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Fabienne Bourgis, Sanja Roje, Michael L. Nuccio, Donald B. Fisher, Mitchell C. Tarczynski, Changjiang Li, Cornelia Herschbach, Heinz Rennenberg, Maria Joao Pimenta, Tun-Li Shen, Douglas A. Gage, Andrew D. Hanson, S-Methylmethionine Plays a Major Role in Phloem Sulfur Transport and Is Synthesized by a Novel Type of Methyltransferase, The Plant Cell, Vol. 11, No. 8 (Aug., 1999), pp. 1485-1497
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Bourgis et al. (1999) studied this question.
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