Wood Hydrolysis by Cellulomonas Fimi Endoglucanase and Exogiucanase Coexpressed as Secreted Enzymes in Saccharomyces Cerevisiae

Synergistic hydrolysis of filter paper by recombinant cellulase cocktails leveraging a key cellobiase, Cba2, of Cellulomonas biazotea

Cellulomonas biazotea, a Gram-positive cellulolytic bacterium isolated from soil, is capable of producing a complete cellulase complex exhibiting endoglucanase, exoglucanase, and cellobiase activities. Despite the presence of a full complement of all three types of cellulases, samples prepared from both cell lysates and culture media of C. biazotea showed only weak synergistic activities formed among the cellulase components, as reflected by their inefficient performance in filter paper hydrolysis. However, when the five previously characterized recombinant cellobiases of C. biazotea were mixed individually or in different combinations with recombinant enzyme preparations (CenA/Cex) containing an endoglucanase, CenA, and an exoglucanase, Cex, of another Cellulomonas species, C. fimi, the cellulase cocktails exhibited not only much higher but also synergistic activities in filter paper hydrolysis. Among the 5 C. biazotea cellobiases studied, Cba2 was shown to perform 2.8 to 3.8 times better than other homologous isozymes when acting individually with CenA/Cex. More noteworthy is that when Cba2 and Cba4 were added together to the reaction mixture, an even better synergistic effect was achieved. The filter paper activities resulting from Cba2 and Cba4 interacting with CenA/Cex are comparable to those obtained from some commercial fungal cellulase mixtures. To our knowledge, our results represent the first demonstration of synergistic effects on filter paper hydrolysis achieved using recombinant bacterial cellulases.

Citrobacter freundii as a test platform for recombinant cellulose degradation systems

Cellulosic biomass represents a huge reservoir of renewable carbon, but converting it into useful products is challenging. Attempts to transfer cellulose degradation capability to industrially useful micro-organisms have met with limited success, possibly due to poorly understood synergy between multiple cellulases. This is best studied by co-expression of many combinations of cellulases and associated proteins. Here, we describe the development of a test platform based on Citrobacter freundii, a cellobiose-assimilating organism closely related to Escherichia coli. Standard E. coli cloning vectors worked well in Cit. freundii. Expression of cellulases CenA and Cex of Cellulomonas fimi in Cit. freundii gave recombinant strains which were able to grow at the expense of cellulosic filter paper or microcrystalline cellulose (Avicel) in a mineral medium supplemented with a small amount of yeast extract. Periodic physical agitation of the cultures was highly beneficial for growth at the expense of filter paper. This provides a test platform for the expression of combinations of genes encoding biomass-degrading enzymes to develop effective genetic cassettes for degradation of different biomass streams.

SIGNIFICANCE AND IMPACT OF THE STUDY

Biofuels have been shown to be the best sustainable and alternative source of fuel to replace fossil fuels. Of the different types of feedstocks used for producing biofuels, lignocellulosic biomass is the most abundant. Converting this biomass to useful products has met with little success. Different approaches are being used and microbial platforms are the most promising and sustainable method. This study shows that Citrobacter freundii is a better test platform than Escherichia coli for testing various combinations of cellulases for the development of microbial systems for biomass conversion.

Cloning and characterization of a novel cellobiase gene, cba3, encoding the first known β-glucosidase of glycoside hydrolase family 1 of Cellulomonas biazotea

A novel cellobiase gene, designated cba3, was cloned from Cellulomonas biazotea. Although cellobiase genes of C. biazotea were previously cloned, published and/or patented, they encoded β-glucosidases all belonging to glycoside hydrolase family 3 (GH3); the new Cba3 cellobiase was identified to be a glycoside hydrolase family 1 (GH1) member, which represents the first discovered GH1 β-glucosidase of C. biazotea. Escherichia coli transformants expressing recombinant Cba3 were shown to grow readily in minimal media using cellobiose as the sole carbon source, supporting the conclusion that Cba3 is a genuine cellobiase. The full-length cba3 gene was revealed by sequencing to be 1344 bp long. Cba3 deletants lacking either the N-terminal 10 amino acids or the C-terminal 10 residues were found to be biologically inactive, supporting the importance of both ends in catalysis. Like other GH1 β-glucosidases, Cba3 was shown to contain the highly conserved NEP and ENG motifs, which are crucial for enzymatic activity. Despite lacking a classical N-terminal signal peptide, Cba3 was demonstrated to be a secretory protein. The findings that Cba3 is a cellobiase, and that it was expressed well as an extracellular protein in E. coli, support the potential of Cba3 for use with other cellulases in the hydrolysis of cellulosic biomass.

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.

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 and characterization of a major secretory cellobiase, Cba2, from Cellulomonas biazotea

A novel cellobiase (Cba2) was purified from the culture supernatant of Cellulomonas biazotea and characterized. Cba2 appeared to be a major secretory cellobiase in C. biazotea as its enzymatic activity was estimated to represent over 40% of the total extracellular beta-glucosidase activity. The enzyme was purified over 260-fold subsequent to ammonium sulfate precipitation, gel-filtration chromatography, anion-exchange chromatography, and reversed-phase high-performance liquid chromatography. Cba2 was shown by SDS-PAGE to have a large molecular mass of 109 kDa, which makes it one of the largest secretory cellobiases characterized. Its homogeneity was confirmed by N-terminal amino acid sequencing. The K(m) and V(max) values were 0.025 mM and 0.0048 mM min(-1), respectively, for the Cba2 hydrolysis of p-nitrophenyl-beta-d-glucopyranoside, and 0.73 mM and 0.00033 mM min(-1), respectively, for the hydrolysis of cellobiose (at 37 degrees C and pH 7.0). The purified enzyme has a pH optimum of 4.8 and the optimum temperature for activity is 70 degrees C. In view of the secretory nature of Cba2 and the fact that it is a major component of secretory cellobiases of C. biazotea, it is potentially important in the enzymatic degradation of cellulose, and its availability as a recombinant protein may facilitate the studies of its biotechnological applications.

Genetic immobilization of proteins on the yeast cell surface

A genetic system has been exploited to immobilize proteins in their active and functional forms on the cell surface of yeast, Saccharomyces cerevisiae. DNAs encoding proteins with a secretion signal peptide were fused with the genes encoding yeast agglutinins, a- and alpha-type proteins involved in mating. The fusion gene was introduced into S. cerevisiae and expressed under the control of several promoters. Appearance of the fused proteins expressed on the cell surface was demonstrated biochemically and by immunofluorescence and immunoelectron microscopy techniques. Alpha-galactosidase from Cyamopsis tetragonoloba seeds, peptide libraries including scFv and variable regions of the T cell receptor from mammalian cells have been successfully immobilized on the yeast cell wall in the active form. Recently, surface-engineered yeasts have been constructed by immobilizing the enzymes and a functional protein, for example, green fluorescent protein (GFP) from Aequorea victoria. The yeasts were termed 'arming yeasts' with biocatalysts or functional proteins. Such arming cells displaying glucoamylase from Rhizopus oryzae and alpha-amylase from Bacillus stearothermophilus, or carboxymethylcellulase and beta-glucosidase from Aspergillus acleatus, could assimilate starch or cellooligosaccharides as the sole carbon source, although S. cerevisiae cannot intrinsically assimilate these substrates. GFP-arming cells can emit green fluorescence from the cell surface in response to the environmental conditions. The approach described in this review will enable us to endow living cells, including yeast cells, with novel additional abilities and to open new dimensions in the field of biotechnology.

Construction of an efficient Bacillus subtilis system for extracellular production of heterologous proteins

An efficient expression/secretion vector, designated pM2Veg, was constructed for extracellular production of heterologous proteins in Bacillus subtilis. To construct pM2Veg, a synthetic cassette, the Veg cassette carrying: (1) the strong vegetative vegI promoter from B. subtilis, (2) the Escherichia coli lac operator, (3) the B. subtilis consensus ribosome-binding site, (4) the Staphylococcal protein A leader sequence, (5) a cloning region for insertion of foreign genes, (6) translational stop codons in all three reading frames, and (7) the gnt transcriptional terminator, was cloned into a derivative of the stable pRB373 B. subtilis/E. coli shuttle plasmid, the pM2 vector. The application of pM2Veg to effect secretory production of heterologous proteins was illustrated using two widely different proteins: the endoglucanase (Eng) encoded by the cenA gene of Cellulomonas fimi and human epidermal growth factor (hEGF). Levels of Eng and hEGF measured in culture supernatant samples of B. subtilis transformants harboring recombinant constructs formed between pM2Veg and the cenA and hEGF genes were 8.3 U ml-1 and 7.0 mg l-1, respectively. The Eng activity is more than four times higher than the yield from the best cenA recombinant construct previously reported, and the hEGF data represents the first successful expression of the factor in B. subtilis.

The cloning, expression and characterization of a cellobiase gene encoding a secretory enzyme from Cellulomonas biazotea

A 4.7-kb DNA insert encoding a secretory cellobiase (Cba) was cloned from Cellulomonas biazotea in Escherichia coli using an excretion vector, pM. Host cells transformed with the recombinant construct, designated pBZ4.7, were able to utilize cellobiose as the sole carbon source. Part of the Cba activity encoded by pBZ4.7 could be detected in the periplasm and even in the culture supernatant. The Cba protein was purified from the culture supernatant and analyzed by SDS-PAGE to have an apparent M(r) of 86,000. The insert consisted of two PstI fragments with lengths of 0.75 and 3.95 kb, both of which were found to be crucial for expressing the Cba activity. Sequencing of the first 3.95 kb of the insert revealed that the coding sequence for Cba, designated the cba gene, was 2484 bp long. Comparison of the deduced Cba sequence with those of published beta-glucosidases revealed a potential active site located at the N-terminal portion of the former. The cba gene has a high G + C content of 76.4% and is flanked by a putative ribosome-binding site and potential transcriptional termination signals upstream and downstream from its coding sequence, respectively.

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.

Mode of action and substrate specificity of a purified exo-1,4-beta-D-glucosidase cloned from the cellulolytic bacterium Ruminococcus albus AR67

A gene encoding exo-1,4-beta-D-glucosidase, from Ruminococcus albus AR67, was cloned in Escherichia coli, restriction mapped, and shown to be expressed from sequences within the insert that function as a promoter in E. coli. The cloned enzyme was located predominantly in the cytoplasm (40%) and attached to insoluble cell components (48%). After purification to homogeneity, the enzyme (Mr = 64,000, monomeric) was specific for substrates with beta-D-glucopyranosyl configuration and was inactive against alpha-glucosides, lactosides and xylosides. Km values of the enzyme decreased with increasing chain length (G2-G5). Glucose was the major product of hydrolysis from cellodextrins. Preference for longer chain cellodextrins is consistent with exo-1,4-beta-D-glucan glucohydrolase mode of action [E.C. 3.2.1.74].

Expression of proteins encoded by foreign genes in Saccharomyces cerevisiae

The yeast, Saccharomyces cerevisiae is currently used for the production of recombinant DNA-generated proteins derived from a variety of eukaryotic organisms. The applications of a yeast-based technology in the production of proteins for pharmaceutical and industrial purposes is discussed including current methods for introducing recombinant genes into yeast and strategies for maximizing their expression.