Lipopolysaccharides of Vibrio cholerae. I. Physical and chemical characterization

Vibrio cholerae is the causative organism of the disease cholera. The lipopolysaccharide (LPS) of V. cholerae plays an important role in eliciting the antibacterial immune response of the host and in classifying the vibrios into some 200 or more serogroups. This review presents an account of our up-to-date knowledge of the physical and chemical characteristics of the three constituents, lipid-A, core-polysaccharide (core-PS) and O-antigen polysaccharide (O-PS), of the LPS of V. cholerae of different serogroups including the disease-causing ones, O1 and O139. The structure and occurrence of the capsular polysaccharide (CPS) on V. cholerae O139 have been discussed as a relevant topic. Similarity and dissimilarity between the structures of LPS of different serogroups, and particularly between O22 and O139, have been analysed with a view to learning their role in the causation of the epidemic form of the disease by avoiding the host defence mechanism and in the evolution of the newer pathogenic strains in future. An idea of the emerging trends of research involving the use of immunogens prepared from synthetic oligosaccharides that mimic terminal epitopes of the O-PS of V. cholerae O1 in the development of a conjugate anti cholera vaccine is also discussed.

The intestinal zonula occludens toxin (ZOT) receptor recognises non-native ZOT conformers and localises to the intercellular contacts

A preliminary structural analysis of Vibrio cholerae zonula occludens toxin (ZOT) was made by equilibrium denaturation and circular dichroism. ZOT is a structurally unstable protein in aqueous solution (DeltaG((H2O)) 3.82 kcal/mol), the putative intra- and extracellular domains unfold co-operatively, with complete denaturation via observed conformational intermediates. Refolding of denatured ZOT is not dependent on disulphide bridge formation. Partial refolding of a maltose binding protein-ZOT fusion did not prevent its specific binding to the ZOT receptor on Caco-2 cells. Immuno-gold labelling showed that the ZOT receptor localises to the intercellular contacts between cells in a confluent monolayer.

Activation and suppression of the proinflammatory immune response by Vibrio cholerae toxins

Vibrio cholerae induces either non-inflammatory diarrhea or inflammatory gastroenteritis, depending on the presence of cholera toxin, a fluid secretion inducer and a modulator of host immunity. In the absence of cholera toxin, other toxins induce inflammation, resulting in gastroenteritis. Thus, multiple toxins likely affect the safety of live attenuated vaccines.

Electron ionization mass spectrometric study of monomeric models of O-polysaccharides of Vibrio cholerae O:1, serotypes Ogawa and Inaba

Fragmentation mechanisms of electron ionization (EI) mass spectrometry of the title compounds have been elucidated by high-resolution (HR) mass spectrometric measurements of the elemental composition and measurements of the metastable transitions (B(2)/E, CID). The experimental results were interpreted with the help of Mass Frontier 3.0 software, which aided the elucidation of fragmentation mechanisms and helped to deduce structures of the ions formed. Characteristic under the conditions of EI-MS measurement was the production of protonated adducts. Three distinct pathways observed include the formation of oxonium type ions, the conjugated transfer of electrons in the pyranose ring, and cleavage of the acylamide side chains. By applying the results obtained, the molecular mass, as well as the structures of both the saccharide and acylamide side chain involved in related substances, can be determined.

Structure of MsbA from Vibrio cholera: a multidrug resistance ABC transporter homolog in a closed conformation

The spread of multidrug resistance (MDR) is a world health crisis that presents a significant challenge to the treatment of cancer and infection. MDR can be caused by a group of ABC (MDR-ABC) transporters that move hydrophobic drug molecules and lipids across the cell membrane. To gain insight into the conformational changes these transporters undergo when flipping hydrophobic substrates across the lipid bilayer, we have determined the structure of the lipid flippase MsbA from Vibrio cholera (VC-MsbA) to 3.8A. Structural comparison of VC-MsbA to MsbA from Escherichia coli reveals that the transporters share a structurally conserved core of transmembrane alpha-helices, but differ in the relative orientations of their nucleotide-binding domains (NBD). The transmembrane domain of VC-MsbA is captured in a closed conformation and the structure supports a "power stroke" model of transporter dynamics where opposing NBDs associate upon ATP binding. The separation of the alpha and beta domains of the NBD suggests the possibility that their association could make them competent to bind ATP and gives further insight into the structural basis for catalytic regulation.

Solution structure of Vibrio cholerae protein VC0424: a variation of the ferredoxin-like fold

The structure of Vibrio cholerae protein VC0424 was determined by NMR spectroscopy. VC0424 belongs to a conserved family of bacterial proteins of unknown function (COG 3076). The structure has an alpha-beta sandwich architecture consisting of two layers: a four-stranded antiparallel beta-sheet and three side-by-side alpha-helices. The secondary structure elements have the order alphabetaalphabetabetaalphabeta along the sequence. This fold is the same as the ferredoxin-like fold, except with an additional long N-terminal helix, making it a variation on this common motif. A cluster of conserved surface residues on the beta-sheet side of the protein forms a pocket that may be important for the biological function of this conserved family of proteins.

Characterization of four orthologs of stringent starvation protein A

Orthologous proteins can be beneficial for X-ray crystallographic studies when a protein from an organism of choice fails to crystallize or the crystals are not suitable for structure determination. Their amino-acid sequences should be similar enough that they will share the same fold, but different enough so that they may crystallize under alternative conditions and diffract to higher resolution. This multi-species approach was employed to obtain diffraction-quality crystals of the RNA polymerase (RNAP) associated stringent starvation protein A (SspA). Although Escherichia coli SspA could be crystallized, the crystals failed to diffract well enough for structure determination. Therefore, SspA proteins from Yersinia pestis, Vibrio cholerae and Pseudomonas aeruginosa were cloned, expressed, purified and subjected to crystallization trials. The V. cholerae SspA protein failed to crystallize under any conditions tested and the P. aeruginosa SspA protein did not form crystals suitable for data collection. On the other hand, Y. pestis SspA crystallized readily and the crystals diffracted to 2.0 A.

Inhibition by apple polyphenols of ADP-ribosyltransferase activity of cholera toxin and toxin-induced fluid accumulation in mice

The effects of crude polyphenol extracted from immature apples on the enzymatic and biological activities of a cholera toxin (CT) were investigated. When the apple polyphenol extract (APE) was examined for properties to inhibit CT-catalyzed ADP-ribosylation of agmatine, it was found that APE inhibited it in a dose-dependent manner. The concentration of APE to inhibit 50% of the enzymatic activity of CT (15 microg/ml) was approximately 8.7 microg/ml. The APE also diminished CT-induced fluid accumulation in two diarrhea models for in vivo mice. In the ligated ileum loops, 25 microg of APE significantly inhibited fluid accumulation induced by 500 ng of CT. In a sealed mouse model, even when APE was administered orally 10 min after a toxin injection, fluid accumulation was significantly inhibited at a comparable dosage. Lineweaver-Burk analysis demonstrated that APE had negative allosteric effects on CT-catalyzed NAD: agmatine ADP-ribosyltransferase. We fractionated the APE into four fractions using LH-20 Sephadex resin. One of the fractions, FAP (fraction from apple polyphenol) 1, which contains non-catechin polyphenols, did not significantly inhibit the CT-catalyzed ADP-ribosylation of agmatine. FAP2, which contains compounds with monomeric, dimeric, and trimeric catechins, inhibited the ADP-ribosylation only partially, but significantly. FAP3 and FAP4, which consist of highly polymerized catechin compounds, strongly inhibited the ADP-ribosylation, indicating that the polymerized structure of catechin is responsible for the inhibitory effect that resides in APE. The results suggest that polymerized catechin compounds in APE inhibit the biological and enzymatic activities of CT and can be used in a precautionary and therapeutic manner in the treatment of cholera patients.

Neoglycoconjugates from synthetic tetra- and hexasaccharides that mimic the terminus of the O-PS of Vibrio cholerae O:1, serotype Inaba

A glycosyl acceptor and a glycosyl donor having the N-3-deoxy-L-glycero-tetronic acid side chain already attached have been prepared and used for the synthesis of the di-through to the hexasaccharide that mimic the upsteam terminus of the O-specific polysaccharide of Vibrio cholerae O:1, serotype Inaba. The target tetra- and the hexasaccharide, which were obtained in the form of 5-methoxycarbonylpentyl glycosides, were linked to BSA using squaric acid diester chemistry. The conjugation reactions were monitored by surface enhanced laser desorption ionization-time of flight mass spectrometry (SELDI-TOF MS). This allowed the progression of the conjugation of the synthetic oligosaccharides in a controlled way and termination of the reaction when the desired molar hapten/BSA ratio had been reached, yielding neoglycoconjugates with predetermined carbohydrate/carrier ratios. The ability to monitor the conjugation by the SELDI-TOF MS technique made it possible to prepare, from one hapten in a one-pot reaction, several neoglycoconjugates having different, predetermined carbohydrate/carrier ratios.

Vibrio cholerae cytolysin is composed of an alpha-hemolysin-like core

The enteric pathogen Vibrio cholerae secretes a water-soluble 80-kD cytolysin, Vibrio cholerae cytolysin (VCC) that assembles into pentameric channels following proteolytic activation by exogenous proteases. Until now, VCC has been placed in a unique class of pore-forming toxins, distinct from paradigms such as Staphyloccal alpha-hemolysin. However, as reported here, amino acid sequence analysis and three-dimensional structure modeling indicate that the core component of the VCC toxin is related in sequence and structure to a family of hemolysins from Staphylococcus aureus that include leukocidin F and alpha-hemolysin. Furthermore, our analysis has identified the channel-forming region of VCC and a potential lipid head-group binding site, and suggests a conserved mechanism of assembly and lysis. An additional domain in the VCC toxin is related to plant lectins, conferring additional target cell specificity to the toxin.

The binding of synthetic analogs of the upstream, terminal residue of the O-polysaccharides (O-PS) of Vibrio cholerae O:1 serotypes Ogawa and Inaba to two murine monoclonal antibodies (MAbs) specific for the Ogawa lipopolysaccharide (LPS)

The binding of nineteen analogues of the upstream, terminal, monosaccharide residue of each of the O-polysaccharide (O-PS) of Vibrio cholerae O:1, serotype Ogawa and Inaba, with two murine monoclonal IgG antibodies both specific for the Ogawa LPS were measured using fluorescence spectroscopy. The use of the deoxy and the deoxyfluoro analogs allowed further refinement of the hydrogen-bonding pattern involved in the binding. Based on the binding characteristics observed for some of the ligands in the Inaba series, the binding of the monosaccharide that represents the upstream, terminal unit of the O-PS of V. cholerae O:1 serotype Inaba was redefined. We show for the first time that the upstream, terminal monosaccharide of the Inaba O-PS shows weak binding with these two anti-Ogawa antibodies. The results obtained allow further rationalization of the structural basis for the binding of V. cholerae O:1 antigens to their homologous antibodies.

The wavB gene of Vibrio cholerae and the waaE of Klebsiella pneumoniae codify for a beta-1,4-glucosyltransferase involved in the transfer of a glucose residue to the L-glycero-D-manno-heptose I in the lipopolysaccharide inner core

Vibrio cholerae WavB protein showed some similarity to WaaE of Klebsiella pneumoniae and Serratia marcescens. From previous data obtained by us and by chemical analyses of a K. pneumoniae non-polar waaE mutant from strain 889 (08:K69), its lipopolysaccharide (LPS) core structure has recently been elucidated. We demonstrated that WaaE is a beta-1,4-glucosyltransferase involved in the transfer of a glucose residue to the L-glycero-D-manno-heptose I in the LPS inner core. Complementation of this K. pneumoniae non-polar waaE mutant with gene wavB obtained, either from V. cholerae or V. mimicus, showed a full complementation either by chemical studies or by a biological test (susceptibility to non-immune serum). The V. cholerae wavB gene is located in a putative core oligosaccharide (OS) gene cluster and the V. cholerae OS core structure showed the same beta-1,4-glucose residue attached to Hep I as is observed for the K. pneumoniae 889 OS core structure. No other glucose residue is found in the ligosaccharide core structure of K. pneumoniae 889. We concluded that WavB protein is able to perform the same function as WaaE.

Vibrio cholerae hemolysin. Implication of amphiphilicity and lipid-induced conformational change for its pore-forming activity

Vibrio cholerae hemolysin (HlyA), a water-soluble protein with a native monomeric relative molecular mass of 65 000, forms transmembrane pentameric channels in target biomembranes. The HlyA binds to lipid vesicles nonspecifically and without saturation; however, self-assembly is triggered specifically by cholesterol. Here we show that the HlyA partitioned quantitatively to amphiphilic media irrespective of their compositions, indicating that the toxin had an amphiphilic surface. Asialofetuin, a beta1-galactosyl-terminated glycoprotein, which binds specifically to the HlyA in a lectin-glycoprotein type of interaction and inhibits carbohydrate-independent interaction of the toxin with lipid, reduced effective amphiphilicity of the toxin significantly. Resistance of the HlyA to proteases together with the tryptophan fluorescence emission spectrum suggested a compact structure for the toxin. Fluorescence energy transfer from the HlyA to dansyl-phosphatidylethanolamine required the presence of cholesterol in the lipid bilayer and was synchronous with oligomerization. Phospholipid bilayer without cholesterol caused a partial unfolding of the HlyA monomer as indicated by the transfer of tryptophan residues from the nonpolar core of the protein to a more polar region. These observations suggested: (a) partitioning of the HlyA to lipid vesicles is driven by the tendency of the amphiphilic toxin to reduce energetically unfavorable contacts with water and is not affected significantly by the composition of the vesicles; and (b) partial unfolding of the HlyA at the lipid-water interface precedes and promotes cholesterol-induced oligomerization to an insertion-competent configuration.

Synthesis of cholera toxin B subunit gene: cloning and expression of a functional 6XHis-tagged protein in Escherichia coli

Cholera toxin B subunit (CTB) has been extensively studied as immunogen, adjuvant, and oral tolerance inductor depending on the antigen conjugated or coadministered. It has been already expressed in several bacterial and yeast systems. In this study, we synthesized a versatile gene coding a 6XHis-tagged CTB (359bp). The sequence was designed according to codon usage of Escherichia coli, Lactobacillus casei, and Salmonella typhimurium. The gene assembly was based on a polymerase chain reaction, in which the polymerase extends DNA fragments from a pool of overlapping oligonucleotides. The synthetic gene was amplified, cloned, and expressed in E. coli in an insoluble form, reaching levels about 13 mg of purified active pentameric rCTB per liter of induced culture. Western blot and ELISA analyses showed that recombinant CTB is strongly and specifically recognized by polyclonal antibodies against the cholera toxin. The ability to form the functional pentamers was observed in cell culture by the inhibition of cholera toxin activity on Y1 adrenal cells in the presence of recombinant CTB. The 6XHis-tagged CTB provides a simple way to obtain functional CTB through Ni(2+)-charged resin after refolding and also free of possible CTA contaminants as in the case of CTB obtained from Vibrio cholerae cultures.

Effect of mild acid pH on the functioning of bacterial membranes in Vibrio cholerae

In this paper, we initiated the first two-dimensional electrophoresis map of Vibrio cholerae, the aetiological agent of cholera disease. In this pathogen the efficient adaptation to detrimental conditions plays an important role in its survival in both the aquatic reservoir and human intestine. By proteome analysis we investigated the effect of mild acid treatment on the physiology of V. cholerae. More than 50 proteins were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and database searching. Amongst them, pH regulated proteins belong to various functional classes such as intermediary metabolism and bacterial envelope. Several proteins whose accumulation level was decreased in response to acidic pH are known to be involved in the organization and the functioning of membranes, including lipopolysaccharide. Consistent with this, we observed an increased susceptibility to hydrophobic drugs, a loss of motility and a reduction in the ability to form a biofilm in cells grown at pH 6. Our results suggest that V. cholerae is able to sense a moderate decrease in pH and to modify accordingly its structure and physiology.

Structure of the O-polysaccharide from the lipopolysaccharide from Vibrio cholerae O6

The O-polysaccharide from Vibrio cholerae O6 was isolated from the LPS by mild-acid hydrolysis and has been investigated by sugar and methylation analysis and NMR spectroscopy. The polysaccharide was also depolymerized with aqueous hydrofluoric acid to give the repeating unit and multiples thereof. The O-polysaccharide had the following tetrasaccharide repeating unit. Two O-acetyl groups are present, one of them making the GlcNAc residue fully substituted and the steric crowding considerable at the branching residue.

Cloning, expression and characterization of the CHO cell elongating factor (Cef) from Vibrio cholerae O1

CHO cell-elongating factor (Cef) is a recently identified putative virulence factor of Vibrio cholerae. Our previous studies show that this 85 kDa protein elongates CHO cells, causes fluid accumulation in suckling mice and has esterase activity. In this study, the cef gene was cloned in Escherichia coli using a yeast vector and subsequently expressed in the yeast Pichia pastoris. The cef genes from V. cholerae candidate vaccine strains JBK 70 and CVD 103-HgR were sequenced and found to be nearly identical (100 and 99.9% respectively) with an open reading frame (ORF) from the published sequence of V. cholerae N16961. Cloned toxin was purified to homogeneity in 3 steps using anion exchange, hydrophobic interaction and gel filtration chromatography. The size of cloned Cef on SDS-PAGE gels was 114 kDa. The increased size was probably due to glycosylation by the yeast since cloned protein reacted strongly with a glycoprotein stain. The cloned protein could not be directly sequenced, but when treated with trypsin, yielded a protein fragment with an amino acid sequence that matched the sequence predicted for the Cef protein. The purified cloned protein had esterase and CHO cell activity, but no suckling mouse activity.

Model of Vibrio cholerae toxin coregulated pilin capable of filament formation

A complete three-dimensional model (RCSB001169; PDB code 1qqz ) for the Vibrio cholerae toxin coregulated pilus protein (TcpA), including residues 1-197, is presented. We have used the crystal structure of the Neisseria gonorrhoeae pilin (PilE), available biochemical data about TcpA, variations in the primary sequences of TcpA among various Vibrio cholerae strains and secondary structure prediction, hydrophilicity, surface probability and antigenicity plots for TcpA to build our model. In our TcpA model, the first 137 residues possess a structure similar to the PilE, but the remainder is different. Though the ladle shape is still preserved, TcpA possesses a larger ladle head or globular domain compared to PilE. Using this model, it has been possible to identify two kinds of conserved residues: (i) those forming the core of the TcpA monomer and (ii) those involved in the monomer-monomer interactions leading to fibre formation. Residues on the fibre exterior, important in the mediation of bacterium (pilus)-bacterium (pilus) and bacterium (pilus)-host interactions, show more variability in comparison to those of (i) and (ii).

[Aspects of Vibrio cholerae lipopolysaccharide]

In this review information on the chemical structure, biosynthesis, antigenic and biological properties of V. cholerae lipopolysaccharide (LPS) is presented. The specific structural feature of this LPS is a small size of the polysaccharide chain of O-antigen. In vibrios of serogroup O 139 it is oligosaccharide. The modification of the O-chain (methylation of individual sugars, shortened chain, etc.) plays an essential role in the antigenic specificity of V. cholerae LPS. All these factors affect of endotoxin function, the microbial resistance to external influences. V. cholerae LPS takes part in the formation of microcapsules and biofilms. The evolutional development of V. cholerae in this direction determines, to some extent, their increased resistance in the environment. In human body the heterogeneity of the LPS composition permits the preservation of vibrios and ensures, together with cholerogen, their pathogenetic action.

Gene analysis of Vibrio cholerae NAGV14 pilus and its distribution

Adhesive pilus of Vibrio cholerae 034, strain NAGV14, was genetically analyzed. The deduced amino acid (aa) sequence of the major pilin structural gene (VcfA) was 67% homologous to the MshA pilin in the N-terminal region, but no homology was found in the C-terminal region which contained the antigenic epitopes. Upstream and downstream flanking regions examined were highly homologous to mshB and mshC of the MSHA (mannose-sensitive hemagglutinin) gene locus. A short leader sequence and a pair of cysteines near the C-terminus which are the characteristics of type 4a pilus family were found. The major pilin structural gene of NAGV14 was compared to that of a strain V10 producing non-adhesive pili. The deduced aa sequences showed 60% homology, and the distance between two cysteines in the C-terminal region was different. A total of 177 V. cholerae strains were investigated for the presence of a type 4 pilus gene locus by PCR, and 95% were positive. The major pilin gene of NAGV14 was detected in 4 of 93 V. cholerae non-O1, non-0139 strains tested, but none of the V. cholerae O1 and O139 (72 and 12 strains, respectively). Our result suggested that a type 4 pilus gene locus similar to the MSHA gene locus is widely distributed among V. cholerae strains. We proposed naming this type 4 pilus gene locus the VCF (for V. cholerae flexible pili) gene locus.

Structures of two O-chain polysaccharides of Citrobacter gillenii O9a,9b lipopolysaccharide. A new homopolymer of 4-amino-4,6-dideoxy-D-mannose (perosamine)

Mild acid degradation of the lipopolysaccharide of Citro- bacter gillenii O9a,9b released a polysaccharide (PS), which was found to consist of a single monosaccharide, 4- acetamido-4,6-dideoxy-d-mannose (d-Rha4NAc, N-acetyl-d-perosamine). PS was studied by methylation analysis and (1)H-NMR and (13)C-NMR spectroscopy, using two-dimensional (1)H,(1)H COSY, TOCSY, NOESY, and H-detected (1)H,(13)C heteronuclear correlation experiments. It was found that PS includes two structurally different polysaccharides: an alpha1-->2-linked homopolymer of N-acetyl-d-perosamine [-->2)-alpha-d-Rhap4NAc-(1-->, PS2] and a polysaccharide composed of tetrasaccharide repeating units (PS1) with the following structure: -->3)-alpha-d-Rhap4NAc-(1-->2)-alpha-d-Rhap4NAc-(1-->2)-alpha-d-Rhap4NAc-(1-->3)-alpha-d-Rhap4 N Ac2Ac-(1--> where the degree of O-acetylation of a 3-substituted Rha4NAc residue at position 2 is approximately 70%. PS could be fractionated into PS1 and PS2 by gel-permeation chromatography on TSK HW-50S. Matrix-assisted laser desorption ionization MS data indicate sequential chain elongation of both PS1 and PS2 by a single sugar unit, with O-acetylation in PS1 beginning at a certain chain length. Anti-(C. gillenii O9a,9b) serum reacted with PS1 in double immunodiffusion and immunoblotting, whereas neither PS2 nor the lipopolysaccharide of Vibrio cholerae O1 with a structurally related O-chain polysaccharide were reactive.

Towards a reliable objective function for multiple sequence alignments

Multiple sequence alignment is a fundamental tool in a number of different domains in modern molecular biology, including functional and evolutionary studies of a protein family. Multiple alignments also play an essential role in the new integrated systems for genome annotation and analysis. Thus, the development of new multiple alignment scores and statistics is essential, in the spirit of the work dedicated to the evaluation of pairwise sequence alignments for database searching techniques. We present here norMD, a new objective scoring function for multiple sequence alignments. NorMD combines the advantages of the column-scoring techniques with the sensitivity of methods incorporating residue similarity scores. In addition, norMD incorporates ab initio sequence information, such as the number, length and similarity of the sequences to be aligned. The sensitivity and reliability of the norMD objective function is demonstrated using structural alignments in the SCOP and BAliBASE databases. The norMD scores are then applied to the multiple alignments of the complete sequences (MACS) detected by BlastP with E-value<10, for a set of 734 hypothetical proteins encoded by the Vibrio cholerae genome. Unrelated or badly aligned sequences were automatically removed from the MACS, leaving a high-quality multiple alignment which could be reliably exploited in a subsequent functional and/or structural annotation process. After removal of unreliable sequences, 176 (24 %) of the alignments contained at least one sequence with a functional annotation. 103 of these new matches were supported by significant hits to the Interpro domain and motif database.

A novel method to calculate the G+C content of genomic DNA sequences

The base composition of a DNA fragment or genome is usually measured by the proportion of A+T or G+C in the sequence. The G+C content along genomic sequences is usually calculated using an overlapping or non-overlapping sliding window method. The result and accuracy of such an approach depends on the size of the window and the moving distance adopted. In this paper, a novel windowless technique to calculate the G+C content of genomic sequences is proposed. By this method, the G+C content can be calculated at different "resolution". In an extreme case, the G+C content may be computed at a specific point, rather than in a window of finite size. This is particularly useful to analyze the fine variation of base composition along genomic sequences. As the first example, the variation of G+C content along each of 16 yeast chromosomes is analyzed. The G+C-rich regions with length larger than 5 kb sequences are detected and listed in details. It is found that each chromosome consists of several G+C-rich and G+C-poor regions alternatively, i.e., a mosaic structure. Another example is to analyze the G+C content for each of the two chromosomes of the Vibrio cholerae genome. Based on the variations of the G+C content in each chromosome, it is shown that some fragments in the Vibrio cholerae genome may have been transferred from other species. Especially, the position and size of the large integron island on the smaller chromosome was precisely predicted. This method would be a useful tool for analyzing genomic sequences.

Synthetic explorations towards 3-deoxy-3-fluoro derivatives of D-perosamine

Based on a literature precedent, preparation of methyl 4-azido-3,4,6-trideoxy-3-fluoro-alpha-D-mannopyranoside (18) was attempted via fluorination of methyl 4-azido-2-O-benzyl-4,6-dideoxy-alpha-D-altropyranoside with diethylaminosulfur trifluoride (DAST). Contrary to expectations, the reaction took place with retention of configuration at the site of the fluorination yielding methyl 4-azido-2-O-benzyl-3,4,6-trideoxy-3-fluoro-alpha-D-altropyranoside. Treatment with DAST of methyl 4-azido-2-O-benzyl-4,6-dideoxy-alpha-D-allopyranoside (8), or its 2-(p-methoxybenzyl) analog 9 resulted in fluorination with inversion of configuration at position 3, to give the corresponding 3-deoxy-3-fluoro glucopyranosides 10 and 11, respectively. Accordingly, compound 18 was prepared from 11, by de-p-methoxybenzylation at O-2, followed by inversion of configuration at C-2 in the resulting methyl 4-azido-3,4,6-trideoxy-3-fluoro-alpha-D-glucopyranoside. The 2-O-methyl analog of 18 (19) was prepared by methylation of 18. Compounds 18 and 19 were converted, conventionally, into the 3-fluoro analogs of the terminal determinants of the O-PS of Vibrio cholerae O:1, serotype Inaba and Ogawa, respectively.