Cloning and nucleotide sequence of a cDNA encoding S-adenosyl-L-methionine synthetase from mustard (Brassica juncea [L.] Czern & Coss).

Ethylene, a gaseous hormone in plants, is involved in the regulation of a wide range of plant physiological processes, including leaf abscission, fruit ripening, and flower senescence (Yang and Hoffman, 1984). SAM or Adomet synthetase (ATP:L-methionine S-adenosyltransferase; EC 2.5.1.6) catalyzes the conversion of Met to SAM in the presence of ATP and Mg2+, which is the first step in ethylene biosynthesis. Changes in the leve1 of endogenous SAM can greatly affect ethylene synthesis. In tomato (Lycopersicon esculentum), transgenic plants that overproduced bacteriophage T3 SAM hydrolase, the enzyme that degrades SAM to a-ketobutyrate and ammonia, showed a 60 to 80% decrease in ethylene production (Kellogg et al., 1994). Apart from being a precursor in ethylene biosynthesis, SAM also serves as a propylamine group donor in polyamine synthesis and as a methyl group donor in a variety of reactions including transmethylation of proteins, nucleic acids, lipids, and polysaccharides (Tabor and Tabor, 1984). SAM synthetase is ubiquitous in both prokaryotes and eukaryotes. Genes encoding this enzyme have been cloned from Escherichia coli, yeast, and rat (see Van Doorsselaere et al., 1993) and severa1 plant species including Avabidopsis thaliana (Peleman et al., 1989), parsley (Kawalleck et al., 1992), poplar (Van Doorsselaere et al., 1993), carnation (Larsen et al., 1991), and tomato (Espartero et al., 1994). In plants, SAM synthetase is encoded by a small gene family (Peleman et al., 1989; Van Doorsselaere et al., 1993). Two SAM synthetase homologs, sam-1 and sam-3, isolated from A. thaliana exhibited a similar expression pattern. Further study revealed that the sam-2 gene was preferentially expressed in vascular tissues, stem sclerenchyma, and root cortex in transgenic plants (Peleman et al., 1989). In addition, environmental stresses and hormones may have a regulatory role in sam expression. This is suggested by the accumulation of sam-1 and sam-3 transcripts in tomato roots in response to NaC1, mannitol, or ABA treatment (Espartero et al., 1994).