Molecular cloning and characterization of the nontypeable Haemophilus influenzae 2019 rfaE gene required for lipopolysaccharide biosynthesis

htrB of Haemophilus influenzae: determination of biochemical activity and effects on virulence and lipooligosaccharide toxicity

The htrB mutant of Haemophilus influenzae (strain B29) has been shown to lack secondary (non-hydroxylated) acyl groups in its lipid A. We have determined through in vitro biochemical assays that the HtrB protein acts as a specific acyltransferase in the late stages of lipid A biosynthesis and that the preferred acyl group donor is myristoyl-acyl carrier protein. Under the conditions employed, the Escherichia coli precursor, Kdo2-lipid IVA, functions as a myristate acceptor. Introduction of the Haemophilus htrB gene into an E. coli mutant lacking htrB complements the biochemical and physiological defects associated with the E. coli htrB mutation.

Tumor necrosis factor α (TNFα) assays using murine and human macrophage cells indicated that nontypeable H. influenzae (NtHi) strain 2019 and H. influenzae type b strain A2 elicit levels of expression of TNFα that are 30-40 times greater than levels induced by the isogenic htrB mutants (B29 and A2B29). Studies using cell-free LOS indicated that the LOS from wild type strain 2019 elicits levels of TNFα expression that are 6-8-fold higher than those of B29. In situ hybridization studies of a primary human bronchial epithelial cell line demonstrated a greater increase of TNFα message produced in the presence of 2019 LOS than in the presence of B29 LOS. TNFα levels of the cell supernatant of cells stimulated with 2019 LOS were found to be 7-8-fold higher than levels in B29 stimulated supernatants. Using the Limulus amoebocyte lysate for assessment of endotoxic activity, we found that wild type LOS was 8-fold higher in endotoxic activity compared with the mutant LOS. In virulence assays using intraperitoneal inoculation of infant rats, the htrB isogenic strain caused bacteremia at 50% the frequency of the wild type strain. In intranasal inoculation studies, the htrB mutant strain was unable to cause bacteremia whereas the wild type b parent produced bacteremia in 40-60% of the animals. These findings suggest that the htrB gene of H. influenzae is important for virulence and that host TNFα expression is attenuated in response to htrB mutant strains.

Deletion of the rfaE gene in Haemophilus parasuis SC096 strain attenuates serum resistance, adhesion and invasion

In Haemophilus parasuis, the lipooligosaccharide (LOS) has been identified as an important virulence factor. The rfa gene cluster encodes enzymes for LOS core biosynthesis. In order to investigate the role of the rfaE gene, we generated an rfaE deficient mutant (ΔrfaE) of a H. parasuis SC096 by a natural transformation method. The purified preparation of LOS from the ΔrfaE mutant strain showed truncated LOS structure on silver-stained SDS-PAGE. Compared to the wild-type SC096 strain, the generation time of ΔrfaE mutant strain was significantly extended from 59 min to 69 min. The ΔrfaE mutant strain caused an approximately 30-fold reductions in survival rate in 50% sera and 36-fold reductions in survival rate in 90% sera, respectively (p < 0.001). In adhesion and invasion assays, the ΔrfaE mutant strain had 10-fold less efficient adherence and 12-fold reductions in invasion of the porcine umbilicus vein endothelial cells (PUVEC) and porcine kidney epithelial cells (PK-15), respectively (p < 0.001). However, the complemented strain could restore the above phenotypes. Hence, the above results suggested that the rfaE gene participated in the pathogenicity of H. parasuis SC096 strain.

Shielding of a lipooligosaccharide IgM epitope allows evasion of neutrophil-mediated killing of an invasive strain of nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae is a frequent cause of noninvasive mucosal inflammatory diseases but may also cause invasive diseases, such as sepsis and meningitis, especially in children and the elderly. Infection by nontypeable Haemophilus influenzae is characterized by recruitment of neutrophilic granulocytes. Despite the presence of a large number of neutrophils, infections with nontypeable Haemophilus influenzae are often not cleared effectively by the antimicrobial activity of these immune cells. Herein, we examined how nontypeable Haemophilus influenzae evades neutrophil-mediated killing. Transposon sequencing (Tn-seq) was used on an isolate resistant to neutrophil-mediated killing to identify genes required for its survival in the presence of human neutrophils and serum, which provided a source of complement and antibodies. Results show that nontypeable Haemophilus influenzae prevents complement-dependent neutrophil-mediated killing by expression of surface galactose-containing oligosaccharide structures. These outer-core structures block recognition of an inner-core lipooligosaccharide epitope containing glucose attached to heptose HepIII-β1,2-Glc by replacement with galactose attached to HepIII or through shielding HepIII-β1,2-Glc by phase-variable attachment of oligosaccharide chain extensions. When the HepIII-β1,2-Glc-containing epitope is expressed and exposed, nontypeable Haemophilus influenzae is opsonized by naturally acquired IgM generally present in human serum and subsequently phagocytosed and killed by human neutrophils. Clinical nontypeable Haemophilus influenzae isolates containing galactose attached to HepIII that are not recognized by this IgM are more often found to cause invasive infections.

Neutrophils are white blood cells that specialize in killing pathogens and are recruited to sites of inflammation. However, despite the presence of large numbers of neutrophils in the middle ear cavity and lungs of patients with otitis media or chronic obstructive pulmonary disease, respectively, the bacterium nontypeable Haemophilus influenzae is often not effectively cleared from these locations by these immune cells. In order to understand how nontypeable Haemophilus influenzae is able to cause inflammatory diseases in the presence of neutrophils, we determined the mechanism that underlies resistance to neutrophil-mediated killing. We have shown that nontypeable Haemophilus influenzae prevents binding of antibodies of the IgM subtype through changes in their surface lipooligosaccharide structure, thereby preventing complement activation and clearance by human neutrophils.

Effect of lipooligosaccharide mutations of Haemophilus influenzae on the middle and inner ears

OBJECTIVE

The purpose of this study was to determine the virulence of nontypeable Haemophilus influenzae 2019 (NTHi 2019) and its two lipooligosaccharide (LOS) mutant strains, B29 (gene htrB) and DK1 (gene rfaD), and compare their effect on the middle ear, round window membrane, and inner ear.

RESULTS

Fifteen chinchillas were divided into three equal groups and their bullas inoculated bilaterally with 0.5 ml of 10(2)CFU/ml of parent NTHi 2019, B29 or DK1 mutant strains. Two days after inoculation all animals had otitis media and inflamed middle ear mucosa. There was a trend of greater thickness and infiltration of the round window membrane in animals inoculated with the wild-type NTHi strain compared to the mutant strains and a significant increase in both inflammatory cell infiltration and bacteria presence in the scala tympani area of the inner ear. Strial edema was only observed in the wild-type-inoculated group.

CONCLUSIONS

LOS mutants of NTHi appear to have a reduced ability to pass through the round window membrane resulting in less inner ear inflammation and pathological changes.

Differential expression of cytokine genes and iNOS induced by nonviable nontypeable Haemophilus influenzae or its LOS mutants during acute otitis media in the rat

OBJECTIVE

We have previously demonstrated that the disruptions of nontypeable Haemophilus influenzae (NTHi) lipooligosaccharide (LOS) htrB and rfaD genes may play a role in the pathogenesis of otitis media (OM). The purpose of this study was to determine whether NTHi LOS gene disruptions influence the induction of gene expression for proinflammatory mediators in vivo using the rat model of acute OM.

METHODS

At 3, 6, 12, 24, 48 and 72 h after transbullar inoculation with nonviable NTHi, expression of genes for the cytokines and chemolines; tumor necrosis factor alpha (TNF-alpha), interleukin-lbeta (IL-1beta), and IL-6, IL-1alpha, IL-8, IL-10, and inducible nitric oxide synthase (iNOS) were quantitated by real-time PCR. Enzyme-linked immunosorbent assay was performed to confirm the gene expression data as determined by real-time PCR. The middle ear inflammatory responses were also evaluated.

RESULTS

The NTHi 2019 parent and its isogenic LOS htrB (B29) and rfaD (DK-1) mutant strains induced a significant up-regulation in gene expression for the cytokines examined compared to the sham-inoculated controls at 3, 6 and 12 h post-inoculation (P<0.05 in all cases). However, the NTHi 2019 cohort demonstrated a significant increase in gene expression for TNF-alpha (up to 6 h), IL-1alpha and IL-8 (up to 24 h), IL-1beta and IL-6 (up to 48 h), and IL-10 and iNOS (up to 72 h) relative to the animals inoculated with NTHi B29 (P<0.05, in all cases), Moreover, the concentrations of inflammatory cells in the middle ear lavage fluid samples from the NTHi 2019 cohort were 2.8-5.3-fold higher than those of the B29 cohort. There were no significant differences in mRNA expression of the cytokines between the NTHi 2019 and the DK-1-treated groups.

CONCLUSIONS

Data from this study indicate that the disruption of the NTHi htrB gene may impact the temporal mRNA expression of inflammatory mediators and inflammation within the middle ear.

Identification, cloning and characterization of rfaE of Actinobacillus pleuropneumoniae serotype 1, a gene involved in lipopolysaccharide inner-core biosynthesis

Actinobacillus pleuropneumoniae is the causative agent of porcine pleuropneumonia and its lipopolysaccharides (LPS) have been identified as important adhesins involved in adherence to host cells. To better understand the role of LPS core in the virulence of this organism, the aim of the present study was to identify and clone genes involved in LPS core biosynthesis by complementation with Salmonella enterica serovar Typhimurium mutants (rfaC, rfaD, rfaE and rfaF). Complementation with an A. pleuropneumoniae 4074 genomic library was successful with Salmonella mutant SL1102. This Salmonella deep-rough LPS mutant is defective for the rfaE gene, which is an ADP-heptose synthase. Novobiocin was used to select transformants that had the smooth-LPS type, since Salmonella strains with wild-type smooth-LPS are less permeable, therefore more resistant to hydrophobic antibiotics like novobiocin. We obtained a clone that was able to restore the wild-type smooth-LPS Salmonella phenotype after complementation. The wild-type phenotype was confirmed using phage (Felix-O, P22c.2 and Ffm) susceptibility and SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis). One of the open reading frames contained in the 3.3-kb insert in the plasmid encoded a 475-amino-acid protein with 71% identity and 85% similarity to the RfaE protein of S. enterica. We then attempted to generate an A. pleuropneumoniae rfaE mutant by gene replacement. The rfaE gene seems essential in A. pleuropneumoniae viability as we were unable to isolate a heptose-less knockout mutant.

Molecular cloning and functional expression of the rfaE gene required for lipopolysaccharide biosynthesis in Salmonella typhimurium

The rfaE (WaaE) gene of Salmonella typhimurium is known to be located at 76min on the genetic map outside of the rfa gene cluster encoding core oligosaccharide biosynthesis of lipopolysaccharide(LPS). The rfaE mutant synthesizes heptose-deficient LPS; its LPS consists of only lipid A and 3-deoxy-D-manno-octulosonic acid (KDO), and the rfaE gene is believed to be involved in the formation of ADP-L-glycero-D-manno-heptose. Mutants, which make incomplete LPS, are known as rough mutants. Salmonella typhimurium deep-rough mutants affected in the heptose region of the inner core often show reduced growth rate, sensitivity to high temperature and hypersensitivity to hydrophobic antibiotics. We have cloned the rfaE gene of S. typhimurium. The chromosomal region carrying this gene was isolated by screening a genomic library of S. typhimurium using the complementation of S. typhimurium rfaE mutant. The 2.6-Kb insert in the plasmid pHEPs appears to carry a functional rfaE gene. SL1102 (rfaE543) makes heptose-deficient LPS and has a deep rough phenotype, but pHEPs complement the rfaE543 mutation to give the smooth phenotype. The sensitivity of SL1102 to bacteriophages (P22.c2, Felix-O, Br60) which use LPS as their receptor for adsorption is changed to that of wild-type strain. The permeability barrier of SL1102 to hydrophobic antibiotics (novobiocin) is restored to that of wild-type. LPS produced by SL1102 (rfaE543) carrying pHEPs makes LPS indistinguishable from that of smooth strains. The rfaE gene encoded a polypeptide of 477 amino acid residues highly homologous to the S. enterica rfaE protein (98% identity), E. coli (93% identity), Yersenia pestis (85% identity), Haemophilus influenzae (70% identity) and Helicobacter pyroli (41% identity) with a molecular weight 53 kDa.

gmhX, a novel gene required for the incorporation of L-glycero-D-manno-heptose into lipooligosaccharide in Neisseria meningitidis

Lipooligosaccharide (LOS) is a critical virulence factor of Neisseria meningitidis. A Tn916 insertion mutant, designated 469, was found to exhibit a markedly truncated LOS of 2.9 kDa when compared by Tricine/SDS-PAGE to the parental LOS (4.6 kDa). Electrospray mass spectrometry analysis of 469 LOS revealed that it consisted of the deep rough, heptose-deficient structure, Kdo(2)-lipid A. Sequencing of chromosomal DNA flanking the Tn916 insertion in mutant 469 revealed that the transposon had inserted into an ORF predicted to encode a 187 aa protein with sequence homology to the histidinol-phosphate phosphatase domain of Escherichia coli HisB and to a family of genes of unknown function. The gene, designated gmhX, is part of a polycistronic operon (ice-2) containing two other genes, nlaB and orfC. nlaB encodes a lysophosphatidic-acid acyltransferase and orfC is predicted to encode a N-acetyltransferase. Specific polar and non-polar gmhX mutations in the parental strain, NMB, exhibited the truncated LOS structure of mutant 469, and repair of gmhX mutants by homologous recombination with the wild-type gmhX restored the LOS parental phenotype. GmhX mutants demonstrated increased sensitivity to polymyxin B. GmhX mutants and other Kdo(2)-lipid A mutants also demonstrated increased sensitivity to killing by normal human serum but were not as sensitive as inner-core mutants containing heptose. In the genomes of Helicobacter pylori and Synechocystis, gmhX homologues are associated with heptose biosynthesis genes; however, in N. meningitidis, gmhX was found in a location distinct from that of gmhA, rfaD, rfaE, aut and rfaC. GmhX is a novel enzyme required for the incorporation of L-glycero-D-manno-heptose into meningococcal LOS, and is a candidate for the 2-D-glycero-manno-heptose phosphatase of the heptose biosynthesis pathway.

Expression of cytokine and chemokine genes by human middle ear epithelial cells induced by formalin-killed Haemophilus influenzae or its lipooligosaccharide htrB and rfaD mutants

To define the role of nontypeable Haemophilus influenzae (NTHI) lipooligosaccharide (LOS) in the induction of proinflammatory cytokine gene expression during otitis media, we compared the abilities of formalin-killed NTHI strain 2019 and its LOS htrB and rfaD mutants to stimulate human middle ear epithelial (HMEE) cell cytokine and chemokine gene expression and production in vitro. Strain DK-1, an rfaD gene mutant, expresses a truncated LOS consisting of only three deoxy-D-manno-octulosonic acid residues, a single heptose, and lipid A. Strain B29, an isogenic htrB mutant, possesses an altered oligosaccharide core and an altered lipid A. HMEE cells were incubated with formalin-killed NTHI 2019, B29, or DK-1. The supernatants and the cells were collected at 2, 4, 8, and 24 h after stimulation. Expression of genes for the cytokines tumor necrosis factor alpha (TNF-alpha), interleukin lbeta (IL-1beta), and IL-6 and for the chemokines macrophage inflammatory protein 1beta (MIP-1beta), monocyte chemotactic peptide 1 (MCP-1), and IL-8 was quantitated by real-time PCR. NTHI B29 did not significantly stimulate any cytokine or chemokine mRNA expression in HMEE cells. In striking contrast, NTHI 2019 induced up to 105-, 139-, and 187-fold increases in HMEE cell expression of IL-1beta, TNF-alpha, and MIP-1beta, respectively (P < 0.01 [2019 versus B29]). NTHI 2019 also induced upregulation of IL-8, IL-6, and MCP-1 mRNA expression (by 26-, 44-, and 14-fold, respectively [P < 0.05 (2019 versus B29)]). The significant induction of cytokine genes was confirmed by quantitating the secretion of cytokines in culture supernatants with an enzyme-linked immunosorbent assay. There were no significant differences in mRNA expression of IL-8, IL-6, and MCP-1 between the 2019- and DK-1-treated groups. The low levels of gene transcripts observed after incubation of HMEE cells with B29 indicate that products of the disrupted NTHI htrB LOS gene may play a major role in induction of these particular inflammatory mediators.

Evaluation of phase variation of nontypeable Haemophilus influenzae lipooligosaccharide during nasopharyngeal colonization and development of otitis media in the chinchilla model

Nontypeable Haemophilus influenzae (NTHI) has four loci, lic-1 to lic-3 and lgtC, that generate phase-variable lipooligosaccharide (LOS) structures. lic-1, which is required for the expression of phosphorylcholine (ChoP), is the best characterized and is associated with an enhanced ability of H. influenzae to persist within the nasopharynges of infant rats. Recent data indicate that LOS impacts various aspects of NTHI virulence in the chinchilla model of nasopharyngeal colonization and otitis media (OM). In this study the effects of ChoP expression and the sequences of lic-1 to lic-3 and lgtC of NTHI strain 2019 were evaluated in the chinchilla OM model. Nasopharyngeal colonization data showed that a switch from the ChoP(-) to the ChoP(+) phenotype was observed as early as day 3 after intranasal inoculation. Chinchillas colonized by strains with the ChoP(+) phenotype demonstrated a significantly higher level of NTHI 2019 per milliliter of nasal lavage fluid than chinchillas colonized with predominantly the ChoP(-) variant (P < 0.05). The concentration of cells with the ChoP(+) phenotype in the middle ear was 3 log units higher than that of cells with the ChoP(-) variant (P < 0.01). There was a statistically significant association between ChoP(+) expression in the nasal lavage and the development of OM with culture-positive middle ear fluids in this model. These data suggest that expression of the ChoP(+) phenotype promotes enhanced nasopharyngeal colonization and development of OM.

The rfaE gene from Escherichia coli encodes a bifunctional protein involved in biosynthesis of the lipopolysaccharide core precursor ADP-L-glycero-D-manno-heptose

The intermediate steps in the biosynthesis of the ADP-L-glycero-D-manno-heptose precursor of inner core lipopolysaccharide (LPS) are not yet elucidated. We isolated a mini-Tn10 insertion that confers a heptoseless LPS phenotype in the chromosome of Escherichia coli K-12. The mutation was in a gene homologous to the previously reported rfaE gene from Haemophilus influenzae. The E. coli rfaE gene was cloned into an expression vector, and an in vitro transcription-translation experiment revealed a polypeptide of approximately 55 kDa in mass. Comparisons of the predicted amino acid sequence with other proteins in the database showed the presence of two clearly separate domains. Domain I (amino acids 1 to 318) shared structural features with members of the ribokinase family, while Domain II (amino acids 344 to 477) had conserved features of the cytidylyltransferase superfamily that includes the aut gene product of Ralstonia eutrophus. Each domain was expressed individually, demonstrating that only Domain I could complement the rfaE::Tn10 mutation in E. coli, as well as the rfaE543 mutation of Salmonella enterica SL1102. DNA sequencing of the rfaE543 gene revealed that Domain I had one amino acid substitution and a 12-bp in-frame deletion resulting in the loss of four amino acids, while Domain II remained intact. We also demonstrated that the aut::Tn5 mutation in R. eutrophus is associated with heptoseless LPS, and this phenotype was restored following the introduction of a plasmid expressing the E. coli Domain II. Thus, both domains of rfaE are functionally different and genetically separable confirming that the encoded protein is bifunctional. We propose that Domain I is involved in the synthesis of D-glycero-D-manno-heptose 1-phosphate, whereas Domain II catalyzes the ADP transfer to form ADP-D-glycero-D-manno-heptose.

Evaluation of the virulence of nontypeable Haemophilus influenzae lipooligosaccharide htrB and rfaD mutants in the chinchilla model of otitis media

Considerable evidence has implicated nontypeable Haemophilus influenzae (NTHi) lipooligosaccharide (LOS) in the pathogenesis of otitis media (OM); however, its exact role has not been conclusively established. Recently, two NTHi LOS-deficient mutants have been created and described. Strain 2019-DK1, an rfaD gene mutant, expresses a truncated LOS consisting of only three deoxy-D-manno-octulosonic acid residues, a single heptose, and lipid A. Strain 2019-B29, an isogenic htrB mutant, possesses an altered oligosaccharide core and an altered lipid A. Each strain's ability to colonize the nasopharynx and to induce OM subsequent to transbullar inoculation was evaluated in the chinchilla model. Nasopharyngeal colonization data indicate that the parent strain and both mutants are able to colonize the nasopharynx and exhibit comparable clearance kinetics. Compared with the parent and each other, however, the mutants demonstrated marked differences in virulence regarding their relative abilities to induce OM and persist in the middle ear post-transbullar inoculation. Strain B29 required a 3-log-greater dose to induce OM than the parent strain and did not exhibit evidence of sustained multiplication but persisted for the same duration as the parent. Conversely, strain-DK1, even when inoculated at a dose 4 logs greater than the parent dose, was eliminated from the middle ear 72 h after challenge. A comparison of the relative pathogenicities of these isolates provides the opportunity to address fundamental questions regarding the contribution of LOS to pathogenesis issues at the molecular level. Specifically, the impact of these LOS gene disruptions on OM pathogenesis can be defined and may thus provide potential new targets for future protection and intervention strategies.

Isolation and characterization of two genes, waaC (rfaC) and waaF (rfaF), involved in Pseudomonas aeruginosa serotype O5 inner-core biosynthesis

Recent studies have provided evidence to implicate involvement of the core oligosaccharide region of Pseudomonas aeruginosa lipopolysaccharide (LPS) in adherence to host tissues. To better understand the role played by LPS in the virulence of this organism, the aim of the present study was to clone and characterize genes involved in core biosynthesis. The inner-core regions of P. aeruginosa and Salmonella enterica serovar Typhimurium are structurally very similar; both contain two main chain residues of heptose linked to lipid A-Kdo2 (Kdo is 3-deoxy-D-manno-octulosonic acid). By electrotransforming a P. aeruginosa PAO1 library into Salmonella waaC and waaF (formerly known as rfaC and rfaF, respectively) mutants, we were able to isolate the homologous heptosyltransferase I and II genes of P. aeruginosa. Two plasmids, pCOREc1 and pCOREc2, which restored smooth LPS production in the waaC mutant, were isolated. Similarly, plasmid pCOREf1 was able to complement the Salmonella waaF mutant. Sequence analysis of the DNA insert of pCOREc2 revealed one open reading frame (ORF) which could code for a protein of 39.8 kDa. The amino acid sequence of the deduced protein exhibited 53% identity with the sequence of the WaaC protein of S. enterica serovar Typhimurium. pCOREf1 contained one ORF capable of encoding a 38.4-kDa protein. The sequence of the predicted protein was 49% identical to the sequence of the Salmonella WaaF protein. Protein expression by the Maxicell system confirmed that a 40-kDa protein was encoded by pCOREc2 and a 38-kDa protein was encoded by pCOREf1. Pulsed-field gel electrophoresis was used to determine the map locations of the cloned waaC and waaF genes, which were found to lie between 0.9 and 6.6 min on the PAO1 chromosome. Using a gene-replacement strategy, we attempted to generate P. aeruginosa waaC and waaF null mutants. Despite multiple attempts to isolate true knockout mutants, all transconjugants were identified as merodiploids.

Identification of the ADP-L-glycero-D-manno-heptose-6-epimerase (rfaD) and heptosyltransferase II (rfaF) biosynthesis genes from nontypeable Haemophilus influenzae 2019

Haemophilus influenzae is an important human pathogen. The lipooligosaccharide (LOS) of H. influenzae has been implicated as a virulence determinant. To better understand the assembly of LOS in nontypeable H. influenzae (NtHi), we have cloned and characterized the rfaD and rfaF genes of NtHi 2019, which encode the ADP-L-glycero-D-manno-heptose-6-epimerase and heptosyltransferase II enzymes, respectively. This cloning was accomplished by the complementation of Salmonella typhimurium lipopolysaccharide (LPS) biosynthesis gene mutants. These deep rough mutants are novobiocin susceptible until complemented with the appropriate gene. In this manner, we are able to use novobiocin resistance to select for specific NtHi LOS inner core biosynthesis genes. Such a screening system yielded a plasmid with a 4.8-kb insert. This plasmid was able to complement both rfaD and rfaF mutants of S. typhimurium. The LPS of these complemented strains appeared identical to the wild-type Salmonella LPS. The genes encoding the rfaD and rfaF genes from NtHi 2019 were sequenced and found to be similar to the analogous genes from S. typhimurium and Escherichia coli. The rfaD gene encodes a polypeptide of 35 kDa and the rfaF encodes a protein of 39 kDa, as demonstrated by in vitro transcription-translation studies. Isogenic mutants which demonstrated truncated LOS consistent with inner core biosynthesis mutants were constructed in the NtHi strain 2019. Primer extension analysis demonstrated the presence of a strong promoter upstream of rfaD but suggested only a very weak promoter upstream of rfaF. Complementation studies, however, suggest that the rfaF gene does have an independent promoter. Mass spectrometric analysis shows that the LOS molecules expressed by H. influenzae rfaD and rfaF mutant strains have identical molecular masses. Additional studies verified that in the rfaD mutant strain, D-glycero-D-manno-heptose is added to the LOS molecule in place of the usual L-glycero-D-manno-heptose. Finally, the genetic organizations of the inner core biosynthesis genes of S. typhimurium, E. coli, and several strains of H. influenzae were examined, and substantial differences were uncovered.

Molecular cloning of the Haemophilus influenzae gmhA (lpcA) gene encoding a phosphoheptose isomerase required for lipooligosaccharide biosynthesis

We have determined that gene HI#1181 of Haemophilus influenzae is a homolog of Escherichia coli gmhA (previously designated lpcA) (J. S. Brooke and M. A. Valvano, J. Biol. Chem. 271:3608-3614, 1996), which encodes a phosphoheptose isomerase catalyzing the first step of the biosynthesis of ADP-L-glycero-D-manno heptose. Mutations in this gene are associated with a heptoseless core lipopolysaccharide which determines an increased outer membrane permeability to hydrophobic compounds. The cloned H. influenzae gmhA restored the synthesis of a complete core in the gmhA-deleted E. coli strain chi711. Amino acid sequence comparisons of the GmhA proteins of E. coli and H. influenzae with other proteins in the databases revealed the existence of a novel family of phosphosugar a1do-keto isomerases.

The lipooligosaccharides of pathogenic gram-negative bacteria

Lipooligosaccharides (LOSs) are the major glycolipids expressed on mucosal Gram-negative bacteria, including members of the genera Neisseria, Haemophilus, Bordetella, and Branhamella. They can also be expressed on some enteric bacteria such as Campylobacter jejuni and Campylobacter coli strains. LOS is analogous to the lipopolysaccharide (LPS) found in other Gram-negative families. LOSs share similar lipid A structures with an identical array of functional activities as LPSs. LOSs lack O-antigen units with the LOS oligosaccharide structures limited to 10 saccharide units. The LOS species of pathogenic Neisseria can play a major role in pathogenesis through enhancing the resistance of the organism to killing by normal human serum. Other distinguishing characteristics of LOS are the structural and antigenic similarity of some LOS species to human glycolipids and the potential for certain LOSs to be modified in vivo by host substances or secretions. These modifications of LOS in different environments of the host result in synthesis of new LOS structures that probably benefit the survival of the pathogen. The LOS of N. gonorrhoeae can act as a ligand of human receptors, promoting invasion of host cells. It is becoming clearer that LOSs are crucial factors in the pathogenesis of bacteria that express them.

Altered lipopolysaccharide characteristic of the I69 phenotype in Haemophilus influenzae results from mutations in a novel gene, isn

The 169 phenotype of Haemophilus influenzae results from a mutation leading to a lipopolysaccharide molecule consisting only of lipid A and a single phosphorylated 2-keto-3-deoxyoctulosonic acid residue. In this paper we describe the identification of a gene which, when mutated, results in the 169 phenotype. We have named the gene isn. The predicted amino acid sequence of Isn is homologous to the product of the lmbN gene involved in the biosynthesis of the sugar-containing antibiotic lincomycin by Streptomyces lincolnensis. lsn is situated between two loci that are homologous to the dpp and art periplasmic permease systems in Escherichia coli. Northern (RNA) blot and primer extension analyses reveal that isn is transcribed as a monocistronic mRNA. Potential functions of Isn protein are discussed.