A cDNA Encoding S-Adenosyl-L-Methionine Synthetase from Poplar

Abstract

Adomet synthetase (EC 2.5.1.6) catalyzes the formation of Adomet, which serves as a methyl group donor in highly specific transmethylation reactions involving a11 kinds of acceptor molecules such as proteins, nucleic acids, phenylpropanoid derivatives, polysaccharides, cyclic fatty acids, etc. Adomet also plays a role in amino acid biosynthesis and polyamine biosynthesis. In plants, Adomet has been studied mainly in relation to the biosynthesis of various phenylpropanoid derivatives and as an intermediate in the biosynthesis of the phytohormone ethylene (Goodwin and Mercer, 1983). To date, cDNA or genomic clones encoding Adomet synthetase have been isolated from Arabidopsis thaliana (Peleman et al., 1989), parsley (Kawalleck et al., 1992), rat (Horikawa et al., 1989), Saccharomyces cerevisiae (Thomas and SurdinKerjan, 1987), and Escherichia coli (Markham et al., 1984). In Arabidopsis, tissue-specific Adomet synthetase gene expression was shown (mainly in vascular tissue), and it was suggested that Adomet synthetase gene expression is correlated with lignin biosynthesis (Peleman et al., 1989). In parsley and alfalfa cells, Adomet synthetase gene expression was induced in response to funga1 elicitor and yeast cell walls, respectively (Gowri et al., 1991; Kawalleck et al., 1992). Here we report on the isolation of a poplar (Populus deltoides X Populus trichocarpa) cDNA for Adomet synthetase. A poplar leaf cDNA library was screened using the A. thaliana saml cDNA clone (Peleman et al., 1989) as a probe. A cDNA clone, designated sam.PdxPt.2 (Table I) and containing a 1435-bp insert, was sequenced. The cDNA contains one open reading frame of 1185 nucleotides encoding a protein of 395 amino acids (calculated mo1 wt 43,271; isoelectric point 5.48). The coding region is flanked on the 5‘ end by 60 bp of noncoding sequence and on the 3’ end by 190 bp. No in-frame stop codons were found upstream from the ATG (positions 60-62). The poplar Adomet synthetase has 90, 91,92, 60, 59, and 49% similarity on the protein leve1