Improvement of valine and isobutanol production in sake yeast by Ala31Thr substitution in the regulatory subunit of acetohydroxy acid synthase

Increased production of isobutanol from xylose through metabolic engineering of Saccharomyces cerevisiae overexpressing transcription factor Znf1 and exogenous genes

Only trace amount of isobutanol is produced by the native Saccharomyces cerevisiae via degradation of amino acids. Despite several attempts using engineered yeast strains expressing exogenous genes, catabolite repression of glucose must be maintained together with high activity of downstream enzymes, involving iron-sulfur assimilation and isobutanol production. Here, we examined novel roles of nonfermentable carbon transcription factor Znf1 in isobutanol production during xylose utilization. RNA-seq analysis showed that Znf1 activates genes in valine biosynthesis, Ehrlich pathway and iron-sulfur assimilation while coupled deletion or downregulated expression of BUD21 further increased isobutanol biosynthesis from xylose. Overexpression of ZNF1 and xylose-reductase/dehydrogenase (XR-XDH) variants, a xylose-specific sugar transporter, xylulokinase, and enzymes of isobutanol pathway in the engineered S. cerevisiae pho13gre3Δ strain resulted in the superb ZNXISO strain, capable of producing high levels of isobutanol from xylose. The isobutanol titer of 14.809 ± 0.400 g/L was achieved, following addition of 0.05 g/L FeSO4.7H2O in 5 L bioreactor. It corresponded to 155.88 mg/g xylose consumed and + 264.75% improvement in isobutanol yield. This work highlights a new regulatory control of alternative carbon sources by Znf1 on various metabolic pathways. Importantly, we provide a foundational step toward more sustainable production of advanced biofuels from the second most abundant carbon source xylose.

Glazing Affects the Fermentation Process of Sake Brewed in Pottery

Sake (Japanese rice wine) was fermented in pottery for more than a millennium before wooden barrels were adopted to obtain a greater brewing capacity. Although a recently conducted analysis of sake brewed in pottery indicated that sake brewed in unglazed pottery contains more ethanol than that brewed in glazed pottery, little is known about the characteristics of sake brewed in pottery. In this study, we used two types of ceramic containers of identical size, one glazed and one unglazed, for small-scale sake brewing to evaluate the effects of glazing on fermentation properties. The following parameters were measured continuously in the sake samples over 3 weeks of fermentation: temperature, weight, ethanol concentration, and glucose concentration in sake mash. Taste-sensory values, minerals, and volatile components were also quantified in the final fermented sake mash. The results show that, in the unglazed containers, the temperature of the sake mash was lower and the weight loss was higher compared to the sake mash in the glazed containers. The quantity of ethanol and the levels of Na+, Fe3+, and Al3+ tended to be higher in the sake brewed in the unglazed pottery. A taste-sensory analysis revealed that umami and saltiness were also higher in the samples brewed in the unglazed pottery. These results suggest that glazing affects multiple fermentation parameters and the flavor of sake brewed in pottery. They may also suggest that the materials of the containers used in sake brewing generally affect the fermentation properties.