Involved in the biosynthesis of the antibiotic indolmycin, an inhibitor of the bacterial tryptophan-tRNA synthetases. Catalyzes the NADH-dependent reduction of beta-methylindolepyruvate to yield indolmycenate. (2S,3R)-2-hydroxy-3-(indol-3-yl)butanoate + NAD(+) = (R)-3-(indol-3-yl)-2-oxobutanoate + H(+) + NADH Cells laking this gene are unable to produce indolmycenate. Belongs to the LDH/MDH superfamily.