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Journal of Bacteriology, October 1999, p. 6556-6559, Vol. 181, No. 20
Department of Preventive Dentistry,
Received 7 June 1999/Accepted 30 July 1999
Gene rgpG is required for biosynthesis of
rhamnose-glucose polysaccharide (RGP) in Streptococcus
mutans. Its deduced amino acid sequence had similarity to WecA,
which initiates syntheses of enterobacterial common antigen and some O
antigens in Escherichia coli. Gene rgpG
complemented a wecA mutation of E. coli,
suggesting that rgpG may function similarly in RGP synthesis.
Cell wall antigens of
Streptococcus mutans are rhamnose-glucose polysaccharides
(RGPs), which are composed of We recently cloned three loci that are related to RGP synthesis. Four
rml genes (rmlA, rmlB,
rmlC, and rmlD) that are directly involved in the
synthesis of dTDP-L-rhamnose (22, 23) and a gene
(gluA) encoding glucose-1-phosphate uridylyltransferase, which synthesizes UDP-D-glucose (28), were
identified in these loci. The two nucleotide sugars were found to
be immediate precursors for RGP synthesis (22, 23, 28).
In addition, we found six rgp genes (rgpA,
rgpB, rgpC, rgpD, rgpE, and
rgpF) required for RGP synthesis in the region downstream
from rmlD (27). Some of these rgp
genes are probably involved in the transport and assembly of RGP.
Here, we describe the identification and characterization of an
additional gene required for RGP synthesis, which is located in a
new locus.
Isolation of an RGP-defective mutant of S. mutans.
A
complete Sau3AI digest of the S. mutans Xc
chromosome was ligated to BamHI- and
BglII-digested pResEmBBN. pResEmBBN was produced by Shiroza
and Kuramitsu in the process of constructing pResEmMCS11
(18) to prepare an S. mutans genomic
library and is equivalent to pResEmMCS11, except that it lacks the
restriction sites from XbaI to NotI in the
multicloning site. S. mutans Xc (22) was randomly
mutated by transformation with the S. mutans genomic
library. Transformants were selected on tryptic soy agar plates
containing 10 µg of erythromycin per ml. Both rml and
rgp mutants of S. mutans, which are completely
unable to synthesize RGP, have a characteristic colony morphology
(22, 23, 27). The fact enables us to visually distinguish a
mutant defective in RGP synthesis from other mutants.
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
A Novel Gene Required for Rhamnose-Glucose
Polysaccharide Synthesis in Streptococcus mutans
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1,2- and
1,3-linked rhamnan
backbones and glucose side chains (8, 14). The biofunction of RGP is receiving increasing attention because of the facts that the
serotype f-specific RGP induces the release of inflammatory cytokines (19) and also provokes nitric oxide production
(9).
Characterization of plasmid insertion point in Xc51. Southern blotting with a DIG-labeled PCR probe specific for the Emr gene revealed that the probe hybridized with a 4.6-kb EcoRI fragment from Xc51 but not with any fragments in the wild-type strain Xc. A PCR probe specific for the Emr gene was prepared by PCR with a set of primers (5'-CTTAGAAGCAAACTTAAG-3' and 5'-TTATTTCCTCCCGTTAAA-3'), using pResEmBBN as a template. Strain Xc was transformed with the chromosome DNA prepared from Xc51. Every Emr transformant showed the same colony morphology as that of Xc51. Five randomly selected transformants were further confirmed to be defective in RGP synthesis. Southern blot analysis with the specific probe for the Emr gene of these transformants gave identical results to that for Xc51. These findings indicated that integration of pResEmBBN in a specific locus on the Xc chromosome, which was different from the rml and rgp loci involved in RGP synthesis, produced a novel defect in RGP synthesis.
Cloning and sequencing of the region flanking the plasmid insertion in strain Xc51. The EcoRI-digested chromosome of Xc51 was self-ligated. Escherichia coli DH5 was transformed with this DNA, and transformants were isolated on Luria-Bertani agar plates containing 200 µg of erythromycin per ml. All plasmids isolated from transformants were identical (4.6 kb) in size. One of them was designated pKU51. The plasmid labeled with DIG-dUTP via random primer labeling hybridized with the 4.6-kb EcoRI fragment in Xc51 and a 2.6-kb EcoRI fragment in Xc. The 2.6-kb EcoRI-fragment of Xc was cloned in pBluescript SKII+ by colony hybridization. The resulting plasmid, pBluescript SKII+ carrying the 2.6-kb EcoRI-fragment, was designated pKU52 and the nucleotide sequence of the insert was determined with a 373 STRETCH automated sequencer (Applied Biosystems, Inc., Foster City, Calif.) as described previously (26).
The nucleotide sequence analysis revealed the presence of two open reading frames (ORFs), as shown in Fig. 1. They were oriented in the same direction as the plasmid lacZ gene. No ORF on the opposite strand of this region was more than 250 bp in length. A possible Shine-Dalgarno sequence was identified just upstream from the potential initiation codons of orf1. A consensus
10 and
35
E. coli promoter-like sequence
(TTGAAA-N17-TATAAT; positions 302 to 330) for
orf1 and an inverted repeat structure (positions 2257 to
2274) with a free energy of
16.8 kcal/mol followed by a polyT
sequence, which may act as a transcriptional terminator for
orf2, were recognized. On the other hand, there was no
typical ribosomal binding motif for orf2.
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Insertional inactivation of orf2. The 0.9-kb PpuMI fragment within orf2 on pKU52 was replaced with the 1.0-kb Emr gene (18). The resultant plasmid was linearized by EcoRI digestion and used to transform strain Xc. Four randomly selected Emr transformants were examined by immunodiffusion analysis, and all were ascertained to be defective in RGP synthesis. An appropriate disruption of orf2 in these transformants was confirmed by PCR amplification of the 1.2-kb region spanning orf2 with primers RgpGF2 and RgpGR1 (Fig. 1) and the subsequent HincII digestion of this fragment, because there is a HincII site within the Emr gene but none in orf2. One of the transformants was designated Xc52.
When the total amount of hexosamine in the purified cell wall preparation of Xc52 was determined by using the colorimetric method of Strominger et al. (21), the hexosamine content in the cell wall of Xc52 (163 ± 13 µg per mg [dry weight] of the purified cell wall preparation) was not very different from those in the cell walls of Xc and the purified cell wall preparations of its rml mutants (23). It seems unlikely that rgpG is involved in peptidoglycan synthesis.Complementation analysis of rgpG. We constructed a new E. coli-streptococcus shuttle plasmid, pKU55, containing a tetracycline resistance marker, the tetM916 gene derived from pLN2 (2), which functions both in E. coli and in streptococci, and the pUC and pC194 replicons for maintenance in E. coli and in streptococci, respectively, which are derived from pTH10 (4). Expression of rgpG was ensured by the promoter sequence in the 0.35-kb upstream region of the Emr gene from pAM77 (3). The 0.35-kb region was amplified by PCR with primers EmF2, 5'-AGAGAGTCTAGAGAAGCAAACTTAAGAGTGTG-3' (XbaI site underlined), and EmR2, 5'-GTGTGTCTGCAGTTTCGTCGTTAAATGCC-3' (PstI site underlined), using pResEmBBN as a template. The PCR fragment was digested with XbaI and PstI and ligated to the XbaI and PstI sites of pKU52, producing pKU54, which contains the streptococcal promoter sequence at the 5' terminus of rgpG. The fragment containing both promoter sequence and rgpG in the same direction was excised from pKU54 by digestion with XbaI and HindIII and ligated to the XbaI and HindIII sites of pKU55; the resulting plasmid was designated pKU56.
Direct transformation of the rgpG mutant with pKU56 was abandoned because we tried unsuccessfully to transform Xc52 with shuttle plasmid pKU55 and also with a tetracycline resistance marker on the chromosomal DNA from strain GS5DD (2). The surface structure of cells with normal RGP synthesis might be critical to the genetic competence of S. mutans. As an alternative, wild-type strain Xc was initially transformed with pKU56 and subsequently transformed with EcoRI-digested pKU53. Three transformants which were randomly selected retained their serotype c antigenicity and had normal rhamnose and glucose contents in cell wall preparations, suggesting that the rgpG gene located on the shuttle plasmid complemented the inactivated chromosomal rgpG. The result ruled out a possible polar effect of the pResBBN insertion and proved a direct effect of the disruption of rgpG on the phenotype of Xc52.Functional analysis of rgpG. We examined whether rgpG complements wecA. A plasmid, pKU58, which has only rgpG on the insert fragment, was constructed by PstI digestion of pKU52 and self-ligation for removal of orf1 (Fig. 1). The plasmid was introduced into E. coli 21548, which is a wecA-defective mutant of strain K-12 (10). Strain 21548 was kindly provided by P. D. Rick, Department of Microbiology, Uniformed Services University of Health Sciences, Bethesda, Md. ECA and O9a antigen production in E. coli strains were detected by immunoblotting analysis with anti-ECA serum and anti-O9a monoclonal antibody (MAb F719), respectively, as previously described (5, 12), because the wecA gene is known to be involved in the syntheses of ECA (10) and O9a antigen (5). Strain 21548 transformed with pKU58 and its parental strain AB1133 produced ECA, whereas strain 21548 transformed with pBluescript SKII+ did not (Fig. 2A). Furthermore, O9a antigen production was observed in strain 21548 transformed with both pKU58 and pNKB26 and in strain AB1133 transformed with only pNKB26 but not in 21548 transformed with pNKB26 (Fig. 2B). These findings indicate that rgpG complemented the wecA-deficient phenotype of E. coli, suggesting a functional similarity between the gene products of rgpG and wecA.
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Nucleotide sequence accession number. The 2,614-bp nucleotide sequence described in this paper has been submitted to the EMBL/GenBank/DDBJ data bank under accession no. AB022909.
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ACKNOWLEDGMENTS |
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This work was supported in part by a Grant-in-Aid for Developmental Scientific Research [(B)09470474] (Y.Y.) from the Ministry of Education, Science, Sports and Culture of Japan and the Kyushu University Interdisciplinary Programs in Education and Projects in Research Development.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Preventive Dentistry, Kyushu University Faculty of Dentistry, Fukuoka 812-8582, Japan. Phone: 81-92-642-6353. Fax: 81-92-642-6354. E-mail: yoshidha{at}mbox.nc.kyushu-u.ac.jp.
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