Haemophilus ducreyi produces a novel sialyltransferase. Identification of the sialyltransferase gene and construction of mutants deficient in the production of the sialic acid-containing glycoform of the lipooligosaccharide.

Haemophilus ducreyi, the cause of the sexually transmitted disease chancroid produces a lipooligosaccharide (LOS) containing a terminal sialyl N-acetyllactosamine trisaccharide. Previously, we reported the identification and characterization of the N-acetylneuraminic acid cytidylsynthetase gene (neuA). Forty-nine base pairs downstream of the synthetase gene is an open reading frame (ORF) encoding a protein with a predicted molecular weight of 34,646. This protein has weak homology to the polysialyltransferase of Escherichia coli K92. Downstream of this ORF is the gene encoding the H. ducreyi homologue of the Salmonella typhimurium rmlB gene. Mutations were constructed in the neuA gene and the gene encoding the second ORF by insertion of an Omega kanamycin cassette, and isogenic strains were constructed. LOS was isolated from each strain and characterized by SDS-polyacrylamide gel electrophoresis, carbohydrate, and mass spectrometric analysis. LOS isolated from strains containing a mutation in neuA or in the second ORF, designated lst, lacked the sialic acid-containing glycoform. Complementation studies were performed. The neuA gene and the lst gene were each cloned into the shuttle vector pLS88 after polymerase chain reaction amplification. Complementation of the mutation in the lst gene was observed, but we were unable to complement the neuA mutation. Since it is possible that transcription of the neuA gene and the lst gene were coupled, we constructed a nonpolar mutation in the neuA gene. In this construct, the neuA mutation was complemented, suggesting transcriptional coupling of the neuA gene and the lst gene. Sialyltransferase activity was detected by incorporation of 14C-labeled NeuAc from CMP-NeuAc into trichloroacetic acid-precipitable material when the lst gene was overexpressed in the nonpolar neuA mutant. We conclude that the lst gene encodes the H. ducreyi sialyltransferase. Since the lst gene product has little, if any, structural relationship to other sialyltransferases, this protein represents a new class of sialyltransferase.