Characterization of levJ, a sucrase/fructanase-encoding gene from Actinomyces naeslundii T14V, and comparison of its product with other sucrose-cleaving enzymes

The Complex GH32 Enzyme Orchestra from Priestia megaterium Holds the Key to Better Discriminate Sucrose-6-phosphate Hydrolases from Other β-Fructofuranosidases in Bacteria

Inulin is widely used as a prebiotic and emerging as a priming compound to counteract plant diseases. We isolated inulin-degrading strains from the lettuce phyllosphere, identified as Bacillus subtilis and Priestia megaterium, species hosting well-known biocontrol organisms. To better understand their varying inulin degradation strategies, three intracellular β-fructofuranosidases from P. megaterium NBRC15308 were characterized after expression in Escherichia coli: a predicted sucrose-6-phosphate (Suc6P) hydrolase (SacAP1, supported by molecular docking), an exofructanase (SacAP2), and an invertase (SacAP3). Based on protein multiple sequence and structure alignments of bacterial glycoside hydrolase family 32 enzymes, we identified conserved residues predicted to be involved in binding phosphorylated (Suc6P hydrolases) or nonphosphorylated substrates (invertases and fructanases). Suc6P hydrolases feature positively charged residues near the structural catalytic pocket (histidine, arginine, or lysine), whereas other β-fructofuranosidases contain tryptophans. This correlates with our phylogenetic tree, grouping all predicted Suc6P hydrolases in a clan associated with genomic regions coding for transporters involved in substrate phosphorylation. These results will help to discriminate between Suc6P hydrolases and other β-fructofuranosidases in future studies and to better understand the interaction of B. subtilis and P. megaterium endophytes with sucrose and/or fructans, sugars naturally present in plants or exogenously applied in the context of defense priming.

Characterization of an exo-inulinase from Arthrobacter: a novel NaCl-tolerant exo-inulinase with high molecular mass

A glycoside hydrolase family 32 exo-inulinase gene was cloned from Arthrobacter sp. HJ7 isolated from saline soil located in Heijing town. The gene encodes an 892-residue polypeptide with a calculated mass of 95.1 kDa and a high total frequency of amino acid residues G, A, and V (30.0%). Escherichia coli BL21 (DE3) cells were used as hosts to express the exo-inulinase gene. The recombinant exo-inulinase (rInuAHJ7) showed an apparently maximal activity at pH 5.0-5.5 and 40-45°C. The addition of 1.0 and 10.0 mM Zn(2+) and Pb(2+) had little or no effect on the enzyme activity. rInuAHJ7 exhibited good salt tolerance, retaining more than 98% inulinase activity at a concentration of 3.0%-20.0% (w/v) NaCl. Fructose was the main product of inulin, levan, and Jerusalem artichoke tubers hydrolyzed by the enzyme. The present study is the first to report the identification and characterization of an Arthrobacter sp exo-inulinase showing a high molecular mass of 95.1 kDa and NaCl tolerance. These results suggest that the exo-inulinase might be an alternative material for potential applications in processing seafood and other foods with high saline contents, such as marine algae, pickles, and sauces.

Functional characterization and evolutionary implication of the internal 157-amino-acid sequence of an exoinulinase from Penicillium sp. strain TN-88

An extracellular exoinulinase from the filamentous fungus Penicillium sp. strain TN-88 has a 14-fold higher specific activity of 743 U/mg toward inulin than its equivalent from the Aspergillus niger strain 12 and possesses an internal 157-amino-acid sequence whose corresponding region is absent in the A. niger enzyme. On the basis of sequence alignment, the internal region D' encoding the 157-amino-acid sequence in the Penicillium exoinulinase gene inuD cDNA was inserted into the site between the nucleotides 897 and 898 of the A. niger exoinulinase gene inuE cDNA. The resultant inuE::D' fusion was expressed in the methylotrophic yeast Pichia pastoris. The K(m) value of the secreted hybrid enzyme InuE::D' for inulin hydrolysis was about 1/15 that of the A. niger InuE, whereas its k(cat) value did not differ greatly from that of the InuE. These observations indicate that the Penicillium exoinulinase has evolved by the horizontal transfer and integration of a relevant DNA segment and that the internal sequence D' functions as an additional noncatalytic inulin-affinity region.

Molecular cloning and characterization of an exoinulinase gene from Aspergillus niger strain 12 and its expression in Pichia pastoris

A genomic DNA segment and cDNAs encoding an extracellular exoinulinase from Aspergillus niger strain 12 were cloned and sequenced. Southern blot analysis indicated that the exoinulinase gene (inuE) was present as a single copy in the genome. An open reading frame of 1611 by was interrupted by a single intron of 60 bp, and encoded a 19-amino acid signal peptide and a 518-amino acid mature protein. The mature protein contained a single Cys residue and nine potential N-linked glycosylation sites. Three distinct transcription start points were observed at positions -41 (A), -35 (A), and -31 (A) from the start codon. The 5'-noncoding region had a putative TATA at position -75 (TATAAA). Transcription of the inuE gene was induced by inulin or sucrose and repressed by fructose or glucose. The inuE cDNA was functionally expressed under the control of the alcohol oxidase gene promoter in the methylotrophic yeast Pichia pastoris. The deduced amino acid sequence of the inuE gene product was 91% identical to that of an exoinulinase from Aspergillus awamori. A neighbor-joining tree showed that exo- and endoinulinases found in Aspergillus and Penicillium spp. have independently evolved the respective hydrolytic activities toward terminal and internal beta-2,1-fructofuranosidic linkages in inulin.

Roles of fructosyltransferase and levanase-sucrase of Actinomyces naeslundii in fructan and sucrose metabolism

The ability of Actinomyces naeslundii to convert sucrose to extracellular homopolymers of fructose and to catabolize these types of polymers is suspected to be a virulence trait that contributes to the initiation and progression of dental caries and periodontal diseases. Previously, we reported on the isolation and characterization of the gene, ftf, encoding the fructosyltransferase (FTF) of A. naeslundii WVU45. Allelic exchange mutagenesis was used to inactivate ftf, revealing that FTF-deficient stains were completely devoid of the capacity to produce levan-type (beta2,6-linked) polysaccharides. A polyclonal antibody was raised to a histidine-tagged, purified A. naeslundii FTF, and the antibody was used to localize the enzyme in the supernatant fluid. A sensitive technique was developed to detect levan formation by proteins that had been separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the method was used to confirm that the levan-synthesizing activity of A. naeslundii existed predominantly in a cell-free form, that a small amount of the activity was cell associated, and that the ftf mutant was unable to produce levans. By using the nucleotide sequence of the levanase gene of a genospecies 2 A. naeslundii, formerly Actinomyces viscosus, a portion of a homologue of this gene (levJ) was amplified by PCR and inserted into a suicide vector, and the resulting construct was used to inactivate the levJ gene in the genospecies 1 strain WVU45. A variety of physiologic and biochemical studies were performed on the wild-type and LevJ-deficient strains to demonstrate that (i) this enzyme was the dominant levanase and sucrase of A. naeslundii; (ii) that LevJ was inducible by growth in sucrose; (iii) that the LevJ activity was found predominantly (>90%) in a cell-associated form; and (iv) that there was a second, fructose-inducible fructan hydrolase activity produced by these strains. The data provide the first detailed molecular analysis of fructan production and catabolism in this abundant and important oral bacterium.

Cloning, expression, and characterization of a neuraminidase gene from Arcanobacterium pyogenes

Arcanobacterium pyogenes is an opportunistic pathogen, associated with suppurative infections in domestic animals. In addition to pyolysin, a pore-forming, cholesterol-binding toxin, A. pyogenes expresses a number of putative virulence factors, including several proteases and neuraminidase activity. A 3,009-bp gene, nanH, was cloned and sequenced and conferred neuraminidase activity on an Escherichia coli host strain. The predicted 107-kDa NanH protein displayed similarity to a number of bacterial neuraminidases and contained the RIP/RLP motif and five copies of the Asp box motif found in all bacterial neuraminidases. Recombinant His-tagged NanH was found to have pH and temperature optima of 5.5 to 6.0 and 55 degrees C, respectively. Insertional deletion of the nanH gene resulted in the reduction, but not absence, of neuraminidase activity, indicating the presence of a second neuraminidase gene in A. pyogenes. NanH was localized to the A. pyogenes cell wall. A. pyogenes adhered to HeLa, CHO, and MDBK cells in a washing-resistant manner. However, the nanH mutant was not defective for adherence to epithelial cells. The role of NanH in host epithelial cell adherence may be masked by the presence of a second neuraminidase in A. pyogenes.

Molecular cloning and sequence analysis of an endoinulinase gene from Penicillium sp. strain TN-88

A genomic DNA segment and cDNAs encoding an extracellular endoinulinase of Penicillium sp. strain TN-88 were cloned and sequenced. Southern blot analysis indicated that the endoinulinase gene (inuC) was present as a single copy in the genome. An open reading frame, consisting of 1,545 bp, was not interrupted by introns, and it encoded a 25 amino acid signal peptide and a 490 amino acid mature protein. The mature protein contained three Cys residues and ten potential N-linked glycosylation sites. Three distinct transcriptional start points were observed at positions -242 (A), -215 (A), and -75 (C) from the start codon. The 5'-noncoding region had a putative TATA box at position -120 (TATATATA) and two contiguous CAAT sequences at -159 to -151. The deduced amino acid sequence showed 72 and 85% identities with those of Aspergillus niger and Penicillium purpurogenum endoinulinase genes, respectively. A neighbor-joining tree showed that fungal endoinulinases form a distinct cluster from other members of the beta-fructofuranosidase superfamily and that they are more closely related to bacterial levanases than to a fungal fructosyltransferase, yeast invertases, or a yeast exoinulinase.

Characterization of the fructosyltransferase gene of Actinomyces naeslundii WVU45

Oral actinomycetes produce fructosyltransferase (FTF) enzymes which convert sucrose into polymers of D-fructose, known as levans, and these polymers are thought to contribute to the persistence and virulence of the organisms. A gene encoding FTF was isolated from Actinomyces naeslundii WVU45; the deduced amino acid sequence showed significant similarity to known levansucrases of gram-negative environmental isolates but was less similar to FTFs from gram-positive bacteria. A transcriptional start site was mapped by primer extension 70 bp 5' from the putative start codon. Promoter fusions to a chloramphenicol acetyltransferase gene were used to confirm that there was a functional promoter driving ftf expression and to show that sequences located 86 to 218 bp upstream of the transcription initiation site were required for optimal ftf expression. Quantitative slot blot analysis against total RNA from cells grown on different sugars or from different growth phases revealed that ftf was constitutively transcribed. Thus, the A. naeslundii FTF is more similar in primary sequence and the regulation of expression to levansucrases of gram-negative bacteria than gram-positive bacteria.

Molecular and genetic analyses of Actinomyces spp

Members of the genus Actinomyces are predominant primary colonizers of the oral cavity and play an important role in initiating plaque development. These bacteria have evolved unique mechanisms that favor colonization and persistence in this micro-environment. The expression of cell-surface fimbriae is correlated with the ability of these bacteria to adhere to specific receptors on the tooth and mucosal surfaces, and to interact with other plaque bacteria. The elaboration of sialidase is thought to enhance fimbriae-mediated adherence by unmasking the fimbrial receptors on mammalian cells. The presence of certain cell-associated or extracellular enzymes, including those involved in sucrose or urea metabolism, may provide the means for these bacteria to thrive under conditions when other growth nutrients are not available. Moreover, these enzyme activities may influence the distribution of other plaque bacteria and promote selection for Actinomyces spp. in certain ecological niches. The recent development of a genetic transfer system for Actinomyces spp. has allowed for studies the results of which demonstrate the existence of multiple genes involved in fimbriae synthesis and function, and facilitated the construction of allelic replacement mutants at each gene locus. Analyses of these mutants have revealed a direct correlation between the synthesis of assembled fimbriae and the observed adherence properties. Further genetic analysis of the various enzyme activities detected from strains of Actinomyces should allow for an assessment of the role of these components in microbial ecology, and their contribution to the overall success of Actinomyces spp. as a primary colonizer and a key player in oral health and disease.

Genetic and physiologic characterization of urease of Actinomyces naeslundii

Ammonia production from urea by ureolytic oral bacteria is believed to have a significant impact on oral health and the ecological balance of oral microbial populations. In this study we cloned and characterized the urease gene cluster of Actinomyces naeslundii, which is one of the pioneer organisms in the oral cavity and a significant constituent of supragingival and subgingival dental plaque in children and adults. An internal fragment of the ureC gene of A. naeslundii WVU45 was initially amplified by PCR with degenerate primers derived from conserved amino acid sequences of the large catalytic subunit of urease in bacteria and plants. The PCR product was then used as a probe to identify recombinant bacteriophages carrying the A. naeslundii urease gene cluster and roughly 30 kbp of flanking DNA. Nucleotide sequence analysis demonstrated that the gene cluster was comprised of seven contiguously arranged open reading frames with significant homologies at the protein and nucleotide sequence levels to the ureABCEFGD genes from other organisms. By using primer extension, a putative transcription initiation site was mapped at 66 bases 5' to the start codon of ureA. A urease-deficient strain was constructed by insertion of a kanamycin resistance determinant within the ureC gene via allelic replacement. In contrast to the wild-type organism, the isogenic mutant was unable to grow in a semidefined medium supplemented with urea as the nitrogen source and was not protected by the addition of urea against killing in moderately acidic environments. These data indicated that urea can be effectively utilized as a nitrogen source by A. naeslundii via a urease-dependent pathway and that ureolysis can protect A. naeslundii against environmental acidification at physiologically relevant pH values. Therefore, urease could confer to A. naeslundii critical selective advantages over nonureolytic organisms in dental plaque, constituting an important determinant of plaque ecology.

Cloning and sequence analysis of the gene encoding invertase (INV1) from the yeast Candida utilis

The gene INV1 encoding invertase from the yeast Candida utilis has been cloned using a homologous PCR hybridization probe, amplified with two sets of degenerate primers designed considering sequence comparisons between yeast invertases. The cloned gene was sequenced and found to encode a polypeptide of 533 amino acids that contain a 26 amino-acid signal peptide and 12 potential N-glycosylation sites. The nucleotide sequences of the 5' and 3' non-coding regions were found to contain motifs probably involved in initiation, regulation and termination of gene transcription. The amino-acid sequence shows significant identity with other yeast, bacterial and plant beta-fructofuranosidases. The INV1 gene from C. utilis was able to complement functionally the suc2 mutation of S. cerevisiae.

Production of 1-kestose in transgenic yeast expressing a fructosyltransferase from Aspergillus foetidus

Sucrose-inducible secretory sucrose:sucrose 1-fructosyltransferase (1-SST) from Aspergillus foetidus has been purified and subjected to N-terminal amino acid sequence determination. The enzyme is extensively glycosylated, and the active form is probably represented by a dimer of identical subunits with an apparent molecular mass of 180 kDa as judged from mobility in seminative acrylamide gels. The enzyme catalyzes fructosyl transfer from sucrose to sucrose producing glucose and 1-kestose. Oligosaccharides with a higher degree of polymerization are not obtained with sucrose as the substrate. The cDNA encoding the A. foetidus 1-SST has been cloned and sequenced. Sequence homology was found to be highest to levanases, but no hydrolytic activity was observed when levan was incubated with the enzyme. Expression of the cloned gene in an invertase-deficient mutant of Saccharomyces cerevisiae resulted in 1-kestose production, with 6-kestose and neokestose being side products of the reaction. Products were well distinguishable from those formed by yeast transformants expressing a cytosolic invertase.

Characterization of a novel endo-levanase and its gene from Bacillus sp. L7

A novel endo-levanase producing bacterium belonging to the Bacillus family has been isolated from soil. The enzyme was characterized and found to have no exo-beta-fructofuranosidase activity. The endo-levanase gene was cloned and sequenced. Homology searches have shown that the C-terminal domain of the enzyme is homologous to a number of known beta-fructofuranosidases, including exo-levanase from Bacillus subtilis and yeast invertases. The N-terminal region of the endo-levanase which is homologous to the C-terminal sequence of the B. subtilis levanase appears to be a levan-binding domain.

Synthesis and function of Actinomyces naeslundii T14V type 1 fimbriae require the expression of additional fimbria-associated genes

The nucleotide sequence of the chromosomal DNA flanking the Actinomyces naeslundii (formerly A. viscosus) T14V type 1 fimbrial structural subunit gene (fimP) was determined. Six open reading frames (ORFs), in the order 5' ORF3, ORF2, ORF1,fimP, ORF4, ORF5, ORF6 3', were identified. ORF1 encoded a protein of 408 amino acid residues (Mr = 39,270) and had significant sequence homology with the A. naeslundii T14V type 1 and A. naeslundii WVU45 type 2 fimbrial structural subunits. An in-frame fusion of ORF1 to the malE gene of the expression vector, pMAL-c2, yielded a protein that was immunostained with antibodies raised against the maltose binding protein and A. naeslundii T14V whole bacteria. Digestion of the fusion protein with factor Xa released a protein (apparent molecular mass of 34 kDa) that was immunostained only with the antibody directed against A. naeslundii T14V whole bacterial cells. Integration plasmids carrying a kanamycin resistance gene (kan) that was used to substitute for ORF1 or for DNA fragments internal to the coding region of the other five ORFs were used to transform A. naeslundii T14V. Neither type 1 fimbriae nor the 65-kDa fimbrial structural subunit was detected in mutants obtained by allelic replacement of ORF1 or ORF2. Mutants obtained by allelic replacement of ORF3 or ORF4 expressed only the 65-kDa fimbrial structural subunit. These mutants did not bind, in vitro, to proline-rich proteins that serve as the receptors for Actinomyces type 1 fimbriae. In contrast, a mutant in which the integration plasmid DNA had been inserted at a site close to the carboxyl terminus of ORF6 expressed type 1 fimbriae and had adherence properties similar to those observed in the wild-type strain. These results demonstrate the existence of additional genes near fimP that are likely to be involved in the synthesis and function of cell surface fimbriae of A. naeslundii T14V.

Biochemical studies on LevJ, a fructanase from Actinomyces naeslundii T14V

The Actinomyces naeslundii T14V gene levJ encodes a sucrase with fructanase activity and may be responsible for the fructanase activity observed bound to the surface of A. naeslundii T14V cells. A large proportion of LevJ expressed in Escherichia coli was translocated to the periplasm, and translocation and enzymatic activity were not affected by deletion of a putative cell-wall anchor sequence. The pH optimum of the enzyme was found to be between 5.5 and 6.5 whether the substrate was sucrose or inulin, although inulinase activity was more sensitive than sucrose activity to perturbation of the pH from the optimum. The relation between LevJ inulinase activity and pH was similar to that of A. naeslundii whole cells. LevJ exhibited standard saturation kinetics with sucrose, and the K(m) was calculated to be 89 mM, but it was not possible to calculate a K(m) for inulin. Evidence for inhibition of inulinase activity but not sucrase activity by high concentrations of inulin was obtained.