Substrate specificity of N-methyltransferase involved in purine alkaloids synthesis is dependent upon one amino acid residue of the enzyme
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Caffeine (1,3,7-trimethylxanthine) and theobromine (3,7-dimethylxanthine) are the major purine alkaloids in plants. To investigate the diversity of N-methyltransferases involved in purine alkaloid biosynthesis, we isolated the genes homologous for caffeine synthase from theobromine-accumulating plants. The predicted amino acid sequences of N-methyltransferases in theobromine-accumulating species in Camellia were more than 80% identical to caffeine synthase in C. sinensis. However, there was a little homology among the N-methyltransferases between Camellia and Theobroma. The recombinant enzymes derived from theobromine-accumulating plants had only 3-N-methyltransferase activity. The accumulation of purine alkaloids was, therefore, dependent on the substrate specificity of N-methyltransferase determined by one amino acid residue in the central part of the protein.
KeywordsCaffeine Caffeine synthase Camellia, Theobromine Theobromine synthase Theobroma cacao
We deeply thank Dr. Y. Takeda (National Institute of Plant Vegetable and Tea Science) for the generous supply of C. irrawadiensis. We also would like to thank Dr. A. Takeuchi (National Institute of Plant Vegetable and Tea Science) for providing cDNA library of C. sinensis and Mr. Y. Tomoda (Meiji Seika Kaisya Co Ltd.) for the supply of cacao trees. This work is supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No.14540590 to MK., No.15780076 to KM.).
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