For sustainable and economic production of fuels and chemicals, various technologies to utilize plant-based renewable resources, so called cellulosic biomass, are being developed. One important consideration for economic and scale-up production is a choice of a type of fermentation processes: batch vs. continuous. While continuous processes confer various benefits over batch processes by reducing capital cost, labor cost, and energy demand, simultaneous co-fermentation of cellulosic sugars (glucose and xylose) under continuous culture conditions has not been demonstrated.
Continuous fermentation of the mixed sugars by microorganisms is difficult because of two reasons. One reason is a sequential consumption of mixed sugars due to glucose repression on other sugars. Especially, xylose fermentation is severely inhibited by the presence of glucose. Therefore, when a mixture of glucose and xylose are fermented, glucose and xylose are sequentially utilized. The other reason is an inefficient xylose uptake system. While xylose can be transported by hexose transporters, this non-specific xylose transport becomes extremely slow when xylose concentration is low. Nevertheless, sugar concentrations need to be kept near zero during continuous processes to minimize sugar loss through output stream.
This presentation will describe a series of genetic perturbations in yeast for efficient fermentation of glucose and xylose during continuous processes. First, mutations in hexokinases were introduced into a xylose-fermenting engineered yeast for simultaneous co-fermentation of glucose and xylose. Second, putative xylose transporter genes were overexpressed to efficiently uptake xylose at low concentrations. The resulting strains facilitated simultaneous co-fermentation of glucose and xylose during continuous culture conditions.