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Genetic and functional analyses of the lgtH gene, a member of the -1,4-galactosyltransferase gene family in the genus Neisseria

Peixuan Zhu

Division of Bacterial, Parasitic and Allergenic Products, Center for Biologics Evaluation and Research, FDA, 8800 Rockville Pike, Bethesda, MD 20892, USA

Correspondence
Peixuan Zhu
pzhu{at}creatvmicrotech.com

Lipooligosaccharide (LOS) is a major virulence factor of the pathogenic Neisseria. Three galactosyltransferase genes, lgtB, lgtE and lgtH, responsible for the biosynthesis of LOS oligosaccharide chains, were analysed in five Neisseria species. The function of lgtH in Neisseria meningitidis 6275 was determined by mutagenesis and chemical characterization of the parent and mutant LOS chains. The chemical characterization included SDS-PAGE, immunoblot, hexose and mass spectrometry analyses. Compared with the parent LOS, the mutant LOS lacked galactose, and its oligosaccharide decreased by three or four sugar units in matrix-assisted laser desorption ionization (MALDI)-MS analysis. The results show that lgtH encodes a -1,4-galactosyltransferase, and that the glucose moiety linked to heptose (Hep) in the chain is the acceptor site in the biosynthesis of Neisseria LOS. To understand the sequence diversity and relationships of lgtB, lgtE and lgtH, the entire lgt-1 locus was further sequenced in three N. meningitidis strains and three commensal Neisseria strains, and compared with the previously reported lgt genes from Neisseria species. Comparison of the protein sequences of the three enzymes LgtB, LgtE and LgtH showed a conserved N-terminal region, and a highly variable C-terminal region, suggesting functional constraint for substrate and acceptor specificity, respectively. The analyses of allelic variation and evolution of 23 lgtB, 12 lgtE and 14 lgtH sequences revealed a distinct evolutionary history of these genes in Neisseria. For example, the splits graph of lgtE displayed a network evolution, indicating frequent DNA recombination, whereas splits graphs of lgtB and lgtH displayed star-tree-like evolution, indicating the accumulation of point mutations. The data presented here represent examples of the evolution and variation of prokaryotic glycosyltransferase gene families. These imply the existence of multiple enzyme isoforms for biosynthesis of a great diversity of oligosaccharides in nature.

The GenBank/EMBL/DDBJ accession numbers for the nucleotide sequences of the lgt genes reported in this paper are: strains 2001-083 and 2001-117, accession nos AY869717 and AY869718; strains 81186, 85071 and 87043, accession nos AY134876–AY134878.

Protein polymorphisms and physical properties for LgtB, LgtE and LgtH are shown in Supplementary Table S1, confirmation of the lgtH isogenic mutant in Supplementary Fig. S1, hexose composition analysis of oligosaccharide preparations from wild-type strain 6275 and isogenic mutant 6275lgtH in Supplementary Fig. S2, and detailed polymorphisms of 47 sequences of LgtB, LgtE and LgtH in Supplementary Fig. S3, available with the online version of this paper.

Present address: Creatv MicroTech, Inc., 11609 Lake Potomac Drive, Potomac, MD 20854, USA.