Understanding the mechanism of glucose-induced relief of Rgt1-mediated repression in yeast

Combinational manipulation of transcription factors, CreA and ClbR, is a viable strategy to improve cellulolytic enzyme production in Aspergillus aculeatus

The production of cellulolytic enzymes in response to inducible carbon sources is mainly regulated at the transcriptional level in filamentous fungi. We have identified a cellobiose-response regulator (ClbR) controlling the expression of cellulolytic enzyme-encoding genes in Aspergillus aculeatus. However, the engineering potential of combining the deletion of transcriptional repressors with the overexpression of transcriptional activators to enhance enzyme production has not been analyzed. Here, we investigated the effect of the deletion of the transcriptional repressor creA and the overexpression of the transcriptional activator clbR in enzyme production in A. aculeatus. Here, we verified that a combination of creA deletion and clbR overexpression (Δc&OE) improved cellulase, β-1,4-xylanase, and β-glucosidase production. Cellulase and β-1,4-xylanase production increased 3.4- and 8.0-fold in Δc&OE compared with the host strain (MR12) at 96-h incubation, respectively. β-Glucosidase production in ΔcreA and Δc&OE increased approximately 5.0-fold compared with that in MR12 at 240-h incubation. Transcriptional analysis revealed that the increase in enzyme production was due to increased expression of cellobiohydrolase, endo-β-1,4-glucanase, β-1,4-xylanase, and β-glucosidase 1 (bgl1). Interestingly, bgl1 expression in ΔcreA increased in a dose-dependent manner in response to glucose. Thus, combinational manipulation of transcription factors improved cellulase, xylanase, and β-glucosidase production in A. aculeatus.

An extracellular glucose sensor for substrate-dependent secretion and display of cellulose-degrading enzymes

The efficient hydrolysis of lignocellulosic biomass into fermentable sugars is key for viable economic production of biofuels and biorenewable chemicals from second-generation feedstocks. Consolidated bioprocessing (CBP) combines lignocellulose saccharification and chemical production in a single step. To avoid wasting valuable resources during CBP, the selective secretion of enzymes (independent or attached to the surface) based on the carbon source available is advantageous. To enable enzyme expression and secretion based on extracellular glucose levels, we implemented a G-protein-coupled receptor (GPCR)-based extracellular glucose sensor; this allows the secretion and display of cellulases in the presence of the cellulosic fraction of lignocellulose by leveraging cellobiose-dependent signal amplification. We focused on the glucose-responsiveness of the HXT1 promoter and engineered PHXT1 by changing its core to that of the strong promoter PTHD3 , increasing extracellular enzyme activity by 81%. We then demonstrated glucose-mediated expression and cell-surface display of the β-glucosidase BglI on the surface of Saccharomyces cerevisiae. The display system was further optimized by re-directing fatty acid pools from lipid droplet synthesis toward formation of membrane precursors via knock-out of PAH1. This resulted in an up to 4.2-fold improvement with respect to the baseline strain. Finally, we observed cellobiose-dependent signal amplification of the system with an increase in enzymatic activity of up to 3.1-fold when cellobiose was added.

Glucose Inhibits Yeast AMPK (Snf1) by Three Independent Mechanisms

Snf1, the fungal homologue of mammalian AMP-dependent kinase (AMPK), is a key protein kinase coordinating the response of cells to a shortage of glucose. In fungi, the response is to activate respiratory gene expression and metabolism. The major regulation of Snf1 activity has been extensively investigated: In the absence of glucose, it becomes activated by phosphorylation of its threonine at position 210. This modification can be erased by phosphatases when glucose is restored. In the past decade, two additional independent mechanisms of Snf1 regulation have been elucidated. In response to glucose (or, surprisingly, also to DNA damage), Snf1 is SUMOylated by Mms21 at lysine 549. This inactivates Snf1 and leads to Snf1 degradation. More recently, glucose-induced proton export has been found to result in Snf1 inhibition via a polyhistidine tract (13 consecutive histidine residues) at the N-terminus of the Snf1 protein. Interestingly, the polyhistidine tract plays also a central role in the response to iron scarcity. This review will present some of the glucose-sensing mechanisms of S. cerevisiae, how they interact, and how their interplay results in Snf1 inhibition by three different, and independent, mechanisms.

Sugar accumulation may be regulated by a transcriptional cascade of ABA-VvGRIP55-VvMYB15-VvSWEET15 in grape berries under root restriction

In the southern of China, precipitation is abundant during the grape growing season, which results in lower sugar content, and finally reduces the quality and yield of grape berries and leads to lower economic benefits. The root restriction cultivation method is an important abiotic stress that limits the disordered growth and development of roots, and it favors the accumulation of sugar and abscisic acid. However, the relationship between ABA and sugar accumulation under root restriction remains unclear. Here, we tested the expression levels of several transcription factors and sugar metabolism-related genes and found that root restriction cultivation could induce higher expression of VvMYB15 and VvSWEET15. The VvMYB15 transcription factor was found to bind to the promoter of VvSWEET15 and activate its expression, furthermore, transient overexpression of VvMYB15 in strawberry fruits and grape berries can promote sugar accumulation and increase the expression level of sugar metabolism-related genes, indicating that VvMYB15 is a positive regulator of sugar accumulation. In addition, the endogenous ABA content and expression level of VvGRIP55, which is highly responsive to ABA, were significantly increased under root restriction, and VvGRIP55 could bind to the promoter of VvMYB15 and activate its expression. Therefore, our results demonstrated that the ABA-responsive factor VvGRIP55 can promote sugar accumulation through VvMYB15 and VvSWEET15, suggesting a mechanism by which ABA regulates sugar accumulation under root restriction.

Identification of genes related to hydrolysis and assimilation of Agave fructans in Candida apicola NRRL Y-50540 and Torulaspora delbrueckii NRRL Y-50541 by de novo transcriptome analysis

Fructans are the main sugar in agave pine used by yeasts during mezcal fermentation processes, from which Candida apicola NRRL Y-50540 and Torulaspora delbrueckii NRRL Y-50541 were isolated. De novo transcriptome analysis was carried out to identify genes involved in the hydrolysis and assimilation of Agave fructans (AF). We identified a transcript annotated as SUC2, which is related to β-fructofuranosidase activity, and several differential expressed genes involved in the transcriptional regulation of SUC2 such as: MIG1, MTH1, SNF1, SNF5, REG1, SSN6, SIP1, SIP2, SIP5, GPR1, RAS2, and PKA. Some of these genes were specifically expressed in some of the yeasts according to their fructans assimilation metabolism. Different hexose transporters that could be related to the assimilation of fructose and glucose were found in both the transcriptomes. Our findings provide a better understanding of AF assimilation in these yeasts and provide resources for further metabolic engineering and biotechnology applications.

Genetic analysis argues for a coactivator function for the Saccharomyces cerevisiae Tup1 corepressor

The Tup1-Cyc8 corepressor complex of Saccharomyces cerevisiae is recruited to promoters by DNA-binding proteins to repress transcription of genes, including the a-specific mating-type genes. We report here a tup1(S649F) mutant that displays mating irregularities and an α-predominant growth defect. RNA-Seq and ChIP-Seq were used to analyze gene expression and Tup1 occupancy changes in mutant vs wild type in both a and α cells. Increased Tup1(S649F) occupancy tended to occur upstream of upregulated genes, whereas locations with decreased occupancy usually did not show changes in gene expression, suggesting this mutant not only loses corepressor function but also behaves as a coactivator. Based upon studies demonstrating a dual role of Tup1 in both repression and activation, we postulate that the coactivator function of Tup1(S649F) results from diminished interaction with repressor proteins, including α2. We also found that large changes in mating-type-specific gene expression between a and α or between mutant and wild type were not easily explained by the range of Tup1 occupancy levels within their promoters, as predicted by the classic model of a-specific gene repression by Tup1. Most surprisingly, we observed Tup1 occupancy upstream of the a-specific gene MFA2 and the α-specific gene MF(ALPHA)1 in cells in which each gene was expressed rather than repressed. These results, combined with the identification of additional mating-related genes upregulated in the tup1(S649F) α strain, illustrate that the role of Tup1 in distinguishing mating types in yeast appears to be both more comprehensive and more nuanced than previously appreciated.

Determinants of water source use, quality of water, sanitation and hygiene perceptions among urban households in North-West Ethiopia: A cross-sectional study

BACKGROUND

Clean water is an essential part of human healthy life and wellbeing. More recently, rapid population growth, high illiteracy rate, lack of sustainable development, and climate change; faces a global challenge in developing countries. The discontinuity of drinking water supply forces households either to use unsafe water storage materials or to use water from unsafe sources. The present study aimed to identify the determinants of water source types, use, quality of water, and sanitation perception of physical parameters among urban households in North-West Ethiopia.

METHODS

A community-based cross-sectional study was conducted among households from February to March 2019. An interview-based a pre-tested and structured questionnaire was used to collect the data. Data collection samples were selected randomly and proportional to each of the kebeles' households. MS Excel and R Version 3.6.2 were used to enter and analyze the data; respectively. Descriptive statistics using frequencies and percentages were used to explain the sample data concerning the predictor variable. Both bivariate and multivariate logistic regressions were used to assess the association between independent and response variables.

RESULTS

Four hundred eighteen (418) households have participated. Based on the study undertaken,78.95% of households used improved and 21.05% of households used unimproved drinking water sources. Households drinking water sources were significantly associated with the age of the participant (x2 = 20.392, df = 3), educational status (x2 = 19.358, df = 4), source of income (x2 = 21.777, df = 3), monthly income (x2 = 13.322, df = 3), availability of additional facilities (x2 = 98.144, df = 7), cleanness status (x2 = 42.979, df = 4), scarcity of water (x2 = 5.1388, df = 1) and family size (x2 = 9.934, df = 2). The logistic regression analysis also indicated that those factors are significantly determining the water source types used by the households. Factors such as availability of toilet facility, household member type, and sex of the head of the household were not significantly associated with drinking water sources.

CONCLUSION

The uses of drinking water from improved sources were determined by different demographic, socio-economic, sanitation, and hygiene-related factors. Therefore; the local, regional, and national governments and other supporting organizations shall improve the accessibility and adequacy of drinking water from improved sources in the area.

Rewired cellular signaling coordinates sugar and hypoxic responses for anaerobic xylose fermentation in yeast

Microbes can be metabolically engineered to produce biofuels and biochemicals, but rerouting metabolic flux toward products is a major hurdle without a systems-level understanding of how cellular flux is controlled. To understand flux rerouting, we investigated a panel of Saccharomyces cerevisiae strains with progressive improvements in anaerobic fermentation of xylose, a sugar abundant in sustainable plant biomass used for biofuel production. We combined comparative transcriptomics, proteomics, and phosphoproteomics with network analysis to understand the physiology of improved anaerobic xylose fermentation. Our results show that upstream regulatory changes produce a suite of physiological effects that collectively impact the phenotype. Evolved strains show an unusual co-activation of Protein Kinase A (PKA) and Snf1, thus combining responses seen during feast on glucose and famine on non-preferred sugars. Surprisingly, these regulatory changes were required to mount the hypoxic response when cells were grown on xylose, revealing a previously unknown connection between sugar source and anaerobic response. Network analysis identified several downstream transcription factors that play a significant, but on their own minor, role in anaerobic xylose fermentation, consistent with the combinatorial effects of small-impact changes. We also discovered that different routes of PKA activation produce distinct phenotypes: deletion of the RAS/PKA inhibitor IRA2 promotes xylose growth and metabolism, whereas deletion of PKA inhibitor BCY1 decouples growth from metabolism to enable robust fermentation without division. Comparing phosphoproteomic changes across ira2Δ and bcy1Δ strains implicated regulatory changes linked to xylose-dependent growth versus metabolism. Together, our results present a picture of the metabolic logic behind anaerobic xylose flux and suggest that widespread cellular remodeling, rather than individual metabolic changes, is an important goal for metabolic engineering.

Jailed Parents and Their Young Children: Residential Instability, Homelessness, and Behavior Problems

This study examined family disruption in the form of jailed parents' housing instability in the year leading up to their most recent incarceration, including periods of homelessness with and without their children, and links between parental housing instability and children's behavior problems. Using the Family Stress Proximal Process Model to understand the links between stressors related to family disruption and child outcomes, the study analyzed data from interviews and surveys with 165 jailed fathers and mothers with young children (age 2-6 years) regarding jailed parents' reports of housing instability during the 12 months prior to their incarceration and child behavior problems. Analyses showed that housing instability, homelessness, and recidivism in jailed parents were relatively common, with a significant proportion of the disruptions occurring with young children, although many disruptions involved parental absence from children. Results indicated that the more months that parents lived with their children prior to incarceration in jail during the past year, the less housing instability the parents experienced. Additionally, multiple regression analyses revealed that more housing instability experienced by parents in the year leading up to their incarceration in jail were associated with elevations in children's internalizing and externalizing behavior problems. These results have implications for future research that explores family disruption as a mechanism in understanding recidivism and homelessness among adults and risk for child behavior problems in families affected by parental incarceration.

Superior protein titers in half the fermentation time: Promoter and process engineering for the glucose-regulated GTH1 promoter of Pichia pastoris

Protein production in Pichia pastoris is often based on the methanol‐inducible P _AOX1 promoter which drives the expression of the target gene. The use of methanol has major drawbacks, so there is a demand for alternative promoters with good induction properties such as the glucose‐regulated P _GTH1 promoter which we reported recently. To further increase its potential, we investigated its regulation in more details by the screening of promoter variants harboring deletions and mutations. Thereby we could identify the main regulatory region and important putative transcription factor binding sites of P _GTH1. Concluding from that, yeast metabolic regulators, monomeric Gal4‐class motifs, carbon source‐responsive elements, and yeast GC‐box proteins likely contribute to the regulation of the promoter. We engineered a P _GTH1 variant with greatly enhanced induction properties compared with that of the wild‐type promoter. Based on that, a model‐based bioprocess design for high volumetric productivity in a limited time was developed for the P _GTH1 variant, to employ a glucose fed‐batch strategy that clearly outperformed a classical methanol fed‐batch of a P _AOX1 strain in terms of titer and process performance.

Low RNA Polymerase III activity results in up regulation of HXT2 glucose transporter independently of glucose signaling and despite changing environment

Background

Saccharomyces cerevisiae responds to glucose availability in the environment, inducing the expression of the low-affinity transporters and high-affinity transporters in a concentration dependent manner. This cellular decision making is controlled through finely tuned communication between multiple glucose sensing pathways including the Snf1-Mig1, Snf3/Rgt2-Rgt1 (SRR) and cAMP-PKA pathways.

Results

We demonstrate the first evidence that RNA Polymerase III (RNAP III) activity affects the expression of the glucose transporter HXT2 (RNA Polymerase II dependent—RNAP II) at the level of transcription. Down-regulation of RNAP III activity in an rpc128-1007 mutant results in a significant increase in HXT2 mRNA, which is considered to respond only to low extracellular glucose concentrations. HXT2 expression is induced in the mutant regardless of the growth conditions either at high glucose concentration or in the presence of a non-fermentable carbon source such as glycerol. Using chromatin immunoprecipitation (ChIP), we found an increased association of Rgt1 and Tup1 transcription factors with the highly activated HXT2 promoter in the rpc128-1007 strain. Furthermore, by measuring cellular abundance of Mth1 corepressor, we found that in rpc128-1007, HXT2 gene expression was independent from Snf3/Rgt2-Rgt1 (SRR) signaling. The Snf1 protein kinase complex, which needs to be active for the release from glucose repression, also did not appear perturbed in the mutated strain.

Conclusions/Significance

These findings suggest that the general activity of RNAP III can indirectly affect the RNAP II transcriptional machinery on the HXT2 promoter when cellular perception transduced via the major signaling pathways, broadly recognized as on/off switch essential to either positive or negative HXT gene regulation, remain entirely intact. Further, Rgt1/Ssn6-Tup1 complex, which has a dual function in gene transcription as a repressor-activator complex, contributes to HXT2 transcriptional activation.

Nutrient Sensing at the Plasma Membrane of Fungal Cells

To respond to the changing environment, cells must be able to sense external conditions. This is important for many processes including growth, mating, the expression of virulence factors, and several other regulatory effects. Nutrient sensing at the plasma membrane is mediated by different classes of membrane proteins that activate downstream signaling pathways: nontransporting receptors, transceptors, classical and nonclassical G-protein-coupled receptors, and the newly defined extracellular mucin receptors. Nontransporting receptors have the same structure as transport proteins, but have lost the capacity to transport while gaining a receptor function. Transceptors are transporters that also function as a receptor, because they can rapidly activate downstream signaling pathways. In this review, we focus on these four types of fungal membrane proteins. We mainly discuss the sensing mechanisms relating to sugars, ammonium, and amino acids. Mechanisms for other nutrients, such as phosphate and sulfate, are discussed briefly. Because the model yeast Saccharomyces cerevisiae has been the most studied, especially regarding these nutrient-sensing systems, each subsection will commence with what is known in this species.

Regulation of carotenogenesis in the red yeast Xanthophyllomyces dendrorhous: the role of the transcriptional co-repressor complex Cyc8-Tup1 involved in catabolic repression

BACKGROUND

The yeast Xanthophyllomyces dendrorhous produces carotenoids of commercial interest, including astaxanthin and β-carotene. Although carotenogenesis in this yeast and the expression profiles of the genes controlling this pathway are known, the mechanisms regulating this process remain poorly understood. Several studies have demonstrated that glucose represses carotenogenesis in X. dendrorhous, suggesting that this pathway could be regulated by catabolic repression. Catabolic repression is a highly conserved regulatory mechanism in eukaryotes and has been widely studied in Saccharomyces cerevisiae. Glucose-dependent repression is mainly observed at the transcriptional level and depends on the DNA-binding regulator Mig1, which recruits the co-repressor complex Cyc8-Tup1, which then represses the expression of target genes. In this work, we studied the regulation of carotenogenesis by catabolic repression in X. dendrorhous, focusing on the role of the co-repressor complex Cyc8-Tup1.

RESULTS

The X. dendrorhous CYC8 and TUP1 genes were identified, and their functions were demonstrated by heterologous complementation in S. cerevisiae. In addition, cyc8 - and tup1 - mutant strains of X. dendrorhous were obtained, and both mutations were shown to prevent the glucose-dependent repression of carotenogenesis in X. dendrorhous, increasing the carotenoid production in both mutant strains. Furthermore, the effects of glucose on the transcript levels of genes involved in carotenogenesis differed between the mutant strains and wild-type X. dendrorhous, particularly for genes involved in the synthesis of carotenoid precursors, such as HMGR, idi and FPS. Additionally, transcriptomic analyses showed that cyc8 - and tup1 - mutations affected the expression of over 250 genes in X. dendrorhous.

CONCLUSIONS

The CYC8 and TUP1 genes are functional in X. dendrorhous, and their gene products are involved in catabolic repression and carotenogenesis regulation. This study presents the first report involving the participation of Cyc8 and Tup1 in carotenogenesis regulation in yeast.

The glucose metabolite methylglyoxal inhibits expression of the glucose transporter genes by inactivating the cell surface glucose sensors Rgt2 and Snf3 in yeast

Methylglyoxal (MG) is a cytotoxic by-product of glycolysis. MG inhibits the growth of glucose-fermenting yeast cells by inhibiting glycolysis. MG does so by inducing endocytosis and degradation of the cell-surface glucose sensors Rgt2 and Snf3, which are required for glucose induction of HXT (glucose transporter) gene expression.

Methylglyoxal (MG) is a cytotoxic by-product of glycolysis. MG has inhibitory effect on the growth of cells ranging from microorganisms to higher eukaryotes, but its molecular targets are largely unknown. The yeast cell-surface glucose sensors Rgt2 and Snf3 function as glucose receptors that sense extracellular glucose and generate a signal for induction of expression of genes encoding glucose transporters (HXTs). Here we provide evidence that these glucose sensors are primary targets of MG in yeast. MG inhibits the growth of glucose-fermenting yeast cells by inducing endocytosis and degradation of the glucose sensors. However, the glucose sensors with mutations at their putative ubiquitin-acceptor lysine residues are resistant to MG-induced degradation. These results suggest that the glucose sensors are inactivated through ubiquitin-mediated endocytosis and degraded in the presence of MG. In addition, the inhibitory effect of MG on the glucose sensors is greatly enhanced in cells lacking Glo1, a key component of the MG detoxification system. Thus the stability of these glucose sensors seems to be critically regulated by intracellular MG levels. Taken together, these findings suggest that MG attenuates glycolysis by promoting degradation of the cell-surface glucose sensors and thus identify MG as a potential glycolytic inhibitor.

Sugar and Glycerol Transport in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the process of transport of sugar substrates into the cell comprises a complex network of transporters and interacting regulatory mechanisms. Members of the large family of hexose (HXT) transporters display uptake efficiencies consistent with their environmental expression and play physiological roles in addition to feeding the glycolytic pathway. Multiple glucose-inducing and glucose-independent mechanisms serve to regulate expression of the sugar transporters in yeast assuring that expression levels and transporter activity are coordinated with cellular metabolism and energy needs. The expression of sugar transport activity is modulated by other nutritional and environmental factors that may override glucose-generated signals. Transporter expression and activity is regulated transcriptionally, post-transcriptionally and post-translationally. Recent studies have expanded upon this suite of regulatory mechanisms to include transcriptional expression fine tuning mediated by antisense RNA and prion-based regulation of transcription. Much remains to be learned about cell biology from the continued analysis of this dynamic process of substrate acquisition.

Cross-Talk between Carbon Metabolism and the DNA Damage Response in S. cerevisiae

Yeast cells with DNA damage avoid respiration, presumably because products of oxidative metabolism can be harmful to DNA. We show that DNA damage inhibits the activity of the Snf1 (AMP-activated) protein kinase (AMPK), which activates expression of genes required for respiration. Glucose and DNA damage upregulate SUMOylation of Snf1, catalyzed by the SUMO E3 ligase Mms21, which inhibits SNF1 activity. The DNA damage checkpoint kinases Mec1/ATR and Tel1/ATM, as well as the nutrient-sensing protein kinase A (PKA), regulate Mms21 activity toward Snf1. Mec1 and Tel1 are required for two SNF1-regulated processes-glucose sensing and ADH2 gene expression-even without exogenous genotoxic stress. Our results imply that inhibition of Snf1 by SUMOylation is a mechanism by which cells lower their respiration in response to DNA damage. This raises the possibility that activation of DNA damage checkpoint mechanisms could contribute to aerobic fermentation (Warburg effect), a hallmark of cancer cells.

Glucose repression in Saccharomyces cerevisiae

Glucose is the primary source of energy for the budding yeast Saccharomyces cerevisiae. Although yeast cells can utilize a wide range of carbon sources, presence of glucose suppresses molecular activities involved in the use of alternate carbon sources as well as it represses respiration and gluconeogenesis. This dominant effect of glucose on yeast carbon metabolism is coordinated by several signaling and metabolic interactions that mainly regulate transcriptional activity but are also effective at post-transcriptional and post-translational levels. This review describes effects of glucose repression on yeast carbon metabolism with a focus on roles of the Snf3/Rgt2 glucose-sensing pathway and Snf1 signal transduction in establishment and relief of glucose repression.

The role of Snf1 signaling in glucose repression and carbon metabolism in Saccharomyces cerevisae.

Assessing glucose uptake through the yeast hexose transporter 1 (Hxt1)

The transport of glucose across the plasma membrane is mediated by members of the glucose transporter family. In this study, we investigated glucose uptake through the yeast hexose transporter 1 (Hxt1) by measuring incorporation of 2-NBDG, a non-metabolizable, fluorescent glucose analog, into the yeast Saccharomyces cerevisiae. We find that 2-NBDG is not incorporated into the hxt null strain lacking all glucose transporter genes and that this defect is rescued by expression of wild type Hxt1, but not of Hxt1 with mutations at the putative glucose-binding residues, inferred from the alignment of yeast and human glucose transporter sequences. Similarly, the growth defect of the hxt null strain on glucose is fully complemented by expression of wild type Hxt1, but not of the mutant Hxt1 proteins. Thus, 2-NBDG, like glucose, is likely to be transported into the yeast cells through the glucose transport system. Hxt1 is internalized and targeted to the vacuole for degradation in response to glucose starvation. Among the mutant Hxt1 proteins, Hxt1^N370A and HXT1^W473A are resistant to such degradation. Hxt1^N370A, in particular, is able to neither uptake 2-NBDG nor restore the growth defect of the hxt null strain on glucose. These results demonstrate 2-NBDG as a fluorescent probe for glucose uptake in the yeast cells and identify N370 as a critical residue for the stability and function of Hxt1.

The Valley-of-Death: reciprocal sign epistasis constrains adaptive trajectories in a constant, nutrient limiting environment

The fitness landscape is a powerful metaphor for describing the relationship between genotype and phenotype for a population under selection. However, empirical data as to the topography of fitness landscapes are limited, owing to difficulties in measuring fitness for large numbers of genotypes under any condition. We previously reported a case of reciprocal sign epistasis (RSE), where two mutations individually increased yeast fitness in a glucose-limited environment, but reduced fitness when combined, suggesting the existence of two peaks on the fitness landscape. We sought to determine whether a ridge connected these peaks so that populations founded by one mutant could reach the peak created by the other, avoiding the low-fitness "Valley-of-Death" between them. Sequencing clones after 250 generations of further evolution provided no evidence for such a ridge, but did reveal many presumptive beneficial mutations, adding to a growing body of evidence that clonal interference pervades evolving microbial populations.