Control of nitrogenase in a photosynthetic autotrophic bacterium, Ectothiorhodospira sp

Poly(hydroxyalkanoic acid) Biosynthesis inEctothiorhodospirashaposhnikovii: Characterization and Reactivity of a Type III PHA Synthase

Ectothiorhodospira shaposhnikovii is able to accumulate polyhydroxybutyrate (PHB) photoautotrophically during nitrogen-limited growth. The activity of polyhydroxyalkanoate (PHA) synthase in the cells correlates with PHB accumulation. PHA synthase samples collected during the light period do not show a lag phase during in vitro polymerization. Synthase samples collected in the dark period displays a significant lag phase during in vitro polymerization. The lag phase can be eliminated by reacting the PHA synthase with the monomer, 3-hydroxybutyryl-CoA (3HBCoA). The PHA synthase genes (phaC and phaE) were cloned by screening a genomic library for PHA accumulation in E. coli cells. The PHA synthase expressed in the recombinant E. coli cells was purified to homogeneity. Both sequence analysis and biochemical studies indicated that this PHA synthase consists of two subunits, PhaE and PhaC and, therefore, belongs to the type III PHA synthases. Two major complexes were identified in preparations of purified PHA synthase. The large complex appears to be composed of 12 PhaC subunits and 12 PhaE subunits (dodecamer), whereas the small complex appears to be composed of 6 PhaC and 6 PhaE subunits (hexamer). In dilute aqueous solution, the synthase is predominantly composed of hexamer and has low activity accompanied with a significant lag period at the initial stage of reaction. The percentage of dodecameric complex increases with increasing salt concentration. The dodecameric complex has a greatly increased specific activity for the polymerization of 3HBCoA and a negligible lag period. The results from in vitro polymerizations of 3HBCoA suggest that the PHA synthase from E. shaposhnikovii may catalyze a living polymerization and demonstrate that two PhaC and two PhaE subunits comprise a single catalytic site in the synthase complex.

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.

Plasmid from photosynthetic bacterium Ectothiorhodospira Sp. carries a transposable streptomycin resistance gene

Centrifugation through a cesium chloride density gradient and agarose gel electrophoresis of the DNA from the purple non-sulfur photosynthetic bacterium Ectothiorhodospira sp. resolved a single extrachromosomal element, plasmid pDG1. Its size was estimated to be 13.2 kilobases by restriction endonuclease mapping. Plasmid pDG1 and two restriction fragments thereof were cloned in Escherichia coli C600 with plasmid pBR327 as a vector to form mixed plasmids pDGBR1, pDGBR2, and pDGBR3. The resistance to streptomycin and mercury found in Ectothiorhodospira sp. was transferred to E. coli C600 after transformation with pDGBR1 but not with pDGBR2 and pDGBR3. The replication origin of pDG1 was estimated to be within a 2-kilobase restriction fragment of pDG1 by monitoring its replication in E. coli HB101, using a kanamycin resistance reporter gene. High stringency molecular hybridization with 32P-labeled pDG1 identified specific fragments of genomic DNA, suggesting the integration of some plasmid sequences. In accordance with the hypothesis that this integration is due to a transposon, we tested the transfer of streptomycin resistance from pDG1 into plasmid pVK100 used as a target. For this test, we regrouped in the same cells of E. coli HB101, pDGBR1 and mobilizable plasmid pVK100 (tetr,kmr). We used the conjugation capacity of the pVK100/pRK2013 system to rescue the target plasmid pVK100 into nalidixic acid-resistant E. coli DH1. The transfer frequency of streptomycin resistance into pVK100 was 10(-5), compatible with a transposition event. In line with the existence of a transposon on pDG1, heteroduplex mapping indicated the presence of inverted repeats approximately 7.5 kb from one another.

Ammonium inhibition of nitrogenase activity in Herbaspirillum seropedicae

The effect of oxygen, ammonium ion, and amino acids on nitrogenase activity in the root-associated N2-fixing bacterium Herbaspirillum seropedicae was investigated in comparison with Azospirillum spp. and Rhodospirillum rubrum. H. seropedicae is microaerophilic, and its optimal dissolved oxygen level is from 0.04 to 0.2 kPa for dinitrogen fixation but higher when it is supplied with fixed nitrogen. No nitrogenase activity was detected when the dissolved O2 level corresponded to 4.0 kPa. Ammonium, a product of the nitrogenase reaction, reversibly inhibited nitrogenase activity when added to derepressed cell cultures. However, the inhibition of nitrogenase activity was only partial even with concentrations of ammonium chloride as high as 20 mM. Amides such as glutamine and asparagine partially inhibited nitrogenase activity, but glutamate did not. Nitrogenase in crude extracts prepared from ammonium-inhibited cells showed activity as high as in extracts from N2-fixing cells. The pattern of the dinitrogenase and the dinitrogenase reductase revealed by the immunoblotting technique did not change upon ammonium chloride treatment of cells in vivo. No homologous sequences were detected with the draT-draG probe from Azospirillum lipoferum. There is no clear evidence that ADP-ribosylation of the dinitrogenase reductase is involved in the ammonium inhibition of H. seropedicae. The uncoupler carbonyl cyanide m-chlorophenylhydrazone decreased the intracellular ATP concentration and inhibited the nitrogenase activity of whole cells. The ATP pool was not significantly disturbed when cultures were treated with ammonium in vivo. Possible mechanisms for inhibition by ammonium of whole-cell nitrogenase activity in H. seropedicae are discussed.

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.

Fluorometric assay for ADP-ribosylarginine cleavage enzymes

A continuous fluorometric assay for enzyme activities which remove ADP-ribose linked to proteins at arginine was developed. The substrate analog, N alpha-dansyl-N omega-(1,N6-etheno-ADP-ribosyl)arginine methyl ester, was used to assay the catalytic activities of dinitrogenase reductase activating glycohydrolase from Rhodospirillum rubrum and nucleotide pyrophosphatase from Crotalus adamanteus. The assay is based on the increase in fluorescent emission by ethenoadenine accompanying the enzyme-catalyzed hydrolysis of the substrate. The assay has been used to detect activities of 10 fmol substrate cleaved per minute. The substrate anomerizes to give a 40:60 equilibrium of alpha:beta ribosylguanidinium anomers, allowing the determination of enzyme stereospecificity. The substrate was used to determine the kinetic parameters and products of the N-glycohydrolase and the pyrophosphatase.

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.

Regulation of nitrogenase activity by covalent modification in Chromatium vinosum

Nitrogenase in Chromatium vinosum was rapidly, but reversibly inhibited by NH4+. Activity of the Fe protein component of nitrogenase required both Mn2+ and activating enzyme. Activating enzyme from Rhodospirillum rubrum could replace Chromatium chromatophores in activating the Chromatium Fe protein, and conversely, a protein fraction prepared from Chromatium chromatophores was effective in activating R. rubrum Fe protein. Inactive Chromatium Fe protein contained a peptide covalently modified by a phosphate-containing molecule, which migrated the same in SDS-polyacrylamide gels as the modified subunit of R. rubrum Fe protein. In sum, these observations suggest that Chromatium nitrogenase activity is regulated by a covalent modification of the Fe protein in a manner similar to that of R. rubrum.

Adenine nucleotide levels in Rhodospirillum rubrum during switch-off of whole-cell nitrogenase activity

Adenine nucleotide pools were measured in Rhodospirillum rubrum cultures that contained nitrogenase. The average energy charge [([ATP] + 1/2[ADP])/([ATP] + [ADP] + [AMP])] was found to be 0.66 and 0.62 in glutamate-grown and N-limited cultures respectively. Treatment of glutamate-grown cells with darkness, ammonia, glutamine, carbonyl cyanide m-chlorophenylhydrazone, or phenazine methosulphate resulted in perturbations in the adenine nucleotide pools, and led to loss of whole-cell nitrogenase activity and modification in vivo of the Fe protein. Treatment of N-limited cells resulted in similar changes in adenine nucleotide pools but not enzyme modification. No correlations were found between changes in adenine nucleotide pools or ratios of these pools and switch-off of nitrogenase activity by Fe protein modification in vivo. Phenazine methosulphate inhibited whole-cell activity at low concentrations. The effect on nitrogenase activity was apparently independent of Fe protein modification.

Oxido-reduction of B800-850 and B880 holochromes isolated from three species of photosynthetic bacteria as studied by electron-paramagnetic resonance and optical spectroscopy

Certain redox properties of bacteriochlorophyll alpha were used to probe the structure of several light-harvesting pigment-protein complexes or holochromes. To attribute redox properties unequivocally to a given holochrome, we worked with purified holochromes. We developed purification procedures for the B880 holochromes from Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Ectothiorhodospira sp. and for the B800-850 holochromes from the latter two species. In all these holochromes, bacteriochlorophyll alpha could be oxidized by ferricyanide as witnessed by the bleaching of their near-infrared absorption bands. However, only in B880 holochromes was this oxidation reversible. Another important difference between the B800-850 and the B880 holochromes is that oxidation of the latter gives rise to a g = 2.0025 electron paramagnetic resonance (EPR) signal with linewidth varying, according to species, from 0.37 mT to 0.48 mT. Both the reversible EPR signal and absorption changes titrate with a midpoint redox potential (pH 8.0) of approximately 570 mV. Linewidth narrowing can be interpreted by delocalization of the free electron spin over approximately 12 bacteriochlorophyll molecules. While the B880 holochromes from the three species considered had indistinguishable redox properties, the B800-850 holochromes differed from one another by their circular dichroic spectra and by the relative ease of oxidation of their 800-nm and 850-nm bands. This indicates that, contrary to the B880 holochromes, the B800-850 holochromes may not form a homogeneous class.

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.