Molecular and biochemical characterization of the selenocysteine Se-methyltransferase gene and Se-methylselenocysteine synthesis in broccoli

Selenium (Se) plays an indispensable role in human nutrition and has been implicated to have important health benefits, including being a cancer preventative agent. While different forms of Se vary in their anticarcinogenic efficacy, Se-methylselenocysteine (SeMSC) has been demonstrated to be one of the most effective chemopreventative compounds. Broccoli (Brassica oleracea var. italica) is known for its ability to accumulate high levels of Se with the majority of the selenoamino acids in the form of Semethylselenocysteine. Therefore, it serves as a good model to study the regulation of SeMSC accumulation in plants. A cDNA encoding selenocysteine Se-methyltransferase, the key enzyme responsible for SeMSC formation, was cloned from broccoli using a homocysteine S-methyltransferase gene probe from Arabidopsis (Arabidopsis thaliana). This clone, designated as BoSMT, was functionally expressed in Escherichia coli, and its identity was confirmed by its substrate specificity in the methylation of selenocysteine. The BoSMT gene represents a single copy sequence in the broccoli genome. Examination of BoSMT gene expression and SeMSC accumulation in response to selenate, selenite, and sulfate treatments showed that the BoSMT transcript and SeMSC synthesis were significantly up-regulated in plants exposed to selenate but were low in plants supplied with selenite. Simultaneous treatment of selenate with selenite significantly reduced SeMSC production. In addition, high levels of sulfate suppressed selenate uptake, resulting in a dramatic reduction of BoSMT mRNA level and SeMSC accumulation. Our results reveal that SeMSC accumulation closely correlated with the BoSMT gene expression and the total Se status in tissues and provide important information for maximizing the SeMSC production in this beneficial vegetable plant.