Department of Microbiology and Immunology, and Division of Clinical Microbiology, University of Western Ontario, London, Ontario, N6A 5C1, Canada1
Instituto de Investigaciones Bioquímicas, Fundación Campomar, Antonio Machado 151, 1405 Buenos Aires, Argentina2
Author for correspondence: Miguel A. Valvano. Tel: +1 519 661 3996. Fax: +1 519 661 3499. e-mail: mvalvano{at}julian.uwo.ca
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ABSTRACT |
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Keywords: Escherichia coli, wb (rfb) gene cluster, lipopolysaccharide (LPS), O antigen, PCR serotyping
Abbreviations: ECA, enterobacterial common antigen; FucN, 4-amino-4,6-dideoxy-D-galactose; FucNac, 4-acetamido-4,6-dideoxy-D-galactose; GlcNAc, N-acetylglucosamine; Vio, viosamine, 4-amino-4,6-dideoxyglucose; VioNAc, N-acetylviosamine (4-acetamido-4,6-dideoxyglucose)
The GenBank accession number for the DNA sequence reported in this paper is AF125322.
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INTRODUCTION |
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The biosynthesis of LPS is very complex and requires the activity of many genes, most of which are clustered in different regions of the chromosomal map. The synthesis of the O antigen involves the formation of a subunit that is polymerized while remaining attached to undecaprenyl phosphate, and later transferred to the lipid A-core, which is assembled by a separate pathway (Raetz, 1996 ). Several O antigen gene clusters have been cloned and sequenced (Reeves et al., 1996
). During the past 10 years, our laboratory has used the O7 antigen genes of E. coli O7:K1 strains as a model system to investigate the biosynthesis, assembly and regulation of O antigens. Previous work by other investigators has shown that the presence of O7 in E. coli K1 strains contributes to the overall virulence of these micro-organisms, commonly found in extraintestinal infections in humans, especially neonatal meningitis and urinary tract infections (Pluschke & Achtman, 1984
).
The O7 antigen subunit contains five sugars, N-acetylglucosamine (GlcNAc), galactose, mannose, 4-acetamido-4,6-dideoxyglucose (N-acetylviosamine; VioNAc) and rhamnose, and its chemical structure has been elucidated (Fig. 1; Lvov et al., 1984
). We have cloned and characterized by transposon mutagenesis two cosmids encoding O7 LPS in E. coli K-12 strains (Marolda et al., 1990
; Valvano & Crosa, 1989
). These laboratory strains usually lack O antigen but produce a complete lipid A-core that serves as an acceptor for O-specific LPS if the genes for its synthesis are supplied in trans (Valvano, 1992
). We discovered that the synthesis of the O7 subunit is initiated in the inner membrane by the activity of the wecA (rfe) gene product (Alexander & Valvano, 1994
), which is an enzyme responsible for the attachment of GlcNAc-1-phosphate to undecaprenyl phosphate via a phosphodiester linkage. wecA is part of a cluster of genes involved in the synthesis of enterobacterial common antigen (ECA) that maps outside the O7 LPS gene cluster (Rick & Silver, 1996
). We also characterized molecularly and biochemically four genes of the O7 biosynthesis gene cluster that are responsible for the synthesis of dTDP-rhamnose (Marolda & Valvano, 1995
), and two genes involved in the GDP-mannose pathway (Marolda & Valvano, 1993
). The genes for the dTDP-rhamnose and GDP-mannose pathways are located at each end of the O7 LPS cluster. In this work, we report the complete characterization of the intervening sequence, revealing the existence of two genes for the dTDP-VioNAc pathway, four presumptive sugar transferases for the assembly of the O7 subunit, the O antigen translocase gene, and the O antigen polymerase gene. The sequence information was also used to develop a pair of DNA primers that provide a specific detection of O7 LPS genes by PCR.
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METHODS |
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DNA sequencing analysis.
DNA was sequenced by the dideoxy method (Sanger et al., 1977 ) modified for use with the Pharmacia T7 DNA Sequencing Kit. Overlapping fragments spanning a 6·9 kb central region of the O7 LPS gene cluster (Fig. 2
) were cloned into either pGEM3 or pGEM4. The recombinant plasmids pCM29, pCM37, pCM38, pCM39, pCM44, pCM59, pCM93, pCM111 and pLD2 (Fig. 2
, Table 1
) were sequenced with T7 or SP6 promoter primers as sequencing end-points. Nested deletions from pCM111, pCM29, pCM38, pCM39, pCM44 and pLD2 (Fig. 2
, Table 1
) were obtained by the ExoIII endonuclease method (Henikoff, 1984
), and used to complete gaps and confirm the DNA sequence of some regions. The junctions of the pJHCV32::Tn3HoHo1 insertions 21, 47, 58, 128 and 136 (Fig. 2
,
Table 1
) were sequenced using a specific primer annealing at one of the ends of the transposon (Marolda & Valvano, 1993
).
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PCR.
Plasmids pMF21 (carrying wzxEcO7) and pMF25 (carrying wzyEcO7) were constructed by amplification of the coding regions of these genes using primers containing unique restriction endonuclease sites to facilitate their cloning into the expression vector pEXT21 (Dykxhoorn et al., 1996 ). Primers 5'-atgagctcatgaataacaaactagccag-3' (SacI site underlined) and 5'-taggatccccaggtaaggcatgaattc-3' (BamHI site underlined), and primers 5'-atggatccatgagaaaaaatttttacacga-3' (BamHI site underlined) and 5'-taaagcttagtcctaagtgtgtac-3' (HindIII site underlined) were used to amplify wzxEcO7 and wzyEcO7, respectively. In both cases PCR amplifications were carried out with Pwo since this enzyme has a high fidelity. PCR products were digested with either SacI/BamHI or BamHI/HindIII, gel purified, and ligated to pEXT21 DNA, which was also digested with the same combination of enzymes in each case.
Chromosomal DNA from strains with different O serogroups was used as a template for the PCR reaction with wzxEcO7- and galF-specific primers. The primers were as follows: wzx, 5'-tcattagctcaggttct-3' (located 50 bases downstream of the initiation codon) and 5'-agactaatggcttcgaga-3' (located 927 bases from the initiation codon, on the complementary strand); galF, 5'-ccggagctcaggtggatgaaataatgacg-3' (located at the initiation codon) and 5'-cgggaatttgttacttcgcttaa-3' (located at the stop codon, on the complementary strand). PCR reactions were carried out with TaqI polymerase with an annealing temperature of 55 °C for both primer pairs, using 28 amplification cycles. Ten-microlitre aliquots from the PCR reaction were examined in ethidium-bromide-stained 0·7% agarose gels.
LPS analysis.
LPS purification, electrophoresis, and immunoblotting with O7 antiserum were all carried out as described elsewhere (Marolda et al., 1990 ).
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RESULTS |
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The product of ORF7 is similar to several proteins in the databases, especially those of the NodL-LacA family of acetyltransferases. These proteins include, among others, WbeO from V. cholerae (accession S28479; Stroeher et al., 1995), WbbJ from E. coli K-12 (accession P37750; Yao & Valvano, 1994 ), CapG from Staphylococcus aureus (accession P39856; Lin et al., 1994
), NodL from Rhizobium (Sinorhizobium) meliloti (accession P28266; Baev & Kondorosi, 1992
) and Cat4 from Agrobacterium tumefaciens (accession P23364; Tennigkeit & Matzura, 1991
). The proteins from this family are involved in the transfer of acetyl groups from acetyl-CoA and share a region of homology along the C-terminal 5070 amino acids (Fig. 4
). One of the main features of this common region is the presence of an hexapeptide motif, also termed the isoleucine patch (Dicker & Seetharam, 1992
), which is present in some transferases (Vaara, 1982
). Each motif starts with an isoleucine, leucine or valine residue and often contains a glycine in the second position. This repeating motif is present in all the proteins of this family (Fig. 4
). We propose that the product of ORF7 is involved in the N-acetylation of dTDP-Vio to complete the formation of dTDP-VioNAc as indicated in Fig. 5
, and have designated this gene as vioB.
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Glycosyltransferases (wbbABCD). We expected to find four transferases since we had already established that wecA is required for the initiation of the synthesis of the O7 subunit (Alexander & Valvano, 1994 ). In a previous report, we identified wbbD (ORF275, Marolda & Valvano, 1993
) as a putative glycosyltransferase gene. A blast search revealed that WbbD is homologous to a diverse family of bacterial glycosyltransferases, transferring sugar from UDP-glucose, UDP-N-acetylgalactosamine, GDP-mannose or CDP-abequose to a range of substrates. The strongest homologies are with AmsE (accession Q46635) and ORF6 (accession S27582) from the lsg locus of Haemophilus influenzae. ORF6 of H. influenzae is involved in the synthesis of lipooligosaccharide (McLaughlin et al., 1992
) but its specific function remains unknown. AmsE is involved in the synthesis of amylovoran (Bugert & Geider, 1995
), an exopolysaccharide of Erwinia amylovora, which contains a repeating unit made of one glucuronic acid and four galactose residues decorated with pyruvate and acetate groups (Smith et al., 1990
). Tentatively, we have assigned wbbD as a galactosyltransferase involved in adding galactose to GlcNAc-P-P-undecaprenol. ORF11 has a strong conservation with mannosyltransferases, especially with WbaU (accession P26402), WbaW (accession X61917), WbdB (accession I76776) and ORF469 (accession AB010150), which are involved in the synthesis of O antigen in two Salmonella enterica serogroups, E. coli O9 and E coli O8, respectively. These findings suggest that ORF11 encodes the
-mannosyltransferase responsible for adding the third sugar to the O7 repeat (Fig. 1
), and it has been designated as wbbC. No significant homologies in the database were found for ORF8 and ORF9. In E. coli and Salmonella O antigen clusters that have been sequenced, the order of transferase genes is in inverse relation to the order of synthesis of the subunit. Therefore, we tentatively assigned ORF8 as wbbA and ORF9 as wbbB, encoding the N-acetylviosaminyl- and rhamnosyltransferases, respectively (Fig. 2
).
Identification of the O antigen chain regulator wzz
Using transposon mutagenesis, we identified an insertion located outside the O7 LPS gene cluster that conferred a unimodal O antigen polysaccharide distribution (Fig. 3, lane I). This phenotype is consistent with a mutation in the wzz gene, which encodes a protein involved in the regulation of the O-side chain length (Batchelor et al., 1992
). To confirm these results and determine the gene structure of the region between gnd and his we performed spotted sequencing using the DNA inserts in plasmids pCM37, pCM59 and pCM93 (Fig. 2
). The results confirmed the existence of two genes in this region, ugd and wzz, both highly homologous to the E. coli K-12 counterparts. ugd encodes the UDP-glucose dehydrogenase, which is an enzyme involved in the synthesis of UDP-glucuronic acid, a precursor for the synthesis of the colanic acid capsular polysaccharide (Stevenson et al., 1996
). By restriction endonuclease analysis, the insertion 134 was mapped to approximately the middle of the wzz gene (Fig. 2
), in agreement with the O7 LPS phenotype determined by pJHCV32::Tn3HoHo1-134.
Design of O7-specific PCR primers for serotyping
The availability of the complete DNA sequence of the O7 cluster permitted us to design O7-specific primers. The sequence of wzx was targeted for this purpose since this gene has great variability among different O antigens. We designed a pair of wzxEcO7-specific primers and used them in PCR reactions with chromosomal DNA templates prepared from E. coli strains of different O serogroups. As a control we used primers specific for the galF gene, which is highly conserved in most E. coli strains examined to date (Marolda & Valvano, 1996 ; C. L. Marolda & M. A. Valvano, unpublished results). The results of these experiments showed that the control strain VW187, as well as other O7 isolates from different clonal groups, were all PCR positive as evidenced by the detection of a 775 bp band (data not shown). In contrast, chromosomes from E. coli strains belonging to other O types, including O1, O2, O4, O6, O8, O14, O16, O18, O25, O28, O39, O39, O45, O68, O75, O78, O100, O111, O116, O124, O136, O138, O141 and O157, did not afford any PCR product. This was also the case for the laboratory E. coli strains B (strain ATCC 113301) and C (strain C1a), which do not form O antigen. A 900 bp fragment corresponding to the amplification of part of the galF gene was found in all cases, demonstrating that the lack of detection of O7-specific amplification products was not due to technical problems with the PCR reactions. On the basis of DNADNA hybridization, analysis of outer-membrane protein profiles, and LPS migration patterns, we have previously reported that Shigella boydii strains that had been classified as serotype O12 constitute two different clonal groups, SB1 and SB2 (Valvano & Marolda, 1991
). The strains belonging to the SB1 clone produce LPS that is identical to O7 and they have genes highly homologous to the wbEcO7 genes in E. coli VW187 (Valvano & Marolda, 1991
). PCR assays with the O7-specific primers confirmed these results since chromosomal DNA of the strain N9BC12 (SB1 clonal group) used as template gave an amplicon similar to that afforded by the chromosomal DNA of the other E. coli O7 strains. In contrast, chromosomal DNA of strains SB504-58 and SB123 belonging to the SB2 clonal group did not afford any PCR product. Therefore, we concluded that our primers are specific and can serve for the identification of strains carrying O7-specific LPS.
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DISCUSSION |
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The genes for the synthesis of the remaining precursors of the O7 subunit, UDP-galactose and UDP-GlcNAc, were not expected to be found in the wbEcO7 cluster. These genes are usually present in other locations of the E. coli chromosome, since these precursors are used in a variety of metabolic pathways, and they are also involved in the synthesis of other cell envelope molecules such as peptidoglycan, core oligosaccharide, colanic acid and ECA (Berlyn, 1998 ). These findings are in agreement with similar results obtained by other investigators suggesting that only O-antigen-specific genes are present in the wb clusters (Reeves et al., 1996
).
The other genes identified in the wbEcO7 cluster encode proteins involved in the assembly of the O7 subunit. The presence of wzy, encoding a polymerase, confirms that the O7 antigen is synthesized by a wzy-dependent pathway, and therefore it is likely that O7 polymerization occurs on the periplasmic side of the inner membrane, as has been proposed for this class of O antigens (Whitfield, 1995 ). Consistent with this idea is the fact that the Wzy protein has predicted topological features similar to those described for the Shigella flexneri homologue, including a relatively large periplasmic loop (Daniels et al., 1998
). The wbEcO7 cluster also contains a wzx-like gene, which appears to play an as yet unidentified role in the translocation of the O antigen subunit across the inner membrane (Liu et al., 1996
). An interesting observation made with wzx genes and their products is the lack of strong amino acid sequence similarities despite the apparent common function in the translocation of the O antigen subunit. Nevertheless, we assigned the wzxEcO7 gene based on weak amino acid sequence similarities with other proteins also predicted to be involved in the translocation of oligosaccharides, as well as the predicted structural topology as an integral membrane protein with multiple transmembrane domains. More importantly, we observed that a non-polar transposon insertion in this gene (insertion 128) expressed a complete O7 LPS in E. coli K-12 strains such as DH5
, LE392 and HB101, albeit at a lower level than wild-type expression by the cosmid pJHCV32 (Marolda et al., 1990
). This observation ruled out the possibility that the insertion 128 was in a gene for the synthesis or the transfer of an essential precursor of the O7 antigen subunit. Many E. coli K-12 strains cannot make O antigen because of an insertion in a rhamnosyltransferase gene (rfb50 mutation; Liu & Reeves, 1994
). However, when this mutation is complemented, E. coli K-12 makes O16 LPS (Stevenson et al., 1994
; Yao & Valvano, 1994
), suggesting that the remaining genes of the O16 LPS cluster are functional. Since a wzx gene is also present in the wbEcO16 cluster, we suggest that this gene can at least partially complement the phenotype of insertion 128. This explains the lack of significant amounts of O7 LPS in strain CLM4, which has a large deletion eliminating the O16 LPS gene cluster (Marolda & Valvano, 1993
). The O7 and O16 subunits are structurally unrelated (Lvov et al., 1984
; Stevenson et al., 1994
) except for the fact that both contain GlcNAc, which is the first sugar added to undecaprenol phosphate (Alexander & Valvano, 1994
; Yao et al., 1992
). Preliminary evidence in our laboratories indicates that a cloned wzxEcO16 gene can also complement the insertion 128 in strain CLM4, and provide the basis for a functional analysis of Wzx proteins (M. Feldman, C. L. Marolda, M. A. Monteiro, A. J. Parodi & M. A. Valvano, unpublished).
In a previous study we determined that the synthesis of the O7 subunit is initiated by the function of wecA, resulting in the transfer of GlcNAc-1-phosphate to undecaprenyl phosphate (Alexander & Valvano, 1994 ). We also determined that this step is very common in the synthesis of O polysaccharides containing GlcNAc in the O subunit (Alexander & Valvano, 1994
). wecA is the first gene of the biosynthesis cluster for ECA, which is present at a distant location in the chromosome with respect to the wb cluster (Rick & Silver, 1996
). Based on this information, we predicted that the wbEcO7 cluster should contain genes for the remaining four glycosyltransferases to complete the synthesis of the O7 subunit. The galactosyl- and mannosyltransferase genes, wbbD and wbbC, respectively, were assigned based on homologies of the predicted amino acid sequences with known glycosyltransferases. The two remaining genes, wbbA and wbbB, are predicted to be involved in the transfer of VioNAc and rhamnose to complete the O7 subunit. However, no homologues were found in the database, and these assignments are tentative until we can confirm them biochemically, work already in progress in our laboratories.
The completion of the genetic characterization of the wbEcO7 cluster also permitted us to design a set of primers for detection of O7-specific sequences. Using this primer set we were able to confirm our previous assignment of clonal groups in Shigella boydii strains serotype 12, some of which produce an O antigen identical to O7 (Valvano & Marolda, 1991 ). We also showed that E. coli B and C, a pair of O-antigen-deficient strains used in genetic studies like E. coli K-12, do not contain O7-specific genes. Therefore, our primers may be useful, alone or in combination with other O antigen-specific primers, to develop a molecular diagnostic kit for determination of O types. Recently, it has been shown that the use of rapid genomic techniques for cloning and sequencing of wb clusters is feasible (Wang & Reeves, 1998
). This progress makes possible the rapid generation of a database of gene sequences that can be used for the design of specific primers for diagnostic purposes and epidemiological surveillance of E. coli infections.
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ACKNOWLEDGEMENTS |
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Received 9 February 1999;
revised 5 May 1999;
accepted 17 May 1999.