The enzymes leading to the methylation of homocysteine have been examined in three micro-organisms: a cobalamin-producing bacterium, Bacillus megaterium; a yeast, Candida utilis; and a basidiomycete fungus, Coprinus lagopus. The yeast and the fungus contain negligible endogenous cobalamin. 2. Extracts of each organism catalyse C(1)-transfer from serine to homocysteine with a polyglutamate folate coenzyme. 3. The enzymes generating the methyl group of methionine from C-3 of serine have similar properties in each case, but different mechanisms of homocysteine transmethylation from 5-methyltetrahydrofolates were found. 4. B. megaterium contains an enzyme with properties suggestive of a vitamin B(12)-dependent homocysteine transmethylase, whereas Cand. utilis and Cop. lagopus transfer the methyl group by a reaction characteristic of the cobalamin-independent mechanism established for Escherichia coli. 5. The specificity of each transmethylase for a 5-methyltetrahydropteroylpolyglutamate is consistent with the results of analyses of endogenous folates in these organisms, which showed only conjugated forms. 6. None of the extracts catalysed methionine production from S-adenosylmethionine and homocysteine. 7. These results are compared with results now available for methionine synthesis in other organisms, which show a considerable diversity of mechanisms.
