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Journal of Bacteriology, June 2006, p. 4125-4130, Vol. 188, No. 11
0021-9193/06/$08.00+0 doi:10.1128/JB.01843-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Soumya Ganguly,2
C. Allen Bush,2 and
John O. Cisar1*
Oral Infection and Immunity Branch, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, Maryland 20892,1 Department of Chemistry and Biochemistry, University of MarylandBaltimore County, Baltimore, Maryland 212502
Received 2 December 2005/ Accepted 14 March 2006
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-GalNAc in type 1Gn RPS (14).
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FIG. 1. Structural types of RPS produced by different streptococci in the present study.
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-GalNAc and
-Rha to Rhaß1-4Glc (20). The specific roles of wefA and wefB in synthesis of the resulting branched structure is, however, unclear, as neither gene has a well-studied homologue in the database.
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FIG. 2. ORF diagrams of RPS gene clusters indicating the homology between strains and the predicted roles of different genes in RPS biosynthesis. Each cluster contains four common regulatory genes (stippled arrows), six or seven genes for glycosyl or glycosyl-1-phosphotransferases (hatched arrows), and additional genes for a polysaccharide polymerase (solid arrows), a repeat unit transporter (open arrows), and enzymes for nucleotide sugar biosynthesis (brick-pattern arrows). Flanking genes (arrows with dashed outlines) are also identified, as are the positions of putative promoters (bent arrows) and rho-independent terminators (lollipops).
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FIG. 3. Proposed pathway for biosynthesis of types 2G, 2Gn, 1G, and 1Gn RPS beginning with the WchA-catalyzed transfer of Glc-1-PO4 to carrier lipid (L). The present findings define the roles of WefA and WefB in RPS biosynthesis and reveal a difference in the specificities of WefC and WefH for branched versus linear acceptors.
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-GalNAc in types 1Gn, 2Gn, and 2G RPS (Fig. 1) and wefB with synthesis of the Rha branches in type 2Gn and 2G RPS. To test these hypotheses, we replaced various specific genes in S. gordonii 38, S. oralis 34, or S. mitis J22 as previously described (21) by transforming these bacteria with DNA constructs containing the ermAM cassette (12) flanked by appropriate 0.5- to 1-kb gene-targeting sequences. The constructs used as transforming DNA were prepared by overlap extension PCR (9, 11). The location of the cassette in the resulting erythromycin-resistant transformants (Table 1) was verified by amplification of specific PCR products across the upstream and downstream boundaries of the ermAM insertion (results not shown). Cell surface RPS production by wild-type and mutant strains was characterized and compared by the binding of different RPS-specific probes to decreasing numbers of bacteria immobilized on nitrocellulose membranes (21). The probes utilized included anti-type 1 RPS specific immunoglobulin G (IgG) from antiserum against S. oralis 34, which was purified by elution from coupled type 1Gn RPS (16), anti-type 2 RPS specific IgG from antiserum R103 against S. gordonii 38 (6), which was purified by elution (20) from coupled type 2G RPS of S. oralis J22, and biotin-labeled A. naeslundii 12104 (4, 18), which binds both Gn and G types of streptococcal cell surface RPS (5, 21).
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TABLE 1. Streptococci and plasmids used in this study
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FIG. 4. RPS production by wild-type and mutant streptococci detected by dot immunoblotting with RPS-specific rabbit IgG or binding of biotin-labeled A. naeslundii 12104. Nitrocellulose membranes were spotted with streptococci, incubated with RPS-specific rabbit IgG or biotin-labeled A. naeslundii, washed, and developed with goat anti-rabbit IgG or horseradish peroxidase-conjugated avidin, followed by a substrate to detect bound IgG or A. naeslundii, respectively. Partial ORF diagrams of wild-type and mutant streptococci indicate the presence of genes from S. oralis 34 (yellow arrows), S. gordonii 38 (white arrows), S. oralis J22 (black arrows), or ermAM (red arrows).
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The finding that wefB directs the synthesis of Rha branches in type 2Gn and 2G RPS leaves wefA for the transfer of
-GalNAc to Rhaß1-4Glc in these polysaccharides. Experimental evidence supporting this role has been obtained in recent molecular studies of S. oralis 10557 type 3G RPS, which contains
-Gal rather than
-GalNAc as in type 2Gn RPS (Y. Yoshida et al., manuscript in preparation). While further studies are needed to verify the order of addition, we suspect that the WefA-catalyzed formation of GalNAc
1-3Rhaß in linear type 1Gn RPS precedes the WefB-catalyzed addition of Rha
to Rhaß (Fig. 3), as the alternative would require that WefA utilize subterminal Rha as an acceptor in biosynthesis of type 2Gn and 2G RPS and terminal Rha as an acceptor in biosynthesis of type 1Gn RPS.
Based on the above findings, we expected that the expression of wefB in S. oralis 34 would switch RPS production from type 1Gn to type 2Gn. To examine this possibility, we isolated a wefB-inserted transformant of S. oralis 34 (i.e., S. oralis OC5) following transformation of this wild-type strain with wefB linked to the selectable ermAM cassette. Transforming DNA was prepared by two rounds of overlap extension PCR, the first to link wefB to ermAM and the second to link the resulting wefB-ermAM fragment to targeting sequences for insertion between wefA of S. oralis 34 and the downstream gene, which is identified below as wefH. The precise insertion of the wefB-ermAM fragment between wefA and wefH in S. oralis 34 was confirmed by DNA sequencing of this region in S. oralis OC5. Surprisingly, this mutant, which expresses wefB from the promoter at the 5' end of the RPS gene cluster and downstream genes for RPS biosynthesis from the promoter in the ermAM cassette, as well a similar mutant in which the order of wefB and ermAM was reversed (results not shown), reacted with anti-type 1 but not anti-type 2 RPS specific IgG in dot immunoblotting (Fig. 5). Consistent with these reactions, the resonances in the HSQC NMR spectrum of the RPS purified from S. oralis OC5 corresponded exactly to the 1H and 13C assignment for S. oralis 34 type 1Gn RPS (3), with no indication (i.e., less than 5% [data not shown]) of Rha branches.
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FIG. 5. RPS production by wild-type and mutant streptococci detected by dot immunoblotting with anti-type 1 or anti-type 2 RPS specific rabbit IgG. Nitrocellulose membranes were spotted with decreasing numbers of streptococci, incubated with RPS-specific rabbit IgG, washed, and developed with goat anti-rabbit IgG followed by a substrate to detect bound IgG. Partial ORF diagrams of wild type and mutant streptococci strains indicate the presence of genes from S. oralis 34 (yellow arrows), S. gordonii 38 (white arrows), or ermAM (red arrows).
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In contrast, S. gordonii GC48, obtained by replacement of wefC in S. gordonii 38 with the corresponding gene from S. oralis 34, reacted with anti-type 1 but not anti-type 2 RPS specific IgG (Fig. 5), thereby associating the production of type 1Gn RPS with the presence of a unique gene (i.e., wefH) in the latter strain. The antigenic identification of type 1Gn RPS on S. gordonii GC48 was confirmed by the HSQC NMR spectrum of the RPS purified from this strain (results not shown). In support of these findings, the plasmid-based expression of wefH in S. gordonii 38 altered the RPS-specific immunoreactivity of this strain. Whereas S. gordonii 38 harboring control plasmid pYY101 reacted with anti-type 2 but not anti-type 1 RPS specific IgG, strain 38 harboring pYY110, which expresses wefH, reacted strongly with anti-type 1 and weakly with anti-type 2 RPS specific IgG (Fig. 5). Thus, the WefH-mediated transfer of Gal
l-PO4 to GalNAc
(Fig. 3) prevents WefB from acting, presumably by eliminating the acceptor of this enzyme. This effect may, in turn, limit the accumulation of lipid-linked branched tetrasaccharide in S. gordonii GC48, S. gordonii 38 (pYY110), and S. oralis OC5 by the action of WefB in these strains. Conversely, the branched acceptor formed by the WefB-catalyzed transfer of Rha
to subterminal Rhaß may not be recognized by WefH, a possibility consistent with the known ability of the Rha branch to block the binding of anti-type 1 antibody (14, 17) or the Codium fragile lectin (5) to adjacent GalNAc
of type 2Gn RPS.
WefC contributes to the synthesis of linear type 1Gn RPS in wefB-deficient S. gordonii GC12 (Fig. 4) and of branched-type 2Gn RPS in wefB-containing S. gordonii 38 (Fig. 3). The preference of WefC for the branched acceptor structure synthesized by strain 38 can, however, be inferred from the NMR spectra of type 2Gn RPS (17), which indicate the absence of detectable (<5%) linear hexasaccharide repeats in this polysaccharide. Likewise, WefF of S. oralis J22 utilizes a branched acceptor in synthesis of type 2G RPS and a linear acceptor in synthesis of type 1G RPS by wefB-deficient S. oralis MC2 (Fig. 3 and 4). The identification and structural characterization of type 1G RPS in the present investigation should facilitate the identification of wild-type streptococci that produce this polysaccharide. Once they are identified, it will be of interest to determine whether these bacteria utilize WefF to transfer GalNAc
l-PO4 to GalNAc
or a closely related enzyme that only acts on the linear acceptor formed in the absence of WefB (Fig. 3).
The proteins encoded by wefH of S. oralis 34, wefC of S. gordonii 38, and wefF of S. oralis J22 have pivotal roles in RPS biosynthesis, linking the recognition and antigenic regions of these polysaccharides. This biological role is a direct reflection of the donor specificities of these enzymes for Gal
l-PO4 or GalNAc
l-PO4 and their acceptor specificities for linear or branched structures. In view of the relatively high homology that exists between these proteins (Fig. 2), their ability to discriminate between different substrates is likely to depend on minor differences in amino acid sequence. The identification of such sequences and their association with the synthesis of different types of RPS would contribute to the further characterization of these proteins as members of a recently recognized group of glycosyl-1-phosphotransferases (19) and of the corresponding genes as genetic markers of oral biofilm development.
Nucleotide sequence accession number. The S. oralis 34 RPS gene cluster and flanking regions have been deposited in GenBank under accession no. AB181234.
We thank Kelly Ten Hagen and John Thompson for helpful comments during preparation of the manuscript.
Supplemental material for this article may be found at http://jb.asm.org/. ![]()
Present address: Department of Dental Pharmacology, Iwate Medical University School of Dentistry, 1-3-27 Chuo-dori, Morioka, Iwate 020-8505, Japan. ![]()
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