Plant and Cell Physiology Advance Access originally published online on December 13, 2006
Plant and Cell Physiology 2007 48(2):232-242; doi:10.1093/pcp/pcl055
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Functional Characterization of a Methionine 
-Lyase in Arabidopsis and its Implication in an Alternative to the Reverse Trans-sulfuration Pathway
1Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA
2Department of Plant Biology, Michigan State University, East Lansing, MI 48824, USA
*Corresponding author: E-mail, agoyer{at}pars.ars.usda.gov; Fax, +1-509-786-9370.
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Abstract |
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Methionine 
-lyase (MGL) catalyzes the degradation of L-methionine to 
-ketobutyrate, methanethiol and ammonia. The Arabidopsis (Arabidopsis thaliana) genome includes a single gene (At1g64660) encoding a protein (AtMGL) with 
35% identity to bacterial and protozoan MGLs. When overexpressed in Escherichia coli, AtMGL allowed growth on L-methionine as sole nitrogen source and conferred a high rate of methanethiol emission. The purified recombinant protein exhibited a spectrum typical of pyridoxal 5'-phosphate enzymes, and had high activity toward L-methionine, L-ethionine, L-homocysteine and seleno-L-methionine, but not L-cysteine. Quantitation of mRNA showed that the AtMGL gene is expressed in aerial organs and roots, and that its expression in leaves was increased 2.5-fold by growth on low sulfate medium. Emission of methanethiol from Arabidopsis plants supplied with 10 mM L-methionine was undetectable (<0.5 nmol min–1 g–1 FW), suggesting that AtMGL is not an important source of volatile methanethiol. Knocking out the AtMGL gene significantly increased leaf methionine content (9.2-fold) and leaf and root S-methylmethionine content (4.7- and 7-fold, respectively) under conditions of sulfate starvation, indicating that AtMGL carries a significant flux in vivo. In Arabidopsis plantlets fed L-[35S]methionine on a low sulfate medium, label was incorporated into protein-bound cysteine as well as methionine, but incorporation into cysteine was significantly (30%) less in the knockout mutant. These data indicate that plants possess an alternative to the reverse trans-sulfuration pathway (methionine
homocysteinecystathioninecysteine) in which methanethiol is an intermediate.
Keywords: Arabidopsis thaliana - Cysteine - Methionine
Abbreviations:
EST, expressed sequence tag; IPTG, isopropyl-ß-D-thiogalactopyranoside; MGL, methionine -lyase; PLP, pyridoxal 5'-phosphate; RT–PCR, reverse transcription–PCR; SMM, S-methylmethionine.
3 Present address: Oregon State University/USDA-ARS, Prosser, WA 99350, USA.
(Received September 18, 2006; Accepted December 8, 2006)
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