Donor Substrate Regeneration for Efficient Synthesis of Globotetraose and Isoglobotetraose

Here we describe the efficient synthesis of two oligosaccharide moieties of human glycosphingolipids, globotetraose (GalNAcβ1→3Galα1→4Galβ1→4Glc) and isoglobotetraose (GalNAcβ1→3Galα1→3Galβ1→4Glc), with in situ enzymatic regeneration of UDP-N-acetylgalactosamine (UDP-GalNAc). We demonstrate that the recombinant β-1,3-N-acetylgalactosaminyltransferase from Haemophilus influenzae strain Rd can transfer N-acetylgalactosamine to a wide range of acceptor substrates with a terminal galactose residue. The donor substrate UDP-GalNAc can be regenerated by a six-enzyme reaction cycle consisting of phosphoglucosamine mutase, UDP-N-acetylglucosamine pyrophosphorylase, phosphate acetyltransferase, pyruvate kinase, and inorganic pyrophosphatase from Escherichia coli, as well as UDP-N-acetylglucosamine C4 epimerase from Plesiomonas shigelloides. All these enzymes were overexpressed in E. coli with six-histidine tags and were purified by one-step nickel-nitrilotriacetic acid affinity chromatography. Multiple-enzyme synthesis of globotetraose or isoglobotetraose with the purified enzymes was achieved with relatively high yields.     

Synthesis of Fucosyl-N-Acetylglucosamine Disaccharides by Transfucosylation Using α-l-Fucosidases from Lactobacillus casei

AlfB and AlfC α-l-fucosidases from Lactobacillus casei were used in transglycosylation reactions, and they showed high efficiency in synthesizing fucosyldisaccharides. AlfB and AlfC activities exclusively produced fucosyl-α-1,3-N-acetylglucosamine and fucosyl-α-1,6-N-acetylglucosamine, respectively. The reaction kinetics showed that AlfB can convert 23% p-nitrophenyl-α-l-fucopyranoside into fucosyl-α-1,3-N-acetylglucosamine and AlfC at up to 56% into fucosyl-α-1,6-N-acetylglucosamine.     

UDP-N-acetylglucosamine Transporter (SLC35A3) Regulates Biosynthesis of Highly Branched N-Glycans and Keratan Sulfate

SLC35A3 is considered the main UDP-N-acetylglucosamine transporter (NGT) in mammals. Detailed analysis of NGT is restricted because mammalian mutant cells defective in this activity have not been isolated. Therefore, using the siRNA approach, we developed and characterized several NGT-deficient mammalian cell lines. CHO, CHO-Lec8, and HeLa cells deficient in NGT activity displayed a decrease in the amount of highly branched tri- and tetraantennary N-glycans, whereas monoantennary and diantennary ones remained unchanged or even were accumulated. Silencing the expression of NGT in Madin-Darby canine kidney II cells resulted in a dramatic decrease in the keratan sulfate content, whereas no changes in biosynthesis of heparan sulfate were observed. We also demonstrated for the first time close proximity between NGT and mannosyl (α-1,6-)-glycoprotein β-1,6-N-acetylglucosaminyltransferase (Mgat5) in the Golgi membrane. We conclude that NGT may be important for the biosynthesis of highly branched, multiantennary complex N-glycans and keratan sulfate. We hypothesize that NGT may specifically supply β-1,3-N-acetylglucosaminyl-transferase 7 (β3GnT7), Mgat5, and possibly mannosyl (α-1,3-)-glycoprotein β-1,4-N-acetylglucosaminyltransferase (Mgat4) with UDP-GlcNAc.     

A Trypanosoma brucei β3 glycosyltransferase superfamily gene encodes a β1-6 GlcNAc-transferase mediating N-glycan and GPI anchor modification

The parasite Trypanosoma brucei exists in both a bloodstream form (BSF) and a procyclic form (PCF), which exhibit large carbohydrate extensions on the N-linked glycans and glycosylphosphatidylinositol (GPI) anchors, respectively. The parasite''s glycoconjugate repertoire suggests at least 38 glycosyltransferase (GT) activities, 16 of which are currently uncharacterized. Here, we probe the function(s) of the uncharacterized GT67 glycosyltransferase family and a β3 glycosyltransferase (β3GT) superfamily gene, TbGT10. A BSF-null mutant, created by applying the diCre/loxP method in T. brucei for the first time, showed a fitness cost but was viable in vitro and in vivo and could differentiate into the PCF, demonstrating nonessentiality of TbGT10. The absence of TbGT10 impaired the elaboration of N-glycans and GPI anchor side chains in BSF and PCF parasites, respectively. Glycosylation defects included reduced BSF glycoprotein binding to the lectin ricin and monoclonal antibodies mAb139 and mAbCB1. The latter bind a carbohydrate epitope present on lysosomal glycoprotein p67 that we show here consists of (-6Galβ1-4GlcNAcβ1-)_≥4 poly-N-acetyllactosamine repeats. Methylation linkage analysis of Pronase-digested glycopeptides isolated from BSF wild-type and TbGT10 null parasites showed a reduction in 6-O-substituted- and 3,6-di-O-substituted-Gal residues. These data define TbGT10 as a UDP-GlcNAc:βGal β1-6 GlcNAc-transferase. The dual role of TbGT10 in BSF N-glycan and PCF GPI-glycan elaboration is notable, and the β1-6 specificity of a β3GT superfamily gene product is unprecedented. The similar activities of trypanosome TbGT10 and higher-eukaryote I-branching enzyme (EC 2.4.1.150), which belong to glycosyltransferase families GT67 and GT14, respectively, in elaborating N-linked glycans, are a novel example of convergent evolution.     

Characterization of the Stimulation of Ethylene Production by Galactose in Tomato (Lycopersicon esculentum Mill.) Fruit

We have characterized the stimulation of ethylene production by galactose in tomatoes (Lycopersicon esculentum Mill.). The effect of concentration was studied by infiltrating 0, 4, 40, 100, 200, 400, or 800 micrograms galactose for each gram of fresh fruit weight into mature green `Rutgers' fruit. Both 400 and 800 micrograms per gram fresh weight consistently stimulated a transient increase in ethylene approximately 25 hours after infiltration; the lower concentrations did not. Carbon dioxide evolution of fruit infiltrated with 400 to 800 micrograms per gram fresh weight was greater than that of lower concentrations. The ripening mutants, rin and nor, also showed the transient increase in ethylene and elevated CO₂ evolution by 400 micrograms per gram fresh weight galactose. 1-Aminocyclopropane-1-carboxylic acid (ACC) content and ACC-synthase activity increased concurrently with ethylene production. However, galactose did not stimulate ACC-synthase activity in vitro. The infiltrated galactose in pericarp tissue was rapidly metabolized, decreasing to endogenous levels within 50 hours. Infiltrated galacturonic acid, dulcitol, and mannose stimulated transient increases in ethylene production similar to that of galactose. The following sugars produced no response: sucrose, fructose, glucose, rhamnose, arabinose, xylose, raffinose, lactose, and sorbitol.     

Heparan Sulfate Containing Unsubstituted Glucosamine Residues: BIOSYNTHESIS AND HEPARANASE-INHIBITORY ACTIVITY*

Degradation of heparan sulfate (HS) in the extracellular matrix by heparanase is linked to the processes of tumor invasion and metastasis. Thus, a heparanase inhibitor can be a potential anticancer drug. Because HS with unsubstituted glucosamine residues accumulates in heparanase-expressing breast cancer cells, we assumed that these HS structures are resistant to heparanase and can therefore be utilized as a heparanase inhibitor. As expected, chemically synthetic HS-tetrasaccharides containing unsubstituted glucosamine residues, GlcAβ1–4GlcNH₃⁺(6-O-sulfate)α1–4GlcAβ1–4GlcNH₃⁺(6-O-sulfate), inhibited heparanase activity and suppressed invasion of breast cancer cells in vitro. Bifunctional NDST-1 (N-deacetylase/N-sulfotransferase-1) catalyzes the modification of N-acetylglucosamine residues within HS chains, and the balance of N-deacetylase and N-sulfotransferase activities of NDST-1 is thought to be a determinant of the generation of unsubstituted glucosamine. We also report here that EXTL3 (exostosin-like 3) controls N-sulfotransferase activity of NDST-1 by forming a complex with NDST-1 and contributes to generation of unsubstituted glucosamine residues.     

Structural basis for the substrate specificity of a novel β-N-acetylhexosaminidase StrH protein from Streptococcus pneumoniae R6

The β-N-acetylhexosaminidase (EC 3.2.1.52) from glycoside hydrolase family 20 (GH20) catalyzes the hydrolysis of the β-N-acetylglucosamine (NAG) group from the nonreducing end of various glycoconjugates. The putative surface-exposed N-acetylhexosaminidase StrH/Spr0057 from Streptococcus pneumoniae R6 was proved to contribute to the virulence by removal of β(1,2)-linked NAG on host defense molecules following the cleavage of sialic acid and galactose by neuraminidase and β-galactosidase, respectively. StrH is the only reported GH20 enzyme that contains a tandem repeat of two 53% sequence-identical catalytic domains (designated as GH20-1 and GH20-2, respectively). Here, we present the 2.1 Å crystal structure of the N-terminal domain of StrH (residues Glu-175 to Lys-642) complexed with NAG. It adopts an overall structure similar to other GH20 enzymes: a (β/α)₈ TIM barrel with the active site residing at the center of the β-barrel convex side. The kinetic investigation using 4-nitrophenyl N-acetyl-β-d-glucosaminide as the substrate demonstrated that GH20-1 had an enzymatic activity (k_cat/K_m) of one-fourth compared with GH20-2. The lower activity of GH20-1 could be attributed to the substitution of active site Cys-469 of GH20-1 to the counterpart Tyr-903 of GH20-2. A complex model of NAGβ(1,2)Man at the active site of GH20-1 combined with activity assays of the corresponding site-directed mutants characterized two key residues Trp-443 and Tyr-482 at subsite +1 of GH20-1 (Trp-876 and Tyr-914 of GH20-2) that might determine the β(1,2) substrate specificity. Taken together, these findings shed light on the mechanism of catalytic specificity toward the β(1,2)-linked β-N-acetylglucosides.     

Localization of cytokinin biosynthetic sites in pea plants and carrot roots

The biosynthesis of cytokinins was examined in pea (Pisum sativum L.) plant organs and carrot (Daucus carota L.) root tissues. When pea roots, stems, and leaves were grown separately for three weeks on a culture medium containing [8-¹⁴C]adenine without an exogenous supply of cytokinin and auxin, radioactive cytokinins were synthesized by each of these organs. Incubation of carrot root cambium and noncambium tissues for three days in a liquid culture medium containing [8-¹⁴C]adenine without cytokinin demonstrates that radioactive cytokinins were synthesized in the cambium but not in the noncambium tissue preparation. The radioactive cytokinins extracted from each of these tissues were analyzed by Sephadex LH-20 columns, reverse phase high pressure liquid chromatography, paper chromatography in various solvent systems, and paper electrophoresis. The main species of cytokinins detectable by these methods are N⁶-(Δ²-isopentyl_adenine-5′-monophosphate, 6-(4-hydroxy-3-methyl-2-butenyl-amino)-9-β-ribofuranosylpurine-5′- monophosphate, N⁶-(Δ²-isopentenyl)adenosine, 6-(4-hydroxy-3-methyl-2-butenylamino)-9-β-ribofuranosylpurine, N⁶-(Δ²-isopentenyl)adenine, and 6-(4-hydroxy-3-methyl-2-butenylamino)purine. On the basis of the amounts of cytokinin synthesized per gram fresh tissues, these results indicate that the root is the major site, but not the only site, of cytokinin biosynthesis. Furthermore, cambium and possibly all actively dividing tissues are responsible for the synthesis of this group of plant hormones.     

beta-Galactosidases in Ripening Tomatoes

Tomatoes (Lycopersicon esculentum L.) contained a high level of β-galactosidase activity which was due to three forms of the enzyme. During tomato ripening, the sum of their activities remained relatively constant, but the levels of the individual forms of β-galactosidase changed markedly. The three enzymes were separated by a combination of chromatography of DEAE-Sephadex A-50 and Sephadex G-100. During ripening of tomatoes, β-galactosidases I and III levels decreased but the β-galactosidase II level increased more than 3-fold. The three enzymes were optimally active near pH 4, and all were inhibited by galactose and galactonolactone. However, the enzymes differed in molecular weight, K_m value with p-nitrophenyl-β-galactoside, and stability with respect to pH and temperature. β-Galactosidase II was the only enzyme capable of hydrolyzing a polysaccharide that was isolated from tomatoes and that consisted primarily of β-1, 4-linked galactose. The ability of β-galactosidase II to degrade the galactan and the increase in its activity during tomato ripening suggest a possible role for this enzyme in tomato softening.     

Enzymic hydrolysis of sphingolipids: Hydrolysis of ceramide lactoside by an enzyme from rat brain

Ceramide lactoside [1-O-(galactosido-4-β-glucosido)-2-N-acyl-sphingosine] was hydrolysed to ceramide glucoside and galactose by β-galactosidase of rat brain. The reaction was not reversible, required cholate or taurocholate, had optimum pH5·0 and K_m 2·2×10⁻⁵m. It was inhibited by γ-galactonolactone and galactose as well as by ceramide, sphingosine and fatty acid. Ceramide lactoside could be degraded to ceramide, galactose and glucose by mixtures of rat-brain β-galactosidase and ox-brain β-glucosidase.     

Levels of Free and Conjugated Abscisic Acid in Developing Floral Organs of the Navel Orange (Citrus sinensi [L.] Osbeck cv Washington)

The levels of abscisic acid (ABA) and alkaline-hydrolyzable ABA-conjugate (putatively identified as the glucosyl ester, abscisyl-β-d-glucopyranoside) were determined by enzyme immunoassay in the organs of developing navel orange (Citrus sinensis [L.] Osbeck cv Washington) flowers. Although both compounds were detected in every tissue, developmentally related differences between organs in the total and relative contents were observed. The highest ABA levels were observed in the stigma/style shortly after anthesis (11.5 ± 0.6 nanomoles ABA per gram fresh weight and 4.8 ± 0.6 nanomoles ABA-conjugate per gram fresh weight); whereas, the highest ABA-conjugate levels were observed at the same time in the floral disc (hypogynous disc plus calyx; 3.5 ± 0.1 nmol nanomols ABA per gram fresh weight and 11.8 ± 0.9 nanomoles ABA-conjugate per gram fresh weight). These results suggest that differences in ABA content reflect tissue-specific variation in the facility for ABA conjugation. Increased ABA levels were observed in the stigma/style near anthesis; however, a relationship with pollination is discounted, since `Washington' navel orange flowers are male sterile and devoid of pollen.     

Expression of glycoproteins bearing complex human-like glycans with galactose terminal in Hansenula polymorpha

Glycoproteins derived from Hansenula polymorpha can not be used for therapeutic purposes due to their high-mannose type asparagine-linked (N-linked) glycans, which result in immune reactions and poor pharmacokinetic behaviors in human body. Previously, we reported that the trimannosyl core N-linked glycans (Man₃GlcNAc₂) intermediate can be generated in endoplasmic reticulum in HpALG3 and HpALG11 double-mutant H. polymorpha. Here, we describe the further modification of the glycosylation pathway in this double-defect strain to express glycoproteins with complex human-like glycans. After eliminating the impact of HpOCH1, three glycosyltransferases were introduced into this triple-mutant strain. When human β-1,2-N-acetylglucosaminyltransferase I (hGnTI) was efficiently targeted in early Golgi, more than 95 % glycans attached to the glycoproteins were added one N-acetylglucosamine (GlcNAc). With subsequently introduction of rat β-1,2-N-acetylglucosaminyltransferase II (rGnTII) and human β-1,4-galactosyltransferase I (hGalTI), several glycoengineered strains can produce glycoproteins bearing glycans with terminal N-acetylglucosamine or galactose. The expression of glycoproteins with glycan Gal₂GlcNAc₂Man₃GlcNAc₂ represents a significant step toward the ability to express fully humanized glycoproteins in H. polymorpha. Furthermore, several shake-flask and bioreactor fermentation experiments indicated that, although the cells do display a reduction in growth rate, the glycoengineered strains are still suitable for high-density fermentation.     

Terminal N-Acetylgalactosamine-Specific Leguminous Lectin from Wisteria japonica as a Probe for Human Lung Squamous Cell Carcinoma

Millettia japonica was recently reclassified into the genus Wisteria japonica based on chloroplast and nuclear DNA sequences. Because the seed of Wisteria floribunda expresses leguminous lectins with unique N-acetylgalactosamine-binding specificity, we purified lectin from Wisteria japonica seeds using ion exchange and gel filtration chromatography. Glycan microarray analysis demonstrated that unlike Wisteria floribunda and Wisteria brachybotrys lectins, which bind to both terminal N-acetylgalactosamine and galactose residues, Wisteria japonica lectin (WJA) specifically bound to both α- and β-linked terminal N-acetylgalactosamine, but not galactose residues on oligosaccharides and glycoproteins. Further, frontal affinity chromatography using more than 100 2-aminopyridine-labeled and p-nitrophenyl-derivatized oligosaccharides demonstrated that the ligands with the highest affinity for Wisteria japonica lectin were GalNAcβ1-3GlcNAc and GalNAcβ1-4GlcNAc, with K _a values of 9.5 × 10⁴ and 1.4 × 10⁵ M^-1, respectively. In addition, when binding was assessed in a variety of cell lines, Wisteria japonica lectin bound specifically to EBC-1 and HEK293 cells while other Wisteria lectins bound equally to all of the cell lines tested. Wisteria japonica lectin binding to EBC-1 and HEK293 cells was dramatically decreased in the presence of N-acetylgalactosamine, but not galactose, mannose, or N-acetylglucosamine, and was completely abrogated by β-hexosaminidase-digestion of these cells. These results clearly demonstrate that Wisteria japonica lectin binds to terminal N-acetylgalactosamine but not galactose. In addition, histochemical analysis of human squamous cell carcinoma tissue sections demonstrated that Wisteria japonica lectin specifically bound to differentiated cancer tissues but not normal tissue. This novel binding characteristic of Wisteria japonica lectin has the potential to become a powerful tool for clinical applications.     

Kinetics and mechanism of catalysis by proteolytic enzymes. A comparison of the kinetics of hydrolysis of synthetic substrates by bovine α- and β-trypsin

Several esters of the α-N-toluene-p-sulphonyl and α-N-benzoyl derivatives of S-(3-aminopropyl)-l-cysteine and the methyl ester of S-(4-aminobutyl)-N-toluene-p-sulphonyl-l-cysteine were synthesized. The kinetics of hydrolysis of these and esters of the α-N-toluene-p-sulphonyl and α-N-benzoyl derivatives of l-arginine, l-lysine, S-(2-aminoethyl)-l-cysteine and esters of γ-guanidino-l-α-toluene-p-sulphonamidobutyric acid and α-N-toluene-p-sulphonyl-l-homoarginine by α- and β-trypsin were compared. On the basis of values of the specificity constants (k_cat./K_m), the two enzymes display similar catalytic efficiency towards some substrates. In other cases α-trypsin is less efficient than β-trypsin. It is possible that α-trypsin possesses greater molecular flexibility than β-trypsin.     

The metabolism of d-galactosamine and N-acetyl-d-galactosamine in rat liver

d-[1-¹⁴C]Galactosamine appears to be utilized mainly by the pathway of galactose metabolism in rat liver, as evidenced by the products isolated from the acid-soluble fraction of perfused rat liver. These products were eluted in the following order from a Dowex 1 (formate form) column and were characterized as galactosamine 1-phosphate, sialic acid, UDP-glucosamine, UDP-galactosamine, N-acetylgalactosamine 1-phosphate, N-acetylglucosamine 6-phosphate, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine and an unidentified galactosamine-containing compound. In addition, [1-¹⁴C]glucosamine was found in the glycogen, an incorporation previously shown to result from the substitution of UDP-glucosamine for UDP-glucose in the glycogen synthetase reaction. Analysis of the [1-¹⁴C]glucosamine-containing disaccharides released from glycogen by β-amylase provided additional evidence that they consist of a mixture of glucose and glucosamine in a 1:1 ratio, but with glucose predominating on the reducing end. UDP-N-acetylgalactosamine was shown to result from the reaction of UTP with N-acetylgalactosamine 1-phosphate in the presence of a rat liver extract.     

Induction and Purification of Endo-β-N-Acetylglucosaminidase from Arthrobacter protophormiae Grown in Ovalbumin

Arthrobacter protophormiae produced a high level of extracellular endo-β-N-acetylglucosaminidase when cells were grown in a medium containing ovalbumin. The enzyme was induced by the glycopeptide fraction of ovalbumin prepared by pronase digestion. Production of the enzyme was also induced by glycoproteins such as yeast invertase and bovine ribonuclease B but not by monosaccharides such as mannose, N-acetylglucosamine, and galactose. The enzyme was purified to homogeneity as demonstrated by polyacrylamide gel electrophoresis and has an apparent molecular weight of about 80,000. The enzyme showed a broad optimum pH in the range of pH 5.0 to 11.0. The enzyme hydrolyzed all heterogeneous ovalbumin glycopeptides, although the hydrolysis rates for hybrid type glycopeptides were very low. The substrate specificity of A. protophormiae endo-β-N-acetylglucosaminidase was very similar to that of Endo-C_II from Clostridium perfringens. Therefore, the enzyme induction by A. protophormiae seems to have a close relation to the substrate specificity of the enzyme.     

P1 Trisaccharide (Galα1,4Galβ1,4GlcNAc) Synthesis by Enzyme Glycosylation Reactions Using Recombinant Escherichia coli

The frequency of Escherichia coli infection has lead to concerns over pathogenic bacteria in our food supply and a demand for therapeutics. Glycolipids on gut cells serve as receptors for the Shiga-like toxin produced by E. coli. Oligosaccharide moiety analogues of these glycolipids can compete with receptors for the toxin, thus acting as antibacterials. An enzymatic synthesis of the P1 trisaccharide (Galα1,4Galβ1,4GlcNAc), one of the oligosaccharide analogues, was assessed in this study. In the proposed synthetic pathway, UDP-glucose was generated from sucrose with an Anabaena sp. sucrose synthase and then converted with an E. coli UDP-glucose 4-epimerase to UDP-galactose. Two molecules of galactose were linked to N-acetylglucosamine subsequently with a Helicobacter pylori β-l,4-galactosyltransferase and a Neisseria meningitidis α-1,4-galactosyltransferase to produce one molecule of P1 trisaccharide. The four enzymes were coexpressed in a single genetically engineered E. coli strain that was then permeabilized and used to catalyze the enzymatic reaction. P1 trisaccharide was accumulated up to 50 mM (5.4 g in a 200-ml reaction volume), with a 67% yield based on the consumption of N-acetylglucosamine. This study provides an efficient approach for the preparative-scale synthesis of P1 trisaccharide with recombinant bacteria.     

Lacto-N-biosidase Encoded by a Novel Gene of Bifidobacterium longum Subspecies longum Shows Unique Substrate Specificity and Requires a Designated Chaperone for Its Active Expression

Infant gut-associated bifidobacteria possess species-specific enzymatic sets to assimilate human milk oligosaccharides, and lacto-N-biosidase (LNBase) is a key enzyme that degrades lacto-N-tetraose (Galβ1–3GlcNAcβ1–3Galβ1–4Glc), the main component of human milk oligosaccharides, to lacto-N-biose I (Galβ1–3GlcNAc) and lactose. We have previously identified LNBase activity in Bifidobacterium bifidum and some strains of Bifidobacterium longum subsp. longum (B. longum). Subsequently, we isolated a glycoside hydrolase family 20 (GH20) LNBase from B. bifidum; however, the genome of the LNBase⁺ strain of B. longum contains no GH20 LNBase homolog. Here, we reveal that locus tags BLLJ_1505 and BLLJ_1506 constitute LNBase from B. longum JCM1217. The gene products, designated LnbX and LnbY, respectively, showed no sequence similarity to previously characterized proteins. The purified enzyme, which consisted of LnbX only, hydrolyzed via a retaining mechanism the GlcNAcβ1–3Gal linkage in lacto-N-tetraose, lacto-N-fucopentaose I (Fucα1–2Galβ1–3GlcNAcβ1–3Galβ1–4Glc), and sialyllacto-N-tetraose a (Neu5Acα2–3Galβ1–3GlcNAcβ1–3Galβ1–4Gal); the latter two are not hydrolyzed by GH20 LNBase. Among the chromogenic substrates examined, the enzyme acted on p-nitrophenyl (pNP)-β-lacto-N-bioside I (Galβ1–3GlcNAcβ-pNP) and GalNAcβ1–3GlcNAcβ-pNP. GalNAcβ1–3GlcNAcβ linkage has been found in O-mannosyl glycans of α-dystroglycan. Therefore, the enzyme may serve as a new tool for examining glycan structures. In vitro refolding experiments revealed that LnbY and metal ions (Ca²⁺ and Mg²⁺) are required for proper folding of LnbX. The LnbX and LnbY homologs have been found only in B. bifidum, B. longum, and a few gut microbes, suggesting that the proteins have evolved in specialized niches.     

Dynamics of Endogenous Cytokinins during the Growth Cycle of a Hormone-Autotrophic Genetic Tumor Line of Tobacco

The profile of endogenous cytokinins in a genetic tumor line of tobacco, namely, Nicotiana glauca (Grah.) × Nicotiana langsdorffii (Weinm.), following 1 to 10 weeks of growth on solid medium was determined by radioimmunoassay. ³H-labeled cytokinins of high specific activity were added during tissue extraction to correct for the purification losses. Following subculture (of 4-week-old tissues when their cytokinin content is high) onto fresh medium the total cytokinin content continued to be high during the first week (1470 picomoles per gram fresh weight) when the tissue fresh weight remained essentially unchanged (lag phase). The cytokinin levels then declined by about half in 2- and 3-week-old tissues (626 and 675 picomoles per gram fresh weight, respectively), a period when rapid increase in tissue fresh weight was recorded. Increments of 840% and 2780% over initial fresh weight were obtained in 2- and 3-week-old cultures, respectively. The cytokinin content then increased to initial high levels in 4-week-old tissues (1384 picomoles per gram fresh weight) after which it gradually declined with tissue age. The lowest cytokinin levels (432 picomoles per gram fresh weight) were observed in 10-week-old tissues. Maximal tissue fresh weight (4030% increase over initial fresh weight) was recorded in 5-week-old cultures after which it decreased slowly to 77.5% of the highest tissue fresh weight in 10-week-old cultures. Zeatin appeared to be the dominant endogenous cytokinin in tissues of all ages. Other cytokinins quantified were dihydrozeatin, zeatin riboside, and dihydrozeatin riboside; the values may include contributions from aglucones derived from the hydrolysis of corresponding O-glucosides, since the entire basic fraction was treated with β-glucosidase before analysis. In addition the levels of isopentenyladenine, isopentenyladenosine, and the nucleotides of zeatin riboside, dihydrozeatin riboside, and isopentenyladenosine were also determined.     

The immunochemistry of Shigella flexneri O-antigens. The biochemical basis of smooth to rough mutation

1. Smooth to rough mutation has the same biochemical basis in Shigella as in Salmonella. It is the result of enzyme defects blocking the incorporation of the O-specific side chains that characterize the smooth lipopolysaccharide with the consequent exposure of the underlying basal structures that determine `rough'-specificity. 2. The Shigella flexneri basal structure resembles its Salmonella analogue in that it has the same qualitative sugar composition, and enzyme defects in its biosynthetic pathway give rise to `rough'-lipopolysaccharides that are indistinguishable from those of Salmonella chemotypes Ra, Rb, Rc and Rd. However, the Salmonella and Shigella basal structures are not identical as judged by quantitative analysis and the absence of serological cross-reaction. 3. The Sh. flexneri basal structure side chain has been isolated and characterized as an α-N-acetylglucosaminyl-(1→4)-galactosyl-(1→3)-glucose sequence with α-glucosyl radicals substituted on the 3- and 4-positions of the galactose and glucose respectively. The different sugar types in this side chain are incorporated into the growing molecule in the same order as in Salmonella, which explains why the enzyme defects associated with smooth to rough mutation produce the same series of R-chemotypes from both genera. The terminal α-glucosyl and α-N-acetylglucosaminyl-(1→4)-galactosyl residues of the Sh. flexneri basal structure are sufficiently different from the terminal α-galactosyl and α-N-acetylglucosaminylglucosyl residues of the Salmonella analogue that they offer an explanation for the absence of serological cross-reaction between these two basal structures.