In Vivo Regulation of De Novo Methionine Biosynthesis in a Higher Plant (Lemna)

Abstract

Administration of methionine to growing Lemna had essentially no effect on accumulation of sulfate S in protein cysteine, but decreased accumulation into cystathionine and its products (homocysteine, methionine, S-methylmethioninesulfonium salt, S-adenosylmethionine and S-adenosylhomocysteine) to as low as 21% that of control plants, suggesting that methionine regulates its own de novo synthesis at cystathionine synthesis. Methionine caused only a slight reduction (to 80% that of control plants) in the accumulation of sucrose C into the 4-C moieties of cystathionine and products. This observation was puzzling since cystathionine synthesis proceeds by incorporation of equivalent amounts of S (from cysteine) and 4-C moieties (from O-phosphohomoserine). The apparent inconsistency was resolved by the demonstration in Lemna (Giovanelli et al., 1983) that de novo synthesis of the methionine 4-C moiety occurs not only via the established transsulfuration route from O-phosphohomoserine, but also via the ribose moiety of 5''-methylthioadenosine. The more accurate assessment of the flux of S (and 4-C moieties) through transsulfuration is provided by the amount of 35S from 35SO42- that accumulates in cystathionine and its products, rather than by the corresponding measurements with 14C. In higher plants methionine does indeed feedback regulate its own de novo synthesis in vivo, and cystathionine synthesis is a locus for this regulation.