| Efficient utilization of pentose sugars (xylose and arabinose) is an essential requirement for economically viable ethanol production from cellulosic biomass. The desirable pentose-fermenting ethanologenic biocatalysts are the native microorganisms or the engineered derivatives without recruited exogenous gene(s). In this study, we used a metabolic evolution (adaptive selection) approach to improve a non-transgenic homoethanol Escherichia coli strain SZ420 (ldhA pflB ackA frdBC pdhR:: pflBp6-aceEF-lpd ) for xylose fermentation. An improved mutant, KC01, was evolved through a three-month metabolic evolution process. This evolved mutant increased pyruvate dehydrogenase activity by 100%, cell growth rate (h-1) by 23%, volumetric ethanol productivity by 65% and ethanol tolerance by 200%. These improvements enable KC01 to complete 50 g L-1 of xylose fermentation with an ethanol titer of 23 g L-1 and a yield of 90%. The improved cell growth and ethanol production of KC01 are likely attributed to its threefold increased ethanol tolerance. |
