Structure and transcription analysis of the gene encoding a cellobiase from Agrobacterium sp. strain ATCC 21400

Glycosyl Cations versus Allylic Cations in Spontaneous and Enzymatic Hydrolysis

Enzymatic prenyl and glycosyl transfer are seemingly unrelated reactions that yield molecules and protein modifications with disparate biological functions. However, both reactions employ diphosphate-activated donors and each proceed via cationic species: allylic cations and oxocarbenium ions, respectively. In this study, we explore the relationship between these processes by preparing valienyl ethers to serve as glycoside mimics that are capable of allylic rather than oxocarbenium cation stabilization. Rate constants for spontaneous hydrolysis of aryl glycosides and their analogous valienyl ethers were found to be almost identical, as were the corresponding activation enthalpies and entropies. This close similarity extended to the associated secondary kinetic isotope effects (KIEs), indicating very similar transition state stabilities and structures. Screening a library of over 100 β-glucosidases identified a number of enzymes that catalyze hydrolysis of these valienyl ethers with k_cat values up to 20 s^-1. Detailed analysis of one such enzyme showed that ether hydrolysis occurs via the analogous mechanisms found for glycosides, and through a very similar transition state. This suggests that the generally lower rates of enzymatic cleavage of the cyclitol ethers reflects evolutionary specialization of these enzymes toward glycosides rather than inherent reactivity differences.

Proteolytic Cleavage Driven by Glycosylation

Proteolytic processing of human host cell factor 1 (HCF-1) to its mature form was recently shown, unexpectedly, to occur in a UDP-GlcNAc-dependent fashion within the transferase active site of O-GlcNAc-transferase (OGT) (Lazarus, M. B., Jiang, J., Kapuria, V., Bhuiyan, T., Janetzko, J., Zandberg, W. F., Vocadlo, D. J., Herr, W., and Walker, S. (2013) Science 342, 1235-1239). An interesting mechanism involving formation and then intramolecular rearrangement of a covalent glycosyl ester adduct of the HCF-1 polypeptide was proposed to account for this unprecedented proteolytic activity. However, the key intermediate remained hypothetical. Here, using a model enzyme system for which the formation of a glycosyl ester within the enzyme active site has been shown unequivocally, we show that ester formation can indeed lead to proteolysis of the adjacent peptide bond, thereby providing substantive support for the mechanism of HCF-1 processing proposed.

Non-Stick Sugars: Synthesis of Difluorosugar Fluorides as Potential Glycosidase Inactivators

Four new difluorosugar fluorides, 2-deoxy-2,5-difluoro-α-l-idopyranosyl fluoride, 1,5-difluoro-d-glucopyranosyl fluoride, 1,5-difluoro-l-idopyranosyl fluoride, and 2-deoxy-1,2-difluoro-d-glucopyranosyl fluoride, were synthesized from known precursors by a radical bromination/fluoride displacement sequence, followed by deprotection. The compounds were tested as time-dependent inactivators of the β-glucosidase from Agrobacterium sp. (Abg, EC 3.2.1.21) and, while they were shown to bind to the enzyme active site as reversible competitive inhibitors, the only time-dependent inactivation observed was traced to the presence of an extremely small amount (<0.1%) of a highly reactive contaminating impurity.

Directed Evolution of a Glycosynthase from Agrobacterium sp. Increases Its Catalytic Activity Dramatically and Expands Its Substrate Repertoire

The Agrobacterium sp. beta-glucosidase (Abg) is a retaining beta-glycosidase and its nucleophile mutants, termed Abg glycosynthases, catalyze the formation of glycosidic bonds using alpha-glycosyl fluorides as donor sugars and various aryl glycosides as acceptor sugars. Two rounds of random mutagenesis were performed on the best glycosynthase to date (AbgE358G), and transformants were screened using an on-plate endocellulase coupled assay. Two highly active mutants were obtained, 1D12 (A19T, E358G) and 2F6 (A19T, E358G, Q248R, M407V) in the first and second rounds, respectively. Relative catalytic efficiencies (kcat/Km) of 1:7:27 were determined for AbgE358G, 1D12, and 2F6, respectively, using alpha-D-galactopyranosyl fluoride and 4-nitrophenyl beta-D-glucopyranoside as substrates. The 2F6 mutant is not only more efficient but also has an expanded repertoire of acceptable substrates. Analysis of a homology model structure of 2F6 indicated that the A19T and M407V mutations do not interact directly with substrates but exert their effects by changing the conformation of the active site. Much of the improvement associated with the A19T mutation seems to be caused by favorable interactions with the equatorial C2-hydroxyl group of the substrate. The alteration of torsional angles of Glu-411, Trp-412, and Trp-404, which are components of the aglycone (+1) subsite, is an expected consequence of the A19T and M407V mutations based on the homology model structure of 2F6.

Investigation of the microheterogeneity and aglycone specificity-conferring residues of black cherry prunasin hydrolases

In black cherry (Prunus serotina Ehrh.) seed homogenates, (R)-amygdalin is degraded to HCN, benzaldehyde, and glucose by the sequential action of amygdalin hydrolase (AH), prunasin hydrolase (PH), and mandelonitrile lyase. Leaves are also highly cyanogenic because they possess (R)-prunasin, PH, and mandelonitrile lyase. Taking both enzymological and molecular approaches, we demonstrate here that black cherry PH is encoded by a putative multigene family of at least five members. Their respective cDNAs (designated Ph1, Ph2, Ph3, Ph4, and Ph5) predict isoforms that share 49% to 92% amino acid identity with members of glycoside hydrolase family 1, including their catalytic asparagine-glutamate-proline and isoleucine-threonine-glutamate-asparagine-glycine motifs. Furthermore, consistent with the vacuolar/protein body location and glycoprotein character of these hydrolases, their open reading frames predict N-terminal signal sequences and multiple potential N-glycosylation sites. Genomic sequences corresponding to the open reading frames of these PHs and of the previously isolated AH1 isoform are interrupted at identical positions by 12 introns. Earlier studies established that native AH and PH display strict specificities toward their respective glucosidic substrates. Such behavior was also shown by recombinant AH1, PH2, and PH4 proteins after expression in Pichia pastoris. Three amino acid moieties that may play a role in conferring such aglycone specificities were predicted by structural modeling and comparative sequence analysis and tested by introducing single and multiple mutations into isoform AH1 by site-directed mutagenesis. The double mutant AH ID (Y200I and G394D) hydrolyzed prunasin at approximately 150% of the rate of amygdalin hydrolysis, whereas the other mutations failed to engender PH activity.

Cloning, sequencing, and characterization of the cgmB gene of Sinorhizobium meliloti involved in cyclic beta-glucan biosynthesis

Periplasmic cyclic beta-glucans of Rhizobium species provide important functions during plant infection and hypo-osmotic adaptation. In Sinorhizobium meliloti (also known as Rhizobium meliloti), these molecules are highly modified with phosphoglycerol and succinyl substituents. We have previously identified an S. meliloti Tn5 insertion mutant, S9, which is specifically impaired in its ability to transfer phosphoglycerol substituents to the cyclic beta-glucan backbone (M. W. Breedveld, J. A. Hadley, and K. J. Miller, J. Bacteriol. 177:6346-6351, 1995). In the present study, we have cloned, sequenced, and characterized this mutation at the molecular level. By using the Tn5 flanking sequences (amplified by inverse PCR) as a probe, an S. meliloti genomic library was screened, and two overlapping cosmid clones which functionally complement S9 were isolated. A 3.1-kb HindIII-EcoRI fragment found in both cosmids was shown to fully complement mutant S9. Furthermore, when a plasmid containing this 3.1-kb fragment was used to transform Rhizobium leguminosarum bv. trifolii TA-1JH, a strain which normally synthesizes only neutral cyclic beta-glucans, anionic glucans containing phosphoglycerol substituents were produced, consistent with the functional expression of an S. meliloti phosphoglycerol transferase gene. Sequence analysis revealed the presence of two major, overlapping open reading frames within the 3.1-kb fragment. Primer extension analysis revealed that one of these open reading frames, ORF1, was transcribed and its transcription was osmotically regulated. This novel locus of S. meliloti is designated the cgm (cyclic glucan modification) locus, and the product encoded by ORF1 is referred to as CgmB.

Cloning and expression of a beta-glycosidase gene from Thermus thermophilus. Sequence and biochemical characterization of the encoded enzyme

A 3.2 kilobase pair DNA fragment from Thermus thermophilus HB27 coding for a beta-galactosidase activity was cloned and sequenced. A gene and a truncated open reading frame orf1 encoding respectively a beta-glycosidase (ttbeta-gly) and probably a sugar permease were located directly adjacent to each other. The deduced aminoacid sequence of the enzyme Ttbeta-gly showed strong identity with those of beta-glycosidases belonging to the glycosyl hydrolase family 1. The enzyme was overexpressed in Escherichia coli and was purified by a two-step purification procedure. The recombinant enzyme is monomeric with a molecular mass of 49-kDa. It catalyzes the hydrolysis of beta-D-galactoside, beta-D-glucoside and beta-D-fucoside derivatives. However, the kcat/Km ratio is much higher for p-nitrophenyl-beta-D-glucoside and p-nitrophenyl-beta-D-fucoside than for p-nitrophenyl-beta-D-galactoside. The specificity towards linkage positions of the disaccharides tested decreased in the following order: beta1-3 (100%) > beta1-2 (71%) > beta1-4 (40%) > beta1-6 (10%). Ttbeta-gly is a thermostable enzyme displaying an optimum temperature of 88 degrees C and a half life of 10 min at 90 degrees C. It performs transglycosylation reactions at high temperature with a yield exceeding 63% for transfucosylation reactions. On the basis of this work, the enzyme appears to be an attractive tool in the synthesis of fucosyl adducts and fucosyl sugars.

Mechanistic consequences of replacing the active-site nucleophile Glu-358 in Agrobacterium sp. beta-glucosidase with a cysteine residue

Retaining glycosidases achieve the hydrolysis of glycosidic bonds through the assistance of two key active-site carboxyls. One carboxyl functions as a nucleophile/leaving group, and the other acts as the acid-base catalyst. It has been suggested that a cysteine residue could fulfil the role of the active site nucleophile [Hardy and Poteete (1991) Biochemistry 30, 9457-9463]. To test the validity of this proposal, a kinetic evaluation was conducted on the active-site nucleophile cysteine mutant (Glu-358-->Cys) of the retaining beta-glucosidase from Agrobacterium sp. The Glu-358-->Cys mutant was able to complete the first step (glycosylation) of the enzymic mechanism, forming a covalent glycosyl-enzyme intermediate, but the rate constant for this step was decreased to 1/10(6) of that of the native enzyme. The subsequent hydrolysis (deglycosylation) step was also severely affected by the replacement of Glu-358 with a cysteine residue, with the rate constant being depressed to 1/10(7) or less. Thus Cys-358 functions inefficiently in both the capacity of catalytic nucleophile and leaving group. On the basis of these results it seems unlikely that the role of the active-site nucleophile in retaining glycosidases could successfully be filled by a cysteine residue.

Identification of the active site nucleophile in the thermostable beta-glycosidase from the archaeon Sulfolobus solfataricus expressed in Escherichia coli

Sulfolobus solfataricus beta-glycosidase expressed in Escherichia coli was fully inactivated at 65 degrees C, according to pseudo-first-order kinetics, by [3H]conduritol B epoxide (DL-1,2 anhydro-myo-inositol) synthesized as the active site directed inhibitor by a slight modification of Legler's procedure [Legler, G. (1977) Methods Enzymol. 46, 368-381]. The determination of kinetic constants for the inactivation showed that the process took place through the formation of a stabilized inhibitor-enzyme intermediate. Inactivation and reactivation studies suggested that the inhibitor-enzyme intermediate complex was formed more rapidly and hydrolyzed at a lower rate than it was for other glycosidases. Moreover, the stoichiometry of the binding, determined by electrospray mass spectrometric analysis, revealed that one molecule of the inhibitor was covalently bound to each enzyme subunit. The binding site for [3H]conduritol B epoxide was identified by the isolation and partial sequence analysis of the radioactive peptide obtained by cyanogen bromide and pepsin digests. Electrospray tandem mass analysis of the labeled peptide showed that the inhibitor was covalently bound to E387. This result, in agreement with data obtained from sequence alignments of S. solfataricus beta-glycosidase with other gluco- and galactosidases of the glycosyl hydrolase family 1 [Henrissat, B. (1991) Biochem. J. 280, 309-316], indicates that the conserved E387 is the nucleophilic amino acid residue in the active site of the enzyme.

Comparison of a beta-glucosidase and a beta-mannosidase from the hyperthermophilic archaeon Pyrococcus furiosus. Purification, characterization, gene cloning, and sequence analysis

Two distinct exo-acting, beta-specific glycosyl hydrolases were purified to homogeneity from crude cell extracts of the hyperthermophilic archaeon Pyrococcus furiosus: a beta-glucosidase, corresponding to the one previously purified by Kengen et al. (Kengen, S. W. M., Luesink, E. J., Stams, A. J. M., and Zehnder, A. J. B. (1993) Eur. J. Biochem. 213, 305-312), and a beta-mannosidase. The beta-mannosidase and beta-glucosidase genes were isolated from a genomic library by expression screening. The nucleotide sequences predicted polypeptides with 510 and 472 amino acids corresponding to calculated molecular masses of 59.0 and 54.6 kDa for the beta-mannosidase and the beta-glucosidase, respectively. The beta-glucosidase gene was identical to that reported by Voorhorst et al. (Voorhorst, W. G. B., Eggen, R. I. L., Luesink, E. J., and deVos, W. M. (1995) J. Bacteriol. 177, 7105-7111; GenBank accession no. U37557U37557). The deduced amino acid sequences showed homology both with each other (46.5% identical) and with several other glycosyl hydrolases, including the beta-glycosidases from Sulfolobus solfataricus, Thermotoga maritima, and Caldocellum saccharolyticum. Based on these sequence similarities, the beta-mannosidase and the beta-glucosidase can both be classified as family 1 glycosyl hydrolases. In addition, the beta-mannosidase and beta-glucosidase from P. furiosus both contained the conserved active site residues found in all family 1 enzymes. The beta-mannosidase showed optimal activity at pH 7.4 and 105 degrees C. Although the enzyme had a half-life of greater than 60 h at 90 degrees C, it is much less thermostable than the beta-glucosidase, which had a reported half-life of 85 h at 100 degrees C. Km and Vmax values for the beta-mannosidase were determined to be 0.79 mM and 31.1 micromol para-nitrophenol released/min/mg with p-nitrophenyl-beta-D-mannopyranoside as substrate. The catalytic efficiency of the beta-mannosidase was significantly lower than that reported for the P. furiosus beta-glucosidase (5.3 versus 4, 500 s-1 mM-1 with p-nitrophenyl-beta-D-glucopyranoside as substrate). The kinetic differences between the two enzymes suggest that, unlike the beta-glucosidase, the primary role of the beta-mannosidase may not be disaccharide hydrolysis. Other possible roles for this enzyme are discussed.

Molecular cloning and bacterial expression of a cDNA encoding furostanol glycoside 26-O-beta-glucosidase of Costus speciosus

Furostanol glycoside 26-O-beta-glucosidase (F26G) purified from Costus speciosus rhizomes was digested with endoproteinase, and several internal peptide fragments were obtained. Degenerate oligonucleotide primers based on amino acid sequences of the peptides were used for amplification of F26G cDNA fragments by applying nested polymerase chain reactions to cDNAs from in vitro cultured plantlets of C. speciosus. Using primers based on sequences of the cDNA fragments, the 5'- and 3'-end clones were isolated by rapid amplification of cDNA ends (RACE) methods. Finally, the entire coding portion of F26G cDNA was cloned by using primers designed from sequences of the RACE products. The deduced amino acid sequence of CSF26G1, the protein encoded by the cloned cDNA, consists of 562 amino acids and shows high homology to a widely distributed family of beta-glucosidases (BGA family). Cell-free homogenate of Escherichia coli expressing CSF26G1 cDNA showed beta-glucosidase activity specific for cleavage of the C-26 glucosidic bond of furostanol glycosides.

Construction of chimeric beta-glucosidases with improved enzymatic properties

The amino acid sequences of beta-glucosidases from Cellvibrio gilvus and Agrobacterium tumefaciens show about 40% similarity. The pH/temperature optima and stabilities and substrate specificities of the two enzymes are quite different. C. gilvus beta-glucosidase exhibits an optimum pH of 6.2-6.4 and temperature of 35 degrees C, whereas the corresponding values for A. tumefaciens are 7.2-7.4 and 60 degrees C, respectively. The substrate specificity of A. tumefaciens enzyme toward different aryl glycosides is broader than C. gilvus enzyme. To analyze these properties further, three chimeric beta-glucosidases were constructed by substituting segments from the C-terminal homologous region of C. gilvus beta-glucosidase gene with that of A. tumefaciens. The chimeric enzymes were characterized with respect to pH/temperature activity and stability and substrate specificity. Chimeric enzymes exhibited chromatographic behavior similar to that of C. gilvus enzyme. However, enzymatic properties of chimeras were admixtures of those of the two parents. The chimeric enzymes were optimally active at 45-50 degrees C and pH 6.6-7.0. Km values of chimeric enzymes for the various saccharides were admixtures of both parental enzymes. These results suggest that the two domains of C. gilvus and A. tumefaciens enzymes probably can fold independently. The homologous C-terminal region in beta-glucosidase appears to play an important role in determining enzyme characteristics. Changes in the properties on substitution of segments in this region might be related to the enzyme specificity, and beta-glucosidases with improved properties can be prepared by manipulating this region.

Temporal and spatial expression of amygdalin hydrolase and (R)-(+)-mandelonitrile lyase in black cherry seeds

In black cherry (Prunus serotina Ehrh.) macerates, the cyanogenic diglucoside (R)-amygdalin undergoes stepwise degradation to HCN catalyzed by amygdalin hydrolase (AH), prunasin hydrolase, and (R)-(+)-mandelonitrile lyase (MDL). A near full-length AH cDNA clone (pAH1), whose insert encodes the isozyme AH I, has been isolated and sequenced. AH I exhibits several features characteristic of beta-glucosidases of the BGA family, including their likely nucleophile center (isoleucine-threonine-glutamic acid-asparagine-glycine) and acid catalyst (asparagine-glutamic acid-proline/isoleucine) motifs. The temporal expression of AH and MDL in ripening fruit was analyzed by northern blotting. Neither mRNA was detectable until approximately 40 days after flowering (DAF), when embryos first became visible to the naked eye. Both mRNAs peaked at approximately 49 DAF before declining to negligible levels when the fruit matured (82 DAF). Taken together with enzyme activity data, these time courses suggest that AH and MDL expression may be under transcriptional control during fruit maturation. In situ hybridization analysis indicated that AH transcripts are restricted to the procambium, whereas MDL transcripts are localized within cotyledonary parenchyma cells. These tissue-specific distributions are consistent with the major locations of AH and MDL protein in mature seeds previously determined by immunocytochemistry (E. Swain, C.P. Li, and J.E. Poulton [1992] Plant Physiol 100:291-300).

Utilization of cellobiose and other beta-D-glucosides in Agrobacterium tumefaciens

Agrobacterium tumefaciens strain C58 was able to utilize carbon from cellobiose and some other beta-D-glucosides as efficiently as from glucose. beta-D-glucoside utilization was partially inducible and the induction was subject to catabolite repression by glucose, independently of the presence of cyclic AMP in the medium. It was also independent of Ti plasmid-encoded functions. beta-D-glucosides were hydrolysed by a single, cytoplasmic and constitutively expressed beta-glucosidase, which was active on non-phosphorylated substrates and insensitive to glucose inhibition.

The pTugA and pTugAS vectors for high-level expression of cloned genes in Escherichia coli

Plasmids pTugA and pTugAS, designed for expression of cloned genes in Escherichia coli, possess the features of high-level inducible transcription, enhanced RNA translation, portability, high copy number, stability and versatility. In addition, pTugAS can be used to produce fusion proteins comprising a target protein and a cellulose-binding domain. Such fusion proteins can be purified in a single step by affinity chromatography on cellulose. Expression of two model gene fusions using the pTug plasmids resulted in yields of 500 mg of intracellular and 250 mg of extracellular recombinant protein per liter.

Cloning and characterization of a gene encoding a cell-bound, extracellular beta-glucosidase in the yeast Candida wickerhamii

The ability of yeasts to ferment cellodextrins is rare. Candida wickerhamii is able to use these sugars for alcohol production because of a cell-bound, extracellular, beta-glucosidase that is unusual by not being inhibited by glucose. A cDNA expression library in lambda phage was prepared with mRNA isolated from cellobiose-grown C. wickerhamii. Immunological screening of the library with polyclonal antibodies against purified C. wickerhamii cell-bound, extracellular beta-glucosidase yielded 12 positive clones. Restriction endonuclease analysis and sequence data revealed that the clones could be divided into two groups, bglA and bglB, which were shown to be genetically distinct by Southern hybridization analyses. Efforts were directed at the study of bglB since it appeared to code for the cell-bound beta-glucosidase. Sequence data from both cDNA and genomic clones showed the absence of introns in bglB. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting of cell lysates from Escherichia coli bglB clones confirmed the presence of an expressed protein with an apparent molecular mass of 72 kDa, which is consistent with that expected for an unglycosylated form of the enzyme. Amino acid comparisons of BglB with other beta-glucosidase sequences suggest that it is a member of family 1 glycosyl hydrolases but is unusual in that it contains an additional 100 to 130 amino acids at the N terminus. This sequence did not have homologies to other known protein sequences and may impart unique properties to this beta-glucosidase.

Construction and characterization of a chimeric beta-glucosidase

The amino acid sequences of beta-glucosidases from Cellvibrio gilvus and Agrobacterium tumefaciens show significant similarity in most of the parts. However, the pH/temperature optima and stabilities of the two enzymes are quite different. C. gilvus beta-glucosidase exhibits an optimum pH of 6.2-6.4 and temperature of 35 degrees C, whereas the corresponding values for A. tumefaciens are 7.2-7.4 and 60 degrees C respectively. To analyse these properties further, a chimeric beta-glucosidase was constructed by replacing a segment from the C-terminal region of C. gilvus beta-glucosidase gene with that of A. tumefaciens. The partially purified chimeric enzyme was characterized with respect to pH/temperature activity and stability and substrate affinity. Our results suggest that C-terminal segment(s) might be important in beta-glucosidase specificity, and shuffling of even a small segment of gene in this region might significantly alter or improve the enzymic properties such as thermal stability.

A beta-glucosidase gene (bgl3) from Streptomyces sp. strain QM-B814. Molecular cloning, nucleotide sequence, purification and characterization of the encoded enzyme, a new member of family 1 glycosyl hydrolases

A beta-glucosidase gene (bgl3) from Streptomyces sp. QM-B814 (American Type Culture Collection 11238) has been cloned by functional complementation of a beta-glucosidase-negative mutant of Streptomyces lividans. An open-reading frame of 1440 nucleotides encoding a polypeptide of 479 amino acids was found by sequencing. The encoded protein (Bgl3) shows extensive similarity (over 45% identity) with beta-glycosidases from family-1 glycosyl hydrolases. The cloned enzyme, purified following ammonium sulphate precipitation and two chromatographic steps, is monomeric with molecular mass 52.6 kDa, as determined by mass spectrometry, and an isoelectric point of pI 4.4. The enzyme appears to be a beta-glucosidase with broad substrate specificity, is active on cellooligomers, and performs transglycosylation reactions. The estimated apparent Km values for p-nitrophenyl-beta-D-glucopyranoside and cellobiose are 0.27 mM and 7.9 mM, respectively. The Ki values for glucose and delta-gluconolactone, using p-nitrophenyl-beta-D-glucopyranoside as a substrate, are 65 mM and 0.08 mM, respectively. The purified enzyme has a pH optimum of pH 6.5 and the temperature optimum for activity is 50 degrees C.

Barley beta-glucosidase: expression during seed germination and maturation and partial amino acid sequences

Unlike most of the hydrolytic enzymes that participate in endosperm mobilization, beta-glucosidase of barley (Hordeum vulgare) seeds does not increase during germination, even in the presence of exogenously added gibberellic acid. However, the germination process affects the physical properties of beta-glucosidase in terms of charge and apparent molecular weight. Analysis of developing barley grains shows that the enzyme is synthesized two weeks before maturation and is stored in the endosperm of the dry dormant seed. Partial amino acid sequencing of the purified beta-glucosidase demonstrates significant similarity between the barley enzyme and beta-glycosidases that belong to family 1 of glycosyl hydrolases.

Purification, characterization, gene cloning, and sequencing of a new beta-glucosidase from Bacillus circulans subsp. alkalophilus

An intracellular beta-glucosidase was purified from cell extracts of Bacillus circulans subsp. alkalophilus by NAD affinity and high-performance anion-exchange chromatographies. The enzyme was active against a wide range of aryl-beta-glucosides and beta-linked disaccharides. The structural gene for beta-glucosidase was cloned in Escherichia coli. The beta-glucosidase gene consisted of an open reading frame of 1,350 bp encoding a protein of 450 amino acids with a calculated M(r) of 51,303. The enzyme exhibited from 45 to 66% identity with five bacterial beta-glucosidases.

Characterization of a Brassica napus myrosinase pseudogene: myrosinases are members of the BGA family of beta-glycosidases

Myrosinase isoenzymes are known to be encoded by two different families of genes denoted MA and MB. Nucleotide sequence analysis of a Brassica napus genomic clone containing a gene for myrosinase revealed it to be a pseudogene of the MA family. The gene spans more than 5 kb and contains at least 12 exons. The exon sequence of the gene is highly similar to myrosinase cDNA sequences. However, the gene displays three potential or actual pseudogene characters. Southern blot analysis using probes from the 3' portions of the genomic and B. napus MA and MB cDNA clones showed that MA type myrosinases are encoded by approximately 4 genes, while MB type myrosinases are encoded by more than 10 genes in B. napus. Northern blots with mRNA from seeds and young leaves probed with the MA- and MB-specific probes showed that the MA and MB myrosinase gene families are differentially expressed. Myrosinases are highly similar to proteins of a beta-glycosidase enzyme family comprising both beta-glycosidases and phospho-beta-glycosidases of as diverged species as archaebacteria, bacteria, mammals and plants. By homology to these beta-glycosidases, putative active site residues in myrosinase are discussed on the basis of the similarity between beta-glycosidases and cellulases.

Cloning, characterization, and nucleotide sequence of a gene encoding Microbispora bispora BglB, a thermostable beta-glucosidase expressed in Escherichia coli

Genomic DNA fragments encoding beta-glucosidase activities of the thermophilic actinomycete Microbispora bispora were cloned into Escherichia coli. Transformants expressing beta-glucosidase activity were selected by their ability to hydrolyze the fluorogenic substrate 4-methylumbelliferyl-beta-D-glucoside. Two genes encoding beta-glucosidase activity were isolated and distinguished by restriction analysis, Southern hybridization, and the substrate specificities of the encoded enzymes. One gene, bglB, encoded a beta-glucosidase that was expressed intracellularly in E. coli. It exhibited a molecular mass of approximately 52,000 Da by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) and 51,280 Da by nondenaturing gradient PAGE, a pI of 4.6, and temperature and pH optima of 60 degrees C and 6.2, respectively. Cloned BglB showed greater activity against cellobiose than against aryl-beta-D-glucosides and was thermostable, retaining about 70% of its activity after 48 h at 60 degrees C. BglB activity is activated two- to threefold in the presence of 2 to 5% (0.1 to 0.3 M) glucose. The DNA sequence of the 2.2-kb insert carrying bglB has been determined. An open reading frame which codes for a protein of 473 amino acids with a predicted molecular mass of 52,227 Da showed significant homology (40 to 47% identity) with beta-glucosidases from glycosal hydrolase family 1.

Characterization of the gene celD and its encoded product 1,4-beta-D-glucan glucohydrolase D from Pseudomonas fluorescens subsp. cellulosa

A genomic library of Pseudomonas fluorescens subsp. cellulosa DNA constructed in pUC18 and expressed in Escherichia coli was screened for recombinants expressing 4-methylumbelliferyl beta-D-glucoside hydrolysing activity (MUGase). A single MUGase-positive clone was isolated. The MUGase hydrolysed cellobiose, cellotriose, cellotetraose, cellopentaose and cellohexaose to glucose, by sequentially cleaving glucose residues from the non-reducing end of the cello-oligosaccharides. The Km values for cellobiose and cellohexaose hydrolysis were 1.2 mM and 28 microM respectively. The enzyme exhibited no activity against soluble or insoluble cellulose, xylan and xylobiose. Thus the MUGase is classified as a 1,4-beta-D-glucan glucohydrolase (EC 3.2.1.74) and is designated 1,4-beta-D-glucan glucohydrolase D (CELD). When expressed by E. coli, CELD was located in the cell-envelope fraction; a significant proportion of the native enzyme was also associated with the cell envelope when synthesized by its endogenous host. The nucleotide sequence of the gene, celD, which encodes CELD, revealed an open reading frame of 2607 bp, encoding a protein of M(r) 92,000. The deduced primary structure of CELD was confirmed by the M(r) of CELD (85,000) expressed by E. coli and P. fluorescens subsp. cellulosa, and by the experimentally determined N-terminus of the enzyme purified from E. coli, which showed identity with residues 52-67 of the celD translated sequence. The structure of the N-terminal region of full-length CELD was similar to the signal peptides of P. fluorescens subsp. cellulosa plant-cell-wall hydrolases. Deletion of the N-terminal 47 residues of CELD solubilized MUGase activity in E. coli. CELD exhibited sequence similarity with beta-glucosidase B of Clostridium thermocellum, particularly in the vicinity of the active-site aspartate residue, but did not display structural similarity with the mature forms of cellulases and xylanases expressed by P. fluorescens subsp. cellulosa.

Cloning and sequencing of an Agrobacterium tumefaciens beta-glucosidase gene involved in modifying a vir-inducing plant signal molecule

Induction of Agrobacterium tumefaciens virulence genes by plant phenolic compounds is essential for successful T-DNA transfer to a host plant. In Douglas fir needles, the major virulence region inducer is the glycoside coniferin (J. W. Morris and R. O. Morris, Proc. Natl. Acad. Sci. USA 87:3612-3618, 1990). Agrobacterium strains with high beta-glucosidase activity respond to coniferin and infect Douglas fir seedlings, whereas most strains with low beta-glucosidase activity fail to respond to coniferin and are avirulent on this host. We have cloned two beta-glucosidase genes from A. tumefaciens B3/73 and sequenced one of them, cbg1. It appears to be part of a polycistronic unit and shows a high bias for GC-rich codons. When expressed in Escherichia coli, Cbg1 beta-glucosidase hydrolyzes coniferin but not cellobiose. The 88-kDa predicted product of cbg1 is highly similar to one other bacterial beta-glucosidase and several fungal beta-glucosidases. There is little homology between Cbg1 and other bacterial beta-glucosidases, including an Agrobacterium cellobiase.

Nucleotide sequences of the arb genes, which control beta-glucoside utilization in Erwinia chrysanthemi: comparison with the Escherichia coli bgl operon and evidence for a new beta-glycohydrolase family including enzymes from eubacteria, archeabacteria, and humans

The phytopathogenic bacterium Erwinia chrysanthemi, unlike other members of the family Enterobacteriaceae, is able to metabolize the beta-glucosides, arbutin, and salicin. A previous genetic analysis of the E. chrysanthemi arb genes, which mediate beta-glucoside metabolism, suggested that they were homologous to the Escherichia coli K-12 bgl genes. We have now determined the nucleotide sequence of a 5,065-bp DNA fragment containing three genes, arbG, arbF, and arbB. Deletion analysis, expression in minicell systems, and comparison with sequences of other proteins suggest that arbF and arbB encode a beta-glucoside-specific phosphotransferase system-dependent permease and a phospho-beta-glucosidase, respectively. The ArbF amino acid sequence shares 55% identity with that of the E. coli BglF permease and contains most residues thought to be important for a phosphotransferase. One change, however, was noted, since BglF Arg-625, presumably involved in phosphoryl transfer, was replaced by a Cys residue in ArbF. An analysis of the ArbB sequence led to the definition of a protein family which contained enzymes classified as phospho-beta-glucosidases, phospho-beta-galactosidases, beta-glucosidases, and beta-galactosidases and originating from gram-positive and gram-negative bacteria, archebacteria, and mammals, including humans. An analysis of this family allowed us (i) to speculate on the ways that these enzymes evolved, (ii) to identify a glutamate residue likely to be a key amino acid in the catalytic activity of each protein, and (iii) to predict that domain II of the human lactate-phlorizin hydrolase, which is involved in lactose intolerance, is catalytically nonactive. A comparison between the untranslated regions of the E. chrysanthemi arb cluster and the E. coli bgl operon revealed the conservation of two regions which, in the latter, are known to terminate transcription under noninducing conditions and be the target of the BglG transcriptional antiterminator under inducing conditions. ArbG was found to share a high level of similarity with the BglG antiterminator as well as with Bacillus subtilis SacT and SacY antiterminators, suggesting that ArbG functions as an antiterminator in regulating the expression of the E. chrysanthemi arb genes.

Structure of the beta-glucosidase gene bglA of Clostridium thermocellum. Sequence analysis reveals a superfamily of cellulases and beta-glycosidases including human lactase/phlorizin hydrolase

The nucleotide sequence of the Clostridium thermocellum gene bglA, coding for the thermostable beta-glucosidase A, has been determined. The coding region of 1344 bp was identified by comparison with the N-terminal amino acid squence of recombinant beta-glucosidase A purified from Escherichia coli. The deduced amino acid sequence corresponds to a protein of 51,482 Da. The coding region is flanked by putative promoter and transcription terminator sequences. The protein is unrelated to beta-glucosidase B of C. thermocellum, but has a high level of similarity with other bacterial beta-glucosidases and phospho-beta-glucosidases. Similarity is also observed with the beta-galactosidase of the archaebacterium Sulfolobus solfataricus. Unexpectedly, it was found that human lactase-phlorizin hydrolase contains three copies of a sequence closely related to C. thermocellum beta-glucosidase A (up to 40% sequence identity). These diverse beta-glucosidases can therefore be grouped into an enzyme family (BGA) of common structural design. Sequence comparison by hydrophobic cluster analysis revealed that all BGA enzymes share a well conserved region which is homologous to the catalytic domain of the widely distributed cellulase family A. A distinctive feature of this domain is the sequence motif His-Asn-Glu-Pro in which the catalytic residues His and Glu are separated by 35-55 amino acid residues. The cellulase family A and the beta-glucosidase family BGA might thus be considered as members of a protein super-family comprising beta-glucanases and beta-glycosidases from all three primary kingdoms of living organisms.

Nucleotide and derived amino acid sequence of the cyanogenic beta-glucosidase (linamarase) from white clover (Trifolium repens L.)

The nucleotide sequence and derived amino acid sequence of two different beta-glucosidase cDNA clones were determined. One clone (TRE104) was identified as the cyanogenic beta-glucosidase by homology with the N-terminal and internal peptide amino acid sequence of the purified enzyme. The biological function of the other beta-glycosidase (TRE361) is not known. Co-segregation of genomic restriction fragments uniquely identified by each cDNA clone shows that these two genes are linked in the white clover genome. Both TRE104 and TRE361 fragments co-segregate with cyanogenic beta-glucosidase activity. Extensive homology was found between the white clover beta-glucosidase sequences and a group of prokaryote and mammalian beta-glycosidases. This group of sequences has no homology with a separate set of beta-glucosidase genes isolated from fungi and the thermophilic bacterium Clostridium thermocellum.

Cloning and amplification of the gene encoding an extracellular beta-glucosidase from Trichoderma reesei: evidence for improved rates of saccharification of cellulosic substrates

We have cloned and determined the nucleotide sequence of the gene encoding an extracellular beta-glucosidase (bgl1) from the cellulolytic fungus Trichoderma reesei. The predicted open reading frame of the bgl1 gene is interrupted by two putative introns of 70 and 64 bp and encodes a protein with a calculated molecular weight of 75,341. The genomic segment encoding bgl1 was cloned into a vector that contained the selectable marker gene, amdS. Transformation of T. reesei with this vector resulted in several stable transformant strains all possessing an increased copy number of the bgl1 gene integrated into the genome together with elevated rates of glucose production from avicel. One transformant produced an extracellular cellulase with a five-fold increase in the rate of production of glucose from cellobiose, a 33% rate increase from avicel, and a 17% increase from phosphoric acid swollen cellulose. These data suggest that the cellulolytic activity of T. reesei strains may be specifically improved by transformation with cloned cellulase genes.

Molecular cloning, expression and nucleotide sequence of the endo-beta-1,3-1,4-D-glucanase gene from Bacillus licheniformis. Predictive structural analyses of the encoded polypeptide

A Bacillus licheniformis gene coding for an endo-beta-1,3-1,4-D-glucanase have been cloned in Escherichia coli and sequenced. The open reading frame contains a sequence of 731 bp, encoding a polypeptide of 243 amino acid residues, with a molecular mass of 27404 Da (24418 Da without the putative signal peptide), which corresponds to the enzyme we had previously isolated and characterized. The signal peptide is functional in E. coli. More than 60% of the endo-beta-1,3-1,4-D-glucanase activity is extracellular or periplasmic. The polypeptide is highly similar to other reported Bacillus beta-glucanases. Several structural predictive analyses (secondary structure, hydropathic plots, similarity with other related enzymes, etc.) have been performed. From these analyses we assign a tentative three-functional-domain structure for the enzyme (signal peptide, substrate binding and catalytic domains) and a putative lysozyme-like active site.

Sequences and homology analysis of two genes encoding beta-glucosidases from Bacillus polymyxa

The nucleotide sequences of the bglA and bglB genes encoding beta-glucosidases from Bacillus polymyxa have been determined. Both genes contain coding regions of 1344 bp, corresponding to polypeptides with Mrs of 51,643 and 51,547, respectively. Patterns of codon usage indicate that both genes are expressed at a low frequency. Previous data suggested that the proteins encoded by bglA and bglB were intra- and extracellular enzymes, respectively; however, neither of the two deduced amino acid sequences has N termini with the typical features of a leader peptide. The proteins encoded by bglA and bglB show remarkable homology to each other and to other beta-glucosidases (Bgl) and beta-galactosidases (beta Gal). On the basis of the observed homologies, we can define two groups of microbial Bgl: one of them, type I, including most bacterial Bgl, and type II, including enzymes from different yeast species and one from Clostridium thermocellum. Likewise, at least two groups of beta Gal can be distinguished: type I, including enzymes homologous to type-I Bgl, and type II, showing no homology to any of the previous groups.

CelS: a novel endoglucanase identified from Erwinia carotovora subsp. carotovora

A plasmid clone expressing a beta(1,4)-glucan glucanohydrolase (EC 3.2.1.4; endoglucanase) in Escherichia coli was isolated from a genomic library of Erwinia carotovora subsp. carotovora. The DNA segment carrying the corresponding structural gene, named celS, contained an open reading frame encoding a 264-amino acid (aa) polypeptide. The N-terminal aa sequence of CelS showed that the protein was synthesized with a 32-aa cleavable signal peptide. The mature 232-aa CelS had a calculated Mr of 26,228 and pI of 5.5. The pH optimum was about 6.8 and the temperature optimum was between 45 and 55 degrees C. Comparison of the aa sequence of CelS by hydrophobic cluster analysis with a range of cellulases and other quasi-isofunctional enzymes revealed only very limited sequence similarities, suggesting that the CelS protein may represent the first member of an additional cellulase family.

Nucleotide sequence of the Clostridium thermocellum bgIB gene encoding thermostable beta-glucosidase B: homology to fungal beta-glucosidases

The nucleotide sequence of the bglB gene, coding for the thermostable beta-glucosidase B of Clostridium thermocellum was determined. The coding region of 2265 bp was identified by comparison with the N-terminal amino acid sequence of beta-glucosidase B purified from Escherichia coli. The derived amino acid sequence corresponding to a polypeptide of Mr 84,100 was confirmed by sequencing of the C-terminal peptide generated by cleavage with cyanogen bromide. The protein bears no resemblance to other bacterial beta-glucosidase sequences. However, extensive regions of homology were identified between the C. thermocellum enzyme and fungal beta-glucosidases. The N-terminal homologous region contains an amino acid sequence very similar to the active site of beta-glucosidase A3 from Aspergillus wentii. The striking sequence similarities between C. thermocellum beta-glucosidase B and Kluyveromyces fragilis beta-glucosidase suggest the possibility of a genetic exchange between thermophilic anaerobic bacteria and yeasts.