Functions required for vitamin B12-dependent ethanolamine utilization in Salmonella typhimurium

When B12 is available, Salmonella typhimurium can degrade ethanolamine to provide a source of carbon and nitrogen. B12 is essential since it is a cofactor for ethanolamine ammonia-lyase, the first enzyme in ethanolamine breakdown. S. typhimurium makes B12 only under anaerobic conditions; in the presence of oxygen, exogenous B12 must be provided to permit ethanolamine utilization. Genes required for ethanolamine utilization are encoded in the eut operon. For complementation testing, an F' plasmid containing the eut genes was constructed by transposition of the eut operon (flanked by two Tn10 elements) to an existing F plasmid. Complementation tests defined six genes in the eut operon. Three of these genes encode enzymes known to be involved in degradation of ethanolamine: ethanolamine ammonia-lyase (eutB and eutC) and acetaldehyde dehydrogenase (eutE). One gene (eutR) seems to encode a positive regulatory protein required for induction of transcription of eut. The function of one of the remaining two genes (eutA) was shown to be required for ethanolamine utilization only when cyano-B12 or hydroxy-B12 were the precursors of the adenosyl-B12 cofactor of ethanolamine ammonia-lyase; eutA mutants could use ethanolamine as the nitrogen source only when adenosyl-B12 was provided. No function has been assigned to the eutD gene, which is required for use of ethanolamine as a carbon source. Ethanolamine uptake assays of eut mutants suggest that no ethanolamine permease is encoded in the eut operon.