Purification and characterization of endoglucanase C of Cellulomonas fimi, cloning of the gene, and analysis of in vivo transcripts of the gene

Chitin-Active Lytic Polysaccharide Monooxygenases Are Rare in Cellulomonas Species

Cellulomonas flavigena is a saprotrophic bacterium that encodes, within its genome, four predicted lytic polysaccharide monooxygenases (LPMOs) from Auxiliary Activity family 10 (AA10). We showed previously that three of these cleave the plant polysaccharide cellulose by oxidation at carbon-1 (J. Li, L. Solhi, E.D. Goddard-Borger, Y. Mattieu et al., Biotechnol Biofuels 14:29, 2021, https://doi.org/10.1186/s13068-020-01860-3). Here, we present the biochemical characterization of the fourth C. flavigena AA10 member (CflaLPMO10D) as a chitin-active LPMO. Both the full-length CflaLPMO10D-Carbohydrate-Binding Module family 2 (CBM2) and catalytic module-only proteins were produced in Escherichia coli using the native general secretory (Sec) signal peptide. To quantify chitinolytic activity, we developed a high-performance anion-exchange chromatography-pulsed amperometric detection (HPAEC-PAD) method as an alternative to the established hydrophilic interaction liquid ion chromatography coupled with UV detection (HILIC-UV) method for separation and detection of released oxidized chito-oligosaccharides. Using this method, we demonstrated that CflaLPMO10D is strictly active on the β-allomorph of chitin, with optimal activity at pH 5 to 6 and a preference for ascorbic acid as the reducing agent. We also demonstrated the importance of the CBM2 member for both mediating enzyme localization to substrates and prolonging LPMO activity. Together with previous work, the present study defines the distinct substrate specificities of the suite of C. flavigena AA10 members. Notably, a cross-genome survey of AA10 members indicated that chitinolytic LPMOs are, in fact, rare among Cellulomonas bacteria. IMPORTANCE Species from the genus Cellulomonas have a long history of study due to their roles in biomass recycling in nature and corresponding potential as sources of enzymes for biotechnological applications. Although Cellulomonas species are more commonly associated with the cleavage and utilization of plant cell wall polysaccharides, here, we show that C. flavigena produces a unique lytic polysaccharide monooxygenase with activity on β-chitin, which is found, for example, in arthropods. The limited distribution of orthologous chitinolytic LPMOs suggests adaptation of individual cellulomonads to specific nutrient niches present in soil ecosystems. This research provides new insight into the biochemical specificity of LPMOs in Cellulomonas species and related bacteria, and it raises new questions about the physiological function of these enzymes.

Four cellulose-active lytic polysaccharide monooxygenases from Cellulomonas species

BACKGROUND

The discovery of lytic polysaccharide monooxygenases (LPMOs) has fundamentally changed our understanding of microbial lignocellulose degradation. Cellulomonas bacteria have a rich history of study due to their ability to degrade recalcitrant cellulose, yet little is known about the predicted LPMOs that they encode from Auxiliary Activity Family 10 (AA10).

RESULTS

Here, we present the comprehensive biochemical characterization of three AA10 LPMOs from Cellulomonas flavigena (CflaLPMO10A, CflaLPMO10B, and CflaLPMO10C) and one LPMO from Cellulomonas fimi (CfiLPMO10). We demonstrate that these four enzymes oxidize insoluble cellulose with C1 regioselectivity and show a preference for substrates with high surface area. In addition, CflaLPMO10B, CflaLPMO10C, and CfiLPMO10 exhibit limited capacity to perform mixed C1/C4 regioselective oxidative cleavage. Thermostability analysis indicates that these LPMOs can refold spontaneously following denaturation dependent on the presence of copper coordination. Scanning and transmission electron microscopy revealed substrate-specific surface and structural morphological changes following LPMO action on Avicel and phosphoric acid-swollen cellulose (PASC). Further, we demonstrate that the LPMOs encoded by Cellulomonas flavigena exhibit synergy in cellulose degradation, which is due in part to decreased autoinactivation.

CONCLUSIONS

Together, these results advance understanding of the cellulose utilization machinery of historically important Cellulomonas species beyond hydrolytic enzymes to include lytic cleavage. This work also contributes to the broader mapping of enzyme activity in Auxiliary Activity Family 10 and provides new biocatalysts for potential applications in biomass modification.

Characterisation of novel biomass degradation enzymes from the genome of Cellulomonas fimi

•

Identified over 90 putative polysaccharide degrading ORFs in C. fimi genome.

•

Cloned 14 putative cellulolytic ORFs as BioBricks, screened them for activity.

•

Partially purified AfsB, BxyF, BxyH and XynF and characterised them further.

•

BxyH proved highly temperature and alkaline pH tolerant.

•

BioBricks are an easy method for screening genes for specific activities.

Recent analyses of genome sequences belonging to cellulolytic bacteria have revealed many genes potentially coding for cellulosic biomass degradation enzymes. Annotation of these genes however, is based on few biochemically characterised examples. Here we present a simple strategy based on BioBricks for the rapid screening of candidate genes expressed in Escherichia coli. As proof of principle we identified over 70 putative biomass degrading genes from bacterium Cellulomonas fimi, expressing a subset of these in BioBrick format. Six novel genes showed activity in E. coli. Four interesting enzymes were characterised further. α-l-arabinofuranosidase AfsB, β-xylosidases BxyF and BxyH and multi-functional β-cellobiosidase/xylosidase XynF were partially purified to determine their optimum pH, temperature and kinetic parameters. One of these enzymes, BxyH, was unexpectedly found to be highly active at strong alkaline pH and at temperatures as high as 100 °C. This report demonstrates a simple method of quickly screening and characterising putative genes as BioBricks.

Characterization of a thermostable endoglucanase from Cellulomonas fimi ATCC484

Bacteria in the genus Cellulomonas are well known as secretors of a variety of mesophilic carbohydrate degrading enzymes (e.g., cellulases and hemicellulases), active against plant cell wall polysaccharides. Recent proteomic analysis of the mesophilic bacterium Cellulomonas fimi ATCC484 revealed uncharacterized enzymes for the hydrolysis of plant cell wall biomass. Celf_1230 (CfCel6C), a secreted protein of Cellulomonas fimi ATCC484, is a novel member of the GH6 family of cellulases that could be successfully expressed in Escherichia coli. This enzyme displayed very little enzymatic/hydrolytic activity at 30 °C, but showed an optimal activity around 65 °C, and exhibited a thermal denaturation temperature of 74 °C. In addition, it also strongly bound to filter paper despite having no recognizable carbohydrate binding module. Our experiments show that CfCel6C is a thermostable endoglucanase with activity on a variety of β-glucans produced by an organism that struggles to grow above 30 °C.

Proteomic Analysis of the Secretome of Cellulomonas fimi ATCC 484 and Cellulomonas flavigena ATCC 482

The bacteria in the genus Cellulomonas are known for their ability to degrade plant cell wall biomass. Cellulomonas fimi ATCC 484 and C. flavigena ATCC 482 have been the subject of much research into secreted cellulases and hemicellulases. Recently the genome sequences of both C. fimi ATCC 484 and C. flavigena ATCC 482 were published, and a genome comparison has revealed their full spectrum of possible carbohydrate-active enzymes (CAZymes). Using mass spectrometry, we have compared the proteins secreted by C. fimi and C. flavigena during growth on the soluble cellulose substrate, carboxymethylcellulose (CMC), as well as a soluble xylan fraction. Many known C. fimi CAZymes were detected, which validated our analysis, as were a number of new CAZymes and other proteins that, though identified in the genome, have not previously been observed in the secretome of either organism. Our data also shows that many of these are co-expressed on growth of either CMC or xylan. This analysis provides a new perspective on Cellulomonas enzymes and provides many new CAZyme targets for characterization.

Cyclic AMP regulates the biosynthesis of cellobiohydrolase in Cellulomonas flavigena growing in sugar cane bagasse

Cellulomonas flavigena produces a battery of cellulase components that act concertedly to degrade cellulose. The addition of cAMP to repressed C. flavigena cultures released catabolic repression, while addition of cAMP to induced C. flavigena cultures led to a cellobiohydrolase hyperproduction. Exogenous cAMP showed positive regulation on cellobiohydrolase production in C. flavigena grown on sugar cane bagasse. A C. flavigena cellobiohydrolase gene was cloned (named celA), which coded for a 71- kDa enzyme. Upstream, a repressor celR1, identified as a 38 kDa protein, was monitored by use of polyclonal antibodies.

Directed Evolution of a Glycosynthase from Agrobacterium sp. Increases Its Catalytic Activity Dramatically and Expands Its Substrate Repertoire

The Agrobacterium sp. beta-glucosidase (Abg) is a retaining beta-glycosidase and its nucleophile mutants, termed Abg glycosynthases, catalyze the formation of glycosidic bonds using alpha-glycosyl fluorides as donor sugars and various aryl glycosides as acceptor sugars. Two rounds of random mutagenesis were performed on the best glycosynthase to date (AbgE358G), and transformants were screened using an on-plate endocellulase coupled assay. Two highly active mutants were obtained, 1D12 (A19T, E358G) and 2F6 (A19T, E358G, Q248R, M407V) in the first and second rounds, respectively. Relative catalytic efficiencies (kcat/Km) of 1:7:27 were determined for AbgE358G, 1D12, and 2F6, respectively, using alpha-D-galactopyranosyl fluoride and 4-nitrophenyl beta-D-glucopyranoside as substrates. The 2F6 mutant is not only more efficient but also has an expanded repertoire of acceptable substrates. Analysis of a homology model structure of 2F6 indicated that the A19T and M407V mutations do not interact directly with substrates but exert their effects by changing the conformation of the active site. Much of the improvement associated with the A19T mutation seems to be caused by favorable interactions with the equatorial C2-hydroxyl group of the substrate. The alteration of torsional angles of Glu-411, Trp-412, and Trp-404, which are components of the aglycone (+1) subsite, is an expected consequence of the A19T and M407V mutations based on the homology model structure of 2F6.

The glucanases of Cellulomonas

Cellulomonas is a unique bacterium possessing not only the capacity to degrade various carbohydrates, such as starch, xylan and cellulose, but crystalline cellulose as well. It has developed a complex battery of glucanases to deal with substrates possessing such extensive microheterogeneities. Some of these enzymes are multifunctional, as well as cross inducible, possessing a multi-domain structure; these enzymes are thought to have arisen by the shuffling of these domains. Intergeneric hybrids have been constructed between Cellulomonas and Zymomonas so as to enhance the industrial potential of this organism. This review examines the unique features of this microorganism and evaluates its key role in the conversion of complex wastes to useful products, by virtue of its unusual attributes.

Purification, characterization, and molecular analysis of thermostable cellulases CelA and CelB from Thermotoga neapolitana

Two thermostable endocellulases, CelA and CelB, were purified from Thermotoga neapolitana. CelA (molecular mass, 29 kDa; pI 4.6) is optimally active at pH 6.0 at 95 degreesC, while CelB (molecular mass, 30 kDa; pI 4.1) has a broader optimal pH range (pH 6.0 to 6.6) at 106 degreesC. Both enzymes are characterized by a high level of activity (high Vmax value and low apparent Km value) with carboxymethyl cellulose; the specific activities of CelA and CelB are 1,219 and 1,536 U/mg, respectively. With p-nitrophenyl cellobioside the Vmax values of CelA and CelB are 69.2 and 18.4 U/mg, respectively, while the Km values are 0.97 and 0.3 mM, respectively. The major end products of cellulose hydrolysis, glucose and cellobiose, competitively inhibit CelA, and CelB. The Ki values for CelA are 0.44 M for glucose and 2.5 mM for cellobiose; the Ki values for CelB are 0.2 M for glucose and 1.16 mM for cellobiose. CelB preferentially cleaves larger cellooligomers, producing cellobiose as the end product; it also exhibits significant transglycosylation activity. This enzyme is highly thermostable and has half-lives of 130 min at 106 degreesC and 26 min at 110 degreesC. A single clone encoding the celA and celB genes was identified by screening a T. neapolitana genomic library in Escherichia coli. The celA gene encodes a 257-amino-acid protein, while celB encodes a 274-amino-acid protein. Both proteins belong to family 12 of the glycosyl hydrolases, and the two proteins are 60% similar to each other. Northern blots of T. neapolitana mRNA revealed that celA and celB are monocistronic messages, and both genes are inducible by cellobiose and are repressed by glucose.

Analysis of molecular size distributions of cellulose molecules during hydrolysis of cellulose by recombinant Cellulomonas fimi beta-1,4-glucanases

Four beta-1,4-glucanases (cellulases) of the cellulolytic bacterium Cellulomonas fimi were purified from Escherichia coli cells transformed with recombinant plasmids. Previous analyses using soluble substrates had suggested that CenA and CenC were endoglucanases while CbhA and CbhB resembled the exo-acting cellobiohydrolases produced by cellulolytic fungi. Analysis of molecular size distributions during cellulose hydrolysis by the individual enzymes confirmed these preliminary findings and provided further evidence that endoglucanase CenC has a more processive hydrolytic activity than CenA. The significant differences between the size distributions obtained during hydrolysis of bacterial microcrystalline cellulose and acid-swollen cellulose can be explained in terms of the accessibility of beta-1,4-glucan chains to enzyme attack. Endoglucanases and cellobiohydrolases were much more easily distinguished when the acid-swollen substrate was used.

Cellobiohydrolase B, a second exo-cellobiohydrolase from the cellulolytic bacterium Cellulomonas fimi

The gene cbhB from the cellulolytic bacterium Cellulomonas fimi encodes a polypeptide of 1090 amino acids. Cellobiohydrolase B (CbhB) is 1037 amino acids long, with a calculated molecular mass of 109765 Da. The enzyme comprises five domains: an N-terminal catalytic domain of 643 amino acids, three fibronectin type III repeats of 97 amino acids each, and a C-terminal cellulose-binding domain of 104 amino acids. The catalytic domain belongs to family 48 of glycosyl hydrolases. CbhB has a very low activity on CM-cellulose. Viscometric analysis of CM-cellulose hydrolysis indicates that the enzyme is an exoglucanase. Cellobiose is the major product of hydrolysis of cellulose. In common with two other exoglycanases from C. fimi, CbhB has low but detectable endoglucanase activity. CbhB is the second exo-cellobiohydrolase found in C. fimi. Therefore, the cellulase system of C. fimi resembles those of fungi in comprising multiple endoglucanases and cellobiohydrolases.

Transcriptional induction and expression of the endoglucanase celA gene from a ruminal Clostridium sp. ("C. longisporum")

Northern (RNA) blot analysis of RNA from Clostridium sp. revealed induction of transcription of the celA gene when barley beta-glucan was used as carbon source, while no celA mRNA was detected after growth on cellobiose. Western blots (immunoblots), prepared by using a rabbit antiserum raised against CelA protein purified from Escherichia coli, revealed the extracellular location of CelA in Clostridium sp. Despite the absence of detectable celA mRNA, significant quantities of CelA were detected in the culture supernatant during growth on cellobiose. This finding indicated a low constitutive expression of celA. A 6.7-fold increase in the total beta-glucanase specific activity in the extracellular fraction was observed during growth on beta-glucan. The transcriptional start site of celA was mapped by extension and was found to be the same in Clostridium sp. and in E. coli expressing the cloned celA gene. A consensus E. coli -10 promoter region (AATAAT), but not a -35 promoter region, could be identified. Two direct repeats (TATTGAATTTAT) separated by 15 nucleotides flank the region where the consensus -35 promoter regions would have been. The size of the celA mRNA transcript corresponded with the size of the open reading frame. A potential stem-loop structure was found 18 nucleotides downstream of the 3' stop codon, which could be responsible for termination of transcription.

Transcriptional analysis of the Clostridium cellulovorans endoglucanase gene, engB

An endoglucanase gene, which was shown to be identical to the previously sequenced engB gene [Attwood et al. (1993) Abstr. Ann. Meet. Am. Soc. Microbiol.], was isolated from a Clostridium cellulovorans genomic library. Because of the lack of transcriptional information concerning engB we examined its expression in C. cellulovorans and in the heterologous hosts Escherichia coli and C. acetobutylicum following transformation of engB. Northern analysis suggested that both E. coli and C. acetobutylicum produced several transcripts of various sizes. C. cellulovarans produced a single transcript of 1600 bp with the relative amount of engB mRNA from cellulose-grown cells being much greater than that from cellobiose-grown cells. Primer extensions showed that engB was transcribed from a single transcription initiation site in C. cellulovorans preceded by sequences similar to promoter sequences found in Gram-positive bacteria. Primer extensions from both E. coli and C. acetobutylicum strains containing the engB gene showed multiple transcription initiation sites, none of which corresponded to the site determined in C. cellulovorans. We conclude that transcriptional control of the engB gene is less stringent in heterologous backgrounds and postulate that expression of the engB gene in C. cellulovorans is increased in the presence of cellulose.

Changes in the molecular-size distribution of insoluble celluloses by the action of recombinant Cellulomonas fimi cellulases

Specific patterns of attacks of cotton, bacterial cellulose and bacterial microcrystalline cellulose (BMCC) by recombinant cellulases of Cellulomonas fimi were investigated. Molecular-size distributions of the celluloses were determined by high-performance size-exclusion chromatography. Chromatography of cotton and bacterial celluloses revealed single major peaks centered over progressively lower molecular-mass positions during attack by endoglucanase CenA. In advanced stages, a second peak appeared at very low average size (approx. 11 glucosyl units); ultimate weight losses were approximately 30%. The isolated catalytic domain of CenA, p30, gave results very similar to those with complete CenA. CenA did not effectively depolymerize or solubilize BMCC significantly. Molecular-size distributions of cotton and bacterial cellulose incubated with endoglucanases CenB or CenD exhibited one major peak regardless of incubation time; low-molecular-mass fragments did not accumulate. Weight losses were 40 and 35% respectively. The single peak shifted to lower-molecular-mass positions as incubation continued, but high-molecular-mass material persisted. CenB and CenD readily attacked and solubilized BMCC (approx. 70%). We conclude that CenA attacks cellulose by preferentially cleaving completely through the cellulose microfibrils at the amorphous sites, and much more slowly by degrading the crystalline surfaces. Conversely, CenB and CenD cleave the amorphous regions much less efficiently while vigorously degrading the surfaces of the crystalline regions of the microfibrils.

Cellobiohydrolase A (CbhA) from the cellulolytic bacterium Cellulomonas fimi is a beta-1,4-exocellobiohydrolase analogous to Trichoderma reesei CBH II

The gene cbhA from the cellulolytic bacterium Cellulomonas fimi encodes a protein of 872 amino acids designated cellobiohydrolase A (CbhA). Mature CbhA contains 832 amino acid residues and has a predicted molecular mass of 85,349 Da. It is composed of five domains: an N-terminal catalytic domain, three repeated sequences of 95 amino acids, and a C-terminal cellulose-binding domain typical of other C. fimi glycanases. The structure and enzymatic activities of the CbhA catalytic domain are closely related to those of CBH II, an exocellobiohydrolase in the glycosyl hydrolase family B from the fungus Trichoderma reesei. CbhA is the first such enzyme to be characterized in bacteria. The data support the proposal that extended loops around the active site distinguish exohydrolases from endohydrolases in this enzyme family.

The biological degradation of cellulose

Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the beta-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.

DNA sequences and expression in Streptomyces lividans of an exoglucanase gene and an endoglucanase gene from Thermomonospora fusca

Two genes encoding cellulases E1 and E4 from Thermomonospora fusca have been cloned in Escherichia coli, and their DNA sequences have been determined. Both genes were introduced into Streptomyces lividans, and the enzymes were purified from the culture supernatants of transformants. E1 and E4 were expressed 18- and 4-fold higher, respectively, in S. lividans than in E. coli. Thin-layer chromatography of digestion products showed that E1 digests cellotriose, cellotetraose, and cellopentaose to cellobiose and a trace of glucose. E4 is poor at degrading cellotriose and cleaves cellopentaose to cellotetraose and glucose or cellotriose and cellobiose. It readily cleaves cellotetraose to cellobiose. E1 shows 59% identity to Cellulomonas fumi CenC in a 689-amino-acid overlap, and E4 shows 80% identity to the N terminus of C. fimi CenB in a 441-amino-acid overlap; all of these proteins are members of cellulase family E. Alignment of the amino acid sequences of Clostridium thermocellum celD, E1, E4, and four other members of family E demonstrates a clear relationship between their catalytic domains, although there is as little as 25% identity between some of them. Residues in celD that have been identified by site-directed mutagenesis and chemical modification to be important for catalytic activity are conserved in all seven proteins. The catalytic domains of E1 and E4 are not similar to those of T. fusca E2 or E5, but all four enzymes share similar cellulose-binding domains and have the same 14-bp inverted repeat upstream of their initiation codons. This sequence has been identified previously as the binding site for a protein that regulates induction.

Cellulose-binding polypeptides from Cellulomonas fimi: endoglucanase D (CenD), a family A beta-1,4-glucanase

Five cellulose-binding polypeptides were detected in Cellulomonas fimi culture supernatants. Two of them are CenA and CenB, endo-beta-1,4-glucanases which have been characterized previously; the other three were previously uncharacterized polypeptides with apparent molecular masses of 120, 95, and 75 kDa. The 75-kDa cellulose-binding protein was designated endoglucanase D (CenD). The cenD gene was cloned and sequenced. It encodes a polypeptide of 747 amino acids. Mature CenD is 708 amino acids long and has a predicted molecular mass of 74,982 Da. Analysis of the predicted amino acid sequence of CenD shows that the enzyme comprises four domains which are separated by short linker polypeptides: an N-terminal catalytic domain of 405 amino acids, two repeated sequences of 95 amino acids each, and a C-terminal domain of 105 amino acids which is > 50% identical to the sequences of cellulose-binding domains in Cex, CenA, and CenB from C. fimi. Amino acid sequence comparison placed the catalytic domain of CenD in family A, subtype 1, of beta-1,4-glycanases. The repeated sequences are more than 40% identical to the sequences of three repeats in CenB and are related to the repeats of fibronectin type III. CenD hydrolyzed the beta-1,4-glucosidic bond with retention of anomeric configuration. The activities of CenD towards various cellulosic substrates were quite different from those of CenA and CenB.

The binding of Cellulomonas fimi endoglucanase C (CenC) to cellulose and Sephadex is mediated by the N-terminal repeats

Endoglucanase C (CenC) from Cellulomonas fimi binds to cellulose and to Sephadex. The enzyme has two contiguous 150-amino-acid repeats (N1 and N2) at its N-terminus and two unrelated contiguous 100-amino-acid repeats (C1 and C2) at its C-terminus. Polypeptides corresponding to N1, N1N2, C1, and C1C2 were produced by expression of appropriate cenC gene fragments in Escherichia coli. N1N2, but not N1 alone, binds to Sephadex; both polypeptides bind to Avicel, (a heterogeneous cellulose preparation containing both crystalline and non-crystalline components). Neither C1 nor C1C2 binds to Avicel or Sephadex. N1N2 and N1 bind to regenerated ('amorphous') cellulose but not to bacterial crystalline cellulose; the cellulose-binding domain of C. fimi exoglucanase Cex binds to both of these forms of cellulose. Amino acid sequence comparison reveals that N1 and N2 are distantly related to the cellulose-binding domains of Cex and C. fimi endoglucanases A and B.

DNA sequence and transcription of an endoglucanase gene from Prevotella (Bacteroides) ruminicola AR20

The endoglucanase gene was sequenced from Prevotella ruminicola AR20, isolated as clone pJW4. The endoglucanase (BrEND) is encoded by an open reading frame (ORF1) of 501 codons, corresponding to a protein of calculated molecular weight 55.7 kDa. Analysis of proteins on SDS-PAGE revealed a protein corresponding to the calculated molecular weight of the processed BrEND. The protein showed substantial homology to members of the A4 sub-family cellulases. Primer extension studies revealed that transcription of celA is initiated at different sites in Escherichia coli and Prevotella ruminicola. E. coli sigma 70 recognition sequences were identified, which were located upstream from the transcription initiation site (TIS) functional in E. coli. A longer extension product was identified using RNA from P. ruminicola, indicating that the gene may normally be transcribed as part of a polycistronic message. The end of the primer extension product corresponded to a site beyond the 5' boundary of the cloned fragment, thus preventing identification of native promoter sequences. A second ORF of 110 codons (ORF2) was identified on the antisense strand, and primer extension indicated that transcription through ORF2 was initiated at an identical site in both E. coli and P. ruminicola. E. coli-like consensus sequences were located at positions -10 and -35 upstream from this site, suggesting that some promoter sequences in P. ruminicola are similar to E. coli consensus sequences, although others recognized by E. coli are non-functional in P. ruminicola.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemistry and genetics of actinomycete cellulases

The order Actinomycetales includes a number of genera that contain species that actively degrade cellulose and these include both mesophilic and facultative thermophilic species. Cellulases produced by strains from two of the genera containing thermophilic organisms have been studied extensively: Microbispora bispora and Thermomonospora fusca. Fractionation of M. bispora cellulases has identified six different enzymes, all of which were purified to near homogeneity and partially characterized. Two of these enzymes appear to be exocellulases and gave synergism with each other and with the endocellulases. The structural genes of five M. bispora cellulases have been cloned and one was sequenced. Fractionation of T. fusca cellulases has identified five different enzymes, all of which were purified to near homogeneity and partially characterized. One of the T. fusca enzymes gives synergism in the hydrolysis of crystalline cellulose with several T. fusca endocellulases and with Trichoderma reesei CBHI but not with T. reesei CBHII. Each T. fusca cellulase contains distinct catalytic and cellulose binding domains. The structural genes of four of the T. fusca endoglucanases have been cloned and sequenced, while three cellulase genes have been cloned from "T. curvata". The T. fusca cellulase genes are expressed at a low level in Escherichia soli, but at a high level in Streptomyces lividans. Sequence comparisons have shown that there are no significant amino acid homologies between any of the catalytic domains of the four T. fusca cellulases, but each of them shows extensive homology to several other cellulases and fits in one of the five existing cellulase gene families. There have been extensive studies of the regulation of the synthesis of these cellulases and a number of regulatory mutants have been isolated. This work has shown that the different T. fusca cellulases are coordinately regulated over a 100-fold range by two independent controls; induction by cellobiose and repression by any good carbon source.

Cloning of three endoglucanase genes from Thermomonospora curvata into Escherichia coli

A BamHI genomic library from Thermomonospora curvata was constructed in E. coli using cosmid vector pHC79. Four clones able to hydrolyze CMC were isolated. Restriction digests and Southern gel analysis revealed the presence of three different endoglucanase genes. DNA fragments contained in all of the endoglucanase cosmids hybridized to T. curvata chromosomal DNA. The cellulase genes were expressed in E. coli, but at rather low levels.

Nucleotide sequence of the endoglucanase C gene (cenC) of Cellulomonas fimi, its high-level expression in Escherichia coli, and characterization of its products

The cenC gene of Cellulomonas fimi, encoding endoglucanase CenC, has an open reading frame of 1101 codons closely followed by a 9 bp inverted repeat. The predicted amino acid sequence of mature CenC, which is 1069 amino acids long, is very unusual in that it has a 150-amino-acid tandem repeat at the N-terminus and an unrelated 100-amino-acid tandem repeat at the C-terminus. CenC belongs to subfamily E1 of the beta-1,4-glycanases. High-level expression in Escherichia coli of cenC from a 3.6 kbp fragment of C. fimi DNA leads to levels of CenC which exceed 10% of total cell protein. Most of the CenC is in the cytoplasm in an inactive form. About 60% of the active fraction of CenC is in the periplasm. The catalytic properties of the active CenC are indistinguishable from those of native CenC from C. fimi. The Mr of CenC from E. coli and C. fimi is approximately 130 kDa. E. coli and C. fimi also produce an endoglucanase, CenC', of approximate Mr 120kDa and with the same N-terminal amino acid sequence and catalytic properties as CenC. CenC' appears to be a proteolytic product of CenC. CenC and CenC' can bind to cellulose and to Sephadex. CenC is the most active component of the C. fimi cellulase system isolated to date.

Transcription of Clostridium thermocellum endoglucanase genes celF and celD

Transcripts of the Clostridium thermocellum endoglucanase genes celF and celD, encoding endoglucanases F and D, respectively, were characterized. The size of the mRNAs was about 2.35 kb for celF and 2.1 kb for celD, indicating monocistronic transcription of both genes. A unique 5' end, located 218 bp upstream from the initiation codon, was found for celF mRNA. No convincing homology could be identified between the sequence upstream from the celF 5' end and other procaryotic promoters. Two 5' ends, located 124 and 294 bp upstream from the initiation codon, were mapped for celD mRNA. The -10 and the -35 sequences preceding the ATG-distal 5' end of celD mRNA were homologous to the consensus sequence of Bacillus subtilis sigma 43 promoters. The sequence upstream from the ATG-proximal 5' end had some similarity with the -10 sequence of B. subtilis sigma 28 promoters. During growth on cellobiose, the 5' end of celD transcripts was found predominantly at the -124 site during the late exponential phase but almost exclusively at the -294 site during the early stationary phase. The kinetics of appearance of celA, celC, celD, and celF mRNA was followed by dot blot analysis. Transcripts of celA, celD, and celF were detected during late exponential and early stationary phase. In contrast, the celC transcript was detected almost exclusively during early stationary phase. Since growth was limited by the availability of cellobiose, the results suggest that the genes are regulated by a mechanism analogous to catabolite repression.

The N-terminal region of an endoglucanase from Pseudomonas fluorescens subspecies cellulosa constitutes a cellulose-binding domain that is distinct from the catalytic centre

The substrate specificity of an endoglucanase (EGB) from Pseudomonas fluorescens subspecies cellulosa was determined. The enzyme was most active against barley beta-glucan, but showed significant activity against amorphous and crystalline cellulose. EGB was purified to homogeneity by affinity chromatography with crystalline cellulose (Avicel). The Mr of the purified enzyme was 50,000, which is in good agreement with the size of EGB deduced from the nucleotide sequence of the celB gene, coding for EGB. The N-terminal region of the mature form of EGB showed strong homology to another endoglucanase and to a xylanase expressed by the same organism; homologous sequences included highly conserved serine-rich regions. Truncated forms of celB, in which the gene sequence encoding the conserved domain had been deleted, directed the synthesis of a functional endoglucanase that did not bind to crystalline cellulose. This indicates that the conserved region of endoglucanases and xylanases expressed by P. fluorescens subsp. cellulosa constitutes a cellulose-binding domain, which is distinct from the active centre. The possible role of this substrate-binding region is discussed.