Biosynthesis of bioactive seleno-metabolites in <I>Saccharomyces cerevisiae</I>

Selenium is an essential micronutrient for animals, exploited by biological systems as a two-electron detoxification system for peroxides. Under the form of Seleno-cysteine (SeCys), selenium is present in at least 25 human proteins involved in the protection of cells from oxidative stress. The main source of dietary selenium comes from edible plants that are able to accumulate selenium from the soil by metabolizing and storing it under organic forms, such as Se-methionie (SeMet), SeCys, Seleno-methyl-cysteine (SeMCys) and gamma-glutamyl-SeMCys. These metabolites have been shown to have beneficial effects in clinical trials, in which supplementation of Se-metabolites reduced cancer incidence and mortality rate by nearly 50% and SeMCys has been shown to be the most effective molecule. An insufficient intake of Se-organic compounds is related to the fact that the diet represents the sole source of such compounds, whose content in plant is highly susceptible to environmental, seasonal and food processing factors. The main objective was the construction of a microbial cell factory based on the yeast <I>Saccharomyces cerevisiae</I> for the production of health promoting Se-metabolites. S.cerevisiae is able to accumulate SeMet when growing in the presence of Se through the sulphur assimilation pathway. However, the aim of this work was to generate a genetically engineered strain able to synthesize the most effective health promoting Se-metabolites, i.e. SeMCys and gamma-glutamyl-SeMCys, by introducing specific plant genes from Se-accumulator plants. The uptake of Se by yeast requires yeast growing under limiting concentration of sulphur that it is competing with Se for the same transporter and assimilation pathway. Since in these conditions Se becomes toxic and inhibits yeast growth, fed-batch cultivations have demonstrated to keep Se assimilation levels high, while minimizing Se toxic effects. The introduction of plant genes for the biosynthesis of SeMCys and derivatives demonstrated to redirect Se fluxes towards the biosynthesis of Se-methylated compounds. The results obtained demonstrate that the coupling of genetic engineering strategies with optimization of cultivation system is a promising approach for the establishment of a yeast cell factory for the production of yeast enriched with health promoting Se-compounds. Further optimization of the system might lead to the development of a novel nutraceutical preparation