MUTANTS OF ESCHERICHIA COLI REQUIRING METHIONINE OR VITAMIN B 12

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Abstract
A number of mutants of E. coli have been isolated that require vitamin B12 or methionine. These do not respond to homocysteine, the immediate precursor of methionine. Furthermore, mutants responding to homocysteine do not respond to B12. Most of the B12 auxotrophs grow equally rapidly on the vitamin or methionine, but 2 grow rapidly on methionine and slowly on B12- Tests for syntrophism show that the B12-requiring mutants accumulate a precursor of methionine that feeds the homocysteineless mutants. An excess of B12 abolishes this accumulation with the fast B12, but not the slow B12 mutants. In contrast to the behavior of many B12-requiring lactic acid bacteria, Bl2-requiring E. coli do not respond to thymidine. It is concluded that B12 is concerned with the methylation of homocysteine. The nature of the genetic block resulting in slow growth on B12 is less clear but may involve slow conversion of B12 to a coenzyme of methylation.