Abstract
Vanillin is a potent fermentation inhibitor derived from the lignocellulosic biomass in biofuel production, and high concentrations of vanillin result in the pronounced repression of bulk translation in Saccharomyces cerevisiae. Studies on genes that are efficiently translated even in the presence of high concentrations of vanillin will be useful for improving yeast vanillin tolerance and fermentation efficiency. The BDH1 and BDH2 genes encode putative medium-chain alcohol dehydrogenase/reductases and their amino acid sequences are very similar to each other. Although BDH2 was previously suggested to be involved in vanillin tolerance, it has yet to be clarified whether Bdh1/Bdh2 actually contribute to vanillin tolerance and reductions in vanillin. Therefore, we herein investigated the effects of Bdh1 and Bdh2 on vanillin tolerance. bdh2Δ cells exhibited hypersensitivity to vanillin and slower reductions in vanillin than wild-type cells and bdh1Δ cells. Additionally, the overexpression of the BDH2 gene improved yeast tolerance to vanillin more efficiently than that of BDH1. Only BDH2 mRNA was efficiently translated under severe vanillin stress, however, both BDH genes were transcriptionally up-regulated. These results reveal the importance of Bdh2 in vanillin detoxification and confirm the preferential translation of the BDH2 gene in the presence of high concentrations of vanillin. The BDH2 promoter also enabled the expression of non-native genes under severe vanillin stress and furfural stress, suggesting its availability to improve of the efficiency of bioethanol production through modifications in gene expression in the presence of fermentation inhibitors.
Highlights
Biomass conversion inhibitors including vanillin, furans, and organic acids are inevitably formed during the saccharification pretreatment of the lignocellulosic biomass
We demonstrated the importance of BDH1 and BDH2 genes in tolerance to vanillin stress even though they were not detected as genes involved in vanillin tolerance in previous genome-wide studies (Endo et al, 2008; Pereira et al, 2011)
Since Adh6 and Adh7 catalyze the reduction of vanillin to vanillyl alcohol (Larroy et al, 2002), it appears to be quite feasible that Bdh1 and Bdh2 catalyze the reduction of vanillin to vanillyl alcohol
Summary
Biomass conversion inhibitors including vanillin, furans, and organic acids are inevitably formed during the saccharification pretreatment of the lignocellulosic biomass They are significant barriers to decreasing the production cost of bioethanol because of their toxicity to yeast cells (Palmqvist and Hahn-Hägerdal, 2000; Helle et al, 2003; Klinke et al, 2004; Jonsson et al, 2013). We recently reported that ADH7 mRNA, which encodes a NADPH-dependent medium-chain alcohol dehydrogenase, was efficiently translated under severe vanillin stress conditions (Larroy et al, 2002; Nguyen et al, 2015). We showed that the BDH2 promoter enabled the protein synthesis of non-native genes such as ADH6 and GFP under severe vanillin stress, indicating that the BDH2 promoter is useful for improving the stress tolerance and fermentation efficiency of yeast cells by modifying gene expression in lignocellulose hydrolysates
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