S-methylmethionine plays a major role in phloem sulfur transport and is synthesized by a novel type of methyltransferase

All flowering plants produce S-methylmethionine (SMM) from Met and have a s eparate mechanism to convert SMM back to Mel. The functions of SMM and the reasons for its interconversion with Met are not known. In this study, by u sing the aphid stylet collection method together with mass spectral and rad iolabeling analyses, we established that L-SMM is a major constituent of th e phloem sap moving to wheat ears. The SMM level in the phloem (similar to 2% Of free amino acids) was 1.5-fold that of glutathione, indicating that S MM could contribute approximately half the sulfur needed for grain protein synthesis. Similarly, L-SMM was a prominently labeled product in phloem exu dates obtained by EDTA treatment of detached leaves from plants of the Poac eae, Fabaceae, Asteraceae, Brassicaceae, and Cucurbitaceae that were given L-S-35-Met. CDNA clones for the enzyme that catalyzes SMM synthesis (S-aden osylMet:Met S-methyltransferase; EC 2.1.1.12) were isolated from Wollastoni a biflora, maize, and Arabidopsis. The deduced amino acid sequences reveale d the expected methyltransferase domain (similar to 300 residues at the N t erminus), plus an 800-residue C-terminal region sharing significant similar ity with aminotransferases and other pyridoxal 5'-phosphate-dependent enzym es. These results indicate that SMM has a previously unrecognized but often major role in sulfur transport in flowering plants and that evolution of S MM synthesis in this group involved a gene fusion event. The resulting bipa rtite enzyme is unlike any other known methyltransferase.