Isoamyl alcohol-induced morphological change in Saccharomyces cerevisiae involves increases in mitochondria and cell wall chitin content

Spoilage Potential of Contaminating Yeast Species Kluyveromyces marxianus, Pichia kudriavzevii and Torulaspora delbrueckii during Cold Storage of Skyr

This study investigated the spoilage potential of yeast strains Kluyveromyces marxianus (Km1, Km2 and Km3), Pichia kudriavzevii Pk1 and Torulaspora delbrueckii Td1 grown in skyr in cold storage. Yeast strains were isolated from skyr and identified by sequencing of the 26S rRNA gene. K. marxianus yeasts were grown in skyr to high numbers, generating large amounts of volatile organic compounds (VOC) associated with off-flavours, among them were alcohols (3-methyl-1-butanol, 2-methyl-1-propanol and 1-hexanol), esters (ethyl acetate and 3-methylbutyl acetate) and aldehydes (hexanal, methylbutanal and methylpropanal). Growth of P. kudriavzevii Pk1 led to moderate increases in several alcohols and esters (mostly, 3-methyl-1-butanol and ethyl acetate), whereas only minor shifts in VOCs were associated with T. delbrueckii Td2. The levels of the key aroma compounds, diacetyl and acetoin, were significantly decreased by all K. marxianus strains and P. kudriavzevii Pk1. In contrast to the other yeast species, K. marxianus was able to utilize lactose, producing ethanol and carbon dioxide. Based on the overall results, K. marxianus was characterised by the highest spoilage potential. The study revealed the differences between the yeast species in fermentative and spoilage activities, and clarified the role of yeast metabolites for off-flavour formation and quality defects in skyr during cold storage.

Comparative Genomic and Metabolomic Analysis of Termitomyces Species Provides Insights into the Terpenome of the Fungal Cultivar and the Characteristic Odor of the Fungus Garden of Macrotermes natalensis Termites

Macrotermitinae termites have domesticated fungi of the genus Termitomyces as food for their colony, analogously to human farmers growing crops. Termites propagate the fungus by continuously blending foraged and predigested plant material with fungal mycelium and spores (fungus comb) within designated subterranean chambers. To test the hypothesis that the obligate fungal symbiont emits specific volatiles (odor) to orchestrate its life cycle and symbiotic relations, we determined the typical volatile emission of fungus comb biomass and Termitomyces nodules, revealing α-pinene, camphene, and d-limonene as the most abundant terpenes. Genome mining of Termitomyces followed by gene expression studies and phylogenetic analysis of putative enzymes related to secondary metabolite production encoded by the genomes uncovered a conserved and specific biosynthetic repertoire across strains. Finally, we proved by heterologous expression and in vitro enzymatic assays that a highly expressed gene sequence encodes a rare bifunctional mono-/sesquiterpene cyclase able to produce the abundant comb volatiles camphene and d-limonene.

IMPORTANCE The symbiosis between macrotermitinae termites and Termitomyces is obligate for both partners and is one of the most important contributors to biomass conversion in the Old World tropic’s ecosystems. To date, research efforts have dominantly focused on acquiring a better understanding of the degradative capabilities of Termitomyces to sustain the obligate nutritional symbiosis, but our knowledge of the small-molecule repertoire of the fungal cultivar mediating interspecies and interkingdom interactions has remained fragmented. Our omics-driven chemical, genomic, and phylogenetic study provides new insights into the volatilome and biosynthetic capabilities of the evolutionarily conserved fungal genus Termitomyces, which allows matching metabolites to genes and enzymes and, thus, opens a new source of unique and rare enzymatic transformations.

[Elucidation of Phenomena Involving Cyanobacteria in Freshwater Ecosystem by Chemically Ecological Approach]

Lakes Sagami and Tsukui are reservoirs constructed by connecting to the Sagami River. Because of eutrophication of the lakes, cyanobacteria have appeared every year. This review deals with phenomena related to occurrence of cyanobacteria that have been observed for 40 years since 1974 at the lakes. These 40 years of observations raised three interesting issues including the retention of cyanobacteria on their surfaces. These phenomena have been attributed to the usual factors, such as illuminance, nutrition and water temperature, but our research results suggested that they cannot be resolved without the introduction of another factor. We have attempted to elucidate various phenomena involving cyanobacteria in lake ecosystems by chemical ecological methods using volatile organic compounds (VOCs) produced by the cyanobacteria as indicators. One of the VOCs, β-cyclocitral, was significantly involved in the above phenomena, which was considered to be produced by the carotenoid cleavage dioxygenase (CCD) of the cyanobacteria. β-Cyclocitral was not produced in the two known CCDs, but two additional CCDs to Microcystis aeruginosa participated to produce the β-cyclocitral. These CCDs did not directly produce β-cyclocitral, but it was accumulated in cells as their precursors. The released β-cyclocitral underwent a Baeyer-Villiger-like oxidation. It was speculated that Microcystis activated the CCD genes through density stress and produced β-cyclocitral, which acted as an allelopathic substance. As a result, the number of cells of cyanobacteria decreased, and the resulting nitrogen and phosphorus were fed to the living cyanobacteria. It is postulated that this "quorum sensing" was functioning in the above-mentioned issues.

Genomic, transcriptomic, and metabolic characterizations of Escherichia coli adapted to branched-chain higher alcohol tolerance

Microbial-produced branched-chain higher alcohols (BCHAs), such as isopropanol, isobutanol, and isopentanol in Escherichia coli, have emerged as promising alternative biofuels under development. Elucidating and improving the tolerance of E. coli to BCHAs are important issues for microbial production of BCHAs due to their physiological inhibitory effect. Previous works aimed at understanding the genetic basis of E. coli tolerance to BCHAs with a comparative genome, reverse engineering, or transcriptome approach have gained some important insights into the mechanism of tolerance. However, investigation on BCHA tolerance from the whole-genomic, transcriptomic, and metabolic levels via a systematic approach has not yet been completely elucidated. Here, in this study, genomic, transcriptomic, and ¹³C-metabolic flux analyses (¹³C-MFA) of an evolved E. coli strain adapted to BCHA tolerance were conducted. Genome mutation of negative regulation factor (rssB, acrB, and clpX) of RpoS level suggested upregulation of RpoS activity in BCHA tolerance of E. coli. From a more detailed perspective, enhanced energy metabolism was observed to be the main characteristic of E. coli strain tolerant to BCHAs. Enhanced energy metabolism has been achieved through several routes, which included redistribution of the central carbon metabolism, upregulation of the energy generation machinery, and facilitating the operation of electron transferring chain. Evidence of multiple solutions of genotype modification toward BCHA tolerance was also revealed through comparative analysis of previous works from different groups.

Small-Molecule Morphogenesis Modulators Enhance the Ability of 14-Helical β-Peptides To Prevent Candida albicans Biofilm Formation

Candida albicans is an opportunistic fungal pathogen responsible for mucosal candidiasis and systemic candidemia in humans. Often, these infections are associated with the formation of drug-resistant biofilms on the surfaces of tissues or medical devices. Increased incidence of C. albicans resistance to current antifungals has heightened the need for new strategies to prevent or eliminate biofilm-related fungal infections. In prior studies, we designed 14-helical β-peptides to mimic the structural properties of natural antimicrobial α-peptides (AMPs) in an effort to develop active and selective antifungal compounds. These amphiphilic, cationic, helical β-peptides exhibited antifungal activity against planktonic C. albicans cells and inhibited biofilm formation in vitro and in vivo Recent studies have suggested the use of antivirulence agents in combination with antifungals. In this study, we investigated the use of compounds that target C. albicans polymorphism, such as 1-dodecanol, isoamyl alcohol, and farnesol, to attempt to improve β-peptide efficacy for preventing C. albicans biofilms. Isoamyl alcohol, which prevents hyphal formation, reduced the minimum biofilm prevention concentrations (MBPCs) of β-peptides by up to 128-fold. Combinations of isoamyl alcohol and antifungal β-peptides resulted in less than 10% hemolysis at the antifungal MBPCs. Overall, our results suggest potential benefits of combination therapies comprised of morphogenesis modulators and antifungal AMP peptidomimetics for preventing C. albicans biofilm formation.

Comparative genomic analysis revealed rapid differentiation in the pathogenicity-related gene repertoires between Pyricularia oryzae and Pyricularia penniseti isolated from a Pennisetum grass

BACKGROUND

A number of Pyricularia species are known to infect different grass species. In the case of Pyricularia oryzae (syn. Magnaporthe oryzae), distinct populations are known to be adapted to a wide variety of grass hosts, including rice, wheat and many other grasses. The genome sizes of Pyricularia species are typical for filamentous ascomycete fungi [~ 40 Mbp for P. oryzae, and ~ 45 Mbp for P. grisea]. Genome plasticity, mediated in part by deletions promoted by recombination between repetitive elements [Genome Res 26:1091-1100, 2016, Nat Rev Microbiol 10:417-430,2012] and transposable elements [Annu Rev Phytopathol 55:483-503,2017] contributes to host adaptation. Therefore, comparisons of genome structure of individual species will provide insight into the evolution of host specificity. However, except for the P. oryzae subgroup, little is known about the gene content or genome organization of other Pyricularia species, such as those infecting Pennisetum grasses.

RESULTS

Here, we report the genome sequence of P. penniseti strain P1609 isolated from a Pennisetum grass (JUJUNCAO) using PacBio SMRT sequencing technology. Phylogenomic analysis of 28 Magnaporthales species and 5 non-Magnaporthales species indicated that P1609 belongs to a Pyricularia subclade, which is genetically distant from P. oryzae. Comparative genomic analysis revealed that the pathogenicity-related gene repertoires had diverged between P1609 and the P. oryzae strain 70-15, including the known avirulence genes, other putative secreted proteins, as well as some other predicted Pathogen-Host Interaction (PHI) genes. Genomic sequence comparison also identified many genomic rearrangements relative to P. oryzae.

CONCLUSION

Our results suggested that the genomic sequence of the P. penniseti P1609 could be a useful resource for the genetic study of the Pennisetum-infecting Pyricularia species and provide new insight into evolution of pathogen genomes during host adaptation.

Branched-Chain Aminotransferases Control TORC1 Signaling in Saccharomyces cerevisiae

The conserved target of rapamycin complex 1 (TORC1) integrates nutrient signals to orchestrate cell growth and proliferation. Leucine availability is conveyed to control TORC1 activity via the leu-tRNA synthetase/EGOC-GTPase module in yeast and mammals, but the mechanisms sensing leucine remain only partially understood. We show here that both leucine and its α-ketoacid metabolite, α-ketoisocaproate, effectively activate the yeast TORC1 kinase via both EGOC GTPase-dependent and -independent mechanisms. Leucine and α-ketoisocaproate are interconverted by ubiquitous branched-chain aminotransferases (BCAT), which in yeast are represented by the mitochondrial and cytosolic enzymes Bat1 and Bat2, respectively. BCAT yeast mutants exhibit severely compromised TORC1 activity, which is partially restored by expression of Bat1 active site mutants, implicating both catalytic and structural roles of BCATs in TORC1 control. We find that Bat1 interacts with branched-chain amino acid metabolic enzymes and, in a leucine-dependent fashion, with the tricarboxylic acid (TCA)-cycle enzyme aconitase. BCAT mutation perturbed TCA-cycle intermediate levels, consistent with a TCA-cycle block, and resulted in low ATP levels, activation of AMPK, and TORC1 inhibition. We propose the biosynthetic capacity of BCAT and its role in forming multicomplex metabolons connecting branched-chain amino acids and TCA-cycle metabolism governs TCA-cycle flux to activate TORC1 signaling. Because mammalian mitochondrial BCAT is known to form a supramolecular branched-chain α-keto acid dehydrogenase enzyme complex that links leucine metabolism to the TCA-cycle, these findings establish a precedent for understanding TORC1 signaling in mammals.

In all organisms from yeasts to mammals the target of rapamycin TORC1 pathway controls growth in response to nutrients such as leucine, but the leucine sensing mechanisms are only partially characterized. We show that both leucine and its α-ketoacid metabolite, α-ketoisocaproate, are similarly capable of activating TORC1 kinase via EGOC GTPase-dependent and -independent mechanisms. Activation of TORC1 by leucine or α-ketoisocaproate is only partially mediated via EGOC-GTPase. Leucine and α-ketoisocaproate are interconverted by ubiquitous branched-chain aminotransferases (BCAT). Disruption of BCAT caused reduced TORC1 activity, which was partially restored by expression of BCAT active site mutants, arguing for both structural and catalytic roles of BCAT in TORC1 control. We find BCAT interacts with several branched-chain amino acid metabolic enzymes, and in a leucine-dependent fashion with the tricarboxylic acid (TCA)-cycle enzyme aconitase. Both aconitase mutation or TCA-cycle inhibition impaired TORC1 activity. Mutation of BCAT resulted in a TCA-cycle intermediate profile consistent with a TCA-cycle block, low ATP levels, activation of AMPK, and TORC1 inhibition. Our results suggest a model whereby BCAT coordinates leucine and TCA cycle metabolism to control TORC1 signaling. Taken together, our findings forge key insights into how the TORC1 signaling cascade senses nutrients to control cell growth.

Mitochondrial signaling in Saccharomyces cerevisiae pseudohyphae formation induced by butanol

Yeasts growing limited for nitrogen source or treated with fusel alcohols form elongated cells--pseudohyphae. Absence of mitochondrial DNA or anaerobic conditions inhibits this process, but the precise role of mitochondria is not clear. We found that a significant percentage of pseudohyphal cells contained mitochondria with different levels of membrane potential within one cell. An uncoupler carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP), but not the ATP-synthase inhibitor oligomycin D, prevented pseudohyphal growth. Interestingly, repression of the MIH1 gene encoding phosphatase activator of the G2/M transition partially restores the ability of yeast to form pseudohyphal cells in the presence of FCCP or in the absence of mitochondrial DNA. At the same time, retrograde signaling (the one triggered by dysfunctional mitochondria) appeared to be a positive regulator of butanol-induced pseudohyphae formation: the deletion of any of the retrograde signaling genes (RTG1, RTG2, or RTG3) partially suppressed pseudohyphal growth. Together, our data suggest that two subpopulations of mitochondria are required for filamentous growth: one with high and another with low transmembrane potential. These mitochondria-activated signaling pathways appear to converge at Mih1p level.

Cell aggregations in yeasts and their applications

Yeasts can display four types of cellular aggregation: sexual, flocculation, biofilm formation, and filamentous growth. These cell aggregations arise, in some yeast strains, as a response to environmental or physiological changes. Sexual aggregation is part of the yeast mating process, representing the first step of meiotic recombination. The flocculation phenomenon is a calcium-dependent asexual reversible cellular aggregation that allows the yeast to withstand adverse conditions. Biofilm formation consists of multicellular aggregates that adhere to solid surfaces and are embedded in a protein matrix; this gives the yeast strain either the ability to colonize new environments or to survive harsh environmental conditions. Finally, the filamentous growth is the ability of some yeast strains to grow in filament forms. Filamentous growth can be attained by two different means, with the formation of either hyphae or pseudohyphae. Both hyphae and pseudohyphae arise when the yeast strain is under nutrient starvation conditions and they represent a means for the microbial strain to spread over a wide area to survey for food sources, without increasing its biomass. Additionally, this filamentous growth is also responsible for the invasive growth of some yeast.

Dysfunctional mitochondria modulate cAMP-PKA signaling and filamentous and invasive growth of Saccharomyces cerevisiae

Mitochondrial metabolism is targeted by conserved signaling pathways that mediate external information to the cell. However, less is known about whether mitochondrial dysfunction interferes with signaling and thereby modulates the cellular response to environmental changes. In this study, we analyzed defective filamentous and invasive growth of the yeast Saccharomyces cerevisiae strains that have a dysfunctional mitochondrial genome (rho mutants). We found that the morphogenetic defect of rho mutants was caused by specific downregulation of FLO11, the adhesin essential for invasive and filamentous growth, and did not result from general metabolic changes brought about by interorganellar retrograde signaling. Transcription of FLO11 is known to be regulated by several signaling pathways, including the filamentous-growth-specific MAPK and cAMP-activated protein kinase A (cAMP-PKA) pathways. Our analysis showed that the filamentous-growth-specific MAPK pathway retained functionality in respiratory-deficient yeast cells. In contrast, the cAMP-PKA pathway was downregulated, explaining also various phenotypic traits observed in rho mutants. Thus, our results indicate that dysfunctional mitochondria modulate the output of the conserved cAMP-PKA signaling pathway.

Volatile organic compounds derived from 2-keto-acid decarboxylase in Microcystis aeruginosa

Volatile organic compounds (VOCs), 2-methyl-1-butanol, 3-methyl-1-butanol and 2-phenylethanol, were detected together with β-cyclocitral from the cyanobacterium Microcystis aeruginosa NIES-843. These alcohols were optimally produced after 35 d of culture, during which nitrate nitrogen in the cultured broth became exhausted. Additionally, these alcohols were definitely produced using the 2-keto-acid decarboxylase (MaKDC) in Microcystis strains. These results suggested that these VOCs from Microcystis are significant for their lifecycle, because these compounds are not produced by any other genus of cyanobacteria. This is the first report of 2-keto-acid decarboxylase producing 3-methyl-1-butanol and 2-phenylethanol by an oxygenic photosynthetic microorganism.

Genetics of yeast impacting wine quality

The availability of the sequence of the Saccharomyces genome in combination with the development of chemical analytical technologies with dynamic ranges sensitive enough to detect volatile aromatic compounds has generated a renewed interest in defining the role of yeast in the generation of wine aroma and flavor. Genetic differences among wine strains are well documented and aroma profiles also appear to vary, implying that specific allelic alterations may exist and impact the production of compounds associated with flavor. Partial or complete sequencing data on several wine strains are available and reveal underlying genetic differences across strains in key genes implicated in flavor formation. This review discusses the current understanding of the roles of Saccharomyces in wine flavor with an emphasis on positive contributions to flavor and highlights the discoveries of the underlying enzymatic and metabolic mechanisms responsible for the yeast contribution to wine quality.

Genome evolution in the eremothecium clade of the Saccharomyces complex revealed by comparative genomics

We used comparative genomics to elucidate the genome evolution within the pre-whole-genome duplication genus Eremothecium. To this end, we sequenced and assembled the complete genome of Eremothecium cymbalariae, a filamentous ascomycete representing the Eremothecium type strain. Genome annotation indicated 4712 gene models and 143 tRNAs. We compared the E. cymbalariae genome with that of its relative, the riboflavin overproducer Ashbya (Eremothecium) gossypii, and the reconstructed yeast ancestor. Decisive changes in the Eremothecium lineage leading to the evolution of the A. gossypii genome include the reduction from eight to seven chromosomes, the downsizing of the genome by removal of 10% or 900 kb of DNA, mostly in intergenic regions, the loss of a TY3-Gypsy-type transposable element, the re-arrangement of mating-type loci, and a massive increase of its GC content. Key species-specific events are the loss of MNN1-family of mannosyltransferases required to add the terminal fourth and fifth α-1,3-linked mannose residue to O-linked glycans and genes of the Ehrlich pathway in E. cymbalariae and the loss of ZMM-family of meiosis-specific proteins and acquisition of riboflavin overproduction in A. gossypii. This reveals that within the Saccharomyces complex genome, evolution is not only based on genome duplication with subsequent gene deletions and chromosomal rearrangements but also on fungi associated with specific environments (e.g. involving fungal-insect interactions as in Eremothecium), which have encountered challenges that may be reflected both in genome streamlining and their biosynthetic potential.

Fusel alcohols regulate translation initiation by inhibiting eIF2B to reduce ternary complex in a mechanism that may involve altering the integrity and dynamics of the eIF2B body

This study highlights a connection between the eIF2B body and the regulation of translation initiation as a response to stress in Saccharomyces cerevisiae. Fusel alcohols are involved in signaling nitrogen scarcity to the cell and they inhibit protein synthesis by preventing the movement of the eIF2B body throughout the cell.

Recycling of eIF2-GDP to the GTP-bound form constitutes a core essential, regulated step in eukaryotic translation. This reaction is mediated by eIF2B, a heteropentameric factor with important links to human disease. eIF2 in the GTP-bound form binds to methionyl initiator tRNA to form a ternary complex, and the levels of this ternary complex can be a critical determinant of the rate of protein synthesis. Here we show that eIF2B serves as the target for translation inhibition by various fusel alcohols in yeast. Fusel alcohols are endpoint metabolites from amino acid catabolism, which signal nitrogen scarcity. We show that the inhibition of eIF2B leads to reduced ternary complex levels and that different eIF2B subunit mutants alter fusel alcohol sensitivity. A DNA tiling array strategy was developed that overcame difficulties in the identification of these mutants where the phenotypic distinctions were too subtle for classical complementation cloning. Fusel alcohols also lead to eIF2α dephosphorylation in a Sit4p-dependent manner. In yeast, eIF2B occupies a large cytoplasmic body where guanine nucleotide exchange on eIF2 can occur and be regulated. Fusel alcohols impact on both the movement and dynamics of this 2B body. Overall, these results confirm that the guanine nucleotide exchange factor, eIF2B, is targeted by fusel alcohols. Moreover, they highlight a potential connection between the movement or integrity of the 2B body and eIF2B regulation.

Environmental and genetic determinants of colony morphology in yeast

Nutrient stresses trigger a variety of developmental switches in the budding yeast Saccharomyces cerevisiae. One of the least understood of such responses is the development of complex colony morphology, characterized by intricate, organized, and strain-specific patterns of colony growth and architecture. The genetic bases of this phenotype and the key environmental signals involved in its induction have heretofore remained poorly understood. By surveying multiple strain backgrounds and a large number of growth conditions, we show that limitation for fermentable carbon sources coupled with a rich nitrogen source is the primary trigger for the colony morphology response in budding yeast. Using knockout mutants and transposon-mediated mutagenesis, we demonstrate that two key signaling networks regulating this response are the filamentous growth MAP kinase cascade and the Ras-cAMP-PKA pathway. We further show synergistic epistasis between Rim15, a kinase involved in integration of nutrient signals, and other genes in these pathways. Ploidy, mating-type, and genotype-by-environment interactions also appear to play a role in the controlling colony morphology. Our study highlights the high degree of network reuse in this model eukaryote; yeast use the same core signaling pathways in multiple contexts to integrate information about environmental and physiological states and generate diverse developmental outputs.

Filament formation in Saccharomyces cerevisiae--a review

Many yeasts can produce filamentous elongated cells identifiable as hyphae, pseudohyphae or invasive filaments. Filament formation has been understood as a foraging response that occurs in nutrient-poor conditions. However, fusel alcohols were observed to induce filament formation in rich nutrient conditions in every yeast species examined. Fusel alcohols, e.g., 3-methyl-1-butanol (3Me-BuOH; 'isoamyl alcohol'), 2-methyl-1-propanol (isobutyl alcohol), (-)-2-methyl-1-butanol ('active amyl alcohol'), 2-phenylethanol and 3-(2-hydroxyethyl)indole (tryptophol) (the end products of leucine, valine, isoleucine, phenylalanine and tryptophan catabolism, respectively) are the end products of amino acid catabolism that accumulate when nutrients become limiting. Thus, yeast responds to its own metabolic by-products. Considerable effort was made to define the cell biological and biochemical changes that take place during 3Me-BuOH-induced filamentation. In Saccharomyces cerevisiae filaments contain significantly greater mitochondrial mass and increased chitin content in comparison with yeast-form cells. The global transcriptional response of S. cerevisiae during the early stages of 3Me-BuOH-induced filament formation has been described. Four ORFs displayed very significant (more than 10-fold) increases in their RNA species, and 12 ORFs displayed increases in transcription of more than 5-fold. The transcription of five genes (all of which encode transporters) decreased by similar amounts. Where examined, the activity of the proteins encoded reflected the transcriptional pattern of their respective mRNAs. To understand this regulation, studies were performed to see whether deletion or overexpression of key genes affects the ability to filament and invade solid YEPD medium. This has led to identification of those proteins that are essential for filament formation, repressors and those which are simply not required. It also leads to the conclusion that 3Me-BuOH-induced filament formation is not a foraging response but a response to reduced growth rate.

A transcriptome analysis of isoamyl alcohol-induced filamentation in yeast reveals a novel role for Gre2p as isovaleraldehyde reductase

A transcriptome analysis was performed of Saccharomyces cerevisiae undergoing isoamyl alcohol-induced filament formation. In the crucial first 5 h of this process, only four mRNA species displayed strong and statistically significant increases in their levels of more than 10-fold. Two of these (YEL071w/DLD3 and YOL151w/GRE2) appear to play important roles in filamentation. The biochemical activities ascribed to these two genes (d-lactate dehydrogenase and methylglyoxal reductase, respectively) displayed similarly timed increases to those of their respective mRNAs. Mutants carrying dld3 mutations displayed reduced filamentation in 0.5% isoamyl alcohol and needed a higher concentration of isoamyl alcohol to effect more complete filament formation. Hence, DLD3 seems to be required for a full response to isoamyl alcohol, but is not absolutely essential for it. Mutants carrying gre2 mutations were derepressed for filament formation and formed large, invasive filaments even in the absence of isoamyl alcohol. These results indicate a previously unsuspected and novel role for the GRE2 gene product as a suppressor of filamentation by virtue of encoding isovaleraldehyde reductase activity.

Global gene expression profiling reveals widespread yet distinctive translational responses to different eukaryotic translation initiation factor 2B-targeting stress pathways

Global inhibition of protein synthesis is a hallmark of many cellular stress conditions. Even though specific mRNAs defy this (e.g., yeast GCN4 and mammalian ATF4), the extent and variation of such resistance remain uncertain. In this study, we have identified yeast mRNAs that are translationally maintained following either amino acid depletion or fusel alcohol addition. Both stresses inhibit eukaryotic translation initiation factor 2B, but via different mechanisms. Using microarray analysis of polysome and monosome mRNA pools, we demonstrate that these stress conditions elicit widespread yet distinct translational reprogramming, identifying a fundamental role for translational control in the adaptation to environmental stress. These studies also highlight the complex interplay that exists between different stages in the gene expression pathway to allow specific preordained programs of proteome remodeling. For example, many ribosome biogenesis genes are coregulated at the transcriptional and translational levels following amino acid starvation. The transcriptional regulation of these genes has recently been connected to the regulation of cellular proliferation, and on the basis of our results, the translational control of these mRNAs should be factored into this equation.

Are yeasts free-living unicellular eukaryotes?

Yeasts are defined as unicellular fungi, yet many recent observations suggest their whole lifestyle is anything but unicellular. This review surveys the evidence that yeasts are really social organisms with cell-to-cell communication.