Evidence for the B12-dependent enzyme ethanolamine deaminase in Salmonella.

Abstract

THE enteric bacteria Escherichia coli and Salmonella typhimurium, like man, do not synthesise significant amounts of cobalamin (B12) compounds and thus depend on exogenous vitamin B12 for their B12-dependent enzymes1. In E. coli and S. typhimurium the only reported B12-dependent enzyme catalyses the final step in methionine biosynthesis—the methylation of homocysteine to give methionine. These bacteria do not depend on exogenous B12 for growth, however, for they have an alternative B12-independent homocysteine transmethylase (non-B12 enzyme) which can catalyse the same reaction. The non-B12 transmethylase is much less efficient than the corresponding B12-dependent enzyme: in bacteria grown in the absence of vitamin B12, the non-B12-dependent enzyme comprises 3–5 % of the total soluble protein2. If this enzyme is blocked by mutation, the mutant bacteria require either methionine itself or exogenous B12, which allows them to utilise the B12-dependent enzyme for methionine biosynthesis3. We have sought other B12-dependent enzymes in S. typhimurium and found evidence for an ethanolamine deaminase. We did not find other examples of parallel non-B12-dependent and B12-dependent enzymes in S. typhimurium. E. coli also seems to have an ethanolamine deaminase.