A putative pathway for biosynthesis of the O-antigen component, 3-deoxy-L-glycero-tetronic acid, based on the sequence of the Vibrio cholerae O1 rfb region

Deletion of AS87_03730 gene changed the bacterial virulence and gene expression of Riemerella anatipestifer

Riemerella anatipestifer is an important pathogen of waterfowl, which causes septicemia anserum exsudativa in ducks. In this study, an AS87_03730 gene deletion R. anatipestifer mutant Yb2ΔAS87_03730 was constructed to investigate the role of AS87_03730 on R. anatipestifer virulence and gene regulation. By deleting a 708-bp fragment from AS87_03730, the mutant Yb2ΔAS87_03730 showed a significant decreased growth rate in TSB and invasion capacity in Vero cells, compared to wild-type strain Yb2. Moreover, the median lethal dose (LD₅₀) of Yb2ΔAS87_03730 was 1.24 × 10⁷ colony forming units (CFU), which is about 80-fold attenuated than that of Yb2 (LD₅₀ = 1.53 × 10⁵ CFU). Furthermore, RNA-Seq analysis and Real-time PCR indicated 19 up-regulated and two down-regulated genes in Yb2ΔAS87_03730. Functional analysis revealed that 12 up-regulated genes were related to “Translation, ribosomal structure and biogenesis”, two were classified into “Cell envelope biogenesis, outer membrane”, one was involved in “Amino acid transport and metabolism”, and the other four had unknown functions. Polymerase chain reaction and sequence analysis indicated that the AS87_03730 gene is highly conserved among R. anatipestifer strains, as the percent sequence identity was over 93.5%. This study presents evidence that AS87_03730 gene is involved in bacterial virulence and gene regulation of R. anatipestifer.

Phase variable O antigen biosynthetic genes control expression of the major protective antigen and bacteriophage receptor in Vibrio cholerae O1

The Vibrio cholerae lipopolysaccharide O1 antigen is a major target of bacteriophages and the human immune system and is of critical importance for vaccine design. We used an O1-specific lytic bacteriophage as a tool to probe the capacity of V. cholerae to alter its O1 antigen and identified a novel mechanism by which this organism can modulate O antigen expression and exhibit intra-strain heterogeneity. We identified two phase variable genes required for O1 antigen biosynthesis, manA and wbeL. manA resides outside of the previously recognized O1 antigen biosynthetic locus, and encodes for a phosphomannose isomerase critical for the initial step in O1 antigen biosynthesis. We determined that manA and wbeL phase variants are attenuated for virulence, providing functional evidence to further support the critical role of the O1 antigen for infectivity. We provide the first report of phase variation modulating O1 antigen expression in V. cholerae, and show that the maintenance of these phase variable loci is an important means by which this facultative pathogen can generate the diverse subpopulations of cells needed for infecting the host intestinal tract and for escaping predation by an O1-specific phage.

The genetics of glycosylation in Gram-negative bacteria

In recent years there has been a dramatic increase in reports of glycosylation of proteins in various Gram-negative systems including Neisseria meningitidis, Neisseria gonorrhoeae, Campylobacter jejuni, Pseudomonas aeruginosa, Escherichia coli, Caulobacter crescentus, Aeromonas caviae and Helicobacter pylori. Although this growing list contains many important pathogens (reviewed by Benz and Schmidt [Mol. Microbiol. 45 (2002) 267-276]) and the glycosylations are found on proteins important in pathogenesis such as pili, adhesins and flagella the precise role(s) of the glycosylation of these proteins remains to be determined. Furthermore, the details of the glycosylation biosynthetic process have not been determined in any of these systems. The definition of the precise role of glycosylation and the mechanism of biosynthesis will be facilitated by a detailed understanding of the genes involved.

Distribution of IS1358 and linkage to rfb-related genes in Vibrio anguillarum

The insertion sequence IS1358 is linked to the rfb regions of both Vibrio cholerae O1 and O139, and its location was suggestive of a role in generating new combinations of rfb genes. This provoked an examination of the distribution and localization of IS1358 in Vibrio anguillarum. S11358 was widely distributed in a number of V. anguillarum serogroups. In particular, when cosmid clones of V. anguillarum O1 were screened with IS1358 and subsequently subcloned and sequenced, it was found that rfb-like genes were linked to this region. Furthermore, when the previously identified genes virA and virB from V. anguillarum O1, now known to be involved in LPS biosynthesis, were used as probes, it was discovered that they too are present on the same large EcoRI fragment as IS1358. This clearly indicated that IS1358 was linked to the rfb region of V. anguillarum O1. Further analysis of the location of IS1358 in other serotypes indicated that V. anguillarum O2 also has IS1358 associated with rfb-like genes. In V. anguillarum O2 there is more than one copy of IS1358, suggesting that this element is a site for recombination, gene duplication or that it may be capable of transposition. Following this latter premise, IS1358 elements from a variety of V. anguillarum strains have been cloned and sequenced. Only those strains with multiple copies of IS1358 produce a full-length putative transposase, as shown by protein overexpression, further strengthening the argument that the element is transposing within these strains.

Genetic organization of the regions associated with surface polysaccharide synthesis in Vibrio cholerae O1, O139 and Vibrio anguillarum O1 and O2: a review

Vibrio cholerae and V. anguillarum are recognized as aquatic-borne human and fish pathogens, respectively. Based upon analyses of several genes and the presence of novel genetic elements it seems that these two species are very closely related. Studies in this laboratory have identified an association of IS1358 with rfb and capsule loci in these two species. The most recent findings suggest that IS1358 is associated with the rfb region in V. cholerae O1 and O139 and in V. anguillarum O1 and O2. In addition, the rfb region in both V. cholerae serogroups and in V. anguillarum O1 is limited at one end by gmhD. These features make it feasible to envisage a mechanism by which the evolution of new rfb genes is taking place involving IS1358 and the region around gmhD. Furthermore, it is possible to envisage that there is or has been an exchange of genetic material between these species leading to new rfb/capsule regions. This review examines the genetics and biosynthesis of the O-antigen and capsule of V. cholerae O1 and O139, as well as the V. anguillarum serogroup O1 and the role of IS1358. Throughout this review we have used the new nomenclature for rfb genes proposed by.

Identification of additional genes required for O-antigen biosynthesis in Vibrio cholerae O1

The cloning and expression of the genes encoding the Vibrio cholerae O1 lipopolysaccharide O antigen in a heterologous host have been described previously (P. A. Manning, M. W. Heuzenroeder, J. Yeadon, D. I. Leavesley, P. R. Reeves, and D. Rowley, Infect. Immun. 53:272-277, 1986). It was thus assumed that all the genes required for O-antigen expression were located on a 20-kb SacI restriction fragment. We present evidence for a number of other as yet undescribed genes that are essential for O-antigen biosynthesis in V. cholerae O1 and that these genes are somehow complemented in Escherichia coli K-12. The two genes termed Vibrio cholerae rfbV and rfbU are transcribed in the opposite orientation from the rest of the rfb operon, whereas the galE dehydratase and rfbP (Salmonella enterica) homologs, designated ORF35x7 and rfbW, respectively, are transcribed in the same orientation. The evidence presented here, using chromosomal insertion mutants, clearly shows that the three genes now designated rfbV, rfbU, and rfbW appear to be accessory rfb genes and are essential for O-antigen biosynthesis in V. cholerae but that ORF35x7 is not.

A putative pathway for perosamine biosynthesis is the first function encoded within the rfb region of Vibrio cholerae O1

The first four genes (rfbA,B,D,E) of the rfb region of Vibrio cholerae O1 are predicted to encode the enzymes required for the biosynthesis of perosamine, which constitutes the backbone structure of the O-antigen of the lipopolysaccharide. Based on homology to known proteins/protein families, the following functions are predicted: RfbA, phosphomannose isomerase-guanosine diphosphomannose pyrophosphorylase; RfbB, phosphomanno-mutase; RfbD, oxido reductase and RfbE, perosamine synthetase (amino-transferase). Thus, perosamine is synthesized from fructose 6-phosphate via the intermediates mannose 6-phosphate by RfbA, to mannose 1-phosphate by RfbB, to GDP-mannose by RfbA, to GDP-4-keto-6-dideoxymannose by RfbD and to GDP-perosamine by RfbE. This final product would then serve as the substrate for the addition of the tetronate, which could then be polymerized into the O-antigen for transfer to the lipid A plus core oligosaccharide and export to the cell surface. The organization of these genes are such that one would expect them to be translationally coupled as part of the rfb operon. However, the absence of readily detectable promoter sequences suggests low levels of transcription, in line with other studies. The nucleotide sequence of these genes is absolutely conserved in the two isolates 569B (classical, Inaba) and O17 (El Tor, Ogawa) which were expected to show maximal sequence variation. This suggests very tight constraints on the micro-evolution within these sequences.