Queuine metabolism and cadmium toxicity in drosophila melanogaster

Queuine is enzymatically inserted to replace guanine in the first position of the anticodon in certain tRNAs. It has been previously observed that when Drosophila melanogaster are dosed with cadmium, the percentage of tRNA containing queuine ((Q+)tRNA) dramatically increases. In the first part of this study, a series of experiments was conducted in which Drosophila eggs were counted into vials of cadmium-dosed medium which either contained or lacked exogenous queuine. In three experiments, considerable statistical evidence suggests that exogenous queuine increases the chance that a given pupa will mature into an adult. Methyl paraben was added to the medium in these three experiments. From an RPC-5 chromatogram of cadmium-dosed control adults, it appears that methyl paraben prevents the characteristic augmentation of (Q+)tRNA associated with cadmium. It may be that because these cadmium-dosed control flies were relatively low in (Q+)tRNA, a protective effect of exogenous queuine was observable. However, since under the conditions of other experiments, no protective effects were seen, the question of whether or not queuine offers Drosophila any resistance to cadmium toxicity remains unanswered. In the second part of this study, a culture of axenic Drosophila was developed and maintained on a chemically defined medium. From their RPC-5 chromatograms, it was seen that the tRNA of these flies was entirely (Q-). From this and further evidence, it was concluded that Drosophila do not synthesize queuine de novo. The final part of this paper dealt with the paradoxical observations that larval Drosophila possess considerable amounts of active queuine insertion enzyme, and that the percentage of (Q+)tRNA decreases throughout the larval stage. Different concentrations of exogenous queuine were titrated into jars of growth medium. Eggs were inoculated onto this medium, the late third instars were collected, and their tRNA was then isolated. The RPC-5 chromatograms revealed that the greater the concentration of queuine in their growth medium, the higher was the percentage of their (Q+)tRNA. Thus, it was concluded that in Drosophila larvae, the insertion of queuine into tRNA is a substrate-dependent reaction.