ARS | Publication request: Engineering industrial Saccharomyces cerevisiae strains for xylose fermentation and comparison for switchgrass conversion

Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 2010
Publication Date: August 1, 2011
Citation: Hector, R.E., Dien, B.S., Cotta, M.A., Qureshi, N. 2011. Engineering industrial Saccharomyces cerevisiae strains for xylose fermentation and comparison for switchgrass conversion. Journal of Industrial Microbiology and Biotechnology. 38(9):1193-1202.

Interpretive Summary: Intense interest in achieving energy security and reducing carbon emissions has led to a global research effort towards developing renewable sources of liquid transportation fuels. Most of the bioethanol produced in the United States is from fermentation of corn starch. Expanding biofuel production further in the United States will largely depend upon developing lignocellulosic materials as feedstocks. Industrial-scale fermentation of lignocellulosic materials will require commercially-robust strains that are genetically stable and possess high ethanol yields and productivities. Additionally, they must be capable of fermenting all biomass-derived sugars, including the five-carbon sugar, xylose. Saccharomyces (brewer’s yeast) physiology and fermentation related properties vary broadly among industrial strains and genetic background has been shown to significantly influence these properties. The objectives of this study were to better understand the influence of genetic background on xylose fermentation in engineered brewer’s yeast and to screen for candidate strains for further strain development. Six new xylose-fermenting yeast strains were constructed from industrial parent strains. Genetic background strongly influenced the growth rate, xylose consumption, and ethanol yield. The three best strains were evaluated using switchgrass as a feedstock. The ability to ferment xylose to ethanol is considered essential for an economical bioethanol process. The new industrial strains developed in this study readily fermented the biomass-derived sugars and produced up to 17% more ethanol than the parent strains because of their ability to ferment xylose.

Technical Abstract: Saccharomyces physiology and fermentation related properties vary broadly among industrial strains. In this study, six industrial strains of varied genetic background were engineered to ferment xylose. Aerobic growth rates on xylose were 0.040 h**-1 to 0.167 h**-1. Fermentation of xylose, glucose/xylose mixtures, and ammonium pretreated switchgrass also showed a wide range of performance between strains. During xylose fermentation, xylose consumption rates were 0.17 to 0.31 g/l/h, with ethanol yields between 0.18 to 0.27 g/g and xylitol yields from 0.44 to 0.53 g/g. Yields of ethanol and the metabolite xylitol were correlated, indicating all of the strains had downstream limitations to xylose metabolism. The better performing engineered and parental strains were compared for conversion of alkaline pretreated switchgrass to ethanol. The engineered strains produced 13-17% more ethanol than the parental control strains because of their ability to ferment xylose.