Involved in the biosynthesis of the antibiotic indolmycin, an inhibitor of the bacterial tryptophan-tRNA synthetases. Catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to position 3 of the aliphatic side chain of (indol-3-yl)pyruvate to yield 3-methylindolepyruvate. indole-3-pyruvate + S-adenosyl-L-methionine = (R)-3-(indol-3-yl)-2-oxobutanoate + H(+) + S-adenosyl-L-homocysteine Strongly inhibited by the thiol reagents p-chloromercuribenzoate and N-ethylmaleimide. Partially inhibited by o-phenanthroline and 2,2'-dipyridyl. Competitively inhibited by L-tryptophan and indolmycin. Optimum pH is 7.5-8.5. Loss of methyltransferase activity at pH 5.5 and below. Cells laking this gene are unable to produce indolmycenate. Belongs to the methyltransferase superfamily.