Exploring improved endoglucanase expression in Saccharomyces cerevisiae strains

Fungal cellulases: protein engineering and post-translational modifications

Enzymatic degradation of lignocelluloses into fermentable sugars to produce biofuels and other biomaterials is critical for environmentally sustainable development and energy resource supply. However, there are problems in enzymatic cellulose hydrolysis, such as the complex cellulase composition, low degradation efficiency, high production cost, and post-translational modifications (PTMs), all of which are closely related to specific characteristics of cellulases that remain unclear. These problems hinder the practical application of cellulases. Due to the rapid development of computer technology in recent years, computer-aided protein engineering is being widely used, which also brings new opportunities for the development of cellulases. Especially in recent years, a large number of studies have reported on the application of computer-aided protein engineering in the development of cellulases; however, these articles have not been systematically reviewed. This article focused on the aspect of protein engineering and PTMs of fungal cellulases. In this manuscript, the latest literatures and the distribution of potential sites of cellulases for engineering have been systematically summarized, which provide reference for further improvement of cellulase properties. KEY POINTS: •Rational design based on virtual mutagenesis can improve cellulase properties. •Modifying protein side chains and glycans helps obtain superior cellulases. •N-terminal glutamine-pyroglutamate conversion stabilizes fungal cellulases.

Heterologous production of cellulose- and starch-degrading hydrolases to expand Saccharomyces cerevisiae substrate utilization: Lessons learnt

Selected strains of Saccharomyces cerevisiae are used for commercial bioethanol production from cellulose and starch, but the high cost of exogenous enzymes for substrate hydrolysis remains a challenge. This can be addressed through consolidated bioprocessing (CBP) where S. cerevisiae strains are engineered to express recombinant glycoside hydrolases during fermentation. Looking back at numerous strategies undertaken over the past four decades to improve recombinant protein production in S. cerevisiae, it is evident that various steps in the protein production "pipeline" can be manipulated depending on the protein of interest and its anticipated application. In this review, we briefly introduce some of the strategies and highlight lessons learned with regards to improved transcription, translation, post-translational modification and protein secretion of heterologous hydrolases. We examine how host strain selection and modification, as well as enzyme compatibility, are crucial determinants for overall success. Finally, we discuss how lessons from heterologous hydrolase expression can inform modern synthetic biology and genome editing tools to provide process-ready yeast strains in future. However, it is clear that the successful expression of any particular enzyme is still unpredictable and requires a trial-and-error approach.

Isolation and Characterization of Yeasts from Rumen Fluids for Potential Use as Additives in Ruminant Feeding

Saccharomyces cerevisiae is a yeast strain often used to improve the feed quality of ruminants. However, S. cerevisiae has limited capacity to provide biomass when inoculated with carbon sources and a low ability to produce cellulase enzymes. Here, we hypothesized that yeast in the rumen produces a large amount of biomass and could release cellulase enzymes to break down fiber content. Therefore, the aim of this study was to screen, isolate and identify yeast from the rumen fluids of Holstein Friesian steers and measure the efficiency of biomass production and cellulase activity. A fermentation medium containing sugarcane molasses as a carbon source and urea as a nitrogen source was optimized. Two fistulated–crossbred Holstein Friesian steers averaging 350 ± 20 kg body weight were used to screen and isolate the ruminal yeast. Two experiments were designed: First, a 12 × 3 × 3 factorial was used in a completely randomized design to determine biomass and carboxymethyl cellulase activity. Factor A was the isolated yeast and S. cerevisiae. Factor B was sugarcane molasses (M) concentration. Factor C was urea (U) concentration. In the second experiment, potential yeasts were selected, identified, and analyzed for 7 × 4 factorial use in a completely randomized design. Factor A was the incubation times. Factor B was the isolated yeast strains, including codes H-Khon Kaen University (KKU) 20 (as P. kudriavzevii-KKU20), I-KKU20 (C. tropicalis-KKU20), and C-KKU20 (as Galactomyces sp.-KKU20). Isolation was imposed under aerobic conditions, resulting in a total of 11 different colonies. Two appearances of colonies including asymmetric colonies of isolated yeast (indicated as A, B, C, E, and J) and ovoid colonies (coded as D, F, G, H, I, and K) were noted. Isolated yeast from the rumen capable of providing a high amount of biomass when inoculant consisted of the molasses 15% + urea 3% (M15 + U3), molasses 25% + urea 1% (M25 + U1), molasses 25% + urea 3% (M25 + U3), and molasses 25% + urea 5% (M25 + U5) when compared to the other media solution (p < 0.01). In addition, 11 isolated biomass-producing yeasts were found in the media solution of M25 + U1. There were 4 isolates cellulase producing yeasts discovered in the media solution of M25 + U1 and M25 + U5 whereas molasses 5% + urea 1% (M5 + U1), molasses 5% + urea 3% (M5 + U3), molasses 5% + urea 5% (M5 + U5), molasses 15% + urea 1% (M15 + U1), molasses 15% + urea 3% (M5 + U3), and M25 + U3 were found with 2, 3, 1, 2, 1, and 2 isolates, respectively. Ruminal yeast strains H-KKU20, I-KKU20, and C-KKU20 were selected for their ability to produce biomass. Identification of isolates H-KKU20 and I-KKU20 revealed that those isolates belonged to Pichia kudriavzevii-KKU20 and Candida tropicalis-KKU20 while C-KKU20 was identified as Galactomyces sp.-KKU20. Two strains provided maximum cell growth: P. kudriavzevii-KKU20 (9.78 and 10.02 Log cell/mL) and C. tropicalis-KKU20 (9.53 and 9.6 Log cells/mL) at 60 and 72 h of incubation time, respectively. The highest ethanol production was observed in S. cerevisiae at 76.4, 77.8, 78.5, and 78.6 g/L at 36, 48, 60, and 72 h of incubation time, respectively (p < 0.01). The P. kudriavzevii-KKU20 yielded the least reducing sugar at about 30.6 and 29.8 g/L at 60 and 72 h of incubation time, respectively. The screening and isolation of yeasts from rumen fluids resulted in 11 different yeasts being obtained. The potential yeasts discovered in the rumen fluid of cattle were Pichia kudriavzevii-KKU20, Candida tropicalis-KKU20, and Galactomyces sp.-KKU20. P. kudriavzevii-KKU20 had higher results than the other yeasts in terms of biomass production, cellulase enzyme activity, and cell number.

Responses of bacterial communities and their carbon dynamics to subsoil exposure on the Loess Plateau

Subsoil exposure due to factors including erosion and terracing, evidently decreases soil organic carbon storage and productivity, but the responses of bacterial communities and their carbon dynamics remain unclear. Soils from 0-20 cm, 20-60 cm and 60-100 cm were collected from three 100 cm profiles in bare land on the Loess Plateau, and incubated in buried pots for a year (July 2016 to July 2017) to simulate subsoil exposure, with ongoing monitoring of the microbial mineralization rate of soil organic carbon (K_c), using Li-Cor 8100. At the end of the incubation period, the exposed soil and the in situ control soil were sampled to investigate changes in bacterial community composition, as represented by 16S rRNA, and the activities of enzymes involved in soil carbon cycling. Both copiotrophic (Actinobacteria and Alphaproteobacteria) and oligotrophic (Thermoleophilia) groups were stimulated in the exposed vs. control soil at 20-60 and 60-100 cm. The exposed vs. control soil from 60 to 100 cm produced the greatest bacterial responses, such as greater diversity and altered keystone groups (Thermoleophilia vs. unidentified Acidobacteria). Enzyme activities were greater in the exposed vs. control soil at both 20-60 cm (β-D-xylosidase and cellobiohydrolase) and 60-100 cm (β-D-xylosidase and β-D-glucosidase). The exposed soil from 20-60 cm and 60-100 cm had lower K_c and Q₁₀ values than those at 0-20 cm. Our findings revealed the existence of bacterial depth-specific responses to subsoil exposure, and highlight the effect of anthropogenic soil redistribution on soil carbon flux and its potential responses to future climate change.

Chimeric cellobiohydrolase I expression, activity, and biochemical properties in three oleaginous yeast

Consolidated bioprocessing using oleaginous yeast is a promising modality for the economic conversion of plant biomass to fuels and chemicals. However, yeast are not known to produce effective biomass degrading enzymes naturally and this trait is essential for efficient consolidated bioprocessing. We expressed a chimeric cellobiohydrolase I gene in three different oleaginous, industrially relevant yeast: Yarrowia lipolytica, Lipomyces starkeyi, and Saccharomyces cerevisiae to study the biochemical and catalytic properties and biomass deconstruction potential of these recombinant enzymes. Our results showed differences in glycosylation, surface charge, thermal and proteolytic stability, and efficacy of biomass digestion. L. starkeyi was shown to be an inferior active cellulase producer compared to both the Y. lipolytica and S. cerevisiae enzymes, whereas the cellulase expressed in S. cerevisiae displayed the lowest activity against dilute-acid-pretreated corn stover. Comparatively, the chimeric cellobiohydrolase I enzyme expressed in Y. lipolytica was found to have a lower extent of glycosylation, better protease stability, and higher activity against dilute-acid-pretreated corn stover.

Rapid optimisation of cellulolytic enzymes ratios in Saccharomyces cerevisiae using in vitro SCRaMbLE

BACKGROUND

For the economic production of biofuels and other valuable products from lignocellulosic waste material, a consolidated bioprocessing (CBP) organism is required. With efficient fermentation capability and attractive industrial qualities, Saccharomyces cerevisiae is a preferred candidate and has been engineered to produce enzymes that hydrolyze cellulosic biomass. Efficient cellulose hydrolysis requires the synergistic action of several enzymes, with the optimum combined activity ratio dependent on the composition of the substrate.

RESULTS

In vitro SCRaMbLE generated a library of plasmids containing different ratios of a β-glucosidase gene (CEL3A) from Saccharomycopsis fibuligera and an endoglucanase gene (CEL5A) from Trichoderma reesei. S. cerevisiae, transformed with the plasmid library, displayed a range of individual enzyme activities and synergistic capabilities. Furthermore, we show for the first time that 4,6-O-(3-ketobutylidene)-4-nitrophenyl-β-D-cellopentaoside (BPNPG5) is a suitable substrate to determine synergistic Cel3A and Cel5A action and an accurate predictive model for this synergistic action was devised. Strains with highest BPNPG5 activity had an average CEL3A and CEL5A gene cassette copy number of 1.3 ± 0.6 and 0.8 ± 0.2, respectively (ratio of 1.6:1).

CONCLUSIONS

Here, we describe a synthetic biology approach to rapidly optimise gene copy numbers to achieve efficient synergistic substrate hydrolysis. This study demonstrates how in vitro SCRaMbLE can be applied to rapidly combine gene constructs in various ratios to allow screening of synergistic enzyme activities for efficient substrate hydrolysis.

Pyrolysis biochar has negligible effects on soil greenhouse gas production, microbial communities, plant germination, and initial seedling growth

Biochar has been promoted as a soil amendment that enhances soil quality and agronomic productivity and reduces greenhouse gas production. However, these benefits are not always realized. A major hurdle to the beneficial use of biochar is our limited knowledge regarding the mechanisms directing its effects on soil systems. This project aimed to eliminate some of this uncertainty by examining the biological responses (plant productivity, greenhouse gas production, soil microbial community structure) of a suite of soils (10) to the addition of biochars produced by different processes (pyrolysis, gasification, burning) from a range of feedstocks (corn stalks, hardwood, grass). Results indicated that these three responses were not significantly impacted by the addition of pyrolysis biochars from different feedstocks at 1 and 5% (w/w) addition levels. On the other hand, both an open-air burned corn stalk (5%) and raw corn stalks (1 and 5%) additions did alter the measured soil functionality. For example, the 5% burnt corn stalks addition reduced total above ground plant biomass (∼30%), increased observed N₂O production by an order of magnitude, and altered soil bacterial community structure. The bacterial groups that increased in relative abundance in the burnt corn stalks-amended soils included families associated with cellulose decomposition (Chitinophagaceae), plant pathogens (Xanthomonadaceae), and biochar/charcoal-amended media (Gemmatimonadetes). In contrast, the abundance of these bacterial groups was not impacted by the pyrolysis biochars. Therefore, this research suggests that pyrolysis biochar represents a stabilized form of carbon that is resistant to microbial mineralization and has negligible effects on soil biological responses.

Spatial variations of soil respiration and temperature sensitivity along a steep slope of the semiarid Loess Plateau

The spatial heterogeneity of soil respiration and its temperature sensitivity pose a great challenge to accurately estimate the carbon flux in global carbon cycling, which has primarily been researched in flatlands versus hillslope ecosystems. On an eroded slope (35°) of the semiarid Loess Plateau, soil respiration, soil moisture and soil temperature were measured in situ at upper and lower slope positions in triplicate from 2014 until 2016, and the soil biochemical and microbial properties were determined. The results showed that soil respiration was significantly greater (by 44.2%) at the lower slope position (2.6 μmol m^–2 s^–1) than at the upper slope position, as were soil moisture, carbon, nitrogen fractions and root biomass. However, the temperature sensitivity was 13.2% greater at the upper slope position than at the lower slope position (P < 0.05). The soil fungal community changed from being Basidiomycota-dominant at the upper slope position to being Zygomycota-dominant at the lower slope position, corresponding with increased β-D-glucosidase activity at the upper slope position than at the lower slope position. We concluded that soil respiration was enhanced by the greater soil moisture, root biomass, carbon and nitrogen contents at the lower slope position than at the upper slope position. Moreover, the increased soil respiration and decreased temperature sensitivity at the lower slope position were partially due to copiotrophs replacing oligotrophs. Such spatial variations along slopes must be properly accounted for when estimating the carbon budget and feedback of future climate change on hillslope ecosystems.

Improving the activity of endoglucanase I (EGI) from Saccharomyces cerevisiae by DNA shuffling

To enhance the endo-β-1,4-glucanase activity of three mixedTrichodermasp. (Trichoderma reesei, Trichoderma longibrachiatum, andTrichoderma pseudokoningii), we optimized the efficiency of the encoding gene using DNA shuffling andSaccharomyces cerevisiaeINVSc1 as a host.

A novel treatment for weight reduction by the recombinant "Pichia pastoris" yeast expressing the hybrid protein of "irisin-furin-transferrin"

Obesity is a major health problem across the world, but there are few ways to effectively treat or manage it in the long term. Researchers are searching for more convenient, cost-effective and noninvasive therapies for overweight and obese people. Recent studies have illustrated that the microbiome of the body's different organs can be used as a vehicle for in-situ gene therapy. We suggest that the recombinant form of "Pichia pastoris" yeast expressing the hybrid protein of "irisin-furin-transferrin" under the control of the enolase 1 promoter is a new nutraceutical strategy to absorb fewer calories from intestinal nutrients, and induce a higher metabolic rate to expend more calories, similar to that from engaging in physical activity. By comparison, this method can be a long-term, noninvasive treatment and can be used for obese patients who have movement limitations.

Challenges for the production of bioethanol from biomass using recombinant yeasts

Lignocellulose biomass, one of the most abundant renewable resources on the planet, is an alternative sustainable energy source for the production of second-generation biofuels. Energy in the form of simple or complex carbohydrates can be extracted from lignocellulose biomass and fermented by microorganisms to produce bioethanol. Despite 40 years of active and cutting-edge research invested into the development of technologies to produce bioethanol from lignocellulosic biomass, the process remains commercially unviable. This review describes the achievements that have been made in generating microorganisms capable of utilizing both simple and complex sugars from lignocellulose biomass and the fermentation of these sugars into ethanol. We also provide a discussion on the current "roadblocks" standing in the way of making second-generation bioethanol a commercially viable alternative to fossil fuels.

Effects of different types of N deposition on the fungal decomposition activities of temperate forest soils

Nitrogen (N) deposition significantly affects soil microbial activities and litter decomposition processes in forest ecosystems. However, the changes in soil fungi during litter decomposition remain unclear. In this study, ammonium nitrate was selected as inorganic N (IN), whereas urea and glycine were selected as organic N (ON). N fertilizer with different IN-to-ON ratios (1:4, 2:3, 3:2, 4:1, and 5:0) was mixed in equal amounts and then added to temperate forest soils. Half of each treatment was simultaneously added with streptomycin to inhibit soil bacteria. The activities of enzymes involved in litter decomposition (invertase, β-glucosidase, cellulase, polyphenol oxidase, and phosphatase) were assayed after a three-year field experiment. The results showed that enzymatic activities were inhibited by IN addition but accelerated by ON addition in the non-antibiotic addition treatments. An increase in ON in the mixed N fertilizer also shifted enzymatic activities from N inhibition to N stimulation. Similarly, in the antibiotic addition treatments, fungal activities revealed the same trends, but they were seriously inhibited by IN and significantly accelerated by ON. These results indicated that soil fungi were more sensitive to N deposition, particularly to ON. A large amount of ON may convert soil microbial communities into a fungi-dominated system. However, excessive ON deposition (20% IN+80% ON) caused N saturation and repressed fungal activities. These results suggested that soil fungi were sensitive to N type and that different IN-to-ON ratios may induce diverse ecological effects on soil fungi.

Expression and functional analysis of a glycoside hydrolase family 45 endoglucanase from Rhizopus stolonifer

A novel endoglucanase gene was cloned from Rhizopus stolonifer and expressed in Escherichia coli, the gene product EG II (45 kDa) was assigned to Glycoside Hydrolase Family 45 (GH45), and its specific activity on phosphoric acid-swollen cellulose (PASC) was 48 IU/mg. To solve the problem of substrate accumulation in the cellulose hydrolysis and enhance the catalytic efficiency of endoglucanase, the eg2 gene was modified by site directed mutagenesis. Mutations generated by overlapping PCR have been proven to increase its catalytic activity on carboxymenthyl cellulose, microcrystalline cellulose (Avicel) and PASC, among which the mutant EG II-E containing all 6 mutations (N39S, V136D, T251G, D255G, P256S and E260D) peaked 121 IU/mg on PASC. The bioinformatic analysis showed that 2 key catalytic residues (D136 and D260) moved closer with the opening of a loop after mutagenesis, and a tunnel was formed by structural transformation. This structure was conducive for the substrate to access the active centre, and D136 played an indispensable role in the substrate recognition.

Metabolic engineering of yeasts by heterologous enzyme production for degradation of cellulose and hemicellulose from biomass: a perspective

This review focuses on current approaches to metabolic engineering of ethanologenic yeast species for the production of bioethanol from complex lignocellulose biomass sources. The experimental strategies for the degradation of the cellulose and xylose-components of lignocellulose are reviewed. Limitations to the current approaches are discussed and novel solutions proposed.

Expression of three Trichoderma reesei cellulase genes in Saccharomyces pastorianus for the development of a two-step process of hydrolysis and fermentation of cellulose

AIMS

To compare the production of recombinant cellulase enzymes in two Saccharomyces species so as to ascertain the most suitable heterologous host for the degradation of cellulose-based biomass and its conversion into bioethanol.

METHOD AND RESULTS

cDNA copies of genes representing the three major classes of cellulases (Endoglucanases, Cellobiohydrolases and β-glucosidases) from Trichoderma reesei were expressed in Saccharomyces pastorianus and Saccharomyces cerevisiae. The recombinant enzymes were secreted by the yeast hosts into the medium and were shown to act in synergy to hydrolyse cellulose. The conditions required to achieve maximum release of glucose from cellulose by the recombinant enzymes were defined and the activity of the recombinant enzymes was compared to a commercial cocktail of T. reesei cellulases.

CONCLUSIONS

We demonstrate that significantly higher levels of cellulase activity were achieved by expression of the genes in S. pastorianus compared to S. cerevisiae. Hydrolysis of cellulose by the combined activity of the recombinant enzymes was significantly better at 50°C than at 30°C, the temperature used for mesophilic yeast fermentations, reflecting the known temperature profiles of the native enzymes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results demonstrate that host choice is important for the heterologous production of cellulases. On the basis of the low activity of the T. reesei recombinant enzymes at fermentation temperatures, we propose a two-step process for the hydrolysis of cellulose and its fermentation into alcohol using cellulases produced in situ.

Challenges and advances in the heterologous expression of cellulolytic enzymes: a review

Second generation biofuel development is increasingly reliant on the recombinant expression of cellulases. Designing or identifying successful expression systems is thus of preeminent importance to industrial progress in the field. Recombinant production of cellulases has been performed using a wide range of expression systems in bacteria, yeasts and plants. In a number of these systems, particularly when using bacteria and plants, significant challenges have been experienced in expressing full-length proteins or proteins at high yield. Further difficulties have been encountered in designing recombinant systems for surface-display of cellulases and for use in consolidated bioprocessing in bacteria and yeast. For establishing cellulase expression in plants, various strategies are utilized to overcome problems, such as the auto-hydrolysis of developing plant cell walls. In this review, we investigate the major challenges, as well as the major advances made to date in the recombinant expression of cellulases across the commonly used bacterial, plant and yeast systems. We review some of the critical aspects to be considered for industrial-scale cellulase production.

Linking microbial community structure to β-glucosidic function in soil aggregates

To link microbial community 16S structure to a measured function in a natural soil, we have scaled both DNA and β-glucosidase assays down to a volume of soil that may approach a unique microbial community. β-Glucosidase activity was assayed in 450 individual aggregates, which were then sorted into classes of high or low activities, from which groups of 10 or 11 aggregates were identified and grouped for DNA extraction and pyrosequencing. Tandem assays of ATP were conducted for each aggregate in order to normalize these small groups of aggregates for biomass size. In spite of there being no significant differences in the richness or diversity of the microbial communities associated with high β-glucosidase activities compared with the communities associated with low β-glucosidase communities, several analyses of variance clearly show that the communities of these two groups differ. The separation of these groups is partially driven by the differential abundances of members of the Chitinophagaceae family. It may be observed that functional differences in otherwise similar soil aggregates can be largely attributed to differences in resource availability, rather than to the presence or absence of particular taxonomic groups.

Ethanol production from high cellulose concentration by the basidiomycete fungus Flammulina velutipes

Ethanol production by Flammulina velutipes from high substrate concentrations was evaluated. F. velutipes produces approximately 40-60 g l(-1) ethanol from 15% (w/v) D-glucose, D-fructose, D-mannose, sucrose, maltose, and cellobiose, with the highest conversion rate of 83% observed using cellobiose as a carbon source. We also attempted to assess direct ethanol fermentation from sugarcane bagasse cellulose (SCBC) by F. velutipes. The hydrolysis rate of 15% (w/v) SCBC with commercial cellulase was approximately 20%. In contrast, F. velutipes was able to produce a significant amount of ethanol from 15% SCBC with the production of β-glucosidase, cellobohydrolase, and cellulase, although the addition of a small amount of commercial cellulase to the culture was required for the conversion. When 9 mg g(-1) biomass of commercial cellulase was added to cultures, 0.36 g of ethanol was produced from 1 g of cellulose, corresponding to an ethanol conversion rate of 69.6%. These results indicate that F. velutipes would be useful for consolidated bioprocessing of lignocellulosic biomass to bioethanol.

High ethanol titers from cellulose by using metabolically engineered thermophilic, anaerobic microbes

This work describes novel genetic tools for use in Clostridium thermocellum that allow creation of unmarked mutations while using a replicating plasmid. The strategy employed counter-selections developed from the native C. thermocellum hpt gene and the Thermoanaerobacterium saccharolyticum tdk gene and was used to delete the genes for both lactate dehydrogenase (Ldh) and phosphotransacetylase (Pta). The Δldh Δpta mutant was evolved for 2,000 h, resulting in a stable strain with 40:1 ethanol selectivity and a 4.2-fold increase in ethanol yield over the wild-type strain. Ethanol production from cellulose was investigated with an engineered coculture of organic acid-deficient engineered strains of both C. thermocellum and T. saccharolyticum. Fermentation of 92 g/liter Avicel by this coculture resulted in 38 g/liter ethanol, with acetic and lactic acids below detection limits, in 146 h. These results demonstrate that ethanol production by thermophilic, cellulolytic microbes is amenable to substantial improvement by metabolic engineering.

High level secretion of cellobiohydrolases by Saccharomyces cerevisiae

BACKGROUND

The main technological impediment to widespread utilization of lignocellulose for the production of fuels and chemicals is the lack of low-cost technologies to overcome its recalcitrance. Organisms that hydrolyze lignocellulose and produce a valuable product such as ethanol at a high rate and titer could significantly reduce the costs of biomass conversion technologies, and will allow separate conversion steps to be combined in a consolidated bioprocess (CBP). Development of Saccharomyces cerevisiae for CBP requires the high level secretion of cellulases, particularly cellobiohydrolases.

RESULTS

We expressed various cellobiohydrolases to identify enzymes that were efficiently secreted by S. cerevisiae. For enhanced cellulose hydrolysis, we engineered bimodular derivatives of a well secreted enzyme that naturally lacks the carbohydrate-binding module, and constructed strains expressing combinations of cbh1 and cbh2 genes. Though there was significant variability in the enzyme levels produced, up to approximately 0.3 g/L CBH1 and approximately 1 g/L CBH2 could be produced in high cell density fermentations. Furthermore, we could show activation of the unfolded protein response as a result of cellobiohydrolase production. Finally, we report fermentation of microcrystalline cellulose (Avicel™) to ethanol by CBH-producing S. cerevisiae strains with the addition of beta-glucosidase.

CONCLUSIONS

Gene or protein specific features and compatibility with the host are important for efficient cellobiohydrolase secretion in yeast. The present work demonstrated that production of both CBH1 and CBH2 could be improved to levels where the barrier to CBH sufficiency in the hydrolysis of cellulose was overcome.

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.

Fermentation of cellobiose to ethanol by industrial Saccharomyces strains carrying the β-glucosidase gene (BGL1) from Saccharomycopsis fibuligera

Constructs carrying the Saccharomycopsis fibuligera β-glucosidase gene (BGL1) under the control of a constitutive actin or a galactose-inducible promoter were introduced into eleven Saccharomyces strains. In ten of these recombinant strains, BGL1 expression driven by the actin promoter was between 1.6- and 18-fold higher than that obtained with the galactose-inducible promoter. Strains carrying the actin promoter yielded ethanol concentrations from cellobiose of between 0.5% and 14%, depending on their ability to accumulate Bgl1 (between 30 and 250 mU/mL) but also on their genetic background. Comparative analysis of a S. cerevisiae strain and its corresponding petite version showed similar ethanol yields, despite a 3-fold lower β-glucosidase production of the latter, suggesting that respiratory activity could be one of the factors influencing ethanol production when using carbon sources other than glucose. This study provides a selection of strains that may be good candidates as hosts for ethanol biosynthesis from cellulosic substrates.