Low structural diversity of the O-polysaccharides of Photorhabdus asymbiotica subspp. asymbiotica and australis and their similarity to the O-polysaccharides of taxonomically remote bacteria including Francisella tularensis

The rare sugar N-acetylated viosamine is a major component of Mimivirus fibers

The giant virus Mimivirus encodes an autonomous glycosylation system that is thought to be responsible for the formation of complex and unusual glycans composing the fibers surrounding its icosahedral capsid, including the dideoxyhexose viosamine. Previous studies have identified a gene cluster in the virus genome, encoding enzymes involved in nucleotide-sugar production and glycan formation, but the functional characterization of these enzymes and the full identification of the glycans found in viral fibers remain incomplete. Because viosamine is typically found in acylated forms, we suspected that one of the genes might encode an acyltransferase, providing directions to our functional annotations. Bioinformatic analyses indicated that the L142 protein contains an N-terminal acyltransferase domain and a predicted C-terminal glycosyltransferase. Sequence analysis of the structural model of the L142 N-terminal domain indicated significant homology with some characterized sugar acetyltransferases that modify the C-4 amino group in the bacillosamine or perosamine biosynthetic pathways. Using mass spectrometry and NMR analyses, we confirmed that the L142 N-terminal domain is a sugar acetyltransferase, catalyzing the transfer of an acetyl moiety from acetyl-CoA to the C-4 amino group of UDP-d-viosamine. The presence of acetylated viosamine in vivo has also been confirmed on the glycosylated viral fibers, using GC-MS and NMR. This study represents the first report of a virally encoded sugar acetyltransferase.

Structure of the O-specific polysaccharide from the lipopolysaccharide of Aeromonas hydrophila strain K691 containing 4-acetamido-4,6-dideoxy-d-glucose

The O-specific polysaccharide (OPS) was isolated from the lipopolysaccharide of Aeromonas hydrophila strain K691 and studied by chemical methods and ¹H and ¹³C NMR spectroscopy, including 2D ¹H,¹H COSY, TOCSY, NOESY, ¹H-detected heteronuclear ¹H,¹³C HSQC, and HMBC experiments. It was found that the O-specific polysaccharide was built up of pentasaccharide repeating units composed of β-GlcpNAc, 2-O-acetylated α-Rhap, and β-Quip4NAc residues. The following structure of the OPS was established: →3)-α-l-Rha2OAc-(1→3)-β-d-GlcNAc-(1→3)-α-l-Rha2OAc-(1→3)-β-d-GlcNAc-(1→2)-β-d-Qui4NAc-(1→.

Photorhabdus asymbiotica as an Insect and Human Pathogen

Photorhabdus asymbiotica is a species of bacterium that is pathogenic to humans whilst retaining the ability to infect insect hosts. Currently, there are two recognised subspecies, P. asymbiotica subsp. asymbiotica and P. asymbiotica subsp. australis with strains isolated from various locations in the USA, Australia, Thailand, Nepal and Europe. Like other species of Photorhabdus, P. asymbiotica subsp. australis was shown to form a symbiotic relationship with a Heterorhabditis nematode. In contrast to most strains of Photorhabdus luminescens, P. asymbiotica can grow at 37 °C and this is a defining factor in its ability to cause human disease. Insights into other adaptations it has undergone that have enabled host switching to occur have come from whole genome sequencing and transcriptomic studies. P. asymbiotica has a smaller genome compared to P. luminenscens with a lower diversity of insecticidal toxins. However, it has acquired plasmids and several pathogenicity islands in its genome. These encode genes with similarity to effectors or systems found in other known human pathogens such as Salmonella and Yersinia and are therefore likely to contribute to human pathogenicity. Of crucial importance to virulence is the fact that P. asymbiotica undergoes a large metabolic shift at the human host temperature.

Structure of a zwitterionic O-polysaccharide from Photorhabdus temperata subsp. cinerea 3240

A phosphorylated O-polysaccharide was isolated from the lipopolysaccharide of an entomopathogenic bacterium Photorhabdus temperata subsp. cinerea 3240 and studied by sugar analysis, dephosphorylation, and (1)H and (13)C NMR spectroscopy. The following structure of the linear trisaccharide repeating unit of the O-polysaccharide was established: →3)-β-D-GalpNAc4PEtN-(1→4)-β-D-GlcpA-(1→3)-β-D-FucpNAc4N-(1→ where GlcA indicates glucuronic acid, FucNAc4N 2-acetamido-4-amino-2,4,6-trideoxygalactose, and PEtN 2-aminoethyl phosphate.

Structure of the O-polysaccharide of Photorhabdus temperata subsp. temperata XlNach(T) containing a novel branched monosaccharide, 3,6-dideoxy-4-C-[(S)-1,2-dihydroxyethyl]-d-xylo-hexose

O-Polysaccharide was isolated from the lipopolysaccharide of an entomopathogenic bacterium Photorhabdus temperata subsp. temperata XlNach(T). Sugar analysis after full acid hydrolysis of the polysaccharide revealed D-glucose, D-mannose, D-galactose, D-GalNAc, and a branched monosaccharide, 3,6-dideoxy-4-C-[(S)-1',2'-dihydroxyethyl]-D-xylo-hexose (Sug), which was isolated as a 1,2'-anhydro furanose derivative. The following structure of the polysaccharide was established by 1D and 2D 1H and 13C NMR spectroscopy:

Giant DNA virus mimivirus encodes pathway for biosynthesis of unusual sugar 4-amino-4,6-dideoxy-D-glucose (Viosamine)

Mimivirus is one the largest DNA virus identified so far, infecting several Acanthamoeba species. Analysis of its genome revealed the presence of a nine-gene cluster containing genes potentially involved in glycan formation. All of these genes are co-expressed at late stages of infection, suggesting their role in the formation of the long fibers covering the viral surface. Among them, we identified the L136 gene as a pyridoxal phosphate-dependent sugar aminotransferase. This enzyme was shown to catalyze the formation of UDP-4-amino-4,6-dideoxy-D-glucose (UDP-viosamine) from UDP-4-keto-6-deoxy-D-glucose, a key compound involved also in the biosynthesis of L-rhamnose. This finding further supports the hypothesis that Mimivirus encodes a glycosylation system that is completely independent of the amoebal host. Viosamine, together with rhamnose, (N-acetyl)glucosamine, and glucose, was found as a major component of the viral glycans. Most of the sugars were associated with the fibers, confirming a capsular-like nature of the viral surface. Phylogenetic analysis clearly indicated that L136 was not a recent acquisition from bacteria through horizontal gene transfer, but it was acquired very early during evolution. Implications for the origin of the glycosylation machinery in giant DNA virus are also discussed.