Robust high-performance S. cerevisiae strains for C5 fermentation in lignocellulosic ethanol production

The process consolidation of lignocellulose based ethanol production at industrial scale is now becoming a reality. Feedstocks, including agricultural crop residues, and wood and paper mill discards, represent a relatively cheap and abundant source of biomass. However, the sustainable production of ethanol requires the combination of efficient hydrolysis and successful fermentation. During biomass pretreatment and hydrolysis, several potential inhibitory compounds including weak acids, furaldehydes and phenolics are released, which reduces the microbial fermentation performance. While fermentation of lignocellulose based sugars such as glucose and xylose by recombinant yeasts have frequently been demonstrated in laboratory media, efficient fermentation in realistic process conditions is much more challenging.

The yeast Saccharomyces cerevisiae is currently the preferred microorganism for the fermentation step of lignocellulose ethanol production. While S. cerevisiae is naturally able to metabolize hexose sugars, genetic engineering is required to develop strains harboring heterologous pathways for the consumption of pentose sugars. However, lignocellulosic hydrolysates from different raw materials are very different in both sugar composition and inhibitor content. To meet the demand for strains suitable for different raw materials, we have developed a tool-box with a flexible modular system to generate collections of strains based on various S. cerevisiae strain backgrounds. Our xylose assimilation pathway based on a mutated xylose reductase (XR) eliminates xylitol formation and results in close to theoretical ethanol yields. C5/C6 fermentation in different lignocellulosic hydrolysates will be presented.