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

Abstract

S-Adenosyl-l-methionine:salicylic acid carboxyl methyltransferase (SAMT) was partially purified from petals of the annual California plant Clarkia breweri. SAMT catalyzes the formation of methylsalicylate, an important floral scent compound in C. breweri, from salicylic acid and S-adenosyl-l-methionine (SAM). The native enzyme is a dimer with a subunit molecular weight of 40.3 kDa, and it has a Km for salicylic acid of 24 μM and a Km for SAM of 9 μM. A cDNA encoding SAMT was isolated from a C. breweri cDNA library prepared from floral mRNA. The sequence of the protein encoded by SAMT cDNA shows no significant sequence similarity to any protein in the data bank whose biochemical function is known. It does show significant sequence similarity (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.