Anaerobic Fungus Neocallimastix Research Articles

The anaerobic fungus Neocallimastix frontalis: isolation and properties of a cellulosome-type enzyme fraction with the capacity to solubilize hydrogen-bond-ordered cellulose

A minor component isolated from the extra-cellular cellulase of the anaerobic rumen fungus Neocallimastix frontalis by adsorption on cellulose had a remarkable capacity to degrade crystalline hydrogen-bond-ordered cellulose. When produced in a semi-defined medium the component comprised normally less than 4% of the total protein and only 0.3% of the protein in cultures containing rumen fluid. The minor component showed endoglucanase (carboxymethylcellulase) and β-glucosidase activity and effected the extensive hydrolysis of “crystalline” cellulose in the form of the cotton fibre when acting alone. Glucose was the sole product of hydrolysis. The specific activity of the crystalline cellulose solubilizing factor (CCSF) in degrading cotton fibre was much higher than any other cellulase or cellulase component reported so far. The activity of the CSSF to crystalline hydrogen-bond-ordered cellulose resides in a high molecular mass complex of 670 kDa, that comprised a number of subunits ranging in size from 68 to 135 kDa.

Studies on the cellulase of the rumen anaerobic fungus Neocallimastix frontalis, with special reference to the capacity of the enzyme to degrade crystalline cellulose

By using cotton fiber, carboxymethylcellulose (CM-cellulose), and o -nitrophenyl-β- d -glucoside as substrates, it was possible to demonstrate that there were at least three different types of enzyme present in culture filtrates of Neocallimastix frontalis RK21. The activity to crystalline cellulose (cotton fiber) resided in a high-molecular-weight complex that comprised endoglucanase activity, β-glucosidase activity, and another enzyme. However, synergism between the components in the high-molecular-weight complex and between the complex and low-molecular-weight endoglucanases and β-glucosidases was also apparent in the solubilization of crystalline cellulose. The composition of the complex varied according to the growth conditions: it was, however, between 750 and 1000 kDa in size. Cultures containing rumen fluid contained only small amounts of the high-molecular-weight complex. Cultures grown on defined medium were rich in high-molecular-weight complex, but only when the concentration of the carbon source was less than 1.0%. Treatment of the crude culture filtrates with chitinase under conditions that had little effect on the activity of the enzyme to endoglucanase (CM-cellulose) or β-glucosidase completely destroyed the activity to crystalline cellulose. It is tentatively suggested that an enzyme crucial for the activity to crystalline cellulose may be cell wall-bound and may be dependent on its association with the cell wall for the maintenance of its conformation for attacking crystalline cellulose. The enzyme involved in degrading crystalline cellulose is much more thermolabile than the CM-cellulase or the β-glucosidase: activity is optimal at pH 6.0 and 40°C.

Sequence of the phosphoenolpyruvate carboxykinase-encoding cDNA from the rumen anaerobic fungus Neocallimastix frontalis: comparison of the amino acid sequence with animals and yeast

The nucleotide sequence of the cDNA of the phosphoenolpyruvate carboxykinase-encoding gene from the fungus Neocallimastix frontalis, was determined. The deduced amino acid sequence (608 residues) and the predicted protein structure were compared to their counterparts in animals and yeast. Catalytic regions (substrate-binding site and nucleotide-binding domains) are highly conserved among fungal and animal organisms. The yeast sequence showed no similarity to the fungal sequence.

Isolation and characterization of p-coumaroyl esterase from the anaerobic fungus Neocallimastix strain MC-2

An extracellular p-coumaroyl esterase produced by the anaerobic fungus Neocallimastix strain MC-2 released p-coumaroyl groups from 0-[5-0-((E)-p-coumaroyl)-alpha-L-arabinofuranosyl]-(1----3)-0-beta -D-xylopyranosyl-(1----4)-D-xylopyranose (PAXX). The esterase was purified 121-fold from culture medium in successive steps involving ultrafiltration column chromatography on S-sepharose and hydroxylapatite, isoelectric focusing, and gel filtration. The native enzyme had an apparent mass of 11 kDa under nondenaturing conditions and a mass of 5.8 kDa under denaturing conditions, suggesting that the enzyme may exist as a dimer. The isoelectric point was 4.7, and the pH optimum was 7.2. The purified esterase had 100 times more activity towards PAXX than towards the analogous feruloyl ester (FAXX). The apparent Km and Vmax of the purified p-coumaroyl esterase for PAXX at pH 7.2 and 40 degrees C were 19.4 microM and 5.1 microM min(-1), respectively. p-Coumaroyl tetrasaccharides isolated from plant cell walls were hydrolyzed at rates similar to that for PAXX, whereas a dimer of PAXX was hydrolyzed at a rate 20-fold lower, yielding 4,4'-dihydroxy-alpha-truxillic acid as an end product. Ethyl and methyl p-coumarates were hydrolyzed at very slow rates, if at all. The purified esterase released p-coumaroyl groups from finely, but not coarsely, ground plant cell walls, and this activity was enhanced by the addition of xylanase and other cell wall-degrading enzymes.

Glucose metabolic pathways in the anaerobic rumen fungus Neocallimastix frontalis EB188

Primary pathways for glucose metabolism were established in the anaerobic rumen fungus Neocallimastix frontalis EB188. This highly capable cellulolytic organism demonstrated a strict anaerobic integration of metabolic pathways. Glycolysis in N. frontalis EB188 was coupled to malate dehydrogenase, 'malic' enzyme and specified hydrogenosome reactions. Pyruvate, as in most life forms, was a pivotal compound. The major fermentation products of N. frontalis EB188 were acetate, ethanol and lactate, with the concomitant generation of H2. On the basis of its unique characteristics and streamlined fermentation pathways, it was concluded that N. frontalis EB188 should be an important contributor to programs generating energy and selected chemicals from currently intractable biomass.

Molecular cloning of genes from the rumen anaerobic fungus Neocallimastix frontalis: expression during hydrolase induction

Glycoside and polysaccharide hydrolase production by the rumen anaerobic fungus, Neocalimastix frontalis is induced by the presence of crystalline cellulose. A differential screening of a cDNA library was used to isolate DNA sequences transcribed at high levels under growth conditions which induce enzyme production. Seven clones were isolated that preferentially hybridized to the induced cDNA probe versus the non-induced cDNA probe. Southern analysis showed that a cDNA clone (118) hybridized to a DNA probe encoding part of the exo-cellobiohydrolase I (CBH I) gene of Trichoderma reesei. Northern analysis demonstrated that the cDNA 118 was transcribed to yield a 2.1 kb RNA. This transcript was induced in the presence of cellulose.

Assay for trans-p-coumaroyl esterase using a specific substrate from plant cell walls

Cell walls of Coastal Bermuda grass ( Cynodon dactylon) were treated with polysaccharide hydrolases to release O-[5- O-( trans-p-coumaroyl)-α- l-arabinofuranosyl]-(1→3)- O-β- d-xylopyranosyl-(1→4)- d-xylopyranose (PAXX) which was isolated by liquid chromatography. The isolated PAXX was greater than 95% pure as determined by 1H NMR and was used as substrate for a sensitive assay of trans-p-coumaroyl esterase. PAXX was hydrolyzed by culture filtrates from the anaerobic fungus Neocallimastix MC-2. The trans-p-coumaric acid released by enzymatic hydrolysis was assayed by reverse-phase HPLC, and as little as 100 ng of acid could be determined. Steady-state velocities for the release of the acid obeyed Michaelis-Menten kinetics. V max was determined to be 1.17 μmol min −1 mg −1 and K m 13.2 μ m at pH 7.5 and 30°C.

Purification and characterization of an extracellular β-xylosidase from the rumen anaerobic fungus Neocallimastix frontalis

The purification of β-xylosidase (gb- d-xyloside xylohydrolase, EC 3.2.1.37) from Neocallimastix frontalis was performed by ammonium sulphate precipitation, ion exchange chromatography, gel filtration and preparative isoelectric focusing. The enzyme has a molecular mass of 180 000 Da, an isoelectric point at pH 4.35 and catalysed the hydrolysis of p- nitrophenyl-β- d- xylopyranoside optimally at pH 6.5 and 35°C with K m of 0.33 mg ml −1. The enzymatic activity was strongly increased by the presence of Ca 2+, Mn 2+, Zn 2+, Co 2+ or Mg 2+ and completely inhibited by Hg 2+ and p-chloromercuribenzoate. The purified protein also had a low level of xylanase activity.

Supernatant protein and cellulase activities of the anaerobic ruminal fungus Neocallimastix frontalis EB188

Protein and cellulose activities were measured in culture supernatants of the anaerobic ruminal fungus Neocallimastix frontalis EB188 established in glucose medium and switched to either glucose, cellobiose, or cellulose media. Polyacrylamide gel electrophoresis was used to show differences caused by changing medium carbon source. Culture supernatants contained proteins with molecular weights ranging from greater than 116,000 to about 19,000. Low levels of cellulose activity were evident in glucose-grown cultures. Increased amounts of slowly migrating cellulase activities appeared in the supernatants of glucose-grown cultures switched to cellulose. Cellulase activities which reacted differentially during colorimetric and in situ assays were produced. Isoelectric points of cellulase activities varied from 3.7 to 8.3, and activities possessed optimal pHs of between 5.9 and 6.5.

Production of xylanase by the ruminal anaerobic fungus Neocallimastix frontalis

Xylanase (1,4-beta-D-xylan xylanohydrolase, EC 3.2.1.8) production was investigated in the ruminal anaerobic fungus Neocallimastix frontalis. The enzyme was released principally into the culture fluid and had pH and temperature optima of 5.5 and 55 degrees C, respectively. In the presence of low concentrations of substrate, the enzyme was stabilized at 50 degrees C. Xylobiose was the principal product of xylanase action, with lesser amounts of longer-chained xylooligosaccharides. No xylose was detected, indicating that xylobiase activity was absent. Activities of xylanase up to 27 U ml-1 (1 U represents 1 micromol of xylose equivalents released min-1) were obtained for cultures grown on xylan (from oat spelt) at 2.5 mg ml-1 in shaken cultures. No growth occurred in unshaken cultures. Xylanase production declined with elevated concentrations of xylan (less than 2.5 mg ml-1), and this was accompanied by an accumulation of xylose and, to a lesser extent, arabinose. Addition of either pentose to cultures grown on low levels of xylan in which neither sugar accumulated suppressed xylanase production, and in growth studies with the paired substrates xylan-xylose, active production of the enzyme occurred during growth on xylan only after xylose had been preferentially utilized. When cellobiose, glucose, and xylose were tested as growth substrates for the production of xylanase (each initially at 2.5 mg ml-1), they were found to be less effective than xylan, and use of xylan from different origins (birch wood or larch wood) as the growth substrate or in the assay system resulted in only marginal differences in enzyme activity. However, elevated production of xylanase occurred during growth on crude hemicellulose (barley straw leaf). The results are discussed in relation to the role of the anaerobic fungi in the ruminal ecosystem, and the possible application of the enzyme in bioconversion processes is also considered.

Production of α-Amylase by the Ruminal Anaerobic Fungus Neocallimastix frontalis

alpha-Amylase production was examined in the ruminal anaerobic fungus Neocallimastix frontalis. The enzyme was released mainly into the culture fluid and had temperature and pH optima of 55 degrees C and 5.5, respectively, and the apparent K(m) for starch was 0.8 mg ml. The products of alpha-amylase action were mainly maltotriose, maltotetraose, and longer-chain oligosaccharides. No activity of the enzyme was observed towards these compounds or pullulan, but activity on amylose was similar to starch. Evidence for the endo action of alpha-amylase was also obtained from experiments which showed that the reduction in iodine-staining capacity and release in reducing power by action on amylose was similar to that for commercial alpha-amylase. Activities of alpha-amylase up to 4.4 U ml (1 U represents 1 mumol of glucose equivalents released per min) were obtained for cultures grown on 2.5 mg of starch ml in shaken cultures. No growth occurred in unshaken cultures. With elevated concentrations of starch (>2.5 mg ml), alpha-amylase production declined and glucose accumulated in the cultures. Addition of glucose to cultures grown on low levels of starch, in which little glucose accumulated, suppressed alpha-amylase production, and in bisubstrate growth studies, active production of the enzyme only occurred during growth on starch after glucose had been preferentially utilized. When cellulose, cellobiose, glucose, xylan, and xylose were tested as growth substrates for the production of alpha-amylase (initial concentration, 2.5 mg ml), they were found to be less effective than starch, but maltose was almost as effective. The fungal alpha-amylase was found to be stable at 60 degrees C in the presence of low concentrations of starch (</=5%), suggesting that it may be suitable for industrial application.

The effect of lasalocid on glucose uptake, hydrogen production and the solubilization of straw by the anaerobic rumen fungus Neocallimastix frontalis

Three isolates of Neocallimastix frontalis grown in pure culture with glucose as substrate differed in their response to the presence of lasalocid in the growth medium. For two strains (RE1 and RK21) hydrogen production and glucose uptake were reduced to 50% or less of the control values in the presence of lasalocid at 0–25 μg/ ml. A third strain (PNK2) retained significant fermentative ability in the presence of at least four times this amount of lasalocid. Lasalocid was rather less inhibitory to the solubilization of straw by these fungi, PNK2 again providing the most resistant strain. It is concluded that the use of lasalocid in ruminant feeds is likely to have a strain selective effect on the rumen population of Neocallimastix.

Hydrogenosomes in the rumen fungus Neocallimastix patriciarum.

Sedimentable hydrogenase activity was demonstrated in cell-free extracts from both zoospores and vegetative growth of the anaerobic rumen fungus Neocallimastix patriciarum. Electron micrographs of the fraction enriched in hydrogenase activity contained finely granular microbody-like organelles, about 0.5 micron in diameter and having an equilibrium density of about 1.2 g X ml-1 in sucrose, 1.12 g X ml-1 in Percoll and 1.27-1.28 g X ml-1 in Metrizamide. These organelles, which are sedimentable at 10(5) g-min, bear no similarity to mitochondria, but are morphologically similar to hydrogen-evolving organelles possessed by certain anaerobic protozoa and termed 'hydrogenosomes'. Other typical hydrogenosomal enzymes, namely 'malic' enzyme, pyruvate:ferredoxin oxidoreductase and NADPH:ferredoxin oxidoreductase, were enriched in the same particle fraction as hydrogenase. The synthesis of pyruvate:ferredoxin oxidoreductase was found to be suppressed when the organism was cultured under an atmosphere of CO2, and an alternative pathway is proposed for growth under these conditions.

Proteolytic activity of a rumen anaerobic fungus

A strain of the anaerobic phycomycetous fungus Neocallimastix frontalis isolated from the rumen of a sheep had a high proteolytic activity which became predominantly extracellular during growth. Proteolytic activity appeared to be due to a metalloprotease, as it was inhibited by 1,10-phenanthroline, EDTA and other chelators but not by phenylmethylsulphonyl fluoride (PMSF). Inhibition by EDTA was fully reversed by the addition of Zn2+, Ca2+ or Co2+, whereas addition of metal ions in the presence of 1,10-phenanthroline restored only a little activity. p-Chloromercuribenzoate (PCMB) was also inhibitory in dialysed supernatant fluid. N-α-p-Tosyl-l-lysine chloromethylketone (TLCK) inhibited proteolysis, suggesting that the protease(s) has a trypsin-like specificity, but benzoylarginine p-nitroanilide was not hydrolysed. Protease activity has a broad pH profile with a maximum at pH 7.5. Gel fractionation indicated that most of the activity was in a high-Mr form.

Lipid composition of an obligately anaerobic fungus Neocallimastix frontalis isolated from a bovine rumen

The lipid composition of an obligately anaerobic bovine rumen fungus Neocallimastix frontalis, belonging to the Neo-callimasticaceae of the Chytridiomycetes, has been investigated. It consisted of 44% neutral lipids (triacylglycerol, fatty acid, and sterols) and 56% polar lipids. The fatty acid composition of this unusual fungus differed markedly from that of other members of Chytridiomycetes by the absence of long-chain polyenoic fatty acids and by the presence of a large proportion (48%) of a series of cis-monoenoic fatty acids between 14:1 and 24:1. The absence of polyunsaturated fatty acids is explained by the anaerobic life style of this organism. However, the formation of the large proportions of monoenoic fatty acids under anaerobic conditions constitutes a major difference which cannot be explained by known pathways of fatty acid biosynthesis in fungi. This result indicates that N. frontalis differs from other fungi in possessing an anaerobic pathway for monoenoic fatty acid biosynthesis.

Production and regulation of cellulase by two strains of the rumen anaerobic fungus Neocallimastix frontalis.

Cellulase production was examined in two strains of Neocallimastix frontalis, namely, PN-1 isolated from the ovine rumen, and PN-2 from the bovine rumen. For both strains, carboxymethylcellulase (CMCase) had a pH optimum of 6.0 and a temperature optimum of 50 degrees C. CMCase resided mainly in the culture fluid, and activities up to 170 U ml-1 (1 U represents 1 microgram of glucose equivalents released per min) were obtained for cultures grown on 2.5 mg of cellulose ml-1. For resting cultures of strain PN-1, the yield of CMCase increased from 9.9 X 10(3) to 10.4 X 10(4) U per g of cellulose degraded, as the initial cellulose concentration decreased from 10 to 0.58 mg ml-1. The range for PN-2 was 8.1 X 10(3) to 11 X 10(4) U g-1. Shaking cultures improved yields for strain PN-1 but not for PN-2. Decreased CMCase production at high initial cellulose concentrations concurred with accumulation of glucose, and addition of glucose (4 mg ml-1) to cultures grown on low cellulose in which none of the sugar accumulated repressed CMCase. Adsorption of CMCase was excluded as a likely explanation for decreased yields at high initial cellulose as only a low proportion (less than 20%) of the enzyme was adsorbed onto the growth substrate. Exoglucanase, measured with alkali-treated Sigmacell or Avicel, gave low levels of activity in the culture fluid (less than 2 U ml-1) and did not appear to be associated with the fungal rhizoid, as treatment with various solubilizing agents failed to give increased activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycosidases of the rumen anaerobic fungus Neocallimastix frontalis grown on cellulosic substrates.

The rumen anaerobic fungus Neocallimastix frontalis was grown on cellulosic substrates, and the cellular distribution and types of glycosidases produced by the organism were studied. Fungal cultures were fractionated into extracellular, insoluble (membrane), and intracellular fractions and assayed for glycosidase activity by using Avicel, carboxymethylcellulose, xylan, starch, polygalacturonic acid, and the p-nitrophenyl derivatives of galactose, glucose, and xylose as substrates. Enzymic activity was highest in the extracellular fraction; however, the membrane fraction also displayed appreciable activity. The intracellular fraction was inactive towards all substrates. Polygalacturonic acid was the only substrate not hydrolyzed by the active fractions, indicating that pectinase was absent. The results show that N. frontalis, a common rumen anaerobic fungus, produces enzymes for degrading cellulose and hemicellulose, key components of plant fiber.