Effect of sulfur availability on the integrity of amino acid biosynthesis in plants

Amino acid levels in plants are regulated by a complex interplay of regulatory circuits at the level of enzyme activities and gene expression. Despite the diversity of precursors involved in amino acid biosynthesis as providing the carbon backbones, the amino groups and, for the amino acids methionine and cysteine, the sulfhydryl group and despite the involvement of amino acids as substrates in various downstream metabolic processes, the plant usually manages to provide relatively constant levels of all amino acids. Here we collate data on how amino acid homeostasis is shifted upon depletion of one of the major biosynthetic constituents, i.e., sulfur. Arabidopsis thaliana seedlings exposed to sulfate starvation respond with a set of adaptation processes to achieve a new balance of amino acid metabolism. First, metabolites containing reduced sulfur (cysteine, glutathione, S-adenosylmethionine) are reduced leading to a number of downstream effects. Second, the relative excess accumulation of N over S triggers processes to dump nitrogen in asparagine, glutamine and further N-rich compounds like ureides. Third, the depletion of glutathione affects the redox and stress response system of the glutathione-ascorbate cycle. Thus, biosynthesis of aromatic compounds is triggered to compensate for this loss, leading to an increased flux and accumulation of aromatic amino acids, especially tryptophan. Despite sulfate starvation, the homeostasis is kept, though shifted to a new state. This adaptation process keeps the plant viable even under an adverse nutritional status.

Mechanisms to account for maintenance of the soluble methionine pool in transgenic Arabidopsis plants expressing antisense cystathionine gamma-synthase cDNA

To investigate the role of cystathionine gamma-synthase (CGS) in the regulation of methionine synthesis Arabidopsis plants were transformed with a full-length antisense CGS cDNA and transformants analysed. Plants that were heterozygous for the transgene showed a 20-fold reduction of CGS activity that was accompanied by severe growth retardation and morphological abnormalities, from germination to flowering. Application of exogenous methionine to the transgenic lines restored normal growth. Surprisingly, transformed Arabidopsis plants exhibited a modest decrease in methionine content (35% reduction of the wild-type level) but a seven-fold decrease in the soluble pool of S-methylmethionine (SMM), a compound that plays a major role in storage and transport of reduced sulphur and labile methyl moieties. Several mechanisms can account for the maintenance of the soluble pool of methionine. First, the observed 20-fold increase in O-phosphohomoserine, a substrate of CGS, could compensate for the depressed level of CGS polypeptide by increasing the net rate of catalysis supported by the remaining enzyme. Second, the transgenic plants exhibited a two-fold increased level of cystathionine beta-lyase, the second enzyme in the methionine biosynthetic pathway. This indicates that enzymes other than CGS are subjected to a regulatory control by methionine or one of its metabolites. In addition to these mechanisms affecting de novo methionine synthesis, the recruitment of SMM to produce methionine may account for the small change of methionine levels in transgenic lines.

The specific features of methionine biosynthesis and metabolism in plants

Plants, unlike other higher eukaryotes, possess all the necessary enzymatic equipment for de novo synthesis of methionine, an amino acid that supports additional roles than simply serving as a building block for protein synthesis. This is because methionine is the immediate precursor of S-adenosylmethionine (AdoMet), which plays numerous roles of being the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of polyamines and of the phytohormone ethylene. In addition, AdoMet has regulatory function in plants behaving as an allosteric activator of threonine synthase. Among the AdoMet-dependent reactions occurring in plants, methylation of cytosine residues in DNA has raised recent interest because impediment of this function alters plant morphology and induces homeotic alterations in flower organs. Also, AdoMet metabolism seems somehow implicated in plant growth via an as yet fully understood link with plant-growth hormones such as cytokinins and auxin and in plant pathogen interactions. Because of this central role in cellular metabolism, a precise knowledge of the biosynthetic pathways that are responsible for homeostatic regulation of methionine and AdoMet in plants has practical implications, particularly in herbicide design.

Cystathionine gamma-synthase from Arabidopsis thaliana: purification and biochemical characterization of the recombinant enzyme overexpressed in Escherichia coli

Cystathionine gamma-synthase catalyses the first reaction specific for methionine biosynthesis in plants, the gamma-replacement of the phosphoryl substituent of O-phosphohomoserine by cysteine. A cDNA encoding cystathionine gamma-synthase from Arabidopsis thaliana has been cloned and used to overexpress the enzyme in Escherichia coli. The native recombinant enzyme is a homotetramer composed of 53 kDa subunits, each being tightly associated with one molecule of pyridoxal 5'-phosphate that binds at lysine-379 of the protein precursor. The replacement reaction follows a Ping Pong mechanism with a Vmax of 33.6 units/mg and Km values of 2.5 mM and 460 microM for O-phosphohomoserine and cysteine respectively. The protective effect of O-phosphohomoserine against enzyme inactivation by propargylglycine indicated that the Kd for the substrate is approx. 1/2500 of its Km value. Thus most of these biochemical properties are similar to those previously reported for plant and bacterial cystathionine gamma-synthases. However, the plant enzyme differs markedly from its enterobacterial counterparts because it catalyses a very faint gamma-elimination of O-phosphohomoserine in the absence of cysteine, this process being about 1/2700 as fast as the gamma-replacement reaction and approx. 1/1500 as fast as the gamma-elimination catalysed by the E. coli enzyme. This huge difference could be attributed to the inability of the A. thaliana cystathionine gamma-synthase to accumulate a long-wavelength-absorbing species that is characteristic for the efficient gamma-elimination reaction catalysed by the enterobacterial enzyme.