Evolutionary engineering: A powerful tool for improving industrially relevant properties of engineered Saccharomyces cerevisiae strains for cellulosic ethanol production

Under laboratory conditions, evolutionary engineering involves a systematic approach that is usually defined as a phenomenon of long term adaptation of cells under selective pressure, where variants of a cell population with a selective advantage gradually outcompete the initially dominating cells. The use of evolutionary engineering has proven to be very valuable for obtaining phenotypes of industrial microorganisms with improved properties, such as expanded substrate range, increased stress tolerance and efficient substrate utilization. Also, for Saccharomyces cerevisiae, the preferred organism for commercial bioethanol production from lignocellulosic feedstocks, adaptive evolution has been extensively used to select for industrially relevant phenotypes. For cellulosic ethanol production by S. cerevisiae, one of the main challenges is the efficient and fast conversion of pentoses (xylose and arabinose) to ethanol, especially in the presence of inhibitors. DSM has developed industrial advanced yeast strains that have been genetically engineered to enable the rapid fermentation of both pentoses and hexoses into ethanol. Evolutionary engineering techniques are part of the DSM R&D toolbox, aiming at continuous improvement of engineered yeast strains contributing in this way to the development of cost-effective, sustainable and efficient bioethanol production. This paper describes the work on improving the consumption rates of C5-sugars and the tolerance towards inhibitors in S. cerevisiae strains by applying adaptive evolution approaches, such as sequential batch (proprietary) and prolonged chemostat cultivations, under selective conditions. The improved ethanol fermentation kinetics of C5-sugars in synthetic media as well as in lignocellulosic hydrolysates, at an economically relevant dry matter content, will be presented.