Novel mutant of Anabaena sp. strain CA which growns on N2 but not on combined nitrogen

A mutant has been isolated from Anabaena sp. strain CA by treatment with N-methyl-N'-nitro-N-nitrosoguanidine, which has the unusual phenotypic characteristic of growth only under N2-fixing conditions. Growth of the mutant was completely inhibited by NO3- or NH4+ at concentrations routinely used for growth of the wild type, and sensitivity to NH4+ was especially pronounced. The inhibitory effect of NH4+ could not be overcome by glutamine, glutamate, or casein hydrolysate. Ammonia had no immediate inhibitory effect on protein synthesis, CO2 fixation, or O2 evolution, and the gradual inhibition of C2H2 reduction activity by NH4+ resembled a repression phenomenon. The glutamine synthetase activity of N2-fixing cultures appeared normal, yet the mutant was incapable of utilizing exogenous NH4+ for growth. Preliminary evidence suggests a possible alteration of glutamine synthetase, which could result in sensitivity to exogenous NH4+ by progressive inactivation of the enzyme or repression of its synthesis.

Activation of ribulose 1,5-bisphosphate carboxylase from Rhodopseudomonas sphaeroides: probable role of the small subunit

The activation properties of the form I and form II ribulose 1,5-bisphosphate carboxylases from Rhodopseudomonas sphaeroides were examined. Both enzymes have a requirement of Mg2+ for optimal activity. Mn2+, Ni2+, and Co2+ can also support activity of the form I enzyme, whereas only Mn2+ can substitute for Mg2+ with the form II enzyme. The effect of different preincubations on the carboxylase reaction was also examined. Both enzymes exhibited a lag when preincubated with other than Mg2+ and CO2 before assay, but the lag was much more pronounced and the rate of the reaction was slower with the form I enzyme under these conditions. Activation of the form I carboxylase By Mg2+ and CO2 occurred more rapidly than that of the form II enzyme. The results obtained with the two distinct forms of carboxylase from R. sphaeroides, as well as studies with the spinach and Rhodospirillum rubrum enzymes, thus indicate that the presence of the small subunit affects the rate of activation by Mg2+ and CO2 as well as the rate of reactivation of ribulose bisphosphate-inactivated enzyme.

Synthesis of nitrogenase and heterocysts by Anabaena sp. CA in the presence of high levels of ammonia

Anabaena sp. CA fails to synthesize heterocysts and nitrogenase when grown with KNO3 as the nitrogen source. By contrast, both heterocysts and proheterocysts are synthesized in NH4Cl-containing media to a level nearly commensurate with cells grown in the absence of combined nitrogen. The growth rate of the organism in NH4Cl-containing media was similar to that obtained with KNO3 as the nitrogen source and was independent of the presence of N2 in the atmosphere. Thus, our results indicate that the organism assimilated nitrate and ammonium nitrogen equally well to meet the nitrogen requirements for growth. Moreover, in contrast to previous studies with other cyanobacteria, the repressor singal for heterocyst differentiation in Anabaena sp. CA is not derived from the metabolism of ammonia but appears to be involved with nitrate metabolism. Nitrogenase activity was partially expressed in NH4Cl-grown cultures. Increasing the level of nitrogenase activity to a value representative of a N2-grown culture required both the inhibition of ammonia assimilation and de novo protein synthesis. An increase in the number of mature heterocysts was not required. The fact that high levels of exogenous ammonia only partially repress the synthesis of proteins required for the maximum expression of nitrogenase activity in Anabaena sp. CA has important implications.

Carbon dioxide assimilation in cyanobacteria: regulation of ribulose, 1,5-bisphosphate carboxylase

Cyanobacteria assimilate carbon dioxide through the Calvin cycle and therefore must regulate the activity of ribulose 1,5-bisophosphate carboxylase. Using an in situ assay, as well as measuring the activity in crude, partially purified, and homogeneous preparations, we can show that a number of phosphorylated intermediates exert a regulatory role. Three diverse organisms, Agmenellum quadruplicatum, Aphanocapsa 6714, and Anabaena sp. CA, were studied, and it was found that the in situ and cell-free carboxylase activities were particularly affected by low levels of phosphogluconate and reduced nicotinamide adenine dinucleotide phosphate. There was a marked activation by these ligands when the inactive enzyme was assayed in the presence of low levels of bicarbonate, a result significantly different from a previous report. Moreover, the fully activated enzyme was inhibited by phosphogluconate. In situ Anabaena CA carboxylase activity exhibited a particular capacity for activation by phosphogluconate and reduced nicotinamide adenine dinucleotide phosphate. However, activation of the crude, partially purified, or homogeneous Anabaena CA carboxylase by phosphogluconate and reduced nicotinamide adenine dinucleotide phosphate was significantly decreased when compared with enzyme activity in permeabilized cells. It appears that the microenvironment or the conformation of the enzyme within the cell may be significantly different from that of the isolated enzyme.

Mutants of Anabaena strain CA altered in their ability to grow under nitrogen-fixing conditions

Mutants of Anabaena strain CA impaired in nitrogenase activity and growth on N2 were isolated and characterized. One mutant was selected for resistance to L-methionine-DL-sulfoximine, and others were selected for resistance to DL-7-azatryptophan or for requirements for combined nitrogen. The mutants varied in sensitivity of growth and nitrogenase activity to atmospheric 02. Several of the mutants whose growth on N2 was impaired under aerobic conditions could grow and reduce acetylene at rates comparable to the wild type when grown microaerobically under N2-CO2 (99:1). The acetylene reduction activity of some of the strains grown under N2-CO2 was immediately and completely lost upon exposure to atmospheric O2, but in at least one strain this loss was reversed when the O2 concentration was lowered, even after 10 h of exposure to air. The characteristics of the O2-sensitive mutants suggest that there may be several sites sensitive to O2 and that the protective mechanism involves several different phenomena.

Activation and regulation of ribulose bisphosphate carboxylase-oxygenase in the absence of small subunits

Ribulose 1,5-bisphosphate carboxylase from Rhodospirillum rubrum requires CO2 and Mg2+ for activation of both CO2, both the carboxylase and oxygenase activities are stimulated by 6-phoshpo-D-gluconate, fructose 1,6-bisphosphate, 2-phosphoglycolate, 3-phosphoglycerate, NADPH, and fructose 6-phosphate. The carboxylase activity is not activated by ribose 5-phosphate. The substrate, ribulose bisphosphate, neither activates nor inhibits the CO2 and Mg2+ activation of this enzyme. Activation by CO2 and Mg2+ is rapid and results in increased susceptibility to active-site-directed protein modification reagents. Because the R. rubrum carboxylase-oxygenase is a dimer of large subunits and contains no small subunits, these results suggest that the effector binding sites of the higher plant enzyme may also be found on the large subunit.

Differential effects of metal ions on Rhodospirillum rubrum ribulosebisphosphate carboxylase/oxygenase and stoichiometric incorporation of HCO3- into a cobalt(III)--enzyme complex

Mg2+ or Mn2+ ions supported both the carboxylase and oxygenase activities of the Rhodospirillum rubrum ribulosebisphosphate carboxylase/oxygenase. For the carboxylase reaction, Mn2+ supported 25% of the maximum activity obtained with Mg2+; oxygenase activity, however, was twice as great with Mn2+ as compared to that with Mg2+. A further differential effect was obtained with Co2+. Co2+ did not support carboxylase activity and, in fact, was a strong inhibitor of Mg2+-dependent carboxylase activity, with a Ki of 10 microM. Co2+ did, however, support oxygenase activity, eliciting about 40% of the Mg2+-dependent oxygenase activity. No other divalent cations supported either activity. With high concentrations of Mg2+ or Mn2+, maximum carboxylase activity was seen after a 5-min activation period; activity decreased to about half of maximum after 30-min activation. A similar time dependence of activation was observed with Mn2+-dependent oxygenase activity but was not seen for Mg2+- or Co2+-dependent activity. Both carboxylase and oxygenase activities were inactivated by the oxidation of Co2+ to Co(III) with the resultant formation of a stable Co(III)--enzyme complex. In the presence of HCO3- (CO2), Co(III) modification was stoichiometric, with two cobalt atoms bound per enzyme dimer. Carbon dioxide was also incorporated into this Co(III)--enzyme complex, but only one molecule per enzyme dimer was bound, indicative of half-the-sites activity. These results thus indicate that there are substantial differences in the metal ion sites of the carboxylase and oxygenase activities of R, rubrum ribulosebisphosphate carboxylase/oxygenase.

The purification of glutamine synthetase from Azotobacter and other procaryotes by blue sepharose chromatography

We report the facile purification of glutamine synthetase (L-glutamate: ammonia ligase (adenosine 5'-diphosphate-forming), EC 6.3.1.2) in both the adenylylated and unadenylylated form, from Azotobacter vinelandii ATCC 12837. A general affinity column, which used as an affinity ligand Reactive blue 2 dye (Cibacron blue) covalently linked to Agarose, was employed as an efficient first step of purification. Further purification to electrophoretic homogeneity employed DEAE-cellulose chromatography and an additional Affigel chromatographic step. The method was used successfully to prepare glutamine synthetase from Escherichia coli, Rhodopseudomonas sphaeroides and Anabaena sp. strain CA.

Control of heterocyst and nitrogenase synthesis in cyanobacteria

The development of the heterocyst by filamentous nitrogen-fixing cyanobacteria provides an attractive model system for studying cellular differentiation. Heterocyst synthesis is repressed by the presence of exogenous combined nitrogen. In this report, it is shown that the tryptophan analog, D,L-7-azatryptophan (Aza-T), is capable of relieving the repressive effect of exogenous NH4NO3 on heterocyst and nitrogenase synthesis. In nitrogen-fixing cultures, the presence of 20 micron Aza-T increases the heterocyst frequency twofold. The glutamate analog, L-methionine-D,L-sulfoximine (MSX), has also been shown to cause a derepression in the synthesis of heterocysts and nitrogenase. However, unlike MSX, Aza-T does not appear to exert its effects by inhibiting the activity of glutamine synthetase. Therefore, glutamine synthetase may not be the sole key to the derepression of heterocyst and nitrogenase development in the cyanobacteria. It is hoped that a study of Aza-T action may lead to the elucidation of a novel control mechanism.

Modification of Rhodospirillum rubrum ribulose bisphosphate carboxylase with pyridoxal phosphate. 2. Stoichiometry and kinetics of inactivation

Rhodospirillum rubrum ribulose bisphosphate carboxylase contains two high affinity binding sites for pyridoxal phosphate and two catalytic sites per dimer. However, pyridoxal phosphate binding at only one site is sufficient for inactivation of both catalytic sites. In the presence of 20 mM bicarbonate, 10 mM magnesium, and pyridoxal phosphate, the rates of inactivation and Schiff base formation are pseudo-first-order and show saturation kinetics. These observations provide additional evidence that pyridoxal phosphate binds at the active site of the R. rubrum carboxylase. It is also proposed that the large subunit may contain regulatory as well as catalytic properties.

Modification of Rhodospirillum rubrum ribulose bisphosphate carboxylase with pyridoxal phosphate. 1. Identification of a lysyl residue at the active site

Ribulose 1,5-bisphosphate carboxylase isolated from Rhodospirillum rubrum was strongly inhibited by low concentrations of pyridoxal 5'-phosphate. Activity was protected by the substrate ribulose bisphosphate and to a lesser extent by other phosphorylated compounds. Pyridoxal phosphate inhibition was enhanced in the presence of magnesium and bicarbonate, but not in the presence of either compound alone. Concomitant with inhibition of enzyme activity, pyridoxal phosphate forms a Schiff base with the enzyme which is reversible upon dialysis and reducible with sodium borohydride. Subsequent to reduction of the Schiff base with tritiated sodium borohydride, tritiated N6-pyridoxyllysine could be identified in the acid hydrolysate of the enzyme. Only small amounts of this compound were present when the reduction was performed in the presence of carboxyribitol bisphosphate, an analogue of the intermediate formed during the carboxylation reaction. Therefore, it is concluded that pyridoxal phosphate modifies a lysyl residue close to or at the active site of ribulose bisphosphate carboxylase.

Carbon isotope fractionation by ribulose-1,5-bisophosphate carboxylase from various organisms

Carbon isotope fractionation by structurally and catalytically distinct ribulose-1,5-bisphosphate carboxylases from one eucaryotic and four procaryotic organisms has been measured under nitrogen. The average fractionation for 40 experiments was -34.1 per thousand with respect to the delta(13)C of the dissolved CO(2) used, although average fractionations for each enzyme varied slightly: spinach carboxylase, -36.5 per thousand; Hydrogenomonas eutropha, -38.7 per thousand; Agmenellum quadruplicatum, -32.2 per thousand; Rhodospirillum rubrum, -32.1 per thousand; Rhodopseudomonas sphaeroides peak I carboxylase, -31.4 per thousand; and R. sphaeroides peak II carboxylase, -28.3 per thousand. The carbon isotope fractionation value was largely independent of method of enzyme preparation, purity, or reaction temperature, but in the case of spinach ribulose-1,5-bisphosphate carboxylase fractionation, changing the metal cofactor used for enzyme activation had a distinct effect on the fractionation value. The fractionation value of -36.5 per thousand with Mg(2+) as activator shifted to -29.9 per thousand with Ni(2+) as activator and to -41.7 per thousand with Mn(2+) as activator. These dramatic metal effects on carbon isotope fractionation may be useful in examining the catalytic site of the enzyme.

Isolation and preliminary characterization of two forms of ribulose 1,5-bisphosphate carboxylase from Rhodopseudomonas capsulata

The presence of two distinct forms of ribulose 1,5-bisphosphate carboxylase has been demonstrated in extracts of Rhodopseudomonas capsulata, similar to the form I (peak I) and form II (peak II) carboxylases previously described from R. sphaeroides (J. Gibson and F. R. Tabita, J. Biol. Chem 252:943-949, 1977). The two activities, separated by diethylaminoethyl-cellulose chromatography, were shown to be of different molecular size after assay on polyacrylamide gels. The higher-molecular-weight carboxylase from R. capsulata was designated form I-C, whereas the smaller enzyme was designated form II-C. Catalytic studies revealed significant differences between the two enzymes in response to pH and the effector 6-phosphogluconate. Immunological studies with antisera directed against the carboxylases from R. sphaeroides demonstrated antigenic differences between the two R. capsulata enzymes; cross-reactivity was observed only between R. sphaeroides anti-form II serum and the corresponding R. capsulata enzyme, form II-C.

Nitrogen and ammonia assimilation in the cyanobacteria: purification of glutamine synthetase from Anabaena sp. strain CA

Glutamine synthetase was purified from the cyanobacterium Anabaena sp. strain CA, a newly isolated marine organism. This organism grows rapidly under nitrogen-fixing conditions and therefore is ideally suited for studies concerning cyanobacterial nitrogen metabolism. Studies were conducted to optimize the production of glutamine synthetase by Anabaena CA. The highest specific activities were obtained from cells grown in the presence of atmospheric N(2) or KNO(3) (13 mM); when NH(4)Cl was used as the nitrogen source, the specific activity was reduced by approximately 40%. Furthermore, through the use of a whole-cell gamma-glutamylhydroxamate transferase assay, it was found that the maximum number of enzyme units is obtained in the late logarithmic stage of growth. Glutamine synthetase purification requires only three steps and results in a preparation that is electrophoretically homogeneous. The transferase specific activity (units per milligram of protein) of the purified enzyme is 78, whereas the biosynthetic specific activity is 2.2. The molecular weight of the native protein was found to be approximately 590,000, and the subunit molecular weight was determined to be about 50,000. Thus, this cyanobacterial enzyme closely resembles the enzyme obtained from other procaryotic sources, at least with regard to size. The purification of glutamine synthetase from Anabaena CA should stimulate a more detailed study of this enzyme and its role in cyanobacterial nitrogen metabolism.

Different molecular forms of D-ribulose-1,5-bisphosphate carboxylase from Rhodopseudomonas sphaeroides

Ribulose-1,5-bisphosphate (Rbu-P2) carboxylase isolated from Rhodopseudomonas sphaeroides 2.4.1.Ga was separated into two different forms by DEAE-cellulose column chromatography. Both forms, designated Peak I and Peak II have been purified to homogeneity by the criterion of polyacrylamide disc-gel electrophoresis. The Peak I carboxylase has a molecular weight of 550,000, while the Peak II carboxylase is a smaller protein having a molecular weight of approximately 360,000. Sodium dodecyl sulfate electrophoresis revealed a large subunit for both enzymes which migrates similarly to the large subunit of spinach Rbu-P2 carboxylase. The Peak I enzyme also exhibited a small subunit having a molecular weight of 11,000. No evidence for a smaller polypeptide was found associated with the Peak II enzyme. Antisera prepared against the Peak I enzyme inhibited Peak I enzymatic activity, but had no effect on the activity of the Peak II enzyme. The two enzymes exhibited marked differences in catalytic properties. The Peak I enzyme exhibits optimal activity at pH 8.0 and is inhibited by low concentrations of 6-phosphogluconate, while the Peak II enzyme has a pH optimum of 7.2 and is relatively insensitive to 6-phosphogluconate.

Molecular and catalytic properties of ribulose 1,5-bisphosphate carboxylase from the photosynthetic extreme halophile Ectothiorhodospira halophila

D-Ribulose 1,5-bisphosphate (RuBP) carboxylase has been purified from the photosynthetic extreme halophile Ectothiorhodospira halophila. Despite a growth requirement for almost saturating sodium chloride in the medium, both crude and homogeneous preparations of RuBP carboxylase obtained from this organism were inhibited by salts. Sedimentation equilibrium analyses showed the enzyme to be large (molecular weight: 601,000). The protein was composed of two types of polypeptide chains of 56,000 and of 18,000 daltons. The small subunit appeared to be considerably larger than the small subunit obtained from the RuBP carboxylase isolated from Chromatium, an organism related to E. halophila. Amino acid analyses of hydrolysates of both E. halophilia and Chromatium RuBP carboxylases were very similar. Initial velocity experiments showed that the E. halophila RuBP carboxylase had a Km for ribulose diphosphate of 0.07 mM and a Km for HCO3- of 10 mM. Moreover, 6-phospho-D-gluconate was found to markedly inhibit the E. halophila carboxylase; a Ki for phosphogluconate of 0.14 mM was determined.

Carbon dioxide assimilation in blue-green algae: initial studies on the structure of ribulose 1,5-bisphosphate carboxylase

D-Ribulose 1,5-bisphosphate carboxylase was purified from the blue-green alga Anabaena cylindrica (Lemm) by procedures involving acid precipitation, ammonium sulfate fractionation, and Sephadex G-200 gel filtration. The enzyme was homogeneous by the criterion of polyacrylamide disc gel electrophoresis and was a multimer of a single-size polypeptide chain of 54,000 daltons. The carboxylases from four species of blue-green algae (Anabaena, Nostoc strain MAC, Agmenellum quadruplicatum strain PR-6, and Anacystis nidulans strain TX20) were closely similar in molecular size, since enzyme activity was eluted at the same volume after sucrose gradient centrifugation. Further analysis by gel filtration indicated that the four blue-green algal carboxylases were nearly identical in molecular weight, ranging from 449 to 453,000. The amino acid composition of the Anabaena carboxylase was determined and was found to resemble closely the composition of the large subunit from eukaryotic photosynthetic organisms.