SYNTHESIS OF THE ESSENTIAL AMINO-ACID TRYPTOPHAN IN THE PEA APHID (ACYRTHOSIPHON-PISUM) SYMBIOSIS

Pea aphids (Acyrthosiphon pisum) containing their symbiotic bacteria do not require a dietary supply of the essential amino acid tryptophan over two generations on chemically-defined diets; but aphids, whose bacteria are disrupted by chlortetracycline, die as first-generation larvae on tryptophan-free diet. Consistent with the interpretation that the symbiotic bacteria in the aphids synthesize tryptophan, tryptophan synthetase activity was detected in both symbiont-bearing aphids and isolated bacteria, but not in chlortetracycline-treated aphids. Aphids reared on tryptophan-free diet produced honeydew containing 0.32 mM tryptophan, from which it can be calculated that each bacterial cell in the aphid releases at least 0.04 fmol tryptophan d-1.