S-adenosyl-L-methionine : salicylic acid carboxyl methyltransferase, an enzyme involved in floral scent production and plant defense, represents a new class of plant methyltransferases

S-Adenosyl-L-methionine:salicylic acid carboxyl methyltransferase (SAMT) wa s partially purified from petals of the annual California plant Clarkia bre weri. SAMT catalyzes the formation of methylsalicylate, an important floral scent compound in C. breweri, from salicylic acid and S-adenosyl-L-methion ine (SAM). The native enzyme is a dimer with a subunit molecular weight of 40.3 kDa, and it has a K-m for salicylic acid of 24 mu M and a K-m for SAM of 9 mu M. A cDNA encoding SAMT was isolated from a C. breweri cDNA library prepared from floral mRNA. The sequence of the protein encoded by SAMT cDN A shows no significant sequence similarity to any protein in the data bank whose biochemical function is known. It does show significant sequence simi larity (20-40% identity) to proteins encoded by at least seven Arabidopsis thaliana genes whose sequences have recently been determined in large-scale sequencing projects. The C. breweri SAMT cDNA was expressed in E. coli and the bacterial cells synthesized a functional SAMT protein with properties nearly identical to those of the plant-purified enzyme. (C) 1999 Academic P ress.