1. A growth factor for Mycobacterium johnei has been isolated from Mycobacterium tuberculosis as an iron complex; it has been named ferric mycobactin T. 2. Ferric mycobactin T as isolated is a mixture of at least four closely related components differing only in their fatty acid side chains. 3. Mycobactin T resembles mycobactin P in structure but differs in the following respects: (a) the aromatic acid residue is salicylic acid in place of 2-hydroxy-6-methylbenzoic acid; (b) the hydroxy acid residue is (+)-beta-hydroxybutyric acid in place of (-)-3-hydroxy-2-methylpentanoic acid; (c) its components have fatty acid side chains that are mostly different from the side chains of the components in mycobactin P preparations. One minor component in mycobactin T has probably a cis-octadec-2-enoyl side chain. The major side-chain components correspond to acids of chain length C(18)-C(21) that are not n-saturated or n-Delta(2)-unsaturated acids. 4. The differences in biosynthesis between mycobactins T and P are discussed.

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