Symbiotic Association of Aphids: Biochemical Role of the Procaryotic Endosymbiont

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Aphids are small syringe-like insects which penetrate plants and suck up plant juices for nutrients. Acting like unsterilized needles these insects transmit a variety of agriculturally important plant diseases. Within the body cavity of aphids are specialized cells which contain gram-negative bacteria (endosymbionts) which cannot be cultured outside the host cells. The aphid is dependent on the endosymbionts, their elimination by treatment with antibiotics results in sterility or death of the host. Plant sap is deficient in a variety of amino acids which insects cannot synthesize and evidence suggests that the endosymbionts supply these amino acids for the aphid host. Perhaps the best available evidence is for the overproduction of tryptophan by the endosymbiont; there is also evidence that some endosymbionts may overproduce cysteine. Our study involves the cloning and sequencing of genes coding for enzymes of tryptophan and cysteine biosynthesis, the overproduction of selected key enzymes and a study of their kinetic properties. These results may indicate that the two key amino acids, tryptophan and cysteine are overproduced by the endosymbiont and utilized by the aphid host. This information will establish a function for the endosymbiont and may suggest approaches for the controlling of aphid pests by the elimination of their endosymbionts.