Ethanol production from cellobiose by Zymobacter palmae carrying the Ruminocuccus albus β-glucosidase gene

Its metabolic characteristics suggest Zymobacter palmae gen. nov., sp. nov. could serve as a useful new ethanol-fermenting bacterium, but its biotechnological exploitation would require certain genetic improvements. We therefore established a method for transforming Z. palmae using the broad-host vector plasmids pRK290, pMFY31 and pMFY40 as a source of transforming DNA. Using electroporation, the frequency of transformation was 105 to 106 transformants/μg of DNA. To confer the ability to ferment cellobiose, which is a hydrolysis product from cellulosic materials treated enzymatically or with acid, the β-glucosidase gene from Ruminococcus albus was introduced into Z. palmae, where its expression was driven by its endogenous promoter. About 56% of the enzyme expressed was localized on the cell-surface or in the periplasm. The recombinant Z. palmae could ferment 2% cellobiose to ethanol, producing 95% of the theoretical yield with no accumulation of organic acids as metabolic by-products. Thus, expression of β-glucosidase in Z. palmae expanded the substrate spectrum of the strain, enabling ethanol production from cellulosic materials.