Vitamin‐B₁₂‐Independent Methionine Synthase from a Higher Plant (Catharanthus Roseus)

Abstract

Methionine synthases catalyze the formation of methionine by the transfer of a methyl group from 5‐methyltetrahydrofolate to homocysteine. This reaction is the last step in l‐methionine biosynthesis, and it also serves to regenerate the methyl group of 5‐adenosylmethionine, a cofactor required for biological methylation reactions. We describe the cloning, expression and characterization of a methionine synthase from the higher plant Catharanthus roseus. cDNAs were identified that encoded a protein of 85 kDa sharing 50 % identity with the cobalamin‐independent methionine synthase from Escherichia coli (MetE) and 41 % identity with a partial sequence of a yeast homolog of MetE. The C. roseus protein was expressed at high levels in E. coli. The enzyme accepts the triglutamate form of methyltetrahydrofolate as a methyl donor but not the monoglutamate form, and it does not require 5‐adenosylmethionine or cobalamin for activity. The properties indicate that the enzyme is a cobalamin‐independent methionine synthase (EC 2.1.1.14). In contrast to the E. coli MetE, the plant protein does not require phosphate or magnesium ions for activity. Immunoblots of plant extracts showed that the protein was localized in the cytosol, and was present in a variety of plant species. A nutritional downshift of the C. roseus cell culture revealed a strong, transient transcriptional activation, but no significant increment in the total level of the protein. The availability of the protein and the cDNA now provide tools to investigate the complexities of methionine biosynthesis in plants.

Mutualistic interactions between vitamin B12‐dependent algae and heterotrophic bacteria exhibit regulation

“…In prokaryotes, there are over 20 enzymes that have a cobalamin cofactor (Marsh, 1999), while in animals there are just two B 12 ‐dependent enzymes, methylmalonyl‐CoA mutase, involved in odd‐chain‐fatty‐acid metabolism in the mitochondria, and methionine synthase (METH), which catalyses the C1 transfer from methylhydrofolate to homocysteine to make methionine (Drummond et al ., 1993). Vitamin B 12 is not found in, or required by, land plants or fungi, because they contain no cobalamin‐dependent enzymes (Eichel et al ., 1995), and instead encode a B 12 ‐independent form of methionine synthase (METE). In contrast, the vitamin is widespread throughout the algal kingdom, and algae such as the seaweed Porphyra yezoensis (nori), are an extremely rich dietary source of cobalamin (Watanabe et al .…”

Alteration in the gene expression ofGlehnia littoralisseedlings exposed to culture filtrate ofPenicillium citrinumKACC43900

“…These enzymes are required for the regeneration of the methyl group of S-adenosyl-Lmethionine, a methyl donor for several biological methylation reactions (Eichel et al 1995). Expression of microspores specific WRKY34 was enhanced by more than 2-fold in treated plant samples.…”

Fire and Brimstone: Molecular Interactions between Sulfur and Glucosinolate Biosynthesis in Model and Crop Brassicaceae

“…Initially, it had been thought that the synthesis of Met required homocysteine to be transported from plastids to the cytoplasm, as there was no evidence of a Met synthase enzyme in the plastids (Eichel et al, 1995). However, following the identification of plastid-localized Met synthase, it was concluded that plastids are indeed capable of de novo synthesis of Met (Ravanel et al, 2004), and so a pool of Met could be sequestered within plastids that is not available for synthesis of aliphatic GSLs ( Figure 5 ).…”