Involved in the biosynthesis of the antibiotic indolmycin, an inhibitor of the bacterial tryptophan-tRNA synthetases. Catalyzes the methylation of N-demethylindolmycin to yield indolmycin, with S-adenosylmethionine (AdoMet) acting as the methyl donor. N-demethylindolmycin + S-adenosyl-L-methionine = H(+) + indolmycin + S-adenosyl-L-homocysteine Cells lacking this gene accumulate N-demethylindolmycin and are unable to produce indolmycin. Belongs to the methyltransferase superfamily.