Plant Physiology Preview
Published on March 13, 2003; 10.1104/pp.102.018846
Received December 8, 2002
Returned for revision December 26, 2002
Accepted December 28, 2002
Insertional Inactivation of the Methionine S-Methyltransferase Gene Eliminates the S-Methylmethionine Cycle and Increases the Methylation Ratio
Michael G. Kocsis , Philippe Ranocha , Douglas A. Gage , Eric S. Simon , David Rhodes , Gregory J. Peel , Stefan Mellema , Kazuki Saito , Motoko Awazuhara , Changjiang Li , Robert B. Meeley , Mitchell C. Tarczynski , Conrad Wagner , and Andrew D. Hanson *
Horticultural Sciences Department, University of Florida, Gainesville, Florida 32611 (M.G.K., P.R., A.D.H.); Biochemistry Department, Michigan State University, East Lansing, Michigan 48824 (D.A.G., E.S.S.); Center for Plant Environmental Stress Physiology, Department of Horticulture and Landscape Architecture, Purdue University, West Lafayette, Indiana 47907 (D.R., G.J.P.); Institute of Plant Sciences, University of Berne, 3013 Berne, Switzerland (S.M.); Graduate School of Pharmaceutical Sciences, Department of Molecular Biology and Biotechnology, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan (K.S., M.A.); Pioneer Hi-Bred International, 7300 NM 62nd Avenue, Johnston, Iowa (C.L., R.B.M., M.C.T.); and Department of Biochemistry, Medical Center, Vanderbilt University, Nashville, Tennessee 37232 (C.W.)
* Corresponding author; email: adha{at}mail.ifas.ufl.edu.
Methionine (Met) S-methyltransferase (MMT) catalyzes the synthesis of S-methyl-Met (SMM) from Met and S-adenosyl-Met (Ado-Met). SMM can be reconverted to Met by donating a methyl group to homocysteine (homo-Cys), and concurrent operation of this reaction and that mediated by MMT sets up the SMM cycle. SMM has been hypothesized to be essential as a methyl donor or as a transport form of sulfur, and the SMM cycle has been hypothesized to guard against depletion of the free Met pool by excess Ado-Met synthesis or to regulate Ado-Met level and hence the Ado-Met to S-adenosylhomo-Cys ratio (the methylation ratio). To test these hypotheses, we isolated insertional mmt mutants of Arabidopsis and maize (Zea mays). Both mutants lacked the capacity to produce SMM and thus had no SMM cycle. They nevertheless grew and reproduced normally, and the seeds of the Arabidopsis mutant had normal sulfur contents. These findings rule out an indispensable role for SMM as a methyl donor or in sulfur transport. The Arabidopsis mutant had significantly higher Ado-Met and lower S-adenosylhomo-Cys levels than the wild type and consequently had a higher methylation ratio (13.8 versus 9.5). Free Met and thiol pools were unaltered in this mutant, although there were moderate decreases (of 30%-60%) in free serine, threonine, proline, and other amino acids. These data indicate that the SMM cycle contributes to regulation of Ado-Met levels rather than preventing depletion of free Met.
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