The enzymes involved in synthesis and utilization of carbamylphosphate in the deep-sea tube worm Riftia pachyptila

The obligate symbiosis of the deep-sea tube worm Riftia pachyptila with a s ulphur-oxidizing bacterium raises important questions concerning its metabo lism and metabolic exchanges. In this study, the presence and properties of the enzymes synthesizing and utilizing carbamylphosphate in the arginine a nd pyrimidine nucleotide pathways were investigated in this worm. The resul ts show that the ammonium-dependent carbamylphosphate synthetase and ornith ine transcarbamylase, enzymes involved in the arginine pathway, are present in all body parts of the worm. In contrast, the glutamine-dependent carbam ylphosphate synthetase and aspartate transcarbamylase, enzymes involved in the de novo pathway for pyrimidine nucleotides biosynthesis, are present on ly in the trophosome, the symbiont-harbouring tissue. Although the bacteria l nature of these enzymes is not unambigously established, these results st rongly suggest that the de novo biosynthesis of pyrimidine nucleotides is l imited to the trophosome, the organ where the production of metabolic energ y takes place, while the other parts of the worm's body rely on the salvage pathway for the production of the pyrimidine triphosphate nucleotides.