14C-labelling of glutamine synthetase and Fe protein of nitrogenase in toluene-treated cells of Rhodopseudomonas capsulata

Potential of Phototrophic Purple Nonsulfur Bacteria to Fix Nitrogen in Rice Fields

Biological nitrogen fixation catalyzed by Mo-nitrogenase of symbiotic diazotrophs has attracted interest because its potential to supply plant-available nitrogen offers an alternative way of using chemical fertilizers for sustainable agriculture. Phototrophic purple nonsulfur bacteria (PNSB) diazotrophically grow under light anaerobic conditions and can be isolated from photic and microaerobic zones of rice fields. Therefore, PNSB as asymbiotic diazotrophs contribute to nitrogen fixation in rice fields. An attempt to measure nitrogen in the oxidized surface layer of paddy soil estimates that approximately 6–8 kg N/ha/year might be accumulated by phototrophic microorganisms. Species of PNSB possess one of or both alternative nitrogenases, V-nitrogenase and Fe-nitrogenase, which are found in asymbiotic diazotrophs, in addition to Mo-nitrogenase. The regulatory networks control nitrogenase activity in response to ammonium, molecular oxygen, and light irradiation. Laboratory and field studies have revealed effectiveness of PNSB inoculation to rice cultures on increases of nitrogen gain, plant growth, and/or grain yield. In this review, properties of the nitrogenase isozymes and regulation of nitrogenase activities in PNSB are described, and research challenges and potential of PNSB inoculation to rice cultures are discussed from a viewpoint of their applications as nitrogen biofertilizer.

Short-term regulation of nitrogenase activity by NH4+ in Rhodobacter capsulatus: multiple in vivo nitrogenase responses to NH4+ addition

The photosynthetic bacterium Rhodobacter capsulatus has been shown to carry out nitrogenase "switch-off," a rapid, reversible inhibition of in vivo activity. Here, we demonstrate that highly nitrogen-limited cultures of both the wild-type strain and a draT draG mutant are capable of nitrogenase switch-off while moderately nitrogen-limited cultures show instead a "magnitude" response, with a decrease in in vivo nitrogenase activity that is proportional to the amount of added NH4+.

Posttranslational regulation of nitrogenase in Rhodobacter capsulatus: existence of two independent regulatory effects of ammonium

In the photosynthetic bacterium Rhodobacter capsulatus, nitrogenase activity is regulated by ADP-ribosylation of component II in response to the addition of ammonium to cultures or to the removal of light. The ammonium stimulus results in a fast and almost complete inhibition of the in vivo acetylene reduction activity, termed switch-off, which is reversed after the ammonium is exhausted. In the present study of the response of cells to ammonium, ADP-ribosylation of component II occurred but could not account for the extent and timing of the inhibition of activity. The presence of an additional response was confirmed with strains expressing mutant component II proteins; although these proteins are not a substrate for ADP-ribosylation, the strains continued to exhibit a switch-off response to ammonium. This second regulatory response of nitrogenase to ammonium was found to be synchronous with ADP-ribosylation and was responsible for the bulk of the observed effects on nitrogenase activity. In comparison, ADP-ribosylation in R. capsulatus was found to be relatively slow and incomplete but responded independently to both known stimuli, darkness and ammonium. Based on the in vitro nitrogenase activity of both the wild type and strains whose component II proteins cannot be ADP-ribosylated, it seems likely that the second response blocks either the ATP or the electron supply to nitrogenase.

Mutations affecting nitrogenase switch-off in Rhodobacter capsulatus

In vivo 'switch-off' and subsequent reactivation of nitrogenase activity in Rhodobacter capsulatus or Rhodospirillum rubrum in response to a variety of environmental stimuli, including the addition of fixed nitrogen, is thought to be due to the action of two nitrogenase Fe protein modifying activities; DRAT (dinitrogenase reductase ADP-ribosyl transferase) and DRAG (dinitrogenase reductase-activating glycohydrolase). Here it is demonstrated that strains, including one mutated in glnB, that constitutively express nif in the presence of fixed nitrogen are never-the-less capable of Fe protein modification. Thus the regulation of Fe protein modification is separate from that of its expression. The observations that Mn-deficient cultures are unable to fix nitrogen and that DRAG activity requires a divalent metal cation, most notably Mn2+, prompted the search for mutants (pseudo-prototrophs) capable of in vivo nitrogen fixation under Mn-deficient conditions. In the present study the isolation and partial characterization of several putative mutants is described. One, AF1, was shown to be altered in the in vivo regulation of N2ase activity in response to fixed nitrogen and to have an altered in vitro activity in glutamate grown cells. However, this strain was shown to possess in vitro DRAT activity and to have a modifiable Fe protein. Two-dimensional gel analysis indicates that this strain is altered in the synthesis of a 48 kDa protein of as yet unknown function. Thus, the mutation in this strain must affect, in an as yet undetermined manner, the response of the modifying system to fixed nitrogen.

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.

Reversible regulation of the nitrogenase iron protein from Rhodospirillum rubrum by ADP-ribosylation in vitro

Nitrogenase activity in the photosynthetic bacterium Rhodospirillum rubrum is reversibly regulated by interconversion of the Fe protein between a modified and an unmodified form. Since the discovery of the activation process in 1976, investigators have been unable to demonstrate the inactivation (modification) reaction in vitro. In this study, NAD-dependent modification and concomitant inactivation of the Fe protein were demonstrated in crude extracts of R. rubrum. Activation of the in vitro-modified Fe protein by activating enzyme and structural similarity between the in vivo and in vitro modifications are presented as evidence that the in vitro modification is the physiologically relevant ADP-ribosylation reaction. Using a partially purified preparation, we showed that the inactivating enzyme activity is stimulated by divalent metal ions and ADP, that O2-denatured Fe protein will not serve as a substrate, and that dithionite inhibits the modification reaction.

Role of glutamine synthetase adenylylation in the self-protection of Pseudomonas syringae subsp. "tabaci" from its toxin, tabtoxinine-beta-lactam

Selected pathovars of Pseudomonas syringae produce an extracellular phytotoxin, tabtoxinine-beta-lactam, that irreversibly inhibits its known physiological target, glutamine synthetase (GS). Pseudomonas syringae subsp. "tabaci" retains significant amounts of glutamine synthetase activity during toxin production in culture. As part of our investigation of the self-protection mechanism(s) used by these pathovars, we have determined that GS becomes adenylylated after toxin production is initiated and that the serine released from the zinc-activated hydrolysis of tabtoxin is a factor in the initiation of this adenylylation. The adenylylation state of this GS was estimated to range from E5.0-7.5. The irreversible inactivation by tabtoxinine-beta-lactam of unadenylylated and adenylylated glutamine synthetase purified from P. syringae subsp. "tabaci" was investigated. Adenylylated GS was inactivated by tabtoxinine-beta-lactam at a slower rate than was unadenylylated enzyme. Adenylylated GS (E7.5-10.5) was significantly protected from this inactivation in the presence of the enzyme effectors, AMP, Ala, Gly, His, and Ser. Thus, the combination of the adenylylation of GS after toxin production is initiated and the presence of the enzyme effectors in vivo could provide part of the self-protection mechanism used by subsp. "tabaci".

Regulation of nitrogen fixation in Rhodospirillum rubrum grown under dark, fermentative conditions

Rhodospirillum rubrum was shown to grow fermentatively on fructose with N2 as a nitrogen source. The nitrogenase activity of these cells was regulated by the NH4+ switch-off/switch-on mechanism in a manner identical to that for photosynthetically grown cells. In vitro, the inactive nitrogenase Fe protein from fermenting cells was reactivated by an endogenous membrane-bound, Mn2+-dependent activating enzyme that was interchangeable with the activating enzyme isolated from photosynthetic membranes.

Effect of ammonia, darkness, and phenazine methosulfate on whole-cell nitrogenase activity and Fe protein modification in Rhodospirillum rubrum

A procedure for the immunoprecipitation of Fe protein from cell extracts was developed and used to monitor the modification of Fe protein in vivo. The subunit pattern of the isolated Fe protein after sodium dodecyl sulfate-polyacrylamide gel electrophoresis was assayed by Coomassie brilliant blue protein staining and autoradiographic 32P detection of the modifying group. Whole-cell nitrogenase activity was also monitored during Fe protein modification. The addition of ammonia, darkness, oxygen, carbonyl cyanide m-chlorophenylhydrazone, and phenazine methosulfate each resulted in a loss of whole-cell nitrogenase activity and the in vivo modification of Fe protein. For ammonia and darkness, the rate of loss of nitrogenase activity was similar to that for Fe protein modification. The reillumination of a culture incubated in the dark brought about a rapid recovery of nitrogenase activity and the demodification of Fe protein. Cyclic dark-light treatments resulted in matching cycles of nitrogenase activity and Fe protein modification. Carbonyl cyanide m-chlorophenylhydrazone and phenazine methosulfate treatments caused an immediate loss of nitrogenase activity, whereas Fe protein modification occurred at a slower rate. Oxygen treatment resulted in a rapid loss of activity but only an incomplete modification of the Fe protein.

Mapping of Rhodopseudomonas capsulata nif genes

The endogenous gene transfer system of Rhodopseudomonas capsulata was used to analyze mutations which block the ability to use molecular nitrogen as the sole nitrogen source (nif). With this fine-structure mapping tool, linkage of nif mutations could be reliably established if separated by 2,700 base pairs or less. Eleven independent mutations were analyzed, and five linkage groups were found. The overall chromosomal arrangement of these groups awaits conjugational or physical analysis. A candidate for the inactive subunit of R. capsulata Fe protein was located in gels at a position of about 38,000 molecular weight, 5,000 more than that of the presumed active subunit.

Glutamine synthetase, glutamate synthase and glutamate dehydrogenase in Rhizobium japonicum strains grown in cultures and in bacteroids from root nodules of Glycine max

The growth yields of three strains of Rhizobium japonicum (CB 1809, CC 723, CC 705) in culture solutions containing L-glutamate were about twice those grown with ammonium. The activities of glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GDH; EC 1.4.1.4) were dependent on the nitrogen source in the medium and also varied with growth. Both NADPH-and NADH-dependent glutamate synthase (GOGAT; EC 1.4.1.13) and NADPH-dependent GDH were found in strains grown with either glutamate or ammonium but NADH-linked GDH was only detected in glutamate-grown cells. Glutamine synthetase was adenylylated in cells grown with NH 4 (+) (90%) and to lesser extent in those grown with L-glutamate (50%). In root nodules produced by the three strains in Glycine max (L.) Merr., the bulk of GS was located in the nodule cytosol (60-85%). The enzyme was adenylylated in bacteroids (43-75%) and in the nodule tissues (52-68%). The enzyme in cell-free extracts of Rh. japonicum (CC 705) grown in culture solutions containing glutamate and in bacteroids (CC 705) was deadenylylated by snake-venom phosphodiesterase. L-methionine-DL-sulfoximine restricted the incoporation of (15)N-labelled (NH4)2SO4 into cells of strains CB 1809 and CC 705, as well as in bacteroids of strain CC 705. It is noteworthy that appreciable activities for GDH were found in the free-living rhizobia grown on glutamate. Thus the presence of an enzyme does not necessarily imply that a particular pathway is operative in assimilating ammonium into cell nitrogen. Based on (15)N studies, the GS-GOGAT pathway of rhizobia (strains CB 1809 and CC 705) is important when grown in culture solutions as well as in bacteroids from root nodules of G. max.

H2 metabolism in photosynthetic bacteria and relationship to N2 fixation

The photosynthetic bacteria can evolve H2 in the light through a nitrogenase-mediated reaction. The nitrogenase enzyme in the photosynthetic bacteria is similar to other nitrogenases. It is made of two soluble components: a) the Fe protein (dinitrogenase reductase or Component II) which receives electrons from ferredoxin, and b) the Mo-Fe protein (dinitrogenase or Component I) on which the substrates (including protons) are reduced. In photosynthetic bacteria, the physiological regulation of nitrogenase activity involves inactivation by covalent modification of the nitrogenase Fe protein. This inactivation can be reversed by an activating factor (or activating enzyme) which is an extrinsic membrane protein. After an ammonia shock, both the Fe protein of nitrogenase, and the glutamine synthetase, become adenylylated in vivo. In the adenylylation state, glutamine synthetase has AMP moieties bound to the protein by phosphate linkage. In toluene-treated cells of Rhodopseudomas capsulata preincubated with radioactive ATP, labelled either by 14C on the adenine or by 32P on the P alpha of ATP and then submitted to an ammonia shock, the Fe protein becomes covalently labelled only with [14C]ATP ad not with [32P]alpha ATP, while glutamine synthetase becomes labelled with both radioactive ATP molecules. This indicates that a different type of linkage is involved in the binding of the modifying group to Fe protein and to glutamine synthetase. Like other N2 fixers, the photosynthetic bacteria also contain a hydrogenase. In R. capsulata, the hydrogenase is an intrinsic membrane protein which protrudes in the cytoplasmic space and is not accessible to anti-hydrogenase antibodies from the periplasmic side. The hydrogenase can transfer electrons from H2 to the electron transport chain. It functions physiologically as an uptake-hydrogenase and may contribute to the recycling of electrons to nitrogenase. In the presence of excess carbon compounds, its main role may be to maintain an anaerobic microenvironment for the nitrogenase. Ferredoxin has been isolated from photosynthetic bacteria. Rhodospirillum rubrum and Rhodopseudomonas capsulata each contain two different soluble ferredoxin molecules. Reduced Fd I from R. capsulata has been shown to donate its electrons to nitrogenase.