D expression is dependent on the homeotic geneSex combs reduced (Scr). In the absence of Scr function, WRS-85D expression is lost in the salivary gland primordia; conversely, ectopic expression ofScr results in expression of WRS-85D in new locations. Despite the fact that WRS-85D is a housekeeping gene essential for protein synthesis, we detected bothWRS-85D mRNA and protein at elevated levels in the developing salivary gland. WRS-85D is required for embryonic survival; embryos lacking the maternal contribution were unrecoverable, whereas larvae lacking the zygotic component died during the third instar larval stage. We showed that recombinant WRS-85D protein specifically charges tRNATrp, andWRS-85D is likely to be the only tryptophanyl-tRNA synthetase gene in Drosophila. We characterized the expression patterns of all 20 aminoacyl-tRNA synthetases and found that of the four aminoacyl-tRNA synthetase genes expressed at elevated levels in the salivary gland primordia, WRS-85D is expressed at the highest level throughout embryogenesis. We also discuss the potential noncanonical activities of tryptophanyl-tRNA synthetase in immune response and regulation of cell growth.