Biology of Pentose-Fermenting Yeasts

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On hydrolysis, lignocellulosic materials from plant biomass yield a mixture of hexose (glucose, mannose, galactose) and pentose (xylose, arabinose) sugars. Among these, glucose is the most abundant, followed by xylose, while the other sugars are present in much lower concentrations. To improve the economics of a biomass conversion process, it is imperative that potential bioconversion microorganisms be able to ferment both the hexoses and pentoses efficiently into fuels or chemicals. Unfortunately, the well-known and well-studied fermentative yeast Saccharomyces cerevisiae is unable to utilize xylose as a carbon source for growth, despite the fact that it possesses all the enzymes needed for xylose metabolism. The discovery of pentose-fermenting yeasts in the early 1980s by Canadian and US researchers was a significant milestone as it gave people the hope that pentoses can be fermented. However, studies with pentose-fermenting yeasts found that they suffer from glucose repression, low ethanol tolerance and poor performance in the presence of pretreatment-derived inhibitors. I have been studying pentose-fermenting yeasts for 32 years and I continue to be amazed at some of the new things I can learn from these fascinating microorganisms. My research has been focused on the biochemistry, physiology and genetic improvement of these yeasts, and this continues to be my long term goal. In this proposal, I plan to study two fundamental issues related to these yeasts. The first is related to the low inhibitor tolerance of some pentose-fermenting yeasts. I plan to use classical random mutagenesis method to improve the tolerance of the xylitol-producing Candida guilliermondii FTI 20037 to inhibitors found in lignocellulosic hydrolysates. Improved mutants will be subjected to whole genome sequencing analysis to identify all the mutations that collectively confer inhibitor tolerance. The second is related to the question as to why some pentose-fermenting yeasts exhibit such low ethanol tolerance. Many pentose-fermenting yeasts can produce up to 5% (w/v) ethanol only while some wine yeasts can produce up to 10% (w/v) ethanol. I plan to investigate if the plasma membrane bound enzyme ATPase may play a role in ethanol tolerance and hence in determining the maximum amount of ethanol that can be produced by different yeasts. I plan to hire two graduate students to help me tackle these fun projects. The students will be trained in yeast physiology, fermentation as well as biomass conversion. Results forthcoming from these fundamental studies will provide valuable knowledge about these interesting microorganisms and the information may ultimately be useful to devise strategies to improve pentose fermentation. The results should also be relevant to parties interested in alcohol toxicity and tolerance in general.