A major route of ethylene synthesis in higher plants involves the following metabolic sequence: methionine ^ S-adenosylmethionine (AdoMet) ^ ACC ^ ethylene. Methionine is converted to AdoMet [ATP:L-methionine S-adenosyltransferase (AdoMet synthetase, EC 126.96.36.199); Giovanelli et al, 1980], AdoMet to ACC [AdoMet methylthioadenosinelyase (ACC synthase, EC 188.8.131.52); Kende, 1989] and ACC to ethylene (ACC oxidase, also called ethylene-forming enzyme; John, 1991). The enzymes catalyzing these reactions and the genes encoding them have been demonstrated in plants (Giovannoni, 2001). The genes encoding these enzymes belong to multigene families (see Fluhr and Mattoo, 1996). Generally, the rate-limiting steps in this pathway are catalyzed by ACC synthase and ACC oxidase.
ACC is only one product of ACC synthase activity, the other product produced in stoichiometric amounts is 5'-methylthioadenosine (MTA) (Adams and Yang, 1979). MTA is also a product generated from decarboxylated-AdoMet during the biosynthesis of polyamines (Schlenk, 1983; Cohen, 1998) and from AdoMet during enzymatic methylation of nucleic acids (Grefter etal., 1966). Further, MTA is readily metabolized and recycled to methionine (Kushad, 1990). The recognition of MTA as a common biosynthetic product in these reactions made it apparent that the different pathways might be interlinked and developmentally regulated.
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