In vivo regulation of de novo methionine biosynthesis in a higher plant (lemna).

Administration of methionine to growing Lemna had essentially no effect on accumulation of sulfate sulfur 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 carbon into the 4-carbon moieties of cystathionine and products. This observation was puzzling since cystathionine synthesis proceeds by incorporation of equivalent amounts of sulfur (from cysteine) and 4-carbon moieties (from O-phosphohomoserine). The apparent inconsistency was resolved by the demonstration in Lemna (Giovanelli, Datko, Mudd, Thompson 1983 Plant Physiol 71: 319-326) that de novo synthesis of the methionine 4-carbon moiety occurs not only via the established transsulfuration route from O-phosphohomoserine, but also via the ribose moiety of 5'-methylthioadenosine. It is now clear that the more accurate assessment of the flux of sulfur (and 4-carbon moieties) through transsulfuration is provided by the amount of (35)S from (35)SO(4) (2-) that accumulates in cystathionine and its products, rather than by the corresponding measurements with (14)C. These studies therefore unequivocally demonstrate in higher plants that methionine does indeed feedback regulate it own de novo synthesis in vivo, and that cystathionine synthesis is a locus for this regulation.