Toxicity of 2-deoxygalactose to Saccharomyces cerevisiae cells constitutively synthesizing galactose-metabolizing enzymes

GAL Regulon in the Yeast S. cerevisiae is Highly Evolvable via Acquisition in the Coding Regions of the Regulatory Elements of the Network

GAL network in the yeast S. cerevisiae is one of the most well-characterized regulatory network. Expression of GAL genes is contingent on exposure to galactose, and an appropriate combination of the alleles of the regulatory genes GAL3, GAL1, GAL80, and GAL4. The presence of multiple regulators in the GAL network makes it unique, as compared to the many sugar utilization networks studied in bacteria. For example, utilization of lactose is controlled by a single regulator LacI, in E. coli's lac operon. Moreover, recent work has demonstrated that multiple alleles of these regulatory proteins are present in yeast isolated from ecological niches. In this work, we develop a mathematical model, and demonstrate via deterministic and stochastic runs of the model, that behavior/gene expression patterns of the cells (at a population level, and at a single-cell resolution) can be modulated by altering the binding affinities between the regulatory proteins. This adaptability is likely the key to explaining the multiple GAL regulatory alleles discovered in ecological isolates in recent years.

Enhancement of catabolite regulatory genes in Saccharomyces cerevisiae to increase ethanol production using hydrolysate from red seaweed Gloiopeltis furcata

Glucose and galactose are monosaccharides obtained from Gloiopeltis furcata (Red algae). A total monosaccharide yield of 62.3 g/L was obtained from G. furcata using thermal acid hydrolysis and enzymatic saccharification. Activated carbon was used to eliminate hydroxymethylfurfural (HMF) from the hydrolysate. Previously obtained monosaccharides are used for ethanol production by Saccharomyces cerevisiae. S. cerevisiae consumes glucose first, then galactose. The methods for reducing fermentation time and increasing the ethanol yield coefficient using the simultaneous consumption of glucose and galactose have been evaluated. Gal3p and Gal80p of S. cerevisiae act as signal transducers that govern the galactose inducer Gal4p mediated transcriptional activation of the Gal gene family. Gal80p binds to Gal4p for transcription deactivation. Therefore, Gal80p was deleted for Gal4p expression without interruption.

Forward Genetic Dissection of Biofilm Development by Fusobacterium nucleatum: Novel Functions of Cell Division Proteins FtsX and EnvC

Fusobacterium nucleatum is a key member of the human oral biofilm. It is also implicated in preterm birth and colorectal cancer. To facilitate basic studies of fusobacterial virulence, we describe here a versatile transposon mutagenesis procedure and a pilot screen for mutants defective in biofilm formation. Out of 10 independent biofilm-defective mutants isolated, the affected genes included the homologs of the Escherichia coli cell division proteins FtsX and EnvC, the electron transport protein RnfA, and four proteins with unknown functions. Next, a facile new gene deletion method demonstrated that nonpolar, in-frame deletion of ftsX or envC produces viable bacteria that are highly filamentous due to defective cell division. Transmission electron and cryo-electron microscopy revealed that the ΔftsX and ΔenvC mutant cells remain joined with apparent constriction, and scanning electron microscopy (EM) uncovered a smooth cell surface without the microfolds present in wild-type cells. FtsX and EnvC proteins interact with each other as well as a common set of interacting partners, many with unknown function. Last, biofilm development is altered when cell division is blocked by MinC overproduction; however, unlike the phenotypes of ΔftsX and ΔenvC mutants, a weakly adherent biofilm is formed, and the wild-type rugged cell surface is maintained. Therefore, FtsX and EnvC may perform novel functions in Fusobacterium cell biology. This is the first report of an unbiased approach to uncover genetic determinants of fusobacterial biofilm development. It points to an intriguing link among cytokinesis, cell surface dynamics, and biofilm formation, whose molecular underpinnings remain to be elucidated.

Little is known about the virulence mechanisms and associated factors in F. nucleatum, due mainly to the lack of convenient genetic tools for this organism. We employed two efficient genetic strategies to identify F. nucleatum biofilm-defective mutants, revealing FtsX and EnvC among seven biofilm-associated factors. Electron microscopy established cell division defects of the ΔftsX and ΔenvC mutants, accompanied with a smooth cell surface, unlike the microfold, rugged appearance of wild-type bacteria. Proteomic studies demonstrated that FtsX and EnvC interact with each other as well as a set of common and unique interacting proteins, many with unknown functions. Importantly, blocking cell division by MinC overproduction led to formation of a weakly adherent biofilm, without alteration of the wild-type cell surface. Thus, this work links cell division and surface dynamics to biofilm development and lays a foundation for future genetic and biochemical investigations of basic cellular processes in this clinically significant pathogen.

Multiple Conformations of Gal3 Protein Drive the Galactose-Induced Allosteric Activation of the GAL Genetic Switch of Saccharomyces cerevisiae

Gal3p is an allosteric monomeric protein that activates the GAL genetic switch of Saccharomyces cerevisiae in response to galactose. Expression of constitutive mutant of Gal3p or overexpression of wild-type Gal3p activates the GAL switch in the absence of galactose. These data suggest that Gal3p exists as an ensemble of active and inactive conformations. Structural data have indicated that Gal3p exists in open (inactive) and closed (active) conformations. However, a mutant of Gal3p that predominantly exists in inactive conformation and is yet capable of responding to galactose has not been isolated. To understand the mechanism of allosteric transition, we have isolated a triple mutant of Gal3p with V273I, T404A, and N450D substitutions, which, upon overexpression, fails to activate the GAL switch on its own but activates the switch in response to galactose. Overexpression of Gal3p mutants with single or double mutations in any of the three combinations failed to exhibit the behavior of the triple mutant. Molecular dynamics analysis of the wild-type and the triple mutant along with two previously reported constitutive mutants suggests that the wild-type Gal3p may also exist in super-open conformation. Furthermore, our results suggest that the dynamics of residue F237 situated in the hydrophobic pocket located in the hinge region drives the transition between different conformations. Based on this study, we suggest that conformational selection mechanism is the driving force in the allosteric transition of Gal3p, which may have implications in other signaling pathways involving monomeric proteins.

Perturbation of the interaction between Gal4p and Gal80p of the Saccharomyces cerevisiae GAL switch results in altered responses to galactose and glucose

In S. cerevisiae, following the Whole Genome Duplication (WGD), GAL1-encoded galactokinase retained its signal transduction function but lost basal expression. On the other hand, its paralogue GAL3, lost kinase activity but retained its signalling function and basal expression, thus making it indispensable for the rapid induction of the S. cerevisiae GAL switch. However, a gal3Δ strain exhibits delayed growth kinetics due to the redundant signalling function of GAL1. The subfunctionalization between the paralogues GAL1 and GAL3 is due to expression divergence and is proposed to be due to the alteration in the Upstream Activating Sequences (UASG ). We demonstrate that the GAL switch becomes independent of GAL3 by altering the interaction between Gal4p and Gal80p without altering the configuration of UASG . In addition to the above, the altered switch of S. cerevisiae loses ultrasensitivity and stringent glucose repression. These changes caused an increase in fitness in the disaccharide melibiose at the expense of a decrease in fitness in galactose. The above altered features of the ScGAL switch are similar to the features of the GAL switch of K. lactis that diverged from S. cerevisiae before the WGD.

Toxic effects of 2-deoxy-D-galactose on Coptotermes formosanus (Isoptera: Rhinotermitidae) and symbionts

In the interest of developing interventions to infestations by Formosan subterranean termites, Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae), several rare sugars were tested for effects on the termites and symbionts. Among these, the D-galactose analog, 2-deoxy-D-galactose (2deoxyGal) showed promise as a potential control chemical. At a test concentration of 2deoxyGal (320.4 microg/mm3) in water applied to 5-cm filter paper, in bioassays with 20 termite workers, we found that worker termite mortality was significantly affected over a 2-wk period. Subsequent dose-mortality feeding studies confirmed these findings. In addition, consumption of the sugar-treated filter paper by termites caused a significant decrease in hindgut protozoan populations. 2deoxyGal caused dose-dependent termite mortality, taking on average 1 wk to begin killing workers, indicating that it may have promise as a delayed action toxin, which, if added to baits, could allow time after bait discovery for an entire colony to be affected.

Selection systems based on dominant-negative transcription factors for precise genetic engineering

Diverse tools are available for performing genetic modifications of microorganisms. However, new methods still need to be developed for performing precise genomic engineering without introducing any undesirable side-alteration. Indeed for functional analyses of genomic elements, as well as for some industrial applications, only the desired mutation should be introduced at the locus considered. This article describes a new approach fulfilling these requirements, based on the use of selection systems consisting in truncated genes encoding dominant-negative transcription factors. We have demonstrated dominant-negative effects mediated by truncated Gal4p and Arg81p proteins in Saccharomyces cerevisiae, interfering with galactose and arginine metabolic pathways, respectively. These genes can be used as positive and negative markers, since they provoke both growth inhibition on substrates and resistance to specific drugs. These selection markers have been successfully used for precisely deleting HO and URA3 in wild yeasts. This genetic engineering approach could be extended to other microorganisms.

Structural basis for dimerization in DNA recognition by Gal4

Gal4 is a Zn2Cys6 binuclear cluster containing transcription factor that binds DNA as a homodimer and can activate transcription by interacting with the mutant Gal11P protein. Although structures have been reported of the Gal4 dimerization domain and the binuclear cluster domain bound to DNA as a dimer, the structure of the "complete" Gal4 dimer bound to DNA has not previously been described. Here we report the structure of a complete Gal4 dimer bound to DNA and additional biochemical studies to address the molecular basis for Gal4 dimerization in DNA binding. We find that Gal4 dimerization on DNA is mediated by an intertwined helical bundle that deviates significantly from the solution NMR structure of the free dimerization domain. Associated biochemical studies show that the dimerization domain of Gal4 is important for DNA binding and protein thermostability. We also map the interaction surface of the Gal4 dimerization domain with Gal11P.

Molecular characterization of MRG19 of Saccharomyces cerevisiae. Implication in the regulation of galactose and nonfermentable carbon source utilization

We have reported previously that multiple copies of MRG19 suppress GAL genes in a wild-type but not in a gal80 strain of Saccharomyces cerevisiae. In this report we show that disruption of MRG19 leads to a decrease in GAL induction when S. cerevisiae is induced with 0.02% but not with 2.0% galactose. Disruption of MRG19 in a gal3 background (this strain shows long-term adaptation phenotype) further delays the GAL induction, supporting the notion that its function is important only under low inducing signals. As a corollary, disruption of MRG19 in a gal80 strain did not decrease the constitutive expression of GAL genes. These results suggest that MRG19 has a role in GAL regulation only when the induction signal is weak. Unlike the effect on GAL gene expression, disruption of MRG19 leads to de-repression of CYC1-driven beta-galactosidase activity. MRG19 disruptant also showed a twofold increase in the rate of oxygen uptake as compared with the wild-type strain. ADH2, CTA1, DLD1, and CYC7 promoters that are active during nonfermentative growth did not show any de-repression of beta-galactosidase activity in the MRG19 disruptant. Western blot analysis indicated that MRG19 is a glucose repressible gene and is expressed in galactose and glycerol plus lactate. Experiments using green fluorescent protein fusion constructs indicate that Mrg19p is localized in the nucleus consistent with the presence of a consensus nuclear localization signal sequence. Based on the above results, we propose that Mrg19p is a regulator of galactose and nonfermentable carbon utilization.

Genetic manipulation of the pathogenic yeast Candida parapsilosis

Candida parapsilosis is an important human pathogen, responsible for severe cases of systemic candidiasis and one of the leading causes of mortality in neonates. In this report, we describe the first system for genetic manipulation of C. parapsilosis. We isolated and subsequently determined DNA sequences of genes encoding galactokinase ( CpGAL1) and orotidine-5'-phosphate decarboxylase ( CpURA3) from a genomic DNA library of C. parapsilosis by functional complementation of corresponding mutations in Saccharomyces cerevisiae. The predicted protein products, Gal1p and Ura3p, displayed a high degree of homology with corresponding sequences of C. albicans and S. cerevisiae, respectively. A collection of galactokinase-deficient ( gal1) strains of C. parapsilosis was prepared using direct selection of mutagenized cells on media containing 2-deoxy-galactose. Additionally, we constructed a plasmid vector carrying CpGAL1 as a selection marker and a genomic DNA fragment with an autonomously replicating sequence activity that transforms the C. parapsilosis gal1 mutant strain with high efficiency. This system for genetic transformation of C. parapsilosis may significantly advance the study of this human pathogen, greatly improving our understanding of its biology and virulence, with implications for drug development.

A counterselection for the tryptophan pathway in yeast: 5-fluoroanthranilic acid resistance

The ability to counterselect, as well as to select for, a genetic marker has numerous applications in microbial genetics. Described here is the use of 5-fluoroanthranilic acid for the counterselection of TRP1, a commonly used genetic marker in the yeast Saccharomyces cerevisiae. Counterselection using 5-fluoroanthranilic acid involves antimetabolism by the enzymes of the tryptophan biosynthetic pathway, such that trp1, trp3, trp4 or trp5 strains, which lack enzymes required for the conversion of anthranilic acid to tryptophan, are resistant to 5-fluoroanthranilic acid. Commonly used genetic procedures, such as selection for loss of a chromosomally integrated plasmid, and a replica-plating method to rapidly assess genetic linkage in self-replicating shuttle vectors, can now be carried out using the TRP1 marker gene. In addition, novel tryptophan auxotrophs can be selected using 5-fluoroanthranilic acid.

Genetic analysis of transcription-associated mutation in Saccharomyces cerevisiae

High levels of transcription are associated with elevated mutation rates in yeast, a phenomenon referred to as transcription-associated mutation (TAM). The transcription-associated increase in mutation rates was previously shown to be partially dependent on the Rev3p translesion bypass pathway, thus implicating DNA damage in TAM. In this study, we use reversion of a pGAL-driven lys2DeltaBgl allele to further examine the genetic requirements of TAM. We find that TAM is increased by disruption of the nucleotide excision repair or recombination pathways. In contrast, elimination of base excision repair components has only modest effects on TAM. In addition to the genetic studies, the lys2DeltaBgl reversion spectra of repair-proficient low and high transcription strains were obtained. In the low transcription spectrum, most of the frameshift events correspond to deletions of AT base pairs whereas in the high transcription strain, deletions of GC base pairs predominate. These results are discussed in terms of transcription and its role in DNA damage and repair.

Three new dominant drug resistance cassettes for gene disruption in Saccharomyces cerevisiae

Disruption-deletion cassettes are powerful tools used to study gene function in many organisms, including Saccharomyces cerevisiae. Perhaps the most widely useful of these are the heterologous dominant drug resistance cassettes, which use antibiotic resistance genes from bacteria and fungi as selectable markers. We have created three new dominant drug resistance cassettes by replacing the kanamycin resistance (kan(r)) open reading frame from the kanMX3 and kanMX4 disruption-deletion cassettes (Wach et al., 1994) with open reading frames conferring resistance to the antibiotics hygromycin B (hph), nourseothricin (nat) and bialaphos (pat). The new cassettes, pAG25 (natMX4), pAG29 (patMX4), pAG31 (patMX3), pAG32 (hphMX4), pAG34 (hphMX3) and pAG35 (natMX3), are cloned into pFA6, and so are in all other respects identical to pFA6-kanMX3 and pFA6-kanMX4. Most tools and techniques used with the kanMX plasmids can also be used with the hph, nat and patMX containing plasmids. These new heterologous dominant drug resistance cassettes have unique antibiotic resistance phenotypes and do not affect growth when inserted into the ho locus. These attributes make the cassettes ideally suited for creating S. cerevisiae strains with multiple mutations within a single strain.

Approaches to the study of Rox1 repression of the hypoxic genes in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a facultative aerobe that responds to changes in oxygen tension by changing patterns of gene expression. One set of genes that responds to this environmental cue is the hypoxic genes. Oxygen levels are sensed by changes in heme biosynthesis, which controls the transcription of the ROX1 gene, encoding a protein that binds to the regulatory region of each hypoxic gene to repress transcription. Several experimental molecular and genetic approaches are described here to study Rox1 repression. Derepression of the hypoxic genes is rapid, and one model for such a response requires that Rox1 have a short half-life. This was demonstrated to be the case by immunoblotting using a c-myc epitope-tagged protein. Rox1 repression is mediated through the general repressors Ssn6 and Tup1. To explore possible interactions among these proteins, all three were expressed and partially purified using a baculovirus expression system and histidine-tagged proteins. The effect of Ssn6 and Tup1 on the formation of Rox1-DNA complexes was explored using these purified proteins by both electrophoretic mobility shift and DNase I protection assays. We found that Rox1 DNA-binding activity decayed rapidly and that Ssn6 could stabilize and restore lost activity. Finally, genetic selections are described for the isolation of loss-of-function mutations in Rox1. Also, schemes are proposed for the reversion of such mutations. These selections have been extended to genetic analyses of the TUP1 and SSN6 genes.

Galactose induces in Saccharomyces cerevisiae sensitivity of the utilization of hexoses to inhibition by D-glucosamine

Inhibition by glucosamine of the utilization of hexoses by Saccharomyces cerevisiae is induced by growing the cells in media with galactose as carbon source. The intensity of inhibition parallels the induction of the galactose pathway. These findings contrast with the fact that glucosamine is a substrate of the constitutive glucose but not of the inducible. galactose transport and phosphorylation systems. The inhibition by glucosamine is pH dependent; the extent seems to be related with phosphorylation of the hexosamine, as shown by its greater effect with substrates or with conditions that less interfere with the phosphorylation of the inhibitor. Inhibition is not a consequence of ATP depletion of the cell. Intracellular accumulated glucosamine derivatives impair the transport of glucose and mannose in yeast cells grown in galactose-supplemented media but not those grown with glucose or ethanol supplements (i.e., under conditions in which the utilization of these sugars is inhibited). However, impairment of the transport is not enough to explain the characteristics of the observed inhibition. The changes induced by growing the yeast in galactose that render the cells sensitive to glucosamine are under the control of the gal80 and gal4 genes.

Genetic and biochemical characterization of the UGP1 gene encoding the UDP-glucose pyrophosphorylase from Saccharomyces cerevisiae

We report here that the open reading frame YKL248, previously identified during the systematic sequencing of yeast chromosome XI [Purnelle B., Skala, J., Van Dijck, L. & Goffeau, A. (1992) Yeast 8, 977-986] encodes UDP-glucose pyrophosphorylase (UGPase), the enzyme which catalyses the reversible formation of UDP-Glc from glucose 1-phosphate and UTP. Proof for this function come from sequence alignment of the YKL248 product with UGPase of other species, from complementation studies of an Escherichia coli galU mutant deficient in UGPase activity, and from overexpression studies. In particular, the amino acid sequence motifs involved in the binding of glucose 1-phosphate and UDP-Glc are entirely conserved between the yeast, bovine, human and potato tuber UGPases, and multi-copy expression of YKL248 resulted in a 40-fold increase in UGPase activity. This gene was, therefore, renamed UGP1. Gene disruption at the UGP1 locus in a diploid strain, followed by tetrad analysis, showed that UGPase is essential for cell viability. Functional analysis of UGP1 was, therefore, carried out by generating strains in which UGPase could be either overexpressed or depleted. This was done by generating haploid strains carrying either UGP1 on a multicopy vector or the chromosomal deletion of UGP1, and rescued by a vector bearing the wild-type gene under the control of the glucose-repressible galactose-inducible promoter. The effects of overproducing UGPase on the cell metabolism and morphology were carbon-source dependent. On glucose medium, the 40-fold increase of UGPase activity was restricted to a twofold increase in the concentration of glycogen and UDP-Glc, with no significant effect on growth. In contrast, on galactose, the 40-fold increase in UGPase activity was accompanied by several effects, including a threefold reduction of the growth rate, a 3-5-fold increase in the concentrations of UDP-Glc, UDP-Gal and galactose 1-phosphate, a higher sensitivity to calcofluor white and an increase in the degree of protein glycosylation. Depletion of UGPase activity was performed by transferring the mutant strains from galactose to glucose medium. Unexpectedly, growth of these mutants on glucose was as efficient as that of the control, although the mutants contained only 5-10% wild-type UGPase activity, and a growth defect could never been obtained, even after serial transfers of the mutants to a 10% glucose medium. However, the 10-fold reduction of UGPase activity induced a multi-budding pattern, a higher resistance to zymolyase, a slight increase in the calcofluor sensitivity and a decrease in the cell-wall beta-glucan content. All these alterations, induced by manipulating the UGP1 gene, are discussed in the context of the strategic position of UDP-Glc in yeast metabolism.

Genetic and molecular characterization of GAL83: its interaction and similarities with other genes involved in glucose repression in Saccharomyces cerevisiae

Expression of the GAL genes of Saccharomyces cerevisiae is subject to glucose repression, a global regulatory mechanism that requires several gene products. We have isolated GAL83, one of these genes required for glucose repression. The sequence of the predicted Gal83 protein is homologous to two other yeast proteins, Sip1p and Sip2p, which are known to interact with the SNF1 gene product, a protein kinase required for expression of the GAL genes. High-copy clones of SIP1 and SIP2 cross-complement the GAL83-2000 mutation (as well as GAL82-1, a mutation in another gene involved in glucose repression), suggesting that these four genes may perform similar functions in glucose repression. Consistent with this hypothesis, a gal83 null mutation does not affect glucose repression, and only dominant or partially dominant mutations exist in GAL83 (and GAL82). Two other observations were made that suggests that GAL83 functions interdependently with GAL82 and REG1 (another gene involved in glucose repression) to effect glucose repression: 1) REG1 on a low-copy plasmid cross-complements GAL82-1 and GAL83-2000 mutations, and 2) all pairwise combinations of reg1, GAL82-1 and GAL83-2000 fail to complement one another. Such unlinked noncomplementation suggests that Gal83p, Gal82p and Reg1p may interact with one another. Possible roles for GAL83, GAL82 and REG1 are discussed in relation to SNF1, SIP1 and SIP2.

Direct cloning of yeast genes from an ordered set of lambda clones in Saccharomyces cerevisiae by recombination in vivo

We describe a technique that facilitates the isolation of yeast genes that are difficult to clone. This technique utilizes a plasmid vector that rescues lambda clones as yeast centromere plasmids. The source of these lambda clones is a set of clones whose location in the yeast genome has been determined by L. Riles et al. in 1993. The Escherichia coli-yeast shuttle plasmid carries URA3, ARS4 and CEN6, and contains DNA fragments from the lambda vector that flank the cloned yeast insert. When yeast is cotransformed with linearized plasmid and lambda clone DNA, Ura+ transformants are obtained by a recombination event between the lambda clone and the plasmid vector that generates an autonomously replicating plasmid containing the cloned yeast DNA sequences. Genes whose genetic map positions are known can easily be identified and recovered in this plasmid by testing only those lambda clones that map to the relevant region of the yeast genome for their ability to complement the mutant phenotype. This technique facilitates the isolation of yeast genes that resist cloning either because (1) they are underrepresented in yeast genomic libraries amplified in E. coli, (2) they provide phenotypes that are too marginal to allow selection of the gene by genetic complementation or (3) they provide phenotypes that are laborious to score. We demonstrate the utility of this technique by isolating three genes, GAL83, SSN2 and MAK7, each of which presents one of these problems for cloning.

Genetic evidence that an activation domain of GAL4 does not require acidity and may form a beta sheet

Regulation of gene expression in eukaryotes relies on intricate protein-protein interactions. Transcription of the galactose genes in yeast has been a productive model for this type of interaction. The positive activator in this system, GAL4, has a bifunctional C-terminus. It contains both a prototypic acidic activation domain and a region that binds the negative regulator, GAL80. We have taken advantage of this colocalization of functions to subject the region to a constrained mutagenesis analysis: one function was maintained, while the other one was altered. This analysis and the experiments it suggested have led us to two conclusions: first, the acidic amino acids are not, as commonly thought, required for activation; second, this region is not unstructured or alpha helical, but its function may require a beta sheet.

Direct selection of galactokinase-negative mutants of Candida albicans using 2-deoxy-galactose

The galactose analogue 2-deoxy-galactose (2DG) has been widely used to select for mutations in the gene encoding the galactose pathway enzyme galactokinase (GalK). We have tested the effect of 2DG on Candida albicans to see if it could be used to obtain GalK- mutants in this diploid asexual yeast. 2DG was shown to be toxic to wild-type cells. Enzyme assays demonstrated that 2DG can induce GalK as efficiently as galactose. Examination of the initial rate of galactose uptake indicated that the galactose transport system is constitutive. 2DG-resistant mutants were isolated from mutagenized cultures and shown to have very low levels of GalK activity. The potential genetic applications of this system of direct mutant selection are discussed.

Inhibition of glycolysis by 2-deoxygalactose in Saccharomyces cerevisiae

The enzymatic steps involved in the inhibition of glycolysis by 2-deoxygalactose in Saccharomyces cerevisiae have been investigated. Yeast, incubated with 2-deoxygalactose, accumulates up to 8 mM-2-deoxygalactose, 30 mM-2-deoxygalactose-1-phosphate and 0.25 mM-UDP-2-deoxygalactose and UDP-2-deoxyglucose. An inverse correlation between 2-deoxygalactose-1-phosphate content and rate of glycolysis has been observed. The intracellular concentration of glycolytic intermediates and related metabolites point to the hexokinase and phosphofructokinase steps as the targets for the inhibition of glycolysis by 2-deoxygalactose and rule out all other mechanisms that have been proposed to explain this inhibition.

GAL4 mutations that separate the transcriptional activation and GAL80-interactive functions of the yeast GAL4 protein

The carboxy-terminal 28 amino acids of the Saccharomyces cerevisiae transcriptional activator protein GAL4 execute two functions--transcriptional activation and interaction with the negative regulatory protein, GAL80. Here we demonstrate that these two functions are separable by single amino acid changes within this region. We determined the sequences of four GAL4C-mutations, and characterized the abilities of the encoded GAL4C proteins to activate transcription of the galactose/melibiose regulon in the presence of GAL80 and superrepressible GAL80S alleles. One of the GAL4C mutations can be compensated by a specific GAL80S mutation, resulting in a wild-type phenotype. These results support the idea that while the GAL4 activation function tolerates at least minor alterations in the GAL4 carboxyl terminus, the GAL80-interactive function is highly sequence-specific and sensitive even to single amino acid alterations. They also argue that the GAL80S mutations affect the affinity of GAL80 for GAL4, and not the ability of GAL80 to bind inducer.

Isolation of mutations that act in trans to alter expression from a yeast hsp70 promoter

Transcription of SSA1 (formerly YG100), a member of the hsp70 gene family in Saccharomyces cerevisiae, increases dramatically upon heat shock. An expression vector in which the promoter of SSA1 is fused to the Escherichia coli galactokinase gene (galK) was constructed and transformed into a galactokinase-deficient yeast strain. The transformants grew on galactose at 23 degrees C, but increased expression of the SSA1-galK fusion gene inhibited growth of cells on galactose at 37 degrees C. Selection for survivors under nonpermissive conditions yielded a class of mutants, termed HSR (for heat shock regulation), which showed reduced levels of expression of the hsp70-galK gene fusion as determined by measurement of galactokinase activity. Similar effects on beta-galactosidase activity were obtained when an SSA1-lacZ fusion vector was introduced into the mutants, suggesting action in trans through the SSA1 promoter. Analysis of Northern (RNA) blots demonstrated that the reduction in expression was a result of decreased mRNA levels for the fusion gene. In addition, mRNA levels of the endogenous SSA1 gene are reduced in an HSR mutant. Genetic analysis has shown that these mutations act in trans and affect both transcription from the SSA1 promoter and turnover of the fusion transcript. These are the first trans-acting mutations known to affect directly the transcriptional regulation and transcript stability of heat shock genes in eucaryotes.

Saccharomyces cerevisiae acquires resistance to 2-deoxyglucose at a very high frequency

We have found that Saccharomyces cerevisiae acquires spontaneously increasing resistance to 2-deoxyglucose at a very high frequency. This finding allows the easy isolation of different types of resistant strains of interest for metabolic studies with 2-deoxyhexoses. On the other hand, it sounds a note of caution in the widespread use of 2-deoxyglucose as a selective agent for the isolation of yeast mutants with impaired hexose transport or phosphorylation systems.

Transcription of multiple copies of the yeast GAL7 gene is limited by specific factors in addition to GAL4

High levels of the GAL7 gene in the yeast cell appear to titrate regulatory factors and to impair transcription of related sequences. To investigate the role that the GAL regulatory factors GAL4 and GAL80 have in this process we have compared the accumulation of mRNA transcribed from single-copy (plasmid-borne GAL7 and chromosomal GAL10) and high-copy (plasmid-borne GAL7) genes in several GAL regulatory mutants. Our results show that functional GAL4 gene product is required for induction of transcription from the single- and high-copy genes. In a strain containing the GAL4 gene fused to the high expression ADH1 promoter, glucose can replace galactose to induce high levels of transcription of GAL7 and GAL10 genes, although the kinetics of accumulation induced by the two sugars are distinctly different. In the presence of high levels of GAL4, maximum accumulation of mRNA from single and high copy genes is elevated two-fold; disruption of the gal80 gene in combination with high levels of GAL4 results in a further two-fold increase in transcription. In this genetic background, galactose-induced transcription of the high copy GAL7 gene results in a greater than 50-fold increase in the levels of GAL7 mRNA, representing 30%-50% of the total cellular mRNA. Our results are consistent with a cooperative effect of saturation of multiple GAL4 DNA binding sites and with a limiting factor, in addition to GAL4, that is required for transcription of the GAL genes.

Characterization of Streptomyces promoter sequences using the Escherichia coli galactokinase gene

A gene fusion system that uses the Escherichia coli galK gene has been developed to characterize Streptomyces transcriptional regulatory sequences. The system consists of galK-deficient Streptomyces lividans mutants and plasmids containing the E. coli galK gene with its natural ribosome-binding site and sites upstream of galK for insertion of transcription signals. Expression of the E. coli galK gene in S. lividans can be quantitated by either an enzymatic or immunoblot assay or detected by genetic complementation of an S. lividans galK- mutant. The utility of the plasmid to select, detect and assess promoter function was examined using the S. lividans XP55 and S. fradiae aph gene promoters. The potential use of the galK fusion system to isolate and characterize Streptomyces transcription signals is discussed.