Identification of a cDNA fragment encoding a putative Selenocysteine methyltransferase

Se is incorporated into antioxidant selenoproteins, making it an essential micronutrient for animals, microorganisms and some other eukaryotes. In addition to the basic nutritional requirements for Se, it has become increasingly evident that Se also has potential health benefits. These include roles in reducing the incidence of some debilitating disorders, such as in improving male fertility and immune function; in reducing viral infection; and in slowing the aging process. In addition, a large body of convincing evidence has indicated that Se acts as a cancer preventive agent when given in pharmacological amounts. It is well established that supplementation of the diet with Se helps reduce the incidence of certain cancers and boots the immune system. Anticarcinogenic and chemoprotective effects of Se have been demonstrated, too. In a long-term, double-blind study, supplemental dietary Se was associated with significant reductions in lung, colorectal and prostate cancer in humans. Numerous studies have demonstrated the efficacy of methylselenocysteine (MeSeCys) in preventing animal mammary cancer. This non-protein seleno amino acid is produced in certain plants including members of the Brassica and Allium genera. While the specific mechanism for the anticancer activity of Se has not been fully elucidated, multiple studies have demonstrated the ability of Se to affect the cell cycle and induce apoptosis in cancer cell line. However, excess Se can be toxic, and its bioaccumulation is toxic to wildlife.

Se is biochemically similar to S, and given that the characteristics of Se parallel those of S, it may compete with S in many of the key enzymatic steps of the S-assimilation pathway. The two elements are thought to be taken up and metabolized via the same mechanisms. Most plants contain only low foliar concentrations of Se, of less than 25 mg/kg dry weight, and rarely exceed 100 mg/kg dry weight even when grown on high-Se soils. They are termed non-accumulators. However, a limited number of specialized plants, which are often found growing on soils that are naturally enriched in Se, can accumulate high concentrations of Se in their foliage. These accumulating plants can be divided into two groups: primary accumulators (hyperaccumulators) and secondary accumulators (indicator species). Primary accumulators have discrimination coefficients of more than one, and have concentrations of Se in the range of thousands of mg/kg. Secondary accumulators take up Se in proportion to the amount of Se available in the soil, they have a DCi less than one, and tissue concentrations of Se in the hundreds of mg/kg. The genus Astragalus contains the highest number of selenium (Se)-hyperaccumulating species, with some of these species accumulating up to 0.6% of their foliar dry weight as Se.

Structure of Seleno-L-cysteine – CAS 10236-58-5

Astragalus bisulcatus metabolizes>90% of the accumulated Se into MeSeCys in young shoot tissue. MeSeCys is synthesized from selenocysteine and S-methylmethionine by the enzyme, selenocysteine methyltransferase (SMT). SMT with the capacity to use S-methylmethionine to methylate SeCys and produce MeSeCys has been isolated from the Se-hyperaccumulator Astragalus bisulcatus. Heterologous expression of AbSMT in Arabidopsis thaliana and Indian mustard increase selenium tolerance and accumulation. Recently, Sors et al. showed that SMT-like protein is present both in Se accumulating and in non-accumulating Astragalus species. According to this knowledge, in this study, we report the isolation of the fragment that could be the part of SMT-like gene from the secondary accumulator Astragalus chrysochlorus. The sequence of the fragment shows a high degree of homology (92%) with Astragalus bisulcatus SMT gene.

 

Reference:

Sule Arı • Ozgur Cakır • Neslihan Turgut-Kara. Acta Physiol Plant (2010) 32:1085–1092

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