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

Products Released from Structurally Different Dextrans by Bacterial and Fungal Dextranases

Dextran hydrolysis by dextranases is applied in the sugar industry and the medical sector, but it also has a high potential for use in structural analysis of dextrans. However, dextranases are produced by several organisms and thus differ in their properties. The aim of this study was to comparatively investigate the product patterns obtained from the incubation of linear as well as O3- and O4-branched dextrans with different dextranases. For this purpose, genes encoding for dextranases from Bacteroides thetaiotaomicron and Streptococcus salivarius were cloned and heterologously expressed in Escherichia coli. The two recombinant enzymes as well as two commercial dextranases from Chaetomium sp. and Penicillium sp. were subsequently used to hydrolyze structurally different dextrans. The hydrolysis products were investigated in detail by HPAEC-PAD. For dextranases from Chaetomium sp., Penicillium sp., and Bacteroides thetaiotaomicron, isomaltose was the end product of the hydrolysis from linear dextrans, whereas Penicillium sp. dextranase led to isomaltose and isomaltotetraose. In addition, the latter enzyme also catalyzed a disproportionation reaction when incubated with isomaltotriose. For O3- and O4-branched dextrans, the fungal dextranases yielded significantly different oligosaccharide patterns than the bacterial enzymes. Overall, the product patterns can be adjusted by choosing the correct enzyme as well as a defined enzyme activity.

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.

Structural elucidation of dextran degradation mechanism by streptococcus mutans dextranase belonging to glycoside hydrolase family 66

Dextranase is an enzyme that hydrolyzes dextran α-1,6 linkages. Streptococcus mutans dextranase belongs to glycoside hydrolase family 66, producing isomaltooligosaccharides of various sizes and consisting of at least five amino acid sequence regions. The crystal structure of the conserved fragment from Gln(100) to Ile(732) of S. mutans dextranase, devoid of its N- and C-terminal variable regions, was determined at 1.6 Å resolution and found to contain three structural domains. Domain N possessed an immunoglobulin-like β-sandwich fold; domain A contained the enzyme's catalytic module, comprising a (β/α)(8)-barrel; and domain C formed a β-sandwich structure containing two Greek key motifs. Two ligand complex structures were also determined, and, in the enzyme-isomaltotriose complex structure, the bound isomaltooligosaccharide with four glucose moieties was observed in the catalytic glycone cleft and considered to be the transglycosylation product of the enzyme, indicating the presence of four subsites, -4 to -1, in the catalytic cleft. The complexed structure with 4',5'-epoxypentyl-α-d-glucopyranoside, a suicide substrate of the enzyme, revealed that the epoxide ring reacted to form a covalent bond with the Asp(385) side chain. These structures collectively indicated that Asp(385) was the catalytic nucleophile and that Glu(453) was the acid/base of the double displacement mechanism, in which the enzyme showed a retaining catalytic character. This is the first structural report for the enzyme belonging to glycoside hydrolase family 66, elucidating the enzyme's catalytic machinery.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Identification and characterization of a dextranase gene of Streptococcus criceti

The dextranase gene, dex, was identified in Streptococcus criceti strain E49 by degenerate PCR and sequenced completely by the gene-walking method. A sequence of 3,960 nucleotides was determined. The dex gene encodes a 1,200-amino acid protein, which has a calculated molecular mass of 128,129.91 and pI of 4.15 and is predicted to be a cell-surface protein. The deduced amino acid sequence of dex showed homology to S. downei dextranase (63.9% identity). Phylogenetic analysis revealed the similarity of the deduced amino acid sequence of dextranases in S. criceti, S. sobrinus, and S. downei. A recombinant form of the protein with six histidine residues tagged in the C-terminus was partially purified and showed dextranase activity on blue-dextran sodium dodecyl sulfate-polyacrylamide gel electrophoresis (BD-SDSPAGE) followed by renaturation. We also detected dextranase activity in S. criceti cell extracts and culture supernatant by renatured BD-SDS-PAGE, whereas no dextranase activity of the cells was observed on blue-dextran brain heart infusion (BD-BHI) agar plates. Furthermore, PCR-based mutations of dextranase indicated that a deletion mutant of the C-terminal region could hydrolyze blue dextrans and that the D453E mutation, W793L mutation, and double mutations (W793L and deletion of the C-terminal region) resulted in a loss of dextranase activity. These findings suggest that Asp-453 and Trp-793 residues of S. criceti dextranase are critical to the enzyme's activity.

Identification of catalytic amino acids of cyclodextran glucanotransferase from Bacillus circulans T-3040

In glycoside hydrolase family 66 (see http://afmb.cnrs-mrs.fr/CAZY/), cyclodextran glucanotransferase (CITase) is the only transglycosylation enzyme, all the other family 66 enzymes being dextranases. To analyze the catalytic amino acids of CITase, we modified CITase chemically from the T-3040 strain of Bacillus circulans with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). EDC inactivated the enzyme by following pseudo-first order kinetics. In addition, the substrates of an isomaltooligosaccharide and a cyclodextran inhibited EDC-induced enzyme inactivation, implicating the carboxyl groups of CITase as the catalytic amino acids of the enzyme. When two conserved aspartic acid residues, Asp145 and Asp270, were replaced with Asn in T-3040 mature CITase, CIT-D270N was completely inactive, and CIT-D145N had reduced activity. The V(max) of CIT-D145N was 1% of that of wild-type CITase, whereas the K(m) of CIT-D145N was about the same as that of the wild-type enzyme. These findings indicate that Asp145 and Asp270 play an important role in the enzymatic reaction of T-3040 CITase.

Identification of a surface protein of Streptococcus suis and evaluation of its immunogenic and protective capacity in pigs

A Streptococcus suis surface protein reacting with convalescent-phase sera from pigs clinically infected by S. suis type 2 was identified. The apparent 110-kDa protein, designated Sao, exhibits typical features of membrane-anchored surface proteins of gram-positive bacteria, such as a signal sequence and an LPVTG membrane anchor motif. In spite of high identity with the partially sequenced genomes of S. suis Canadian strain 89/1591 and European strain P1/7, Sao does not share significant homology with other known sequences. However, a conserved avirulence domain that is often found in plant pathogens has been detected. Electron microscopy using an Sao-specific antiserum has confirmed the surface location of the Sao protein on S. suis. The Sao-specific antibody reacts with cell lysates of 28 of 33 S. suis serotypes and 25 of 26 serotype 2 isolates in immunoblots, suggesting its high conservation in S. suis species. The immunization of piglets with recombinant Sao elicits a significant humoral antibody response. However, the antibody response is not reflected in protection of pigs that are intratracheally challenged with a virulent strain in our conventional vaccination model.

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.

Diverse dextranase genes from Paenibacillus species

Genes encoding dextranolytic enzymes were isolated from Paenibacillus strains Dex40-8 and Dex50-2. Single, similar but non-identical dex1 genes were isolated from each strain, and a more divergent dex2 gene was isolated from strain Dex50-2. The protein deduced from the Dex40-8 dex1 gene sequence had 716 amino acids, with a predicted M(r) of 80.8 kDa. The proteins deduced from the Dex50-2 dex1 and dex2 gene sequences had 905 and 596 amino acids, with predicted M(r) of 100.1 kDa and 68.3 kDa, respectively. The deduced amino acid sequences of all three dextranolytic proteins had similarity to family 66 glycosyl hydrolases and were predicted to possess cleavable N-terminal signal peptides. Homology searches suggest that the Dex40-8 and Dex50-2 Dex1 proteins have one and two copies, respectively, of a carbohydrate-binding module similar to CBM_4_9 (pfam02018.11). The Dex50-2 Dex2 deduced amino acid sequence had highest sequence similarity to thermotolerant dextranases from thermophilic Paenibacillus strains, while the Dex40-8 and Dex50-2 Dex1 deduced protein sequences formed a distinct sequence clade among the family 66 proteins. Examination of seven Paenibacillus strains, using a polymerase chain reaction-based assay, indicated that multiple family 66 genes are common within this genus. The three recombinant proteins expressed in Escherichia coli possessed dextranolytic activity and were able to convert ethanol-insoluble blue dextran into an ethanol-soluble product, indicating they are endodextranases (EC 3.2.1.11). The reaction catalysed by each enzyme had a distinct temperature and pH dependence.

Roles of Streptococcus mutans dextranase anchored to the cell wall by sortase

In order to clarify the role that sortase (SrtA) plays in anchoring dextranase (Dex) to the cell wall of Streptococcus mutans, both Dex- and SrtA- mutants were constructed by insertional inactivation of the respective genes. Western blot analysis with a Dex antiserum showed that in the srtA mutant the Dex was not bound to the cell wall but was secreted into the culture supernatant. In contrast, in the wild type, Dex remained cell-wall-associated. Biological properties of the srtA mutant were examined in dextran fermentation, colony morphology and adherence to a smooth surface. The srtA mutant, as well as the wild type, retained the ability to ferment dextran. However, the colony morphology of the srtA mutant on Todd Hewitt agar containing sucrose was much larger than that of the wild type and showed a ring-like structure. In addition, the srtA mutant was more adhesive to a smooth surface than the wild type when sucrose was present. However, the adhesion of the srtA mutant remarkably decreased by addition of exogenous dextranase. These studies suggest that the SrtA mediates Dex-anchoring to the cell wall in S. mutans, and cell wall-anchored Dex plays a role in controlling both the adhesive properties of extracellular glucan and the ability to utilize extracellular glucan as a nutrient source. In contrast, extracellular Dex is only responsible for degrading extracellular glucan as a nutrient source.

Species-specific PCR method for identification of Streptococcus downei

AIMS

To establish a rapid method to differentiate Streptococcus downei and S. sobrinus by multiplex PCR.

METHODS AND RESULTS

A PCR primer pair specific to S. downei was designed on the basis of the nucleotide sequence of the dextranase gene of S. downei NCTC 11391T. The primer pair specifically detected S. downei, but none of the other mutans streptococci (16 strains of six species). The PCR procedure was capable of detecting 1 pg of genomic DNA purified from S. downei NCTC 11391 and as few as 14 CFU of S. downei cells. The mixture of primer pairs specific to each S. downei (this study) and S. sobrinus (Igarashi et al. 2000) detected only the strains of these two species among all the mutans streptococcal strains, and concomitantly differentiated the two species by species-specific amplicons of different lengths.

CONCLUSIONS

The present PCR method is highly specific to S. downei and is useful for detection and identification of S. downei.

SIGNIFICANCE AND IMPACT OF THE STUDY

Multiplex PCR using dextranase gene primers is a useful method for simultaneous detection and differentiation of S. downei and S. sobrinus.

Purification and properties of extracellular dextranase from a Bacillus sp

Bacterial strains in the genus Bacillus were isolated from natural soil samples and screened for production of extracellular dextranases (E.C.3.2.1.11). One strain, determined by 16sRNA analysis as Paenibacillus illinoisensis exhibiting stable dextranase activity, was chosen for further analysis, and the dextranase from it was purified 733-fold using salt and PEG precipitations, two-phase extraction and DEAE-Sepharose chromatography with a total yield of 19%. The purified enzyme had three isoforms, with molecular masses of 76, 89 and 110kDa and isoelectric points of 4.95, 4.2 and 4.0, respectively. The mixture of the three dextranase isoforms has a broad pH optimum around pH 6.8 and a temperature optimum at 50 degrees C. The N-terminal sequence (Ala-Ser-Thr-Gly-Lys) was identical between the isoforms. No sequence homology with the known dextranases in the protein databanks was found.

Analysis of a dextran-binding domain of the dextranase of Streptococcus mutans

AIMS

To examine the dextran-binding domain of the dextranase (Dex) of Streptococcus mutans.

METHODS AND RESULTS

Deletion mutants of the Dex gene of Strep. mutans were prepared by polymerase chain reaction and expressed in Escherichia coli cells. Binding of the truncated Dexs to dextran was measured with a Sephadex G-150 gel. Although the Dexs which lacked the N-terminal variable region lost enzyme activity, they still retained dextran-binding ability. In addition, further deletion into the conserved region from the N-terminal did not influence the dextran-binding ability. However, the Dex which carried a deletion in the C-terminus still possessed both enzyme activity and dextran-binding ability. Further deletion into the conserved region from the C-terminal resulted in complete disappearance of both enzyme and dextran-binding activities.

CONCLUSIONS

Deletion analysis of the Dex gene of Strep. mutans showed that the C-terminal side (about 120 amino acid residues) of the conserved region of the Dex was essential for dextran-binding ability.

SIGNIFICANCE AND IMPACT OF THE STUDY

The dextran-binding domain was present in a different area from the catalytic site in the conserved region of the Dex molecule. The amino acid sequence of the dextran-binding domain of the Dex differed from those of glucan-binding regions of other glucan-binding proteins reported.

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.

Isolation of a new Thermoanaerobacterium thermosaccharolyticum strain (FH1) producing a thermostable dextranase

A Gram-positive spore-forming thermophilic strict anaerobic bacterium, designated FH1, was isolated from enrichments at 65 degrees C with dextran as sole carbon and energy source. A sequence analysis of the 16S rRNA gene revealed 99.2% identity of FH1 to Thermoanaerobacterium thermosaccharolyticum. Furthermore, the substrate spectra of both organisms were similar. It was therefore concluded that FH1 represents a new strain within the species T. thermosaccharolyticum. The optimal growth temperature of strain FH1 was 68 degrees C. The isolated organism produced a thermostable and thermoactive dextranase with a native molecular mass of approximately 200,000 Da. The enzyme was concentrated from the cell-free culture supernatant by ammonium sulfate precipitation. The resulting crude dextranase exhibited optimal activity from 65 to 70 degrees C and a pH optimum of 5.5.

Identification of mutans streptococcal species by the PCR products of the dex genes

A pair of polymerase chain reaction (PCR) primers was designed on the basis of the nucleotide sequence homology of dextranase genes (dex) of Streptococcus mutans, S. sobrinus and S. downei. The primer pair amplified a 530-bp DNA fragment on the dex genes of mutans streptococcal species: S. mutans, S. sobrinus, S. downei, S. rattus and S. cricetus. HaeIII digestion of the 530-bp fragments generated species-specific subfragments, which were easily distinguishable from each other by agarose gel electrophoresis. These results suggest that the PCR-amplification of the dex gene followed by the HaeIII digestion is useful for rapid identification of the five species of mutans streptococci.

Identification of Streptococcus salivarius by PCR and DNA probe

AIMS

To establish species-specific PCR and DNA probe methods for Streptococcus salivarius and to clarify the distribution of dextranase in oral isolates of Strep. salivarius.

METHODS AND RESULTS

A pair of PCR primers and a DNA probe were designed based on the nucleotide sequence of the dextranase gene of Strep. salivarius JCM5707. Both the PCR primer and the DNA probe specifically detected Strep. salivarius but none of the other oral streptococci (23 strains of 13 species). The primer and the probe were capable of detecting 1 pg and 1 ng of the genomic DNA, respectively, purified from Strep. salivarius JCM5707. All oral isolates (130 strains from 12 subjects) of Strep. salivarius from human saliva were positive by both methods.

CONCLUSION

The present PCR and DNA probe methods are highly specific to Strep. salivarius and are useful for the its detection and identification of this bacterium. The dextranase widely distributes among oral isolates of Strep. salivarius.

SIGNIFICANCE AND IMPACT OF THE STUDY

The DNA sequence of a dextranase gene present in the genome of Strep. salivarius is useful as the target DNA of the species-specific PCR and DNA probe.

A DNA probe specific to Streptococcus sobrinus

Three DNA fragments (SSB-1, -2 and -3) in the dextranase gene (dex) of Streptococcus sobrinus were amplified by polymerase chain reaction and used as DNA probes. The probes were examined for the specificity and the sensitivity of hybridization with DNA of oral streptococcal species. While probes SSB-1 and SSB-2 were specific to both S. sobrinus and Streptococcus downei, SSB-3 was specific only to S. sobrinus. SSB-3 was able to detect 5 ng of chromosomal DNA purified from S. sobrinus NIDR6715 and DNA extracted from 1 x 10(5) cells of the strain. In addition, SSB-3 could differentiate clinical isolates of S. sobrinus from Streptococcus mutans. These results suggest that SSB-3 is an effective DNA-probe to detect and to identify S. sobrinus.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

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.

Structural model of Dex protein from Penicillium minioluteum and its implications in the mechanism of catalysis

The DEX gene encodes an extracellular dextranase (EC 3.2.1.11); this enzyme hydrolyzes the alpha(1,6) glucosidic bond contained in dextran to release small isomaltosaccharides. Sequence analysis has revealed only one homologous sequence, CB-8 protein, from Arthrobacter sp., with 30% sequence identity. The secondary structure prediction for Dex was corroborated by circular dichroism measurements. To explore the possibility that Dex protein might adopt a fold similar to any known structure, we conducted a threading search of a three-dimensional structure database. This search revealed that the Dex sequence is compatible with the galactose oxidase/methanol dehydrogenase/sialidase fold. A structural model of Dex based on these results is physically and biologically plausible and leads to testable predictions, including the prediction that Asp246 and Glu299 might be catalytic residues. Also, according to this model the Dex enzyme has a mechanism of hydrolysis with net inversion of anomeric configuration.

Cloning and sequencing of a dextranase-encoding cDNA from Penicillium minioluteum

A cDNA from Penicillium minioluteum HI-4 encoding a dextranase (1,6-alpha-glucan hydrolase, EC 3.2.1.11) was isolated and characterized. cDNA clones corresponding to genes expressed in dextran-induced cultures were identified by differential hybridization. Southern hybridization and restriction mapping analysis of selected clones revealed four different groups of cDNAs. The dextranase cDNA was identified after expressing a cDNA fragment from each of the isolated groups of cDNA clones in the Escherichia coli T7 system. The expression of a 2 kb cDNA fragment in E. coli led to the production of a 67 kDa protein which was recognized by an anti-dextranase polyclonal antibody. The cDNA contains 2109 bp plus a poly(A) tail, coding for a protein of 608 amino acids, including 20 N-terminal amino acid residues which might correspond to a signal peptide. There was 29% sequence identity between the P. minioluteum dextranase and the dextranase from Arthrobacter sp. CB-8.

Cloning of the Penicillium minioluteum gene encoding dextranase and its expression in Pichia pastoris

The DEX gene encoding an extracellular dextranase was isolated from the genomic DNA library of Penicillium minioluteum by hybridization using the dextranase cDNA as a probe. Comparison of the gene and cDNA sequences revealed that the DEX gene does not contain introns. Amino acid sequences comparison of P. minioluteum dextranase with other reported dextranases reveals a significant homology (29% identity) with a dextranase from Arthrobacter sp. CB-8. The DEX gene fragment encoding a mature protein of 574 amino acids was expressed in the methylotrophic yeast Pichia pastoris by using the SUC2 gene signal sequence from Saccharomyces cerevisiae under control of the alcohol oxidase-1 (AOX1) promoter. Over 3.2 g/l of enzymatically active dextranase was secreted into the medium after induction by methanol. The yeast product was indistinguishable from the native enzyme in specific activity and the N-terminus of both proteins were identical.

Oral streptococci with genetic determinants similar to the glucosyltransferase regulatory gene, rgg

The Streptococcus gordonii Challis glucosyltransferase structural gene, gtfG, is positively regulated by the upstream gene, rgg, the only described gtf regulatory determinant in oral streptococci. Southern hybridization analyses indicated that rgg-like and gtfG-like determinants were present on the same HindIII fragment in strains of S. gordonii, Streptococcus sanguis, and Streptococcus oralis, whereas no rgg-like determinants were detected in mutans streptococci, Streptococcus mitis, and Streptococcus salivarius.

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.

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.