Biosynthesis of enterobacterial common antigen

Cultures of Salmonella typhimurium pulse-labeled with N-acetyl-D-[3H]glucosamine ([3H]GlcNAc) incorporated isotope into a GlcNAc-linked lipid that was tentatively identified as GlcNAc-pyrophosphorylundecaprenol. The incorporation of [3H]GlcNAc into this compound was abolished when cells were pulse-labeled in the presence of the antibiotic tunicamycin. Tunicamycin also abolished the in vivo synthesis of the haptenic form of enterobacterial common antigen (ECA) in S. typhimurium as determined by the passive hemagglutination test. These data indicated that the synthesis of the GlcNAc-linked lipid is related to ECA synthesis. Support for this conclusion was provided by the following observations. Cultures of Escherichia coli and S. typhimurium incorporated [3H]GlcNAc into cell envelope components that migrated as a homologous series of polymers when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The [3H]GlcNAc-labeled polymers were not detected in mutants of E. coli and S. typhimurium defective in ECA synthesis due to lesions in either the rfe or rff gene clusters. These polymers were identified as ECA based on Western blot analyses employing anti-ECA monoclonal antibody. The incorporation of [3H]GlcNAc into ECA polymers was abolished by tunicamycin when the drug was added to cultures to give a minimum concentration of 3 micrograms/ml. In addition, pulse-chase experiments provided evidence for a precursor-product relationship between the GlcNAc-linked lipid and ECA. These results strongly suggest that the GlcNAc-linked lipid is involved in the biosynthesis of ECA in a manner analogous to the role of carrier lipid in the biosynthesis of O-antigen and peptidoglycan.