Overproduction from a cellulase gene with a high guanosine-plus-cytosine content in Escherichia coli

Advancement of Biotechnology by Genetic Modifications

One of the greatest sources of metabolic and enzymatic diversity are microorganisms. In recent years, emerging recombinant DNA and genomic techniques have facilitated the development of new efficient expression systems, modification of biosynthetic pathways leading to new metabolites by metabolic engineering, and enhancement of catalytic properties of enzymes by directed evolution. Complete sequencing of industrially important microbial genomes is taking place very rapidly, and there are already hundreds of genomes sequenced. Functional genomics and proteomics are major tools used in the search for new molecules and development of higher-producing strains.

Essential role of genetics in the advancement of biotechnology

Microorganisms are one of the greatest sources of metabolic and enzymatic diversity. In recent years, emerging recombinant DNA and genomic techniques have facilitated the development of new efficient expression systems, modification of biosynthetic pathways leading to new metabolites by metabolic engineering, and enhancement of catalytic properties of enzymes by directed evolution. Complete sequencing of industrially important microbial genomes is taking place very rapidly and there are already hundreds of genomes sequenced. Functional genomics and proteomics are major tools used in the search for new molecules and development of higher-producing strains.

Expression in Escherichia coli of the Cellulomonas fimi Structural Gene for Endoglucanase B

Endoglucanase B (EB) of Cellulomonas fimi has an M(r) of 110,000 when it is produced in Escherichia coli. The level of expression of the cenB gene (encoding EB) was significantly increased by replacing its normal transcriptional and translational regulatory signals with those of the E. coli lac operon. EB was purified to homogeneity from the periplasmic fraction of E. coli in one step by affinity chromatography on microcrystalline cellulose (Avicel). Alignment of the NH(2)-terminal amino acid sequence with the partial nucleotide sequence of a fragment of C. fimi DNA showed that EB is preceded by a putative signal polypeptide of 33 amino acids. The signal peptide functions and is processed correctly in E. coli, even when its first 15 amino acids are replaced by the first 7 amino acids of beta-galactosidase. The intact EB polypeptide is not required for enzymatic activity. Active polypeptides with M(r)s of 95,000 and 82,000 also appear in E. coli, and a deletion mutant of cenB encodes an active polypeptide with an M(r) of 72,000.

The cellulose-binding domain (CBD(Cex)) of an exoglucanase from Cellulomonas fimi: production in Escherichia coli and characterization of the polypeptide

The gene fragment encoding the cellulose-binding domain (CBD) of an exoglucanase (Cex) from Cellulomonas fimi was subcloned and expressed in Escherichia coli. Transcription from the lac promoter coupled with translation from a consensus prokaryotic ribosome binding site led to the production of large quantities of CBD(Cex) (up to 25% total soluble cell protein). The polypeptide leaked into the culture supernatant (up to 50 mg . L(-1)), facilitating one-step purification by affinity chromatography on cellulose. The 11-kDa polypeptide reacted with Cex antiserum. Absence of free thiols indicated that the two Cys residues of CBD(Cex) form a disulfide bridge. It had the same N-terminal amino acid sequence as CBD(Cex) prepared from Cex by proteolysis, plus two additional N-terminal amino acid residues (Ala and Ser) encoded by the Nhel site introduced during plasmid construction. CBD(Cex) bound to a variety of beta-1, 4-glycans with different affinities and saturation levels. Adsorption to bacterial microcrystalline cellulose was dependent on the temperature, but not on the pH.

Recombinant organisms for production of industrial products

A revolution in industrial microbiology was sparked by the discoveries of ther double-stranded structure of DNA and the development of recombinant DNA technology. Traditional industrial microbiology was merged with molecular biology to yield improved recombinant processes for the industrial production of primary and secondary metabolites, protein biopharmaceuticals and industrial enzymes. Novel genetic techniques such as metabolic engineering, combinatorial biosynthesis and molecular breeding techniques and their modifications are contributing greatly to the development of improved industrial processes. In addition, functional genomics, proteomics and metabolomics are being exploited for the discovery of novel valuable small molecules for medicine as well as enzymes for catalysis. The sequencing of industrial microbal genomes is being carried out which bodes well for future process improvement and discovery of new industrial products.

Gas phase noncovalent protein complexes that retain solution binding properties: Binding of xylobiose inhibitors to the beta-1, 4 exoglucanase from cellulomonas fimi

Tandem mass spectrometry has been used to compare gas-phase and solution binding of three small-molecule inhibitors to the wild type and three mutant forms of the catalytic domain of Cex, an enzyme that hydrolyses xylan and xylo-oligosaccharides. The inhibitors, xylobiosyl-deoxynojirimycin, xylobiosyl-isofagomine lactam, and xylobiosyl-isofagomine consist of a common distal xylose linked to different proximal aza-sugars. The three mutant forms of the enzyme contain the substitutions Asn44Ala, Gln87Met, and Gln87Tyr that alter the binding interactions between Cex and the distal sugar of each inhibitor. An electrospray ionization (ESI) triple quadrupole MS/MS system is used to measure the internal energies, DeltaE(int), that must be added to gas-phase ions to cause dissociation of the noncovalent enzyme-inhibitor complexes. Collision cross sections of ions of the apo-enzyme and enzyme-inhibitor complexes, which are required for the calculations of DeltaE(int), have also been measured. The results show that, in the gas phase, enzyme-inhibitor complexes have more compact, folded conformations than the corresponding apo-enzyme ions. With the mutant enzymes, the effects of substituting a single residue can be detected. The energies required to dissociate the gas-phase complexes follow the same trend as the values of DeltaG0 for dissociation of the complexes in solution. This trend is observed both with different inhibitors, which probe binding to the proximal sugar, and with mutants of Cex, which probe binding to the distal sugar. Thus the gas-phase complexes appear to retain much of their solution binding characteristics.

Genetic improvement of processes yielding microbial products

Although microorganisms are extremely good in presenting us with an amazing array of valuable products, they usually produce them only in amounts that they need for their own benefit; thus, they tend not to overproduce their metabolites. In strain improvement programs, a strain producing a high titer is usually the desired goal. Genetics has had a long history of contributing to the production of microbial products. The tremendous increases in fermentation productivity and the resulting decreases in costs have come about mainly by mutagenesis and screening/selection for higher producing microbial strains and the application of recombinant DNA technology.

The topology of the substrate binding clefts of glycosyl hydrolase family 10 xylanases are not conserved

The crystal structures of family 10 xylanases indicate that the distal regions of their active sites are quite different, suggesting that the topology of the substrate binding clefts of these enzymes may vary. To test this hypothesis, we have investigated the rate and pattern of xylooligosaccharide cleavage by the family 10 enzymes, Pseudomonas fluorescens subsp. cellulosa xylanase A (XYLA) and Cellulomonas fimi exoglucanase, Cex. The data showed that Cex contained three glycone and two aglycone binding sites, while XYLA had three glycone and four aglycone binding sites, supporting the view that the topologies of substrate binding clefts in family 10 glycanases are not highly conserved. The importance of residues in the substrate binding cleft of XYLA in catalysis and ligand binding were evaluated using site-directed mutagenesis. In addition to providing insight into the function of residues in the glycone region of the active site, the data showed that the aromatic residues Phe-181, Tyr-255, and Tyr-220 play important roles in binding xylose moieties, via hydrophobic interactions, at subsites +1, +3, and +4, respectively. Interestingly, the F181A mutation caused a much larger reduction in the activity of the enzyme against xylooligosaccharides compared with xylan. These data, in conjunction with a previous study (Charnock, S. J., Lakey, J. H., Virden, R., Hughes, N., Sinnott, M. L., Hazlewood, G. P., Pickersgill, R., and Gilbert, H. J. (1997) J. Biol. Chem. 272, 2942-2951), suggest that the binding of xylooligosaccharides at the -2 and +1 subsites ensures that the substrates occupy the -1 and +1 subsites and thus preferentially form productive complexes with the enzyme. Loss of ligand binding at either subsite results in small substrates forming nonproductive complexes with XYLA by binding to distal regions of the substrate binding cleft.

Exploring the cellulose/xylan specificity of the beta-1,4-glycanase cex from Cellulomonas fimi through crystallography and mutation

The retaining beta-1,4-glycanase Cex from Cellulomonas fimi, a family 10 glycosyl hydrolase, hydrolyzes xylan 40-fold more efficiently than cellulose. To gain insight into the nature of its preference for xylan, we determined the crystal structure of the Cex catalytic domain (Cex-cd) trapped as its covalent 2-deoxy-2-fluoroxylobiosyl-enzyme intermediate to 1.9 A resolution. Together with the crystal structure of unliganded Cex-cd [White, A., et al. (1994) Biochemistry 33, 12546-12552] and the previously determined crystal structure of the covalent 2-deoxy-2-fluorocellobiosyl-Cex-cd intermediate [White, A., et al. (1996) Nat. Struct. Biol. 3, 149-154], this structure provides a convincing rationale for the observed substrate specificity in Cex. Two active site residues, Gln87 and Trp281, are found to sterically hinder the binding of glucosides and must rearrange to accommodate these substrates. Such rearrangements are not necessary for the binding of xylobiosides. The importance of this observation was tested by examining the catalytic behavior of the enzyme with Gln87 mutated to Met. This mutation had no measurable effect on substrate affinity or turnover number relative to the wild type enzyme, indicating that the Met side chain could accommodate the glucoside moiety as effectively as the wild type Gln residue. Subsequent mutagenesis studies will address the role of entropic versus enthalpic contributions to binding by introducing side chains that might be more rigid in the unliganded enzyme.

Enhancement of extracellular production of a Cellulomonas fimi exoglucanase in Escherichia coli by the reduction of promoter strength

The enzymatic approach to the treatment of cellulosic wastes depends on the availability of cost-effective means for the production of cellulases. We have engineered an excretion construct, tacIQpar8cex, to investigate the extracellular production of a Cellulomonas fimi exoglucanase (Exg) in Escherichia coli. The overall yield of Exg expressed by the culture of JM101 (tacIQpar8cex) was 2-11 times higher than that obtained using other systems. Over 20% of the activity was detected in the medium. When the culture was induced with IPTG, the overall production of Exg dropped dramatically. The lower yield was found to be caused by both rapid cell death and plasmid curing. A derivative of tacIQpar8cex containing the weaker lacUV5 promoter, designated lacUV5par8cex, was constructed to enhance excretion of Exg from strain JM101. Even with IPTG induction, the JM101 (lacUV5par8cex) culture was found to show a high level of cell viability and plasmid stability as well as the ability to provide efficient expression and excretion of Exg. Upon IPTG induction for 12 h, the activity and specific activity of the excreted Exg obtained from the lacUV5par8cex construct were 143 U ml-1 and 793 U mg-1 protein, respectively, which are 2-5 times higher than that detected from the tacIQpar8cex construct and from the best construct expressing the same gene reported previously.

Mechanistic consequences of mutation of active site carboxylates in a retaining beta-1,4-glycanase from Cellulomonas fimi

The exoglucanase/xylanase Cex from Cellulomonas fimi is a retaining glycosidase which functions via a two-step mechanism involving the formation and hydrolysis of a covalent glycosyl-enzyme intermediate. The roles of three conserved active site carboxylic acids in this enzyme have been probed by detailed kinetic analysis of mutants modified at these three positions. Elimination of the catalytic nucleophile (E233A) results in an essentially inactive enzyme, consistent with the important role of this residue. However addition of small anions such as azide or formate restores activity, but as an inverting enzyme since the product formed under these conditions is the alpha-glycosyl azide. Shortening of the catalytic nucleophile (E233D) reduces the rates of both formation and hydrolysis of the glycosyl-enzyme intermediate some 3000-4000-fold. Elimination of the acid/base catalyst (E127A) yields a mutant for which the deglycosylation step is slowed some 200-300-fold as a consequence of removal of general base catalysis, but with little effect on the transition state structure at the anomeric center. Effects on the glycosylation step due to removal of the acid catalyst depend on the aglycon leaving group ability, with minimal effects on substrates requiring no general acid catalysis but large (> 10(5)-fold) effects on substrates with poor leaving groups. The Brønsted beta 1g value for hydrolysis of aryl cellobiosides was much larger (beta 1g approximately -1) for the mutant than for the wild-type enzyme (beta 1g = -0.3), consistent with removal of protonic assistance. The pH-dependence was also significantly perturbed. Mutation of a third conserved active site carboxylic acid (E123A) resulted in rate reductions of up to 1500-fold on poorer substrates, which could be largely restored by addition of azide, but without the formation of glycosyl azide products. These results suggest a simple strategy for the identification of the key active site nucleophile and acid/base catalyst residues in glycosidases without resort to active site labeling.

The expression of recombinant proteins on the external surface of Escherichia coli. Biotechnological applications

The expression of recombinant proteins on the external surface of Gram-negative bacteria is expected to open the way for a number of significant biotechnological applications, including the development of live bacterial vaccines, the production of whole cell adsorbents, the preparation of whole cell catalysts, and the display and selection of peptide and antibody libraries. We have developed a fusion protein system for the production of active recombinant proteins on the surface of Escherichia coli. Using this system we have expressed beta-lactamase, the Cellulomonas fimi exoglucanase Cex as well as its cellulose binding domain, and an antidigoxin single chain Fv antibody fragment on the cell surface. Recently we have begun to explore some of the potential applications for cell-surface expression.

The acid/base catalyst in the exoglucanase/xylanase from Cellulomonas fimi is glutamic acid 127: evidence from detailed kinetic studies of mutants

The exoglucanase/xylanase Cex from Cellulomonas fimi hydrolyzes beta-1,4-glycosidic bonds with net retention of anomeric configuration, releasing the disaccharides beta-cellobiose or beta-xylobiose. It uses a double-displacement mechanism involving a glycosyl-enzyme intermediate which is formed and hydrolyzed with general acid/base catalytic assistance. Glu127 was proposed as the acid/base catalyst on the basis of sequence alignments, and mutants at this position were constructed in which the glutamic acid is replaced by alanine or glycine. The following kinetic analysis provides firm support for the assignment of Glu127 as the acid/base catalyst and suggests a more general strategy for identification of this residue in other glycosidases. Substrates which do not require protonic assistance for initial bond cleavage exhibit kcat/Km values similar to those of wild-type enzyme, whereas substrates which do require assistance have kcat/Km values over 6000-fold smaller. Thus rate constants for glycosylation are affected to different degrees by this substitution, depending upon their need for acid catalysis. The deglycosylation rate constant is decreased 200-fold by such substitution, due to the removal of general base catalytic assistance. In the presence of sodium azide a new product, beta-cellobiosyl azide, is formed with these mutants whereas only cellobiose is formed with wild-type enzyme or the Glu127Asp mutant under similar conditions. Addition of azide results in very significant increases in kcat values, ranging from 8-fold for 4''-nitrophenyl cellobioside to over 200-fold for 2'',4''-dinitrophenyl cellobioside, whereas kcat/Km values for these substrates remain essentially constant. No effects on rate upon azide addition are seen with substrates containing aglycons of poor leaving group ability.(ABSTRACT TRUNCATED AT 250 WORDS)

Streptomyces lividans glycosylates the linker region of a beta-1,4-glycanase from Cellulomonas fimi

The beta-1,4-glycanase Cex of the gram-positive bacterium Cellulomonas fimi is a glycoprotein comprising a C-terminal cellulose-binding domain connected to an N-terminal catalytic domain by a linker containing only prolyl and threonyl (PT) residues. Cex is also glycosylated by Streptomyces lividans. The glycosylation of Cex produced in both C. fimi and S. lividans protects the enzyme from proteolysis. When the gene fragments encoding the cellulose-binding domain of Cex (CBDCex), the PT linker plus CBDCex (PT-CBDCex), and the catalytic domain plus CBDCex of Cex were expressed in S. lividans, only PT-CBDCex was glycosylated. Therefore, all the glycans must be O linked because only the PT linker was glycosylated. A glycosylated form and a nonglycosylated form of PT-CBDCex were produced by S. lividans. The glycosylated form of PT-CBDCex was heterogeneous; its average carbohydrate content was approximately 10 mol of D-mannose equivalents per mol of protein, but the glycans contained from 4 to 12 alpha-D-mannosyl and alpha-D-galactosyl residues. Glycosylated Cex from S. lividans was also heterogeneous. The presence of glycans on PT-CBDCex increased its affinity for bacterial microcrystalline cellulose. The location of glycosylation only on the linker region of Cex correlates with the properties conferred on the enzyme by the glycans.

Rational design and PCR-based synthesis of an artificial Schizophyllum commune xylanase gene

A synthetic gene encoding the Schizophyllum commune xylanase XynA was constructed by a novel PCR-based procedure. Three long oligonucleotides were synthesized and used in combination with flanking PCR primers to generate a 607 base pair gene which contained 31 unique locations for restriction enzyme cleavage. The amino acid sequence was tailored for expression in Escherichia coli by using only those codons found in highly expressed E. coli genes. The availability of the gene will facilitate analysis of the structure and function of this and other beta-(1,4) xylanases.

Specific adhesion and hydrolysis of cellulose by intact Escherichia coli expressing surface anchored cellulase or cellulose binding domains

The entire Cex exoglucanase from Cellulomonas fimi and the Cex Cellulose Binding Domain (CBDCex) were expressed in Escherichia coli as fusions to an Lpp-OmpA hybrid which had been shown earlier to direct a heterologous protein to the cell surface. Both Cex and CBDCex were readily localized on the cell surface and could be detected by immunofluorescence microscopy, whole cell ELISAs and functional assays. In cells expressing the entire Cex, about 90% of the total cellobiose hydrolase activity was anchored on the external side of the outer membrane and was susceptible to protease (papain) added in the extracellular fluid. Cells expressing either Cex or CBDCex bound tightly and rapidly to cellulosic materials such as cotton fibers. This property can be exploited for the preparation of immobilized microbial biocatalysts via adsorption to cellulose and for cell separation through specific agglutination on inexpensive cellulosic materials. In addition, our results demonstrate the general utility of fusions to lpp-ompA for the efficient display of proteins and the engineering of the surface topology of Gram-negative bacteria.

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.

Runaway–Replication Plasmids as Tools to Produce Large Quantities of Proteins from Cloned Genes in Bacteria

Here we review the properties and uses of runaway-replication vectors, a class of versatile plasmids discovered and developed in Escherichia coli. They are based on the IncFII plasmid, R1, in which an antisense RNA (CopA RNA) negatively controls the formation of a protein that is rate-limiting for replication. The copy number of the plasmid is determined by the balance between the rates of formation of CopA RNA and RepA mRNA. A small increase in the rate of formation of the latter drastically reduces the rate of formation of CopA RNA due to convergent transcription, which may lead to a total loss of copy number control (runaway replication), resulting in massive DNA amplification, and plasmid copy numbers up to 1000 per genome. Since this amplification occurs in the presence of protein synthesis, the protein that is encoded by a cloned gene can also be amplified, and may constitute 10-50% of the total protein.

Multiple domains in endoglucanase B (CenB) from Cellulomonas fimi: functions and relatedness to domains in other polypeptides

Endoglucanase B (CenB) from the bacterium Cellulomonas fimi is divided into five discrete domains by linker sequences rich in proline and hydroxyamino acids (A. Meinke, C. Braun, N. R. Gilkes, D. G. Kilburn, R. C. Miller, Jr., and R. A. J. Warren, J. Bacteriol. 173:308-314, 1991). The catalytic domain of 608 amino acids is at the N terminus. The sequence of the first 477 amino acids in the catalytic domain is related to the sequences of cellulases in family E, which includes procaryotic and eucaryotic enzymes. The sequence of the last 131 amino acids of the catalytic domain is related to sequences present in a number of cellulases from different families. The catalytic domain alone can bind to cellulose, and this binding is mediated at least in part by the C-terminal 131 amino acids. Deletion of these 131 amino acids reduces but does not eliminate activity. The catalytic domain is followed by three domains which are repeats of a 98-amino-acid sequence. The repeats are approximately 50% identical to two repeats of 95 amino acids in a chitinase from Bacillus circulans which are related to fibronectin type III repeats (T. Watanabe, K. Suzuki, K. Oyanagi, K. Ohnishi, and H. Tanaka, J. Biol. Chem. 265:15659-15665, 1990). The C-terminal domain of 101 amino acids is related to sequences, present in a number of bacterial cellulases and xylanases from different families, which form cellulose-binding domains (CBDs). It functions as a CBD when fused to a heterologous polypeptide. Cells of Escherichia coli expressing the wild-type cenB gene accumulate both native CenB and a stable proteolytic fragment of 41 kDa comprising the three repeats and the C-terminal CBD. The 41-kDa polypeptide binds to cellulose but lacks enzymatic activity.

Overproduction of an acetylxylan esterase from the extreme thermophile "Caldocellum saccharolyticum" in Escherichia coli

The xynC gene coding for an acetylxylan esterase from the extreme thermophile "Caldocellum saccharolyticum" was overexpressed in Escherichia coli strain RR28 by cloning the gene downstream from the lacZ promoter region of pUC18 (pNZ1447) or downstream from the temperature-inducible lambda pRpL promoters of pJLA602 (pNZ1600). The protein formed high molecular weight aggregates in induced cells of RR28/pNZ1600 but not in RR28/pNZ1447. The enzyme constituted up to 10% of the total cell protein and was located in the cytoplasmic fraction of RR28/pNZ1447. The acetyl esterase was most active at pH 6.0 and 70-75 degrees C with a half-life of 64 h at 70 degrees C and 30 h at 80 degrees C, respectively.

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.

Xylanase from the extremely thermophilic bacterium "Caldocellum saccharolyticum": overexpression of the gene in Escherichia coli and characterization of the gene product

A xylanase encoded by the xynA gene of the extreme thermophile "Caldocellum saccharolyticum" was overexpressed in Escherichia coli by cloning the gene downstream from the temperature-inducible lambda pR and pL promoters of the expression vector pJLA602. Induction of up to 55 times was obtained by growing the cells at 42 degrees C, and the xylanase made up to 20% of the whole-cell protein content. The enzyme was located in the cytoplasmic fraction in E. coli. The temperature and pH optima were determined to be 70 degrees C and pH 5.5 to 6, respectively. The xylanase was stable for at least 72 h if incubated at 60 degrees C, with half-lives of 8 to 9 h at 70 degrees C and 2 to 3 min at 80 degrees C. The enzyme had high activity on xylan and ortho-nitrophenyl beta-D-xylopyranoside and some activity on carboxymethyl cellulose and para-nitrophenyl beta-D-cellobioside. The gene was probably expressed from its own promoter in E. coli. Translation of the xylanase overproduced in E. coli seemed to initiate at a GTG codon and not at an ATG codon as previously determined.

Expression of tetanus toxin fragment C in E. coli: high level expression by removing rare codons

Tetanus toxin fragment C had been previously expressed in Escherichia coli at 3-4% cell protein. The codon bias for tetanus toxin in Clostridium tetani is very different from that of highly expressed homologous genes in E. coli, resulting in the presence of many rare E. coli codons in the sequence encoding fragment C. We have replaced the coding sequence by sequence optimized for codon usage in E. coli, and show that the expression of fragment C is increased. Although the level of mRNA also increased this appeared to be a secondary consequence of more efficient translation. Complete sequence replacement increased expression to approximately 11-14% cell protein but only after the promoter strength had been improved.

Identification of three distinct Clostridium thermocellum xylanase genes by molecular cloning

Three genes coding for xylanase synthesis in Clostridium thermocellum were cloned and expressed in Escherichia coli. Genomic DNA from Clostridium thermocellum was digested to completion with HindIII, BamHI, and SalI. The fragments were ligated into the corresponding sites of pUC19 and transformed into Escherichia coli. Two of the genes encoded for xylanases which depolymerized xylans but were unable to extensively convert these substrates to reducing sugar. The third gene encoded for an enzyme that extensively hydrolyzed xylan. The insert containing the latter gene was subjected to extensive mapping and was found to encode for a xylanase with a molecular weight of approximately 25,000. The protein product of the cloned gene was obtained in a relatively pure form by heat treatment, ion exchange and gel permeation steps. The enzyme was quite stable to high temperatures with a half-life of 24 h at 70 degrees C.

Hyperexpression of a Bacillus circulans xylanase gene in Escherichia coli and characterization of the gene product

A 4.0-kilobase (kb) fragment of Bacillus circulans genomic DNA inserted into pUC19 and encoding endoxylanase activity was subjected to a series of subclonings. A 1.0-kb HindIII-HincII subfragment was found to code for xylanase activity. Maximum expression levels were observed with a subclone that contained an additional 0.3-kb sequence upstream from the coding region. Enhancer sequences in the upstream region are thought to be responsible for these high expression levels. Southern hybridization analyses revealed that the cloned gene hybridized with genomic DNA from Bacillus subtilis and Bacillus polymyxa. Xylanase activity expressed by Escherichia coli harboring the cloned gene was located primarily in the intracellular fraction. Levels of up to 7 U/ml or 35 mg/liter were obtained. The protein product was purified by ion exchange and gel permeation chromatography. The xylanase had a molecular weight of 20,500 and an isoelectric point of 9.0.

Characterization and expression in Escherichia coli of an endoglucanase gene of Pseudomonas fluorescens subsp. cellulosa

An endoglucanase gene of Pseudomonas fluorescens subsp. cellulosa present on plasmid pRUCL150 and expressed in Escherichia coli was subcloned in plasmid pBR322. Plasmid pRUCL153 contained the smallest DNA insert (2.9 kb) with endoglucanase activity. The plasmids directed the synthesis of a mostly periplasmic enzyme in E. coli and the level of enzyme activity was comparable in several strains. Analysis by non-denaturing polyacrylamide gel electrophoresis of the endoglucanase produced with various recombinant plasmids showed that it was unique. The endoglucanase gene on plasmid pRUCL153 was localized by physical mapping of independent transposon Tn5 insertions. Hence, its size was estimated to be approx. 1.3 kb. In vivo radioactive labelling of plasmid-encoded proteins using minicells, followed by denaturing polyacrylamide gel electrophoresis, allowed us to determine the size of the endoglucanase: Mr 40,000 for the precursor and Mr 38,000 for the mature enzyme. It was demonstrated that no cellulase operon, but a single gene, was cloned. The direction of transcription of the gene was determined by placing it under the control of the promoter of the lactose operon.

Glycosylation of bacterial cellulases prevents proteolytic cleavage between functional domains

Glycosylated cellulases from Cellulomonas fimi were compared with their non-glycosylated counterparts synthesized in Escherichia coli from recombinant DNA. Glycosylation of the enzymes does not significantly affect their kinetic properties, or their stabilities towards heat and pH. However, the glycosylated enzymes are protected from attack by a C. fimi protease when bound to cellulose, while the non-glycosylated enzymes yield active, truncated products with greatly reduced affinity for cellulose.