Structure and gene cluster of the O-antigen of Salmonella enterica O60 containing 3-formamido-3,6-dideoxy-D-galactose

Structure of surface polysaccharides from Aeromonas sp. AMG272, a plant-growth promoting rhizobacterium isolated from rice rhizosphere

Aeromonas sp. AMG272 is a Gram-negative bacterium that has been isolated from agricultural soil and studied for its plant growth-promoting activities. Structures of the O-specific polysaccharide chain of the AMG272 lipopolysaccharide and its capsular polysaccharide were elucidated using GLC-MS and NMR spectroscopy. The structure of the O-specific polysaccharide, →4)-α-l-Rhap-(1 → 3)-β-d-GlcpNAc-(1→, has been found in other Aeromonas strains and related bacteria, whereas the structure of the capsular polysaccharide has not been reported before: →6)[β-d-Fucp3NAc4Ac-(1 → 3)]-α-d-GlcpNAc-(1 → 4)-α-d-Galp-(1 → 3)-α-d-GalpNAc-(1 → 4)-α-d-Galp-(1 → .

Molecular architecture of an N-formyltransferase from Salmonella enterica O60

N-formylated sugars are found on the lipopolysaccharides of various pathogenic Gram negative bacteria including Campylobacter jejuni 81116, Francisella tularensis, Providencia alcalifaciens O30, and Providencia alcalifaciens O40. The last step in the biosynthetic pathways for these unusual sugars is catalyzed by N-formyltransferases that utilize N¹⁰-formyltetrahydrofolate as the carbon source. The substrates are dTDP-linked amino sugars with the functional groups installed at either the C-3' or C-4' positions of the pyranosyl rings. Here we describe a structural and enzymological investigation of the putative N-formyltransferase, FdtF, from Salmonella enterica O60. In keeping with its proposed role in the organism, the kinetic data reveal that the enzyme is more active with dTDP-3-amino-3,6-dideoxy-d-galactose than with dTDP-3-amino-3,6-dideoxy-d-glucose. The structural data demonstrate that the enzyme contains, in addition to the canonical N-formyltransferase fold, an ankyrin repeat moiety that houses a second dTDP-sugar binding pocket. This is only the second time an ankyrin repeat has been shown to be involved in small molecule binding. The research described herein represents the first structural analysis of a sugar N-formyltransferase that specifically functions on dTDP-3-amino-3,6-dideoxy-d-galactose in vivo and thus adds to our understanding of these intriguing enzymes.

Enzymes required for the biosynthesis of N-formylated sugars

The N-formyltransferases, also known as transformylases, play key roles in de novo purine biosynthesis where they catalyze the transfer of formyl groups to primary amine acceptors. These enzymes require N¹⁰-formyltetrahydrofolate for activity. Due to their biological importance they have been extensively investigated for many years, and they are still serving as targets for antifolate drug design. Most of our understanding of the N-formyltransferases has been derived from these previous studies. It is now becoming increasingly apparent, however, that N-formylation also occurs on some amino sugars found on the O-antigens of pathogenic bacteria. This review focuses on recent developments in the biochemical and structural characterization of the sugar N-formyltransferases.

Structure of the Escherichia coli ArnA N-formyltransferase domain in complex with N(5) -formyltetrahydrofolate and UDP-Ara4N

ArnA from Escherichia coli is a key enzyme involved in the formation of 4-amino-4-deoxy-l-arabinose. The addition of this sugar to the lipid A moiety of the lipopolysaccharide of pathogenic Gram-negative bacteria allows these organisms to evade the cationic antimicrobial peptides of the host immune system. Indeed, it is thought that such modifications may be responsible for the repeated infections of cystic fibrosis patients with Pseudomonas aeruginosa. ArnA is a bifunctional enzyme with the N- and C-terminal domains catalyzing formylation and oxidative decarboxylation reactions, respectively. The catalytically competent cofactor for the formylation reaction is N(10) -formyltetrahydrofolate. Here we describe the structure of the isolated N-terminal domain of ArnA in complex with its UDP-sugar substrate and N(5) -formyltetrahydrofolate. The model presented herein may prove valuable in the development of new antimicrobial therapeutics.

Structural diversity in Salmonella O antigens and its genetic basis

This review covers the structures and genetics of the 46 O antigens of Salmonella, a major pathogen of humans and domestic animals. The variation in structures underpins the serological specificity of the 46 recognized serogroups. The O antigen is important for the full function and virulence of many bacteria, and the considerable diversity of O antigens can confer selective advantage. Salmonella O antigens can be divided into two major groups: those which have N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc) and those which have galactose (Gal) as the first sugar in the O unit. In recent years, we have determined 21 chemical structures and sequenced 28 gene clusters for GlcNAc-/GalNAc-initiated O antigens, thus completing the structure and DNA sequence data for the 46 Salmonella O antigens. The structures and gene clusters of the GlcNAc-/GalNAc-initiated O antigens were found to be highly diverse, and 24 of them were found to be identical or closely related to Escherichia coli O antigens. Sequence comparisons indicate that all or most of the shared gene clusters were probably present in the common ancestor, although alternative explanations are also possible. In contrast, the better-known eight Gal-initiated O antigens are closely related both in structures and gene cluster sequences.