Symbiotic Association of Aphids: Biochemical Role of the Procaryotic Endosymbiont

蚜虫是一种像注射器一样的小昆虫，它穿透植物并吸收植物汁液中的营养。这些昆虫就像未经消毒的针一样，传播各种在农业上具有重要意义的植物疾病。在蚜虫的体腔内有专门的细胞，其中含有革兰氏阴性细菌(内共生菌)，这些细菌不能在宿主细胞外培养。蚜虫依赖于内共生菌，用抗生素清除内共生菌会导致寄主不育或死亡。植物汁液缺乏昆虫不能合成的多种氨基酸，有证据表明内共生菌为蚜虫宿主提供这些氨基酸。也许现有的最好证据是内共生体过度产生色氨酸；也有证据表明一些内共生体可能过度产生半胱氨酸。我们的研究涉及色氨酸和半胱氨酸生物合成酶编码基因的克隆和测序，部分关键酶的过量生产及其动力学性质的研究。这些结果可能表明，内生共生菌过度生产了色氨酸和半胱氨酸这两种关键氨基酸，并被蚜虫寄主利用。这一信息将确定内共生菌的功能，并可能提出通过消除内共生菌来控制蚜虫的方法。

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.