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Fig. 1 B12 synthesis and metabolism in mycobacteria. (A) B12 synthesis and metabolism in M. marinum; the cobC gene (in red) was mutated (cobC::fs) to disrupt B12 synthesis in M. marinum MUSA. (B) Reporter assay employing a B12-sensitive pinib4-mCherry construct in a cobC mutant strain. This assay detects incomplete synthesis pathways, producing a fluorescent signal. Bacteria were exposed to varying B12 concentrations, leading to signal reduction with increased B12 levels. M. marinum WT (wild-type) strain and WT expressing the reporter construct were used as controls. (C) The enzymes MetH and MetE convert L-homocysteine into L-methionine. MetH relies on B12 as a cofactor, while MetE operates independently of B12. Translation of metE is regulated by an RNA riboswitch.