Tryptophan pyrrolase activity regulation in Drosophila: role of an isoacceptor tRNA unsettled

Abstract

TRYPTOPHAN pyrrolase is a key enzyme in the biosynthetic pathway leading to the ommochromes¹. In wild-type flies of D. melanogaster or D. hydei, tryptophan is converted by this enzyme into N-formyl kynurenine, which is hydrolysed to formic acid and kynurenine. Further metabolism of kynurenine results in the deposition of the yellow-brown pigments in the eyes of these insects. Tryptophan pyrrolase activity, however, is lacking or greatly reduced in freshly prepared homogenates of living imagines of the vermilion mutants of D. melanogaster^1,2,3 or D. hydei (Table 1). Finding a stimulation of tryptophan pyrrolase activity in homogenates of D. melanogaster vermilion flies by RNase T₁ treatment and an inhibition of this activity by wild-type tRNA preparations or by an isoaccepting tyrosine tRNA (tRNA₂^Tyr) led Jacobson^4,5 to suggest the implication of this specific isoaccepting tRNA in the regulation of tryptophan pyrrolase activity and in the mechanism of genetic suppression of the vermilion mutant. Because of the importance of these conclusions and their bearing on the general understanding of the basic molecular mechanisms of enzyme regulation and suppression in eukaryotes, we reinvestigated the entire system including the assay procedure.