First published online March 13, 2003; 10.1104/pp.102.018846
Plant Physiol, April 2003, Vol. 131, pp. 1808-1815
Insertional Inactivation of the Methionine
S-Methyltransferase Gene Eliminates the
S-Methylmethionine Cycle and Increases the Methylation
Ratio1
Michael G.
Kocsis,
Philippe
Ranocha,2
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.)
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.
1
This work was supported in part by the National
Science Foundation (grant nos. IBN-981399 and MCB-0114117 to A.D.H.
and IBN-9904263 to D.A.G.), by the Department of Energy (grant no.
DE-FG02-99ER20344 to D.R.), by the Japan Society for the Promotion of
Science (grants to K.S. and M.A.), by Core Research for Evolutional
Science and Technology of Japan Science and Technology Corporation (to
K.S.), by an endowment from the C.V. Griffin, Sr. Foundation, and by the Florida Agricultural Experiment Station (journal series no. R-09217).
2
Present address: Unité Mixte de Recherche Centre
National de la Recherche Scientifique/Université de Paris-Sud
5546, Pôle de Biotechnologies Végétales, Castanet
Tolosan, France.
*
Corresponding author; e-mail
adha{at}mail.ifas.ufl.edu; fax 352-392-5653.
© 2003 American Society of Plant Biologists
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