Purification and Characterization of Two Endoxylanases from Clostridium acetobutylicum ATCC 824

Two endoxylanases produced by C. acetobutylicum ATCC 824 were purified to homogeneity by column chromatography. Xylanase A, which has a molecular weight of 65,000, hydrolyzed larchwood xylan randomly, yielding xylohexaose, xylopentaose, xylotetraose, xylotriose, and xylobiose as end products. Xylanase B, which has a molecular weight of 29,000, also hydrolyzed xylan randomly, giving xylotriose and xylobiose as end products. Xylanase A hydrolyzed carboxymethyl cellulose with a higher specific activity than xylan. It also exhibited high activity on acid-swollen cellulose. Xylanase B showed practically no activity against either cellulose or carboxymethyl cellulose but was able to hydrolyze lichenan with a specific activity similar to that for xylan. Both xylanases had no aryl-β-xylosidase activity. The smallest oligosaccharides degraded by xylanases A and B were xylohexaose and xylotetraose, respectively. The two xylanases demonstrated similar K_m and V_max values but had different pH optima and isoelectric points. Ouchterlony immunodiffusion tests showed that xylanases A and B lacked antigenic similarity.     

Characterization of a maltose transport system in Clostridium acetobutylicum ATCC 824

The utilization of maltose by Clostridium acetobutylicum ATCC 824 was investigated. Glucose was used preferentially to maltose, when both substrates were present in the medium. Maltose phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) activity was detected in extracts prepared from cultures grown on maltose, but not glucose or sucrose, as the sole carbon source. Extract fractionation and PTS reconstitution experiments revealed that the specificity for maltose is contained entirely within the membrane in this organism. A putative gene system for the maltose PTS was identified (from the C. acetobutylicum ATCC 824 genome sequence), encoding an enzyme II^Mal and a maltose 6-phosphate hydrolase. Journal of Industrial Microbiology & Biotechnology (2001) 27, 298–306. Received 12 September 2000/ Accepted in revised form 30 November 2000     

Expression of a Cloned Cyclopropane Fatty Acid Synthase Gene Reduces Solvent Formation in Clostridium acetobutylicum ATCC 824

The cyclopropane fatty acid synthase gene (cfa) of Clostridium acetobutylicum ATCC 824 was cloned and overexpressed under the control of the clostridial ptb promoter. The function of the cfa gene was confirmed by complementation of an Escherichia coli cfa-deficient strain in terms of fatty acid composition and growth rate under solvent stress. Constructs expressing cfa were introduced into C. acetobutylicum hosts and cultured in rich glucose broth in static flasks without pH control. Overexpression of the cfa gene in the wild type and in a butyrate kinase-deficient strain increased the cyclopropane fatty acid content of early-log-phase cells as well as initial acid and butanol resistance. However, solvent production in the cfa-overexpressing strain was considerably decreased, while acetate and butyrate levels remained high. The findings suggest that overexpression of cfa results in changes in membrane properties that dampen the full induction of solventogenesis. The overexpression of a marR homologous gene preceding the cfa gene in the clostridial genome resulted in reduced cyclopropane fatty acid accumulation.     

Autolysis of Clostridium acetobutylicum ATCC 824.

The optimum conditions for autolysis of Clostridium acetobutylicum ATCC 824 were determined. Autolysis was optimal at pH 6.3 and 55 degrees C in 0.1 M-sodium acetate/phosphate buffer. The ability of cells to autolyse decreased sharply at the end of the exponential phase of growth. Lysis was stimulated by monovalent cations and compounds that complex divalent cations, and inhibited by divalent cations. The autolysin of C. acetobutylicum, which was mainly cytoplasmic, was purified to homogeneity and characterized as a muramidase. The enzyme was identical to the extracellular muramidase in terms of M(r), isoelectric point and NH2-terminal amino acid sequence. The autolysin was inhibited by lipoteichoic acids and cardiolipin but not by phosphatidylethanolamine and phosphatidylglycerol. A mechanism of regulation and fixation involving lipoteichoic acid, cardiolipin and divalent cations is proposed.     

Characterization of thermostable Xyn10A enzyme from mesophilic Clostridium acetobutylicum ATCC 824

A thermostable xylanase gene, xyn10A (CAP0053), was cloned from Clostridium acetobutylicum ATCC 824. The nucleotide sequence of the C. acetobutylicum xyn10A gene encoded a 318-amino-acid, single-domain, family 10 xylanase, Xyn10A, with a molecular mass of 34 kDa. Xyn10A exhibited extremely high (92%) amino acid sequence identity with Xyn10B (CAP0116) of this strain and had 42% and 32% identity with the catalytic domains of Rhodothermus marinus xylanase I and Thermoascus aurantiacus xylanase I, respectively. Xyn10A enzyme was purified from recombinant Escherichia coli and was highly active toward oat-spelt and Birchwood xylan and slightly active toward carboxymethyl cellulose, arabinogalactouronic acid, and various p-nitrophenyl monosaccharides. Xyn10A hydrolyzed xylan and xylooligosaccharides larger than xylobiose to produce xylose. This enzyme was optimally active at 60°C and had an optimum pH of 5.0. This is one of a number of related activities encoded on the large plasmid in this strain.     

Phosphotransbutyrylase from Clostridium acetobutylicum ATCC 824 and its role in acidogenesis

Phosphotransbutyrylase (phosphate butyryltransferase [EC 2.3.1.19]) from Clostridium acetobutylicum ATCC 824 was purified approximately 200-fold to homogeneity with a yield of 13%. Steps used in the purification procedure were fractional precipitation with (NH4)2SO4, Phenyl Sepharose CL-4B chromatography, DEAE-Sephacel chromatography, high-pressure liquid chromatography with an anion-exchange column, and high-pressure liquid chromatography with a hydrophobic-interaction column. Gel filtration and denaturing gel electrophoresis data were consistent with a native enzyme having eight 31,000-molecular-weight subunits. Within the physiological range of pH 5.5 to 7, the enzyme was very sensitive to pH change in the butyryl phosphate-forming direction and showed virtually no activity below pH 6. This finding indicates that a change in internal pH may be one important factor in the regulation of the enzyme. The enzyme was less sensitive to pH change in the reverse direction. The enzyme could use a number of substrates in addition to butyryl coenzyme A (butyryl-CoA) but had the highest relative activity with butyryl-CoA, isovaleryl-CoA, and valeryl-CoA. The Km values at 30 degrees C and pH 8.0 for butyryl-CoA, phosphate, butyryl phosphate, and CoASH (reduced form of CoA) were 0.11, 14, 0.26, and 0.077 mM, respectively. Results of product inhibition studies were consistent with a random Bi Bi binding mechanism in which phosphate binds at more than one site.     

Thiolase from Clostridium acetobutylicum ATCC 824 and Its Role in the Synthesis of Acids and Solvents

Thiolase (acetyl-coenzyme A [CoA] acetyltransferase, E.C. 2.3.1.19) from Clostridium acetobutylicum ATCC 824 has been purified 70-fold to homogeneity. Unlike the thiolase in Clostridium pasteurianum, this thiolase has high relative activity throughout the physiological range of internal pH of 5.5 to 7.0, indicating that change in internal pH during acid production is not an important factor in the regulation of this thiolase. In the condensation direction, the thiolase is inhibited by micromolar levels of CoA, and this may be an important factor in modulating the net condensation of acetyl-CoA to acetoacetyl-CoA. Other cofactors and metabolites that were tested and shown to be inhibitors are ATP and butyryl-CoA. The native enzyme consists of four 44,000-molecular-weight subunits. The kinetic binding mechanism is ping-pong. The K_m value for acetyl-CoA is 0.27 mM at 30°C and pH 7.4. The K_m values for sulfhydryl-CoA and acetoacetyl-CoA are, respectively, 0.0048 and 0.032 mM at 30°C and pH 8.0. The active site apparently contains a sulfhydryl group, but unlike other thiolases, this thiolase is relatively stable in the presence of 5,5′-dithiobis(2-nitrobenzoic acid). Studies of thiolase specific activity under various types of continuous fermentations show that regulation of this enzyme at both the genetic and enzyme levels is important.     

Resonance assignments of cohesin and dockerin domains from Clostridium acetobutylicum ATCC824

Cohesin and dockerin domains are critical assembling components of cellulosome, a large extracellular multienzyme complex which is used by anaerobic cellulolytic bacteria to efficiently degrade lignocellulose. According to sequence homology, cohesins can be divided into three major groups, whereas cohesins from Clostridium acetobutylicum are beyond these groups and emanate from a branching point between the type I and type III cohesins. Cohesins and dockerins from C. acetobutylicum show low sequence homology to those from other cellulolytic bacteria, and their interactions are specific in corresponding species. Therefore the interactions between cohesins and dockerins from C. acetobutylicum are meaningful to the studies of both cellulosome assembling mechanism and the construction of designer cellulosome. Here we report the NMR resonance assignments of one cohesin from cellulosome scaffoldin cipA and one dockerin from a cellulosomal glycoside hydrolase (family 9) of C. acetobutylicum for further structural determination and functional studies.     

Phosphotransbutyrylase from Clostridium acetobutylicum ATCC 824 and its role in acidogenesis.

Phosphotransbutyrylase (phosphate butyryltransferase [EC 2.3.1.19]) from Clostridium acetobutylicum ATCC 824 was purified approximately 200-fold to homogeneity with a yield of 13%. Steps used in the purification procedure were fractional precipitation with (NH4)2SO4, Phenyl Sepharose CL-4B chromatography, DEAE-Sephacel chromatography, high-pressure liquid chromatography with an anion-exchange column, and high-pressure liquid chromatography with a hydrophobic-interaction column. Gel filtration and denaturing gel electrophoresis data were consistent with a native enzyme having eight 31,000-molecular-weight subunits. Within the physiological range of pH 5.5 to 7, the enzyme was very sensitive to pH change in the butyryl phosphate-forming direction and showed virtually no activity below pH 6. This finding indicates that a change in internal pH may be one important factor in the regulation of the enzyme. The enzyme was less sensitive to pH change in the reverse direction. The enzyme could use a number of substrates in addition to butyryl coenzyme A (butyryl-CoA) but had the highest relative activity with butyryl-CoA, isovaleryl-CoA, and valeryl-CoA. The Km values at 30 degrees C and pH 8.0 for butyryl-CoA, phosphate, butyryl phosphate, and CoASH (reduced form of CoA) were 0.11, 14, 0.26, and 0.077 mM, respectively. Results of product inhibition studies were consistent with a random Bi Bi binding mechanism in which phosphate binds at more than one site.     

Purification and characterization of the extracellular alpha-amylase from Clostridium acetobutylicum ATCC 824.

The extracellular alpha-amylase (1,4-alpha-D-glucanglucanohydrolase; EC 3.2.1.1) from Clostridium acetobutylicum ATCC 824 was purified to homogeneity by anion-exchange chromatography (mono Q) and gel filtration (Superose 12). The enzyme had an isoelectric point of 4.7 and a molecular weight of 84,000, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was a monomeric protein, the 19-amino-acid N terminus of which displayed 42% homology with the Bacillus subtilis saccharifying alpha-amylase. The amino acid composition of the enzyme showed a high number of acidic and hydrophobic residues and only one cysteine residue per mole. The activity of the alpha-amylase was not stimulated by calcium ions (or other metal ions) or inhibited by EDTA, although the enzyme contained seven calcium atoms per molecule. alpha-Amylase activity on soluble starch was optimal at pH 5.6 and 45 degrees C. The alpha-amylase was stable at an acidic pH but very sensitive to thermal inactivation. It hydrolyzed soluble starch, with a Km of 3.6 g . liter-1 and a Kcat of 122 mol of reducing sugars . s-1 . mol-1. The alpha-amylase showed greater activity with high-molecular-weight substrates than with low-molecular-weight maltooligosaccharides, hydrolyzed glycogen and pullulan slowly, but did not hydrolyze dextran or cyclodextrins. The major end products of maltohexaose degradation were glucose, maltose, and maltotriose; maltotetraose and maltopentaose were formed as intermediate products. Twenty seven percent of the glucoamylase activity generally detected in the culture supernatant of C. acetobutylicum can be attributed to the alpha-amylase.     

Isolation and Some Properties of a beta-d-Xylosidase from Clostridium acetobutylicum ATCC 824

A beta-d-xylosidase from C. acetobutylicum ATCC 824 was purified by column chromatography on CM-Sepharose, hydroxylapatite, Phenyl Sepharose, and Sephadex G-200. The enzyme had an apparent molecular weight of 224,000 as estimated by gel filtration. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the enzyme consisted of two subunits of 85,000 and one subunit of 63,000 daltons. It exhibited optimal activity at pH 6.0 to 6.5 and 45 degrees C. the enzyme had an isoelectric point of 5.85. It hydrolyzed p-nitrophenylxyloside readily with a K(m) of 3.7 mM. The enzyme hydrolyzed xylo-oligosaccharides with chain lengths of 2 to 6 units by cleaving a single xylose from the chain end. It showed little or no activity against xylan, carboxymethyl cellulose, and other p-nitrophenylglycosides.     

Host-plasmid interactions in recombinant strains of Clostridium acetobutylicum ATCC 824

Abstract Plasmid-containing strains of Clostridium acetobutylicum produced higher levels of solvents and lower levels of acids than wild-type cells in controlled pH 4.5 batch fermentations. This effect was observed regardless of whether or not the plasmids contained C. acetobutylicum genes. The effect was less prevalent in higher pH fermentations and apparently independent of the actual DNA sequences contained on these plasmids. The plasmid-containing strains were found to have lower growth-rates and higher solventogenic enzyme activities than wild-type cells. However, similar activity levels were found for both butyrate-pathway enzymes.     

Development and Characterization of a Gene Expression Reporter System for Clostridium acetobutylicum ATCC 824

A gene expression reporter system (pHT3) for Clostridium acetobutylicum ATCC 824 was developed by using the lacZ gene from Thermoanaerobacterium thermosulfurogenes EM1 as the reporter gene. In order to test the reporter system, promoters of three key metabolic pathway genes, ptb (coding for phosphotransbutyrylase), thl (coding for thiolase), and adc (coding for acetoacetate decarboxylase), were cloned upstream of the reporter gene in pHT3 in order to construct vectors pHT4, pHT5, and pHTA, respectively. Detection of β-galactosidase activity in time course studies performed with strains ATCC 824(pHT4), ATCC 824(pHT5), and ATCC 824(pHTA) demonstrated that the reporter gene produced a functional β-galactosidase in C. acetobutylicum. In addition, time course studies revealed differences in the β-galactosidase specific activity profiles of strains ATCC 824(pHT4), ATCC 824(pHT5), and ATCC 824(pHTA), suggesting that the reporter system developed in this study is able to effectively distinguish between different promoters. The stability of the β-galactosidase produced by the reporter gene was also examined with strains ATCC 824(pHT4) and ATCC 824(pHT5) by using chloramphenicol treatment to inhibit protein synthesis. The data indicated that the β-galactosidase produced by the lacZ gene from T. thermosulfurogenes EM1 was stable in the exponential phase of growth. In pH-controlled fermentations of ATCC 824(pHT4), the kinetics of β-galactosidase formation from the ptb promoter and phosphotransbutyrylase formation from its own autologous promoter were found to be similar.     

Metabolic engineering of Clostridium acetobutylicum ATCC 824 for isopropanol-butanol-ethanol fermentation

Clostridium acetobutylicum naturally produces acetone as well as butanol and ethanol. Since acetone cannot be used as a biofuel, its production needs to be minimized or suppressed by cell or bioreactor engineering. Thus, there have been attempts to disrupt or inactivate the acetone formation pathway. Here we present another approach, namely, converting acetone to isopropanol by metabolic engineering. Since isopropanol can be used as a fuel additive, the mixture of isopropanol, butanol, and ethanol (IBE) produced by engineered C. acetobutylicum can be directly used as a biofuel. IBE production is achieved by the expression of a primary/secondary alcohol dehydrogenase gene from Clostridium beijerinckii NRRL B-593 (i.e., adh(B-593)) in C. acetobutylicum ATCC 824. To increase the total alcohol titer, a synthetic acetone operon (act operon; adc-ctfA-ctfB) was constructed and expressed to increase the flux toward isopropanol formation. When this engineering strategy was applied to the PJC4BK strain lacking in the buk gene (encoding butyrate kinase), a significantly higher titer and yield of IBE could be achieved. The resulting PJC4BK(pIPA3-Cm2) strain produced 20.4 g/liter of total alcohol. Fermentation could be prolonged by in situ removal of solvents by gas stripping, and 35.6 g/liter of the IBE mixture could be produced in 45 h.     

Production of heterologous and chimeric scaffoldins by Clostridium acetobutylicum ATCC 824

Clostridium acetobutylicum ATCC 824 converts sugars and various polysaccharides into acids and solvents. This bacterium, however, is unable to utilize cellulosic substrates, since it is able to secrete very small amounts of cellulosomes. To promote the utilization of crystalline cellulose, the strategy we chose aims at producing heterologous minicellulosomes, containing two different cellulases bound to a miniscaffoldin, in C. acetobutylicum. A first step toward this goal describes the production of miniCipC1, a truncated form of CipC from Clostridium cellulolyticum, and the hybrid scaffoldin Scaf 3, which bears an additional cohesin domain derived from CipA from Clostridium thermocellum. Both proteins were correctly matured and secreted in the medium, and their various domains were found to be functional.     

Genomic analysis of the protein secretion systems in Clostridium acetobutylicum ATCC 824

Consistent information about protein secretion in Gram-positive bacteria is essentially restricted to the model organism Bacillus subtilis. Among genome-sequenced clostridia, Clostridium acetobutylicum has been the most extensively studied from a physiological point of view and is the organism for which the largest variety of molecular biology tools have been developed. Following in silico analyses, both secreted proteins and protein secretion systems were identified. The Tat (Twin arginine translocation; TC #2.A.64) pathway and ABC (ATP binding cassette) protein exporters (TC #3.A.1.) could not be identified, but the Sec (secretion) pathway (TC #3.A.5) appears to be used prevalently. Similarly, a flagella export apparatus (FEA; TC #3.A.6.), holins (TC #1.E.), and an ESAT-6/WXG100 (early secreted antigen target of 6 kDa/proteins with a WXG motif of approximately 100 residues) secretion system were identified. Here, we report for the first time the identification of a fimbrilin protein exporter (FPE; TC #3.A.14) and a Tad (tight adherence) export apparatus in C. acetobutylicum. This investigation highlights the potential use of this saprophytic bacterium in biotechnological and biomedical applications as well as a model organism for studying protein secretion in pathogenic Gram-positive bacteria.     

Analysis of the elements of catabolite repression in Clostridium acetobutylicum ATCC 824

The PTSH gene, encoding the phosphotransferase protein HPr, from Clostridium acetobutylicum ATCC 824 was identified from the genome sequence, cloned and shown to complement a PTSH mutant of Escherichia coli. The deduced protein sequence shares significant homology with HPr proteins from other low-GC gram-positive bacteria, although the highly conserved sequence surrounding the Ser-46 phosphorylation site is not well preserved in the clostridial protein. Nevertheless, the HPr was phosphorylated in an ATP-dependent manner in cell-free extracts of C. Acetobutylicum. Furthermore, purified His-tagged HPr from Bacillus Subtilis was also a substrate for the clostridial HPr kinase/phosphorylase. This phosphorylation reaction is a key step in the mechanism of carbon catabolite repression proposed to operate in B. Subtilis and other low-GC gram-positive bacteria. Putative genes encoding the HPr kinase/phosphorylase and the other element of this model, namely the catabolite control protein CcpA, were identified from the C. Acetobutylicum genome sequence, suggesting that a similar mechanism of carbon catabolite repression may operate in this industrially important organism.