Contribution of the Bacterial Endosymbiont to the Biosynthesis of Pyrimidine Nucleotides in the Deep-sea Tube Worm Riftia pachyptila

Abstract

The deep-sea tube worm Riftia pachyptila (Vestimentifera) from hydrothermal vents lives in an intimate symbiosis with a sulfur-oxidizing bacterium. That involves specific interactions and obligatory metabolic exchanges between the two organisms. In this work, we analyzed the contribution of the two partners to the biosynthesis of pyrimidine nucleotides through both the "de novo" and "salvage" pathways. The first three enzymes of the de novo pathway, carbamyl-phosphate synthetase, aspartate transcarbamylase, and dihydroorotase, were present only in the trophosome, the symbiont-containing tissue. The study of these enzymes in terms of their catalytic and regulatory properties in both the trophosome and the isolated symbiotic bacteria provided a clear indication of the microbial origin of these enzymes. In contrast, the succeeding enzymes of this de novo pathway, dihydroorotate dehydrogenase and orotate phosphoribosyltransferase, were present in all body parts of the worm. This finding indicates that the animal is fully dependent on the symbiont for the de novo biosynthesis of pyrimidines. In addition, it suggests that the synthesis of pyrimidines in other tissues is possible from the intermediary metabolites provided by the trophosomal tissue and from nucleic acid degradation products since the enzymes of the salvage pathway appear to be present in all tissues of the worm. Analysis of these salvage pathway enzymes in the trophosome strongly suggested that these enzymes belong to the worm. In accordance with this conclusion, none of these enzyme activities was found in the isolated bacteria. The enzymes involved in the production of the precursors of carbamyl phosphate and nitrogen assimilation, glutamine synthetase and nitrate reductase, were also investigated, and it appears that these two enzymes are present in the bacteria.

Arginine Metabolism in the Deep Sea Tube Worm Riftia pachyptila and Its Bacterial Endosymbiont

“…It was previously reported that the carbamoylphosphate syn-thetase specific of the pyrimidine pathway (CPSase-P) 1 is absent in all of the worm tissues and is present only in the bacteria (17). In contrast, the carbamoylphosphate synthetase specific of the arginine biosynthetic pathway (CPSase-A) was found in all of the tissues of the worm (25).…”

“…Although the bacterial symbiont was shown to be able to reduce nitrate to nitrite (17,23,24), the mechanisms by which the worm assimilates and metabolizes nitrogen from its inorganic sources are much less well documented. However, ammonia assimilatory enzymes such as glutamine synthetase and glutamate dehydrogenase were found to be present in both the bacterial symbiont and the host (17,23).…”

“…The assimilation and respiration of the metabolites such as CO 2 , O 2 , H 2 S, NH 3 , NO 3 Ϫ are mainly carried out in the bacteria, which, in return, provides organic compounds to the worm (15)(16)(17). In these processes, the bacteria produces metabolic energy from the oxidation of H 2 S (18).…”

Pyrimidine Nucleoside Salvage Confers an Advantage to Xenorhabdus nematophila in Its Host Interactions

“…To our knowledge, this study presents the first demonstration of the requirement for a nucleoside-salvaging enzyme in bacterium-host interactions, though previous studies have suggested the importance of nucleoside salvaging to intracellular microbes during host interactions (4,7,15,22,23,29). Our data further suggest that, unless Tdk has an as-yet-unidentified alternative substrate, dT or dU is present in both the nematode and insect environments.…”

One-pot Synthesis of Dihydroorotate by Multienzyme Polypeptide CAD

“…unusual multiple reaction has been purified in a homogenous state. The CAD is a dimeric enzyme which requires as a cofactor [7][8][9][10].…”