Reconstruction of thermotolerant yeast by one-point mutation identified through whole-genome analyses of adaptively-evolved strains

Elucidation and engineering mitochondrial respiratory-related genes for improving bioethanol production at high temperature in Saccharomyces cerevisiae

Industrial manufacturing of bioproducts, especially bioethanol, can benefit from high-temperature fermentation, which requires the use of thermotolerant yeast strains. Mitochondrial activity in yeast is closely related to its overall metabolism. However, the mitochondrial respiratory changes in response to adaptive thermotolerance are still poorly understood and have been rarely utilized for developing thermotolerant yeast cell factories. Here, adaptive evolution and transcriptional sequencing, as well as whole-genome-level gene knockout, were used to obtain a thermotolerant strain of Saccharomyces cerevisiae. Furthermore, thermotolerance and bioethanol production efficiency of the engineered strain were examined. Physiological evaluation showed the boosted fermentation capacity and suppressed mitochondrial respiratory activity in the thermotolerant strain. The improved fermentation produced an increased supply of adenosine triphosphate required for more active energy-consuming pathways. Transcriptome analysis revealed significant changes in the expression of the genes involved in the mitochondrial respiratory chain. Evaluation of mitochondria-associated gene knockout confirmed that ADK1, DOC1, or MET7 were the key factors for the adaptive evolution of thermotolerance in the engineered yeast strain. Intriguingly, overexpression of DOC1 with TEF1 promoter regulation led to a 10.1% increase in ethanol production at 42 °C. The relationships between thermotolerance, mitochondrial activity, and respiration were explored, and a thermotolerant yeast strain was developed by altering the expression of mitochondrial respiration-related genes. This study provides a better understanding on the physiological mechanism of adaptive evolution of thermotolerance in yeast.

Experimental evolution of Saccharomyces uvarum at high temperature yields elevation of maximal growth temperature and loss of the mitochondrial genome

An organism's upper thermal tolerance is a major driver of its ecology and is a complex, polygenic trait. Given the wide variance in this critical phenotype across the tree of life, it is quite striking that this trait has not proven very evolutionarily labile in experimental evolution studies of microbes. In stark contrast to recent studies, William Henry Dallinger in the 1880s reported increasing the upper thermal limit of microbes he experimentally evolved by >40°C using a very gradual temperature ramping strategy. Using a selection scheme inspired by Dallinger, we sought to increase the upper thermal limit of Saccharomyces uvarum . This species has a maximum growth temperature of 34-35°C, considerably lower than S. cerevisiae . After 136 passages on solid plates at progressively higher temperatures, we recovered a clone that can grow at 36°C, a gain of ~1.5°C. Additionally, the evolved clone lost its mitochondrial genome and cannot respire. In contrast, an induced rho 0 derivative of the ancestor shows a decrease in thermotolerance. Also, incubation of the ancestor at 34°C for 5 days increased the frequency of petite mutants drastically compared to 22°C, supporting the notion that mutation pressure rather than selection drove loss of mtDNA in the evolved clone. These results demonstrate that S. uvarum 's upper thermal limit can be elevated slightly via experimental evolution and corroborate past observations in S. cerevisiae that high temperature selection schemes can inadvertently lead to production of the potentially undesirable respiratory incompetent phenotype in yeasts.

Evolutionary and reverse engineering to increase Saccharomyces cerevisiae tolerance to acetic acid, acidic pH, and high temperature

Saccharomyces cerevisiae scarcely grows on minimal media with acetic acid, acidic pH, and high temperatures. In this study, the adaptive laboratory evolution (ALE), whole-genome analysis, and reverse engineering approaches were used to generate strains tolerant to these conditions. The thermotolerant strain TTY23 and its parental S288C were evolved through 1 year, in increasing concentrations of acetic acid up to 12 g/L, keeping the pH ≤ 4. Of the 18 isolated strains, 9 from each ancestor, we selected the thermo-acid tolerant TAT12, derived from TTY23, and the acid tolerant AT22, derived from S288C. Both grew in minimal media with 12 g/L of acetic acid, pH 4, and 30 °C, and produced ethanol up to 29.25 ± 6 mmol/g_DCW/h-neither of the ancestors thrived in these conditions. Furthermore, only the TAT12 grew on 2 g/L of acetic acid, pH 3, and 37 °C, and accumulated 16.5 ± 0.5 mmol/g_DCW/h of ethanol. Whole-genome sequencing and transcriptomic analysis of this strain showed changes in the genetic sequence and transcription of key genes involved in the RAS-cAMP-PKA signaling pathway (RAS2, GPA2, and IRA2), the heat shock transcription factor (HSF1), and the positive regulator of replication initiation (SUM1), among others. By reverse engineering, the relevance of the combined mutations in the genes RAS2, HSF1, and SUM1 to the tolerance for acetic acid, low pH, and high temperature was confirmed. Alone, the RAS2 mutation yielded acid tolerance and HSF1 nutation thermotolerance. Increasing the thermo-acidic niche and acetic acid tolerance of S. cerevisiae can contribute to improve economic ethanol production. KEY POINTS: • Thermo-acid tolerant (TAT) yeast strains were generated by adaptive laboratory evolution. • The strain TAT12 thrived on non-native, thermo-acidic harmful conditions. • Mutations in RAS2, HSF1, and SUM1 genes rendered yeast thermo and acid tolerant.

D-Xylose Sensing in Saccharomyces cerevisiae: Insights from D-Glucose Signaling and Native D-Xylose Utilizers

Extension of the substrate range is among one of the metabolic engineering goals for microorganisms used in biotechnological processes because it enables the use of a wide range of raw materials as substrates. One of the most prominent examples is the engineering of baker's yeast Saccharomyces cerevisiae for the utilization of d-xylose, a five-carbon sugar found in high abundance in lignocellulosic biomass and a key substrate to achieve good process economy in chemical production from renewable and non-edible plant feedstocks. Despite many excellent engineering strategies that have allowed recombinant S. cerevisiae to ferment d-xylose to ethanol at high yields, the consumption rate of d-xylose is still significantly lower than that of its preferred sugar d-glucose. In mixed d-glucose/d-xylose cultivations, d-xylose is only utilized after d-glucose depletion, which leads to prolonged process times and added costs. Due to this limitation, the response on d-xylose in the native sugar signaling pathways has emerged as a promising next-level engineering target. Here we review the current status of the knowledge of the response of S. cerevisiae signaling pathways to d-xylose. To do this, we first summarize the response of the native sensing and signaling pathways in S. cerevisiae to d-glucose (the preferred sugar of the yeast). Using the d-glucose case as a point of reference, we then proceed to discuss the known signaling response to d-xylose in S. cerevisiae and current attempts of improving the response by signaling engineering using native targets and synthetic (non-native) regulatory circuits.

From baker's yeast to genetically modified budding yeasts: the scientific evolution of bioethanol industry from sugarcane

The peculiarities of Brazilian fuel ethanol fermentation allow the entry of native yeasts that may dominate over the starter strains of Saccharomyces cerevisiae and persist throughout the sugarcane harvest. The switch from the use of baker's yeast as starter to selected budding yeasts obtained by a selective pressure strategy was followed by a wealth of genomic information that enabled the understanding of the superiority of selected yeast strains. This review describes how the process of yeast selection evolved in the sugarcane-based bioethanol industry, the selection criteria and recent advances in genomics that could advance the fermentation process. The prospective use of genetically modified yeast strains, specially designed for increased robustness and product yield, with special emphasis on those obtained by the CRISPR (clustered regularly interspaced palindromic repeats)-Cas9 (CRISPR-associated protein 9) genome-editing approach, is discussed as a possible solution to confer higher performance and stability to the fermentation process for fuel ethanol production.

Chromosome drives via CRISPR-Cas9 in yeast

Self-propagating drive systems are capable of causing non-Mendelian inheritance. Here, we report a drive system in yeast referred to as a chromosome drive that eliminates the target chromosome via CRISPR-Cas9, enabling the transmission of the desired chromosome. Our results show that the entire Saccharomyces cerevisiae chromosome can be eliminated efficiently through only one double-strand break around the centromere via CRISPR-Cas9. As a proof-of-concept experiment of this CRISPR-Cas9 chromosome drive system, the synthetic yeast chromosome X is completely eliminated, and the counterpart wild-type chromosome X harboring a green fluorescent protein gene or the components of a synthetic violacein pathway are duplicated by sexual reproduction. We also demonstrate the use of chromosome drive to preferentially transmit complex genetic traits in yeast. Chromosome drive enables entire chromosome elimination and biased inheritance on a chromosomal scale, facilitating genomic engineering and chromosome-scale genetic mapping, and extending applications of self-propagating drives.

Engineered Saccharomyces cerevisiae for lignocellulosic valorization: a review and perspectives on bioethanol production

The biorefinery concept, consisting in using renewable biomass with economical and energy goals, appeared in response to the ongoing exhaustion of fossil reserves. Bioethanol is the most prominent biofuel and has been considered one of the top chemicals to be obtained from biomass. Saccharomyces cerevisiae, the preferred microorganism for ethanol production, has been the target of extensive genetic modifications to improve the production of this alcohol from renewable biomasses. Additionally, S. cerevisiae strains from harsh industrial environments have been exploited due to their robust traits and improved fermentative capacity. Nevertheless, there is still not an optimized strain capable of turning second generation bioprocesses economically viable. Considering this, and aiming to facilitate and guide the future development of effective S. cerevisiae strains, this work reviews genetic engineering strategies envisioning improvements in ^2nd generation bioethanol production, with special focus in process-related traits, xylose consumption, and consolidated bioprocessing. Altogether, the genetic toolbox described proves S. cerevisiae to be a key microorganism for the establishment of a bioeconomy, not only for the production of lignocellulosic bioethanol, but also having potential as a cell factory platform for overall valorization of renewable biomasses.

Development and genomic elucidation of hybrid yeast with improved glucose-xylose co-fermentation at high temperature

Enhanced capability of co-fermenting glucose and xylose at high temperature is highly desirable for yeast application in second-generation bioethanol production. Here, we obtained hybrid strains with improved glucose-xylose co-fermentation properties at high temperature by combining genome shuffling and adaptive evolution. Genome resequencing of these strains suggested predominantly inherited genetic information from one parental strain Spathaspora passalidarum SP rather than the other parental strain Saccharomyces cerevisiae ScY01, possibly due to that the CUG codon system of S. passalidarum might have systematically eliminated most of the functional proteins from S. cerevisiae through misfolding. Compared to SP, one-copy loss of a 146-kb fragment was found in the hybrid strain and regained after being evolved for a while, whereas one-copy loss of an 11-kb fragment was only found after being evolved for a longer time. Besides, the genes affected by nonsynonymous variants were also identified, especially the mutation S540F in the endoplasmic reticulum chaperon Kar2. Structural prediction indicated that S540F might change the substrate binding activity of Kar2, and thus play a role in preventing protein aggregation in yeast at high temperature. Our results illustrated genomic alterations during this process and revealed some genomic factors that might be involved to determine yeast thermotolerance.

Development of Robust Yeast Strains for Lignocellulosic Biorefineries Based on Genome-Wide Studies

Lignocellulosic biomass has been widely studied as the renewable feedstock for the production of biofuels and biochemicals. Budding yeast Saccharomyces cerevisiae is commonly used as a cell factory for bioconversion of lignocellulosic biomass. However, economic bioproduction using fermentable sugars released from lignocellulosic feedstocks is still challenging. Due to impaired cell viability and fermentation performance by various inhibitors that are present in the cellulosic hydrolysates, robust yeast strains resistant to various stress environments are highly desired. Here, we summarize recent progress on yeast strain development for the production of biofuels and biochemical using lignocellulosic biomass. Genome-wide studies which have contributed to the elucidation of mechanisms of yeast stress tolerance are reviewed. Key gene targets recently identified based on multiomics analysis such as transcriptomic, proteomic, and metabolomics studies are summarized. Physiological genomic studies based on zinc sulfate supplementation are highlighted, and novel zinc-responsive genes involved in yeast stress tolerance are focused. The dependence of host genetic background of yeast stress tolerance and roles of histones and their modifications are emphasized. The development of robust yeast strains based on multiomics analysis benefits economic bioconversion of lignocellulosic biomass.

Traditional Norwegian Kveik Are a Genetically Distinct Group of Domesticated Saccharomyces cerevisiae Brewing Yeasts

The widespread production of fermented food and beverages has resulted in the domestication of Saccharomyces cerevisiae yeasts specifically adapted to beer production. While there is evidence beer yeast domestication was accelerated by industrialization of beer, there also exists a farmhouse brewing culture in western Norway which has passed down yeasts referred to as kveik for generations. This practice has resulted in ale yeasts which are typically highly flocculant, phenolic off flavor negative (POF-), and exhibit a high rate of fermentation, similar to previously characterized lineages of domesticated yeast. Additionally, kveik yeasts are reportedly high-temperature tolerant, likely due to the traditional practice of pitching yeast into warm (>28°C) wort. Here, we characterize kveik yeasts from 9 different Norwegian sources via PCR fingerprinting, whole genome sequencing of selected strains, phenotypic screens, and lab-scale fermentations. Phylogenetic analysis suggests that kveik yeasts form a distinct group among beer yeasts. Additionally, we identify a novel POF- loss-of-function mutation, as well as SNPs and CNVs potentially relevant to the thermotolerance, high ethanol tolerance, and high fermentation rate phenotypes of kveik strains. We also identify domestication markers related to flocculation in kveik. Taken together, the results suggest that Norwegian kveik yeasts are a genetically distinct group of domesticated beer yeasts with properties highly relevant to the brewing sector.

A Single-Nucleotide Insertion in a Drug Transporter Gene Induces a Thermotolerance Phenotype in Gluconobacter frateurii by Increasing the NADPH/NADP+ Ratio via Metabolic Change

Thermotolerant microorganisms are beneficial to the fermentation industry because they reduce the need for cooling and offer other operational advantages. Previously, we obtained a thermally adapted Gluconobacter frateurii strain by experimental evolution. In the present study, we found only a single G insertion in the adapted strain, which causes a frameshift in a gene encoding a putative drug transporter. A mutant derivative strain with the single G insertion in the transporter gene (Wild-G) was constructed from the wild-type strain and showed increased thermotolerance. We found that the thermotolerant strains accumulated substantial intracellular trehalose and manifested a defect in sorbose assimilation, suggesting that the transporter is partly involved in trehalose efflux and sorbose uptake and that the defect in the transporter can improve thermotolerance. The ΔotsAB strain, constructed by elimination of the trehalose synthesis gene in the wild type, showed no trehalose production but, unexpectedly, much better growth than the adapted strain at high temperatures. The ΔotsAB mutant produced more acetate as the final metabolite than the wild-type strain did. We hypothesized that trehalose does not contribute to thermotolerance directly; rather, a metabolic change including increased carbon flux to the pentose phosphate pathway may be the key factor. The NADPH/NADP⁺ ratio was higher in strain Wild-G, and much higher in the ΔotsAB strain, than in the wild-type strain. Levels of reactive oxygen species (ROS) were lower in the thermotolerant strains. We propose that the defect of the transporter causes the metabolic flux to generate more NADPH, which may enhance thermotolerance in G. frateuriiIMPORTANCE The biorefinery industry has to ensure that microorganisms are robust and retain their viability and function at high temperatures. Here we show that Gluconobacterfrateurii, an industrially important member of the acetic acid bacteria, exhibited enhanced thermotolerance through the reduction of trehalose excretion after thermal adaptation. Although intracellular trehalose may play a key role in thermotolerance, the molecular mechanisms of action of trehalose in thermotolerance are a matter of debate. Our mutated strain that was defective in trehalose synthase genes, producing no trehalose but a larger amount of acetic acid as the end metabolite instead, unexpectedly showed higher thermotolerance than the wild type. Our adapted and mutated thermotolerant strains showed increased NADPH/NADP⁺ ratios and reductions in ROS levels. We concluded that in G. frateurii, trehalose does not contribute to thermotolerance directly; rather, the metabolic change increases the NADPH/NADP⁺ ratio to enhance thermotolerance.

Improved ethanol production at high temperature by consolidated bioprocessing using Saccharomyces cerevisiae strain engineered with artificial zinc finger protein

In this work, the consolidated bioprocessing (CBP) yeast Saccharomyces cerevisiae MNII/cocδBEC3 was transformed by an artificial zinc finger protein (AZFP) library to improve its thermal tolerance, and the strain MNII-AZFP with superior growth at 42°C was selected. Improved degradation of acid swollen cellulose by 45.9% led to an increase in ethanol production, when compared to the control strain. Moreover, the fermentation of Jerusalem artichoke stalk (JAS) by MNII-AZFP was shortened by 12h at 42°C with a concomitant improvement in ethanol production. Comparative transcriptomics analysis suggested that the AZFP in the mutant exerted beneficial effect by modulating the expression of multiple functional genes. These results provide a feasible strategy for efficient ethanol production from JAS and other cellulosic biomass through CBP based-fermentation at elevated temperatures.

Precise genome-wide base editing by the CRISPR Nickase system in yeast

The CRISPR/Cas9 system has been applied to efficient genome editing in many eukaryotic cells. However, the bases that can be edited by this system have been limited to those within the protospacer adjacent motif (PAM) and guide RNA-targeting sequences. In this study, we developed a genome-wide base editing technology, "CRISPR Nickase system" that utilizes a single Cas9 nickase. This system was free from the limitation of editable bases that was observed in the CRISPR/Cas9 system, and was able to precisely edit bases up to 53 bp from the nicking site. In addition, this system showed no off-target editing, in contrast to the CRISPR/Cas9 system. Coupling the CRISPR Nickase system with yeast gap repair cloning enabled the construction of yeast mutants within only five days. The CRISPR Nickase system provides a versatile and powerful technology for rapid, site-specific, and precise base editing in yeast.

Molecular mechanisms of the yeast adaptive response and tolerance to stresses encountered during ethanol fermentation

During ethanol fermentation, yeast cells encounter various stresses including sugar substrates-induced high osmolarity, increased ethanol concentration, oxygen metabolism-derived reactive oxygen species (ROS), and elevated temperature. To cope with these fermentation-associated stresses, appropriate adaptive responses are required to prevent stress-induced cellular dysfunctions and to acquire stress tolerances. This review will focus on the cellular effects of these stresses, molecular basis of the adaptive response to each stress, and the cellular mechanisms contributing to stress tolerance. Since a single stress can cause diverse effects, including specific and non-specific effects, both specific and general stress responses are needed for achieving comprehensive protection. For instance, the high-osmolarity glycerol (HOG) pathway and the Yap1/Skn7-mediated pathways are specifically involved in responses to osmotic and oxidative stresses, respectively. On the other hand, due to the common effect of these stresses on disturbing protein structures, the upregulation of heat shock proteins (HSPs) and trehalose is induced upon exposures to all of these stresses. A better understanding of molecular mechanisms underlying yeast tolerance to these fermentation-associated stresses is essential for improvement of yeast stress tolerance by genetic engineering approaches.