Secretion of a bacterial cellulase by yeast

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.

Protein expression in Pichia pastoris: recent achievements and perspectives for heterologous protein production

Pichia pastoris is an established protein expression host mainly applied for the production of biopharmaceuticals and industrial enzymes. This methylotrophic yeast is a distinguished production system for its growth to very high cell densities, for the available strong and tightly regulated promoters, and for the options to produce gram amounts of recombinant protein per litre of culture both intracellularly and in secretory fashion. However, not every protein of interest is produced in or secreted by P. pastoris to such high titres. Frequently, protein yields are clearly lower, particularly if complex proteins are expressed that are hetero-oligomers, membrane-attached or prone to proteolytic degradation. The last few years have been particularly fruitful because of numerous activities in improving the expression of such complex proteins with a focus on either protein engineering or on engineering the protein expression host P. pastoris. This review refers to established tools in protein expression in P. pastoris and highlights novel developments in the areas of expression vector design, host strain engineering and screening for high-level expression strains. Breakthroughs in membrane protein expression are discussed alongside numerous commercial applications of P. pastoris derived proteins.

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.

Efficient expression of a Paenibacillus barcinonensis endoglucanase in Saccharomyces cerevisiae

The endoglucanase coded by celA (GenBank Access No. Y12512) from Paenibacillus barcinonensis, an enzyme with good characteristics for application on paper manufacture from agricultural fibers, was expressed in Saccharomyces cerevisiae by using different domains of the cell wall protein Pir4 as translational fusion partners, to achieve either secretion or cell wall retention of the recombinant enzyme. Given the presence of five potential N-glycosylation sites in the amino acid sequence coded by celA, the effect of glycosylation on the enzymatic activity of the recombinant enzyme was investigated by expressing the recombinant fusion proteins in both, standard and glycosylation-deficient strains of S. cerevisiae. Correct targeting of the recombinant fusion proteins was confirmed by Western immunoblot using Pir-specific antibodies, while enzymatic activity on carboxymethyl cellulose was demonstrated on plate assays, zymographic analysis and colorimetric assays. Hyperglycosylation of the enzyme when expressed in the standard strain of S. cerevisiae did not affect activity, and values of 1.2 U/ml were obtained in growth medium supernatants in ordinary batch cultures after 24 h. These values compare quite favorably with those described for other recombinant endoglucanases expressed in S. cerevisiae. This is one of the few reports describing the expression of Bacillus cellulases in S. cerevisiae, since yeast expressed recombinant cellulases have been mostly of fungal origin. It is also the first report of the yeast expression of this particular endoglucanase.

Expression and Secretion of a Cellulomonas fimi Exoglucanase in Saccharomyces cerevisiae

We used the yeast MEL1 gene for secreted alpha-galactosidase to construct cartridges for the regulated expression of foreign proteins from Saccharomyces cerevisiae. The gene for a Cellulomonas fimi beta-1,4-exoglucanase was inserted into one cartridge to create a fusion of the alpha-galactosidase signal peptide to the exoglucanase. Yeast transformed with plasmids containing this construction produced active extracellular exoglucanase when grown under conditions appropriate to MEL1 promoter function. The cells also produced active intracellular enzyme. The secreted exoglucanase was N-glycosylated and was produced continuously during culture growth. It hydrolyzed xylan, carboxymethyl cellulose, 4-methylumbelliferyl-beta-d-cellobiose, and p-nitrophenyl-beta-d-cellobiose. A comparison of the recombinant S. cerevisiae enzyme with the native C. fimi enzyme showed the yeast version to have an identical K(m) and pH optimum but to be more thermostable.

Paenibacillus pectinilyticus sp. nov., isolated from the gut of Diestrammena apicalis

During a search for exo-enzyme-producing bacteria in the gut of an insect, Diestrammena apicalis, a novel bacterium capable of degrading pectin was isolated. The isolate, designated strain RCB-08(T), comprised Gram-positive, endospore-forming, motile rods capable of growth at 15-30 degrees C and pH 6.0-8.7. The DNA G+C content of the isolate was 51.5 mol% and the predominant cellular fatty acid was anteiso-C(15 : 0) (74.1 %). Phylogenetic analysis based on 16S rRNA gene sequences showed that strain RCB-08(T) was affiliated with a cluster within the Paenibacillaceae, and was related most closely to Paenibacillus chondroitinus NBRC 15376(T), with a sequence similarity of 96.7 %. The DNA-DNA relatedness value for strain RCB-08(T) with P. chondroitinus NBRC 15376(T) was 15.0 %. Strain RCB-08(T) hydrolysed pectin, but not cellulose, casein, starch or xylan. Strain RCB-08(T) could be clearly distinguished from other Paenibacillus species on the basis of characteristics observed using a polyphasic approach. Therefore strain RCB-08(T) is considered to represent a novel species of the genus Paenibacillus, for which the name Paenibacillus pectinilyticus sp. nov. is proposed. The type strain is RCB-08(T) (=KCTC 13222(T)=CECT 7358(T)).

Engineering a synthetic dual-organism system for hydrogen production

Molecular hydrogen produced biologically from renewable biomass is an attractive replacement for fossil fuels. One potential route for biological hydrogen production is the conversion of biomass into formate, which can subsequently be processed into hydrogen by Escherichia coli. Formate is also a widely used commodity chemical, making its bioproduction even more attractive. Here we demonstrate the implementation of a formate-overproducing pathway in Saccharomyces cerevisiae, a well-established industrial organism. By expressing the anaerobic enzyme pyruvate formate lyase from E. coli, we engineered a strain of yeast that overproduced formate relative to undetectable levels in the wild type. The addition of a downstream enzyme, AdhE of E. coli, resulted in an additional 4.5-fold formate production increase as well as an increase in growth rate and biomass yield. Overall, an 18-fold formate increase was achieved in a strain background whose formate degradation pathway had been deleted. Finally, as a proof of concept, we were able to produce hydrogen from this formate-containing medium by using E. coli as a catalyst in a two-step process. With further optimizations, it may be feasible to use S. cerevisiae on a larger scale as the foundation for yeast-based biohydrogen.

Paenibacillus camelliae sp. nov., isolated from fermented leaves of Camellia sinensis

A novel bacterium, strain blls-2(T) was isolated from Pu'er tea. The isolate was Gram-positive, endospore-forming motile rod that grew at 15 approximately 42 degrees C and pH 6.0 approximately 10.2. The DNA G+C content was 48.3 mol%, the predominant isoprenoid quinone was MK-7, and the predominant cellular fatty acid was anteiso-C15:0 (54.2%) followed by C16:0 (15.5%) and iso-C16:0 (8.2%). The polar lipid pattern of blls-2(T) was characterized by the presence of diphosphatidylglycerol, phosphatidylethanolamine, and phosphatidylglycerol. Phy-logenetic analysis based on 16S rRNA gene sequence showed that the strain was affiliated within the Paenibacillaceae. The strain was most closely related to Paenibacillus granivorans A30(T), with a similarity of 97.1%. Based on the phylogenetic and phenotypic characteristics of strain blls-2(T), the isolate is thought to represent a novel taxon in the genus Paenibacillus. The name Paenibacillus camelliae sp. nov. is proposed for the fermented tea isolate; the type strain is blls-2(T) (= KCTC 13220(T)= CECT 7361(T)).

Expression of recombinant human epidermal growth factor in Escherichia coli and characterization of its biological activity

Recombinant human epidermal growth factor (EGF) was successfully expressed as a fusion protein in Escherichia coli system. This system was used OmpA signal sequence to produce soluble protein into the periplasm of E. coli. Human EGF (hEGF) synthesized in bacterial cell was found to be similar in size with the original protein and molecular weight approximately at 6.8 kDa. Cell proliferation assay was conducted to characterize the biological activity of hEGF on human dermal fibroblasts. The synthesized hEGF was found to be functional as compared with authentic hEGF in stimulating cell proliferation and promoting growth of cell. In comparison of biological activity between synthesized and commercial hEGF on cell proliferation, the results showed there was no significant different. This finding indicates the synthesized hEGF in E. coli system is fully bioactive in vitro.

Enzymatic activities of novel mutant endoglucanases carrying sequential active sites

Novel mutant enzymes of endoglucanase II (EGII) from fungus Trichoderma viride were prepared and their hydrolysis and enzymatic polymerization activities were studied. EGII(core)2 and EGII(core)2-His, which possess sequential two active sites of EGII with a His-tag probe at the N-terminal and with His-tag probes at the N and C terminals, respectively, showed higher hydrolysis activities than EGIIcore with a single active site even in comparison on the active-site concentration basis. These mutant enzymes were applied to the enzymatic polymerization to afford artificial cellulose. The polymerization rates with using EGII(core)2 and EGII(core)2-His were also higher than that with using EGIIcore. The polymerization products were identified as highly crystalline cellulose of type II. The mutant enzymes were also effective to prepare spherulites. EGII(core)2 and EGII(core)2-His are considered to possess higher hydrolysis and polymerization activities than EGIIcore mainly due to the suitably stabilized conformation with the sequential arrangement.

Consolidated bioprocessing for bioethanol production using Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of lignocellulose to bioethanol refers to the combining of the four biological events required for this conversion process (production of saccharolytic enzymes, hydrolysis of the polysaccharides present in pretreated biomass, fermentation of hexose sugars, and fermentation of pentose sugars) in one reactor. CBP is gaining increasing recognition as a potential breakthrough for low-cost biomass processing. Although no natural microorganism exhibits all the features desired for CBP, a number of microorganisms, both bacteria and fungi, possess some of the desirable properties. This review focuses on progress made toward the development of baker's yeast (Saccharomyces cerevisiae) for CBP. The current status of saccharolytic enzyme (cellulases and hemicellulases) expression in S. cerevisiae to complement its natural fermentative ability is highlighted. Attention is also devoted to the challenges ahead to integrate all required enzymatic activities in an industrial S. cerevisiae strain(s) and the need for molecular and selection strategies pursuant to developing a yeast capable of CBP.

Regulated expression of green fluorescent protein in Debaryomyces hansenii

The broad range of environmental conditions under which Debaryomyces hansenii can grow, and its production of lipolytic and proteolytic enzymes, have promoted its widespread use. The present work represents a preliminary characterization of D. hansenii for heterologous expression and secretion of green fluorescent protein (GFP). Six heterologous expression vectors were used to address protein production efficiency under regulated expression conditions. Protein expression in D. hansenii seems to be similar to that in Saccharomyces cerevisiae, with transcription being controlled by almost all of the S. cerevisiae and D. hansenii inducible promoters tested, with the exception of the alcohol dehydrogenase 2 gene promoter from S. cerevisiae. Extracellular protein levels in D. hansenii were lower than in S. cerevisiae when Saccharomyces signal peptides were used.

The development of gene expression systems for filamentous fungi

Filamentous fungi have been used for decades in the commercial production of enzymes, antibiotics, and specialty chemicals. Traditionally, improving the yields of these products has involved either mutagenesis and screening or modification of fermentation conditions. Generally, selective breeding of strains has not been successful, because most of the commercially important fungal species lack a sexual cycle. For a few species, strain improvements have been made possible by employing the parasexual cycle for genetic crosses (30). The recent development of DNA-mediated transformation systems for several industrially important fungal species has spawned a flurry of research activity directed toward the development of gene expression systems for these microorganisms. This technology is now a viable means for novel and more directed approaches to improving existing fungal strains which produce enzymes or antibiotics. In addition, fungal expression systems are now being tested for the production of heterologous gene products such as mammalian pharmaceutical proteins. The goal of this review is to present a summary of the gene expression systems which have recently been developed for some filamentous fungi of commercial importance. To insure that the most recent developments are presented we have included data from not only scientific papers, but also from personal communications, abstracts, symposia, and our own laboratory.

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.

Zooshikella ganghwensis gen. nov., sp. nov., isolated from tidal flat sediments

Two red pigment-producing bacterial strains with a metallic green sheen were isolated from a sediment sample of getbol, the Korean tidal flat. Phylogenetic analysis based on 16S rDNA sequences showed that these isolates represent a phyletic lineage within the gamma-Proteobacteria that is distantly related to the genus Hahella. No bacterial species with validly published names showed > or = 92% 16S rRNA similarity with the getbol isolates. The strains were gram-negative, chemo-organotrophic, aerobic and required NaCl (1-7%) for growth. They produced pigments with maximum absorption at 540 nm, which indicated the presence of prodigiosin, a well-known red pigment previously detected in Serratia marcescens. The major isoprenoid quinone was ubiquinone-9. The predominant cellular fatty acids were saturated and monounsaturated straight-chain fatty acids. The DNA G + C contents ranged from 40 to 42 mol%. The combination of physiological, biochemical and chemotaxonomic data clearly separated the test strains from other phylogenetically related genera in the gamma-Proteobacteria. On the basis of polyphasic evidence from this study, it is proposed that the two getbol isolates should be classified in a novel genus, Zooshikella gen. nov., as Zooshikella ganghwensis sp. nov.

Microbial cellulose utilization: fundamentals and biotechnology

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

Metabolic engineering of Saccharomyces cerevisiae for xylose utilization

Metabolic engineering of Saccharomyces cerevisiae for ethanolic fermentation of xylose is summarized with emphasis on progress made during the last decade. Advances in xylose transport, initial xylose metabolism, selection of host strains, transformation and classical breeding techniques applied to industrial polyploid strains as well as modeling of xylose metabolism are discussed. The production and composition of the substrates--lignocellulosic hydrolysates--is briefly summarized. In a future outlook iterative strategies involving the techniques of classical breeding, quantitative physiology, proteomics, DNA micro arrays, and genetic engineering are proposed for the development of efficient xylose-fermenting recombinant strains of S. cerevisiae.

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.

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.

Cloning of endoglucanase genes from Cellulomonas biazotea into E. coli and S. cerevisiae using shuttle vector YEp24

We constructed a SmaI genomic library of Cellulomonas biazotea DNA in E. coli and in the S. cerevisiae shuttle vector, YEP 24. Three clone were identified that conferred the ability for E. coli or S. cerevisiae transformants to produce carboxymethylcellulase (CMCase). Cells transformed with these clones were compared with one another and with nontransformed cells for hyper-production of CMCase. In vivo and in vitro studies indicated that the CMCase genes were fully expressed and the enzyme activity was located extracellularly. The optimum pH and temperature for the CMCase thus cloned were pH 7 and 50 degrees C, respectively, as was the case for the donor.

New developments in fungal virology

Although viruses are widely distributed in fungi, their biological significance to their hosts is still poorly understood. A large number of fungal viruses are associated with latent infections of their hosts. With the exception of the killer-immune character in the yeasts, smuts, and hypovirulence in the chestnut blight fungus, fungal properties that can specifically be related to virus infection are not well defined. Mycoviruses are not known to have natural vectors; they are transmitted in nature intracellularly by hyphal anastomosis and heterokaryosis, and are disseminated via spores. Because fungi have a potential for plasmogamy and cytoplasmic exchange during extended periods of their life cycles and because they produce many types of propagules (sexual and asexual spores), often in great profusion, mycoviruses have them accessible to highly efficient means for transmission and spread. It is no surprise, therefore, that fungal viruses are not known to have an extracellular phase to their life cycles. Although extracellular transmission of a few fungal viruses have been demonstrated, using fungal protoplasts, the lack of conventional methods for experimental transmission of these viruses have been, and remains, an obstacle to understanding their biology. The recent application of molecular biological approaches to the study of mycoviral dsRNAs and the improvements in DNA-mediated fungal transformation systems, have allowed a clearer understanding of the molecular biology of mycoviruses to emerge. Considerable progress has been made in elucidating the genome organization and expression strategies of the yeast L-A virus and the unencapsidated RNA virus associated with hypovirulence in the chestnut blight fungus. These recent advances in the biochemical and molecular characterization of the genomes of fungal viruses and associated satellite dsRNAs, as they relate to the biological properties of these viruses and to their interactions with their hosts are the focus of this chapter.

Secretion of a Cryptococcus albidus xylanase in Saccharomyces cerevisiae

The xylanase(XLN)-encoding gene(XLN) of Cryptococcus albidus and its cDNA were each inserted into the vector, pVT100, for expression in Saccharomyces cerevisiae. Expression was under the control of either their own promoter or the gene encoding alcohol dehydrogenase (ADH1) promoter. Yeast transformed with plasmids containing the cDNA of the structural XLN gene and the XLN promoter produced active extracellular XLN when grown with galactose as carbon source. However, with glucose as carbon source, XLN was repressed. Using the ADH1 promoter, which is stimulated by glucose, XLN was secreted into the culture medium. In both cases, the secreted 48-kDa enzyme corresponded to the native XLN produced by C. albidus. With the plasmid bearing the genomic XLN gene, there was transcription, but the seven introns interrupting XLN were not spliced out by S. cerevisiae and no enzyme was produced.

Enzymatic degradation of cell wall and related plant polysaccharides

Polysaccharides such as starch, cellulose and other glucans, pectins, xylans, mannans, and fructans are present as major structural and storage materials in plants. These constituents may be degraded and modified by endogenous enzymes during plant growth and development. In plant pathogenesis by microorganisms, extracellular enzymes secreted by infected strains play a major role in plant tissue degradation and invasion of the host. Many of these polysaccharide-degrading enzymes are also produced by microorganisms widely used in industrial enzyme production. Most commerical enzyme preparations contain an array of secondary activities in addition to the one or two principal components which have standardized activities. In the processing of unpurified carbohydrate materials such as cereals, fruits, and tubers, these secondary enzyme activities offer major potential for improving process efficiency. Use of more defined combinations of industrial polysaccharases should allow final control of existing enzyme processes and should also lead to the development of novel enzymatic applications.

Ethanol inhibition of Saccharomyces and Candida enzymes

Ethanol inhibition of several hydrolases (sucrase, maltase, trehalase, melezitase and cellobiase) has been measured in both highly ethanol-tolerant Saccharomyces strains (R) and in Candida strains less tolerant to ethanol (S). Cells were either grown in the presence of ethanol and the activities of the enzymes measured without preincubation in this alcohol ("in situ" inhibition assay), or the culture was grown in the absence of ethanol and the activities of the enzymes were determined after preincubation and in the presence of this compound ("in vitro" inhibition assay). Ethanol inhibition (Ki values) of sucrase, maltase, trehalase, and melezitase was quite different for these different enzymes in the same strain (R or S), but similar for the same enzyme in different strains (R and S). The Ki values for cellobiase, which is absent from the R strain, were higher when induced than at the basal level and higher in in vitro assays than in in situ assays. This suggests that the inhibition observed in situ is mainly the result of an inhibition of other proteins related to cellobiase (i.e., those involved in its synthesis) but not a direct inactivation of the enzyme by ethanol. Accordingly, when hybrids between Saccharomyces (R) and Candida (S) strains were constructed by protoplast fusion, and cellobiase was measured in the parental Candida strain and some of the hybrids, there was an increase in the Ki values in the in situ assays from 2.25% ethanol in Candida to 5.5% in some of the hybrids.

Regulatory aspects of cellulase biosynthesis and secretion

The cellulase enzyme system consists of cellobiohydrolase, endoglucanase, and beta-glucosidase and has been extensively studied with respect to its biosynthesis, properties, mode of action, application, and, most recently, secretion mechanisms. A knowledge of the factors governing the biosynthesis and secretion of these enzymes at the molecular level will be useful in maximizing enzyme productivity in extracellular fluid. Among other topics, the regulatory effects of sorbose (a noninducing sugar which is not a product of cellulose hydrolysis) on cellulase synthesis and release are described. Cellulase genes have recently been cloned into a number of microorganisms with a view to understanding the gene structure and expression and to obtaining the enzyme components in pure form. The factors governing biosynthesis and secretion of cellulases in recombinant cells are also discussed. Cellulases are known to be glycoproteins, therefore, the role of O- and N-linked glycosylation on enzyme stability and secretion is also detailed.

Efficient secretion of two fungal cellobiohydrolases by Saccharomyces cerevisiae

Two different cellobiohydrolases, CBHI and CBHII, of the filamentous fungus Trichoderma reesei both hydrolyse highly crystalline cellulose. Cellulolytic strains of the yeast Saccharomyces cerevisiae were constructed by transferring cDNAs coding for these enzymes into yeast on an expression plasmid. These cellulolytic yeasts were able to secrete efficiently the large, heterologous proteins to the culture medium. The recombinant cellulases were observed to be heterogeneous in Mr due, at least partly, to variable N-glycosylation. Recombinant CBHII was able to bind to crystalline cellulose, although slightly less efficiently than the native enzyme. Both of the two recombinant cellulases were able to degrade amorphous cellulose. In a fermenter cultivation, around 100 micrograms/ml of CBHII was secreted into the yeast growth medium.