All flowering plants produce S-methylmethionine (SMM) from Met and have a separate mechanism to convert SMM back to Met. The functions of SMM and the reasons for its interconversion with Met are not known. In this study, by using the aphid stylet collection method together with mass spectral and radiolabeling analyses, we established that l-SMM is a major constituent of the phloem sap moving to wheat ears. The SMM level in the phloem ( approximately 2% of free amino acids) was 1.5-fold that of glutathione, indicating that SMM could contribute approximately half the sulfur needed for grain protein synthesis. Similarly, l-SMM was a prominently labeled product in phloem exudates obtained by EDTA treatment of detached leaves from plants of the Poaceae, Fabaceae, Asteraceae, Brassicaceae, and Cucurbitaceae that were given l-(35)S-Met. cDNA clones for the enzyme that catalyzes SMM synthesis (S-adenosylMet:Met S-methyltransferase; EC 2.1.1.12) were isolated from Wollastonia biflora, maize, and Arabidopsis. The deduced amino acid sequences revealed the expected methyltransferase domain ( approximately 300 residues at the N terminus), plus an 800-residue C-terminal region sharing significant similarity with aminotransferases and other pyridoxal 5'-phosphate-dependent enzymes. These results indicate that SMM has a previously unrecognized but often major role in sulfur transport in flowering plants and that evolution of SMM synthesis in this group involved a gene fusion event. The resulting bipartite enzyme is unlike any other known methyltransferase.

All flowering plants produce S -methylmethionine (SMM) from Met and have a separate mechanism to convert SMM back to Met. The functions of SMM and the reasons for its interconversion with Met are not known. In this study, by using the aphid stylet collection method together with mass spectral and radiolabeling analyses, we established that L -SMM is a major constituent of the phloem sap moving to wheat ears. The SMM level in the phloem ( ‫ف‬ 2% of free amino acids) was 1.5-fold that of glutathione, indicating that SMM could contribute approximately half the sulfur needed for grain protein synthesis. Similarly, L -SMM was a prominently labeled product in phloem exudates obtained by EDTA treatment of detached leaves from plants of the Poaceae, Fabaceae, Asteraceae, Brassicaceae, and Cucurbitaceae that were given L -35 S-Met. cDNA clones for the enzyme that catalyzes SMM synthesis ( S -adenosylMet:Met S -methyltransferase; EC 2.1.1.12) were isolated from Wollastonia biflora , maize, and Arabidopsis. The deduced amino acid sequences revealed the expected methyltransferase domain ( ‫ف‬ 300 residues at the N terminus), plus an 800-residue C-terminal region sharing significant similarity with aminotransferases and other pyridoxal 5 -phosphate-dependent enzymes. These results indicate that SMM has a previously unrecognized but often major role in sulfur transport in flowering plants and that evolution of SMM synthesis in this group involved a gene fusion event. The resulting bipartite enzyme is unlike any other known methyltransferase. INTRODUCTIONPlant Met metabolism differs from that in other organisms by involving S -methylmethionine (SMM). SMM is a ubiquitous constituent of the free amino acid pool in flowering plants, occurring in leaves, roots, and other organs at levels that typically range from 0.5 to 3 mol g Ϫ 1 dry weight, a concentration that is often higher than those of Met or S -adenosylmethionine (AdoMet) (Giovanelli et al., 1980;Mudd and Datko, 1990;Bezzubov and Gessler, 1992). SMM also has been detected as a metabolite of radiolabeled L -Met in all flowering plants tested ( Ͼ 50 species from Ͼ 20 families; Paquet et al., 1995). As shown in Figure 1, SMM is formed from L -Met via the action of AdoMet:Met S -methyltransferase (MMT; EC 2.1.1.12) and can be reconverted to Met by donating a methyl group to L -homocysteine (Hcy) in a reaction catalyzed by Hcy S -methyltransferase (HMT; EC 2.1.1.10; Giovanelli et al., 1980;Mudd and Datko, 1990). The tandem action of MMT and HMT, together with S -adenosyl-L -Hcy hydrolase, constitutes the SMM cycle, which is apparently futile (Mudd and Datko, 1990).As expected from the universality of SMM, MMT activity has been found in many flowering plants (Giovanelli et al., 1980;Mudd and Datko, 1990). It has been purified from leaves of Wollastonia biflora (James et al., 1995a) and from germinating barley (Pimenta et al., 1998), and it is known to have subunits of ‫ف‬ 115 kD. Because this is approximately three times larger than any other small-molecule methyltransferase (F...

We report a solution phase synthesis of 16 nm CuNi nanoparticles (NPs) with the Cu/Ni composition control. These NPs are assembled on graphene (G) and show Cu/Ni composition-dependent catalysis for methanolysis of ammonia borane (AB) and hydrogenation of aromatic nitro (nitrile) compounds to primary amines in methanol at room temperature. Among five different CuNi NPs studied, the G-Cu 36 Ni 64 NPs are the best catalyst for both AB methanolysis (TOF = 49.1 mol H2 mol CuNi -1 min -1 and E a = 24.4 kJ/mol) and hydrogenation reactions (conversion yield >97%). The G-CuNi represents a unique noble-metal-free catalyst for hydrogenation reactions in a green environment without using pure hydrogen.

No abstract