Antigenic analysis of Methanomicrobiales and Methanobrevibacter arboriphilus.

Preparation of new antisera has permitted more comprehensive immunological analyses of the two families of Methanomicrobiales. Methanomicrobiaceae and Methanosarcinaceae, and the species Methanobrevibacter arboriphilus. Immunological analysis was carried out with antibody probes against 23 strains, including almost all genera and species of methanogens. The absence of cross-reactions between families of methanogens was confirmed. Methanomicrobium and Methanogenium were found to be immunologically related. Extensive cross-reactions occurred among six strains of Methanosarcinaceae, but none occurred among three strains of M. arboriphilus when tested with the S probe, i.e., the last antiserum dilution of the titration curve's plateau.     

Characterization of a Heme-Dependent Catalase from Methanobrevibacter arboriphilus

Recently it was reported that methanogens of the genus Methanobrevibacter exhibit catalase activity. This was surprising, since Methanobrevibacter species belong to the order Methanobacteriales, which are known not to contain cytochromes and to lack the ability to synthesize heme. We report here that Methanobrevibacter arboriphilus strains AZ and DH1 contained catalase activity only when the growth medium was supplemented with hemin. The heme catalase was purified and characterized, and the encoding gene was cloned. The amino acid sequence of the catalase from the methanogens is most similar to that of Methanosarcina barkeri.     

Characterization of a heme-dependent catalase from Methanobrevibacter arboriphilus.

Recently it was reported that methanogens of the genus Methanobrevibacter exhibit catalase activity. This was surprising, since Methanobrevibacter species belong to the order Methanobacteriales, which are known not to contain cytochromes and to lack the ability to synthesize heme. We report here that Methanobrevibacter arboriphilus strains AZ and DH1 contained catalase activity only when the growth medium was supplemented with hemin. The heme catalase was purified and characterized, and the encoding gene was cloned. The amino acid sequence of the catalase from the methanogens is most similar to that of Methanosarcina barkeri.     

A diphytanyl ether analog of phosphatidylserine from a methanogenic bacterium, Methanobrevibacter arboriphilus

Several ninhydrin-positive lipids were found in methanogenic bacteria and the structure of one of them, designated as PNL2 from Methanobrevibacter arboriphilus, was identified as a diphytanyl ether analog of phosphatidylserine. The chromatographic behavior of the lipid on thin-layer plates and on a DEAE-cellulose column was identical to the ester form of phosphatidylserine. The infrared spectra showed the presence of amino, carboxyl, ether, and phosphate groups, and the absence of an ester linkage. The hydrophobic portion of the lipid was identified as diphytanyl glycerol diether on the basis of the mass spectrum of the acetolysis product and gas-liquid chromatography of the iodinated alkyl chain prepared by hydroiodic acid cleavage of PNL2. The fast atom bombardment-ionization and field desorption mass spectrum provided a molecular weight of 819 and several fragment ions consistent with the proposed structure. Hydrofluoric acid hydrolysis resulted in water-soluble products including serine, phosphoserine, and ammonia, which accounted for 95% of hydrolyzed PNL2. The lipid product of the hydrolysis was mainly the diether form of phosphatidic acid. This is the first report on the structural characterization of an amino-containing phospholipid in archaebacteria. Amino lipids have been found in many other methanogenic bacteria.     

Cellulose fermentation by continuous cultures of Ruminococcus albus and Methanobrevibacter smithii

Summary The hydrolysis and fermentation of cellulose (Avicel) by continuous cultures of Ruminococcus albus strain 7 and Methanobrevibacter smithii strain PS were studied. Cellulose destruction ranged from ca. 22% to 71% for 0.25 to 2.27 days solids retention time, respectively. The cellulose hydrolysis rate constant (k) was 1.3 days^–1. Concentrations of soluble reducing sugars were low, showing that cellulose hydrolysis was the rate-limiting step of cellulose fermentation. The estimated methane-based molar growth yield for M. smithii was 2.8 g mol^–1. Its maximum specific growth rate was ca. 4 days^–1. The dissolved H₂ half-saturation constant (K _s ) for methanogenesis was ca. 1 M. The final products of the co-culture were primarily acetate, CH₄ and CO₂ and low levels of ethanol and H₂. The co-culture produced more H₂ (used for reduction of CO₂ to CH₄) and acetate than a monoculture of R. albus. These differences coulb be accounted for by the lower production of ethanol, confirming to the theory of interspecies H₂ transfer. Offprint requests to: M. J. Wolin     

Purification and characterization of Fe-containing superoxide dismutase from Methanobrevibacter arboriphilus strain AZ

Superoxide dismutase (SOD) was purified from cells of the strict anaerobic methanogenic archaeon Methanobrevibacter arboriphilus strain AZ. The four-step purification procedure resulted in enzyme with specific activity of 3970 units/mg and yield of 22%. It was shown that the SOD is a Fe-containing homotetramer composed of subunits of 21.2 kD each. Sodium azide (13.5 mM), unlike KCN, inhibits the activity of the SOD. Hydrogen peroxide (0.5 mM) inactivates the enzyme, which is consistent with the properties of the known Fe-containing SODs from methanogenic Archaea.     

F420H2 oxidase (FprA) from Methanobrevibacter arboriphilus, a coenzyme F420-dependent enzyme involved in O2 detoxification

Cell suspensions of Methanobrevibacter arboriphilus catalyzed the reduction of O₂ with H₂ at a maximal specific rate of 0.4 U (mol/min) per mg protein with an apparent K _m for O₂ of 30 M. The reaction was not inhibited by cyanide. The oxidase activity was traced back to a coenzyme F₄₂₀-dependent enzyme that was purified to apparent homogeneity and that catalyzed the oxidation of 2 F₄₂₀H₂ with 1 O₂ to 2 F₄₂₀ and 2 H₂O. The apparent K _m for F₄₂₀ was 30 M and that for O₂ was 2 M with a V _max of 240 U/mg at 37°C and pH 7.6, the pH optimum of the oxidase. The enzyme did not use NADH or NADPH as electron donor or H₂O₂ as electron acceptor and was not inhibited by cyanide. The 45-kDa protein, whose gene was cloned and sequenced, contained 1 FMN per mol and harbored a binuclear iron center as indicated by the sequence motif H–X–E–X–D–X₆₂–H–X₁₈–D–X₆₀–H. Sequence comparisons revealed that the F₄₂₀H₂ oxidase from M. arboriphilus is phylogenetically closely related to FprA from Methanothermobacter marburgensis (71% sequence identity), a 45-kDa flavoprotein of hitherto unknown function, and to A-type flavoproteins from bacteria (30–40%), which all have dioxygen reductase activity. With heterologously produced FprA from M. marburgensis it is shown that this protein is also a highly efficient F₄₂₀H₂ oxidase and that it contains 1 FMN and 2 iron atoms. The presence of F₄₂₀H₂ oxidase in methanogenic archaea may explain why some methanogens, e.g., the Methanobrevibacter species in the termite hindgut, cannot only tolerate but thrive under microoxic conditions.Dedicated to Hans Schlegel on the occasion of his 80th birthday.     

The Positive Effect of Exogenous Hemin on a Resistance of Strict Anaerobic Archaeon Methanobrevibacter arboriphilus to Oxidative Stresses

Methanogenic archaeon Methanobrevibacter arboriphilus (strains AZ and DH1), which is a strict anaerobic microorganism not able to synthesize heme, possessed a very high catalase activity in the presence of 20-50?μM hemin in a growth medium. We investigated the effect of various oxidative stresses (hydrogen peroxide and oxygenation) on M. arboriphilus cells grown on the standard nutrient medium supplemented with 0.1?% yeast extract, and on the same medium supplemented with hemin. It was demonstrated that 30?μM hemin had a very significant positive effect on the resistance of M. arboriphilus strains to H(2)O(2) and O(2) stresses because of 30- to 40-fold increase of heme catalase activity. Thus, hydrogen peroxide (0.6-1.2?mM) or oxygen (3-5?%) had a strong negative impact on low-catalase cultures grown in the hemin-free standard medium, whereas the presence of 30?μM hemin in the medium results in a high yield of biomass even under conditions of four times stronger H(2)O(2) and two times stronger O(2) stresses. The intracellular catalase activity reached a high level in 30-60?min after hemin was added to the nutrient medium, but the activity already increased about 5-7-fold in 6?min after hemin addition. Our experimental data suggest that exogenous hemin provides an effective antioxidative defense in representatives of the genus Methanobrevibacter, specially playing an important role in the puromycin-insensitive formation of the active heme-containing catalase from presynthesized apoenzyme and heme.     

Modeling the growth and proteinase A production in continuous cultures of recombinant Saccharomyces cerevisiae

Overexpression of the homologous protein proteinase A (PrA) in Saccharomyces cerevisiae has been achieved by inserting the PrA gene (PEP4) with its own promoter on a 2mu multicopy plasmid. With this system the specific PrA production rate was found to be described well by a linear function of the oxidative glucose metabolism, the reductive glucose metabolism, and the oxidative ethanol metabolism, with a significant lower yield resulting from the reductive glucose metabolism compared with the oxidative glucose metabolism. To describe the experimental data, a simple mathematical model has been set up. The model is based on an assumption of a limited respiratory capacity as suggested by Sonnleitner and K?ppeli but extended to describe production of an extracellular protein. The model predicts correctly the critical dilution rate to be between 0.15 and 0.16 h(-1), the decrease in the biomass yield above the critical dilution rate, and the production of proteinase A at different dilution rates. Both the experimental data and model simulations suggest that the optimum operating conditions for protein production is just at the critical dilution rate. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 447-454, 1997.     

Kinetics of biphasic growth of yeast in continuous and fed-batch cultures

The influence of dilution rate on the production of biomass, ethanol, and invertase in an aerobic culture of Saccharomyces carlsbergensis was studied in a glucose-limited chemostat culture. A kinetic model was developed to analyze the biphasic growth of yeast on both the glucose remaining and the ethanol produced in the culture. The model assumes a double effect where glucose regulates the flux of glucose catabolism (respiration and aerobic fermentation) and the ethanol utilization in yeast cells. The model could successfully demonstrate the experimental results of a chemostat culture featuring the monotonic decrease of biomass concentration with an increase of dilution rate higher than 0.2 hr^?1 as well as the maximum ethanol concentration at a particular dilution rate around 0.5 hr^?1. Some supplementary data were collected from an ethanol-limited aerobic chemostat culture and a glucose-limited anaerobic chemostat culture to use in the model calculation. Some parametric constants of cell growth, ethanol production, and invertase formation were determined in batch cultures under aerobic and anaerobic states as summarized in a table in comparison with the chemostat data. Using the constants, a prediction of the optimal control of a glucose fed-batch yeast culture was conducted in connection with an experiment for harvesting a high yield of yeast cells with high invertase activity.     

A model for light-limited continuous cultures: growth, shading, and maintenance

Light-limited growth in continuous cultures of phototrophic organisms is modeled. It is assumed that light energy up-take rate depends hyperbolically on light intensity and that the maintenance costs are proportional to biomass. Modeling the light distribution caused by shading within the vessel is necessary to explain the existence of steady state in light-limited chemostats. The model fits well to experimental data from literature on light-limited chemostats and turbidostats. Attention is given to the implications of the model for the estimation of the specific maintenance rate constant in light-limited continuous cultures.     

Effect of medium osmolality on the growth of murine continuous marrow cultures

The effect of medium osmolality was examined in primary, continuous bone-marrow cultures established from TO strain mice. The non-adherent cell population increased exponentially between weeks 2 and 5 and thereafter declined steadily. The number of CFU-GM followed a similar pattern but showed greater variability. The optimum osmolality in 4 week old cultures was found to be about 345 mosmol/kg which was higher than the plasma osmolality (n = 20; mean = 323.3 mosmol/kg; range = 313-331). Maximum non-adherent cell numbers were found at about 345 mosmol/kg (better than half-maximum between 320 and 370 mosmol/kg). CFU-GM numbers in the culture supernatant were maximal at about 355 mosmol/kg (better than half-maximum between 320 and 400 mosmol/kg). An adherent layer developed over a wider range of osmolality than supported granulopoiesis (better than half-maximum between 258 and 402 mosmol/kg). It was necessary to increase the osmolality of Fischer's medium in order to obtain maximum growth.     

A stereoscopic analysis of the centrosome structure in the cells of continuous and primary cell cultures]

A 3D reconstruction of the centrosome region was made based on series of semithick sections in tissue culture cells. It was shown that: 1) the total number of microtubules attached to the centrosome is about 30-50 of which only 20% or less run farther than 2 microns away from the centrosome; 2) a certain number of short microtubules (less than 1 micron length) is present in the vicinity of the centrosome, the majority of them are attached to the centrosome; 3) many microtubules around the centrosome have no direct contact with either centrioles, or other microtubule-convergent structures; 4) the majority of free microtubules are comparatively long (more than 1 micron length); 5) almost all the microtubules running closer than 2 microns to the centrosome are oriented towards it with their proximal ends. The radial distribution of free microtubules around the centrosome support the supposition that they may appear as a result of their detachment from the microtubule-nucleating centres.     

Cybernetic model of the growth dynamics of Saccharomyces cerevisiae in batch and continuous cultures.

Growth of Saccharomyces cerevisiae on glucose in aerobic batch culture follows the well-documented diauxic pattern of completely fermenting glucose to ethanol during the first exponential growth phase, followed by an intermediate lag phase and a second exponential growth phase consuming ethanol. In continuous cultures over a range of intermediate dilution rates, the yeast bioreactor exhibits sustained oscillations in all the measured concentrations, such as cell mass, glucose, ethanol, and dissolved oxygen, the amounts of intracellular storage carbohydrates, such as glycogen and trehalose, the fraction of budded cells as well as the culture pH. We present here a structured, unsegregated model for the yeast growth dynamics developed from the 'cybernetic' modeling framework, to simulate the dynamic competition between all the available metabolic pathways. This cybernetic model accurately predicts all the key experimentally observed aspects: (i) in batch cultures, duration of the intermediate lag phase, sequential production and consumption of ethanol, and the dynamics of the gaseous exchange rates of oxygen and carbon dioxide; and (ii) in continuous cultures, the spontaneous generation of oscillations as well as the variations in period and amplitude of oscillations when the dilution rate or agitatin rate are changed.     

Regulation of bacteriorhodopsin synthesis by growth rate in continuous cultures of Halobacterium halobium

The independent effects of oxygen tension and growth rate on bacteriorhodopsin synthesis in Halobacterium halobium have been studied in chemostat cultures. Bacteriorhodopsin synthesis occurs only at low growth rates and is stimulated by low oxygen tension. Fast growth rates override the stimulatory effects of oxygen tension, with the result that bacteriorhodopsin can scarcely be detected. Illumination of cultures maintained at low growth rate and low oxygen tension significantly increases the steady state cell yield. This finding suggests that under these conditions the purple membrane proton pump is coupled to energy transduction.