S-methylmethionine is involved in the salinity tolerance of Arabidopsis thaliana plants at germination and early growth stages.

Methionine (Met) is biosynthesized by the activated methyl cycle and S-methylmethionine (SMM) cycle in one-carbon (C1) metabolism in plants. It is converted to S-adenosylmethionine (SAM) which serves as a precursor for many metabolites including glycinebetaine, methylated polyols, polyamines and ethylene which accumulate in plants in response to salinity. We have investigated how the Met biosynthetic pathway is regulated under saline conditions at the transcriptional level in Arabidopsis thaliana plants. Within Met biosynthesis-related genes, the expression of homocysteine methyltransferase (HMT) and methionine methyltransferase (MMT) genes in SMM cycle had altered toward increasing Met production by the presence of NaCl. We have determined the salinity tolerance of an Arabidopsis mmt mutant with an insertional mutation in the single copy of the AtMMT gene. Although the mmt mutant showed comparable germination and shoot growth with wild type under normal conditions, NaCl treatment caused severe repression of germination rate and shoot growth in the mmt mutant compared with in the wild type. These results indicate that the utilization of SMM is important for the salinity tolerance of Arabidopsis plants at the germination and early growth stages.

Changes in saccharide, amino acid and S-methylmethionine content during malting of barley grown with different nitrogen and sulfur status.

BACKGROUND: Changes in saccharide, amino acid and S-methylmethionine (SMM) concentrations and enzyme activities during the malting of barley grown with different nitrogen (N) and sulfur (S) supplementation were investigated in order to clarify their relationship with N and S fertiliser levels. RESULTS: Concentrations of N and S in barley grain were significantly increased by the addition of N to the culture soil. Application of N decreased the starch concentration in grain. On the other hand, higher N fertilisation increased the ß-glucan concentration in grain and malt, thus decreasing the accessibility of ß-glucanase to its substrates. Proteolytic enzyme activity was significantly higher in the absence (-N treatment) than in the presence (+N treatment) of N fertiliser, making the concentration of the majority of amino acids in malt slightly higher in the - N treatment. SMM was synthesised in grain after imbibition, and application of N increased the SMM content in malt. CONCLUSION: Although SMM can be controlled to a certain extent during kilning, a balanced supply of N and S during cultivation can also be helpful for the production of malt with lower SMM concentration. Adequate soil management is desirable to maintain the balance between good agronomic performance and high malt quality.

Purification and properties of S-adenosyl-L-methionine:L-methionine S-methyltransferase from Wollastonia biflora leaves.

The plant enzyme S-adenosylmethionine:methionine S-methyltransferase (EC 2.1.1.12, MMT) catalyzes the synthesis of S-methylmethionine. MMT was purified 620-fold to apparent homogeneity from leaves of Wollastonia biflora. The four-step purification included fractionation with polyethylene glycol, affinity chromatography on adenosine-agarose, anion exchange chromatography, and gel filtration. Protein yield was about 180 micrograms/kg of leaves. Estimates of molecular mass from sodium dodecyl sulfate-polyacrylamide gel electrophoresis and native gel filtration chromatography were, respectively, 115 and 450 kDa, suggesting a tetramer of 115-kDa subunits. The 115-kDa subunit was photoaffinity labeled by S-adenosyl[3H]methionine. Antibodies raised against W. biflora MMT recognized a 115-kDa polypeptide in partially purified MMT preparations from leaves of lettuce, cabbage, clover, and maize. The pH optimum of W. biflora MMT was 7.2. Kinetic analysis of substrate interaction and product inhibition patterns indicated an Ordered Bi Bi mechanism, with S-adenosylmethionine the first reactant to bind and S-adenosylhomocysteine the last product to be released. The enzyme catalyzed methylation of selenomethionine and ethionine, but not of S-methylcysteine, homocysteine, cysteine, or peptidylmethionine. Tests with other substrate analogs indicated that a free carboxyl group was required for enzyme activity, and that a free amino group was not.

Constitutive overexpression of cystathionine gamma-synthase in Arabidopsis leads to accumulation of soluble methionine and S-methylmethionine.

The committing step in Met and S-adenosyl-L-Met (SAM) synthesis is catalyzed by cystathionine gamma-synthase (CGS). Transgenic Arabidopsis plants overexpressing CGS under control of the cauliflower mosaic virus 35S promoter show increased soluble Met and its metabolite S-methyl-Met, but only at specific stages of development. The highest level of Met and S-methyl-Met was observed in seedling tissues and in flowers, siliques, and roots of mature plants where they accumulate 8- to 20-fold above wild type, whereas the level in mature leaves and other tissues is no greater than wild type. CGS-overexpressing seedlings are resistant to ethionine, a toxic Met analog. With these properties the transgenic lines resemble mto1, an Arabidopsis, CGS-mutant inactivated in the autogenous control mechanism for Met-dependent down-regulation of CGS expression. However, wild-type CGS was overexpressed in the transgenic plants, indicating that autogenous control can be overcome by increasing the level of CGS mRNA through transcriptional control. Several of the transgenic lines show silencing of CGS resulting in deformed plants with a reduced capacity for reproductive growth. Exogenous feeding of Met to the most severely affected plants partially restores their growth. Similar morphological deformities are observed in plants cosuppressed for SAM synthetase, even though such plants accumulate 250-fold more soluble Met than wild type and they overexpress CGS. The results suggest that the abnormalities associated with CGS and SAM synthetase silencing are due in part to a reduced ability to produce SAM and that SAM may be a regulator of CGS expression.