Cloning of the Thermomonospora fusca Endoglucanase E2 Gene in Streptomyces lividans: Affinity Purification and Functional Domains of the Cloned Gene Product

Discovery and characterization of a thermostable two-domain GH6 endoglucanase from a compost metagenome

Enzymatic depolymerization of recalcitrant polysaccharides plays a key role in accessing the renewable energy stored within lignocellulosic biomass, and natural biodiversities may be explored to discover microbial enzymes that have evolved to conquer this task in various environments. Here, a metagenome from a thermophilic microbial community was mined to yield a novel, thermostable cellulase, named mgCel6A, with activity on an industrial cellulosic substrate (sulfite-pulped Norway spruce) and a glucomannanase side activity. The enzyme consists of a glycoside hydrolase family 6 catalytic domain (GH6) and a family 2 carbohydrate binding module (CBM2) that are connected by a linker rich in prolines and threonines. MgCel6A exhibited maximum activity at 85°C and pH 5.0 on carboxymethyl cellulose (CMC), but in prolonged incubations with the industrial substrate, the highest yields were obtained at 60°C, pH 6.0. Differential scanning calorimetry (DSC) indicated a T_m(app) of 76°C. Both functional data and the crystal structure, solved at 1.88 Å resolution, indicate that mgCel6A is an endoglucanase. Comparative studies with a truncated variant of the enzyme showed that the CBM increases substrate binding, while not affecting thermal stability. Importantly, at higher substrate concentrations the full-length enzyme was outperformed by the catalytic domain alone, underpinning previous suggestions that CBMs may be less useful in high-consistency bioprocessing.

Proteomics-based metabolic modeling and characterization of the cellulolytic bacterium Thermobifida fusca

Background

Thermobifida fusca is a cellulolytic bacterium with potential to be used as a platform organism for sustainable industrial production of biofuels, pharmaceutical ingredients and other bioprocesses due to its capability of potential to convert plant biomass to value-added chemicals. To best develop T. fusca as a bioprocess organism, it is important to understand its native cellular processes. In the current study, we characterize the metabolic network of T. fusca through reconstruction of a genome-scale metabolic model and proteomics data. The overall goal of this study was to use multiple metabolic models generated by different methods and comparison to experimental data to gain a high-confidence understanding of the T. fusca metabolic network.

Results

We report the generation of three versions of a metabolic model of Thermobifida fusca sp. XY developed using three different approaches (automated, semi-automated, and proteomics-derived). The model closest to in vivo growth was the proteomics-derived model that consists of 975 reactions involving 1382 metabolites and account for 316 EC numbers (296 genes). The model was optimized for biomass production with the optimal flux of 0.48 doublings per hour when grown on cellobiose with a substrate uptake rate of 0.25 mmole/h. In vivo activity of the DXP pathway for terpenoid biosynthesis was also confirmed using real-time PCR.

Conclusions

iTfu296 provides a platform to understand and explore the metabolic capabilities of the actinomycete T. fusca for the potential use in bioprocess industries for the production of biofuel and pharmaceutical ingredients. By comparing different model reconstruction methods, the use of high-throughput proteomics data as a starting point proved to be the most accurate to in vivo growth.

A combined cell-consortium approach for lignocellulose degradation by specialized Lactobacillus plantarum cells

BACKGROUND

Lactobacillus plantarum is an attractive candidate for metabolic engineering towards bioprocessing of lignocellulosic biomass to ethanol or polylactic acid, as its natural characteristics include high ethanol and acid tolerance and the ability to metabolize the two major polysaccharide constituents of lignocellulolytic biomass (pentoses and hexoses). We recently engineered L. plantarum via separate introduction of a potent cellulase and xylanase, thereby creating two different L. plantarum strains. We used these strains as a combined cell-consortium for synergistic degradation of cellulosic biomass.

RESULTS

To optimize enzymatic degradation, we applied the cell-consortium approach to assess the significance of enzyme localization by comparing three enzymatic paradigms prevalent in nature: (i) a secreted enzymes system, (ii) enzymes anchored to the bacterial cell surface and (iii) enzymes integrated into cellulosome complexes. The construction of the three paradigmatic systems involved the division of the production and organization of the enzymes and scaffold proteins into different strains of L. plantarum. The spatial differentiation of the components of the enzymatic systems alleviated the load on the cell machinery of the different bacterial strains. Active designer cellulosomes containing a xylanase and a cellulase were thus assembled on L. plantarum cells by co-culturing three distinct engineered strains of the bacterium: two helper strains for enzyme secretion and one producing only the anchored scaffoldin. Alternatively, the two enzymes were anchored separately to the cell wall. The secreted enzyme consortium appeared to have a slight advantage over the designer cellulosome system in degrading the hypochlorite pretreated wheat straw substrate, and both exhibited significantly higher levels of activity compared to the anchored enzyme consortium. However, the secreted enzymes appeared to be less stable than the enzymes integrated into designer cellulosomes, suggesting an advantage of the latter over longer time periods.

CONCLUSIONS

By developing the potential of L. plantarum to express lignocellulolytic enzymes and to control their functional combination and stoichiometry on the cell wall, this study provides a step forward towards optimal biomass bioprocessing and soluble fermentable sugar production. Future expansion of the preferred secreted-enzyme and designer-cellulosome systems to include additional types of enzymes will promote enhanced deconstruction of cellulosic feedstocks.

High-level overproduction of Thermobifida enzyme in Streptomyces lividans using a novel expression vector

In this study, we constructed a novel Streptomyces-E.coli shuttle vector pZRJ362 combining the xylose isomerase promoter and amylase terminator. A gene encoding the endoglucanase Cel6A in Thermobifida fusca was amplified by PCR, cloned into Streptomyces lividans host strain using the novel expression vector and Pichia pastoris GS115 host strain using the vector pPICZα-C, respectively. Afterwards, the expression pattern and the maximum expression level were comparatively studied in both expression systems. The maximum enzyme activity of Cel6A-(His)₆ secreted in S. lividans supernatant after 84-h of cultivation amounted to 5.56 U/mL, which was dramatically higher than that secreted in P. pastoris about 1.4 U/mL after 96-h of cultivation. The maximum expression level of Cel6A-(His)₆ in S. lividans supernatant reached up to 173 mg/L after 84-h of cultivation. The endoglucanase activity staining SDS-PAGE showed that there were some minor proteins in S. lividans supernatant which may be the Cel6A derivant by proteolytic degradation, while there was no proteolytic product detected in supernatant of P. pastoris.

Establishment of a simple Lactobacillus plantarum cell consortium for cellulase-xylanase synergistic interactions

Lactobacillus plantarum is an attractive candidate for bioprocessing of lignocellulosic biomass due to its high metabolic variability, including its ability to ferment both pentoses and hexoses, as well as its high acid tolerance, a quality often utilized in industrial processes. This bacterium grows naturally on biomass; however, it lacks the inherent ability to deconstruct lignocellulosic substrates. As a first step toward engineering lignocellulose-converting lactobacilli, we have introduced genes coding for a GH6 cellulase and a GH11 xylanase from a highly active cellulolytic bacterium into L. plantarum. For this purpose, we employed the recently developed pSIP vectors for efficient secretion of heterologous proteins. Both enzymes were secreted by L. plantarum at levels estimated at 0.33 nM and 3.3 nM, for the cellulase and xylanase, respectively, in culture at an optical density at 600 nm (OD600) of 1. Transformed cells demonstrated the ability to degrade individually either cellulose or xylan and wheat straw. When mixed together to form a two-strain cell-based consortium secreting both cellulase and xylanase, they exhibited synergistic activity in the overall release of soluble sugar from wheat straw. This result paves the way toward metabolic harnessing of L. plantarum for novel biorefining applications, such as production of ethanol and polylactic acid directly from plant biomass.

Purification and Cooperative Activity of Enzymes Constituting the Xylan-Degrading System of Thermomonospora fusca

The thermophilic actinomycete Thermomonospora fusca produced endoxylanase, alpha-arabinofuranosidase, beta-xylosidase, and acetyl esterase activities maximally during growth on xylan. Growth yields on glucose, xylose, or arabinose were comparable, but production of endoxylanase and beta-xylosidase was not induced on these substrates. The crude xylanase activity was thermostable and relatively resistant to end product inhibition by xylobiose and xylan hydrolysis products. Six proteins with xylanase activity were identified by zymogram analysis of isoelectric focusing gels, but only a 32-kDa protein exhibiting three isomeric forms could be purified by fast protein liquid chromatography. Endoglucanases were also identified in carboxymethylcellulose-grown cultures, and their distinction from endoxylanases was confirmed. alpha-Arabinofuranosidase activity was due to a single dimeric protein of 92 kDa, which was particularly resistant to end product inhibition by arabinose. Three bands of acetyl esterase activity were detected by zymogram analysis, and there was evidence that these mainly consisted of an intracellular 80-kDa protein secreted to yield active 40-kDa subunits in the culture supernatant. The acetyl esterases were found to be responsible for acetyl xylan esterase activity in T. fusca, in contrast to the distinction proposed in some other systems. The addition of purified betaxylosidase to endoxylanase increased the hydrolysis of xylan, probably by relieving end product inhibition. The enhanced saccharification of wheat straw caused by the addition of purified alpha-arabinofuranosidase to T. fusca endoxylanase suggested a truly synergistic relationship, in agreement with proposals that arabinose side groups on the xylan chain participate in cross-linking within the plant cell wall structure.

Thermobifida fusca exoglucanase Cel6B is incompatible with the cellulosomal mode in contrast to endoglucanase Cel6A

Cellulosomes are efficient cellulose-degradation systems produced by selected anaerobic bacteria. This multi-enzyme complex is assembled from a group of cellulases attached to a protein scaffold termed scaffoldin, mediated by a high-affinity protein-protein interaction between the enzyme-borne dockerin module and the cohesin module of the scaffoldin. The enzymatic complex is attached as a whole to the cellulosic substrate via a cellulose-binding module (CBM) on the scaffoldin subunit. In previous works, we have employed a synthetic biology approach to convert several of the free cellulases of the aerobic bacterium, Thermobifida fusca, into the cellulosomal mode by replacing each of the enzymes' CBM with a dockerin. Here we show that although family six enzymes are not a part of any known cellulosomal system, the two family six enzymes of the T. fusca system (endoglucanase Cel6A and exoglucanase Cel6B) can be converted to work as cellulosomal enzymes. Indeed, the chimaeric dockerin-containing family six endoglucanase worked well as a cellulosomal enzyme, and proved to be more efficient than the parent enzyme when present in designer cellulosomes. In stark contrast, the chimaeric family six exoglucanase was markedly less efficient than the wild-type enzyme when mixed with other T. fusca cellulases, thus indicating its incompatibility with the cellulosomal mode of action.

High-level bacterial cellulase accumulation in chloroplast-transformed tobacco mediated by downstream box fusions

The Thermobifida fusca cel6A gene encoding an endoglucanase was fused to three different downstream box (DB) regions to generate cel6A genes with 14 amino acid fusions. The DB-Cel6A fusions were inserted into the tobacco (Nicotiana tabacum cv. Samsun) chloroplast genome for protein expression. Accumulation of Cel6A protein in transformed tobacco leaves varied over approximately two orders of magnitude, dependent on the identity of the DB region fused to the cel6A open reading frame (ORF). Additionally, the DB region fused to the cel6A ORF affected the accumulation of Cel6A protein in aging leaves, with the most effective DB regions allowing for high level accumulation of Cel6A protein in young, mature, and old leaves, while Cel6A protein accumulation decreased with leaf age when less effective DB regions were fused to the cel6A ORF. In the most highly expressed DB-Cel6A construct, enzymatically active Cel6A protein accumulated at up to 10.7% of total soluble leaf protein (%TSP). The strategy used for high-level endoglucanase expression may be useful for expression of other cellulolytic enzymes in chloroplasts, ultimately leading to cost-effective heterologous enzyme production for cellulosic ethanol using transplastomic plants.

Expression patterns of cel5A-cel5B, two endoglucanase encoding genes of Thermobifida fusca

Expression patterns of cel5A and cel5B, two endoglucanase encoding genes of Thermobifida fusca were compared by quantitative real-time PCR. With Avicel as carbon source the transcript level of cel5A continuously increased until the 10th hour of incubation and then a sharp decrease was observed, whereas cel5B presented a slow constitutive expression on this substrate. When the microcrystalline cellulose powder MN300 was used as the inducing carbon source, the expression patterns of the two genes were similar. A low initial level of expression was followed by a rapid increase at the 5th hour of incubation; a transient repression was then observed at the 10th hour but after this sampling time, the expression levels started to increase again. The relative expression levels of cel5A were always higher than those of cel5B. Differences in transcription patterns of these two genes can be explained with the imperfect structure of the CelR binding regulatory region of cel5B.

Conversion of Thermobifida fusca free exoglucanases into cellulosomal components: comparative impact on cellulose-degrading activity

Cellulosomes are multi-enzyme complexes produced by certain anaerobic bacteria that exhibit efficient degradation of plant cell wall polysaccharides. To understand their enhanced levels of hydrolysis, we are investigating the effects of converting a free-cellulase system into a cellulosomal one. To achieve this end, we are replacing the cellulose-binding module of the native cellulases, produced by the aerobic bacterium Thermobifida fusca, with a cellulosome-derived dockerin module of established specificity, to allow their incorporation into defined "designer cellulosomes". In this communication, we have attached divergent dockerins to the two exoglucanases produced by T. fusca exoglucanase, Cel6B and Cel48A. The resultant fusion proteins were shown to bind efficiently and specifically to their matching cohesins, and their activities on several different cellulose substrates were compared. The lack of a cellulose-binding module in Cel6B had a deleterious effect on its activity on crystalline substrates. In contrast, the dockerin-bearing family-48 exoglucanase showed increased levels of hydrolytic activity on carboxymethyl cellulose and on both crystalline substrates tested, compared to the wild-type enzyme. The marked difference in the response of the two exoglucanases to incorporation into a cellulosome, suggests that the family-48 cellulase is more appropriate than the family-6 enzyme as a designer cellulosome component.

Proteomic and transcriptomic analysis of extracellular proteins and mRNA levels in Thermobifida fusca grown on cellobiose and glucose

Thermobifida fusca secretes proteins that carry out plant cell wall degradation. Using two-dimensional electrophoresis, the extracellular proteome of T. fusca grown on cellobiose was compared to that of cells grown on glucose. Extracellular proteins, the expression of which is induced by cellobiose, mainly are cellulases and cellulose-binding proteins. Other major extracellular proteins induced by cellobiose include a xylanase (Xyl10A) and two unknown proteins, the C-terminal regions of which are homologous to a lytic transglycosylase goose egg white lysozyme domain and an NLPC_P60 domain (which defines a family of cell wall peptidases), respectively. Transcriptional analysis of genes encoding cellobiose-induced proteins suggests that their expression is controlled at the transcriptional level and that their expression also is induced by cellulose. Some other major extracellular proteins produced by T. fusca grown on both cellobiose and glucose include Lam81A and three unknown proteins that are homologous to aminopeptidases and xylanases or that contain a putative NLPC_P60 domain.

Genome sequence and analysis of the soil cellulolytic actinomycete Thermobifida fusca YX

Thermobifida fusca is a moderately thermophilic soil bacterium that belongs to Actinobacteria. It is a major degrader of plant cell walls and has been used as a model organism for the study of secreted, thermostable cellulases. The complete genome sequence showed that T. fusca has a single circular chromosome of 3,642,249 bp predicted to encode 3,117 proteins and 65 RNA species with a coding density of 85%. Genome analysis revealed the existence of 29 putative glycoside hydrolases in addition to the previously identified cellulases and xylanases. The glycosyl hydrolases include enzymes predicted to exhibit mainly dextran/starch- and xylan-degrading functions. T. fusca possesses two protein secretion systems: the sec general secretion system and the twin-arginine translocation system. Several of the secreted cellulases have sequence signatures indicating their secretion may be mediated by the twin-arginine translocation system. T. fusca has extensive transport systems for import of carbohydrates coupled to transcriptional regulators controlling the expression of the transporters and glycosylhydrolases. In addition to providing an overview of the physiology of a soil actinomycete, this study presents insights on the transcriptional regulation and secretion of cellulases which may facilitate the industrial exploitation of these systems.

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.

Studies ofThermobifida fusca plant cell wall degrading enzymes

I have been studying the Thermobifida fusca cellulose degrading proteins for the past 25 years. In this period, we have purified and characterized the six extracellular cellulases and an intracellular beta- glucosidase used by T. fusca for cellulose degradation, cloned and sequenced the structural genes encoding these enzymes, and helped to determine the 3-dimensional structures of two of the cellulase catalytic domains. This research determined the mechanism of a novel class of cellulase, family 9 processive endoglucanases, and helped to show that there were two types of exocellulases, ones that attacked the non-reducing ends of cellulose and ones that attacked the reducing ends. It also led to the sequencing of the T. fusca genome by the DOE Joint Genome Institute. We have studied the mechanisms that regulate T. fusca cellulases and have shown that cellobiose is the inducer and that cellulase synthesis is repressed by any good carbon source. A regulatory protein (CelR) that functions in the induction control has been purified, characterized, and its structural gene cloned and expressed in E. coli. I have also carried out research on two rumen bacteria, Prevotella ruminicola and Fibrobacter succinogenes, in collaboration with Professor James Russell, helping to arrange for the genomes of these two organisms to be sequenced by TIGR, funded by a USDA grant to the North American Consortium for Genomics of Fibrolytic Ruminal Biology.

Effects of noncatalytic residue mutations on substrate specificity and ligand binding of Thermobifida fusca endocellulase cel6A

The availability of a high-resolution structure of the Thermobifida fusca endocellulase Cel6A catalytic domain makes this enzyme ideal for structure-based efforts to engineer cellulases with high activity on native cellulose. In order to determine the role of conserved, noncatalytic residues in cellulose hydrolysis, 14 mutations of six conserved residues in or near the Cel6A active-site cleft were studied for their effects on catalytic activity, substrate specificity, processivity and ligand-binding affinity. Eleven mutations were generated by site-directed mutagenesis using PCR, while three were from previous studies. All the CD spectra of the mutant enzymes were indistinguishable from that of Cel6A indicating that the mutations did not dramatically change protein conformation. Seven mutations in four residues (H159, R237, K259 and E263) increased activity on carboxymethyl cellulose (CM-cellulose), with K259H (in glucosyl subsite -2) creating the highest activity (370%). Interestingly, the other mutations in these residues reduced CM-cellulose activity. Only the K259H enzyme retained more activity on acid-swollen cellulose than on filter paper, suggesting that this mutation affected the rate-limiting step in crystalline cellulose hydrolysis. All the mutations lowered activity on cellotriose and cellotetraose, but two mutations, both in subsite +1 (H159S and N190A), had higher kcat/Km values (6.6-fold and 5.0-fold, respectively) than Cel6A on 2,4-dinitrophenyl-beta-D-cellobioside. Measurement of enzyme : ligand dissociation constants for three methylumbelliferyl oligosaccharides and cellotriose showed that all mutant enzymes bound these ligands either to the same extent as or more weakly than Cel6A. These results show that conserved noncatalytic residues can profoundly affect Cel6A activity and specificity.

Fed-batch production of thermomonospora fusca endoglucanase by recombinant streptomyces lividans

The factors affecting the production of a Thermomonospora fusca endoglucanase by a recombinant Streptomyces lividans strain were studied in a fermentor with glucose addition controlled by a pH-stat. The recombinant plasmid was stable for 35 generations with constant endoglucanase productivity. Glucose and peptone were used as the carbon and nitrogen sources. Addition of Tween-80 increased endoglucanase production twofold. A significant decrease in endoglucanase production was observed at low aeration. During fed-batch cultivation, pulse feeding (6 g/L) of a glucose-ammonium sulfate solution was optimal for endoglucanase production. With higher concentrations of glucose (15 g/L), a significant amount of organic acid, including acetic acid, was produced, which inhibited cell growth and endoglucanase production. Under optimum conditions, 1.7 U/mL of endoglucanase were produced. Copyright 1998 John Wiley & Sons, Inc.

Surface residue mutations which change the substrate specificity of Thermomonospora fusca endoglucanase E2

The three dimensional structure of a T. fusca endoglucanase catalytic domain (E2cd) has been determined by X-ray crystallography at 1.0 A resolution (Wilson et al., 1995). The availability of a high resolution structure for E2cd allows us to initiate structure-based efforts to engineer cellulases with a high activity on native cellulose. The low activity on crystalline cellulose suggests that the entry of a cellulose molecule into the active site rather than catalysis may be the rate limiting step for hydrolysis of crystalline cellulose. Movement of a loop upon substrate binding has been proposed to play a crucial role in catalysis. A total of 15 surface mutants and 5 loop mutants were created by site-directed mutagenesis and their effects on activity and substrate specificity were determined. Circular dichroism spectra were used to monitor structural changes, and no major changes were found. The binding constants for two methyl umbelliferyl oligosaccharides and cellotriose were measured for some of the mutants and all of them showed binding similar to wild type E2. These results provide the first direct link between loop movement and catalysis by E2 and show that surface residues can affect its substrate specificity.

Cloning, sequencing, and expression of a Thermomonospora fusca protease gene in Streptomyces lividans

The major Thermomonospora fusca YX extracellular protease gene (tfpA) was cloned into Escherichia coli and Streptomyces lividans and was sequenced. The open reading frame encoded 375 residues, including a 31-residue potential signal sequence, an N-terminal prosequence containing 150 residues, and the 194-residue mature protease that belongs to the chymotrypsin family. The protease was secreted by S. lividans, but evidence suggested that it was bound to an extracellular protease inhibitor. An inhibitor-deficient mutant was selected to produce protease for purification.

Evidence that linker sequences and cellulose-binding domains enhance the activity of hemicellulases against complex substrates

Xylanase A (XYLA) and arabinofuranosidase C (XYLC) from Pseudomonas fluorescens subsp. cellulosa are modular enzymes consisting of discrete cellulose-binding domains (CBDs) and catalytic domains joined by serine-rich linker sequences. To evaluate the role of the CBDs and interdomain regions, the capacity of full-length and truncated derivatives of the two enzymes, lacking either the linker sequences or CBDs, to hydrolyse a range of substrates, and bind to cellulose, was determined. Removal of the CBDs did not affect either the activity of XYLA or XYLC against soluble arabinoxylan. Similarly, deletion of the linker sequences did not alter the affinity of the enzymes for cellulose or their activity against soluble substrates, even when bound to cellulose via the CBDs. Truncated derivatives of XYLA lacking either the linker sequences or the CBD were less active against xylan contained in cellulose-hemicellulose complexes, compared with the full-length xylanase. Similarly, removal of the CBD from XYLC diminished the activity of the enzyme (XYLC''') against plant-cell-wall material containing highly substituted arabinoxylan. The role of CBDs and linker sequences in the catalytic activity of hemicellulases against the plant cell wall is discussed.

Characterization of a Thermomonospora fusca exocellulase

The exocellulase E3 gene was cloned on a 7.1 kb NotI fragment from Thermomonospora fusca genomic DNA into Escherichia coli and expressed in Streptomyces lividans. The E3 gene was sequenced and encoded a 596 residue peptide. The molecular masses of the native and cloned E3s were determined by mass spectrometry, and the value for E. coli E3, 59,797 Da, agreed well with that predicted from the DNA sequence, 59,646 Da. The value of 61,200 Da for T. fusca E3 is consistent with E3 being a glycoprotein. E3 is thermostable, retaining full activity after 16 h at 55 degrees C. It also has a broad pH optimum around 7-8, retaining 90% of its maximal activity between pH 6 and 10. The cloned E3s were identical to the native enzyme in their activity, cellulose binding, and thermostability. Papain digestion produced a 45.7 kDa catalytic domain with 77% of the native activity on amorphous cellulose and 33% on crystalline cellulose. E3 belongs to cellulase family B and retains the residues that have been identified to be crucial for catalytic activity in Trichoderma reesei cellobiohydrolase II and T. fusca E2. The E3 gene contains a 14 bp inverted repeat regulatory sequence 212 bp before the translational start codon instead of the 30-70 bp found for the other T. fusca cellulase genes. An additional copy of this sequence with one base changed is 314 bp before the translational start codon. The transcriptional start site of the E3 gene was shown to be between these two inverted repeats.

Production and secretion of proteins by streptomycetes

Streptomycetes produce a large number of extracellular enzymes as part of their saprophytic mode of life. Their ability to synthesize enzymes as products of their primary metabolism could lead to the production of many proteins of industrial importance. The development of high-yielding expression systems for both homologous and heterologous gene products is of considerable interest. In this article, we review the current knowledge on the various factors that affect the production and secretion of proteins by streptomycetes and try to evaluate the suitability of these bacteria for the large-scale production of proteins of industrial importance.

C1-Cx revisited: intramolecular synergism in a cellulase

Endoglucanase A (CenA) from the bacterium Cellulomonas fimi is composed of a catalytic domain and a nonhydrolytic cellulose-binding domain that can function independently. The individual domains interact synergistically in the disruption and hydrolysis of cellulose fibers. This intramolecular synergism is distinct from the well-known intermolecular synergism between individual cellulases. The catalytic domain corresponds to the hydrolytic Cx system and the cellulose-binding domain corresponds to the nonhydrolytic C1 system postulated by Reese et al. [Reese, E. T., Sui, R. G. H. & Levinson, H. S. (1950) J. Bacteriol. 59, 485-497] to be required for the hydrolysis of cellulose.

Non-S-layer glycoproteins in eubacteria

Glycoproteins are proving to be quite common in prokaryotes. Those in S-layers are the best understood in terms of structure. Numerous eubacteria produce non-S-layer glycoproteins about which relatively little is known. The glycans on such proteins and the nature and sites of their linkages to proteins are novel in those glycoproteins which have been examined in any detail. The possible functions of the glycans are mostly not understood. Eubacterial non-S-layer glycoproteins and the glycosylation systems producing them deserve more attention.

The cellulose-binding domain of endoglucanase A (CenA) from Cellulomonas fimi: evidence for the involvement of tryptophan residues in binding

Cellulomonas fimi endo-beta-1,4-glucanase A (CenA) contains a discrete N-terminal cellulose-binding domain (CBDCenA). Related CBDs occur in at least 16 bacterial glycanases and are characterized by four highly conserved Trp residues, two of which correspond to W14 and W68 of CBDCenA. The adsorption of CBDCenA to crystalline cellulose was compared with that of two Trp mutants (W14A and W68A). The affinities of the mutant CBDs for cellulose were reduced by approximately 50- and 30-fold, respectively, relative to the wild type. Physical measurements indicated that the mutant CBDs fold normally. Fluorescence data indicated that W14 and W68 were exposed on the CBD, consistent with their participation in binding to cellobiosyl residues on the cellulose surface.

A bifunctional affinity linker to couple antibodies to cellulose

We have constructed a fusion protein between the staphylococcal A protein and the cellulose binding domain of an exoglucanase (Cex) from Cellulomonas fimi that can be directly immobilized on cellulose while retaining its capacity to bind immunoglobulin G molecules. The cellulose domain provides binding that does not interfere with the biological activity of the fusion partner, does not involve hazardous chemicals and the matrix does not need to be chemically activated which reduces its cost. We have tested some of the possible applications of the fusion protein and show that it can be used in immunoassays, affinity chromatography and immunoprecipitations.

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.

Disulfide arrangement and functional domains of beta-1,4-endoglucanse E5 from Thermomonospora fusca

Thermomonospora fusca cellulase E5 contains six cysteine residues. The number and location of the disulfide bonds and the effect of reduction of the disulfides and modification of the resulting half-cystine residues on enzymatic activity were determined. No free sulfhydryl groups were found in E5. Reduction and subsequent labeling with iodoacetamide of E5 and of an enzymatically active 32-kDa proteolytic derivative of E5 (E5cd) showed that one of the three disulfides is accessible to reduction under nondenatured conditions while the other two are not accessible. Full reduction of the disulfides and complete carboxymethylation of the six cysteines decrease the specific activity of E5 on CMC by more than half, but reduction of only the exposed disulfide bond does not affect enzymatic activity or binding of E5 to cellulose. A 14-kDa proteolytic fragment of E5 containing 120 amino acids from the N-terminus of the protein was shown to bind to crystalline cellulose. This confirms earlier evidence that the cellulose binding domain of E5 is located at the N-terminus of the protein. This 14-kDa fragment contains the accessible disulfide bond involving Cys93 and Cys100. The location of the two disulfide bonds in the other fragment (E5cd) was determined by cleaving it with cyanogen bromide under conditions that left the disulfide bonds intact. The resulting peptides were separated under both nonreducing and reducing conditions using RP-HPLC. Amino acid analysis of peptide peaks indicated that one disulfide linkage in E5cd joins Cys138 to Cys143 while the other joins Cys166 to Cys406.

Dimerization of Thermomonospora fusca beta-1,4-endoglucanase E2

Unboiled Thermomonospora fusca endoglucanase E2 electrophoresed on SDS-polyacrylamide gels migrated in the range of 80-90 kDa, but when boiled it migrated in the 40-42-kDa range. Sedimentation equilibrium centrifugation as well as chemical cross-linking experiments confirmed that E2 is a dimer. The dimer was reversibly dissociated at low pH. The E2 dimer was stable up to 70 degrees C, but began to dissociate at this temperature after a 30-60-min incubation. A nondimerizing mutant was obtained using region-specific chemical mutagenesis. DNA sequencing of this mutant revealed a single base change that substituted Gly for Glu-263. Chemical modification of carboxylic acid residues in E2 disrupted the dimer interaction.

Disulfide arrangement and chemical modification of beta-1,4-endoglucanase E2 from Thermomonospora fusca

Thermomonospora fusca endoglucanase E2 contains six cysteine residues scattered along the protein sequence. Four of the cysteine residues were shown to participate in two disulfide bonds while the last two form a third disulfide bond. Neither full reduction of the disulfides nor complete carboxymethylation of all six cysteines totally destroys enzymatic activity, but the activity of the reduced enzyme is much lower than the native enzyme and the iodoacetamide-modified enzyme has very low activity. Reduction of only the accessible disulfides drastically decreases the enzyme's thermostability. One disulfide linkage joins Cys80 to Cys125, another joins Cys232 to Cys267, and the third joins Cys315 to Cys407. The first two bonds are similar to those in cellobiohydrolase II, which also belongs to cellulase family B (Rouvinen et al., 1990; Lao et al., 1991; Henrissat et al., 1989). Direct evidence for the involvement of carboxyl groups in catalysis by E2 was demonstrated by chemical modification with carbodiimide.

Streptomyces lividans glycosylates an exoglucanase (Cex) from Cellulomonas fimi

Exoglucanase Cex from Cellulomonas fimi is a glycoprotein [Langsford et al., J. Gen. Microbiol. 130 (1984) 1367-1376]. Cex produced by Streptomyces lividans from the cloned cex gene is also glycosylated. The extent and nature of glycosylation are similar for Cex from both organisms. The glycosylation affords protection against proteolysis for the enzymes from both organisms when they are bound to cellulose, but not in solution. The ability to glycosylate cloned gene products enhances the utility of Streptomyces as a host for the production of heterologous polypeptides.

Properties of a genetically reconstructed Prevotella ruminicola endoglucanase

A pUC19-derived plasmid was constructed that coded for a hybrid cellulase with the Thermomonospora fusca E2 cellulose-binding domain at its C terminus joined to the Prevotella ruminicola 40.5-kDa carboxymethyl cellulase (CMCase). The hybrid enzyme was purified and characterized enzymatically. It bound tightly to cellulose, and its specific activities on carboxymethyl cellulose, amorphous cellulose, and ball-milled cellulose were 1.5, 10, and 8 times that of the 40.5-kDa CMCase, respectively. Furthermore, the modified enzyme gave synergism with an exocellulase in the degradation of filter paper, while the 40.5-kDa CMCase did not.

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.