Purification and partial characterization of the multicomponent dextranase complex of Streptococcus sobrinus and cloning of the dextranase gene

Microbial dextran-hydrolyzing enzyme: Properties, structural features, and versatile applications

Dextran, an α-glucan mainly composed of (α1 → 6) linkages, has been widely applied in the food, cosmetic, and medicine industries. Dextranase can hydrolyze dextran to synthesize oligodextrans, which show prominent properties and promising applications in the food industry. Dextranases are widely distributed in bacteria, yeasts, and fungus, and classified into glycoside hydrolase (GH) 13, 15, 31, 49, and 66 families according to their sequence similarity, structural features, and reaction types. Dextranase, as a dextran-hydrolyzing enzyme, displays great application potential in the sugar-making, oral health care, medicine, and biotechnology industries. Here we mainly focused on presenting the enzymatic properties, structural features, and versatile (potential) applications of dextranase. To date, seven crystal structures of dextranases from GH 13, 15, 31, 49, and 66 families have been successfully solved. However, their molecular mechanisms for hydrolyzing dextran, especially on the size determinants of the hydrolysates, remain largely unknown. Additionally, the classification, microbial distribution, and immobilization technology of dextranase were also discussed in detail. This review discussed dextranase from different aspects with the ambition to present how they constitute the groundwork for promising future developments.

A novel dextranase gene from the marine bacterium Bacillus aquimaris S5 and its expression and characteristics

Dextranase specifically hydrolyzes dextran and is used to produce functional isomalto-saccharide prebiotics. Moreover, dextranase is used as an additive in mouthwash to remove dental plaque. We cloned and expressed the dextranase gene of the marine bacterium Bacillus aquimaris S5. The length of the BaDex gene was 1788 bp, encoding 573 amino acids. Using bioinformatics to predict and analyze the amino acid sequence of BaDex, we found the isoelectric point and instability coefficient to be 4.55 and 29.22, respectively. The average hydrophilicity (GRAVY) was -0.662. The secondary structure of BaDex consisted of 145 alpha helices, accounting for 25.31% of the protein; 126 extended strands, accounting for 21.99%; and 282 random coils, accounting for 49.21%. The 3D structure of the BaDex protein was predicted and simulated using SWISS-MODEL, and BaDex was classified as a Glycoside Hydrolase Family 66 protein. The optimal temperature and pH for BaDex activity were 40°C and 6.0, respectively. The hydrolysates had excellent antioxidant activity, and 8 U/mL of BaDex could remove 80% of dental plaque in MBRC experiment. This recombinant protein thus has great promise for applications in the food and pharmaceutical industries.

Purification and Characterization of a Biofilm-Degradable Dextranase from a Marine Bacterium

This study evaluated the ability of a dextranase from a marine bacterium Catenovulum sp. (Cadex) to impede formation of Streptococcus mutans biofilms, a primary pathogen of dental caries, one of the most common human infectious diseases. Cadex was purified 29.6-fold and had a specific activity of 2309 U/mg protein and molecular weight of 75 kDa. Cadex showed maximum activity at pH 8.0 and 40 °C and was stable at temperatures under 30 °C and at pH ranging from 5.0 to 11.0. A metal ion and chemical dependency study showed that Mn²⁺ and Sr²⁺ exerted positive effects on Cadex, whereas Cu²⁺, Fe³⁺, Zn²⁺, Cd²⁺, Ni²⁺, and Co²⁺ functioned as inhibitors. Several teeth rinsing product reagents, including carboxybenzene, ethanol, sodium fluoride, and xylitol were found to have no effects on Cadex activity. A substrate specificity study showed that Cadex specifically cleaved the α-1,6 glycosidic bond. Thin layer chromatogram and high-performance liquid chromatography indicated that the main hydrolysis products were isomaltoogligosaccharides. Crystal violet staining and scanning electron microscopy showed that Cadex impeded the formation of S. mutans biofilm to some extent. In conclusion, Cadex from a marine bacterium was shown to be an alkaline and cold-adapted endo-type dextranase suitable for development of a novel marine agent for the treatment of dental caries.

Purification, characterization, and application of a thermostable dextranase from Talaromyces pinophilus

Abstract

Dextranase can hydrolyze dextran to low-molecular-weight polysaccharides, which have important medical applications. In the study, dextranase-producing strains were screened from various soil sources. The strain H6 was identified as Talaromyces pinophilus by a standard ITS rDNA analysis. Crude dextranase was purified by ammonium sulfate fractionation and Sepharose 6B chromatography, which resulted in a 6.69-fold increase in the specific activity and an 11.27% recovery. The enzyme was 58 kDa, lower than most dextranase, with an optimum temperature of 45 °C and an optimum pH of 6.0, and identified as an endodextranase. It was steady over a pH range from 3.0 to 10.0 and had reasonable thermal stability. The dextranase activity was increased by urea, which enhanced its activity to 115.35% and was conducive to clinical dextran production. Therefore, T. pinophilus H6 dextranase could show its superiority in practical applications.

Graphical Abstract

Purification and characterization of a novel marine Arthrobacter oxydans KQ11 dextranase

Dextranases can hydrolyze dextran deposits and have been used in the sugar industry. Microbial strains which produce dextranases for industrial use are chiefly molds, which present safety issues, and dextranase production from them is impractically long. Thus, marine bacteria to produce dextranases may overcome these problems. Crude dextranase was purified by a combination of ammonium sulfate fractionation and ion-exchange chromatography, and then the enzyme was characterized. The enzyme was 66.2 kDa with an optimal temperature of 50°C and a pH of 7. The enzyme had greater than 60% activity at 60°C for 1h. Moreover, 10mM Co(2+) enhanced dextranase activity (196%), whereas Ni(2+) and Fe(3+) negatively affected activity. 0.02% xylitol and 1% alcohol enhanced activity (132.25% and 110.37%, respectively) whereas 0.05% SDS inhibited activity (14.07%). The thickness of S. mutans and mixed-species oral biofilm decreased from 54,340 nm to 36,670 nm and from 64,260 nm to 43,320 nm, respectively.

Molecular Characterization of Dextranase fromStreptococcus rattus

The complete nucleotide sequence of the dextranase gene of Streptococcus rattus ATCC19645 was determined. An open reading frame of the dextranase gene was 2,760 bp long and encoded a dextranase protein consisting of 920 amino acids with a molecular weight of 100,163 Da and an isoelectric point of 4.67. The S. rattus dextranase purified from recombinant Escherichia coli cells showed dextran-hydrolyzing activity with optimal pH (5.0) and temperature (40 C) similar to those of dextranases from Streptococcus mutans and Streptococcus sobrinus. The deduced amino acid sequence of the S. rattus dextranase revealed that the dextranase molecule consists of two variable regions and a conserved region. The variable regions contained an N-terminal signal peptide and a C-terminal cell wall sorting signal; the conserved region contained two functional domains, catalytic and dextran-binding sites. This structural feature of the S. rattus dextranase is quite similar to that of other cariogenic species such as S. mutans, S. sobrinus, and Streptococcus downei.

Cloning and characterization of a dextranase gene fromLipomyces starkeyi and its expression inSaccharomyces cerevisiae

A dextranase-encoding cDNA from L. starkeyi KSM22 was isolated and characterized. The 2052 bp cDNA fragment (lsd1) harbouring the dextranase gene exhibited one open reading frame (ORF) composed of 1824 bp flanked by a 41 bp 5'-UTR and a 184 bp 3'-UTR, including a 27 bp poly(A) tail. The lsd1 gene contains no introns. The open reading frame encodes a 608 amino acid polypeptide (LSD1) with a 67.6 kDa predicted molecular mass. There was a 77% deduced amino acid sequence identity between the LSD1 dextranase and the dextranase from Penicillium minioluteum. The primary structure of LSD1 dextranase exhibits distant similarity with the enzymes of the glycosyl hydrolase family 49 that comprises Penicillium dextranase. The optimum pH of LSD1 was 6.0 and the optimum temperature was 37 degrees C. LSD1 dextranase activity was substantially abolished by exposure to 1 mM Hg2+, Ag3+ and Mn2+. LSD1 exhibited high hydrolysing activity towards dextran (100%), soluble starch (22%) and mutan (8%).

Microbial dextran-hydrolyzing enzymes: fundamentals and applications

Dextran is a chemically and physically complex polymer, breakdown of which is carried out by a variety of endo- and exodextranases. Enzymes in many groups can be classified as dextranases according to function: such enzymes include dextranhydrolases, glucodextranases, exoisomaltohydrolases, exoisomaltotriohydrases, and branched-dextran exo-1,2-alpha-glucosidases. Cycloisomalto-oligosaccharide glucanotransferase does not formally belong to the dextranases even though its side reaction produces hydrolyzed dextrans. A new classification system for glycosylhydrolases and glycosyltransferases, which is based on amino acid sequence similarities, divides the dextranases into five families. However, this classification is still incomplete since sequence information is missing for many of the enzymes that have been biochemically characterized as dextranases. Dextran-degrading enzymes have been isolated from a wide range of microorganisms. The major characteristics of these enzymes, the methods for analyzing their activities and biological roles, analysis of primary sequence data, and three-dimensional structures of dextranases have been dealt with in this review. Dextranases are promising for future use in various scientific and biotechnological applications.

Microbial dextran-hydrolyzing enzymes: fundamentals and applications

Dextran is a chemically and physically complex polymer, breakdown of which is carried out by a variety of endo- and exodextranases. Enzymes in many groups can be classified as dextranases according to function: such enzymes include dextranhydrolases, glucodextranases, exoisomaltohydrolases, exoisomaltotriohydrases, and branched-dextran exo-1,2-alpha-glucosidases. Cycloisomalto-oligosaccharide glucanotransferase does not formally belong to the dextranases even though its side reaction produces hydrolyzed dextrans. A new classification system for glycosylhydrolases and glycosyltransferases, which is based on amino acid sequence similarities, divides the dextranases into five families. However, this classification is still incomplete since sequence information is missing for many of the enzymes that have been biochemically characterized as dextranases. Dextran-degrading enzymes have been isolated from a wide range of microorganisms. The major characteristics of these enzymes, the methods for analyzing their activities and biological roles, analysis of primary sequence data, and three-dimensional structures of dextranases have been dealt with in this review. Dextranases are promising for future use in various scientific and biotechnological applications.

Paenibacillus isolates possess diverse dextran-degrading enzymes

AIMS

To isolate and identify dextran-degrading organisms from sugar mill and compost samples, and to examine the diversity of the dextranolytic enzymes produced.

METHODS AND RESULTS

Fifteen dextranolytic prokaryotes were purified at various temperatures from sugar-mill or compost samples, using indicator plates containing blue dextran. A 16S rRNA gene sequence analysis showed that 12 isolates purified at 40, 50 or 70 degrees C were closely aligned to Paenibacillus spp. The three isolates purified at 60 degrees C had identical 16S rDNA sequences, with highest affinity to Bacillus spp. Liquid culture of the 11 isolates purified at 40 or 50 degrees C produced dextranolytic activity in the spent media with maximal activity at 40 or 45 degrees C under the assay conditions used. Hydrolysis of blue dextran in activity gels showed that the 12 Paenibacillus isolates produced from one to five dextranolytic proteins, ranging from 70 to 120 kDa. Based on 16S rDNA sequence, growth habit in liquid culture and dextranolytic enzyme pattern, the 12 Paenibacillus-like isolates could be differentiated into six distinct groups, one of which was capable of growth at 70 degrees C.

CONCLUSIONS

The Bacillales, especially the Paenibacillus, are a valuable environmental repository for dextranolytic enzymes of diverse size and potentially diverse activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

Dextranolytic enzymes produced by Paenibacillus spp. are an exploitable resource for those interested in modifying the structure of dextrans.

An essential amino acid residue for catalytic activity of the dextranase of Streptococcus mutans

Dextranase (Dex) is an enzyme that hydrolyzes glucan, a polymer of glucose synthesized from sucrose by glucosyltransferases (GTFs). By comparing amino acid sequences of Dexs and GTFs, we found that the Dex enzymes of Streptococcus mutans, Streptococcus sobrinus, Streptococcus downei and Streptococcus salivarius had similar amino acid sequences to those of the catalytic sites of GTFs of mutans streptococci. We therefore examined the amino acid essential in Dex catalysis by molecular genetic approaches in this study. Site-directed mutagenesis was used to convert the Asp-385 of the Dex molecule of S. mutans Ingbritt to Glu, Asn, Thr or Val. Replacement of Asp-385 with any of the amino acids resulted in complete disappearance of Dex activity. However, replacement of other Asp residues did not affect the enzyme activity. The inactive enzymes still retained dextran-binding ability. These results suggest that Asp-385 of the Dex of S. mutans Ingbritt was essential for enzyme activity and the catalytic and substrate-binding sites were located at different sites within the Dex molecule.

Nucleotide sequence and molecular characterization of a dextranase gene from Streptococcus downei

DNA fragments encoding the Streptococcus downei dextranase were amplified by PCR and inverse PCR based on a comparison of the dextranase gene (dex) sequences from S. sobrinus, S. mutans, and S. salivarius, and the complete nucleotide sequence of the S. downei dex was determined. An open reading frame (ORF) of dex was 3,891 bp long. It encoded a dextranase protein (Dex) consisting of 1,297 amino acids with a molecular mass of 139,743 Da and an isoelectric point of 4.49. The deduced amino acid sequence of S. downei Dex had homology to those of S. sobrinus, S. mutans and S. salivanus Dex in the conserved region (made of about 540 amino acid residues). DNA hybridization analysis showed that a dex DNA probe of S. downei hybridized to the chromosomal DNA of S. sobrinus as well as that of S. downei, but did not to other species of mutans streptococci. The C terminus of the S. downei Dex had a membrane-anchor region which has been reported as a common structure of C termini of both the S. mutans and S. sobrinus Dex. The recombinant plasmid which harbored the dex ORF of S. downei produced a recombinant Dex enzyme in Escherichia coli cells. The analysis of the recombinant enzyme on SDS-PAGE containing blue dextran showed multiple active forms as well as dextranases of S. mutans, S. sobrinus and S. salivarius.

Detection of dextranase-producing gram-negative oral bacteria

Thirty-one strains of 23 gram-negative oral bacterial species were examined for dextran-degrading activity on agar plates containing blue dextran. One strain each of Capnocytophaga ochracea, Capnocytophaga sputigena, Prevotella loescheii, Prevotella melaninogenica and Prevotella oralis had detectable dextranase activity. The culture supernatants of P. melaninogenica and P. oralis cells contained dextranases of multiple sizes, but those of the other three species had a single size of enzyme. A 56-kDa dextranase was purified from the culture supernatant of P. oralis and the antiserum against the enzyme was prepared with a rabbit. The Ouchterlony test showed that the antibody reacted with the supernatants of both P. melaninogenica and P. oralis but not with the others. Dot-blot hybridization using the dextranase gene of Streptococcus mutans as a probe revealed that there was no significantly homologous sequence in the chromosomal DNA of the five species.

Analysis of gene expression in Streptococcus mutans in biofilms in vitro

The purpose of this study was to develop methods for the consistent production of biofilms of S. mutans containing reporter gene fusions, and to examine the expression of genes involved in sucrose metabolism in adherent populations of this organism. Three strains of S. mutans harboring reporter gene fusions to the gene promoter regions of the gtfBC genes, ftf, and scrA were grown in a Rototorque biofilm fermenter in a tryptone-yeast extract-sucrose medium. Quasi-steady-state levels of reporter gene activity were measured after the biofilms were grown for either 48 hrs of 7 days. Also, induction of gene expression by the addition of sucrose to biofilm cells was monitored. Reporter gene activity was measurable from all gene fusion strains. This study (i) establishes the feasibility of doing detailed molecular and physiologic studies on immobilized populations of S. mutans, (ii) demonstrates that the polysaccharide synthesis machinery of S. mutans is differentially expressed in biofilms, and (iii) opens the way for a more detailed analysis of the environmental signals and signal transduction pathways governing the regulation of gene expression by S. mutans cells that are immobilized on a solid surface.

Rapid identification of mutans streptococcal species

Mutans streptococci are considered the predominant pathogens in dental caries. Three methods, i.e. dot blot hybridization analysis, PCR analysis and SDS-blue dextran-PAGE, were examined for identifying mutans streptococcal species. In dot blot hybridization, DNA probe derived from the dextranse gene (dexA) of Streptococcus mutans hybridized with different intensities under the condition of low stringency against each species of mutans streptococci although the dexA probe was specific for S. mutans under the condition of high stringency. Oligonucleotide primers for polymerase chain reaction (PCR) were designed on the basis of the dexA DNA sequence. The primers amplified species-specific PCR products in the reference species (15 strains of 5 species) of mutants streptococci. An electrophoretic profile of dextranases from the mutans streptococci on SDS-blue dextran-PAGE also showed species-specific behavior. These results suggest that the three identification methods examined here are useful for distinguishing the species of mutans streptococci and also indicate that PCR analysis is suitable for simple, rapid and reliable identification of mutans streptococcal species.

Sequence analysis of the Streptococcus mutans Ingbritt dexA gene encoding extracellular dextranase

The complete nucleotide sequence (3,747 bp) of the dextranase gene (dexA) and flanking regions of the chromosome of Streptococcus mutans Ingbritt (serotype c) were determined. The open reading frame for dexA was 2,550 bp, ending with a stop codon TGA. A putative ribosome-binding site, promoter preceding the start codon, and potential stem-loop structure were identified. The presumed dextranase protein (DexA) consisting of 850 amino acids was estimated to have a molecular size of 94,536 Da and a pI of 4.79. The nucleotide sequence and the deduced amino acid sequences of S. mutans dexA exhibited homologies of 57.8% and 47.0%, respectively, to those of Streptococcus sobrinus dex. The homologous region of dex of S. sobrinus was in the N-terminal half. The C terminus of DexA consisted of a hexapeptide LPQTGD, followed by 7 charged amino acids, 21 amino acids with a strongly hydrophobic character, and a charged hexapeptide tail, which have been reported as a common structure of C termini of not only the surface-associated proteins of Gram-positive cocci but also the extracellular enzymes such as beta-fructosidase of S. mutans and dextranase of S. sobrinus. The DexA protein had no significant homology with the glucosyltransferases, the glucan-binding protein, or the dextranase inhibitor of mutans streptococci.

Characterization of the dextranase gene (dex) of Streptococcus mutans and its recombinant product in an Escherichia coli host

The gene (dex), which encodes the Streptococcus mutans dextranase (Dex), was cloned in Escherichia coli. The E. coli host harboring a recombinant plasmid (pSD2) containing an 8-kb BamHI insert produced a Dex protein of 133 kDa as well as smaller enzymes of 118, 104, and 88 kDa. The Dex produced by the recombinant E. coli was apparently located in the cytoplasmic fraction, not in the periplasmic nor the extracellular fractions. Subcloning and deletion analysis of pSD2 showed that the structural gene of Dex was encoded by a 4-kb BamHI-SalI fragment. The fragment also contained the dex promoter which was effective in the E. coli cell.

Cloning and sequencing of the gene coding for dextranase from Streptococcus salivarius

We cloned and sequenced the dextranase (Dex) (1,6-alpha-glucanhydrolase; EC 3.2.1.11)-encoding gene from Streptococcus salivarius (Ss) strain M-33. Recombinant clones from an Ss genomic library specifying Dex activity were identified as colonies surrounded by transparent halos on blue dextran plates. One of the clones had a 4.3-kb KpnI fragment containing the gene coding for an 826-amino-acid polypeptide with a molecular mass of 87.9 kDa, which corresponds well to that of native Dex from the Ss culture supernatant. There was no sequence homology between the gene encoding Ss Dex and the gene encoding dextran glucosidase of S. mutans, or between their protein products.

Purification, characterization, and specificity of dextranase inhibitor (Dei) expressed from Streptococcus sobrinus UAB108 gene cloned in Escherichia coli

The dextranase inhibitor gene (dei) from Streptococcus sobrinus UAB108 was previously cloned, expressed, and sequenced. Its gene product (Dei) has now been purified as a single band with apparent molecular mass of 43 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The specific activity of Dei increased 121-fold upon purification. Most Dei activity (91.2%) was located in the periplasmic fraction from recombinant Escherichia coli cells. Dei competitively inhibits dextranase (Dex). This competitive inhibition mechanism has been further shown by detection and recovery of the intermediate enzyme-inhibitor (Dex-Dei) complex by gel filtration technology using fast protein liquid chromatography. Calibration of their molecular masses indicated that native Dei exists as a tetramer, Dex exists as dimer, and the Dex-Dei complex consists of two Dex molecules with two Dei molecules. Deletion analysis indicates that the intact Dei molecule is essential for Dei activity but not for glucan binding and immune cross-reaction. Dei is a special kind of glucan-binding protein with ability to inhibit Dex with high specificity. It can inhibit endogenous Dex, which can make more branches in glucan with the cooperation of the glucosyltransferase GTF-I. This inhibition cause the accumulation of water-soluble glucan. The latter reaction product can inhibit plaque formation and adherence of the mutans group of streptococcal cells. Dei derived from S. sobrinus UAB108 can inhibit only Dex from S. sobrinus (serotypes d and g), S. downei (previously S. sobrinus, serotype h), and S. macacae (serotype h). This finding suggests that Dei is another important protein existing in some serotypes of the mutans group of streptococci which participates in sucrose metabolism through its interaction with Dex.

Cloning and DNA sequencing of the dextranase inhibitor gene (dei) from Streptococcus sobrinus

Some dextranase-deficient (Dex-) mutants of Streptococcus sobrinus UAB66 (serotype g) synthesize a substance which inhibits dextranase activity (S.-Y. Wanda, A. Camilli, H. M. Murchison, and R. Curtiss III, J. Bacteriol. 176:7206-7212, 1994). This substance produced by the Dex- mutant UAB108 was designated dextranase inhibitor (Dei) and identified as a protein. The Dei gene (dei) from UAB108 has been cloned into pACYC184 to yield pYA2651, which was then used to generate several subclones (pYA2653 to pYA2657). The DNA sequence of dei was determined by using Tn5seq1 transposon mutagenesis of pYA2653. The open reading frame of dei is 990 bp long. It encodes a signal peptide of 38 amino acids and a mature Dei protein of 292 amino acids with a molecular weight of 31,372. The deduced amino acid sequence of Dei shows various degrees of similarity with glucosyltransferases and glucan-binding protein and contains A and C repeating units probably involved in glucan binding. Southern hybridization results showed that the dei probe from UAB108 hybridized to the same-size fragment in S. sobrinus (serotype d and g) DNA, to a different-size fragment in S. downei (serotype h) and S. cricetus (serotype a), and not at all to DNAs from other mutans group of streptococci.

Overproduction of a dextranase inhibitor by Streptococcus sobrinus mutants

An inhibitor of Streptococcus sobrinus endodextranase was detected in the extracellular fractions of UAB66 mutants identified following ethyl methanesulfonate mutagenesis as either devoid of dextranase activity (Dex-) or overproducing water-soluble glucan. The two groups of mutants had the same phenotype and displayed no dextranase activity in assays of extracellular fractions (H. Murchison, S. Larrimore, and R. Curtiss III, Infect. Immun. 34:1044-1055, 1981) and had been shown to be defective in adherence (Adh-) and capable of inhibiting adherence of wild-type strains during cocultivation in vitro (H. Murchison, S. Larrimore, and R. Curtiss III, Infect. Immun. 50:826-832, 1985) and in vivo in gnotobiotic rats (K. Takada, T. Shiota, R. Curtiss III, and S. M. Michalek, Infect. Immun. 50:833-843, 1985). By analysis of proteins in Western blots (immunoblots) and following blue dextran-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (BD-SDS-PAGE), it was demonstrated that these Dex- mutants did synthesize enzymatically active dextranase. From the results of mixing experiments, it was determined that these Dex- Adh- mutants produced enhanced amounts of a cell surface-localized or a cell-associated dextranase inhibitor (Dei). Dei was heat stable but trypsin sensitive. By adding excess dextranase following BD-SDS-PAGE, Dei was detected as blue bands with apparent molecular masses of 43, 40, 37, 27, and 23 kDa. Dei competitively inhibits dextranase activity and is synthesized by wild-type S. sobrinus strains, with the amount varying depending upon growth medium and stage in the growth cycle. R. M. Hamelik and M. M. McCabe (Biochem. Biophys. Res. Commun. 106:875-880, 1982) previously described a Dei in a wild-type S. sobrinus strain.

Purification and characterization of Streptococcus sobrinus dextranase produced in recombinant Escherichia coli and sequence analysis of the dextranase gene

The plasmid (pYA902) with the dextranase (dex) gene of Streptococcus sobrinus UAB66 (serotype g) produces a C-terminal truncated dextranase enzyme (Dex) with a multicomplex mass form which ranges from 80 to 130 kDa. The Escherichia coli-produced enzyme was purified and characterized, and antibodies were raised in rabbits. Purified dextranase has a native-form molecular mass of 160 to 260 kDa and specific activity of 4,000 U/mg of protein. Potential immunological cross-reactivity between dextranase and the SpaA protein specified by various recombinant clones was studied by using various antisera and Western blot (immunoblot) analysis. No cross-reactivity was observed. Optimal pH (5.3) and temperature (39 degrees C) and the isoelectric points (3.56, 3.6, and 3.7) were determined and found to be similar to those for dextranase purified from S. sobrinus. The dex DNA restriction map was determined, and several subclones were obtained. The nucleotide sequence of the dex gene was determined by using subclones pYA993 and pYA3009 and UAB66 chromosomal DNA. The open reading frame for dex was 4,011 bp, ending with a stop codon TAA. A ribosome-binding site and putative promoter preceding the start codon were identified. The deduced amino acid sequence of Dex revealed the presence of a signal peptide of 30 amino acids. The cleavage site for the signal sequence was determined by N-terminal amino acid sequence analysis for Dex produced in E. coli chi 2831(pYA902). The C terminus consists of a serine- and threonine-rich region followed by the peptide LPKTGD, 3 charged amino acids, 19 amino acids with a strongly hydrophobic character, and a charged pentapeptide tail, which are proposed to correspond to the cell wall-spanning region, the LPXTGX consensus sequence, and the membrane-anchoring domains of surface-associated proteins of gram-positive cocci.

Inactivation of the Streptococcus mutans wall-associated protein A gene (wapA) results in a decrease in sucrose-dependent adherence and aggregation

A 0.8-kb HindIII-BamHI internal fragment of the Streptococcus mutans wall-associated protein A gene (wapA) was ligated to the 5.1-kb HindIII-BamHI fragment of the chimeric Streptococcus-Escherichia coli plasmid pVA891 (Emr Cmr). The resulting construct was used to transform S. mutans GS-5, and erythromycin-resistant mutants were isolated and analyzed. Directed mutagenesis of the wapA gene by plasmid insertion through homologous recombination was demonstrated by Southern blot hybridization with the wapA and pVA891 probes. Stable mutants were obtained, and the alteration of the wapA gene by insertional inactivation was associated with a significant decrease in S. mutans sucrose-dependent aggregation and binding to smooth surfaces. Thus, WapA may play an important role in the colonization of the tooth surface by S. mutans and in the buildup of dental plaque. These findings provided an explanation for previous studies which indicated that WapA was effective in the prevention of dental caries in animal models. Thus, the use of recombinant WapA in the preparation of a safe and effective human dental vaccine should be investigated further.

Dissociation and electrophoretic separation of dextranase and dextranase inhibitor from a tightly bound enzyme-inhibitor complex of Streptococcus sobrinus

Endodextranase was separated from dextranase inhibitor in culture filtrates of Streptococcus sobrinus by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) in gel slabs containing blue dextran. Sample preparation included dissociation of the enzyme from its inhibitor by boiling for 1 min in SDS. During subsequent incubation of the gel, dextranase was located as clear bands on a blue background, and dextranase inhibitor appeared as blue zones on a clear background following incubation in dextranase solution. The enzyme and the inhibitor existed in multiple forms, and the range of molecular masses for dextranase (223-132 kDa) permitted an excellent separation from dextranase inhibitor (49-25 kDa). Although dextranase-negative mutants, and wild type strains grown at low dilution rate in the chemostat, were devoid of free dextranase activity, the enzyme was easily located by analytical SDS-PAGE. Likewise, analysis of filtrates from wild type strains, which contained no free inhibitor activity when growth occurred at high dilution rate, revealed dextranase inhibitor activity on the gels. The total production (free + combined) of dextranase and inhibitor by S. sobrinus was determined by dissociation of enzyme-inhibitor complexes in concentrated cell-free filtrates, their separation by preparative SDS-PAGE and electroelution from the gels, followed by renaturation of protein activity. From a comparison of activity tests of free dextranase and free inhibitor in untreated filtrates with the results of similar tests on renatured electroeluates, the proportion of each constituent bound into a complex under each growth condition could be deduced.

Escherichia coli heat-labile toxin subunit B fusions with Streptococcus sobrinus antigens expressed by Salmonella typhimurium oral vaccine strains: importance of the linker for antigenicity and biological activities of the hybrid proteins

A set of vectors possessing the genes for aspartate semialdehyde dehydrogenase (asd) and the B subunit of the heat-labile enterotoxin of Escherichia coli (LT-B) has been developed. These vectors allow operon or gene fusions of foreign gene epitopes at the C-terminal end of LT-B. Two groups of vectors have been constructed with and without leader sequences to facilitate placing of the foreign antigen in different cell compartments. Two Streptococcus sobrinus genes coding for principal colonization factors, surface protein antigen A (SpaA), and dextranase (Dex), have been fused into the 3' end of the LT-B gene. Resulting protein fusions of approximately 120 to 130 kDa are extremely well recognized by antibodies directed against both SpaA and Dex as well as against LT-B domains and retain the enzymatic activity of dextranase and the biological activity of LT-B in that they bind to GM1 gangliosides. Maximum antigenicity was obtained with the vector possessing an intervening linker of at least six amino acids with two proline residues. Some of the fusion proteins also exhibited another property of LT-B in that they were exported into the periplasm where they oligomerized. LT-B-SpaA and LT-B-Dex hybrid proteins are expressed stably and at a high level in avirulent Salmonella typhimurium vaccine strains which are being used to investigate their immunogenicity and types of induced immune responses. The fusion vectors will also be useful for production and purification of LT-B fusion antigens to be used and evaluated in other vaccine compositions.

Virulence factors of mutans streptococci: role of molecular genetics

Biochemical approaches were utilized initially to identify the virulence factors of the mutans streptococci (primarily Streptococcus mutans and S. sobrinu). Traditional mutant analysis of these organisms further suggested the important role of several of these factors in cariogenicity. However, because these mutations were not clearly defined, the utilization of cloned genes was necessary to verify their significance. The introduction of molecular genetic approaches for characterizing these factors has led not only to a clearer understanding of the role of these virulence factors in cariogenicity but has also suggested some novel approaches for reducing further the incidence of dental caries.

Characterization of the dextranase purified from Streptococcus mutans Ingbritt

We purified dextranase from the culture supernatant of Streptococcus mutans Ingbritt by procedures including ammonium sulfate precipitation, ion-exchange chromatography, and gel filtration. The molecular weight of the enzyme was estimated as 78 kDa by SDS-PAGE. The enzyme degraded dextran at the optimum pH of 5.5, but not other glucans and fructans at all. Paper chromatographic analysis revealed that the enzyme cleaved dextran by an endo-type mechanism. The enzyme was inhibited by Hg2+, Fe3+, Zn2+, and anionic detergents SDS and deoxycholic acid, but not inhibited by non-ionic detergents Triton X-100, Lubrol PX, Nonidet P-40, and Tween 80. SDS-blue dextran-PAGE analysis of the culture supernatant revealed that the enzyme activity detected in the 96 kDa band shifted gradually to the 78 kDa band during handling the supernatant. This shift was inhibited by phenylmethylsulfonyl fluoride, suggesting that the shift of the molecular size is due to proteolytic degradation of the enzyme by serine protease.

Molecular cloning of the extracellular endodextranase of Streptococcus salivarius

We report the cloning in Escherichia coli of the gene encoding an extracellular endodextranase (alpha-1,6-glucanhydrolase, EC 3.2.1.11) from Streptococcus salivarius PC-1. Recombinants from a S. salivarius PC-1-Lambda ZAP II genomic library specifying dextranase activity were identified as plaques surrounded by zones of clearing on blue dextran agar. One such clone, PD1, had a 6.3-kb EcoRI fragment insert which encoded a 190-kDa protein with dextranase activity. The recombinant strain also produced two lower-molecular-mass polypeptides (90 and 70 kDa) that had dextranase activity. Native dextranase was recovered from concentrated culture fluids of S. salivarius as a single 110-kDa polypeptide. PD1 phage lysate and PC-1 culture supernatant fluid extract were used to measure substrate specificity of the recombinant and native forms of dextranase, respectively. Analysis of these reaction products by thin-layer chromatography revealed the expected isomaltosaccharide products yielded by the recombinant-specified enzyme but was unable to resolve the larger polysaccharide products of the native enzyme. Furthermore, S. salivarius utilized neither the substrates nor the products of dextran hydrolysis for growth.

Cross-reactivity between the immunodominant determinant of the antigen I component of Streptococcus sobrinus SpaA protein and surface antigens from other members of the Streptococcus mutans group

Most members of the Streptococcus mutans group of microorganisms specify a major cell surface-associated protein, SpaA, that is defined by its antigenic properties. The region of the spaA gene from Streptococcus sobrinus 6715 encoding the immunodominant determinant of the major antigenic component (antigen I) of the SpaA protein has recently been characterized. This study examined whether recognition of the immunodominant determinant is independent of the immunized animal host and whether antibodies elicited by the immunodominant determinant cross-react with cell surface proteins from S. mutans of various serotypes. Mouse and rabbit antisera to the undenatured SpaA protein reacted similarly both with the immunodominant determinant and with other antigenic structures of the protein in Western immunoblots with SpaA polypeptides that were specified by spaA gene fragments expressed in recombinant Escherichia coli. This suggests that the antibody responses of inbred and outbred animals were similar. Furthermore, antibodies raised against both the S. sobrinus SpaA immunodominant determinant expressed by recombinant E. coli and the purified protein from S. sobrinus displayed similar strain specificities and protein band profiles towards cells surface proteins from S. mutans of various serotypes in immunodot and Western blot analyses, respectively. This suggests that for S. sobrinus serotype g, the immune response against the SpaA protein is governed by the immunodominant determinant of antigen I. In addition, it indicates that the SpaA protein domain containing the immunodominant determinant overlaps the domain conferring cross-reactivity to cell surface proteins of S. mutans of various serotypes.

Genetic approaches to the study of oral microflora: a review

As the study of oral microorganisms intensified almost 2 decades ago, the application of genetic techniques resulted in important contributions to the understanding of this clinically and ecologically important group of bacteria. The isolation and characterization of mutants of cariogenic streptococci helped to focus attention on traits that were important in colonization and virulence. Such classic genetic approaches gave way to molecular genetic techniques, including recombinant DNA methodology in the late 1970s. Gene cloning systems and methods to move DNA into cells have been developed for oral streptococci. Many streptococcal genes thought to be important in colonization and virulence have since been cloned and their nucleotide sequence determined. Mutant strains have been constructed using defective copies of cloned genes in order to create specific genetic lesions on the bacterial chromosome. By testing such mutants in animal models, a picture of the cellular and molecular basis of dental caries is beginning to emerge. These modern genetic methodologies also are being employed to develop novel and efficacious cell-free or whole cell vaccines against this infection. Genetic approaches and analyses are now being used to dissect microorganisms important in periodontal disease as well. Such systems should be able to exploit advances made in genetically manipulating related anaerobes, such as the intestinal Bacteroides. Gene cloning techniques in oral anaerobes, Actinomyces and Actinobacillus, are already beginning to pay dividends in helping understand gene structure and expression. Additional effort is needed to develop facile systems for genetic manipulation of these important groups of microorganisms.

Molecular genetics of glucan metabolism in oral streptococci

Computer analysis of the primary amino acid sequences deduced from nucleotide sequences of cloned genes from Streptococcus mutans and Strep. downei was used to examine features of enzymes involved in formation and degradation of glucans. All 4 glucosyltransferases for which sequence data are available show a common structure characterized by a series of reiterated repeats in the carboxy-terminal one third of the molecule. These repeats are also found in Strep. mutans glucan-binding protein and resemble those found in enzymes from other bacteria with binding properties.

Molecular cloning of the Streptococcus mutans gene specifying antigen A

A gene encoding a Streptococcus mutans surface protein antigen has been isolated from a strain GS-5 gene bank constructed via the Streptococcus-Escherichia coli shuttle vector pSA3. This E. coli recombinant clone, designated 4B2, expressed S. mutans proteins, as shown by Western immunoblot analysis with a specific rabbit antibody to S. mutans surface antigens. Three bands were observed, including a 52-kilodalton (kDa) protein (pI 5.7), a 29-kDa protein (pI 4.2), and a 20-kDa protein usually present in lower amounts. The 52- and 29-kDa proteins both reacted with a monoclonal antibody to S. mutans antigen A, a 29-kDa protein which has been characterized and used as a vaccine for the prevention of induced caries in rodents and monkeys. The 52-kDa protein, but not the 29-kDa protein, showed a capacity to bind to a broad number of carbohydrate polymers. The results from this study suggest that the recombinant 4B2 clone specifies a 52-kDa protein which is a precursor to the 29-kDa antigen A.

Biochemical characterization and evaluation of virulence of a fructosyltransferase-deficient mutant of Streptococcus mutans V403

The Streptococcus mutans extracellular fructosyltransferase (FTF) enzyme may play a role in the formation of dental caries by synthesizing a fructan polymer that serves as an extracellular storage polysaccharide. We sought to determine if an FTF-deficient strain of S. mutans was less virulent than wild-type cells in a rat animal model system. Cloned ftf gene sequences from S. mutans GS5 were used to generate a defective copy of the ftf gene by inserting into the ftf coding region a DNA fragment which encoded erythromycin resistance. The plasmid which carried the defective ftf construct was introduced into S. mutans V403 by using genetic transformation. This defective construct replaced, by allelic exchange, the wild-type copy of the ftf gene carried on the V403 chromosome. FTF activity assays indicated that the recombinant strain, V1741, was deficient in fructan synthesis. However, extracellular protein preparations from this strain displayed an increased ability to generate glucose polymers (glucans) compared with V403 preparations. Levels of adherence to glass and rat tooth surfaces by strain V1741 were similar to those of the V403 strain. Both strains caused moderate decay on rat tooth surfaces; however, the FTF-deficient strain was less pathogenic compared with the wild-type strain. These results suggest that FTF activity contributes to the pathogenicity of S. mutans V403, possibly by generating extracellular fructans which serve as storage compounds.

Analysis of the virulence of Streptococcus mutans serotype c gtfA mutants in the rat model system

The Streptococcus mutans serotype c gtfA gene encodes a 55-kilodalton protein which catalyzes the synthesis of a small glucan (1.5 kilodaltons) from sucrose (J.P. Robeson, R.G. Barletta, and R. Curtiss III, J. Bacteriol. 153:211-221, 1983). To investigate the role of the GtfA enzyme in virulence, we constructed S. mutans gtfA mutants from three cariogenic serotype c strains. A plasmid that carried an erythromycin resistance determinant and an internal fragment of the gtfA gene but that was unable to replicate in streptococci was used to transform S. mutans. The erythromycin-resistant transformants carried a partial duplication of the internal gtfA fragment, because of the integration of plasmid sequences within the S. mutans gtfA gene, which also resulted in the inactivation of the gtfA gene. This was verified by Southern DNA hybridization analysis and Western blot studies of cellular protein extracts of the mutant strains with GtfA antiserum. Mutants were fully virulent in both germfree and conventional rats. These results do not rule out the involvement of the GtfA protein in virulence. Pucci and Macrina (M.J. Pucci and F.L. Macrina, Infect. Immun. 54:77-84, 1986) have suggested that the GtfA enzyme synthesizes a primer for water-insoluble glucans. Another S. mutans protein, presumably a glucosyltransferase, may have a similar function and, thus, may obscure the relevance of the GtfA enzyme in pathogenesis.

Recent advances in defining the cariogenicity of mutans streptococci: molecular genetic approaches

The application of molecular genetic approaches with oral mutans streptococci has resulted in the isolation of several genes which may be involved in the cariogenicity of these organisms. Among these are genes coding for cell surface proteins, sucrose metabolizing enzymes, and glycosyltransferase activities. The isolated genes have been utilized to create specific mutants of Streptococcus mutans to assess the potential roles of the gene products in cariogenicity both in vitro and in vivo. These approaches should prove useful in answering some still unresolved questions at the molecular level regarding the cariogenic properties of the organisms.