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and synthesis of active enzyme

nif V

Homocitrate synthase; organic component of FeMo cofactor

nif N

Subunit of nif N2E2, which provides a transient site for the assembly of FeMo cofactor

nif E

Subunit of nif N2E2

nif B

FeMo cofactor precursor biosynthesis

nifQ

Early step in FeMo cofactor biosynthesis

nif X

Intermediate carrier in FeMo cofactor biosynthesis

nif Y

Intermediate carrier in FeMo cofactor biosynthesis

nif W

Required for synthesis of fully active dinitrogenase

nif Z

Required for synthesis of fully active dinitrogenase

nif A

Positive regulatory protein

nif L

Negative regulatory protein

nif T

Unknown function

naf Y

Probably an intermediate carrier in FeMo cofactor biosynthesis

Modified from Dean and Jacobsen (1992) by permission of the publisher.

Modified from Dean and Jacobsen (1992) by permission of the publisher.

FIGURE 14.2 N2-fixing heterocystous cyanobacteria recovered from the water fern Azolla. Heterocysts are designated with the letter "H". Note the high percentage of heterocysts relative to vegetative cells. This reflects a high rate of N2 fixation by the cyanobacterium in the symbiotic state with the Azolla. Courtesy of J. C. Meeks; used by permission.

FIGURE 14.2 N2-fixing heterocystous cyanobacteria recovered from the water fern Azolla. Heterocysts are designated with the letter "H". Note the high percentage of heterocysts relative to vegetative cells. This reflects a high rate of N2 fixation by the cyanobacterium in the symbiotic state with the Azolla. Courtesy of J. C. Meeks; used by permission.

maintains a low O2 concentration inside the cell. If the O2 level increases quickly, however, nitrogenase interacts with other redox-sensitive proteins in the cell that invoke a conformational change that inactivates nitrogenase and prevents its irreversible damage.

Because NH3, the primary product of N2 fixation, is toxic to cells in high concentrations, N2-fixing bacteria possess highly efficient mechanisms of NH3 assimilation that prevent its accumulation. In most diazotrophs the enzyme combination of glutamine synthetase (GS) and glutamate synthase (GOGAT) carries out this task (Eqs. (1) and (2)):

Glutamate + NH3 + ATP -Glutaimne syilthetase : Glutamine + ADP + P.

Ketoglutarate + Glutamine + NADH-G°-GA-T—, 2 Glutamine + NADox

As can be seen both ATP and reductant are required at this stage of NH3 assimilation and add an additional energy burden to the N2-fixing organism. Because BNF is energetically expensive, bacteria do not usually fix N2 in the presence of reactive N sources. A number of mechanisms have been identified whereby nitrogenase activity is inhibited, and nif gene expression is down-regulated in response to increasing levels of NO-, NH+, and/or amino acid N in the environment (Merrick, 1992).

Many attempts have been made to quantify BNF in both natural and agricultural ecosystems, and the values vary widely (Table 14.5). Two methods are generally

TABLE 14.5 Range of Rates of BNF Measured under Field Conditions by Different Diazotrophic Organisms and Their Associations

Diazotrophic bacteria and their associations

Free-living bacteria (associated with wood decay, straw decomposition, cyanobacterial mats)

Examples of plant-cyanobacterial associations Cryolithic crusts Azolla

Examples of legume-rhizobial associations Soybean Beans Alfalfa White clover

Examples of nonlegume-Frankia associations Alder Ceanothus Hippophae

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