STRUCTURAL ORGANIZATION OF DE-NOVO PURINE BIOSYNTHESIS ENZYMES IN PLANTS - 5-AMINOIMIDAZOLE RIBONUCLEOTIDE CARBOXYLASE AND 5-AMINOIMIDAZOLE-4-N-SUCCINOCARBOXAMIDE RIBONUCLEOTIDE SYNTHETASE CDNAS FROM VIGNA-ACONITIFOLIA

Nodules of tropical legumes generally export symbiotically fixed nitro gen in the form of ureides that are produced by oxidation of de novo s ynthesized purines. To investigate the regulation of de novo purine bi osynthesis in these nodules, we have isolated cDNA clones encoding 5-a minoimidazole ribonucleotide (AIR) carboxylase and 5-aminoimidazole-4- N-succinocarboxamide ribonucleotide (SAICAR) synthetase from a mothbea n (Vigna aconitifolia) nodule cDNA library by complementation of Esche richia coli purE and purC mutants, respectively. Sequencing of these c lones revealed that the two enzymes are distinct proteins in mothbean, unlike in animals where both activities are associated with a single bifunctional polypeptide. As is the case in yeast, the mothbean AIR ca rboxylase has a N-terminal domain homologous to the eubacterial purK g ene product. This PurK-like domain appears to facilitate the binding o f CO2 and is dispensable in the presence of high CO2 concentrations. B ecause the expression of the mothbean PurE cDNA clone in E. coli appar ently generates a truncated polypeptide lacking at least 140 N-termina l amino acids, this N-terminal region of the enzyme may not be essenti al for its CO2-binding activity.