Evidence of multiple branching in the levan elaborated by Streptococcus salivarius strain 51

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.

Genetic analysis of fructan-hyperproducing strains of Streptococcus mutans

Fructan polymer, synthesized from sucrose by the extracellular fructosyltransferase of Streptococcus mutans, is thought to contribute to the progression of dental caries. It may serve as an extracellular storage polysaccharide facilitating survival and acid production. It may also have a role in adherence or accumulation of bacterial cells on the tooth surface. A number of clinical isolates of S. mutans which produce large, mucoid colonies on sucrose-containing agar as a result of increased production of fructan have been discovered. By using eight independent isolates, we sought to determine if such fructan-hyperproducing strains represented a genetically homogeneous group of organisms. Restriction fragment patterns of total cellular DNA were examined by using pulsed-field and conventional gel electrophoresis. Four genetic types which appeared to correlate with the serotype of the organism and the geographic site of isolation were evident. Southern blot analysis of several genetic loci for extracellular enzymes revealed some minor differences between the strains, but the basic genomic organizations of these loci were similar. To evaluate whether the excess fructan produced by these strains enhanced the virulence of these organisms in the oral cavity, it was of interest to create mutants deficient in fructosidase (FruA), the extracellular enzyme which degrades this polymer. The fruA gene was inactivated by allelic exchange in two fructan-hyperproducing strains as well as in S. mutans GS5, a strain which does not hyperproduce fructan. All of the fruA mutant strains were devoid of fructan hydrolase activity when levan was used as a substrate. However, the fructan-hyperproducing strains retained the ability to hydrolyze inulin, suggesting the presence of a second fructosidase with specificity for inulin in these strains.

The fructosyltransferase of Streptococcus salivarius

The nature of the fructans produced by oral streptococci and the β-D-fructosyltransferases (FTFs) that synthesize them from sucrose are considered in this review. Models for the apparent unique surface expression and secretion of the FTF produced by Streptococcus salivarius are discussed in light of recent evidence obtained from the cloning and sequencing of the ftf from this organism.

Characterization of the Streptococcus mutans GS-5 fruA gene encoding exo-beta-D-fructosidase

The complete nucleotide sequence (5,010 bp) of the fructanase gene (fruA) and flanking regions of the chromosome of Streptococcus mutans GS-5 was determined. The fruA gene appears to be the sole transcript arising from a proximal promoter. The presumed precursor of the secreted FruA protein consists of 1,423 amino acids, and it has an M(r) of 158,656 and a pI of 4.82. The N terminus of FruA has characteristics in common with signal peptides of gram-positive organisms. The C terminus consists of a serine- and threonine-rich region, followed by the peptide LPDTGD, 4 charged amino acids, 21 amino acids with a strongly hydrophobic character, and a charged pentapeptide tail, which are proposed to correspond to the wall-spanning region, the LPXTGX consensus sequence, and the membrane-spanning domains of surface-associated proteins of gram-positive cocci. The FruA protein has significant homology with the Bacillus subtilis levanase (SacC), the Bacteroides fragilis levanase (ScrL), yeast invertases, and a number of other beta-fructosidases but not with fructosyltransferase, glucosyltransferases, or glucan-binding proteins of oral streptococci. Genes with homology to fruA were detected in S. mutans serotype c, e, and f strains, Streptococcus rattus, Streptococcus salivarius, and Streptococcus sanguis. A deletion derivative of FruA lacking the C-terminal 437 amino acids was still functional and could hydrolyze beta-(2,6)- and beta-(2,1)-linked sugars, but with altered preference for substrates. The data begin to define functional domains of the FruA protein and potential regulatory sites for induction, repression, growth rate control, and posttranslational localization of this multifunctional enzyme.

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.

Expression, purification, and characterization of an exo-beta-D-fructosidase of Streptococcus mutans

A genetic library of Streptococcus mutans GS-5, constructed in an Escherichia coli plasmid vector, was screened for cells which could utilize sucrose as the sole carbon and energy source. The recombinant plasmid pFRU1, containing a 4.2-kilobase pair insert of S. mutans DNA, was shown to confer this phenotype. Further characterization of the gene product encoded by pFRU1 revealed that the enzyme was a beta-D-fructosidase with the highest specificity for the beta (2----6)-linked fructan polymer levan. The enzyme could also hydrolyze inulin [beta (2----1)-linked fructan], sucrose, and raffinose with 34, 21, and 12%, respectively, of the activity observed for levan. The gene (designated fruA) appeared to be expressed under its own control in E. coli, as judged by the lack of influence on gene product activity of induction or repression of the beta-galactosidase promoter adjacent to the insertion site on the cloning vector. The protein was purified to homogeneity, as judged by silver staining of purified protein in denaturing and reducing conditions in polyacrylamide gels, from sonic lysate of E. coli, as well as from culture supernatants of S. mutans GS-5 grown in a chemostat at low dilution rate with fructose as the sole carbohydrate source. Both purified proteins had an apparent molecular mass of 140,000 daltons in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were immunologically related and comigrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as determined by Western blotting with antisera raised against the cloned gene product, and were identical in all physical and biochemical properties tested. The pH optimum of the enzyme acting on fructan polymers was 5.5, with a significant amount of activity remaining at pH 4.0. The optimum pH for sucrose degradation was broader and lower, with a peak at approximately 4.5. Enzyme activity was inhibited almost completely by Hg2+ and Ag2+, inhibited partially by Cu2+, not inhibited by fluoride ion or Tris, and slightly stimulated by Mn2+ and Co2+. Fructan polymers were attacked exohydrolytically by the enzyme, fructose being the only product released. With sufficient time, both levan and inulin were degraded to completion, with no evidence of product inhibition.

Role of NADH oxidase in the oxidative inactivation of Streptococcus salivarius fructosyltransferase

A cell-associated fructosyltransferase produced by Streptococcus salivarius was irreversibly inactivated in a time-dependent manner when resting or permeabilized cell suspensions were incubated with low concentrations (less than 1.0 microM) of copper. In addition to copper, the inactivation was dependent on oxygen and on a fermentable carbon source (endogenous intracellular polysaccharide or glucose). In starved, permeabilized cell suspensions, the fermentable carbon source could be replaced by NADH but not by NADPH or ATP. Of several other S. salivarius enzymes tested, only fructosyltransferase was inactivated under these conditions. The available evidence indicated that NADH oxidase is the enzyme responsible for fructosyltransferase inactivation. Results from oxygen radical scavenger studies implicated one or more species of oxygen radicals and hydrogen peroxide in the inactivation reaction.

Production and Characterization of a Polymer from Arthrobacter sp

An Arthrobacter sp. isolated from a glucose-sucrose agar plate was found to produce a neutral, extremely viscous, opalescent extracellular polymer. Growth, polymer production, and rheological properties and chemical composition of the isolated polymer were examined. The polymer was found to be substantially different from other arthrobacter polymers. Some unusual properties included irreversible loss of viscosity with high temperature and degradation of the polymer during fermentation and upon storage at 4°C. Other characteristics included dependence on sucrose for polymer production, relative pH stability, increased viscosity with increased salt concentration, and pseudoplasticity. The polymer was found to be composed primarily (if not entirely) of d -fructose. The fructose content and other characteristics suggested that the polymer was a levan.