A dedicated C-6 ß-hydroxyacyltransferase required for biosynthesis of the glycolipid anchor for Vi antigen capsule in typhoidal Salmonella.

Vi antigen is an extracellular polysaccharide produced by Salmonella enterica Typhi, Citrobacter freundii, and some soil bacteria belonging to the Burkholderiales. In Salmonella Typhi, Vi-antigen capsule protects the bacterium against host defenses, and the glycan is used in a current glycoconjugate vaccine to protect against typhoid. Vi antigen is a glycolipid assembled in the cytoplasm and translocated to the cell surface by an export complex driven by an ABC transporter. In Salmonella Typhi, efficient export and cell-surface retention of the capsule layer depend on a reducing terminal acylated-HexNAc moiety. Although the precise structure and biosynthesis of the acylated terminus has not been resolved, it distinguishes Vi antigen from other known glycolipid substrates for bacterial ABC transporters. The genetic locus for Vi antigen-biosynthesis encodes a single acyltransferase candidate (VexE), which is implicated in the acylation process. Here, we determined the structure of the VexE in vitro reaction product by mass spectrometry and NMR spectroscopy to reveal that VexE catalyzes ß-hydroxyacyl-ACP dependent acylation of the activated sugar precursor, uridine-5'-diphospho-GlcNAc, at C-6 to form UDP-6-O-[ß-hydroxymyristoyl]-α-d-GlcNAc. VexE belongs to the lysophosphatidyl acyltransferase family, and comparison of an Alphafold VexE model to solved lysophosphatidyl acyltransferase structures, together with modeling enzyme:substrate complexes, led us to predict an enzyme mechanism. This study provides new insight into Vi terminal structure, offers a new model substrate to investigate the mechanism of glycolipid ABC transporters, and adds biochemical understanding for a novel reaction used in the synthesis of an important bacterial virulence factor.

O-Antigens of Escherichia coli Strains O81 and HS3-104 Are Structurally and Genetically Related, Except O-Antigen Glucosylation in E. coli HS3-104.

Glycerophosphate-containing O-specific polysaccharides (OPSs) were obtained by mild acidic degradation of lipopolysaccharides isolated from Escherichia coli type strain O81 and E. coli strain HS3-104 from horse feces. The structures of both OPSs and of the oligosaccharide derived from the strain O81 OPS by treatment with 48% HF were studied by monosaccharide analysis and one- and two-dimensional 1H- and 13C-NMR spectroscopy. Both OPSs had similar structures and differed only in the presence of a side-chain glucose residue in the strain HS3-104 OPS. The genes and the organization of the O-antigen biosynthesis gene cluster in both strains are almost identical with the exception of the gtr gene cluster responsible for glucosylations in the strain HS3-104, which is located elsewhere in the genome.

O-antigens of bacteria of the genus providencia: structure, serology, genetics, and biosynthesis.

The genus Providencia consists of eight species of opportunistic pathogenic enterobacteria that can cause enteric diseases and urinary tract infections. The existing combined serological classification scheme of three species, P. alcalifaciens, P. stuartii, and P. rustigianii, is based on the specificity of O-antigens (O-polysaccharides) and comprises 63 O-serogroups. Differences between serogroups are related to polymorphism at a specific genome locus, the O-antigen gene cluster, responsible for O-antigen biosynthesis. This review presents data on structures of 36 O-antigens of Providencia, many of which contain unusual monosaccharides and non-carbohydrate components. The structural data correlate with the immunospecificity of the O-antigens and enable substantiation on a molecular level of serological relationships within the genus Providencia and between strains of Providencia and bacteria of the genera Proteus, Escherichia, and Salmonella. Peculiar features of the O-antigen gene cluster organization in 10 Providencia serogroups and biosynthetic pathways of nucleotide precursors of specific monosaccharide components of the O-antigens also are discussed.

Structure of a peptidoglycan-related polysaccharide from Providencia alcalifaciens O45.

A polysaccharide was isolated from the opportunistic human pathogen Providencia alcalifaciens O45:H26 by extraction with aqueous phenol and studied by sugar and methylation analyses along with (1)H and (13)C NMR spectroscopy, including two-dimensional ROESY and H-detected (1)H,(13)C HSQC experiments. The polysaccharide contains N-acetylglucosamine and N-acetylmuramic acid (D-GlcpNAc3Rlac) amidated with L-alanine and has the following structure: →4)-ß-D-GlcpNAc-(1→4)-ß-D-GlcpNAc3(Rlac-L-Ala)-(1→. The polysaccharide possesses a remarkable structural similarity to the bacterial cell wall peptidoglycan. It is not unique to the strain studied but is common to strains of at least four P. alcalifaciens O-serogroups (O3, O24, O38, and O45). No evidence was obtained that the polysaccharide is associated with the LPS, and hence it might represent a bacterial capsule component.

Structure of the O-polysaccharide of Providencia alcalifaciens O25 containing an amide of D-galacturonic acid with N(ε)-[(R)-1-carboxyethyl]-L-lysine.

An acidic O-polysaccharide was isolated by mild acid degradation of the lipopolysaccharide of Providencia alcalifaciens O25 followed by gel-permeation and anion-exchange chromatography. The O-polysaccharide was studied by sugar and methylation analyses along with (1)H and (13)C NMR spectroscopy, including two-dimensional correlation (1)H,(13)C HMBC, and (1)H,(1)H ROESY experiments both in D(2)O and, to detect correlations for NH protons, in a 9 : 1 H(2)O/D(2)O mixture. An amino acid was isolated from the polysaccharide by acid hydrolysis and identified as N(ε)-[(R)-1-carboxyethyl]-L-lysine ("alaninolysine", 2S,8R-alaLys) by determination of the specific optical rotation and (13)C NMR spectroscopy, using the authentic synthetic diastereomers 2S,8R-alaLys and 2S,8S-alaLys for comparison. The structure of the branched tetrasaccharide repeating unit of the O-polysaccharide was established.

[Bacterial antigen polysaccharides. 43. Structure of O-specific polysaccharide of Providencia alcalifaciens O46].

Acid O-specific polysaccharide containing D-glucose, D-glucuronic acid, L-fucose, and 2-acetamido-2-deoxy-D-glucose was obtained by mild acid degradation of lipopolysaccharide from Providencia alcalifaciens O46. Consideration of the data revealed the following structure of the hexasaccharide repeating unit of O-specific polysaccharide under methylation analyses along with (1)H and (13)C NMR spectroscopy, including 2D (1)H, (1)H-COSY, TOCSY-, ROESY-,(1)H, (13)C-HSQC-, and HMQC-TOCSY experiments: [Formula: see text].