Utilizing the O-antigen lipopolysaccharide biosynthesis pathway in Escherichia coli to interrogate the substrate specificities of exogenous glycosyltransferase genes in a combinatorial approach

In previous work, our laboratory generated novel chimeric lipopolysaccharides (LPS) in Escherichia coli transformed with a plasmid containing exogenous lipooligosaccharide synthesis genes (lsg) from Haemophilus influenzae. Analysis of these novel oligosaccharide-LPS chimeras allowed characterization of the carbohydrate structures generated by several putative glycosyltransferase genes within the lsg locus. Here, we adapted this strategy to construct a modular approach to study the synthetic properties of individual glycosyltransferases expressed alone and in combinations. To this end, a set of expression vectors containing one to four putative glycosyltransferase genes from the lsg locus, lsgC-F, were transformed into E. coli K12 (XL-1) which is defective in LPS O-antigen biosynthesis. This strategy relied on the inclusion of the H. influenzae gene product lsgG in every plasmid construct, which partially rescues the E. coli LPS biosynthesis defect by priming uridine diphosphate-undecaprenyl in the WecA-dependent O-antigen synthetic pathway with N-acetyl-glucosamine (GlcNAc). This GlcNAc-undecaprenyl then served as an acceptor substrate for further carbohydrate extension by transformed glycosyltransferases. The resultant LPS-linked chimeric glycans were isolated from their E. coli constructs and characterized by mass spectrometry, methylation analysis and enzyme-linked immunosorbent assays. These structural data allowed the specificity of various glycosyltransferases to be unambiguously assigned to individual genes. LsgF was found to transfer a galactose (Gal) to terminal GlcNAc. LsgE was found to transfer GlcNAc to Gal-GlcNAc, and both LsgF and LsgD were found to transfer Gal to GlcNAc-Gal-GlcNAc but with differing linkage specificities. This method can be generalized and readily adapted to study the substrate specificity of other putative or uncharacterized glycosyltransferases.

Characterization of chimeric lipopolysaccharides from Escherichia coli strain JM109 transformed with lipooligosaccharide synthesis genes (lsg) from Haemophilus influenzae

Previously, we reported the expression of chimeric lipopolysaccharides (LPS) in Escherichia coli strain JM109 (a K-12 strain) transformed with plasmids containing Haemophilus influenzae lipooligosaccharide synthesis genes (lsg) (Abu Kwaik, Y., McLaughlin, R. E., Apicella, M. A., and Spinola, S. M. (1991) Mol. Microbiol. 5, 2475-2480). In this current study, we have analyzed the O-deacylated LPS and free oligosaccharides from three transformants (designated pGEMLOS-4, pGEMLOS-5, and pGEMLOS-7) by matrix-assisted laser desorption ionization, electrospray ionization, and tandem mass spectrometry techniques, along with composition and linkage analyses. These data show that the chimeric LPS consist of the complete E. coli LPS core structure glycosylated on the 7-position of the non-reducing terminal branch heptose with oligosaccharides from H. influenzae. In pGEMLOS-7, the disaccharide Gal1--> 3GlcNAc1--> is added, and in pGEMLOS-5, the structure is extended to Gal1-->4GlcNAc1-->3Gal1-->3GlcNAc1-->. PGEMLOS-5 LPS reacts positively with monoclonal antibody 3F11, an antibody that recognizes the terminal disaccharide of lacto-N-neotetraose. In pGEMLOS-4 LPS, the 3F11 epitope is apparently blocked by glycosylation on the 6-position of the terminal Gal with either Gal or GlcNAc. The biosynthesis of these chimeric LPS was found to be dependent on a functional wecA (formerly rfe) gene in E. coli. By using this carbohydrate expression system, we have been able to examine the functions of the lsg genes independent of the effects of other endogenous Haemophilus genes and expressed proteins.

Characterization of two transposon mutants from Haemophilus influenzae type b with altered lipooligosaccharide biosynthesis

Two isogenic mutants of Haemophilus influenzae type b (Hib) strain A2 were prepared by random m-Tn3(Cm) insertions into the 7.4-kb lsg (lipooligosaccharide synthesis genes) region of Hib DNA, which consists of seven complete and one partial open reading frame (orfs). Compared to the parent A2 strain which produces a complex mixture of lipooligosaccharides (LOS), the mutant strains 281.25 and 276.4 produced only a few LOS species. The precise locations of transposon insertions into the lsg loci of these mutants were determined (base 3546 in orf 4 for strain 281.25 and base 4402 in orf 5 for strain 276.4), and the effects of these mutations on LOS biosynthesis and epitope expression were evaluated. When the O-deacylated LOS were analyzed by mass spectrometry, both strains contained major LOS species of M(r) 2601, 2439, and 2277, which consisted of a common heptose trisaccharide core structure [Hep3(PEA)Kdo(P)-lipid A, where Hep is L-glycero-D-manno-heptose, Kdo is 3-deoxy-D-manno-octulosonic acid, and PEA is phosphoethanolamine] and four, three, or two hexoses, respectively. These species represent the smallest components of the wild-type LOS mixture. The major LOS oligosaccharide obtained from the strain 281.25 by mild acid hydrolysis was dephosphorylated and shown by composition analysis, methylation analysis, mass spectrometry, and 2D NMR studies to be a triantennary structure consisting of a heptose trisaccharide core with two glucose disaccharide branches: Hep alpha 1 --> (Glc beta 1 --> 4Glc alpha 1 --> 3) 2Hep alpha 1 --> (Glc beta 1 --> 4Glc beta 1 --> 4)3Hep alpha 1 --> anhydroKdo. Unlike the parent A2 strain, mutant strain 281.25 cannot add galactoses to the branches of this octasaccharide. Strain 276.4 is similarly deficient, except that it can still utilize a minor biosynthetic pathway leading to the addition of sialyl-N-acetyllactosamine.

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

The lipooligosaccharide (LOS) of nontypeable Haemophilus influenzae (NTHi) is an important factor in pathogenesis and virulence. In an attempt to elucidate the genes involved in LOS biosynthesis, we have cloned the rfaE gene from NTHi 2019 by complementing a Salmonella typhimurium rfaE mutant strain with an NTHi 2019 plasmid library. The rfaE mutant synthesizes lipopolysaccharide (LPS) lacking heptose, and the rfaE gene is postulated to be involved in ADP-heptose synthesis. Retransformation with the plasmid containing 4 kb of NTHi DNA isolated from a reconstituted mutant into rfaE mutants gave wild-type LPS phenotypes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis confirmed the conversion of the rfaE mutant LPS to a wild-type LPS phenotype. Sequence analysis of a 2.4-kb BglII fragment revealed two open reading frames. One open reading frame encodes the RfaE protein with a molecular weight of 37.6 kDa, which was confirmed by in vitro transcription and translation, and the other encodes a polypeptide highly homologous to the Escherichia coli HtrB protein. These two genes are transcribed from the same promoter region into opposite directions. Primer extension analysis of the rfaE gene revealed a single transcription start site at 37 bp upstream of the predicted translation start site. The upstream promoter region contained a sequence (TA AAAT) homologous to the -10 region of the bacterial sigma 70-dependent promoters at an appropriate distance (7 bp), but not sequence resembling the consensus sequence of the -35 region was found. These studies demonstrate the ability to use complementation of defined LPS defects in members of the family Enterobacteriaceae to identify LOS synthesis genes in NTHi.