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J. Biol. Chem., Vol. 277, Issue 1, 327-337, January 4, 2002

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The Genetic Bases for the Variation in the Lipo-oligosaccharide of the Mucosal Pathogen, Campylobacter jejuni

BIOSYNTHESIS OF SIALYLATED GANGLIOSIDE MIMICS IN THE CORE OLIGOSACCHARIDE^*

Michel Gilbert, Marie-France Karwaski, Stéphane Bernatchez, N. Martin Young, Eduardo Taboada, Joseph Michniewicz, Anna-Maria Cunningham, and Warren W. Wakarchuk

From the Institute for Biological Sciences, National Research Council of Canada, 100 Sussex Dr., Ottawa, Ontario K1A 0R6, Canada

Received for publication, August 31, 2001, and in revised form, October 26, 2001

	ABSTRACT

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

We have compared the lipo-oligosaccharide (LOS) biosynthesis loci from 11 Campylobacter jejuni strains expressing a total of 8 different ganglioside mimics in their LOS outer cores. Based on the organization of the genes, the 11 corresponding loci could be classified into three classes, with one of them being clearly an intermediate evolutionary step between the other two. Comparative genomics and expression of specific glycosyltransferases combined with in vitro activity assays allowed us to identify at least five distinct mechanisms that allow C. jejuni to vary the structure of the LOS outer core as follows: 1) different gene complements; 2) phase variation because of homopolymeric tracts; 3) gene inactivation by the deletion or insertion of a single base (without phase variation); 4) single mutation leading to the inactivation of a glycosyltransferase; and 5) single or multiple mutations leading to "allelic" glycosyltransferases with different acceptor specificities. The differences in the LOS outer core structures expressed by the 11 C. jejuni strains examined can be explained by one or more of the five mechanisms described in this work.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Many pathogenic bacteria have variable cell-surface glycoconjugates such as capsules in Streptococcus spp. and Neisseria meningitidis (1), lipopolysaccharides in Gram-negative bacteria (2), and glycosylated surface-layer proteins (3). In mucosal pathogens, the variability of cell-surface polysaccharides has been shown to play a major role in virulence (4). This variation is caused by the diversity of monosaccharide components and the linkages between them, derivatization with noncarbohydrate moieties, and in some cases, by the length and sequence of the repeating units. The variation of these glycan structures can sometimes be correlated with a specific gene complement, but it is probable that other genetic mechanisms are also employed to create variable cell-surface glycoconjugates. The DNA sequencing of the relevant genetic loci from multiple strains of a pathogen can provide insights into the genetic origins of important strain variable traits such as cell-surface glycoconjugates.

The mucosal pathogen Campylobacter jejuni has been recognized as an important cause of acute gastroenteritis in humans (5) and has been shown to have variable cell-surface carbohydrates that are associated with virulence (6, 7). Epidemiological studies have shown that Campylobacter infections are more common than Salmonella infections in developed countries, and they are also an important cause of diarrheal diseases in developing countries. C. jejuni is also considered the most frequent antecedent infection to the development of Guillain-Barré syndrome, a form of neuropathy that is the most common cause of generalized paralysis since the eradication of poliomyelitis in developed countries (8). The core oligosaccharides of low molecular weight lipo-oligosaccharides (LOS)¹ of many C. jejuni strains have been shown to exhibit molecular mimicry of the carbohydrate moieties of gangliosides (Fig. 1). Terminal oligosaccharides identical to those of GM1a, GM2, GM3, GD1a, GD1c, GD3, and GT1a gangliosides have all been found in various C. jejuni strains (see Table I for references). Molecular mimicry of host structures by the saccharide portion of LOS is considered to be a virulence factor of various mucosal pathogens, which may use this strategy to evade the immune response (9). The molecular mimicry between C. jejuni LOS outer core structures and gangliosides has also been suggested to act as a trigger for autoimmune mechanisms in the development of Guillain-Barré syndrome (10).

Aspinall et al. (11-14) and Nam Shin et al. (15) determined the LOS outer core structures of representative C. jejuni reference strains of the Penner serotyping system. The Penner serotyping system of C. jejuni is based on heat-stable antigens, and it was proposed that the specificity is due to LOS and/or lipopolysaccharide-type molecules (16, 17). However, recent biochemical and genetic studies suggest that capsular polysaccharides account for Penner serotype specificity (6, 18). Because the loci responsible for capsule and LOS biosynthesis are distant in the C. jejuni genome (19) and intraspecies gene transfers are known to be frequent in C. jejuni (20, 21), it is possible that strains having the same Penner type could express different LOS outer cores. Consequently, we decided to associate the published LOS outer core structures (Fig. 1) with the specific strain identification numbers (ATCC, NCTC, etc.) rather than with the Penner types, although the latter are also provided for convenient reference (Table I).

The identification of the genes involved in LOS synthesis and the study of their regulation are of considerable interest for a better understanding of the pathogenesis mechanisms used by these bacteria. The availability of the complete genome sequence of C. jejuni NCTC 11168 (19) has facilitated the identification of loci involved in the biosynthesis of cell-surface carbohydrates including LOS (22, 23). The genome sequence was also used to clone the corresponding LOS biosynthesis locus in other C. jejuni strains (24, 25), which allowed the identification of genes involved in the transfer of Gal, GalNAc, and N-acetylneuraminic acid (Neu5Ac or sialic acid) to the LOS outer core.

Because cell-surface structures such as the LOS are recognized as antigens by the host, it is therefore not surprising that microorganisms will modulate these structures to increase the chances of evading the immune system. The C. jejuni strains used in this study were shown to express a total of 8 different sialylated LOS outer cores (Fig. 1 and see Table I for references). The LOS biosynthesis loci of C. jejuni OH4384 and C. jejuni NCTC 11168 were found to have common genes as well genes unique to each strain (24), which provide a basis for differences in LOS outer cores. However, mechanisms other than differences in gene complement are involved in generating a variety of LOS outer cores. In the strain C. jejuni OH4382, the gene involved in the transfer of the GalNAc residue of the LOS outer core was shown to be inactive (a missing A nucleotide causes a premature translation stop). This results in the expression of a truncated LOS outer core when compared with strain OH4384 (13, 24). Parkhill et al. (19) showed that short homopolymeric nucleotide runs of variable length are commonly found in genes involved in the biosynthesis of C. jejuni carbohydrates, which provides a form of on/off regulation of these genes. Linton et al. (22) studied in detail a gene encoding a -1,3-galactosyltransferase that occurs with either an 8- or a 9-G nucleotide tract which results in the expression of either a GM1a or a GM2 ganglioside mimic in C. jejuni NCTC 11168. We reported previously that the cst-II gene occurs as a mono-functional -2,3-sialyltransferase in C. jejuni ATCC 43446 (O:19 serostrain) and as a bi-functional -2,3-/-2,8-sialyltransferase in C. jejuni OH4384 that results in the expression of either a GD1a or GT1a mimic, respectively (24).

In this work we describe the mechanisms used by C. jejuni to generate various sialylated outer core structures. In addition to reporting other examples of on/off expression of genes due to variable homopolymeric tracts, we use enzymatic assays to show that amino acid substitutions are responsible for the expression of glycosyltransferases with different substrate specificities, a "strategy" that further expands the ability of C. jejuni to express various LOS outer cores.

	EXPERIMENTAL PROCEDURES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Bacterial Strains-- The C. jejuni strains used in this study are listed in Table I. The Penner type strains were obtained from the American Type Culture Collection. C. jejuni OH4382, OH4384 and NCTC 11168 were obtained from the Laboratory Center for Disease Control (Health Canada, Winnipeg, Manitoba, Canada). C. jejuni strains were grown on Mueller-Hinton medium under microaerobic conditions. Escherichia coli AD202 (CGSG 7297) was used to express the different cloned glycosyltransferases and was grown using 2YT agar or broth. The recombinant E. coli strains were incubated at 25 °C for a total of 24 h, with induction with 1 mM isopropyl-1-thio--D-galactopyranoside after 6 h for cgtA constructs and with 0.3 mM isopropyl-1-thio--D-galactopyranoside after 4.5 h for cst-II constructs.

Basic Recombinant DNA Method-- Genomic DNA isolation from the C. jejuni strains was performed using the DNeasy Tissue kit (Qiagen Inc., Valencia, CA). Plasmid DNA isolation, restriction enzyme digestions, purification of DNA fragments for cloning, ligations, and transformations were performed as recommended by the enzyme supplier or the manufacturer of the kit used for the particular procedure. Long PCRs (>2 kb) were performed using the Expand^TM long template PCR system as described by the manufacturer (Roche Molecular Biochemicals). PCRs to amplify specific ORFs were performed using the Pwo DNA polymerase as described by the manufacturer (Roche Molecular Biochemicals). Restriction and DNA modification enzymes were purchased from MBI Fermentas Inc. (Hanover, MD). Site-directed mutagenesis of cst-II was performed using a two stage PCR mutagenesis protocol. Two separate PCR reactions were performed to generate two overlapping gene fragments that both contained the mutation due to either the 5' or the 3' primers. The two PCR products were used with the cst-II 5' and 3' primers to amplify the full-length mutated version of cst-II.

Sequencing of the LOS Biosynthesis Loci-- The DNA sequences of the LOS biosynthesis loci of C. jejuni NCTC 11168 (GenBank^TM accession number AL139077) and OH4384 (GenBank^TM accession number AF130984) were used to design primers to amplify the LOS biosynthesis loci of the other strains described in this work. The primers were designed to obtain overlapping PCR products of 2-5 kb that covered completely each of the LOS locus. The PCR products were sequenced by "primer walking," and new primers were synthesized to amplify and sequence the regions that diverge significantly from the NCTC 11168 and OH4384 sequences. DNA sequencing was performed using an Applied Biosystems (Montreal) model 373 automated DNA sequencer and the manufacturer's cycle sequencing kit.

Assays-- Protein concentration was determined using the bicinchoninic acid protein assay kit (Pierce). FCHASE-labeled oligosaccharides were prepared as described previously (26). Extracts were made by sonication, and the enzymatic reactions were performed at 37 °C for 5 min to 2 h. The -1,4-N-acetylgalactosaminyltransferase was assayed using 0.5 mM Neu5Ac-2,3-Gal-1,4-Glc-FCHASE, 1 mM UDP-GalNAc, 50 mM Hepes, pH 7, and 10 mM MnCl₂. The -2,3-sialyltransferase was assayed using 0.5 mM Gal--1,4-Glc-FCHASE, 0.2 mM CMP-Neu5Ac, 50 mM Hepes, pH 7.5, and 10 mM MgCl₂. The -2,8-sialyltransferase was assayed using 0.5 mM Neu5Ac-2,3-Gal-1,4-Glc-FCHASE, 0.2 mM CMP-Neu5Ac, 50 mM Hepes, pH 7.5, and 10 mM MgCl₂. The CMP-Neu5Ac synthetase was assayed using CTP, Neu5Ac, Gal-1,4-GlcNAc-FCHASE, and a purified fusion of the N. meningitidis -2,3-sialyltransferase (MalE-NST)² in a coupled assay that measured the production of Neu5Ac-2,3-Gal-1,4-GlcNAc-FCHASE. The reaction mix included 0.5 mM Gal--1,4-GlcNAc-FCHASE, 3 mM CTP, 3 mM Neu5Ac, 4 milliunits of -2,3-sialyltransferase (MalE-NST), 100 mM Tris, pH 7.5, 10 mM MgCl₂, and 0.2 mM dithiothreitol. All the reactions were stopped by the addition of acetonitrile (25% final concentration) and were diluted with H₂O to get 10-15 µM final concentration of the FCHASE-labeled compounds. The samples were analyzed by capillary electrophoresis performed using the separation and detection conditions as described previously (27). The peaks from the electropherograms were analyzed using manual peak integration with the P/ACE Station software.

	RESULTS

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Organization of the LOS Biosynthesis Loci in the Various C. jejuni Strains-- We have compared the LOS biosynthesis loci of 11 C. jejuni strains (Table I) that include 7 previously unpublished loci and extend our previous limited comparison of 4 C. jejuni strains that included 3 closely related O:19 strains (OH4382, OH4384, and ATCC 43446, the O:19 serostrain) and the genome strain NCTC 11168 (24). The LOS outer core structures were published for 10 of the 11 strains included in this study (Fig. 1 and Table I). The general organization of the LOS biosynthesis genes allows us to group these C. jejuni strains into three classes "A," "B," and "C" (see Fig. 2). The LOS biosynthesis loci of the six class A strains have 13 ORFs, whereas the LOS biosynthesis loci of the two class B strains and of the three class C strains have 14 ORFs. One gene (orf11) is found only in classes A and B, whereas three genes are unique to class C (orf14, orf15, and orf16). Proposed functions for each ORF are described in Table II.

View larger version (19K): [in this window] [in a new window]   Fig. 1.   Outer regions of the LOS structures expressed by the C. jejuni strains studied in this work (see Table I for the references to the original publications of the structures). The numbering system (I-VIII) is not based on any biological phenotype and serves only for referencing purposes. The LOS outer core structures display the oligosaccharide portions of the gangliosides indicated below the structure number.

View larger version (46K): [in this window] [in a new window]   Fig. 2.   Genetic organization of the LOS biosynthesis loci of the different C. jejuni strains. The proposed functions for the ORFs are described in Table II. The * indicates where a premature translation stop is observed for some of the strains. The G indicates where a poly(G) tract is observed (C shows where genes are translated on the complementary strands). Phase-variable ORFs that were observed to be mostly out of frame (see Table IV) are broken in two arrows. There is a gene (orf11, shown in gray), unique to classes A and B. Three genes (orf14, orf15, and orf16, shown with downward stripes) are unique to class C. The gene encoding the -1,4-N-acetylgalactosaminyltransferase (orf5, shown in black) is found in one copy in class A, in two copies in class B, and as an in-frame fusion with the CMP-Neu5Ac synthetase (orf10, shown with horizontal stripes) in class C.

The 11.5-kb DNA sequences of the LOS loci from the six class A strains can be aligned with only minor gaps, the longest being 6 bp. The overall DNA sequence identity is 91% between the six A strains. However, the level of conservation observed in pairwise alignments varies considerably. As reported previously (24) the three O:19 strains (ATCC 43446, OH4382, and OH4384) are closely related. There is only one base difference (a missing A at position 71 of orf5) between the LOS locus of OH4382 and OH4384. There are 68 base differences (20 amino acid differences) between ATCC 43446 (O:19 serostrain) and OH4384. The LOS locus from C. jejuni ATCC 43438 (O:10 serostrain) is primarily responsible for decreasing the overall degree of conservation among the A class strains. When the ATCC 43438 strain is excluded from the class A alignment, the overall DNA sequence identity increases to 96.5%. The highest level of divergence between the LOS locus of ATCC 43438 and the other class A strains is found between nt 4500 and 5700 (66% DNA sequence identity), a region that spans both the orf5 and orf6 which encode a -1,4-N-acetylgalactosaminyltransferase and a -1,3-galactosyltransferase, respectively.

The 12.4-kb LOS biosynthesis locus of the two class B strains (ATCC 43449, the O:23 serostrain, and ATCC 43456, the O:36 serostrain) shows 95.2% DNA sequence identity in a full-length pairwise alignment. However, the sequence identity is only 65.3% in the region from nt 4500 to 5700, whereas it is above 98% in the rest of the locus. It is noticeable that this region corresponds to the same region that was found to diverge considerably between ATCC 43438 and the other class A strains. In fact, ATCC 43438 and ATCC 43449 share 98% DNA sequence identity in the nt 4500-5700 region, whereas the other class A strains and ATCC 43456 share 99% DNA identity in that region.

Class B appears to be an evolutionary intermediate between classes A and C because it has two copies of orf5, with one of them (orf5-I) more similar to orf5 from class A (96% DNA sequence identity) and the second copy (orf5-II) more similar to orf5 from class C (85% DNA sequence identity). The orf5-I in the class B is inactive because of premature translational termination after 28 codons in ATCC 43449 and after 86 codons in ATCC 43456. Transcription reinitiation of orf5-I would theoretically be possible, but a similar frameshift mutation was described in orf5 of OH4382 and resulted in the expression of a truncated LOS, consistent with the absence of active -1,4-N-acetylgalactosaminyltransferase (24). The orf5-II is located just upstream of orf10 in the class B (Fig. 2). Although orf5-II and orf10 are separate ORFs in class B, they are found as an in-frame single ORF (orf5/10) in class C as reported previously (24). A genetic rearrangement is presumed to have occurred that led to the fusion of these two ORFs in class C.

The level of DNA sequence conservation among the loci from the three class C strains (ATCC 43429, the O:1 serostrain, ATCC 43430, the O:2 serostrain and NCTC 11168) is very high with a maximum of 18-base differences between them using pairwise comparisons across the whole 13.5-kb sequence. We describe below how some of the minor DNA sequence differences are responsible for the different LOS outer cores expressed by the three class C strains.

Comparisons among the three classes are more easily made by aligning the corresponding translated genes (Table III). As mentioned above, class C is distinctive by the absence of a homologue of orf11 and the presence of three unique genes (orf14, orf15, and orf16). When comparing the translated ORFs that are common to all classes, it is observed that the most conserved ones are at each end of the locus with ORFs 1, 2, and 13 sharing above 94% protein sequence identity between corresponding homologues. ORFs 3, 4, 8, 9, 10, and 12 share from 66 to 86% protein sequence identity, whereas the most divergent proteins are found in the middle of the locus with ORFs 5-7 sharing from 34 to 50% protein sequence identity.

Gene Inactivation by the Deletion or Insertion of a Single Base (without Phase Variation)-- There are two glycosyltransferase genes that are found as inactive versions in some of the strains due to frameshift mutations. There is a missing A base at position 1,234 of orf3 in four class A strains (ATCC 43432, ATCC 43446, OH4382, and OH4384). Based on BLAST searches, orf3 was proposed to encode a 515-amino acid two-domain glycosyltransferase (The Sanger Center website address: www.sanger.ac.uk/Projects/C_jejuni/, predicted coding sequence Cj1135). The amino acid sequence at the N terminus (residues 1-250) is homologous to LgtF from Haemophilus ducreyi that encodes a -1,4-glucosyltransferase that transfers glucose to heptose (28). The first domain of orf3 is therefore the likely candidate for transferring the -1,4-glucose to the inner heptose (Hep-I) in C. jejuni. The second domain (residues 250-515) of orf3 is homologous to various glycosyltransferases, but it is not possible to deduce its specificity based on sequence homology alone. However, the frameshift mutation observed in four class A strains results in the expression of a 418-amino acid protein which means that the second domain is missing 98 residues. Because the four strains that have this frameshift mutation are also missing the -1,2-glucose residue on the second heptose (Hep-II, see Fig. 1), we suggest that the second domain of orf3 is a -1,2-glucosyltransferase.

The second example of a glycosyltransferase gene that shows inactivation by frameshift mutation is orf5 in strain OH4382 (missing A at base 71), orf5-I in ATCC 43449 (missing A at base 71), and orf5-I in ATCC 43456 (missing G at base 200). We reported previously that this gene encodes a -1,4-N-acetylgalactosaminyltransferase and that its inactivation results in the expression of a truncated LOS in OH4382 (24). However, the inactivation of orf5-I in ATCC 43449 and ATCC 43456 does not result in LOS outer cores without GalNAc because these two strains have a second, functional, copy of this gene (orf5-II).

Phase Variation Due to Homopolymeric G-tracts-- Four of the 11 C. jejuni strains lack G-tracts longer that 5 bases in their LOS biosynthesis locus (Fig. 2). Longer homopolymeric G-tracts are present in five LOS biosynthesis genes distributed among the seven other C. jejuni strains. Some of the G-tracts are unique to one gene and strain (such as in orf7 of ATCC 43449), whereas some others are present in all representatives of a gene (such as in orf16 in the three class C strains). Some of the G-tracts were found to be homogeneous, i.e. with a specific number of G bases that would result in the expression of a specific on or off phenotype. For instance, orf6 (-1,3-galactosyltransferase) is inactive in ATCC 43429 and ATCC 43430 because it is found to have a homogeneous 9 G-tract that causes premature translation termination, consistent with the absence of a terminal -1-3-Gal residue in these strains (Fig. 1, structures VII and VIII, and see Table I). Other G-tracts are heterogeneous with one of the variants being present more frequently. Determining the proportions of each variant was found to be difficult because heterogeneity was sometimes observed even when chromosomal DNA was isolated from single colonies. We defined the "most frequent variant" as the one corresponding to the strongest signal on a DNA sequencing electropherogram when we sequenced a PCR product obtained using as template chromosomal DNA isolated from a confluent plate. Because NCTC 11168 was sequenced from a plasmid library, specific numbers were reported for each variant of orf6 and orf16 for this strain (see Table IV for references). In the case of orf6 from NCTC 11168, the most frequent variant has 8 G (in-frame) which is consistent with the LOS outer core (structure VI) having a terminal -1,3-Gal residue (Ref. 22 and see Fig. 1). However, it is not always possible to correlate the most frequent variants with the published structures. For instance, the LOS outer core structure of ATCC 43449 was reported to be sialylated (Fig. 1, structure V, and see Table I), but it contains an -2,3/2,8-sialyltransferase gene (orf7) mostly as an out-of-frame variant (Table IV). It is possible that the level of active orf7 in ATCC 43449 is sufficient to produce LOS with enough of the sialic acid residue for it to be detected by chemical analysis. Because the phase-variable genes are heterogeneous, it is also probable that the proportion of active/inactive variants will vary between laboratories depending on the number of passages of the strain and whether practices such as sub-culturing from isolated colonies are used or not. We avoided sub-culturing from single colonies because our original stocks were not single colonies and to avoid enriching specific variants.

Single Mutations Leading to the Inactivation of a Glycosyltransferase-- There are only eight base differences between the LOS biosynthesis loci of C. jejuni NCTC11168 and ATCC 43430 although they express different LOS outer cores (Fig. 1, structures VI and VIII, respectively). Six of these base differences cause frameshift changes in orf6 (-1,3-galactosyltransferase) and in orf16 (unknown function, 5 bases are missing in NCTC 11168). One of the base differences causes a silent mutation in orf16, whereas the last base difference causes an amino acid change (Cys-92  Tyr, NCTC 11168  ATCC 43430) in orf5/10 (-1,4-N-acetylgalactosaminyltransferase/CMP-NeuAc synthetase natural fusion). Because the LOS outer core of ATCC 43430 is truncated at the second inner Gal residue (Fig. 1, structure VIII), we suspected that this mutation was responsible for the inactivation of the -1,4-N-acetyl galactosaminyltransferase in ATCC 43430. We cloned orf5/10 from both NCTC 11168 and ATCC 43430 and expressed them in E. coli. We found that both versions have similar CMP-NeuAc synthetase activity, whereas only the NCTC11168 version has -1,4-N-acetylgalactosaminyltransferase activity (Table V).

View this table: [in this window] [in a new window]   Table V Enzymatic assays of various alleles of the -1,4-N-acetylgalactosaminyltransferase (orf5, orf5-II, and orf5/10) expressed as fusions with MalE in E. coli

Mutations Leading to Glycosyltransferases with Different Glycan Acceptor Specificities-- Although the -1,4-N-acetylgalactosaminyltransferase alleles from the three classes are clearly homologous, the level of conservation among them is only 34% (Table III). We expressed representatives from each class as C-terminal fusions with the maltose-binding protein in E. coli. The acceptor preference was found to vary significantly (Table V) with the ATCC 43438 version using only a nonsialylated acceptor, the version from OH4384 using only a mono-sialylated acceptor, and the versions from ATCC 43456 and NCTC 11168 are able to use both a mono-sialylated and a di-sialylated acceptors. In most cases the acceptor specificity correlates with the natural acceptor because only ATCC 43438 has no sialic acid on the inner Gal residue of the LOS outer core, and the three other strains (OH4384, ATCC 43456, and NCTC 11168) have a single sialic acid on the inner Gal residue (Fig. 1). The ability of the -1,4-N-acetylgalactosaminyltransferase from ATCC 43456 and NCTC 11168 to use a di-sialylated acceptor could seem superfluous because these strains express LOS outer cores with a single sialic acid on the inner Gal residue. However, Prendergast and Moran (29) recently reported a C. jejuni outer core mimicking GD2 (i.e. GalNAc--1,4-[Neu5Ac--2,8-Neu5Ac-2,3-]-Gal-inner core), which suggests that this strain could contain a CgtA version that is related to the one found in either ATCC 43456 or NCTC 11168. In addition some C. jejuni strains were reported to express a GQ1b epitope in their LOS outer core based on antibody probing (30). Although the expression of an "authentic" GQ1b mimic requires confirmation by structural analysis, the ability of the -1,4-N-acetylgalactosaminyltransferase from ATCC 43456 and NCTC 11168 to use a di-sialylated acceptor suggests that this structure could exist in the outer core of some C. jejuni strains.

Another example of mutations leading to different acceptor specificities is provided by orf7, which was named cst-II when we cloned it from C. jejuni OH4384 (24). We will use this designation for all of the versions from classes A and B. Gerry et al. (25) showed that orf7 from ATCC 43429 is responsible for transferring the -2,3-sialic acid and named this gene cst-III, a designation that we will use for class C orf7. An alignment of the deduced protein sequences of the orf7 (sialyltransferase) versions from all the classes gave 50% identity. However, when the classes A and B versions are aligned together, the level of protein sequence identity rises to 92% (Fig. 3), whereas the three class C versions are 100% identical between themselves. Pairwise alignments between Cst-III and each variant of Cst-II gave 52% protein sequence identity on average.

View larger version (41K): [in this window] [in a new window]   Fig. 3.   Alignment of the cst-II sialyltransferase (orf7) deduced amino acid sequences. The C. jejuni strain numbers are indicated to the left of the sequences, and the GenBankTM accession numbers are indicated in Table I. The three residues (Asn-51, Leu-54, and Ile-269) that are specific for the bi-functional Cst-II variants are underlined.

Because Cst-II from OH4382 and OH4384 are identical, there are seven distinct Cst-II amino acid sequences (Fig. 3). We cloned and expressed six of them in E. coli and assayed the recombinant Cst-IIs for -2,3-sialyltransferase and -2,8-sialyltransferase activities (Table VI) using Gal--1,4-Glc-FCHASE and Neu5Ac--2,3-Gal--1,4-Glc-FCHASE, respectively, as acceptors. We found four versions (OH4382/84, ATCC 43438, ATCC 43449, and ATCC 43460) that are bi-functional (both -2,3- and -2,8-sialyltransferase activities), and two versions (ATCC 43432 and ATCC 43446) that have only the -2,3-sialyltransferase activity (Table VI). An alignment of the amino acid sequences of the various Cst-II versions (Fig. 3) indicated that only three residues (Asn-51, Leu-54, and Ile-269) were specific for the bi-functional Cst-II versions. We used site-directed mutagenesis to determine which of these residues are essential for bi-functional sialyltransferase activity. An Asn-51  Thr substitution in Cst-II from OH4384 completely abolished the -2,8-sialyltransferase activity (Table VI). The opposite substitution (Thr-51  Asn) in the mono-functional Cst-II from ATCC 43446 conferred it the ability to perform both activities (-2,3- and -2,8-sialyltransferase). The other two residues (Leu-54 and Ile-269) unique to bi-functional Cst-II variants as well as the very variable residue 53 were found to affect the relative ratios of -2,3- and -2,8-sialyltransferase activities (Table VI and data not shown), but only Asn-51 was found to be absolutely essential for -2,8-sialyltransferase activity. Although the in vitro assays with the various recombinant Cst-IIs allowed us to determine which versions are mono- or bi-functional, the levels of activities vary considerably between the various versions (Table VI). SDS-PAGE analyses indicated that all the versions were expressed at similar levels (data not shown). Two Cst-II versions (ATCC 43449 and ATCC 43460) have low -2,3-sialyltransferase activity, whereas the Cst-II from OH4384 has both low -2,3- and low -2,8-sialyltransferase activities (Table VI). The amino acid substitution Ile-53  Gly increased both activities of the OH4384 version (Table VI) which suggests that this residue has an important impact on the level of in vitro activity. It is also noticeable that the two versions (ATCC 43449 and ATCC 43460) that have much lower -2,3- than -2,8-sialyltransferase activity both have the same residue (a serine) at position 53 (Fig. 3).

View this table: [in this window] [in a new window]   Table VI Spectrum of -2,3- and -2,8 sialyltransferase activities of the recombinant Cst-II variants All the constructs were expressed as MalE/Cst-II fusion in E. coli.

	DISCUSSION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Genetic loci involved in the biosynthesis of cell-surface carbohydrates have been identified in many bacteria as a result of the sequencing of entire genomes. However, only a few studies have looked at the corresponding loci of strains expressing distinct carbohydrate structures. Different gene complements and phase variation due to homopolymeric G-tracts were shown to be involved in the variability of LOS outer core structures in N. meningitidis (31). Different gene complements were also observed in the corresponding loci responsible for various inner core structures in E. coli (32). Comparative genomics studies of the capsular polysaccharide biosynthesis loci from Streptococcus pneumoniae strains of different serotypes have shown evidence of recombination events resulting in different gene complements (33-34). Corresponding glycosyltransferase genes that had diverged were also proposed to contribute to the capsular variability by transferring the same sugar unit to create different linkages, although no biochemical data were reported to support the proposed functions (33).

The presence of a large number of ganglioside mimics in various C. jejuni strains prompted us to investigate the genetic basis for this variation. The general organization of the various LOS biosynthesis loci allowed them to be grouped in three classes and demonstrated that not all of the differences in LOS outer cores are due to the different gene complements. Previous work had also shown that phase variation using homopolymeric G tracts (22) and gene inactivation by the deletion or insertion of a single base (without phase variation) were also responsible for some of the variations in LOS outer core structures (24). By combining comparative genomics of LOS biosynthesis loci from strains expressing different LOS outer cores with functional assays, it was possible to determine that C. jejuni also uses glycosyltransferase alleles to produce enzymes that are inactive or that show different acceptor specificities. We propose that each of the differences in the structure of LOS outer cores displayed by the 11 different C. jejuni strains can be explained by either one or more of the five genetic mechanisms we have described. Transcriptional regulation was not examined in this study, and it is possible that the expression of some glycosyltransferases (or of other carbohydrate biosynthesis enzymes) would be induced or repressed under varying growth conditions or during infection. However this would not change the potential of a strain with specific glycosyltransferase alleles to make the LOS outer core structure(s). Because some of the variations are due to amino acid substitutions or frameshift mutations, the regulation of the DNA repair system is also likely to have an impact on the possibility of strain to vary its LOS outer core.

Class B is clearly an evolutionary intermediate between classes A and C as it seems to have evolved from a class A locus by duplication of orf5 (into orf5-I and orf-II, see Fig. 2). At least two more recombination events would have been necessary to generate a class C locus by the insertion of orf14, orf15, and orf16 and the deletion of orf5-I and orf11. The three class C loci studied here also have orf5-II and orf10 as an in-frame fusion. It is certainly possible that other evolutionary intermediates exist with different combinations of inserted/deleted ORFs and with orf5-II and orf10 either as separate ORFs or as an in-frame fusion. Although class C loci have three unique genes, there seems to be a need for only two additional glycosyltransferases when the outer core structures are compared. Class C outer cores have two linkages (a Gal-1,3-Gal and a Gal-1,2-Gal, see Fig. 1) that are not present in classes A and B outer cores. The orf14 and orf15 both show homology with various glycosyltransferases (data not shown) and would be good candidates to make these two linkages although current experimental evidence is not sufficient to confirm these assignments. The orf16 is a hypothetical ORF with no homologue in GenBank^TM, and it is not possible to determine its role, if any, in LOS biosynthesis. In families A and B, orf11 has no clear function in LOS biosynthesis. It shows homology with various acetyltransferases (data not shown), but acetylation of the C. jejuni LOS structures was not reported, although it could have been overlooked.

The divergence observed between ATCC 43438 and the other class A loci in the region from nt 4500 to 5700 is interesting from both the functional and evolutionary aspects. In this region, ATCC 43438 is much more similar to ATCC 43449 from class B than to the other class A loci. The other class A loci are themselves closer to ATCC 43456 (the other class B locus) in the corresponding region. Consequently, ATCC 43438 could either be an evolutionary intermediate between classes A and B, and ATCC 43456 would have acquired the class A corresponding region by lateral gene transfer. The opposite sequence of events is as likely, i.e. ATCC 43456 would be an intermediate between classes A and B loci and ATCC 43438 would have acquired the region from nt 4500 to 5700 by lateral gene transfer.

It is also noteworthy that the region from nt 4500 to 5700 spans both the orf5 and orf6 that encode a -1,4-N-acetylgalactosaminyltransferase and a -1,3-galactosyltransferase, respectively. Because orf5 and orf6 are translated in opposite orientations, the divergence of the region from nt 4500 to 5700 results in the large number of amino acid substitutions observed in the C terminus of both the -1,4-N-acetylgalactosaminyltransferase and the -1,3-galactosyltransferase of ATCC 43438 when they are compared with the corresponding glycosyltransferases from the other loci (data not shown). These two genes seem to have evolved to accommodate the presence of a nonsialylated acceptor in the inner core of C. jejuni ATCC 43438. A functional assay of the recombinant -1,4-N-acetylgalactosaminyltransferase from ATCC 43438 confirmed that it is specific for a nonsialylated acceptor (Table V).

The absence or presence of activity of a specific glycosyltransferase can also have an impact on the activity of other glycosyltransferases. Based on the examination of the LOS outer core structures, Nam Shin et al. (15) have suggested that the presence of a -1,2-glucosyl residue on Hep-II would prevent sialylation of the inner -1,3-galactosyl residue, possibly because of steric hindrance. Consequently the inactivation of the second domain of orf3 by a frameshift mutation results in both the absence of a -1,2-glucosyl residue on Hep-II and makes possible the sialylation of the inner -1,3-Gal residue by Cst-II (orf7). We suggested previously (24) that the inner sialic acid was added by the product of cst-I, a gene that was cloned from C. jejuni OH4384 by activity screening and found downstream of the prfB gene (Cj1455), i.e. outside of the LOS biosynthesis locus. However, cst-I was shown to be absent from some strains that have a sialic acid on the inner -1,3-Gal residue of their LOS outer core (data not shown), and consequently it is unlikely to be responsible for LOS sialylation.

We reported previously (24) that orf7 from C. jejuni ATCC 43446 (the O:19 type strain) encoded an -2,3-sialyltransferase, whereas the version from C. jejuni OH4382/84 had both -2,3- and -2,8-sialytransferase activities. We named these two versions mono-functional Cst-II and bi-functional Cst-II, respectively. Guerry et al. (25) showed that the corresponding gene in ATCC 43429 is responsible for sialylation of the LOS outer core and named it Cst-III because it only showed 53% protein sequence identity with Cst-II from C. jejuni OH4384. When extending the comparison of orf7 to the other strains, it appears that classes A and B all have slightly divergent Cst-II versions, whereas class C strains all have an identical version of Cst-III. We showed that one of the variable residues among the Cst-II versions results in either a mono-functional Cst-II (Thr-51) or a bi-functional Cst-II (Asn-51). Although Cst-III also has Asn-51, it seems to have only -2,3-sialyltransferase activity (mono-functional) as observed in the LOS outer core structures (Fig. 1) and from in vitro assays (data not shown). Because Cst-II and Cst-III have diverged significantly, it is not too surprising that the presence of Asn-51 in Cst-III is not sufficient to confer it -2,8-sialyltransferase activity. The low level of protein sequence conservation between Cst-II and Cst-III might be a consequence of adaptation to significantly different acceptor environments: in classes A and B the acceptor -1,3-Gal residue is next to an un-substituted sugar residue (either GalNAc, Hep, or Glc), whereas in class C the acceptor -1,3-Gal is attached to a Gal residue that is substituted with an -1,2-Gal residue (Fig. 1).

The in vitro assays allowed us to determine which Cst-IIs are mono- or bi-functional, although the levels of activities vary considerably between the Cst-II versions. Comparison of the amino acid sequences and site-directed mutagenesis suggested that some residues (such as a glycine at position 53) have a large impact on the level of in vitro activities. It is unclear if these residues affect the stability of the recombinant enzyme or the efficiency of catalysis. Because the wild type Cst-II versions are known to be functional in their respective strains, it is probable that the less active versions are still active enough in vivo to carry efficient LOS sialylation.

The availability of variable cell-surface structures should be advantageous to a pathogen in order to evade the immune system. The five different modulation mechanisms described in this work can be effective over various time scales. The different gene complements are probably a result of evolution as well as of lateral gene transfers. There are at least two other distinct classes of LOS biosynthesis loci based on the sequences reported for C. jejuni LIO87 (GenBank^TM accession number AF400669), C. jejuni ATCC 43431 (GenBank^TM accession number AF411225), and C. jejuni 81116 (GenBank^TM accession numbers AF343914 and AJ131360). These LOS loci were not included in this study because the corresponding LOS outer core structures have not been determined for C. jejuni LIO87 and 81116, whereas the LOS outer core of C. jejuni ATCC 43431 does not mimic ganglioside structures (14). Nevertheless these types of LOS loci certainly expand the pool of genes that could be recombined in the LOS biosynthesis loci of C. jejuni.

Although no study has shown directly that phase variation provides an advantage during the course of C. jejuni infection, the high level of heterogeneity of some of the homopolymeric G-tracts suggests that a mixture of LOS outer core structures is likely to be expressed in many cases.³ Although some of the frameshift mutations were found to be more stable than the homopolymeric G-tracts, the "one-base difference" between C. jejuni OH4382 and OH4384 has occurred during the course of an infection because these two strains were isolated from siblings (35). Because a one amino acid substitution can change Cst-II from a mono- to bi-functional sialyltransferase (and vice versa), it is also possible that such mutations could occur during the course of an infection or an outbreak.

In this work we have shown that C. jejuni can use up to five mechanisms to vary its LOS outer cores. These mechanisms can involve as little as a 1-base or a one amino acid change or be more substantive, as in the acquisition of new genes. It will be interesting to determine whether the expression of other C. jejuni cell-surface carbohydrates involves as many different regulatory and modulating mechanisms or if other pathogens have the same repertoire of modulating mechanisms.

	ACKNOWLEDGEMENTS

We thank Dr. Christine Szymanski for helpful discussion, Julie Bellefeuille for technical help, and Simon Foote and Dr. John Nash for help with bio-informatics analyses.

	FOOTNOTES

^* The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed. Tel.: 613-991-9956; Fax: 613-941-1327; E-mail: michel.gilbert@nrc.ca.

Published, JBC Papers in Press, October 31, 2001, DOI 10.1074/jbc.M108452200

² M. Gilbert, M.-F. Karwaski, S. Bernatchez, N. M. Young, E. Taboada, J. Michniewicz, A.-M. Cunningham, and W. W. Wakarchuk, unpublished data.

³ St. Michael et al., personal communication.

	ABBREVIATIONS

The abbreviations used are: LOS, lipo-oligosaccharides; CMP-Neu5Ac, cytidine monophosphate-N-acetylneuraminic acid; FCHASE, 6-(5-fluorescein-carboxamido)-hexanoic acid succimidyl ester; ORF, open reading frame; Hep, heptose; nt, nucleotide. The abbreviated designations of glycolipids are according to IUPAC-IUC nomenclature.

	REFERENCES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

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