Efficient recombinant expression and secretion of a thermostable GH26 mannan endo-1,4-beta-mannosidase from Bacillus licheniformis in Escherichia coli

A review of the principles and biotechnological applications of glycoside hydrolases from extreme environments

It is apparent that Biocatalysts are shaping the future by providing a more sustainable approach to established chemical processes. Industrial processes rely heavily on the use of toxic compounds and high energy or pH reactions, factors that both contributes to the worsening climate crisis. Enzymes found in bacterial systems and other microorganisms, from the glaciers of the Arctic to the sandy deserts of Abu Dhabi, provide key tools and understanding as to how we can progress in the biotechnology sector. These extremophilic bacteria harness the adaptive enzymes capable of withstanding harsh reaction conditions in terms of stability and reactivity. Carbohydrate-active enzymes, including glycoside hydrolases or carbohydrate esterases, are extremely beneficial for the presence and future of biocatalysis. Their involvement in the industry spans from laundry detergents to paper and pulp treatment by degrading oligo/polysaccharides into their monomeric products in almost all detrimental environments. This includes exceedingly high temperatures, pHs or even in the absence of water. In this review, we discuss the structure and function of different glycoside hydrolases from extremophiles, and how they can be applied to industrial-scale reactions to replace the use of harsh chemicals, reduce waste, or decrease energy consumption.

Engineered Alcaligenes sp. by chemical mutagen produces thermostable and acido-alkalophilic endo-1,4-β-mannanases for improved industrial biocatalyst

This study reported physicochemical properties of purified endo-1,4-β-mannanase from the wild type, Alcaligenes sp. and its most promising chemical mutant. The crude enzymes from fermentation of wild and mutant bacteria were purified by ammonium sulfate precipitation, ion exchange and gel-filtration chromatography followed by an investigation of the physicochemical properties of purified wild and mutant enzymes. β-mannanase from wild and mutant Alcaligenes sp. exhibited 1.75 and 1.6 purification-folds with percentage recoveries of 2.6 and 2.5% and molecular weights of 61.6 and 80 kDa respectively. The wild and mutant β-mannanase were most active at 40 and 50 °C with optimum pH 6.0 for both and were thermostable with very high percentage activity but the wild-type β-mannanase showed better stability over a broad pH activity. The β-mannanase activity from the parent strain was stimulated in the presence of Mn2+, Co2+, Zn2+, Mg2+ and Na+. Vmax and Km for the wild type and its mutant were found to be 0.747 U//mL/min and 5.2 × 10-4 mg/mL, and 0.247 U/mL/min and 2.47 × 10-4 mg/mL, respectively. Changes that occurred in the nucleotide sequences of the most improved mutant may be attributed to its thermo-stability, thermo-tolerant and high substrate affinity- desired properties for improved bioprocesses.

Chemical and nutritional characteristics, and microbial degradation of rapeseed meal recalcitrant carbohydrates: A review

Approximately 35% of rapeseed meal (RSM) dry matter (DM) are carbohydrates, half of which are water-soluble carbohydrates. The cell wall of rapeseed meal contains arabinan, galactomannan, homogalacturonan, rhamnogalacturonan I, type II arabinogalactan, glucuronoxylan, XXGG-type and XXXG-type xyloglucan, and cellulose. Glycoside hydrolases including in the degradation of RSM carbohydrates are α-L-Arabinofuranosidases (EC 3.2.1.55), endo-α-1,5-L-arabinanases (EC 3.2.1.99), Endo-1,4-β-mannanase (EC 3.2.1.78), β-mannosidase (EC 3.2.1.25), α-galactosidase (EC 3.2.1.22), reducing-end-disaccharide-lyase (pectate disaccharide-lyase) (EC 4.2.2.9), (1 → 4)-6-O-methyl-α-D-galacturonan lyase (pectin lyase) (EC 4.2.2.10), (1 → 4)-α-D-galacturonan reducing-end-trisaccharide-lyase (pectate trisaccharide-lyase) (EC 4.2.2.22), α-1,4-D-galacturonan lyase (pectate lyase) (EC 4.2.2.2), (1 → 4)-α-D-galacturonan glycanohydrolase (endo-polygalacturonase) (EC 3.2.1.15), Rhamnogalacturonan hydrolase, Rhamnogalacturonan lyase (EC 4.2.2.23), Exo-β-1,3-galactanase (EC 3.2.1.145), endo-β-1,6-galactanase (EC 3.2.1.164), Endo-β-1,4-glucanase (EC 3.2.1.4), α-xylosidase (EC 3.2.1.177), β-glucosidase (EC 3.2.1.21) endo-β-1,4-glucanase (EC 3.2.1.4), exo-β-1,4-glucanase (EC 3.2.1.91), and β-glucosidase (EC 3.2.1.21). In conclusion, this review summarizes the chemical and nutritional compositions of RSM, and the microbial degradation of RSM cell wall carbohydrates which are important to allow to develop strategies to improve recalcitrant RSM carbohydrate degradation by the gut microbiota, and eventually to improve animal feed digestibility, feed efficiency, and animal performance.

Select Streptococci Can Degrade Candida Mannan To Facilitate Growth

Multiple studies have found that streptococci have a synergistic relationship with Candida species, but the details of these interactions are still being discovered. Candida species are covered by mannan, a polymer of mannose, which could serve as a carbon source for certain microbes. We hypothesized that streptococci that possess mannan-degrading glycosyl hydrolases would be able to enzymatically cleave mannose residues, which could serve as a primary carbohydrate source to support growth. We analyzed 90 streptococcus genomes to predict the capability of streptococci to transport and utilize mannose and to degrade diverse mannose linkages found on mannan. The genome analysis revealed mannose transporters and downstream pathways in most streptococci, but only <50% of streptococci harbored the glycosyl hydrolases required for mannan degradation. To confirm the ability of streptococci to use mannose or mannan, we grew 6 representative streptococci in a chemically defined medium lacking glucose supplemented with mannose, yeast extract, or purified mannan isolated from Candida and Saccharomyces strains. Although all tested Streptococcus strains could use mannose, Streptococcus salivarius and Streptococcus agalactiae, which did not possess mannan-degrading glycosyl hydrolases, could not use yeast extract or mannan to enhance their growth. In contrast, we found that Streptococcus mitis, Streptococcus parasanguinis, Streptococcus sanguinis, and Streptococcus pyogenes possessed the necessary glycosyl hydrolases to use yeast extract and isolated mannan, which promoted robust growth. Our data indicate that several streptococci are capable of degrading fungal mannans and harvesting mannose for energy. IMPORTANCE This work highlights a previously undescribed aspect of streptococcal Candida interactions. Our work identifies that certain streptococci possess the enzymes required to degrade mannan, and through this mechanism, they can release mannose residues from the cell wall of fungal species and use them as a nutrient source. We speculate that streptococci that can degrade fungal mannan may have a competitive advantage for colonization. This finding has broad implications for human health, as streptococci and Candida are found at multiple body sites.

Manno-Oligosaccharide Production from Biomass Hydrolysis by Using Endo-1,4-β-Mannanase (ManNj6-379) from Nonomuraea jabiensis ID06-379

A novel endo-β-1,4-mannanase gene was cloned from a novel actinomycetes, Nonomuraea jabiensis ID06-379, isolated from soil, overexpressed as an extracellular protein (47.8 kDa) in Streptomyces lividans 1326. This new endo-1,4-β-mannanase gene (manNj6-379) is encoded by 445-amino acids. The ManNj6-379 consists of a 28-residue signal peptide and a carbohydrate-binding module of family 2 belonging to the glycoside hydrolase (GH) family 5, with 59–77% identity to GH5 mannan endo-1,4-β-mannanase. The recombinant ManNj6-379 displayed an optimal pH of 6.5 with pH stability ranging between 5.5 and 7.0 and was stable for 120 min at 50 °C and lower temperatures. The optimal temperature for activity was 70 °C. An enzymatic hydrolysis assay revealed that ManNj6-379 could hydrolyze commercial β-mannan and biomass containing mannan.

Enhanced β-mannanase production by Bacillus licheniformis by optimizing carbon source and feeding regimes

Bacillus licheniformis HDYM-04 was isolated in flax retting water and showed β-mannanase activity. Carbon sources for β-mannanase production, as well as the fermentation conditions and feeding strategy, were optimized in shake flasks. When glucose or konjac powder was used as the carbon source, the β-mannanase activity was 288.13 ± 21.59 U/mL and 696.35 ± 23.47 U/mL at 24 h, respectively, which was approximately 4.4- to 10.68-fold higher than the values obtained with wheat powder. When 0.5% (w/v) glucose and 1% (w/v) konjac powder were added together, maximum enzyme activities of 789.07 ± 25.82 U/mL were obtained, an increase of 13.35% compared to the unoptimized cultures with only 1% (w/v) konjac powder. The enzyme activity decreased in the presence of 1% (w/v) konjac powder, but the highest enzyme activity was 1,533.26 ± 33.74 U/mL, a 1.2-fold increase compared with that in nonoptimized cultures; when 0.5% (w/v) glucose was used, the highest enzyme activity was 966.53 ± 27.84 U/mL, an increase in β-mannanase activity of 38.79% compared with control cultures. In this study, by optimizing fed-batch fermentation conditions, the yield of β-mannanase produced by HDYM-04 was increased, laying the foundation for the industrial application and further research of B. licheniformis HDYM-04.

Ideal Feedstock and Fermentation Process Improvements for the Production of Lignocellulolytic Enzymes

The usage of lignocellulosic biomass in energy production for biofuels and other value-added products can extensively decrease the carbon footprint of current and future energy sectors. However, the infrastructure in the processing of lignocellulosic biomass is not well-established as compared to the fossil fuel industry. One of the bottlenecks is the production of the lignocellulolytic enzymes. These enzymes are produced by different fungal and bacterial species for degradation of the lignocellulosic biomass into its reactive fibers, which can then be converted to biofuel. The selection of an ideal feedstock for the lignocellulolytic enzyme production is one of the most studied aspects of lignocellulolytic enzyme production. Similarly, the fermentation enhancement strategies for different fermentation variables and modes are also the focuses of researchers. The implementation of fermentation enhancement strategies such as optimization of culture parameters (pH, temperature, agitation, incubation time, etc.) and the media nutrient amendment can increase the lignocellulolytic enzyme production significantly. Therefore, this review paper summarized these strategies and feedstock characteristics required for hydrolytic enzyme production with a special focus on the characteristics of an ideal feedstock to be utilized for the production of such enzymes on industrial scales.

Expression, Characterization and Structure Analysis of a New GH26 Endo-β-1, 4-Mannanase (Man26E) from Enterobacter aerogenes B19

β-mannanase is one of the key enzymes to hydrolyze hemicellulose. At present, most β-mannanases are not widely applied because of their low enzyme activity and unsuitable enzymatic properties. In this work, a new β-mannanase from Enterobacter aerogenes was studied, which laid the foundation for its further application. Additionally, we will further perform directed evolution of the enzyme to increase its activity, improve its temperature and pH properties to allow it more applications in industry. A new β-mannanase (Man26E) from Enterobacter aerogenes was successfully expressed in Escherichia coli. Man26E showed about 40 kDa on SDS-PAGE gel. The SWISS-MODEL program was used to model the tertiary structure of Man26E, which presented a core (α/β)8-barrel catalytic module. Based on the binding pattern of CjMan26 C, Man26E docking Gal1Man4 was investigated. The catalytic region consisted of a surface containing four solvent-exposed aromatic rings, many hydrophilic and charged residues. Man26E displayed the highest activity at pH 6.0 and 55 °C, and high acid and alkali stability in a wide pH range (pH 4–10) and thermostability from 40 to 50 °C. The enzyme showed the highest activity on locust bean gum, and the Km and Vmax were 7.16 mg mL−1 and 508 U mg−1, respectively. This is the second β-mannanase reported from Enterobacter aerogenes B19. The β-mannanase displayed high enzyme activity, a relatively high catalytic temperature and a broad range of catalytic pH values. The enzyme catalyzed both polysaccharides and manno-oligosaccharides.

Comparative time-course transcriptome analysis in contrasting Carex rigescens genotypes in response to high environmental salinity

Soil salinization is one of most crucial environmental problems around the world and negatively affects plant growth and production. Carex rigescens is a turfgrass with favorable stress tolerance and great application prospect in salinity soil remediation and utilization; however, the molecular mechanisms behind its salt stress response are unknown. We performed a time-course transcriptome analysis between salt tolerant 'Huanghua' (HH) and salt sensitive 'Beijing' (BJ) genotypes. Physiological changes within 24 h were observed, with the HH genotype exhibiting increased salt tolerance compared to BJ. 5764 and 10752 differentially expressed genes were approved by transcriptome in BJ and HH genotype, respectively, and dynamic analysis showed a discrepant profile between two genotypes. In the BJ genotype, genes related to carbohydrate metabolism and stress response were more active and ABA signal transduction pathway might play a more important role in salt stress tolerance than in HH genotype. In the HH genotype, unique increases in the regulatory network of transcription factors, hormone signal transduction, and oxidation-reduction processes were observed. Moreover, trehalose and pectin biosynthesis and chitin catabolic related genes were specifically involved in the HH genotype, which may have contributed to salt tolerance. Moreover, some candidate genes like mannan endo-1,4-beta-mannosidase and EG45-like domain-containing protein are highlighted for future research about salt stress resistance in C. rigescens and other plant species. Our study revealed unique salt adaptation and resistance characteristics of two C. rigescens genotypes and these findings could help to enrich the currently available knowledge and clarify the detailed salt stress regulatory mechanisms in C. rigescens and other plants.

A Recombinant β-Mannanase from Thermoanaerobacterium aotearoense SCUT27: Biochemical Characterization and Its Thermostability Improvement

β-Mannanase was expressed in Thermoanaerobacterium aotearoense SCUT27 induced by locust bean gum (LBG). The open reading frame encoding a GH26 β-mannanase was identified and encoded a preprotein of 515 amino acids with a putative signal peptide. The enzyme without a signal sequence (Man25) was overexpressed in Escherichia coli with a specific activity of 1286.2 U/mg. Moreover, a facile method for β-mannanase activity screening was established based on agar plates. The optimum temperature for the purified Man25 using LBG as a substrate was 55 °C. The catalytic activity and thermostability of Man25 displayed a strong dependence on calcium ions. Through saturation mutagenesis at the putative Ca²⁺ binding sites in Man25, the best mutant ManM3-3 (D143A) presented improvements in thermostability with 3.6-fold extended half-life at 55 °C compared with that of the wild-type. The results suggest that mutagenesis at metal binding sites could be an efficient approach to increase enzyme thermostability.

Constitutive expression and cell-surface display of a bacterial β-mannanase in Lactobacillus plantarum

BACKGROUND

Lactic acid bacteria (LAB) are important microorganisms in the food and beverage industry. Due to their food-grade status and probiotic characteristics, several LAB are considered as safe and effective cell-factories for food-application purposes. In this present study, we aimed at constitutive expression of a mannanase from Bacillus licheniformis DSM13, which was subsequently displayed on the cell surface of Lactobacillus plantarum WCFS1, for use as whole-cell biocatalyst in oligosaccharide production.

RESULTS

Two strong constitutive promoters, Pgm and SlpA, from L. acidophilus NCFM and L. acidophilus ATCC4356, respectively, were used to replace the inducible promoter in the lactobacillal pSIP expression system for the construction of constitutive pSIP vectors. The mannanase-encoding gene (manB) was fused to the N-terminal lipoprotein anchor (Lp_1261) from L. plantarum and the resulting fusion protein was cloned into constitutive pSIP vectors and expressed in L. plantarum WCFS1. The localization of the protein on the bacterial cell surface was confirmed by flow cytometry and immunofluorescence microscopy. The mannanase activity and the reusability of the constructed L. plantarum displaying cells were evaluated. The highest mannanase activities on the surface of L. plantarum cells obtained under the control of the Pgm and SlpA promoters were 1200 and 3500 U/g dry cell weight, respectively, which were 2.6- and 7.8-fold higher compared to the activity obtained from inducible pSIP anchoring vectors. Surface-displayed mannanase was shown to be able to degrade galactomannan into manno-oligosaccharides (MOS).

CONCLUSION

This work demonstrated successful displaying of ManB on the cell surface of L. plantarum WCFS1 using constitutive promoter-based anchoring vectors for use in the production of manno-oligosaccharides, which are potentially prebiotic compounds with health-promoting effects. Our approach, where the enzyme of interest is displayed on the cell surface of a food-grade organism with the use of strong constitutive promoters, which continuously drive synthesis of the recombinant protein without the need to add an inducer or change the growth conditions of the host strain, should result in the availability of safe, stable food-grade biocatalysts.

Secretion of the recombination α-amylase in Escherichia coli and purification by the gram-positive enhancer matrix (GEM) particles

α-Amylases are important enzymes in industry. A recombinant α-amylase with a secretion signal peptide and an AcmA tag was expressed in Escherichia coli to improve the yield. The induction concentrations were optimized, and the temperature had a significant influence on soluble expression and secretion. A visible band could be obtained when the induction was conducted at 16 °C. The gram-positive enhancer matrix (GEM) particles could separate and purify the recombinant α-amylase with the AcmA tag, and no visible band could be seen in the culture even after the culture was concentrated ten times. The solution and concentration of the recombinant α-amylase could be adjusted by GEM particles. The recombinant untagged α-amylase was obtained after digestion. The α-amylase was characterized. The recombinant α-amylase was a thermophilic enzyme with a broad pH tolerance. In addition, the enzyme activity of the recombinant α-amylase was independent of Ca²⁺. The recombinant α-amylase contained the OmpA signal peptide and the AcmA tag and was expressed and purified quickly and easily.

Characterization and high-efficiency secreted expression in Bacillus subtilis of a thermo-alkaline β-mannanase from an alkaliphilic Bacillus clausii strain S10

Background

β-Mannanase catalyzes the cleavage of β-1,4-linked internal linkages of mannan backbone randomly to produce new chain ends. Alkaline and thermostable β-mannanases provide obvious advantages for many applications in biobleaching of pulp and paper, detergent industry, oil grilling operation and enzymatic production of mannooligosaccharides. However, only a few of them are commercially exploited as wild or recombinant enzymes, and none heterologous and secretory expression of alkaline β-mannanase in Bacillus subtilis expression system was reported. Alkaliphilic Bacillus clausii S10 showed high β-mannanase activity at alkaline condition. In this study, this β-mannanase was cloned, purified and characterized. The high-level secretory expression in B. subtilis was also studied.

Results

A thermo-alkaline β-mannanase (BcManA) gene encoding a 317-amino acid protein from alkaliphilic Bacillus clausii strain was cloned and expressed in Escherichia coli. The purified mature BcManA exhibited maximum activity at pH 9.5 and 75 °C with good stability at pH 7.0–11.5 and below 80 °C. BcManA demonstrated high cleavage capability on polysaccharides containing β-1,4-mannosidic linkages, such as konjac glucomannan, locust bean gum, guar gum and sesbania gum. The highest specific activity of 2366.2 U mg⁻¹ was observed on konjac glucomannan with the K_m and k_cat value of 0.62 g l⁻¹ and 1238.9 s⁻¹, respectively. The hydrolysis products were mainly oligosaccharides with a higher degree of polymerization than biose. BcManA also cleaved manno-oligosaccharides with polymerization degree more than 3 without transglycosylation. Furthermore, six signal peptides and two strong promoters were used for efficiently secreted expression optimization in B. subtilis WB600 and the highest extracellular activity of 2374 U ml⁻¹ with secretory rate of 98.5% was obtained using SP_lipA and P43 after 72 h cultivation in 2 × SR medium. By medium optimization using cheap nitrogen and carbon source of peanut meal and glucose, the extracellular activity reached 6041 U ml⁻¹ after 72 h cultivation with 6% inoculum size by shake flask fermentation.

Conclusions

The thermo-alkaline β-mannanase BcManA showed good thermal and pH stability and high catalytic efficiency towards konjac glucomannan and locust bean gum, which distinguished from other reported β-mannanases and was a promising thermo-alkaline β-mannanase for potential industrial application. The extracellular BcManA yield of 6041 U ml⁻¹, which was to date the highest reported yield by flask shake, was obtained in B. subtilis with constitutive expression vector. This is the first report for secretory expression of alkaline β-mannanase in B. subtilis protein expression system, which would significantly cut down the production cost of this enzyme. Also this research would be helpful for secretory expression of other β-mannanases in B. subtilis.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0973-0) contains supplementary material, which is available to authorized users.

Kinetic properties analysis of beta-mannanase from Klebsiella oxytoca KUB-CW2-3 expressed in Escherichia coli

Endo-1,4-β-mannanase is an enzyme that can catalyze the random hydrolysis of β-1,4-mannosidic linkages in the main chain of mannans, glucomannans and galactomannans and offers many applications in different biotechnology industries. Purification and kinetic properties of the endo-1,4-β-mannanase from recombinant Escherichia coli strain KMAN-3 were examined. Recombinant β-mannanase (KMAN-3) was purified 50.5 fold using Ni-NTA Agarose resin and specific activity of 11900 U/mg protein was obtained. Purified KMAN-3 showed a single band on SDS-PAGE with a molecular weight of 43 kDa. K_m and V_max values of KMAN-3 on ivory nut mannan, locust bean gum, defatted copra meal and konjac glucomannan were 243, 3.83 × 10⁵ 37 and 2.13 × 10⁶ mg ml^-1 and 2940, 61,100, 3930 and 1.56 × 10¹⁰ mg^-1, respectively. Carboxymethyl cellulose was not digested by KMAN-3.

High-Efficiency Secretion of β-Mannanase in Bacillus subtilis through Protein Synthesis and Secretion Optimization

The manno endo-1,4-mannosidase (β-mannanase, EC. 3.2.1.78) catalyzes the random hydrolysis of internal (1 → 4)-β-mannosidic linkages in the mannan polymers. A codon optimized β-mannanase gene from Bacillus licheniformis DSM13 was expressed in Bacillus subtilis. When four Sec-dependent and two Tat-dependent signal peptide sequences cloned from B. subtilis were placed upstream of the target gene, the highest activity of β-mannanase was observed using SP_lipA as a signal peptide. Then a 1.25-fold activity of β-mannanase was obtained when another copy of groESL operon was inserted into the genome of host strain. Finally, five different promoters were separately used to enhance the synthesis of the target protein. The results showed that promoter P_mglv, a modified maltose-inducible promoter, significantly elevated the production of β-mannanase. After 72 h of flask fermentation, the enzyme activity of β-mannanase in the supernatant when using locust bean gum as substrate reached 2207 U/mL. This work provided a promising β-mannanase production strain in industrial application.

Identification and Molecular Characterization of Genes Coding Pharmaceutically Important Enzymes from Halo-Thermo Tolerant Bacillus

Purpose: Robust pharmaceutical and industrial enzymes from extremophile microorganisms are main source of enzymes with tremendous stability under harsh conditions which make them potential tools for commercial and biotechnological applications. Methods: The genome of a Gram-positive halo-thermotolerant Bacillus sp. SL1, new isolate from Saline Lake, was investigated for the presence of genes coding for potentially pharmaceutical enzymes. We determined gene sequences for the enzymes laccase (CotA), l-asparaginase (ansA3, ansA1), glutamate-specific endopeptidase (blaSE), l-arabinose isomerase (araA2), endo-1,4-β mannosidase (gmuG), glutaminase (glsA), pectate lyase (pelA), cellulase (bglC1), aldehyde dehydrogenase (ycbD) and allantoinases (pucH) in the genome of Bacillus sp. SL1. Results: Based on the DNA sequence alignment results, six of the studied enzymes of Bacillus sp. SL-1 showed 100% similarity at the nucleotide level to the same genes of B. licheniformis 14580 demonstrating extensive organizational relationship between these two strains. Despite high similarities between the B. licheniformis and Bacillus sp. SL-1 genomes, there are minor differences in the sequences of some enzyme. Approximately 30% of the enzyme sequences revealed more than 99% identity with some variations in nucleotides leading to amino acid substitution in protein sequences. Conclusion: Molecular characterization of this new isolate provides useful information regarding evolutionary relationship between B. subtilis and B. licheniformis species. Since, the most industrial processes are often performed in harsh conditions, enzymes from such halo-thermotolerant bacteria may provide economically and industrially appealing biocatalysts to be used under specific physicochemical situations in medical, pharmaceutical, chemical and other industries.

Mannoside recognition and degradation by bacteria

Mannosides constitute a vast group of glycans widely distributed in nature. Produced by almost all organisms, these carbohydrates are involved in numerous cellular processes, such as cell structuration, protein maturation and signalling, mediation of protein-protein interactions and cell recognition. The ubiquitous presence of mannosides in the environment means they are a reliable source of carbon and energy for bacteria, which have developed complex strategies to harvest them. This review focuses on the various mannosides that can be found in nature and details their structure. It underlines their involvement in cellular interactions and finally describes the latest discoveries regarding the catalytic machinery and metabolic pathways that bacteria have developed to metabolize them.

Conservation in the mechanism of glucuronoxylan hydrolysis revealed by the structure of glucuronoxylan xylanohydrolase (CtXyn30A) from Clostridium thermocellum

Glucuronoxylan endo-β-1,4-xylanases cleave the xylan chain specifically at sites containing 4-O-methylglucuronic acid substitutions. These enzymes have recently received considerable attention owing to their importance in the cooperative hydrolysis of heteropolysaccharides. However, little is known about the hydrolysis of glucuronoxylans in extreme environments. Here, the structure of a thermostable family 30 glucuronoxylan endo-β-1,4-xylanase (CtXyn30A) from Clostridium thermocellum is reported. CtXyn30A is part of the cellulosome, a highly elaborate multi-enzyme complex secreted by the bacterium to efficiently deconstruct plant cell-wall carbohydrates. CtXyn30A preferably hydrolyses glucuronoxylans and displays maximum activity at pH 6.0 and 70°C. The structure of CtXyn30A displays a (β/α)₈ TIM-barrel core with a side-associated β-sheet domain. Structural analysis of the CtXyn30A mutant E225A, solved in the presence of xylotetraose, revealed xylotetraose-cleavage oligosaccharides partially occupying subsites -3 to +2. The sugar ring at the +1 subsite is held in place by hydrophobic stacking interactions between Tyr139 and Tyr200 and hydrogen bonds to the OH group of Tyr227. Although family 30 glycoside hydrolases are retaining enzymes, the xylopyranosyl ring at the -1 subsite of CtXyn30A-E225A appears in the α-anomeric configuration. A set of residues were found to be strictly conserved in glucuronoxylan endo-β-1,4-xylanases and constitute the molecular determinants of the restricted specificity displayed by these enzymes. CtXyn30A is the first thermostable glucuronoxylan endo-β-1,4-xylanase described to date. This work reveals that substrate recognition by both thermophilic and mesophilic glucuronoxylan endo-β-1,4-xylanases is modulated by a conserved set of residues.

Kinetic study of a β-mannanase from the Bacillus licheniformis HDYM-04 and its decolorization ability of twenty-two structurally different dyes

BACKGROUND

The microbial β-mannanases have been increasingly exploited for bioconversion of biomass materials and various potential industrial applications, such as bleaching of softwood pulps, scouring and desizing, food and feed additive, and oil and textile industries. In this paper, a β-mannanase was characterization from the bacteria, Bacillus licheniformis HDYM-04, which was a high β-mannanase-producing strain (576.16 ± 2.12 U/mL at 48 h during fermentation).

METHODS

The michaelis constant (K_m ) and maximum velocity (V_max ) of β-mannanase were determined. The effect of organic solvents, inhibitors, detergents, chelating agents, oxidizing agents and reducing agents on the stability of enzyme were determined. The degradation of twenty-two structurally different dyes by the purified β-mannanase produced by HDYM-04 was determined by full spectrum scan among 200-1000 nm at 0 min and 10 min, respectively.

RESULTS

β-Mannanase produced by HDYM-04 was highly specific towards glucomannan, where as exhibited low activity towards guar gum. Michaelis constant (K_m ) and maximum velocity (V_max ) of glucomannan substrate were 2.69 mg/ml and 251.41 U/mg, respectively. The activity of different organic solvents showed significantly difference (p < 0.05). It retained > 80 % activity in dimethyl sulfoxide, acetone, chloroform, benzene, hexane. In the presence of solvents, citric acid, ethylene diamine teraacetic acid and potassium iodide, it retained > 80 % residual activity. Twenty-two structurally different dyes could be effectively decolourised by β-mannanase within 12 h, in which methyl orange (99.89 ± 2.87 %), aniline blue (90.23 ± 2.87 %) and alizalin (83.63 ± 2.89 %) had high decolorization rate.

CONLUSION

The obtained results displayed that the β-mannanase produced by HDYM-04 showed high stability under different chemical reagents and was found to be capable of decolorizing synthetic dyes with different structures. So, the reported biochemical properties of the purified β-mannanase and its rapid decolorizations of dyes suggested that it might be suitable for industrial wastewater bioremediation.

Display of a β-mannanase and a chitosanase on the cell surface of Lactobacillus plantarum towards the development of whole-cell biocatalysts

BACKGROUND

Lactobacillus plantarum is considered as a potential cell factory because of its GRAS (generally recognized as safe) status and long history of use in food applications. Its possible applications include in situ delivery of proteins to a host, based on its ability to persist at mucosal surfaces of the human intestine, and the production of food-related enzymes. By displaying different enzymes on the surface of L. plantarum cells these could be used as whole-cell biocatalysts for the production of oligosaccharides. In this present study, we aimed to express and display a mannanase and a chitosanase on the cell surface of L. plantarum.

RESULTS

ManB, a mannanase from Bacillus licheniformis DSM13, and CsnA, a chitosanase from Bacillus subtilis ATCC 23857 were fused to different anchoring motifs of L. plantarum for covalent attachment to the cell surface, either via an N-terminal lipoprotein anchor (Lp_1261) or a C-terminal cell wall anchor (Lp_2578), and the resulting fusion proteins were expressed in L. plantarum WCFS1. The localization of the recombinant proteins on the bacterial cell surface was confirmed by flow cytometry and immunofluorescence microscopy. The highest mannanase and chitosanase activities obtained for displaying L. plantarum cells were 890 U and 1360 U g dry cell weight, respectively. In reactions with chitosan and galactomannans, L. plantarum CsnA- and ManB-displaying cells produced chito- and manno-oligosaccharides, respectively, as analyzed by high performance anion exchange chromatography (HPAEC) and mass spectrometry (MS). Surface-displayed ManB is able to break down galactomannan (LBG) into smaller manno-oligosaccharides, which can support growth of L. plantarum.

CONCLUSION

This study shows that mannanolytic and chitinolytic enzymes can be anchored to the cell surface of L. plantarum in active forms. L. plantarum chitosanase- and mannanase-displaying cells should be of interest for the production of potentially 'prebiotic' oligosaccharides. This approach, with the enzyme of interest being displayed on the cell surface of a food-grade organism, may also be applied in production processes relevant for food industry.

Cloning, Expression and Biochemical Characterization of Endomannanases from Thermobifida Species Isolated from Different Niches

Thermobifidas are thermotolerant, compost inhabiting actinomycetes which have complex polysaccharide hydrolyzing enzyme systems. The best characterized enzymes of these hydrolases are cellulases from T. fusca, while other important enzymes especially hemicellulases are not deeply explored. To fill this gap we cloned and investigated endomannanases from those reference strains of the Thermobifida genus, which have published data on other hydrolases (T. fusca TM51, T. alba CECT3323, T. cellulosilytica TB100^T and T. halotolerans YIM90462^T). Our phylogenetic analyses of 16S rDNA and endomannanase sequences revealed that T. alba CECT3323 is miss-classified; it belongs to the T. fusca species. The cloned and investigated endomannanases belong to the family of glycosyl hydrolases 5 (GH5), their size is around 50 kDa and they are modular enzymes. Their catalytic domains are extended by a C-terminal carbohydrate binding module (CBM) of type 2 with a 23–25 residues long interdomain linker region consisting of Pro, Thr and Glu/Asp rich repetitive tetrapeptide motifs. Their polypeptide chains exhibit high homology, interdomain sequence, which don’t show homology to each other, but all of them are built up from 3–6 times repeated tetrapeptide motifs) (PTDP-Tc, TEEP-Tf, DPGT-Th). All of the heterologously expressed Man5A enzymes exhibited activity only on mannan. The pH optima of Man5A enzymes from T. halotolerans, T. cellulosilytica and T. fusca are slightly different (7.0, 7.5 and 8.0, respectively) while their temperature optima span within the range of 70–75°C. The three endomannanases exhibited very similar kinetic performances on LBG-mannan substrate: 0.9–1.7mM of K_M and 80–120 1/sec of turnover number. We detected great variability in heat stability at 70°C, which was influenced by the presence of Ca²⁺. The investigated endomannanases might be important subjects for studying the structure/function relation behind the heat stability and for industrial applications to hemicellulose degradation.

A thermostable GH26 endo-β-mannanase from Myceliophthora thermophila capable of enhancing lignocellulose degradation

The endomannanase gene em26a from the thermophilic fungus Myceliophthora thermophila, belonging to the glycoside hydrolase family 26, was functionally expressed in the methylotrophic yeast Pichia pastoris. The putative endomannanase, dubbed MtMan26A, was purified to homogeneity (60 kDa) and subsequently characterized. The optimum pH and temperature for the enzymatic activity of MtMan26A were 6.0 and 60 °C, respectively. MtMan26A showed high specific activity against konjac glucomannan and carob galactomannan, while it also exhibited high thermal stability with a half-life of 14.4 h at 60 °C. Thermostability is of great importance, especially in industrial processes where harsh conditions are employed. With the aim of better understanding its structure-function relationships, a homology model of MtMan26A was constructed, based on the crystallographic structure of a close homologue. Finally, the addition of MtMan26A as a supplement to the commercial enzyme mixture Celluclast® 1.5 L and Novozyme® 188 resulted in enhanced enzymatic hydrolysis of pretreated beechwood sawdust, improving the release of total reducing sugars and glucose by 13 and 12 %, respectively.

Secretory production of a beta-mannanase and a chitosanase using a Lactobacillus plantarum expression system

BACKGROUND

Heterologous production of hydrolytic enzymes is important for green and white biotechnology since these enzymes serve as efficient biocatalysts for the conversion of a wide variety of raw materials into value-added products. Lactic acid bacteria are interesting cell factories for the expression of hydrolytic enzymes as many of them are generally recognized as safe and require only a simple cultivation process. We are studying a potentially food-grade expression system for secretion of hydrolytic enzymes into the culture medium, since this enables easy harvesting and purification, while allowing direct use of the enzymes in food applications.

RESULTS

We studied overexpression of a chitosanase (CsnA) and a β-mannanase (ManB), from Bacillus licheniformis and Bacillus subtilis, respectively, in Lactobacillus plantarum, using the pSIP system for inducible expression. The enzymes were over-expressed in three forms: without a signal peptide, with their natural signal peptide and with the well-known OmpA signal peptide from Escherichia coli. The total production levels and secretion efficiencies of CsnA and ManB were highest when using the native signal peptides, and both were reduced considerably when using the OmpA signal. At 20 h after induction with 12.5 ng/mL of inducing peptide in MRS media containing 20 g/L glucose, the yields and secretion efficiencies of the proteins with their native signal peptides were 50 kU/L and 84% for ManB, and 79 kU/L and 56% for CsnA, respectively. In addition, to avoid using antibiotics, the erythromycin resistance gene was replaced on the expression plasmid with the alanine racemase (alr) gene, which led to comparable levels of protein production and secretion efficiency in a suitable, alr-deficient L. plantarum host.

CONCLUSIONS

ManB and CsnA were efficiently produced and secreted in L. plantarum using pSIP-based expression vectors containing either an erythromycin resistance or the alr gene as selection marker.

N-terminal truncation contributed to increasing thermal stability of mannanase Man1312 without activity loss

BACKGROUND

The disordered residues on distal loops affect the molecular structural stability and on some occasions have regulatory roles in catalytic reaction. To increase understanding of the influence of distal residue mutation, this study explored the thermostability and enzymatic activity of mannanase Man1312 deletion mutants. The focus was on residues located on the N-terminal region because they are more disordered and changeable. The effects of N-terminal truncation on enzymatic activity and thermal dynamics were investigated by spectrophotometry, circular dichroism and differential scanning calorimetry assays.

RESULTS

The deletion mutants on V3, N7 and Q11 showed a marked increase in stability, while the enzymatic activity was significantly improved when triplet deletion was carried out. Triplet deletion MandVNQ showed around double the stability of its corresponding single-site and double-site deletion mutants. The Tm value of MandVNP was about 8 °C higher than that of Man1312. MandVNP had improved characteristics of Topt by 10 °C, t1/2 by 10 min and catalytic activity by 11% in comparison with Man1312. Analysis of spectra and modeling showed that MandVNQ had increased helix and strand contents.

CONCLUSION

N-terminal truncation had positive effects on the thermostability and activity of mannanase.

Molecular cloning of kman coding for mannanase from Klebsiella oxytoca KUB-CW2-3 and its hybrid mannanase characters

Gene encoding for β-mannanase (E.C 3.2.1.78) from Klebsiella oxytoca KUB-CW2-3 was cloned and expressed by an E. coli system resulting in 400 times higher mannanase activities than the wild type. A 3314bp DNA fragment obtained revealed an open reading frame of 1164bp, namely kman-2, which encoded for 387 amino acids with an estimated molecular weight of 43.2kDa. It belonged to the glycosyl hydrolase family 26 (GH26) exhibited low similarity of 50-71% to β-mannanase produced by other microbial sources. Interestingly, the enzyme had a broad range of substrate specificity of homopolymer of ivory nut mannan (6%), carboxymethyl cellulose (30.6%) and avicel (5%), and heteropolymer of konjac glucomannan (100%), locust bean gum (92.6%) and copra meal (non-defatted 5.3% and defatted 7%) which would be necessary for in vivo feed digestion. The optimum temperature and pH were 30-50°C and 4-6, respectively. The enzyme was still highly active over a low temperature range of 10-40°C and over a wide pH range of 4-10. The hydrolysates of konjac glucomannan (H-KGM), locust bean gum (H-LBG) and defatted copra meal (H-DCM) composed of compounds which were different in their molecular weight range from mannobiose to mannohexaose and unknown oligosaccharides indicating the endo action of mannanase. Both H-DCM and H-LBG enhanced the growth of lactic acid bacteria and some pathogens except Escherichia coli E010 with a specific growth rate of 0.36-0.83h(-1). H-LBG was more specific to 3 species of Weissella confusa JCM 1093, Lactobacillus reuteri KUB-AC5, Lb salivarius KL-D4 and E. coli E010 while both H-KGM and H-DCM were to Lb. reuteri KUB-AC5 and Lb. johnsonii KUNN19-2. Based on the nucleotide sequence of kman-2 containing two open reading frames of 1 and 2at 5' end of the +1 and +43, respectively, removal of the first open reading frame provided the recombinant clone E. coli KMAN-3 resulting in the mature protein of mannanase composing of 345 amino acid residues confirmed by 3D structure analysis and amino acid sequence at N-terminal namely KMAN (GenBank accession number KM100456). It exhibited 10 times higher extracellular and periplasmic total activities of 17,600 and 14,800 units than E. coli KMAN-2. With its low similarity to mannanases previously proposed, wide range of homo- and hetero-polysaccharide specificity, negative effect to E. coli and most importance of high production, it would be proposed as a novel mannanase source for application in the future.

Isolation and characterization of novel multifunctional recombinant family 26 glycoside hydrolase from Mehsani buffalo rumen metagenome

Rumen microbiota harbor a diverse set of carbohydrate-active enzymes (CAZymes), which play a crucial role in the degradation of a complex plant polysaccharide thereby providing metabolic energy to the host animals. Earlier, we reported CAZYme analysis from the buffalo rumen metagenome by high throughput shotgun sequencing. Among the various CAZymes, glycoside hydrolase family 26 (GH26) enzymes have a number of industrial applications including in paper, oil, biofuel, food, feed, pharmaceutical, coffee, and detergent industries. Here, we report isolation and characterization of GH26 enzyme from the buffalo rumen metagenome. A novel GH26 gene composed of 1,119 base pairs was successfully amplified using the gene-specific primers inferred based on the contig generated from metagenome sequence assembly and cloned in a pET32a (+) expression vector as an N-terminal histidine tag fusion protein. A novel GH26 protein from an unknown rumen microorganism shared a maximum of 68% identity with the Prevotella ruminicola 23 encoded carbohydrate esterase family 7 and 46% with Bacteroides sp. 2_1_33B encoded mannan endo-1, 4-β-mannosidase. The recombinant GH26-histidine tag fusion protein was expressed in Escherichia coli and purified using Ni-NTA affinity chromatography. The purified enzyme displayed multifunctional activities against various carbohydrate substrates including locust bean gum, beechwood xylan, pectin, and carboxymethyl cellulose suggesting mannanase, xylanase, pectin esterase, and endoglucanase activities, respectively.

Bio-chemical characterization of a β-mannanase from Bacillus licheniformis HDYM-04 isolated from flax water-retting liquid and its decolorization ability of dyes

A β-mannanase was purified from the bacteria,Bacillus licheniformisHDYM-04, which was a high β-mannanase-producing strain (576.16 U mL⁻¹at 48 h during fermentation).

Genomics review of holocellulose deconstruction by aspergilli

SUMMARY

Biomass is constructed of dense recalcitrant polymeric materials: proteins, lignin, and holocellulose, a fraction constituting fibrous cellulose wrapped in hemicellulose-pectin. Bacteria and fungi are abundant in soil and forest floors, actively recycling biomass mainly by extracting sugars from holocellulose degradation. Here we review the genome-wide contents of seven Aspergillus species and unravel hundreds of gene models encoding holocellulose-degrading enzymes. Numerous apparent gene duplications followed functional evolution, grouping similar genes into smaller coherent functional families according to specialized structural features, domain organization, biochemical activity, and genus genome distribution. Aspergilli contain about 37 cellulase gene models, clustered in two mechanistic categories: 27 hydrolyze and 10 oxidize glycosidic bonds. Within the oxidative enzymes, we found two cellobiose dehydrogenases that produce oxygen radicals utilized by eight lytic polysaccharide monooxygenases that oxidize glycosidic linkages, breaking crystalline cellulose chains and making them accessible to hydrolytic enzymes. Among the hydrolases, six cellobiohydrolases with a tunnel-like structural fold embrace single crystalline cellulose chains and cooperate at nonreducing or reducing end termini, splitting off cellobiose. Five endoglucanases group into four structural families and interact randomly and internally with cellulose through an open cleft catalytic domain, and finally, seven extracellular β-glucosidases cleave cellobiose and related oligomers into glucose. Aspergilli contain, on average, 30 hemicellulase and 7 accessory gene models, distributed among 9 distinct functional categories: the backbone-attacking enzymes xylanase, mannosidase, arabinase, and xyloglucanase, the short-side-chain-removing enzymes xylan α-1,2-glucuronidase, arabinofuranosidase, and xylosidase, and the accessory enzymes acetyl xylan and feruloyl esterases.

Low-temperature-active and salt-tolerant β-mannanase from a newly isolated Enterobacter sp. strain N18

A low-temperature-active and salt-tolerant β-mannanase produced by a novel mannanase-producer, Enterobacter sp. strain N18, was isolated, purified and then evaluated for its potential application as a gel-breaker in relation to viscosity reduction of guar-based hydraulic fracturing fluids used in oil field. The enzyme could lower the viscosity of guar gum solution by more than 95% within 10 min. The purified β-mannanase with molecular mass of 90 kDa displayed high activity in a broad range of pH and temperature: more than 70% of activity was retained in the pH range of 3.0-8.0 with the optimal pH 7.5, about 50% activity at 20°C with the optimal temperature 50°C. Furthermore, the enzyme retained >70% activity in the presence of 0.5-4.0 M NaCl. These properties implied that the enzyme from strain N18 had potential for serving as a gel-breaker for low temperature oil wells and other industrial fields, where chemical gel breakers were inactive due to low temperature.

Expression of the Thermobifida fusca xylanase Xyn11A in Pichia pastoris and its characterization

BACKGROUND

Xylan is a major component of plant cells and the most abundant hemicellulose. Xylanases degrade xylan into monomers by randomly cleaving β-1,4-glycosidic bonds in the xylan backbone, and have widespread potential applications in various industries. The purpose of our study was to clone and express the endoxylanase gene xynA of Thermobifida fusca YX in its native form and with a C-terminal histidine (His) tag in Pichia pastoris X-33. We analyzed and compared these two forms of the protein and examined their potential applications in various industries.

RESULTS

The xynA gene from T. fusca YX was successfully cloned and expressed using P. pastoris X-33. We produced a recombinant native form of the protein (rXyn11A) and a C-terminal His-tagged form of the desired protein (rXyn11A-(His)6). The specific activities of rXyn11A and rXyn11A-(His)6 in culture supernatants approached 149.4 and 133.4 U/mg, respectively. These activities were approximately 4- and 3.5-fold higher than those for the non-recombinant wild-type Xyn11A (29.3 U/mg). Following purification, the specific activities of rXyn11A and rXyn11A-(His)6 were 557.35 and 515.84 U/mg, respectively. The specific activity of rXyn11A was 8% higher than that of rXyn11A-(His)6. Both recombinant xylanases were optimally active at 80°C and pH 8.0, and exhibited greater than 60% activity between pH 6-9 and 60-80°C. They exhibited similar pH stability, while rXyn11A exhibited better thermostability; N-glycosylation enhanced the thermostability of both recombinant xylanases. The products of beechwood xylan hydrolyzed by both xylanases included xylobiose, xylotriose, xylotetraose and xylopentaose.

CONCLUSIONS

The C-terminal His tag had adverse effects when added to the Xyn11A protein. The thermostability of both recombinant xylanases was enhanced by N-glycosylation. Their stabilities at a high pH and temperature indicate their potential for application in various industries.

A new acidophilic thermostable endo-1,4-β-mannanase from Penicillium oxalicum GZ-2: cloning, characterization and functional expression in Pichia pastoris

Background

Endo-1,4-β-mannanase is an enzyme that can catalyze the random hydrolysis of β-1, 4-mannosidic linkages in the main chain of mannans, glucomannans and galactomannans and has a number of applications in different biotechnology industries. Penicillium oxalicum is a powerful hemicellulase-producing fungus (Bioresour Technol 123:117-124, 2012); however, few previous studies have focused on the cloning and expression of the endo-1,4-β-mannanase gene from Penicillium oxalicum.

Results

A gene encoding an acidophilic thermostable endo-1,4-β-mannanase (E.C. 3.2.1.78) from Penicillium oxalicum GZ-2, which belongs to glycoside hydrolase family 5, was cloned and successfully expressed in Pichia pastoris GS115. A high enzyme activity (84.4 U mL⁻¹) was detected in the culture supernatant. The recombinant endo-1,4-β-mannanase (rPoMan5A) was tagged with 6 × His at its C-terminus and purified using a Ni-NTA Sepharose column to apparent homogeneity. The purified rPoMan5A showed a single band on SDS-PAGE with a molecular mass of approximately 61.6 kDa. The specific activity of the purified rPoMan5A was 420.9 U mg⁻¹ using locust bean gum as substrate. The optimal catalytic temperature (10 min assay) and pH value for rPoMan5A are 80°C and pH 4.0, respectively. The rPoMan5A is highly thermostable with a half-life of approximately 58 h at 60°C at pH 4.0. The K_m and V_max values for locust bean gum, konjac mannan, and guar gum are 7.6 mg mL⁻¹ and 1425.5 μmol min⁻¹ mg⁻¹, 2.1 mg mL⁻¹ and 154.8 μmol min⁻¹ mg⁻¹, and 2.3 mg mL⁻¹ and 18.9 μmol min⁻¹ mg⁻¹, respectively. The enzymatic activity of rPoMan5A was not significantly affected by an array of metal ions, but was inhibited by Fe³⁺ and Hg²⁺. Analytical results of hydrolytic products showed that rPoMan5A could hydrolyze various types of mannan polymers and released various mannose and manno-oligosaccharides, with the main products being mannobiose, mannotriose, and mannopentaose.

Conclusion

Our study demonstrated that the high-efficient expression and secretion of acid stable and thermostable recombinant endo-1, 4-β-mannanase in Pichia pastoris is suitable for various biotechnology applications.

Cloning, secretory expression and characterization of recombinant β-mannanase from Bacillus circulans NT 6.7

The mannanase gene of B. circulans NT 6.7 was cloned and expressed in an Escherichia coli expression system. The B. circulans NT 6.7 mannanase gene consists of 1,083 nucleotides encoding a 360-amino acid residue long polypeptide, belonging to glycoside hydrolase family 26. The full-length mannanase gene including its native signal sequence was cloned into the vector pET21d and expressed in E. coli BL21 (DE3). β-Mannanase activities in the culture supernatant and crude cell extract were 37.10 and 515 U per ml, respectively, with most of the activity in the cell extract attributed to the periplasmic fraction. In contrast, expression of mannanase was much lower when using the B. circulans NT 6.7 mannanase gene without its signal sequence. The optimum temperature of recombinant β-mannanase activity was 50°C and the optimum pH was 6.0. The enzyme was very specific for β-mannan substrates with a preference for galactomannan. Hydrolysis products of locust bean gum were various mannooligosaccharides including mannohexaose, mannopentaose, mannotetraose, mannotriose and mannobiose, while mannose could not be detected. In conclusion, this expression system is efficient for the secretory production of recombinant β-mannanase from B. circulans NT 6.7, which shows good characteristics for various applications.