Cloning and Inactivation of Genes Encoding Ferredoxin- and NADH-Dependent Glutamate Synthases in the Cyanobacterium Plectonema boryanum. Imbalances in Nitrogen and Carbon Assimilations Caused by Deficiency of the Ferredoxin-Dependent Enzyme¹

Hiroaki Okuhara, Tomohiro Matsumura², Yuichi Fujita, and Toshiharu Hase^*

Division of Enzymology, Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka, 565-0871 Japan

Glutamate synthase (GOGAT) is a key enzyme in the assimilation of inorganic nitrogen in photosynthetic organisms. We found that, like higher plants, the facultative heterotrophic cyanobacterium Plectonema boryanum had ferredoxin (Fd)- and NADH-dependent GOGATs. The genes glsF, gltB, and gltD were cloned, and structural analyses and target mutageneses demonstrated that glsF encoded Fd-GOGAT and that gltB and gltD encoded the two subunits of NADH-GOGAT. All three mutants lacking one of the GOGAT genes were able to grow photosynthetically and heterotrophically. However, the Fd-GOGAT mutant exhibited a phenotype of marked nitrogen deficiency when grown under conditions of saturating illumination and CO₂ supply. In these conditions the rate of the ammonia uptake from the culture medium was slower in the Fd-GOGAT mutant than in the wild type or in the NADH-GOGAT mutant, but no significant differences were found in the rate of the CO₂ fixation-dependent O₂ evolution among these strains. Our results suggest that, although both Fd- and NADH-GOGATs were operative in the cells growing in light, the contribution of Fd-GOGAT, which directly utilizes photoreducing power for the catalytic reaction, is essential for balancing photosynthetic nitrogen and carbon assimilation.

Plant Physiol. (1999) 120: 33-42
Copyright Clearance Center:   0032-0889/99/120//10
© 1999 American Society of Plant Physiologists

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