The S ‐methylmethionine cycle in angiosperms: ubiquity, antiquity and activity

The S ‐methylmethionine cycle in angiosperms: ubiquity, antiquity and activity Summary Angiosperms synthesize S‐methylmethionine (SMM) from methionine (Met) and S‐adenosylmethionine (AdoMet) in a unique reaction catalyzed by Met S‐methyltransferase (MMT). SMM serves as methyl donor for Met synthesis from homocysteine, catalyzed by homocysteine S‐methyltransferase (HMT). MMT and HMT together have been proposed to constitute a futile SMM cycle that stops the free Met pool from being depleted by an overshoot in AdoMet synthesis. Arabidopsis and maize have one MMT gene, and at least three HMT genes that belong to two anciently diverged classes and encode enzymes with distinct properties and expression patterns. SMM, and presumably its cycle, must therefore have originated before dicot and monocot lineages separated. Arabidopsis leaves, roots and developing seeds all express MMT and HMTs, and can metabolize (35S)Met to (35S)SMM and vice versa. The SMM cycle therefore operates throughout the plant. This appears to be a general feature of angiosperms, as digital gene expression profiles show that MMT and HMT are co‐expressed in leaves, roots and reproductive tissues of maize and other species. An in silico model of the SMM cycle in mature Arabidopsis leaves was developed from radiotracer kinetic measurements and pool size data. This model indicates that the SMM cycle consumes half the AdoMet produced, and suggests that the cycle serves to stop accumulation of AdoMet, rather than to prevent depletion of free Met. Because plants lack the negative feedback loops that regulate AdoMet pool size in other eukaryotes, the SMM cycle may be the main mechanism whereby plants achieve short‐term control of AdoMet level. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Plant Journal Wiley

The S ‐methylmethionine cycle in angiosperms: ubiquity, antiquity and activity

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Publisher
Wiley
Copyright
Copyright © 2001 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0960-7412
eISSN
1365-313X
D.O.I.
10.1046/j.1365-313x.2001.00988.x
Publisher site
See Article on Publisher Site

Abstract

Summary Angiosperms synthesize S‐methylmethionine (SMM) from methionine (Met) and S‐adenosylmethionine (AdoMet) in a unique reaction catalyzed by Met S‐methyltransferase (MMT). SMM serves as methyl donor for Met synthesis from homocysteine, catalyzed by homocysteine S‐methyltransferase (HMT). MMT and HMT together have been proposed to constitute a futile SMM cycle that stops the free Met pool from being depleted by an overshoot in AdoMet synthesis. Arabidopsis and maize have one MMT gene, and at least three HMT genes that belong to two anciently diverged classes and encode enzymes with distinct properties and expression patterns. SMM, and presumably its cycle, must therefore have originated before dicot and monocot lineages separated. Arabidopsis leaves, roots and developing seeds all express MMT and HMTs, and can metabolize (35S)Met to (35S)SMM and vice versa. The SMM cycle therefore operates throughout the plant. This appears to be a general feature of angiosperms, as digital gene expression profiles show that MMT and HMT are co‐expressed in leaves, roots and reproductive tissues of maize and other species. An in silico model of the SMM cycle in mature Arabidopsis leaves was developed from radiotracer kinetic measurements and pool size data. This model indicates that the SMM cycle consumes half the AdoMet produced, and suggests that the cycle serves to stop accumulation of AdoMet, rather than to prevent depletion of free Met. Because plants lack the negative feedback loops that regulate AdoMet pool size in other eukaryotes, the SMM cycle may be the main mechanism whereby plants achieve short‐term control of AdoMet level.

Journal

The Plant JournalWiley

Published: Mar 1, 2001

References

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