text in heYeast its metabolic fermentation has been used for a long time already, it is involved in brewing of alcohols and baking. It is an old and frequently used microorganism in the biotechnology. Over the years, the yeast applications have become more commercially. Yeast is now used as a factory to produce not only chemicals and fuels, but also pharmaceuticals. In addition, yeast strains have been engineered to produce the compounds at industrial scale. Thanks to genetic engineering, not only native yeast compounds but as well non-native compounds are possible to generate in yeast. Most of these non-native compounds have pharmaceutical applications, like human insulin. Next to that, genetic engineering was involved in the transformation of yeast into a cell factory for production of chemicals and fuels, based on metabolic engineering. Thanks to tremendous increase in the research on yeast, it can now be used as a cell factory with more sustainable processes to produce a wide range of compounds. The yeast cell factories can now produce molecules for several purposes at industrial scale (Hong and Nielsen, 2012; Nielsen, 2015) .
Cell factory platform and synthetic biology
Yeast and its fermentation are core of the biorefineries. Biorefineries produce materials like fuels and chemicals from biomass. It enables production of building-block chemicals. Organic acids are a key group in the building-block chemicals that are produced by microbial processes. Building-blocks serve as precursors for industrially products. Sustainable microbial production of organic acids require cell factory platforms. One well known and widely used microbial cell factory is yeast. Yeast cell factory platforms have great potential in biorefineries, they can be used for organic acid production and the same yeast platform can also be combined with pathways from other organisms to produce complex compounds. It serves the biorefineries a wide range of different products (Sauer et al., 2008; Otero et al., 2013).
The cell factories need to meet the demands for next generation bioprocesses. Synthetic biology can engineer yeast cell factories in an efficient and controllable way. Synthetic biology is used to design and construct new biological parts, devices and systems by the use of genetic information; it introduces new biosynthetic pathways in natural, existing biological systems for useful applications. The research on and the engineering of yeast as novel cell factories for a wide range chemicals demonstrates us that that yeast can serve as a general cell factory platform for chemical production (Kav''ek et al., 2015). There are several examples of yeast engineering resulting in novel cell factories, two of them are summarized below.
Yeast as novel cell factories examples
Otero et al. used S. cerevisiae and engineered this yeast resulting in a novel succinic acid cell factory. Succinic acid is seen as a value chemical building block. This building-block has been produced in a bio-based manner in several prokaryotes, but cost-intensive acidification and precipitation were needed to get succinic acid as end product. Despite the fact that S. cerevisiae does not produce succinic acid naturally, the reporters still chose this yeast as a cell factory. It is often seen that a chosen cell factory does not produce the chemical of interest, due to the importance of several characteristics of the host. Some of these characteristics of yeast are its robustness, both aerobic and anaerobic conditions, scalable industrial production and its low pH tolerance. Next to that, yeast has a broad library of genetic engineering tools available. The metabolic engineering that was used in this experiment included multi-gene deletion and selection, based on directed evolutions, to identify a succinic acid producing mutant. Transcriptome guided metabolic engineering was used for analysis; physiological characterization combined with transcriptome data was used to identify second round metabolic engineering targets. The resulting strain had improved succinic acid titer and yield, with only a small decrease in growth rate. The reporters demonstrated with this that yeast by successful metabolic engineering serves as an attractive cell factory platform to produce chemicals of interest, like organic acids (Otero et al., 2013).
Galanie et al. reported an engineered yeast that produces opioids. They made use of synthetic biology to engineer the yeast in such way that it expressed genes that fulfilled more than 20 enzymatic activities, coming from plants, mammals, bacteria and yeast itself. They demonstrated that yeast can be used for producing complex natural products. Opioids are complex tertiary amines, and their synthesis in yeast through synthetic biology showed the potential of this technology and yeast. The reconstruction of biosynthetic pathways from other organisms requires that these pathways connect to the yeast metabolism. Thanks to the conservation of metabolism pathways between organisms, it has been possible to use a metabolite of the yeast as a precursor for the metabolic pathway from the other organism. The mevalonate pathway (Mev) can be used in this process, recruitment of this pathway serves the yeast to generate compounds that are normally produced and extracted from plants. This technology is further in development by several companies to produce yeast cell factories for compound production of interest (Galanie et al., 2015; Nielsen, 2015).
Strategies, potential and limits
In general there are two strategies for developing a suitable yeast cell factory for production of chemicals or other compounds. The first strategy is to select the best suitable host based on its performance. Several parameters are important in this performance, including the yield and tolerance to the product. This is often based on evolutionary optimization or random mutagenesis, also seen as fishing for the best. The second strategy involves optimization of a chosen well-known species. This optimization is based on engineering of the cell factories; synthetic biology seems an efficient and controllable for this optimization. Yeast is often the host of choice in this strategy. This is because of the huge knowledge about the yeast biology and the availability of many genetic engineering tools. Synthetic biology seems the next generation engineering in cell factories, but the synthetic biology research concerned to yeast lags behind compared to other organisms, like E.coli. Yeast is still an promising candidate as cell factory, but research about synthetic biology in yeast, like genome editing, need to catch up to enhance the potential of yeast as a synthetic host (Kav''ek et al., 2015).
Although the research on the synthetic biology in yeast lags behind, yeast is seen a powerful host for synthetic biology. This gives yeast a great potential to serve as cell factories. It has the ability to be easily engineered to produce small and big, but also complex molecules. As mentioned earlier, large knowledge about the yeast biology makes the yeast a potential cell factory, as well as the great expertise of yeast in the academia and industry. On the other hand, yeast as a cell factory also brings some limits with it. To produce a novel strain can be time- and resource-consuming. The yeast itself has often relatively a low yield, making it economically less attractive. However, it has been showed that engineering the strain can increase the yield (Jouhten et al., 2016).
Yeast has a great ability for sustainable production of chemicals like organic acids. Microbial production of organic acids are expanding the markets. Synthetic biology in cell factories, and especially yeast, has clear potentials for future production of these compounds. Synthetic biology is the next generation in genetic engineering and will allow the biotechnology to produce compounds of interest in sustainable manner. The vast knowledge, robustness, high sugar and low pH tolerance, and the industrial conditions of yeast overweight its relatively low yield and time-consuming work. Yeast can be engineered to produce a range of several compounds and has the potential to serve as a platform cell factory in biorefineries.
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