Decarboxylation performed by particulate fractions of two nitrogen-fixing blue-green algae

Molecular aspects of nitrogen fixation by photosynthetic prokaryotes

The photosynthetic prokaryotes possess diverse metabolic capabilities, both in carrying out different types of photosynthesis and in their other growth modes. The nature of the coupling of these energy-generating processes with the basic metabolic demands of the cell, such as nitrogen fixation, has stimulated research for many years. In addition, nitrogen fixation by photosynthetic prokaryotes exhibits several unique features; the oxygen-evolving cyanobacteria have developed various strategies for protection of the oxygen-labile nitrogenase proteins, and some photosynthetic bacteria have been found to regulate their nitrogenase (N2ase) activity in a rapid response to fixed nitrogen, thus saving substantial amounts of energy. Recent advances in the biochemistry, physiology, and genetics of nitrogen fixation by cyanobacteria and photosynthetic bacteria are reviewed, with special emphasis on the unique features found in these organisms. Several major topics in cyanobacterial nitrogen fixation are reviewed. The isolation and characterization of N2ase and the isolation and sequence of N2ase structural genes have shown a great deal of similarity with other organisms. The possible pathways of electron flow to N2ase, the mechanisms of oxygen protection, and the control of nif expression and heterocyst differentiation will be discussed. Several recent advances in the physiology and biochemistry of nitrogen fixation by the photosynthetic bacteria are reviewed. Photosynthetic bacteria have been found to fix nitrogen microaerobically in darkness. The regulation of nif expression and possible pathways of electron flow to N2ase are discussed. The isolation of N2ase proteins, particularly the covalent modification of the Fe protein, the nature of the modifying group, properties of the activating enzyme, and regulating factors of the inactivation/activation process are reviewed.

The effect of pyruvate on nitrogenase activity in the blue-green alga Anabaena cylindrica

Exogenous pyruvate added to cultures of the blue-green alga, Anabaena cylindrica stimulated nitrogenase activity (measured by acetylene reduction) only in the dark under low pO2 (0.05 atmospheres). Under aerobic conditions or in the light, stimulation was absent and replaced by an inhibition of activity above 5 mM added pyruvate. The curve of nitrogenase activity versus oxygen concentration had a similar maximal value of ethylene production with, or without added pyruvate, but in the presence of pyruvate this maximum occurred at 0.05 atmospheres O2, whilst in the absence of pyruvate the maximum occurred at 0.10 atmospheres O2. Malate, citrate, alpha-ketoglutarate, glucose and fructose were tested also, but none gave a similar effect to pyruvate. Addition of 14C-pyruvate and autoradiography indicated that exogenous pyruvate is metabolized through the interrupted Krebs cycle. These results are explained in terms of the activity of pyruvate: ferredoxin oxidoreductase and the ATP-induced oxygen sensitivity of nitrogenase.