Cloning and nucleotide sequence of a s-adenosylmethionine synthetase cDNA from carnation.

SAM2 serves as a methyl donor in many transmethylation reactions involving a variety of methyl acceptor molecules such as proteins, lipids, polysaccharides, and nucleic acids. In addition, SAM is a precursor in the synthesis of polyamines and ethylene (8). It has been suggested that the dynamic equilibrium between polyamines and ethylene in cells is regulated through the availability of SAM (1, 4). The genes for SAM synthetase which catalyzes the conversion ofmethionine to SAM have recently been isolated and cloned from Arabidopsis thaliana (6, 7). Using a cDNA probe for the SAM synthetase mRNA, Peleman et al. (6) showed strong cellular preference in the expression of these genes. An increase in the rate of ethylene production has not generally been considered to be dependent on increased availability of SAM (10); however, this has not been critically examined. Carnation floral tissues, and particularly styles and petals, produce ethylene at very high rates during senescence and thus provide an excellent model to address the regulation of SAM synthetase in relation to ethylene biosynthesis (2). Here we report the nucleotide sequence of a cDNA clone encoding SAM synthetase (EC 2.5.1.6) from carnation (Dianthus caryophyllus L., cv White Sim). The nucleotide sequence of cDNA clone pSAM2 is 1632 base pairs and contains an open reading frame encoding a polypeptide of 396 amino acids (Fig. 1). The predicted protein is highly homologous to functionally identified SAM synthetase proteins from rat (3) and yeast (9). In addition, the pSAM2 protein shares extensive homology with the predicted proteins from cloned SAM synthetase genes ofArabidopsis (6, 7).