Feedback regulation of methionine biosynthesis in plants

Methionine is an important amino acid because it is used not only as a building block for proteins, but also, after being converted to S-adenosyl-L-methionine (SAM), as a methyl donor in methylation reactions, a substrate of polyamine biosynthesis and, in plants, a substrate of ethylene biosynthesis (Fig. 1). Methionine is a member of the aspartate family of amino acids and is synthesized from aspartic acid. All the aspartate family amino acids (methionine, lysine, threonine and isoleucine) are essential amino acids; however, the methionine content in soybean seeds is not sufficient for the nutrition of non-ruminant animals. Therefore, the biosynthetic pathway and regulation of aspartate family amino acids have been extensively studied. In the 1980s, studies using Lemna revealed that cystathionine -synthase (CGS), which catalyzes the first committed step of methionine biosynthesis, is the regulatory point of methionine biosynthesis. However, unlike many of the key-step enzymes of biosynthetic pathways, CGS is not an allosteric enzyme, and the mechanism for its regulation has been a mystery for over 20 years.

1. mto1 mutants and the MTO1 region

In order to understand the mechanism of regulation of methionine biosynthesis, this laboratory exploited a genetic approach to isolate A. thaliana mutants that overaccumulate soluble methionine. The mutants accumulate 10- to 40-fold higher concentrations of soluble methionine than wild-type plants and were named mto, for methionine overaccumulation. The mto mutations map to 3 loci, and mto1 mutants are impaired in feedback regulation of CGS. CGS is encoded by the CGS1 gene in A. thaliana. Studies using mto1 mutants revealed that CGS is regulated during gene expression, particularly at the step of CGS1 mRNA degradation (1). In wild-type A. thaliana, degradation of CGS1 mRNA was enhanced about 2-fold by methionine feeding, whereas in the mto1 mutant, CGS1 mRNA was stable, irrespective of methionine feeding. mto1 mutations cluster in a short stretch of the CGS N-terminal region, which has an amino acid sequence that is highly conserved among plant CGSs. This region, termed the MTO1 region (77-RRNCSNIGVAQIVAA-91), acts in cis and is necessary for feedback regulation.

2. In vitro studies

The regulation of CGS1 can be reproduced in an in vitro translation system of wheat germ extract (2). Studies using wheat germ extract revealed that it is not methionine but rather SAM that acts as the effector of CGS1 regulation (2, Fig. 1), and that, prior to CGS1 mRNA degradation, SAM induces translation arrest at the Ser-94 codon located immediately downstream of the MTO1 region (3). A newly synthesized peptide passes through the ribosomal exit tunnel that penetrates the ribosomal large subunit. The exit tunnel is about 100 ? long and holds 30-40 amino acid residues, so that the nascent MTO1 peptide resides in the exit tunnel when SAM-induced translation arrest occurs (3). These observations imply that a ribosome translating CGS1 mRNA senses the SAM concentration in the cytosol, and when SAM is present in excess, the ribosome is arrested, which then triggers CGS1 mRNA degradation (Fig. 2). Upon translation arrest, the nascent MTO1 peptide takes on a compact conformation, which is linked with a couple of conformation changes in the ribosomal RNA residues around the exit tunnel.

3. Biological significance of the regulation

CGS is encoded by a nuclear gene and is translocated to chloroplasts. Although most steps in the aspartate family biosynthetic pathway take place in the chloroplasts, methionine and SAM are produced in the cytosol. Therefore, in order to tightly regulate SAM production in the cytosol, feedback regulation during CGS1 mRNA translation in the cytosol would be more beneficial to plants than allosteric regulation based on the SAM concentration in the chloroplast. This laboratory is currently focusing effort on the mechanism of both SAM sensing by the translating ribosome and CGS1 mRNA degradation near the 5′ edge of the arrested ribosome.

Reprinted from " Agricultural Sciences for Human Sustainability - Meeting the Challenges of Food Safety and Stable Food Production - " KAISEI PRESS

Naito Satoshi