SUMMARY: During growth of strain SPA o in the presence of methionine, an intermediate accumulates in the medium. This intermediate reacts with 2,4-dinitro-phenylhydrazine, and can be degraded to ethylene either enzymically or photo-chemically, the latter being stimulated by the addition of a flavin. The pH optimum for the photochemical degradation of this intermediate and 2-keto-4-methylthio-butyric acid (KMBA) is pH 3 whereas the optimum for methional is pH 6. The enzyme which converts the intermediate to ethylene also converts KMBA to ethylene and has many of the properties of a peroxidase including inhibition by catalase, cyanide, azide and anaerobiosis. The enzyme which synthesizes the intermediate is not known but requires oxygen and pyridoxal phosphate. A pathway for ethylene biosynthesis is proposed in which methionine is converted to KMBA which can be degraded either by peroxidase or in a flavin-mediated photochemical reaction. Its relevance to the properties of other ethylene-producing bacteria and to the proposed pathway of ethylene release by higher plants is discussed.


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