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Background  Simultaneous cofermentation of glucose and xylose mixtures would be a cost-effective solution for the conversion of cellulosic biomass to high-value products. However, most yeasts ferment glucose and xylose sequentially due to glucose catabolite repression. A well known thermotolerant yeast,  Kluyveromyces marxianus , was selected for this work because it possesses cost-effective advantages over  Saccharomyces cerevisiae  for biofuel production from cellulosic biomass.    Results  In the present study, we employed a directed evolutionary approach using 2-deoxyglucose to develop a thermotolerant mutant capable of simultaneous cofermentation of glucose and xylose by alleviating catabolite repression. The selected mutant,  K. marxianus  SBK1, simultaneously cofermented 40 g/L glucose and 28 g/L xylose to produce 23.82 g/L ethanol at 40 °C. This outcome corresponded to a yield of 0.35 g/g and productivity of 0.33 g/L h, representing an 84% and 129% improvement, respectively, over the parental strain. Interestingly, following mutagenesis the overall transcriptome of the glycolysis pathway was highly downregulated in  K. marxianus  SBK1, except for glucokinase-1 (GLK1) which was 21-fold upregulated. Amino acid sequence of GLK1 from  K. marxianus  SBK1 revealed three amino acid mutations which led to more than 22-fold lower enzymatic activity compared to the parental strain.    Conclusions  We herein successfully demonstrated that the cofermentation of a sugar mixture is a promising strategy for the efficient utilization of cellulosic biomass by  K. marxianus  SBK1. Through introduction of additional biosynthetic pathways,  K.   marxianus  SBK1 could become a chassis-type strain for the production of fuels and chemicals from cellulosic biomass.    Electronic supplementary material  The online version of this article (10.1186/s13068-019-1431-x) contains supplementary material, which is available to authorized users.

Alleviation of catabolite repression in Kluyveromyces marxianus: the thermotolerant SBK1 mutant simultaneously coferments glucose and xylose