Amino acid discrimination by the nuclear encoded mitochondrial arginyl-tRNA synthetase of the larva of a bruchid beetle (Caryedes brasiliensis) from northwestern Costa Rica

Abstract

l-canavanine, the toxic guanidinooxy analogue of l-arginine, is the product of plant secondary metabolism. The need for a detoxifying mechanism for the producer plant is self-evident but the larvae of the bruchid beetle Caryedes brasiliensis, that is itself a non-producer, have specialized in feeding on the l-canavanine-containing seeds of Dioclea megacarpa. The evolution of a seed predator that can imitate the enzymatic abilities of the host permits us to address the question of whether the same problem of amino acid recognition in two different kingdoms has been solved by the same mechanism. A discriminating arginyl-tRNA synthetase, detected in a crude C. brasiliensis larval extract, was proposed to be responsible for insect's ability to survive the diet of l-canavanine (Rosenthal, G. A., Dahlman, D. L., and Janzen, D. H. (1976) A novel means for dealing with L-canavanine, a toxic metabolite. Science 192, 256–258). Since the arginyl-tRNA synthetase of at least three genetic compartments (insect cytoplasmic, insect mitochondrial and insect gut microflora) may participate in conferring l-canavanine resistance, we investigated whether the nuclear-encoded C. brasiliensis mitochondrial arginyl-tRNA synthetase plays a role in this discrimination. Steady state kinetics of the cloned, recombinant enzyme have revealed and quantified an amino acid discriminating potential of the mitochondrial enzyme that is sufficient to account for the overall l-canavanine misincorporation rate observed in vivo. As in the cytoplasmic enzyme of the l-canavanine producer plant, the mitochondrial arginyl-tRNA synthetases from a specialist seed predator relies on a kinetic discrimination that prevents l-canavanine misincorporation into proteins.