[Metabolic engineering for improving ethanol fermentation of xylose by wild yeast]

One yeast strain, which was isolated from 256 natural samples, was found to be able to utilize D-xylose effectively. On the basis of assimilation physiological and molecular biological tests, the yeast strain was identified as a strain of Candida tropicalis. Furthermore, metabolic engineering breeding strategy was applied to change the metabolic flux in order to increase ethanol productivity. In this study, the C. tropicalis was used as the host strain and the plasmid pYX212-XYL2, which was formerly constructed for over expression of XYL2 gene encoding xylitol dehydrogenase (XDH) from Pichia stipitis, was used as the backbone of the recombinant vector. A hygro gene was inserted into downstream position of XYL2 gene, meanwhile, the result plasmid pXY212-XYL2-Hygro transformed into C. tropicalis by electroporation. Thus, a recombinant yeast C. tropicalis XYL2-7 was obtained through hygromycin B resistance screening and its specific XDH activity was 0.5 u/mg protein, which was 3 times more than that of the parent strain. Additionally, the recombinant yeast was applied in the fermentation of xylose. Compared with the parent yeast, it was concluded that the xylitol yield in the broth decreased by 3 times, however, the ethanol yield increased by 5 times. The feasibility of ethanol production from xylose by C. tropicalis was firstly studied in this paper. These research results are helpful to advance the bioconversion of renewable resources (e. g. straw, wheat bran, and husk) to fuel ethanol.