Expression of an endoglucanase gene fromClostridium cellulolyticum inEscherichia coli

Clostridium cellulolyticum: model organism of mesophilic cellulolytic clostridia

Clostridium cellulolyticum ATCC 35319 is a non-ruminal mesophilic cellulolytic bacterium originally isolated from decayed grass. As with most truly cellulolytic clostridia, C. cellulolyticum possesses an extracellular multi-enzymatic complex, the cellulosome. The catalytic components of the cellulosome release soluble cello-oligosaccharides from cellulose providing the primary carbon substrates to support bacterial growth. As most cellulolytic bacteria, C. cellulolyticum was initially characterised by limited carbon consumption and subsequent limited growth in comparison to other saccharolytic clostridia. The first metabolic studies performed in batch cultures suggested nutrient(s) limitation and/or by-product(s) inhibition as the reasons for this limited growth. In most recent investigations using chemostat cultures, metabolic flux analysis suggests a self-intoxication of bacterial metabolism resulting from an inefficiently regulated carbon flow. The investigation of C. cellulolyticum physiology with cellobiose, as a model of soluble cellodextrin, and with pure cellulose, as a carbon source more closely related to lignocellulosic compounds, strengthen the idea of a bacterium particularly well adapted, and even restricted, to a cellulolytic lifestyle. The metabolic flux analysis from continuous cultures revealed that (i) in comparison to cellobiose, the cellulose hydrolysis by the cellulosome introduces an extra regulation of entering carbon flow resulting in globally lower metabolic fluxes on cellulose than on cellobiose, (ii) the glucose 1-phosphate/glucose 6-phosphate branch point controls the carbon flow directed towards glycolysis and dissipates carbon excess towards the formation of cellodextrins, glycogen and exopolysaccharides, (iii) the pyruvate/acetyl-CoA metabolic node is essential to the regulation of electronic and energetic fluxes. This in-depth analysis of C. cellulolyticum metabolism has permitted the first attempt to engineer metabolically a cellulolytic microorganism.

Isolation, characterization and manipulation of cellulase genes

The complete hydrolysis of cellulose requires a number of different enzymes including endoglucanase, exoglucanase and beta-glucosidase. These enzymes function in concert as part of a 'cellulase'complex called a cellulosome. In order (i) to develop a better understanding of the biochemical nature of the cellulase complex as well as the genetic regulation of its integral components and (ii) to utilize cellulases either as purified enzymes or as part of an engineered organism for a variety of purposes, researchers have, as a first step, used recombinant DNA technology to isolate the genes for these enzymes from a variety of organisms. This review provides some perspective on the current status of the isolation, characterization and manipulation of cellulase genes and specifically discusses (i) strategies for the isolation of endoglucanase, exoglucanase and beta-glucosidase genes; (ii) DNA sequence characterization of the cellulase genes and their accompanying regulatory elements; (iii) the expression of cellulase genes in heterologous host organisms and (iv) some of the proposed uses for isolated cellulase genes.

Carboxymethylcellulase and Avicelase activities from a cellulolytic Clostridium strain A11

The extracellular cellulase enzyme system of Clostridium A11 was fractionated by affinity chromatography on Avicel: 80% of the initial carboxymethylcellulase (CMCase) activity was adhered. This cellulase system was a multicomponent aggregate. Several CMCase activities were detected, but the major protein P1 had no detectable activity. Adhered and unadhered cellulases showed CMCase activity with the highest specific activity in Avicel-adhered fraction. However, only adhered fractions could degrade Avicel. Thus, efficiency of the enzymatic hydrolysis of Avicel was related to the cellulase-adhesion capacity. Carboxymethylcellulase and Avicelase activities were studied with the extracellular enzyme system and cloned cellulases. Genomic libraries from Clostridium A11 were constructed with DNA from this Clostridium, and a new gene cel1 was isolated. The gene(s) product(s) from cel1 exhibited CMCase and p-nitrophenylcellobiosidase (pNPCbase) activities. This cloned cellulase adhered to cellulose. Synergism between "adhered enzyme system" and cloned endoglucanases was observed on Avicel degradation. Conversely, no synergism was observed on CMC hydrolysis. Addition of cloned endoglucanase to cellulase complex led to increase of the Vmax without significant Km variation. Cloned endoglucanases can be added to cellulase complexes to efficiently hydrolyze cellulose.

The effects of tunicamycin on secretion, adhesion and activities of the cellulase complex of Clostridium cellulolyticum, ATCC 35319

The effects of tunicamycin, an inhibitor of N-asparagine-linked glycosylation, on the secretion, adhesion and activities of the cellulase complex produced by Clostridium cellulolyticum have been studied. Tunicamycin at 0.1 micrograms/ml slightly inhibited growth on cellobiose. Endoglucanase, p-nitrophenylcellobiosidase and avicelase activities of the "Avicel"-adsorbed fraction from a culture grown with this drug were decreased 4.4-, 1.4- and 12.2-fold, respectively. During growth on cellulose, tunicamycin considerably inhibited growth and adhesion of cells on their substrate (only 28% of the cells were bound to cellulose). SDS-PAGE mobilities of some proteins excreted during growth with the drug were different from those of proteins from control cultures; the native Avicel-adsorbed fraction (PH2O) consisted of three major components of molecular weights about 135, 90 and 68 kDa, whereas in the presence of tunicamycin (0.1 micrograms/ml), the Avicel-adsorbed fraction (PH2OT) contained only a major band of 105 kDa, and the proteins of 135 and 68 kDa appeared weakly. By using the "Dig Glycan Detection" kit, some proteins appeared to be glycosylated, such as the 135-, 95-, 47- and 40-kDa proteins. Moreover, the affinity for Avicel and the avicelase activity decreased dramatically for the Avicel-adsorbed fraction from a culture grown with the drug. The remaining avicelase activity of the PH2O fraction in the presence of specific P135 antiserum was 50% of the initial activity, whereas CMCase and pNPCbase were not affected. The glycosylated protein of 135 kDa played a prominent role in the adhesion and avicelase activity of C. cellulolyticum. Moreover, the endoglucanase activity in a culture broth from tunicamycin-grown cells was more thermolabile and protease-sensitive than that from control cultures.

Clostridium cellulolyticum Viability and Sporulation under Cellobiose Starvation Conditions

Depending on the moment of cellobiose starvation, Clostridium cellulolyticum cells behave in different ways. Cells starved during the exponential phase of growth sporulate at 30%, whereas exhaustion of the carbon substrate at the beginning of growth does not provoke cell sporulation. Growth in the presence of excess cellobiose generates 3% spores. The response of C. cellulolyticum to carbon starvation involves changes in proteolytic activities; higher activities (20% protein degradation) corresponded to a higher level of sporulation; lower proteolysis (5%) was observed in cells starved during the beginning of exponential growth, when sporulation was not observed; with an excess of cellobiose, an intermediate value (10%), accompanied by a low level of sporulation, was observed in cells taken at the end of the exponential growth phase. The basal percentage of the protein breakdown in nonstarved culture was 4%. Cells lacking proteolytic activities failed to induce sporulation. High concentrations of cellobiose repressed proteolytic activities and sporulation. The onset of carbon starvation during the growth phase affected the survival response of C. cellulolyticum via the sporulation process and also via cell-cellulose interaction. Cells from the exponential growth phase were more adhesive to filter paper than cells from the stationary growth phase but less than cells from the late stationary growth phase.

Colonization of Crystalline Cellulose by Clostridium cellulolyticum ATCC 35319

Cellulose colonization by Clostridium cellulolyticum was studied by using [ methyl- 3 H]thymidine incorporation. The colonization process indicated that a part of the bacterial population was released from cellulose to the liquid phase before binding and colonizing another adhesion site of the cellulose. We postulate that cellulose colonization occurs according to the following process: adhesion, colonization, release, and readhesion.

Adhesion and growth rate of Clostridium cellulolyticum ATCC 35319 on crystalline cellulose

The rate of tritiated-thymidine incorporation into DNA was used to estimate Clostridium cellulolyticum H10 growth rates on Avicel cellulose, taking into consideration both the unattached cells and the cells adhered to the substrate. The generation time on cellobiose calculated from the data on cell density (4.5 h) agreed well with the generation time calculated by tritiated-thymidine incorporation (3.8 h). Growth on Avicel cellulose occurred when bacteria were adhered to their substrate; 80% of the biomass was detected on the cellulose. Taking into consideration attached and free bacteria, the generation time as determined by thymidine incorporation was about 8 h, whereas by bacterial-protein estimation it was about 13 h. In addition to the growth rate of the bacteria on the cellulose, the release of adhered cells constituted an important factor in the efficiency of the cellulolysis. The stage of growth influenced adhesion of C. cellulolyticum; maximum adhesion was found during the exponential phase. Under the conditions used, the end of growth was characterized by an acute release of biomass and cellulase activity from the cellulose. An exhaustion of the accessible cellulose could be responsible for this release.

Purification and properties of two truncated endoglucanases produced in Escherichia coli harbouring Clostridium cellulolyticum endoglucanase gene celCCD

The endoglucanase gene, celCCD, of Clostridium cellulolyticum has been expressed in Escherichia coli. Multiple active polypeptides were detected in the E. coli cells. The relative molecular mass (M(r)) of two major active polypeptides were 56,000 (D56) and 38,000 (D38), which were smaller than the deduced M(r) of the mature protein (63,401). D56 and D38 were purified from the periplasmic fraction. The N-terminal sequences of the two purified polypeptides were identical to that of the mature endoglucanase (Ala-Ile-Asn-Ser-Gln-Asp-Met-Val---) deduced from the nucleotide sequence. These data indicated that these polypeptides were produced by processing the original mature protein in the C-terminal region. The enzymatic properties of these two polypeptides were very similar, except that the specific activity of D38 was 2-3.5-fold higher than that of D56, and D38 was more heat stable than D56.

Sequence analysis of a gene cluster encoding cellulases from Clostridium cellulolyticum

The sequence of a 5633-bp EcoRI-PvuII DNA fragment from Clostridium cellulolyticum was determined. This fragment contains two complete endo-beta-1,4-glucanase-encoding genes, designated celCCC and celCCG. These two genes are flanked by two other partial open reading frames (ORF1 and celCCE) that probably encode two cellulases or related enzymes. The celCCC and celCCG genes appear to be present in a polycistronic transcriptional unit. Northern blot hybridisations with intragenic probes derived from celCCC and celCCG gave similar patterns. Two transcripts of about 5 and 6 kb were identified. The celCCC and celCCG ORFs extend over 1380 bp and 2175 bp, respectively. They are separated by only 87 nt. A typical signal sequence is present at the N terminus of the deduced polypeptides. The mature CelCCC and CelCCG proteins have M(r)s 47,201 and 76,101, respectively. Comparisons between their amino acid (aa) sequences and other known cellulase sequences revealed that: first, they both contain the repeated 24-aa sequence characteristic of clostridial beta-glycanases, secondly, the N-terminal catalytic domains of CelCCC and CelCCG can be classified into the D and E2 families, respectively, and thirdly, the largest CelCCG contains an additional internal domain which is very similar to that of the Bacillus-type cellulose-binding domain (CBD). The ORF1-C-terminal-encoded sequence also contains the clostridial 24-aa repeat. The CelCCE N-terminus consists of a typical signal sequence followed by a 168-aa domain homologous to the N-terminal repeated domain of Cellulomonas fimi CenC. This domain is connected to an incomplete catalytic domain of family E1 by a Pro-rich junction linker.

Nucleotide sequence analysis of the endoglucanase-encoding gene, celCCD, of Clostridium cellulolyticum

The nucleotide sequence of the Clostridium cellulolyticum endo-beta-1,4- glucanase (EGCCD)-encoding gene, celCCD, and its flanking regions, was determined. The open reading frame encodes a protein (Mr 66,061) which consists of 584 amino acids (aa). The N terminus shows the features of the typical signal peptide, with a cleavage site after Gly24. The protein could be divided into N-terminal and C-terminal regions by an intermediate Pro + Thr-rich sequence. Deletion analysis suggests the C-terminal region is not necessary for EG activity. The predicted aa sequence of the mature protein was similar to those of the central catalytic and the following C-terminal regions of the C. thermocellum endoglucanase H (EGH; identity, 58.8%). The N-terminal region resembled that of the endoglucanase, EGCCA, from C. cellulolyticum (identity, 24.7%; 336 aa) and the endoglucanase, EGE, from C. thermocellum (identity, 31.4%; 373 aa). The C-terminal regions ended with two conserved 21-aa stretches which had close similarity to each other. The C-terminal sequence was also highly similar to the reiterated domain of several EG and a xylanase from C. thermocellum, and of an EG from C. cellulolyticum.

Sequence analysis of the Clostridium cellulolyticum endoglucanase-A-encoding gene, celCCA

The nucleotide sequence of a Clostridium cellulolyticum endo-beta-1,4- glucanase (EGCCA)-encoding gene (celCCA) and its flanking regions, was determined. An open reading frame (ORF) of 1425 bp was found, encoding a protein of 475 amino acids (aa). This ORF began with an ATG start codon and ended with a TAA ochre stop codon. The N-terminal region of the EGCCA protein resembled a typical signal sequence of a Gram-positive bacterial extracellular protein. A putative signal peptidase cleavage site was determined. EGCCA, without a signal peptide, was found to be composed of more than 35% hydrophobic aa and to have an Mr of 50715. Comparison of the encoded sequence with other known cellulase sequences showed the existence of various kinds of aa sequence homologies. First, a strong homology was found between the C-terminal region of EGCCA, containing a reiterated stretch of 24 aa, and the conserved reiterated region previously found to exist in four Clostridium thermocellum endoglucanases and one xylanase from the same organism. This region was suspected of playing a role in organizing the cellulosome complex. Second, an extensive homology was found between EGCCA and the N-terminal region of the large endoglucanase, EGE, from C. thermocellum, which suggests that they may have a common ancestral gene. Third, a region, which extended for 21 aa residues beginning at aa + 127, was found to be homologous with regions of cellulases belonging to Bacilli, Clostridia and Erwinia chrysanthemi.

Cloning and characterization of two genes from Bacillus polymyxa expressing beta-glucosidase activity in Escherichia coli

DNA fragments from Bacillus polymyxa which encode beta-glucosidase activity were cloned in Escherichia coli by selection of yellow transformants able to hydrolyze the artificial chromogenic substrate p-nitrophenyl-beta-D-glucopyranoside. Restriction endonuclease maps and Southern analysis of the cloned fragments showed the existence of two different genes. Expression of either one of these genes allowed growth of E. coli in minimal medium with cellobiose as the only carbon source. One of the two enzymes was found in the periplasm of E. coli, hydrolyzed arylglucosides more actively than cellobiose, and rendered glucose as the only product upon cellobiose hydrolysis. The other enzyme was located in the cytoplasm, was more active toward cellobiose, and hydrolyzed this disaccharide, yielding glucose and another, unidentified compound, probably a phosphorylated sugar.