Catabolite repression of the Bacillus subtilis xyl operon involves a cis element functional in the context of an unrelated sequence, and glucose exerts additional xylR-dependent repression.

Catabolite repression (CR) of xylose utilization by Bacillus subtilis involves a 14-bp cis-acting element (CRE) located in the translated region of the gene encoding xylose isomerase (xylA). Mutations of CRE making it more similar to a previously proposed consensus element lead to increased CR exerted by glucose, fructose, and glycerol. Fusion of CRE to an unrelated, constitutive promoter confers CR to beta-galactosidase expression directed by that promoter. This result demonstrates that CRE can function independently of sequence context and suggests that it is indeed a generally active cis element for CR. In contrast to the other carbon sources studied here, glucose leads to an additional repression of xylA expression, which is independent of CRE and is not found when CRE is fused to the unrelated promoter. This repression requires a functional xylR encoding Xyl repressor and is dependent on the concentrations of glucose and the inducer xylose in the culture broth. Potential mechanisms for this glucose-specific repression are discussed.     

Co-fermentation of cellobiose and xylose using beta-glucosidase displaying diploid industrial yeast strain OC-2

The co-utilization of sugars, particularly xylose and glucose, during industrial fermentation is essential for economically feasible processes with high ethanol productivity. However, the major problem encountered during xylose/glucose co-fermentation is the lower consumption rate of xylose compared with that of glucose fermentation. Here, we therefore attempted to construct high xylose assimilation yeast by using industrial yeast strain with high β-glucosidase activity on the cell surface. We first constructed the triple auxotrophic industrial strain OC2-HUT and introduced four copies of the cell-surface-displaying β-glucosidase (BGL) gene and two copies of a xylose-assimilating gene into its genome to generate strain OC2-ABGL4Xyl2. It was confirmed that the introduction of multiple copies of the BGL gene increased the cell-surface BGL activity, which was also correlated to the observed increase in xylose-assimilating ability. The strain OC2-ABGL4Xyl2 was able to consume xylose during cellobiose/xylose co-fermentation (0.38 g/h/g-DW) more rapidly than during glucose/xylose co-fermentation (0.18 g/h/g-DW). After 48 h, 5.77% of the xylose was consumed despite the co-fermentation conditions, and the observed ethanol yield was 0.39 g-ethanol/g-total sugar. Our results demonstrate that a BGL-displaying and xylose-assimilating industrial yeast strain is capable of efficient xylose consumption during the co-fermentation with cellobiose. Due to its high performance for fermentation of mixtures of cellobiose and xylose, OC2-ABGL4Xyl2 does not require the addition of β-glucosidase and is therefore a promising yeast strain for cost-effective ethanol production from lignocellulosic biomass.     

Heterologous expression of D-xylulokinase from Pichia stipitis enables high levels of xylitol production by engineered Escherichia coli growing on xylose

Deletion of the Escherichia coli xylulokinase gene (xylB) is essential for achieving high xylitol titers from xylitol-producing E. coli strains growing on glucose in the presence of xylose. Our study suggests that this is due to XylB-catalyzed toxic synthesis of xylitol-phosphate. This activity prohibits the use of xylose as the sole carbon source during xylitol production by E. coli. To overcome this limitation we turned to the yeast Pichia stipitis, which naturally produces xylitol, as a source of xylulokinase (Xyl3). We examined the effects of plasmid-based expression of Xyl3 versus XylB on growth and xylitol production by engineered E. coli strains. Xylulokinase activity assays show similar levels of functional expression of both enzymes (determined as activity on xylulose), and reveal significantly more activity on xylitol by XylB compared to Xyl3. (31)P NMR confirms the production of xylitol-phosphate from in vitro reactions with XylB. Lastly, the replacement of xylB with XYL3 results in drastically enhanced xylitol titers from E. coli strains co-expressing xylose reductase during growth on xylose.     

The effect of CreA in glucose and xylose catabolism in<Emphasis Type="Italic"> Aspergillus nidulans</Emphasis>

The catabolism of glucose and xylose was studied in a wild type and creA deleted (carbon catabolite de-repressed) strain of Aspergillus nidulans. Both strains were cultivated in bioreactors with either glucose or xylose as the sole carbon source, or in the presence of both sugars. In the cultivations on single carbon sources, it was demonstrated that xylose acted as a carbon catabolite repressor (xylose cultivations), while the enzymes in the xylose utilisation pathway were also subject to repression in the presence of glucose (glucose cultivations). In the wild type strain growing on the sugar mixture, glucose repression of xylose utilisation was observed; with xylose utilisation occurring only after glucose was depleted. This phenomenon was not seen in the creA deleted strain, where glucose and xylose were catabolised simultaneously. Measurement of key metabolites and the activities of key enzymes in the xylose utilisation pathway revealed that xylose metabolism was occurring in the creA deleted strain, even at high glucose concentrations. Conversely, in the wild type strain, activities of the key enzymes for xylose metabolism increased only when the effects of glucose repression had been relieved. Xylose was both a repressor and an inducer of xylanases at the same time. The creA mutation seemed to have pleiotropic effects on carbohydratases and carbon catabolism.     

Induction of cellulose- and xylan-degrading enzyme systems in Aspergillus terreus by homo- and heterodisaccharides composed of glucose and xylose

Synthetic heterodisaccharides composed of glucose and xylose were tested as inducers of cellulose- and xylan-degrading enzymes in Aspergillus terreus, and the inducing abilities were compared with those of sophorose and xylobiose or their positional isomers. Measurement of secreted and cell-associated enzyme activities revealed that the heterodisaccharides induced the synthesis of the cellulolytic and xylanolytic enzymes, 2-O-beta-D-glucopyranosyl D-xylose (Glcbeta 1-2Xyl) being the most powerful inducer. Sophorose and 2-O-beta-D-xylopyranosyl D-Xylose (Xylbeta 1-2Xyl), or their positional isomers, selectively induced the synthesis of cellulases and beta-xylanases, respectively. An analysis of the extracellular enzymes (which were separated by isoelectric focusing followed by detection using chromogenic and fluorogenic substrates) showed that Glcbeta 1-2Xyl initiated the synthesis of specific endo-1,4-beta-glucanases and specific endo-1,4-beta-xylanases identical to those produced separately in response to sophorose or Xylbeta 1-2Xyl. Glcbeta 1-2Xyl also induced specific endo-1,4-beta-glucanases that hydrolysed 4-methylumbelliferyl beta-lactoside at the agluconic bond. The results strengthen the concept of separate regulatory control of the synthesis of cullulases and beta-xylanases. The results also suggest that mixed disaccharides, composed of glucose and xylose moieties, which may occur in nature, could play an important role in regulating the synthesis of wood-degrading enzymes.     

Dissolution of xylose metabolism in Lactococcus lactis

Xylose metabolism, a variable phenotype in strains of Lactococcus lactis, was studied and evidence was obtained for the accumulation of mutations that inactivate the xyl operon. The xylose metabolism operon (xylRAB) was sequenced from three strains of lactococci. Fragments of 4.2, 4.2, and 5.4 kb that included the xyl locus were sequenced from L. lactis subsp. lactis B-4449 (formerly Lactobacillus xylosus), L. lactis subsp. lactis IO-1, and L. lactis subsp. lactis 210, respectively. The two environmental isolates, L. lactis B-4449 and L. lactis IO-1, produce active xylose isomerases and xylulokinases and can metabolize xylose. L. lactis 210, a dairy starter culture strain, has neither xylose isomerase nor xylulokinase activity and is Xyl(-). Xylose isomerase and xylulokinase activities are induced by xylose and repressed by glucose in the two Xyl(+) strains. Sequence comparisons revealed a number of point mutations in the xylA, xylB, and xylR genes in L. lactis 210, IO-1, and B-4449. None of these mutations, with the exception of a premature stop codon in xylB, are obviously lethal, since they lie outside of regions recognized as critical for activity. Nevertheless, either cumulatively or because of indirect affects on the structures of catalytic sites, these mutations render some strains of L. lactis unable to metabolize xylose.     

Effects of cell wall degrading enzymes on carbohydrate fractions and metabolites in stomach and ileum of pigs fed wheat bran based diets

Pigs were fed diets containing 40% wheat bran incubated with a water.‐acetic acid mixture (control, C) and a cellulase (Cel‐i) or xylanase (Xyl‐i) preparation or with addition of the cellulase (Cel‐a) or xylanase (Xyl‐a) preparation immediately before feeding. Stomach and ileal samples were analysed for pH, osmolality, soluble saccharides, volatile fatty acids (VFA) and lactic acid. Incubation of wheat bran resulted in a small reduction of NDF and an increase in the amount of soluble starch, ß‐glucans and saccharides (glucose, xylose and arabinose), especially after incubation with the cellulase preparation. Two hours after feeding, significantly higher arabinose and xylose concentrations were present in the stomach for diets Cel‐i, Cel‐a and Xyl‐i. In the ileum xylose and arabinose concentrations were higher 2 to 4 and 6 to 8 hours after feeding the enzyme‐treated diets. In stomach and ileum there were no differences between the diets in pH, osmolality, VFA and lactic acid concentrations, but ileal VFA concentration from 4 h after feeding tended to be higher for diets Cel‐i and Xyl‐i. It can be concluded that the amount of soluble saccharides in stomach and small intestine and the ileal VFA concentration may be increased by cell wall degrading enzyme preparations.     

Evidence for the existence of O-linked sugar chains consisting of glucose and xylose in bovine thrombospondin.

We have recently discovered unusual sugar chains [xylose-glucose and (xylose)2-glucose] linked to a serine residue in the first epidermal growth factor (EGF)-like domains of human and bovine coagulation factors VII, IX, and protein Z. The sequence surrounding this serine residue has a common -Cys-X-Ser-X-Pro-Cys- structure. Since one (residues 533-538) of the three EGF-like domains found in human thrombospondin contains the conserved sequence, we examined the presence of such O-linked sugar chains in bovine thrombospondin (bTSP) and its 210-kDa fragment. Component sugar analysis after pyridylamination (PA) of the acid hydrolysates of the S-aminoethylated proteins revealed that the proteins contain glucose (Glc) and xylose (Xyl). The oligosaccharide moieties released from intact bTSP by hydrazinolysis followed by pyridylamination were separated into two PA-oligosaccharides by high performance liquid chromatography (HPLC). Component sugar analysis of these PA-oligosaccharides indicated that they consist of Glc and Xyl in molar ratios of 1:1 and 1:2 (or 1:3). The reducing ends of both PA-sugar chains were found to be PA-Glc, as judged from the retention time of the HPLC peak of their hydrolysates. The presence of these PA-sugar chains in bTSP was confirmed by HPLC mapping with two different columns, using standard PA-di- or PA-trisaccharide derived from coagulation factors. From these results, we concluded that bTSP contains O-linked sugar chains consisting of Glc and Xyl in one of its three EGF-like domains.     

Sugar Compositions, Intrinsic Viscosities and Molecular Weights of Hemicellulosic Polysaccharides of the Coleoptile Cell Walls in a Semi-Brachytic and a Normal Type Barley

The sugar compositions and intrinsic viscosities of the hemicellulosicpolysaccharides of the coleoptile cell wall were determinedin a normal type barley and a semi-brachytic type which producedless IAA than the normal type. The major sugar components ofhemicelluloses for both strains were arabinose (Ara), xylose(Xyl) and glucose (Glc). The Ara and Xyl content per unit lengthin the normal type did not change during growth, while thosein the semi-brachytic type decreased during growth. The Glccontents per unit length and per coleoptile decreased duringgrowth in both types of barley. The intrinsic viscosity of hemicellulosesfrom the coleoptile of the normal type was lower than that ofthe semi-brachytic type. These results suggested that the synthesis of arabinoxylan keptpace with the growth of the coleoptile in the normal type butnot in the semi-brachytic type, and that the average mol wtof the hemicelluloses in the normal type was lower than thatin the semibrachytic type. These chemical and physical changesin the hemicellulosic polysaccharides may account for the stuntedcoleoptile of the semi-brachytic barley with its less amountof endogenous IAA. (Received March 14, 1984; Accepted June 8, 1984)     

Enzymatic degradation studies of xylogalacturonans from apple and potato, using xylogalacturonan hydrolase

Action of xylogalacturonan hydrolase (XGH) towards xylogalacturonan (XGA) present in the alkali saponified “modified hairy regions” from potato and apple pectin was studied.

Analysis of enzymatic degradation products from XGA in these complex pectins demonstrated that the degradable xylogalacturonans from both sources have a similar xylose side chain distribution. The disaccharide β-d-Xyl-(1,3)-GalA was the predominant product from these substrates.

The number of enzymatic degradation products from xylogalacturonan present in apple and potato pectin was much lower than the number of products from a xylogalacturonan derived from Gum Tragacanth. This suggests a relatively uniform distribution of xylose in the degradable part of XGA from apple and potato pectin. In addition, dimeric side chains of xylose were observed in digests of XGA from both pectins, which apparently did not hinder the action of XGH. From this it is assumed that Xyl–Xyl as well as Xyl substituted GalA residues are accepted in subsite −1 of xylogalacturonan hydrolase.     

Regulation of the Bacillus subtilis W23 xylose utilization operon : interaction of the Xyl repressor with the xyl operator and the inducer xylose

Summary A crude protein extract of Bacillus subtilis W23 contains a sequence-specific DNA binding activity for the xyl operator as detected by the gel mobility shift assay. A xylR determinant encoded on a multicopy plasmid leads to increased expression of this binding activity. In situ footprinting analysis of the protein-DNA complex in a polyacrylamide gel shows that the xyl operator is sequence-specifically bound and protected from cleavage by copper-phenanthroline at 26 phosphodiester bonds on each strand. Quantitative competition assays for repressor binding reveal that a 25 by synthetic xyl operator cloned into a polylinker is bound with the same affinity as the operator in the wild-type xyl regulatory region. This confirms that no additional sites in the wild-type sequence contribute to repressor binding. The xyl operator consists of ten palindromic base pairs flanking five central non-palindromic base pairs. A mutational analysis shows that the sequence of the central base pairs contributes to recognition by the repressor protein and that the spacing of the palindromic elements is crucial for repressor binding. An operator half site is not bound by the repressor. In vivo and in vitro induction studies suggest that, of several structurally similar sugars, xylose is the only molecular inducer of the Xyl repressor.     

Arabidopsis family GT43 members are xylan xylosyltransferases required for the elongation of the xylan backbone

Xylan is the second most abundant polysaccharide in plant biomass targeted for biofuel production. Therefore, it is imperative to understand the biochemical mechanism underlying xylan biosynthesis. Although previous genetic studies have identified several genes implicated in xylan biosynthesis, biochemical proof of any of their encoded proteins as a xylan xylosyltransferase (XylT) responsible for xylan backbone biosynthesis is still lacking. In this study, we investigated the enzymatic activities of two Arabidopsis thaliana GT43 members, IRX9 (Irregular Xylem9) and IRX14, which have been genetically shown to be non-redundantly involved in the elongation of the xylan backbone. IRX9 and IRX14, alone or simultaneously, were heterologously expressed in tobacco BY2 cells, and microsomes isolated from the transgenic BY2 cells were tested for XylT activity using xylotetraose (Xyl(4)) as an acceptor and UDP-[(14)C]xylose as a donor. It was found that although microsomes with expression of IRX9 or IRX14 alone exhibited little incorporation of radiolabeled xylose, a high level of incorporation of radiolabeled xylose onto Xyl(4) was conferred by microsomes with co-expression of IRX9 and IRX14. Further analysis using fluorescent anthranilic acid-labeled xylotetraose (Xyl(4)-AA) as an acceptor revealed that up to five β-(1,4)-linked xylosyl residues were able to be transferred onto Xyl(4)-AA by microsomes with co-expression of IRX9 and IRX14. Furthermore, it was shown that xylooligomers ranging from Xyl(3)-AA to Xyl(6)-AA could all be used as acceptors for the xylosyl transfer by microsomes with co-expression of IRX9 and IRX14. Together, these findings provide the first biochemical evidence that IRX9 and IRX14 are xylosyltransferases that operate cooperatively in the elongation of the xylan backbone.     

Evaluation of hydrolysis products of xylans by xylan-degrading enzymes from Humicola grisea var. thermoidea and Aspergillus fumigatus Fresenius

Xylan-degrading enzyme activities were isolated from crude extracts of solid-state cultures of Aspergillus fumigatus Fresenius (Xyl I, Xyl III, Xyl IV and Xyl V) and Humicola grisea var. thermoidea (Xyl II) by chromatographic procedures. The pattern of hydrolysis of different xylans and pulps varied from traces of xylose to xylooligomers. The products formed suggest an endo-type enzyme mode of action. Some enzymes showed debranching and transglycosidase activities.     

Mode of action of glycoside hydrolase family 5 glucuronoxylan xylanohydrolase from Erwinia chrysanthemi

The mode of action of xylanase A from a phytopathogenic bacterium, Erwinia chrysanthemi, classified in glycoside hydrolase family 5, was investigated on xylooligosaccharides and polysaccharides using TLC, MALDI-TOF MS and enzyme treatment with exoglycosidases. The hydrolytic action of xylanase A was found to be absolutely dependent on the presence of 4-O-methyl-D-glucuronosyl (MeGlcA) side residues in both oligosaccharides and polysaccharides. Neutral linear beta-1,4-xylooligosaccharides and esterified aldouronic acids were resistant towards enzymatic action. Aldouronic acids of the structure MeGlcA(3)Xyl(3) (aldotetraouronic acid), MeGlcA(3)Xyl(4) (aldopentaouronic acid) and MeGlcA(3)Xyl(5) (aldohexaouronic acid) were cleaved with the enzyme to give xylose from the reducing end and products shorter by one xylopyranosyl residue: MeGlcA(2)Xyl(2), MeGlcA(2)Xyl(3) and MeGlcA(2)Xyl(4). As a rule, the enzyme attacked the second glycosidic linkage following the MeGlcA branch towards the reducing end. Depending on the distribution of MeGlcA residues on the glucuronoxylan main chain, the enzyme generated series of shorter and longer aldouronic acids of backbone polymerization degree 3-14, in which the MeGlcA is linked exclusively to the second xylopyranosyl residue from the reducing end. Upon incubation with beta-xylosidase, all acidic hydrolysis products of acidic oligosaccharides and hardwood glucuronoxylans were converted to aldotriouronic acid, MeGlcA(2)Xyl(2). In agreement with this mode of action, xylose and unsubstituted oligosaccharides were essentially absent in the hydrolysates. The E. chrysanthemi xylanase A thus appears to be an excellent biocatalyst for the production of large acidic oligosaccharides from glucuronoxylans as well as an invaluable tool for determination of the distribution of MeGlcA residues along the main chain of this major plant hemicellulose.     

Extraction, purification and chemical characterisation of xylogalacturonans from pea hulls

Pea hulls contained 925 mg/g sugar including 659 mg/g cellulosic glucose and 90 mg/g uronic acid. They were de-esterified by NaOH (pH>13 at 4°C, 2 h) and treated with HCl (0.1 mol/l, 80°C, 24 h). The HCl-soluble fraction represented 95 mg/g initial pea hulls. It was rich in galacturonic acid (259 mg/g), xylose (93 mg/g) and rhamnose (91 mg/g), which co-eluted in anion-exchange chromatography. The HCl-soluble fraction was degraded by a rhamnogalacturonan-hydrolase and the reaction products were fractionated by size-exclusion chromatography. Two fractions, representing together 18 mg/g initial pea hulls, were composed almost exclusively of galacturonic acid and xylose and could be defined as xylogalacturonans. The first fraction exhibited a high molar mass, a molar ratio Xyl/GalA of 1 and contained almost 5% of rhamnose. The molar mass of the second fraction was much lower and the molar ratio Xyl/GalA was 0.6. Methylation analysis showed the presence in both fractions of a (1→4) galacturonan backbone highly substituted on O-3 either by terminal xylosyl residues or by short side-chains of (1→2) linked xylosyl residues.