Molecular mechanism of regulation of methionine biosynthesis by mRNA stability : Studies using mto1 mutants of Arabidopsis.

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Cystathionine gamma-synthase (CGS) of Arabidopsis has 563 amino acids and the first exon codes for 183 amino acids. Comparison of CGS sequence among higher plants reveals a 35 amino acid region ("MTO1 region") within the exon 1 that is highly conserved among plant species. This suggests that this region has a role in the regulation of CGS gene expression.Sequential deletion mutants within the CGS exon 1 was constructed from both the N-and C-termini. The deletion constructs were fused in-frame with a reporter gene, beta-glucuronidase (GUS), and placed under the control of cauliflower mosaic virus (CaMV) 35S RNA promoter and their response to Met application was analyzed by transient expression experiments. The results of GUS assay indicated that the constructs carrying the MTO1 region respond to Met application while those do not carry the MTO1 region do not. In addition, when mto1-1 mutation was introduced to those constructs that carry the MTO1 region, the response disappeared.Deletions in both sides of MTO1 were also constructed and tested for their response to Met application. The results suggest that the MTO1 region is sufficient for the downregulation in response to Met application.