Enhancement of xylose uptake in 2-deoxyglucose tolerant mutant of Saccharomyces cerevisiae

d-Xylose consumption by nonrecombinant Saccharomyces cerevisiae: A review

Xylose is the second most abundant sugar in nature. Its efficient fermentation has been considered as a critical factor for a feasible conversion of renewable biomass resources into biofuels and other chemicals. The yeast Saccharomyces cerevisiae is of exceptional industrial importance due to its excellent capability to ferment sugars. However, although S. cerevisiae is able to ferment xylulose, it is considered unable to metabolize xylose, and thus, a lot of research has been directed to engineer this yeast with heterologous genes to allow xylose consumption and fermentation. The analysis of the natural genetic diversity of this yeast has also revealed some nonrecombinant S. cerevisiae strains that consume or even grow (modestly) on xylose. The genome of this yeast has all the genes required for xylose transport and metabolism through the xylose reductase, xylitol dehydrogenase, and xylulokinase pathway, but there seems to be problems in their kinetic properties and/or required expression. Self-cloning industrial S. cerevisiae strains overexpressing some of the endogenous genes have shown interesting results, and new strategies and approaches designed to improve these S. cerevisiae strains for ethanol production from xylose will also be presented in this review.

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

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.

ChiNet uncovers rewired transcription subnetworks in tolerant yeast for advanced biofuels conversion

Analysis of rewired upstream subnetworks impacting downstream differential gene expression aids the delineation of evolving molecular mechanisms. Cumulative statistics based on conventional differential correlation are limited for subnetwork rewiring analysis since rewiring is not necessarily equivalent to change in correlation coefficients. Here we present a computational method ChiNet to quantify subnetwork rewiring by statistical heterogeneity that enables detection of potential genotype changes causing altered transcription regulation in evolving organisms. Given a differentially expressed downstream gene set, ChiNet backtracks a rewired upstream subnetwork from a super-network including gene interactions known to occur under various molecular contexts. We benchmarked ChiNet for its high accuracy in distinguishing rewired artificial subnetworks, in silico yeast transcription-metabolic subnetworks, and rewired transcription subnetworks for Candida albicans versus Saccharomyces cerevisiae, against two differential-correlation based subnetwork rewiring approaches. Then, using transcriptome data from tolerant S. cerevisiae strain NRRL Y-50049 and a wild-type intolerant strain, ChiNet identified 44 metabolic pathways affected by rewired transcription subnetworks anchored to major adaptively activated transcription factor genes YAP1, RPN4, SFP1 and ROX1, in response to toxic chemical challenges involved in lignocellulose-to-biofuels conversion. These findings support the use of ChiNet in rewiring analysis of subnetworks where differential interaction patterns resulting from divergent nonlinear dynamics abound.

A unique hexokinase in Cryptosporidium parvum, an apicomplexan pathogen lacking the Krebs cycle and oxidative phosphorylation

Cryptosporidium parvum may cause virtually untreatable infections in AIDS patients, and is recently identified as one of the top four diarrheal pathogens in children in developing countries. Cryptosporidium differs from other apicomplexans (e.g., Plasmodium and Toxoplasma) by lacking many metabolic pathways including the Krebs cycle and cytochrome-based respiratory chain, thus relying mainly on glycolysis for ATP production. Here we report the molecular and biochemical characterizations of a hexokinase in C. parvum (CpHK). Our phylogenetic reconstructions indicated that apicomplexan hexokinases including CpHK were highly divergent from those of humans and animals (i.e., at the base of the eukaryotic clade). CpHK displays unique kinetic features that differ from those in mammals and Toxoplasma gondii (TgHK) in the preference towards various hexoses and its capacity to use ATP and other NTPs. CpHK also displays substrate inhibition by ATP. Moreover, 2-deoxy-D-glucose (2DG) could not only inhibit the CpHK activity, but also the parasite growth in vitro at concentrations nontoxic to host cells (IC(50) = 0.54 mM). While the exact action of 2-deoxy-D-glucose on the parasite is subject to further verification, our data suggest that CpHK and the glycolytic pathway may be explored for developing anti-cryptosporidial therapeutics.

Improving xylose utilization by recombinant Zymomonas mobilis strain 8b through adaptation using 2-deoxyglucose

BACKGROUND

Numerous attempts have been made to improve xylose utilization in Z. mobilis including adaptive approaches. However, no one has yet found a way to overcome the reduced xylose utilization observed in fermentations carried out in the presence of glucose as well as the inhibitory compounds found within pretreated and saccharified biomass. Our goal was to generate Z. mobilis strains that are more robust than the wildtype strain with increased productivity in fermenting the glucose and xylose present in PCS. Through adaptation in the presence of 2-deoxyglucose, we have generated Zymomonas mobilis strain #7, which is better suited to utilizing xylose in pretreated corn stover (PCS) fermentations in the presence of both glucose and model inhibitory compounds of acetate and furfural. Strain #7 over performed the parent strain 8b both on simultaneous saccharification and fermentation (SFF) of PCS and fermentation of saccharified PCS slurry. At 65% neutralized PCS liquor level, strain #7 used 86% of the xylose present in the liquor while strain 8b was not able to ferment the liquor under similar conditions. Similarly, under SSF process conditions with 20% total solids loading of PCS, strain #7 used more than 50% of the xylose present, while strain 8b did not utilize any xylose under this condition. We have further identified genetic alterations in strain #7 in relation to the parental strain 8b that may be responsible for these phenotypic enhancements.

RESULTS

We performed an extended lab-directed evolution of Z. mobilis strain 8b in the presence of acetate and a non-hydrolyzable glucose analogue 2-deoxyglucose. Following the adaptation, we identified and characterized numerous candidate strains and found a dramatic increase in xylose usage not only in shake flask, but also in a controlled PCS fermentation. We re-sequenced the genomes of evolved strains to identify genetic alterations responsible for these improved phenotypes, and identified two mutations that may be key to the improved xylose usage in these strains.

CONCLUSION

We have generated Z. mobilis strain #7, which can ferment xylose efficiently in the presence of toxins present in pretreated corn stover. Genetic alterations responsible for the improvement have been identified.