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Elucidation of the Complete Azorhizobium Nicotinate Catabolism Pathway
Journal of Bacteriology
  • Christopher Kitts, University of California - Santa Cruz
  • Jerome P. Lapointe, University of California - Santa Cruz
  • Van Thai Lam, University of California - Santa Cruz
  • Robert A. Ludwig, University of California - Santa Cruz
Publication Date
A complete pathway for Azorhizobium caulinodans nicotinate catabolism has been determined from mutant phenotype analyses, isolation of metabolic intermediates, and structural studies. Nicotinate serves as a respiratory electron donor to O2 via a membrane-bound hydroxylase and a specific c-type cytochrome oxidase. The resulting oxidized product, 6-hydroxynicotinate, is next reduced to 1,4,5,6-tetrahydro-6-oxonicotinate. Hydrolytic ring breakage follows, with release of pyridine N as ammonium. Decarboxylation then releases the nicotinate C-7 carboxyl group as CO2, and the remaining C skeleton is then oxidized to yield glutarate. Transthioesterification with succinyl coenzyme A (succinyl-CoA) yields glutaryl-CoA, which is then oxidatively decarboxylated to yield crotonyl-CoA. As with general acyl β oxidation, L-β-hydroxybutyryl-CoA, acetoacetyl-CoA, and finally two molecules of acetyl-CoA are produced. In sum, nicotinate is catabolized to yield two CO2 molecules, two acetyl-CoA molecules, and ammonium. Nicotinate catabolism stimulates Azorhizobium N2 fixation rates in culture. Nicotinate catabolism mutants still able to liberate pyridine N as ammonium retain this capability, whereas mutants so blocked do not. From, mutant analyses and additional physiological tests, N2 fixation stimulation is indirect. In N-limited culture, nicotinate catabolism augments anabolic N pools and, as a consequence, yields N2-fixing cells with higher dinitrogenase content.
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Citation Information
Christopher Kitts, Jerome P. Lapointe, Van Thai Lam and Robert A. Ludwig. "Elucidation of the Complete Azorhizobium Nicotinate Catabolism Pathway" Journal of Bacteriology Vol. 174 Iss. 23 (1992) p. 7791 - 7797
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