Lactic acid production from xylose by engineered Saccharomyces cerevisiae without PDC or ADH deletion
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Production of lactic acid from renewable sugars has received growing attention as lactic acid can be used for making renewable and bio-based plastics. However, most prior studies have focused on production of lactic acid from glucose despite that cellulosic hydrolysates contain xylose as well as glucose. Microbial strains capable of fermenting both glucose and xylose into lactic acid are needed for sustainable and economic lactic acid production. In this study, we introduced a lactic acid-producing pathway into an engineered Saccharomyces cerevisiae capable of fermenting xylose. Specifically, ldhA from the fungi Rhizopus oryzae was overexpressed under the control of the PGK1 promoter through integration of the expression cassette in the chromosome. The resulting strain exhibited a high lactate dehydrogenase activity and produced lactic acid from glucose or xylose. Interestingly, we observed that the engineered strain exhibited substrate-dependent product formation. When the engineered yeast was cultured on glucose, the major fermentation product was ethanol while lactic acid was a minor product. In contrast, the engineered yeast produced lactic acid almost exclusively when cultured on xylose under oxygen-limited conditions. The yields of ethanol and lactic acid from glucose were 0.31 g ethanol/g glucose and 0.22 g lactic acid/g glucose, respectively. On xylose, the yields of ethanol and lactic acid were <0.01 g ethanol/g xylose and 0.69 g lactic acid/g xylose, respectively. These results demonstrate that lactic acid can be produced from xylose with a high yield by S. cerevisiae without deleting pyruvate decarboxylase, and the formation patterns of fermentations can be altered by substrates.
KeywordsLactic acid Xylose Saccharomyces cerevisiae Lignocellulose
TLT would like to thank the JBT Graduate Fellowship and acknowledge that this project was supported by the Agriculture and Food Research Initiative Competitive Grant No. 2015-67011-22806 from the USDA National Institute of Food and Agriculture. YSJ is affiliated with the Energy Biosciences Institute (EBI).
Compliance with ethical standards
This study was funded by the Agriculture and Food Research Initiative Competitive Grant No. 2016-67011-22806 from the USDA National Institute of Food and Agriculture. TLT received additional funding support from the University of Illinois at Urbana-Champaign’s JBT Graduate Fellowship.
Conflict of interest
The authors declare that they have no competing interests.
This article does not contain any studies with human participants or animals performed by any of the authors.
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