The N-Terminal Region of Arabidopsis Cystathionine gamma -Synthase Plays an Important Regulatory Role in Methionine Metabolism

Abstract

Cystathionine ␥-synthase (CGS) is a key enzyme of Met biosynthesis in bacteria and plants. Aligning the amino acid sequences revealed that the plant enzyme has an extended N-terminal region that is not found in the bacterial enzyme. However, this region is not essential for the catalytic activity of this enzyme, as deduced from the complementation test of an Escherichia coli CGS mutant. To determine the function of this N-terminal region, we overexpressed full-length Arabidopsis CGS and its truncated version that lacks the N-terminal region in transgenic tobacco (Nicotiana tabacum) plants. Transgenic plants expressing both types of CGS had a significant higher level of Met, S-methyl-Met, and Met content in their proteins. However, although plants expressing full-length CGS showed the same phenotype and developmental pattern as wild-type plants, those expressing the truncated CGS showed a severely abnormal phenotype. These abnormal plants also emitted high levels of Met catabolic products, dimethyl sulfide and carbon disulfide. The level of ethylene, the Met-derived hormone, was 40 times higher than in wild-type plants. Since the alien CGS was expressed at comparable levels in both types of transgenic plants, we further suggest that post-translational modification(s) occurs in this N-terminal region, which regulate CGS and/or Met metabolism. More specifically, since the absence of the N-terminal region leads to an impaired Met metabolism, the results further suggest that this region plays a role in protecting plants from a high level of Met catabolic products such as ethylene.The sulfur-containing amino acid Met is an important essential amino acid in animal nutrition. Apart from its role as a protein constituent and its central function in initiating mRNA translation, Met indirectly regulates a variety of cellular processes as the precursor of S-adenosyl-Met (SAM), the primary biological methyl group donor. SAM is also the precursor of plant metabolites such as ethylene, polyamines, vitamin B1, and the Fe-chelator mugineic acid (Anderson, 1990;Ma et al., 1995;Sun, 1998). In addition, Met also serves as a donor for secondary metabolites through S-methyl-Met (SMM; Mudd and Datko, 1990). As can be expected of its cellular importance, Met biosynthesis is subject to complex regulatory control whose mechanism is only now being gradually clarified. Two main elements of this complex regulation have recently been elucidated in plants. In the first, the Met level is controlled by competition between its first specific enzyme, cystathionine ␥-synthase (CGS), and Thr synthase, for their common substrate, O-phosphohomo-Ser. Evidence of this competition and its role in Met synthesis was recently obtained by analyzing a mto2-1 mutant of Arabidopsis. This mutant, in which the gene encoding Thr synthase is impaired, demonstrated a approximately 22-fold higher accumulation of soluble Met in rosette leaves than wild-type Arabidopsis (Bartlem et al., 2000).A second regulatory mechanism of Met synthesis in plants occurs at the level...

C1 metabolism and chlorophyll synthesis: the Mg‐protoporphyrin IX methyltransferase activity is dependent on the folate status

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Transgenic soybean plants overexpressing O-acetylserine sulfhydrylase accumulate enhanced levels of cysteine and Bowman–Birk protease inhibitor in seeds

“…Analysis of free amino acid content used the protocol of Hacham et al (2002). Briefly, dry seeds of transgenic and untransformed soybean were frozen in liquid nitrogen and ground to a fine powder in the presence of water:chloroform:methanol (3:5:12, by vol.).…”

Regulatory Role of Cystathionine-γ-Synthase and de novo Synthesis of Methionine in Ethylene Production during tomato Fruit Ripening

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