L-Methionine Overproduction by Ethionine-resistant Mutants of Obligate Methylotroph Strain OM 33

Abstract

More than 400 methanol-utilizing microorganisms isolated from soil were examined for L-methionine excretion in methanol medium supplemented with L-homoserine as a precusor of L-methionine. The strain OM 33 excreted 70 μg/ml of L-methionine when L-homoserine (5 mg/ml) and Na₂S•9H₂O (2 mg/ml) were added to the medium. This strain utilized only methanol and monomethylamine as sole sources of carbon, and assimilated methanol via the ribulose monophosphate pathway. Ethionine-resistant mutants were induced from strain OM 33 by treatment with N-methyl-N'-nitro-N-nitrosoguanidine. A mutant, OE 120, excreted L-methionine into the medium without the addition of L-homoserine. Cultural conditions for L-methionine accumulation were investigated. After 4- to 5-days of cultivation, this mutant accumulated 420 μg/ml of L-methionine under optimum conditions.