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PubMed1994

Role of alkali cations (K+ and Na+) in cyanobacterial nitrogen fixation and adaptation to salinity and osmotic stress.

Cyanobacteria occupy almost every possible ecological niche on earth, being tolerant to a large number of environmental stresses, including salinity and drought. Many of them also fix atmospheric nitrogen. They are responsible for a significant share of biosolar energy conversions on this planet and make substantial contributions to the carbon and nitrogen status of both oceans and soils. Sodium and potassium are two of the most prevalent cations on this planet. While K+ is an essential macronutrient in most life-forms, Na+ is strongly discriminated by means of highly selective alkali cation transport systems, favouring K+ over Na+. Although a nutritional requirement for K+ has not been specifically investigated, rapid accumulation of K+ during salt/osmotic stress has been observed in several cyanobacteria. Genes and proteins constituting a membrane-bound, turgor- and osmo-inducible, Kdp-ATPase-like system in Anabaena strains that may help in their early K+ responses to salt/osmotic stress have been identified. An unusual, specific and absolute requirement for trace quantities of sodium has been documented in cyanobacteria. Work done in our laboratory, and elsewhere, has elucidated the mechanisms underlying such a unique requirement. It has long been believed that cyanobacteria scavenge and immobilise sodium. We have, however, shown that sodium exclusion brought about by curtailment of influx and active efflux of Na+ forms the basis of salt tolerance in these microbes and that the inherent salt tolerance can be modified by factors that modulate Na+ fluxes in cyanobacteria. Identification of genes affecting the cation relationships in nitrogen-fixing cyanobacteria is currently in progress.

Adaptation, Physiological↗
PubMed1988

Bacterial alternative nitrogen fixation systems.

The introduction briefly reviews some of the salient features of the well-characterized conventional molybdo-enzyme system for N2 fixation. This is followed by a brief account of the discovery of an alternative N2 fixation system that does not require molybdenum in the N2-fixing bacterum Azotobacter vinelandii. The next section cites observations from the early literature on N2 fixation suggesting may not always require molybdenum. Next, recent evidence for an alternative N2 fixation system in A. vinelandii is discussed. A brief description of our discovery of an alternative nitrogenase which is not a molybdenum or vanadium enzyme is presented, followed by a summary of recent papers describing an alternative vanadium-containing nitrogenase. Available information on the genetics and regulation of alternative N2 fixation systems is discussed. Finally, the possible/probable presence of alternative N2 fixation systems in bacteria other than Azotobacter species is covered.

Bacteria↗
PubMed1975

Nitrogenase. VI. Acetylene reduction assay: Dependence of nitrogen fixation estimates on component ratio and acetylene concentration.

Acetylene reduction, an assay for nitrogenase activity (nitrogen:(acceptor) oxidoreductase, EC 1.7.99.2), Is dependent on the ratio of the two protein components of nitrogenase as well as on C2H2 concentration. As the component I : component II ratio (based on activity) is increased, the C2H2 reduction : N2 fixation ratio decreases to a minimum of 3.4 and then increases. The minimum is found at a ratio near 1 : 1. At a component I : component II ratio of 20 : 1, the C2H2 reduction : N2 fixation ratio is 5.3. Acetylene exhibits substrate inhibition in assays for nitrogenase activity. Both the apparent Km and Ki for acetylene vary as a function of the relative concentrations of components I and II present in the assay. When the more labile component II is limiting in the assay and "saturating" levels of C2H2 (above 0.1 atm) are used, N2-fixation capacity may be greatly under-estimated.

Acetylene↗
PubMed2003

Density functional calculations on the binding of dinitrogen to the FeFe cofactor in Fe-only nitrogenase: FeFeco(mu 6-N2) as intermediate in nitrogen fixation.

The geometries and stabilities of the FeFe cofactor at different oxidation states and its complexes with N(2) have been determined by density functional calculations. These calculations support an EPR-inactive resting state of the FeFe cofactor with four Fe(2+) and four Fe(3+) sites (4Fe(2+)4Fe(3+)). FeFeco(mu(6)-N(2)) with a central dinitrogen ligand is predicted to be the most stable complex of the FeFe cofactor with N(2). It is easily formed by penetration of N(2) into the trigonal Fe(6) prism of the FeFe cofactor with an approximate barrier of 4 kcal mol(-1). The present DFT results suggest that an FeFeco(mu(6)-N(2)) entity is a plausible intermediate in dinitrogen fixation by nitrogenase. CO is calculated to bind even more strongly than N(2) to the FeFe cofactor so that CO may inhibit the reduction of nitrogen by Fe-only nitrogenase.

Iron↗