S-adenosyl-L-methionine-dependent methyltransferase involved in synthesis of the nonproteinogenic amino acid (2S,3S)-beta-methyl-phenylalanine, a building block of the antibiotic mannopeptimycin. 3-phenylpyruvate + S-adenosyl-L-methionine = (3S)-2-oxo-3-phenylbutanoate + H(+) + S-adenosyl-L-homocysteine kcat is 0.8 sec(-1) with S-adenosyl-L-methionine as substrate. kcat is 8.15 sec(-1) with phenylpyruvic acid as substrate. Antibiotic biosynthesis. Belongs to the methyltransferase superfamily.