Clonal expansion of hepatocytes with a selective advantage occurs during all stages of chronic hepatitis B virus infection

Hepatocyte clone size was measured in liver samples of 21 patients in various stages of chronic hepatitis B virus (HBV) infection and from 21 to 76 years of age. Hepatocyte clones containing unique virus-cell DNA junctions formed by the integration of HBV DNA were detected using inverse nested PCR. The maximum hepatocyte clone size tended to increase with age, although there was considerable patient-to-patient variation in each age group. There was an upward trend in maximum clone size with increasing fibrosis, inflammatory activity and with seroconversion from HBV e-antigen (HBeAg)-positive to HBeAg-negative, but these differences did not reach statistical significance. Maximum hepatocyte clone size did not differ between patients with and without a coexisting hepatocellular carcinoma. Thus, large hepatocyte clones containing integrated HBV DNA were detected during all stages of chronic HBV infection. Using laser microdissection, no significant difference in clone size was observed between foci of HBV surface antigen (HBsAg)-positive and HBsAg-negative hepatocytes, suggesting that expression of HBsAg is not a significant factor in clonal expansion. Laser microdissection also revealed that hepatocytes with normal-appearing histology make up a major fraction of the cells undergoing clonal expansion. Thus, preneoplasia does not appear to be a factor in the clonal expansion detected in our assays. Computer simulations suggest that the large hepatocyte clones are not produced by random hepatocyte turnover but have an as-yet-unknown selective advantage that drives increased clonal expansion in the HBV-infected liver.

Isolation and characterization of bacteriophage-resistant mutants of Vibrio cholerae O139

Vibrio cholerae O139 strains produce a capsule which is associated with complement resistance and is used as a receptor by bacteriophage JA1. Spontaneous JA1-resistant mutants were found to have several phenotypes, with loss of capsule and/or O-antigen from the cell surface. Determination of the residual complement resistance and infant mouse colonization potential of each mutant suggested that production of O-antigen is of much greater significance than the presence of capsular material for both of these properties. Two different in vitro assays of complement resistance were compared and the results of one shown to closely reflect the comparative recoveries of bacteria from the colonization experiments. Preliminary complementation studies implicated two rfb region genes, wzz and wbfP, as being essential for the biosynthesis of capsule but not O-antigen.

Surface signaling in ferric citrate transport gene induction: interaction of the FecA, FecR, and FecI regulatory proteins

In Escherichia coli, transcription of the ferric citrate transport genes fecABCDE is controlled by a novel signal transduction mechanism that starts at the cell surface. Binding of ferric citrate to the outer membrane protein FecA initiates a signal that is transmitted by FecR across the cytoplasmic membrane into the cytoplasm where FecI, the sigma factor, is activated. Interaction between the signaling proteins was demonstrated by utilizing two methods. In in vitro binding assays, FecR that was His tagged at the N terminus [(His)(10)-FecR] and bound to a Ni-nitrilotriacetic acid agarose column was able to retain FecA, and FecR that was His tagged at the C terminus [FecR-(His)(6)] retained FecI on the column. An N-terminally truncated, induction-negative but transport-active FecA protein did not bind to (His)(10)-FecR. The in vivo assay involved the determination of the FecA, FecR, and FecI interacting domains with the bacterial two-hybrid Lex-based system. FecA(1-79) interacts with FecR(101-317) and FecR(1-85) interacts with FecI(1-173). These data clearly support a model that proposes interaction of the periplasmic N terminus of FecA with the periplasmic C-terminal portion of FecR and interaction of the cytoplasmic N terminus of FecR with FecI, which results in FecI activation.

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.

Lipopolysaccharide O-antigen expression and the effect of its absence on virulence in rfb mutants of Vibrio cholerae O1

Using defined rfb mutants, defective in the biosynthesis of the O-antigen of the lipopolysaccharide (LPS), and monoclonal antibodies (MAbs) to the A, B and C LPS antigens, we have examined the distribution of the antigens and the effects of their loss. By immunogold electron microscopy, it has been possible to determine the relative amounts of the A, B and C antigens on Inaba and Ogawa cells, confirming previous studies based upon bacterial agglutination and hemagglutination inhibitions. These antigens are absent from rfb::Tn mutants selected as resistant to phages which have been shown to use the O-antigen as their receptor. These mutants were severely attenuated as measured by both LD50 and their ability to compete with the wild-type parents when analyzed in the infant mouse cholera model. These mutants were unchanged in the export of cholera toxin or other secreted proteins but revealed an altered outer membrane protein profile. The competition defect suggested an effect on TCP (toxin-coregulated pilus). An analysis of the rfb::Tn mutants revealed that they were unable to assemble TCP on their surface, but the major subunit, TcpA, could be found as an intracellular pool. These mutants could be complemented back to wild-type using the cloned rfb region, implying that functional TCP assembly is dependent upon an intact LPS.

Novel Vibrio cholerae O139 genes involved in lipopolysaccharide biosynthesis

The sequence of part of the rfb region of Vibrio cholerae serogroup O139 and the physical map of a 35-kb region of the O139 chromosome have been determined. The O139 rfb region presented contains a number of open reading frames which show similarities to other rfb and capsular biosynthesis genes found in members of the Enterobacteriaceae family and in V. cholerae O1. The cloned and sequenced region can complement the defects in O139 antigen biosynthesis in transposon insertions within the O139 rfb cluster. Linkage is demonstrated among IS1358 of V. cholerae O139, the rfb region, and the recently reported otnA and otnB genes (E. M. Bik, A. E. Bunschoten, R. D. Gouw, and F. R. Mooi, EMBO J. 14:209-216, 1995). In addition, the whole of this region has been linked to the rfaD gene. Furthermore, determination of the sequence flanking IS1358 has revealed homology to other rfb-like genes. The exact site of insertion with respect to rfaD is defined for the novel DNAs of both the Bengal and the Argentinian O139 isolates.

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

The nucleotide sequence of a region of the rfb genes, encoding biosynthesis of the Vibrio cholerae (Vc) O1 O-antigen, was determined. Analysis of the open reading frames (ORFs) within this region has revealed similarities with a number of different classes of biosynthetic proteins and enzymes. The ORFs have been designated RfbK, RfbL, RfbM, RfbN and RfbO. RfbK is a small, acidic protein which has similarity to the family of proteins known as acyl-carrier proteins (ACP). The RfbL protein has similarity to a super-family of enzymes which adenylate their substrates as a part of their reaction mechanism. Included in these are several acetyl-CoA ligases. Alignment of RfbL with these proteins reveals a highly conserved domain containing the motif GlyXaaXaaGlyXaaPro. This resembles the ATP-binding site motif and may represent a variant of the usual motif, except that Pro replaces Gly. The VcRfbM protein has similarity with a family of long-chain, iron-containing alcohol dehydrogenases, of which the Escherichia coli K-12 fucO and adhE gene products are also members. The RfbN protein has sequence homology with LuxE and LuxC of Vibrio harveyi (Vh) and other bioluminescent bacterial species. The latter are two components of the enzyme complex which synthesizes the long-chain aldehyde used in the V. harveyi bioluminescence system. Finally, the VcRfbO protein has sequence similarity with acetyl-CoA transferases. We were able to identify a number of the gene products using a T7 expression system, confirming several of the allocated ORFs. A biosynthetic pathway for the Vc O-antigen component 3-deoxy-L-glycero-tetronic acid, based on the enzymatic functions predicted for the RfbK, RfbL, RfbM, RfbN and RfbO proteins, is presented.

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.

Genetic rearrangements in the rfb regions of Vibrio cholerae O1 and O139

The recent emergence of a pathogenic new non-O1 serotype (O139) of Vibrio cholerae has led to numerous studies in an attempt to identify the origins of this new strain. Our studies indicate that O139 strains have clear differences in the surface polysaccharides when compared with O1 strains: the lipopolysaccharide can be described as semi-rough. Southern hybridization with the O1 rfb region demonstrates that O139 strains no longer contain any of the rfb genes required for the synthesis of the O1 O-antigen or its modification and also lack at least 6 kb of additional contiguous DNA. However, O139 strains have retained rfaD and have a single open reading frame closely related to three small open reading frames of the O1 rfb region. This region is closely related to the H-repeat of Escherichia coli and to the transposases of a number of insertion sequence elements and has all the features of an insertion sequence element that has been designated VcIS1. Transposon insertion mutants defective in O139 O-antigen (and capsule) biosynthesis map to the same fragment as VcIS1. Preliminary sequence data of complementing clones indicate that this DNA encodes a galactosyl-transferase and other enzymes for the utilization of galactose in polysaccharide biosynthesis. We propose a mechanism by which both the Ogawa serotype of O1 strains and the O139 serotype strains may have evolved.

Putative O-antigen transport genes within the rfb region of Vibrio cholerae O1 are homologous to those for capsule transport

The nucleotide sequence of that part of the Vibrio cholerae (Vc) O1 rfb region encompassing rfbG, rfbH and rfbI is presented. Expression of these genes has enabled the products for rfbG and rfbI to be confirmed, but the rfbH product has not been detected. Comparisons with the sequences of known proteins reveals that RfbH and RfbI are likely to be involved in the export of lipopolysaccharide (LPS). RfbH shows considerable homology to a number of integral membrane proteins, some of which have been identified as possibly having a role as an export channel for capsular polysaccharides. RfbI corresponds to an ATP-binding protein usually found linked to the membrane protein and is thought to be required for energizing this export process. Thus, we propose that RfbH and RfbI form a complex for the export of Vc O1 LPS. The function of RfbG is unknown, but it would appear to be a relatively hydrophilic protein and we can only speculate that it may be either a specific transferase or possibly the O-antigen polymerase.

In Vibrio cholerae serogroup O1, rfaD is closely linked to the rfb operon

The rfaD gene of Escherichia coli encodes ADP-L-glycero-D-mannoheptose-6- epimerase, an enzyme required for the biosynthesis of the lipopolysaccharide (LPS) precursor ADP-L-glycero-D- mannoheptose, associated with production of the core oligosaccharide. We have identified an rfaD homologue in Vibrio cholerae O1. This gene maps adjacent to the rfb region encoding O-antigen biosynthesis, but is transcribed divergently. The complete nucleotide sequence of rfaD and the flanking DNA has been determined, and rfaD would appear to be the only gene homologous to known LPS core biosynthesis genes in this region. Comparison with the E. coli rfaD shows many similar structural features such as the ADP-binding beta alpha beta fold at the N terminus, as well as a high degree of homology of both the nucleotide and amino-acid sequences. Based on homology, rfaD of V. cholerae may be transcribed using both sigma 70- and sigma 54-dependent promoters.

Gene sequence of recA+ and construction of recA mutants of Vibrio cholerae

The recA+ gene of Vibrio cholerae O1 has been cloned, its nucleotide sequence determined and the product characterized. A deletion mutation was constructed in the recA gene and mutants showed the typical sensitivity to UV and to DNA-damaging agents, as well as an inability to mediate homologous DNA recombination. The chromosomal recA deletion mutants in V. cholerae do not show altered virulence in the infant mouse cholera model and are thus ideal strains for use in complementation studies.

Characterization and sequence of a 33-kDa enterohemolysin (Ehly 1)-associated protein in Escherichia coli

The nucleotide sequence of the 2.1-kb EcoRI-AccI fragment of the enterohemolysin (Ehly)-associated plasmid, pEO21, has been determined. A third of this sequence encodes a 29.6-kDa protein, and coincides with the location of Tn1725 insertions which inactivate the production of Ehly. A protein of similar size (33-35 kDa) was found to be produced in large amounts by JM83[pEO21] and found to be immunologically related to the approximately 65-kDa protein made by the parental strain, C3888. DNA sequences coding for the 29.6-kDa protein of phi C3888, now designated Ehly 1, were found only in some enterohemolytic Escherichia coli indicating the existence of multiple, genetically different Ehlys.

Isolation of enterohemolysin (Ehly2)-associated sequences encoded on temperate phages of Escherichia coli

We have cloned and sequenced the enterohemolysin (ehl)-associated region from a temperate bacteriophage isolated from an Escherichia coli O26:H11 strain. Phage phi C3208 was isolated together with other temperate bacteriophages which transduce the enterohemolytic phenotype to non-hemolytic E. coli O26 strains. The nucleotide sequence of the 1245-bp phi C3208 DNA insert in plasmid pEO39, which mediates Ehly2 production in E. coli K-12, was determined and was found to be partially homologous to DNA of bacteriophage lambda but is completely unrelated to DNA sequences encoding the synthesis of Ehly1 [Stroeher et al. Gene 132 (1993), 89-94]. It was shown that part of this region can be used as an Hly2-associated specific DNA probe.

Serotype conversion in Vibrio cholerae O1

Vibrio cholerae O1 exists as two major serotypes, Inaba and Ogawa, which are associated with the O antigen of the lipopolysaccharide and are capable of unequal reciprocal interconversion. The 20-kilobase rfb regions encoding O-antigen biosynthesis in strains 569B (Inaba) and O17 (Ogawa) have been cloned in Escherichia coli K-12 and the nucleotide sequences have been determined. Besides several base substitutions and a small deletion in the 569B sequence relative to O17, there is a single nucleotide change resulting in a TGA stop codon within the gene for the 32-kDa RfbT protein. We have demonstrated that rfbT is responsible for serotype conversion (Inaba to Ogawa). The construction of a specific rfbT mutation in the Ogawa strain O17, and the ability of the gene from O17 to complement Inaba strains to Ogawa, confirmed rfbT as the gene required for the Ogawa serotype. By Southern hybridization and sequencing of PCR products of a number of strains, we have shown that the changes observed in one Inaba strain (569B) are not conserved in other Inaba strains. This may explain why some Inaba strains are able to convert to Ogawa whereas others are not. The protein encoded by rfbT has been identified and expressed in E. coli K-12 using a phage T7 expression system. Amino-terminal analysis of partially purified protein has identified the translational start of the protein. Primer extension studies have enabled the 5' end of the mRNA to be defined. It exists as a separate transcript from the rest of the rfb region, and the distinctive G + C content of rfbT suggests that it has been acquired from a non-Vibrio source.

Nucleotide sequence of the structural gene, tcpA, for a major pilin subunit of Vibrio cholerae

The toxin co-regulated pilus (Tcp) of Vibrio cholerae appears to be a major protective antigen. By cosmid cloning we have isolated a number of clones capable of converting Tcp- El Tor strains of V. cholerae to Tcp+. A synthetic oligodeoxyribonucleotide probe based upon the N-terminal amino acid sequence of TcpA, has been used to localize the structural gene within the cosmid clones. Using suitable subclones, the nucleotide sequence of the tcpA gene has been determined. The gene encodes a 23.3-kDa pre-protein which in its mature form has a size of 20.3 kDa. The N-terminal leader peptide or signal sequence is atypical and does not conform with the usual rules of such sequences. The TcpA protein shows some similarities to the major pilins of the methylated phenylalanine type or type-4 pili from other bacteria; however, it is sufficiently different that it may represent a new class.

The toxin-coregulated pilus (TCP) of Vibrio cholerae: molecular cloning of genes involved in pilus biosynthesis and evaluation of TCP as a protective antigen in the infant mouse model

A serum containing antibodies to non-lipopolysaccharide (non-LPS) protective antigens of Vibrio cholerae has been used, after extensive absorption, to facilitate the cloning of genes involved in the synthesis of toxin-coregulated pili (TCP). A gene bank was constructed from V. cholerae Z17561 DNA using a mobilizable cosmid vector in Escherichia coli, and subsequently transferred by conjugation into V. cholerae O17. This strain does not produce TCP in vitro and lacks non-LPS protective antigens. Eight positive clones were isolated, and of these, four produced TCP as determined by electron microscopic and immunoblotting analyses. TCP-positive O17 clones were 70-fold more virulent than TCP-negative clones or O17 in the infant mouse cholera model. Only the former could remove protective antibodies from the clone-probing serum by absorption. As a corollary, serum containing antibodies to TCP protected mice from challenge with TCP-positive clones, but not with TCP-negative clones or O17. Our data indicate that TCP can function as both a virulence determinant and a protective antigen in the infant mouse model.

Extracellular proteins of Vibrio cholerae: nucleotide sequence of the structural gene (hlyA) for the haemolysin of the haemolytic El Tor strain 017 and characterization of the hlyA mutation in the non-haemolytic classical strain 569B

The EI T or haemolysin, product of hlyA, is exported from Vibrio cholerae as a Mr 80,000 protein which can be subsequently cleaved to give two proteins of Mr 65,000 and 15,000. Nucleotide sequence analysis has demonstrated that hlyA encodes a protein of Mr 82,250 with a potential 18-amino-acid signal sequence. The non-haemolytic classical strain 569B has been shown to have a structural gene defect rather than a defect in secretion. By non-reciprocal recombination it was possible to transfer this defect onto a plasmid and show that a truncated hlyA product of Mr 27,000 is made in Escherichia coli K-12 minicells. Nucleotide sequence analysis demonstrates an 11-base-pair deletion which would result in a Mr 26,940 protein probably loosely associated with the membrane.