High-level production of Rhodiola rosea characteristic component rosavin from D-glucose and L-arabinose in engineered Escherichia coli.

Rosavin is the characteristic component of Rhodiola rosea L., an important medicinal plant used widely in the world that has been reported to possess multiple biological activities. However, the endangered status of wild Rhodiola has limited the supply of rosavin. In this work, we successfully engineered an Escherichia coli strain to efficiently produce rosavin as an alternative production method. Firstly, cinnamate: CoA ligase from Hypericum calycinum, cinnamoyl-CoA reductase from Lolium perenne, and uridine diphosphate (UDP)-glycosyltransferase (UGT) from Bacillus subtilis (Bs-YjiC) were selected to improve the titer of rosin in E. coli. Subsequently, four UGTs from the UGT91R subfamily were identified to catalyze the formation of rosavin from rosin, with SlUGT91R1 from Solanum lycopersicum showing the highest activity level. Secondly, production of rosavin was achieved for the first time in E. coli by incorporating the SlUGT91R1 and UDP-arabinose pathway, including UDP-glucose dehydrogenase, UDP-xylose synthase, and UDP-xylose 4-epimerase, into the rosin-producing stain, and the titer reached 430.5 ± 91.4 mg/L. Thirdly, a two-step pathway derived from L-arabinose, composed of L-arabinokinase and UDP-sugar pyrophosphorylase, was developed in E. coli to further optimize the supply of the precursor UDP-arabinose. Furthermore, 1203.7 ± 32.1 mg/L of rosavin was produced from D-glucose and L-arabinose using shake-flask fermentation. Finally, the production of rosavin reached 7539.1 ± 228.7 mg/L by fed-batch fermentation in a 5-L bioreactor. Thus, the microbe-based production of rosavin shows great potential for commercialization. This work provides an effective strategy for the biosynthesis of other valuable natural products with arabinose-containing units from D-glucose and L-arabinose.

Bioconversion of citrus waste into mucic acid by xylose-fermenting Saccharomyces cerevisiae.

Mucic acid holds promise as a platform chemical for bio-based nylon synthesis; however, its biological production encounters challenges including low yield and productivity. In this study, an efficient and high-yield method for mucic acid production was developed by employing genetically engineered Saccharomyces cerevisiae expressing the NAD+-dependent uronate dehydrogenase (udh) gene. To overcome the NAD+ dependency for the conversion of pectin to mucic acid, xylose was utilized as a co-substrate. Through optimization of the udh expression system, the engineered strain achieved a notable output, producing 20 g/L mucic acid with a highest reported productivity of 0.83 g/L-h and a theoretical yield of 0.18 g/g when processing pectin-containing citrus peel waste. These results suggest promising industrial applications for the biological production of mucic acid. Additionally, there is potential to establish a viable bioprocess by harnessing pectin-rich fruit waste alongside xylose-rich cellulosic biomass as raw materials.

Profiling of historical rag papers by their non-cellulosic polysaccharide composition.

Hemicellulose and pectin are noteworthy components of historical European rag papers, and have not been studied in detail so far. Rag papers were made from used textiles, and fiber-based utilities, such as ropes and bags. These had been prepared until the mid-19th century from plant-based fibers. Their polysaccharide composition could relate to their condition and history. This information can be expected to hold importance for the preservation and conservation of historical objects. We investigated a collection of rag papers of different age for their composition of non-cellulosic polysaccharides, and compared the findings with modern rag papers and wood pulps. Furthermore, a non-destructive determination of the hemicellulose and pectin content by near-infrared spectroscopy was developed. Historical rag papers had a lower hemicellulose/pectin content than pulps; the fractions of rhamnose, galactose, and arabinose were higher, while xylose was lower. In modern rag papers, xylose tended to be at the higher end of the range, which suggests a degradation of hemicelluloses/pectin over time or a change in raw materials and manufacturing. Rag papers also showed higher crystallinity than wood pulp papers. These findings provide insights into rag paper characteristics and offer potential classification methods.

Xylobiose treatment triggers a defense-related response and alters cell wall composition.

Plant cell wall-derived oligosaccharides, i.e., damage-associated molecular patterns (DAMPs), could be generated after pathogen attack or during normal plant development, perceived by cell wall receptors, and can alter immunity and cell wall composition. Therefore, we hypothesised that xylo-oligosaccharides (XOS) could act as an elicitor and trigger immune responses. To test this, we treated Arabidopsis with xylobiose (XB) and investigated different parameters. XB-treatment significantly triggered the generation of reactive oxygen species (ROS), activated MAPK protein phosphorylation, and induced callose deposition. The combination of XB (DAMP) and flg22 a microbe-associated molecular pattern (MAMP) further enhanced ROS response and gene expression of PTI marker genes. RNA sequencing analysis revealed that more genes were differentially regulated after 30 min compared to 24 h XB-treated leaves, which correlated with ROS response. Increased xylosidase activity and soluble xylose level after 30 min and 3 h of XB-treatment were observed which might have weakened the DAMP response. However, an increase in total cell wall sugar and a decrease in uronic acid level was observed at both 30 min and 24 h. Additionally, arabinose, rhamnose, and xylose levels were increased in 30 min, and glucose was increased in 24 h compared to mock-treated leaves. The level of jasmonic acid, abscisic acid, auxin, and cytokinin were also affected after XB treatment. Overall, our data revealed that the shortest XOS can act as a DAMP, which triggers the PTI response and alters cell wall composition and hormone level.

[Expression of ß-xylosidase An-xyl from Aspergillus niger and characterization of its xylose tolerance].

The hydrolysis of xylo-oligosaccharides catalyzed by ß-xylosidase plays an important role in the degradation of lignocellulose. However, the enzyme is easily inhibited by its catalytic product xylose, which severely limits its application. Based on molecular docking, this paper studied the xylose affinity of Aspergillus niger ß-xylosidase An-xyl, which was significantly differentially expressed in the fermentation medium of tea stalks, through cloning, expression and characterization. The synergistic degradation effect of this enzyme and cellulase on lignocellulose in tea stems was investigated. Molecular docking showed that the affinity of An-xyl to xylose was lower than that of Aspergillus oryzae ß-xylosidase with poor xylose tolerance. The Ki value of xylose inhibition constant of recombinant-expressed An-xyl was 433.2 mmol/L, higher than that of most ß-xylosidases of the GH3 family. The Km and Vmax towards pNPX were 3.6 mmol/L and 10 000 µmol/(min·mL), respectively. The optimum temperature of An-xyl was 65 ℃, the optimum pH was 4.0, 61% of the An-xyl activity could be retained upon treatment at 65 ℃ for 300 min, and 80% of the An-xyl activity could be retained upon treatment at pH 2.0-8.0 for 24 h. The hydrolysis of tea stem by An-xyl and cellulase produced 19.3% and 38.6% higher reducing sugar content at 2 h and 4 h, respectively, than that of using cellulase alone. This study showed that the An-xyl mined from differential expression exhibited high xylose tolerance and higher catalytic activity and stability, and could hydrolyze tea stem lignocellulose synergistically, which enriched the resource of ß-xylosidase with high xylose tolerance, thus may facilitate the advanced experimental research and its application.

Multifactorial interaction of selenium, iron, xylose, and glycine on cordycepin metabolism in Cordyceps militaris.

Cordycepin, a nucleoside analog, is the main antioxidative and antimicrobial substance in Cordyceps militaris. To improve the metabolism of cordycepin, carbon sources, nitrogen sources, trace elements, and precursors were studied by single factor, Plackett-Burman, and central composite designs in C. militaris mycelial fermentation. Under the regulation of the multifactorial interactions of selenite, ferrous chloride, xylose, and glycine, cordycepin production was increased by 5.2-fold compared with the control. The gene expression of hexokinase, ATP phosphoribosyltransferase, adenylosuccinate synthetase, and cns1-3 in the glycolysis, pentose phosphate, and adenosine synthesis pathways were increased by 3.2-7.5 times due to multifactorial interactions, while the gene expression of histidine biosynthesis trifunctional protein and histidinol-phosphate aminotransferase in histidine synthesis pathway were decreased by 23.4%-56.2%. Increasing with cordycepin production, glucose uptake was accelerated, mycelia growth was inhibited, and the cell wall was damaged. Selenomethionine (SeMet), selenocysteine (SeCys), and selenium nanoparticles (SeNPs) were the major Se species in C. militaris mycelia. This study provides a new insight for promoting cordycepin production by regulating glycolysis, pentose phosphate, and histidine metabolism. KEY POINTS: • Cordycepin production in the CCDmax group was 5.2-fold than that of the control. • Glucose uptake of the CCDmax group was accelerated and cell wall was damaged. • The metabolic flux was concentrated to the cordycepin synthesis pathway.

Glutamine Synthetase Contributes to the Regulation of Growth, Conidiation, Sclerotia Development, and Resistance to Oxidative Stress in the Fungus Aspergillus flavus.

The basic biological function of glutamine synthetase (Gs) is to catalyze the conversion of ammonium and glutamate to glutamine. This synthetase also performs other biological functions. However, the roles of Gs in fungi, especially in filamentous fungi, are not fully understood. Here, we found that conditional disruption of glutamine synthetase (AflGsA) gene expression in Aspergillus flavus by using a xylose promoter leads to a complete glutamine deficiency. Supplementation of glutamine could restore the nutritional deficiency caused by AflGsA expression deficiency. Additionally, by using the xylose promoter for the downregulation of AflgsA expression, we found that AflGsA regulates spore and sclerotic development by regulating the transcriptional levels of sporulation genes abaA and brlA and the sclerotic generation genes nsdC and nsdD, respectively. In addition, AflGsA was found to maintain the balance of reactive oxygen species (ROS) and to aid in resisting oxidative stress. AflGsA is also involved in the regulation of light signals through the production of glutamine. The results also showed that the recombinant AflGsA had glutamine synthetase activity in vitro and required the assistance of metal ions. The inhibitor molecule L-α-aminoadipic acid suppressed the activity of rAflGsA in vitro and disrupted the morphogenesis of spores, sclerotia, and colonies in A. flavus. These results provide a mechanistic link between nutrition metabolism and glutamine synthetase in A. flavus and suggest a strategy for the prevention of fungal infection.

Agrobacterium sp. ZX09 ß-Glucan Attenuates Enterotoxigenic Escherichia coli-Induced Disruption of Intestinal Epithelium in Weaned Pigs.

To explore the protective effect of dietary ß-glucan (BGL) supplementation on intestinal epithelium exposure to enterotoxigenic Escherichia coli (ETEC), thirty-two weaned pigs were assigned to four groups. Pigs were fed with a basal diet or basal diet containing 500 mg/kg BGL, and were orally infused with ETEC or culture medium. Results showed BGL supplementation had no influence on growth performance in weaned pigs. However, BGL supplementation increased the absorption of D-xylose, and significantly decreased the serum concentrations of D-lactate and diamine oxidase (DAO) in the ETEC-challenged pigs (p < 0.05). Interestingly, BGL significantly increased the abundance of the zonula occludens-1-(ZO-1) in the jejunal epithelium upon ETEC challenge (p < 0.05). BGL supplementation also increased the number of S-phase cells and the number of sIgA-positive cells, but significantly decreased the number of total apoptotic cells in the jejunal epithelium upon ETEC challenge (p < 0.05). Moreover, BGL significantly increased the duodenal catalase (CAT) activity and the ileal total superoxide dismutase (T-SOD) activity in the ETEC-challenged pigs (p < 0.05). Importantly, BGL significantly decreased the expression levels of critical inflammation related proteins such as the tumor necrosis factor-α (TNF-α), interlukin-6 (IL-6), myeloid differentiation factor 88 (MyD88), and nuclear factor-κB (NF-κB) in the jejunal and ileal mucosa upon ETEC challenge (p < 0.05). BGL also elevated the propanoic acid content and the abundance of Lactobacillus and Bacillus in the colon upon ETEC challenge (p < 0.05). These results suggested BGL could alleviate the ETEC-induced intestinal epithelium injury, which may be associated with suppressed inflammation and improved intestinal immunity and antioxidant capacity, as well as the improved intestinal macrobiotic.

Screening of xylose utilizing and high lipid producing yeast strains as a potential candidate for industrial application.

BACKGROUND: Sustainable production of oil for food, feed, fuels and other lipid-based chemicals is essential to meet the demand of the increasing human population. Consequently, novel and sustainable resources such as lignocellulosic hydrolysates and processes involving these must be explored. In this paper we screened for naturally-occurring xylose utilizing oleaginous yeasts as cell factories for lipid production, since pentose sugar catabolism plays a major role in efficient utilization of lignocellulosic feedstocks. Glycerol utilization, which is also beneficial in yeast-based oil production as glycerol is a common by-product of biodiesel production, was investigated as well. Natural yeast isolates were studied for lipid accumulation on a variety of substrates, and the highest lipid accumulating strains were further investigated in shake flask cultivations and fermenter studies on xylose and hydrolysate. RESULTS: By collecting leaves from exotic plants in greenhouses and selective cultivation on xylose, a high frequency of oleaginous yeasts was obtained (> 40%). Different cultivation conditions lead to differences in fatty acid contents and compositions, resulting in a set of strains that can be used to select candidate production strains for different purposes. In this study, the most prominent strains were identified as Pseudozyma hubeiensis BOT-O and Rhodosporidium toruloides BOT-A2. The fatty acid levels per cell dry weight after cultivation in a nitrogen limited medium with either glucose, xylose or glycerol as carbon source, respectively, were 46.8, 43.2 and 38.9% for P. hubeiensis BOT-O, and 40.4, 27.3 and 42.1% for BOT-A2. Furthermore, BOT-A2 accumulated 45.1% fatty acids per cell dry weight in a natural plant hydrolysate, and P. hubeiensis BOT-O showed simultaneous glucose and xylose consumption with similar growth rates on both carbon sources. The fatty acid analysis demonstrated both long chain and poly-unsaturated fatty acids, depending on strain and medium. CONCLUSIONS: We found various natural yeast isolates with high lipid production capabilities and the ability to grow not only on glucose, but also xylose, glycerol and natural plant hydrolysate. R. toruloides BOT-A2 and P. hubeiensis BOT-O specifically showed great potential as production strains with high levels of storage lipids and comparable growth to that on glucose on various other substrates, especially compared to currently used lipid production strains. In BOT-O, glucose repression was not detected, making it particularly desirable for utilization of plant waste hydrolysates. Furthermore, the isolated strains were shown to produce oils with fatty acid profiles similar to that of various plant oils, making them interesting for future applications in fuel, food or feed production.

Analysis of astragaloside IV metabolism to cycloastragenol in human gut microorganism, bifidobacteria, and lactic acid bacteria.

This study investigated different gut bacteria in an anaerobic environment to identify specific candidates that could transform astragaloside IV (AIV) to cycloastragenol (CA). Two representative gut microbes, lactic acid bacteria (LAB) and bifidobacteria, could metabolize AIV to CA. Multiple screenings showed two metabolic pathways to metabolize AIV in two groups of bacteria. LAB metabolized AIV initiated by removing the C-6 glucose, whereas bifidobacteria indicated the initial removal of C-3 xylose. The final products differed between the two groups as bifidobacteria showed the production of CA, whereas LAB demonstrated preferential production of 20R, 24S-epoxy-6α, -16ß, -25-trihydroxy-9, -19-cycloartan-3-one (CA-2H).

Metabolic and process engineering for microbial production of protocatechuate with Corynebacterium glutamicum.

3,4-Dihydroxybenzoate (protocatechuate, PCA) is a phenolic compound naturally found in edible vegetables and medicinal herbs. PCA is of high interest in the chemical industry and has wide potential for pharmaceutical applications. We designed and constructed a novel Corynebacterium glutamicum strain to enable the efficient utilization of d-xylose for microbial production of PCA. Shake flask cultivation of the engineered strain showed a maximum PCA titer of 62.1 ± 12.1 mM (9.6 ± 1.9 g L-1 ) from d-xylose as the primary carbon and energy source. The corresponding yield was 0.33 C-mol PCA per C-mol d-xylose, which corresponds to 38% of the maximum theoretical yield. Under growth-decoupled bioreactor conditions, a comparable PCA titer and a total amount of 16.5 ± 1.1 g PCA could be achieved when d-glucose and d-xylose were combined as orthogonal carbon substrates for biocatalyst provision and product synthesis, respectively. Downstream processing of PCA was realized via electrochemically induced crystallization by taking advantage of the pH-dependent properties of PCA. This resulted in a maximum final purity of 95.4%. The established PCA production process represents a highly sustainable approach, which will serve as a blueprint for the bio-based production of other hydroxybenzoic acids from alternative sugar feedstocks.

Valorisation of walnut shell and pea pod as novel sources for the production of xylooligosaccharides.

According to the high interest in agro-industrial waste reutilisation, underutilised lignocellulosic materials, such as walnut shell (WS) and pea pod (PP), come in focus. The aim of this paper was to evaluate WS and PP as sources for the production of xylooligosaccharides (XOS). Hemicelluloses from WS and PP were recovered by combining varying parameters of delignification and alkaline extraction. At optimal recovery conditions, the fractions were further hydrolysed to XOS using GH11 endo-xylanase, by varying time and enzyme concentration. Xylose was predominant in the monomeric composition of the obtained hemicelluloses, building low-branched (arabino)glucuronoxylan, in WS exclusively, while in PP some xyloglucan as well. Delignification was essential for high recovery of total xylose from the materials, up to at least 70 %. High xylan conversions were obtained for 24 h hydrolysis, resulting in xylobiose and xylotriose when using low enzyme concentration, while in xylose and xylobiose with high enzyme concentration.

Fermentation characteristics of four non-Saccharomyces yeasts in green tea slurry.

The fermentation characteristics of non-Saccharomyces yeasts (Pichia kluyveri FrootZen, Torulaspora delbrueckii Prelude, Williopsis saturnus var. mrakii NCYC2251 and Torulaspora delbrueckii Biodiva) were evaluated in green tea slurry fermentation. Each yeast showed different fermentation performances: strains Prelude and Biodiva utilized sucrose faster than the other two yeasts; strain NCYC2251 was the only species that metabolized xylose. Strain FrootZen increased the caffeine content significantly and strain Prelude showed the opposite trend, both at a statistical level, while theanine contents in four samples were relatively stable. Biodiva and FrootZen significantly improved polyphenols content and the oxygen radical absorbance capacity of fermented teas. Some endogenous volatiles such as ketones, lactones and aldehydes decreased to lower or undetected levels, but one of the key tea aroma compounds methyl salicylate increased by 34-fold and 100-fold in P. kluyveri and W. saturnus samples respectively. Therefore, green tea fermentation by appropriate non-Saccharomyces yeasts can enhance its antioxidant capacity and alter the aroma compound profile.

Correlation of D-xylose with severity and morbidity-related factors of COVID-19 and possible therapeutic use of D-xylose and antibiotics for COVID-19.

The SARS-Cov-2 pandemic that currently affects the entire world has been shown to be especially dangerous in the elderly (≥65 years) and in smokers, with notably strong comorbidity in patients already suffering from chronic diseases, such as Type 2 diabetes, cancers, chronic respiratory diseases, obesity, and hypertension. Inflammation of the lungs is the main factor leading to respiratory distress in patients with chronic respiratory disease and in patients with severe COVID-19. Several studies have shown that inflammation of the lungs in general and Type 2 diabetes are accompanied by the degradation of glycosaminoglycans (GAGs), especially heparan sulfate (HS). Several studies have also shown the importance of countering the degradation of HS in lung infections and Type 2 diabetes. D-xylose, which is the initiating element for different sulfate GAG chains (especially HS), has shown regeneration properties for GAGs. D-xylose and xylitol have demonstrated anti-inflammatory, antiglycemic, antiviral, and antibacterial properties in lung infections, alone or in combination with antibiotics. Considering the existing research on COVID-19 and related to D-xylose/xylitol, this review offers a perspective on why the association between D-xylose and antibiotics may contribute to significantly reducing the duration of treatment of COVID-19 patients and why some anti-inflammatory drugs may increase the severity of COVID-19. A strong correlation with scurvy, based on gender, age, ethnicity, smoking status, and obesity status, is also reviewed. Related to this, the effects of treatment with plants such as Artemisia are also addressed. CHEMICAL COMPOUNDS: D-xylose; xylitol; l-ascorbic Acid; D-glucuronic acid; N-acetylglucosamine; D-N-acetylglucosamine; N-acetylgalactosamine; galactose.

Producing Biochemicals in Yarrowia lipolytica from Xylose through a Strain Mating Approach.

Enabling xylose catabolism is challenging, especially for unconventional yeasts and previously engineered background strains. In this study, the efficacy of a yeast mating approach with Yarrowia lipolytica that can combine a previously engineering and evolved xylose phenotype with a metabolite overproduction phenotype is demonstrated. Specifically, several engineered Y. lipolytica strains that produce α-linolenic acid (ALA), riboflavin, and triacetic acid lactone (TAL) with an engineered and adapted xylose-utilizing strain to obtain three diploid strains that rapidly produce these molecules directly from xylose are mated. Titers of 0.52 g L-1 ALA, 96.6 mg L-1 riboflavin, and 2.9 g L-1 TAL, are obtained from xylose in flask cultures and 1.42 g L-1 production of ALA is obtained using bioreactor condition. This total production level is generally on par or higher than the parental strain cultivated on glucose, although specific productivities decreased as a result of improved overall cell growth by the diploid strains. In the case of ALA, this lipid content reached similar levels to that of flaxseed oil. This result showcases the first study using strain mating in Y. lipolytica for producing biomolecules from xylose, and thus demonstrates the utility of this approach as a routine tool for metabolic engineering.

Enzyme production and oil palm empty fruit bunch bioconversion to ethanol using a hybrid yeast strain.

Oil palm empty fruit bunch (OPEFB) is a lignocellulosic biomass generated in palm oil mills. It is a sustainable resource for fuels and chemicals. In this study, OPEFB was converted to ethanol by an integrative OPEFB conversion process including dilute alkaline pretreatment, cellulolytic enzyme production, separate OPEFB hydrolysis, and cofermentation using a hybrid xylose-fermenting yeast. OPEFB was pretreated using 1% (w/v) NaOH solution followed by 1% (v/v) H2 O2 . Further, cellulolytic enzymes were produced by submerged fermentation using Trichoderma reesei Rut C30 and used for OPEFB hydrolysis. The filter paper cellulase activity of the crude cellulolytic enzymes was 15.1 IU/mL, which was higher than those obtained by reported Trichoderma strains under laboratory conditions. Glucose and xylose yields reached 66.9% and 74.2%, respectively, at 30 filter paper unit (FPU)/g-biomass enzyme dosage and 10% (w/v) biomass loading. The hybrid yeast strain ScF2 was previously constructed through recursive genome shuffling of Pichia stipitis and Saccharomyces cerevisiae and was used in OPEFB hydrolysate fermentation. About 16.9 g/L ethanol was produced with an ethanol yield of 0.34 g/g sugars, which was 67% of theoretical ethanol yield.

Fabrication of a soluble crosslinked corn bran arabinoxylan matrix supports a shift to butyrogenic gut bacteria.

Insoluble fermentable cell wall matrix fibers have been shown to support beneficial butyrogenic gut Clostridia, but have restricted use in food products. Here, a soluble fiber matrix was developed that exhibited a similar effect. A low arabinose/xylose ratio corn bran arabinoxylan (CAX) was extracted with two concentrations of sodium hydroxide, 0.25 M and 1.5 M, to give soluble polymers with relatively low (L) and high (H) residual levels of bound ferulic acid (FA) (CAX-LFA and CAX-HFA). After laccase treatment to make diferulate crosslinks, soluble matrices were formed with average 3.5 to 4.5 mer. In vitro human fecal fermentation of CAX-LFA, CAX-HFA, soluble crosslinked ∼3.5 mer CAX-LFA (SCCAX-LFA), and ∼4.5 mer SCCAX-HFA revealed that the SCCAX matrices had somewhat slower fermentation properties by measuring the gas production, total short chain fatty acids, and carbohydrate disappearance, with a higher butyrate proportion in SCCAX-HFA. 16S rRNA gene sequencing showed that SCCAX-HFA promoted OTUs associated with butyrate production including unassigned Ruminococcaceae, unassigned Blautia, Fecalibacterium prausnitzii, and unassigned Clostridium. Thus, when the physical form of an individual soluble polysaccharide was changed to a soluble crosslinked matrix, in vitro fermentation was shifted to Clostridial butyrate producers. This study shows that the physical form of the fiber influences the competition for substrate among the gut bacteria. Crosslinking of soluble fibers may be a strategy for developing soluble matrices with good physical functionalities for beverages and other foods to improve gut health.

Metabolic Engineering for Improved Fermentation of L-Arabinose.

L-Arabinose, a five carbon sugar, has not been considered as an important bioresource because most studies have focused on D-xylose, another type of five-carbon sugar that is prevalent as a monomeric structure of hemicellulose. In fact, L-arabinose is also an important monomer of hemicellulose, but its content is much more significant in pectin (3-22%, g/g pectin), which is considered an alternative biomass due to its low lignin content and mass production as juiceprocessing waste. This review presents native and engineered microorganisms that can ferment L-arabinose. Saccharomyces cerevisiae is highlighted as the most preferred engineering host for expressing a heterologous arabinose pathway for producing ethanol. Because metabolic engineering efforts have been limited so far, with this review as momentum, more attention to research is needed on the fermentation of L-arabinose as well as the utilization of pectin-rich biomass.

l-Arabinose induces d-galactose catabolism via the Leloir pathway in Aspergillus nidulans.

l-Arabinose and d-galactose are the principal constituents of l-arabinogalactan, and also co-occur in other hemicelluloses and pectins. In this work we hypothesized that similar to the induction of relevant glycoside hydrolases by monomers liberated from these plant heteropolymers, their respective catabolisms in saprophytic and phytopathogenic fungi may respond to the presence of the other sugar to promote synergistic use of the complex growth substrate. We showed that these two sugars are indeed consumed simultaneously by Aspergillus nidulans, while l-arabinose is utilised faster in the presence than in the absence of d-galactose. Furthermore, the first two genes of the Leloir pathway for d-galactose catabolism - encoding d-galactose 1-epimerase and galactokinase - are induced more rapidly by l-arabinose than by d-galactose eventhough deletion mutants thereof grow as well as a wild type strain on the pentose. d-Galactose 1-epimerase is hyperinduced by l-arabinose, d-xylose and l-arabitol but not by xylitol. The results suggest that in A. nidulans, l-arabinose and d-xylose - both requiring NADPH for their catabolisation - actively promote the enzyme infrastructure necessary to convert ß-d-galactopyranose via the Leloir pathway with its α-anomer specific enzymes, into ß-d-glucose-6-phosphate (the starting substrate of the oxidative part of the pentose phosphate pathway) even in the absence of d-galactose.

Characterisation of pectin-xylan complexes in tomato primary plant cell walls.

The primary plant cell wall is composed of a complex network of pectin, hemicellulose and cellulose. Potential interactions between these polysaccharides were studied for carrot, tomato and strawberry, with a focus on the role of pectin. The Chelating agent Unextractable Solids (ChUS), the residue after water- and EDTA extraction, was ball milled and subsequently water extracted. For tomato and strawberry, pectin and substantial amounts of hemicellulose were solubilised. Anion exchange chromatography (AEC) showed co-elution of pectin and acetylated glucuronoxylan in tomato, representing 18% of solubilised uronic acid and 48% of solubilised xylose by ball milling from ChUS. The existence of a covalently linked pectin-xylan complex was proposed since xylan co-precipitated with pectin under mild alkali conditions. It was proposed that pectin links with xylan through the RG-I region since degradation of HG did not alter AEC elution patterns for RG-I and xylan, suggesting RG-I - xylan interactions.