Med8, a subunit of the mediator CTD complex of RNA polymerase II, directly binds to regulatory elements of SUC2 and HXK2 genes

The mitochondrial iron exporter genes MMT1 and MMT2 in yeast are transcriptionally regulated by Aft1 and Yap1

Budding yeast (Saccharomyces cerevisiae) responds to low cytosolic iron by up-regulating the expression of iron import genes; iron import can reflect iron transport into the cytosol or mitochondria. Mmt1 and Mmt2 are nuclearly encoded mitochondrial proteins that export iron from the mitochondria into the cytosol. Here we report that MMT1 and MMT2 expression is transcriptionally regulated by two pathways: the low-iron-sensing transcription factor Aft1 and the oxidant-sensing transcription factor Yap1. We determined that MMT1 and MMT2 expression is increased under low-iron conditions and decreased when mitochondrial iron import is increased through overexpression of the high-affinity mitochondrial iron importer Mrs3. Moreover, loss of iron-sulfur cluster synthesis induced expression of MMT1 and MMT2 We show that exposure to the oxidant H2O2 induced MMT1 expression but not MMT2 expression and identified the transcription factor Yap1 as being involved in oxidant-mediated MMT1 expression. We defined Aft1- and Yap1-dependent transcriptional sites in the MMT1 promoter that are necessary for low-iron- or oxidant-mediated MMT1 expression. We also found that the MMT2 promoter contains domains that are important for regulating its expression under low-iron conditions, including an upstream region that appears to partially repress expression under low-iron conditions. Our findings reveal that MMT1 and MMT2 are induced under low-iron conditions and that the low-iron regulator Aft1 is required for this induction. We further uncover an Aft1-binding site in the MMT1 promoter sufficient for inducing MMT1 transcription and identify an MMT2 promoter region required for low iron induction.

Proteins interacting with CreA and CreB in the carbon catabolite repression network in Aspergillus nidulans

In Aspergillus nidulans, carbon catabolite repression (CCR) is mediated by the global repressor protein CreA. The deubiquitinating enzyme CreB is a component of the CCR network. Genetic interaction was confirmed using a strain containing complete loss-of-function alleles of both creA and creB. No direct physical interaction was identified between tagged versions of CreA and CreB. To identify any possible protein(s) that may form a bridge between CreA and CreB, we purified both proteins from mycelia grown in media that result in repression or derepression. The purified proteins were analysed by LC/MS and identified using MaxQuant and Mascot databases. For both CreA and CreB, 47 proteins were identified in repressing and derepressing conditions. Orthologues of the co-purified proteins were identified in S. cerevisiae and humans. Gene ontology analyses of A. nidulans proteins and yeast and human orthologues were performed. Functional annotation analysis revealed that proteins that preferentially interact with CreA in repressing conditions include histones and histone transcription regulator 3 (Hir3). Proteins interacting with CreB tend to be involved in cellular transportation and organization. Similar findings were obtained using yeast and human orthologues, although the yeast background generated a number of other biological processes involving Mig1p which were not present in the A. nidulans or human background analyses. Hir3 was present in repressing conditions for CreA and in both growth conditions for CreB, suggesting that Hir3, or proteins interacting with Hir3, could be a possible target of CreB.

Tracking control of concentration profiles in a fed-batch bioreactor using a linear algebra methodology

Based on a linear algebra approach, this paper aims at developing a novel control law able to track reference profiles that were previously-determined in the literature. A main advantage of the proposed strategy is that the control actions are obtained by solving a system of linear equations. The optimal controller parameters are selected through Monte Carlo Randomized Algorithm in order to minimize a proposed cost index. The controller performance is evaluated through several tests, and compared with other controller reported in the literature. Finally, a Monte Carlo Randomized Algorithm is conducted to assess the performance of the proposed controller.

Glucose signaling-mediated coordination of cell growth and cell cycle in Saccharomyces cerevisiae

Besides being the favorite carbon and energy source for the budding yeast Sacchromyces cerevisiae, glucose can act as a signaling molecule to regulate multiple aspects of yeast physiology. Yeast cells have evolved several mechanisms for monitoring the level of glucose in their habitat and respond quickly to frequent changes in the sugar availability in the environment: the cAMP/PKA pathways (with its two branches comprising Ras and the Gpr1/Gpa2 module), the Rgt2/Snf3-Rgt1 pathway and the main repression pathway involving the kinase Snf1. The cAMP/PKA pathway plays the prominent role in responding to changes in glucose availability and initiating the signaling processes that promote cell growth and division. Snf1 (the yeast homologous to mammalian AMP-activated protein kinase) is primarily required for the adaptation of yeast cell to glucose limitation and for growth on alternative carbon source, but it is also involved in the cellular response to various environmental stresses. The Rgt2/Snf3-Rgt1 pathway regulates the expression of genes required for glucose uptake. Many interconnections exist between the diverse glucose sensing systems, which enables yeast cells to fine tune cell growth, cell cycle and their coordination in response to nutritional changes.

Metabolic control of transcription: paradigms and lessons from Saccharomyces cerevisiae

The comparatively simple eukaryote Saccharomyces cerevisiae is composed of some 6000 individual genes. Specific sets of these genes can be transcribed co-ordinately in response to particular metabolic signals. The resultant integrated response to nutrient challenge allows the organism to survive and flourish in a variety of environmental conditions while minimal energy is expended upon the production of unnecessary proteins. The Zn(II)2Cys6 family of transcriptional regulators is composed of some 46 members in S. cerevisiae and many of these have been implicated in mediating transcriptional responses to specific nutrients. Gal4p, the archetypical member of this family, is responsible for the expression of the GAL genes when galactose is utilized as a carbon source. The regulation of Gal4p activity has been studied for many years, but we are still uncovering both nuances and fundamental control mechanisms that impinge on its function. In the present review, we describe the latest developments in the regulation of GAL gene expression and compare the mechanisms employed here with the molecular control of other Zn(II)2Cys6 transcriptional regulators. This reveals a wide array of protein-protein, protein-DNA and protein-nutrient interactions that are employed by this family of regulators.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Moonlighting proteins in yeasts

Proteins able to participate in unrelated biological processes have been grouped under the generic name of moonlighting proteins. Work with different yeast species has uncovered a great number of moonlighting proteins and shown their importance for adequate functioning of the yeast cell. Moonlighting activities in yeasts include such diverse functions as control of gene expression, organelle assembly, and modification of the activity of metabolic pathways. In this review, we consider several well-studied moonlighting proteins in different yeast species, paying attention to the experimental approaches used to identify them and the evidence that supports their participation in the unexpected function. Usually, moonlighting activities have been uncovered unexpectedly, and up to now, no satisfactory way to predict moonlighting activities has been found. Among the well-characterized moonlighting proteins in yeasts, enzymes from the glycolytic pathway appear to be prominent. For some cases, it is shown that despite close phylogenetic relationships, moonlighting activities are not necessarily conserved among yeast species. Organisms may utilize moonlighting to add a new layer of regulation to conventional regulatory networks. The existence of this type of proteins in yeasts should be taken into account when designing mutant screens or in attempts to model or modify yeast metabolism.

Genomic expression patterns in cell separation mutants of Schizosaccharomyces pombe defective in the genes sep10 ( + ) and sep15 ( + ) coding for the Mediator subunits Med31 and Med8

Cell division is controlled by a complex network involving regulated transcription of genes and postranslational modification of proteins. The aim of this study is to demonstrate that the Mediator complex, a general regulator of transcription, is involved in the regulation of the second phase (cell separation) of cell division of the fission yeast Schizosaccharomyces pombe. In previous studies we have found that the fission yeast cell separation genes sep10 ( + ) and sep15 ( + ) code for proteins (Med31 and Med8) associated with the Mediator complex. Here, we show by genome-wide gene expression profiling of mutants defective in these genes that both Med8 and Med31 control large, partially overlapping sets of genes scattered over the entire genome and involved in diverse biological functions. Six cell separation genes controlled by the transcription factors Sep1 and Ace2 are among the target genes. Since neither sep1 ( + ) nor ace2 ( + ) is affected in the mutant cells, we propose that the Med8 and Med31 proteins act as coactivators of the Sep1-Ace2-dependent cell separation genes. The results also indicate that the subunits of Mediator may contribute to the coordination of cellular processes by fine-tuning of the expression of larger sets of genes.

Tpk3 and Snf1 protein kinases regulate Rgt1 association with Saccharomyces cerevisiae HXK2 promoter

Hexokinase 2 is an essential factor for signalling repression through the Saccharomyces cerevisiae high-glucose sensing pathway. The main regulatory mechanism that controls the HXK2 gene expression in yeast is mediated by the Rgt1 and Med8 transcription factors, which repress HXK2 expression in low-glucose containing media. In this study, we show that the repression activity of Rgt1 is regulated by Snf1 and Tpk3 protein kinases. Binding of Rgt1 to the HXK2 promoter requires Rgt1 phosphorylation by Snf1 or by an Snf1-dependent protein kinase. Conversely, Rgt1 hyperphosphorylation by the Tpk3 or by a Tpk3-dependent protein kinase dissociates Rgt1 from the repressor complex. Two-hybrid and chromatin immunoprecipitation experiments indicate that an Snf1-dependent interaction between Rgt1 and Med8 in the repressor complex is also essential for Rgt1 repression. The repression of HXK2 transcription by Rgt1 likely occurs through the formation of a DNA loop in the HXK2 locus, spanning the promoter and coding regions. These results suggest that a novel silent-chromatin loop is responsible for Rgt1-dependent transcriptional regulation of the HXK2 gene.

Glucose signaling in Saccharomyces cerevisiae

Eukaryotic cells possess an exquisitely interwoven and fine-tuned series of signal transduction mechanisms with which to sense and respond to the ubiquitous fermentable carbon source glucose. The budding yeast Saccharomyces cerevisiae has proven to be a fertile model system with which to identify glucose signaling factors, determine the relevant functional and physical interrelationships, and characterize the corresponding metabolic, transcriptomic, and proteomic readouts. The early events in glucose signaling appear to require both extracellular sensing by transmembrane proteins and intracellular sensing by G proteins. Intermediate steps involve cAMP-dependent stimulation of protein kinase A (PKA) as well as one or more redundant PKA-independent pathways. The final steps are mediated by a relatively small collection of transcriptional regulators that collaborate closely to maximize the cellular rates of energy generation and growth. Understanding the nuclear events in this process may necessitate the further elaboration of a new model for eukaryotic gene regulation, called "reverse recruitment." An essential feature of this idea is that fine-structure mapping of nuclear architecture will be required to understand the reception of regulatory signals that emanate from the plasma membrane and cytoplasm. Completion of this task should result in a much improved understanding of eukaryotic growth, differentiation, and carcinogenesis.

Rgt1, a glucose sensing transcription factor, is required for transcriptional repression of the HXK2 gene in Saccharomyces cerevisiae

Expression of HXK2, a gene encoding a Saccharomyces cerevisiae bifunctional protein with catalytic and regulatory functions, is controlled by glucose availability, being activated in the presence of glucose and inhibited when the levels of the sugar are low. In the present study, we identified Rgt1 as a transcription factor that, together with the Med8 protein, is essential for repression of the HXK2 gene in the absence of glucose. Rgt1 represses HXK2 expression by binding specifically to the motif (CGGAAAA) located at -395 bp relative to the ATG translation start codon in the HXK2 promoter. Disruption of the RGT1 gene causes an 18-fold increase in the level of HXK2 transcript in the absence of glucose. Rgt1 binds to the RGT1 element of HXK2 promoter in a glucose-dependent manner, and the repression of target gene depends on binding of Rgt1 to DNA. The physiological significance of the connection between two glucose-signalling pathways, the Snf3/Rgt2 that causes glucose induction and the Mig1/Hxk2 that causes glucose repression, was also analysed.

Gln3p and Nil1p regulation of invertase activity and SUC2 expression in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, sensing and signalling pathways regulate gene expression in response to quality of carbon and nitrogen sources. One such system, the target of rapamycin (Tor) proteins, senses nutrients and uses the GATA activators Gln3p and Nil1p to regulate translation in response to low-quality carbon and nitrogen. The signal transduction, triggered in response to nitrogen nutrition that is sensed by the Tor proteins, operates via a regulatory pathway involving the cytoplasmic factor Ure2p. When carbon and nitrogen are abundant, the phosphorylated Ure2p anchors the also phosphorylated Gln3p and Nil1p in the cytoplasm. Upon a shift from high- to low-quality nitrogen or treatment with rapamycin all three proteins are dephosphorylated, causing Gln3p and Nil1p to enter the nucleus and promote transcription. The genes that code for yeast periplasmic enzymes with nutritional roles would be obvious targets for regulation by the sensing and signalling pathways that respond to quality of carbon and nitrogen sources. Indeed, previous results from our laboratory had shown that the GATA factors Gln3p, Nil1p, Dal80p, Nil2p and also the protein Ure2 regulate the expression of asparaginase II, coded by ASP3. We also had observed that the activity levels of the also periplasmic invertase, coded by SUC2, were 6-fold lower in ure2 mutant cells in comparison to wild-type cells collected at stationary phase. These results suggested similarities between the signalling pathways regulating the expression of ASP3 and SUC2. In the present work we showed that invertase levels displayed by the single nil1 and gln3 and by the double gln3nil1 mutant cells, cultivated in a sucrose-ammonium medium and collected at the exponential phase, were 6-, 10- and 60-fold higher, respectively, in comparison to their wild-type counterparts. RT-PCR data of SUC2 expression in the double-mutant cells indicated a 10-fold increase in the mRNA(SUC2) levels.

SCFGrr1-mediated ubiquitination of Gis4 modulates glucose response in yeast

The F box protein Grr1 is the substrate specificity-determinant of the SCF(Grr1) E3 ubiquitin ligase complex. Genetic analyses of Grr1 mutants have implicated Grr1 in glucose repression, specifically with regard to expression of the SUC2 transcript. To better understand Grr1, we screened for substrates using a mutant version of Grr1 that should not associate with the SCF complex. We identified Gis4 as a novel Grr1 substrate. Gis4 was originally isolated as a multi-copy suppressor of a Gal--phenotype in the triple mutant snf1 mig1 srb8. Here, we show that Gis4 binds Grr1 in vivo and that Grr1 protein levels positively affect the protein levels of Gis4. The Gis4 protein is stable in wild-type cells and in grr1Delta cells; however, Gis4 is ubiquitinated in a Grr1-dependent manner. Furthermore, we show that Gis4 interacts with Snf1 in a Grr1-dependent fashion, and that Gis4 is involved in de-repression of SUC2 and in transcription of other Snf1-dependent transcripts. Gis4 appears to connect the glucose repression and de-repression pathways. We suggest that Gis4 may explain the glucose repression defects in carbon source metabolism for the grr1 mutants.

The glucose-regulated nuclear localization of hexokinase 2 in Saccharomyces cerevisiae is Mig1-dependent

Two major mediators of glucose repression in Saccharomyces cerevisiae are the proteins Mig1 and Hxk2. The mechanism of Hxk2-dependent glucose repression pathway is not well understood, but the Mig1-dependent part of the pathway has been elucidated in great detail. Here we report that Hxk2 has a glucose-regulated nuclear localization and that Mig1, a transcriptional repressor responsible for glucose repression of many genes, is required to sequester Hxk2 into the nucleus. Mig1 and Hxk2 interacted in vivo in a yeast two-hybrid assay and in vitro in immunoprecipitation and glutathione S-transferase pull-down experiments. We found that the Lys(6)-Met(15) decapeptide of Hxk2, which is necessary for nuclear localization of the protein, is also essential for interaction with the Mig1 protein. Our results also show that the Hxk2-Mig1 interaction is of physiological significance because both proteins have been found interacting together in a cluster with DNA fragments containing the MIG1 site of SUC2 promoter. We conclude that Hxk2 operates by interacting with Mig1 to generate a repressor complex located in the nucleus of S. cerevisiae during growth in glucose medium.

Calorimetric determination of thermodynamic parameters of reaction reveals different enthalpic compensations of the yeast hexokinase isozymes

The change in enthalpy and rate constants for the reactions of yeast hexokinase isozymes, PI (Hxk1) and PII (Hxk2), was determined at pH 7.6 and 25 degrees C by isothermal titration calorimetry. The reactions were done in five buffer systems with enthalpy of protonation varying from -1.22 kcal/mol (phosphate) to -11.51 kcal/mol (Tris), allowing the determination of the number of protons released during glucose phosphorylation. The reaction is exothermic for both isozymes with a small, but significant (p < 0.0001), difference in the enthalpy of reaction (Delta HR), with an Delta HR of -5.1 +/- 0.2 (mean +/- S.D.) kcal/mol for Hxk1, and an Delta HR of -3.3 +/- 0.3 (mean +/- S.D.) kcal/mol for Hxk2. The Km for ATP determined by ITC was very similar to those reported in the literature for both isozymes. The effect of NaCl and KCl, from 0 to 200 mM, showed that although the rate of reaction decreases with increasing ionic strength, no change in the Delta HR was observed suggesting an entropic nature for the ionic strength. The differences in Delta HR obtained here for both isozymes strongly suggest that, besides glucose phosphorylation, another side reaction such as ATP hydrolysis and/or enzyme phosphorylation is taking place.

Mediator factor Med8p interacts with the hexokinase 2: implication in the glucose signalling pathway of Saccharomyces cerevisiae

In the presence of glucose the protein hexokinase 2 (Hxk2p), normally resident in the cytosol, is translocated to the nucleus where it impairs the activation of transcription of the glucose-repressed genes HXK1, GLK1 and SUC2, and promotes the activation of transcription of the glucose-induced genes HXK2 and HXT1. Here, we demonstrate the involvement of an heptameric motif, named the MED8 site, in the direct binding of the mediator protein Med8p, either as a monomer or as a homodimer. Because this site was previously involved in the Hxk2p-dependent glucose-induced regulation of gene transcription, we tested whether Hxk2p interacts with Med8p. Our results show that Hxk2 and Med8 proteins are physically associated and that this Hxk2p-Med8p interaction is of physiological significance because both proteins have been found interacting together in a cluster with DNA fragments containing the MED8 site. We conclude that Hxk2p operates through the MED8 site, by interacting with Med8p, in the glucose signal transduction pathway of Saccharomyces cerevisiae.

The hexokinase 2-dependent glucose signal transduction pathway of Saccharomyces cerevisiae

Sugars, predominantly glucose, evoke a variety of responses in Saccharomyces cerevisiae. These responses are elicited through a complex network of regulatory mechanisms that transduce the signal of presence of external glucose to their final intracellular targets. The HXK2 gene, encoding hexokinase 2 (Hxk2), the enzyme that initiates glucose metabolism, is highly expressed during growth in glucose and plays a pivotal role in the control of the expression of numerous genes, including itself. The mechanism of this autocontrol of expression is not completely understood. Hxk2 is found both in the nucleus and in the cytoplasm of S. cerevisiae; the nuclear localization is dependent on the presence of a stretch of amino acids located from lysine-6 to methionine-15. Although serine-14, within this stretch, can be phosphorylated in the absence of glucose, it is still unsettled whether this phosphorylation plays a role in the cellular localization of Hxk2. The elucidation of the mechanism of transport of Hxk2 to and from the nucleus, the influence of the oligomeric state of the protein on the nuclear transport and the fine mechanism of regulation of transcription of HXK2 are among the important unanswered questions in relation with the regulatory role of Hxk2.

A downstream regulatory element located within the coding sequence mediates autoregulated expression of the yeast fatty acid synthase gene FAS2 by the FAS1 gene product

The fatty acid synthase genes FAS1 and FAS2 of the yeast Saccharomyces cerevisiae are transcriptionally co-regulated by general transcription factors (such as Reb1, Rap1 and Abf1) and by the phospholipid-specific heterodimeric activator Ino2/Ino4, acting via their corresponding upstream binding sites. Here we provide evidence for a positive autoregulatory influence of FAS1 on FAS2 expression. Even with a constant FAS2 copy number, a 10-fold increase of FAS2 transcript amount was observed in the presence of FAS1 in multi-copy, compared to a fas1 null mutant. Surprisingly, the first 66 nt of the FAS2 coding region turned out as necessary and sufficient for FAS1-dependent gene expression. FAS2-lacZ fusion constructs deleted for this region showed high reporter gene expression even in the absence of FAS1, arguing for a negatively-acting downstream repression site (DRS) responsible for FAS1-dependent expression of FAS2. Our data suggest that the FAS1 gene product, in addition to its catalytic function, is also required for the coordinate biosynthetic control of the yeast FAS complex. An excess of uncomplexed Fas1 may be responsible for the deactivation of an FAS2-specific repressor, acting via the DRS.

Transcription of eukaryotic protein-coding genes

The past decade has seen an explosive increase in information about regulation of eukaryotic gene transcription, especially for protein-coding genes. The most striking advances in our knowledge of transcriptional regulation involve the chromatin template, the large complexes recruited by transcriptional activators that regulate chromatin structure and the transcription apparatus, the holoenzyme forms of RNA polymerase II involved in initiation and elongation, and the mechanisms that link mRNA processing with its synthesis. We describe here the major advances in these areas, with particular emphasis on the modular complexes associated with RNA polymerase II that are targeted by activators and other regulators of mRNA biosynthesis.

Mediator of transcriptional regulation

Three lines of evidence have converged on a multiprotein Mediator complex as a conserved interface between gene-specific regulatory proteins and the general transcription apparatus of eukaryotes. Mediator was discovered as an activity required for transcriptional activation in a reconstituted system from yeast. Upon resolution to homogeneity, the activity proved to reside in a 20-protein complex, which could exist in a free state or in a complex with RNA polymerase II, termed holoenzyme. A second line of evidence came from screens in yeast for mutations affecting transcription. Two-thirds of Mediator subunits are encoded by genes revealed by these screens. Five of the genetically defined subunits, termed Srbs, were characterized as interacting with the C-terminal domain of RNA polymerase II in vivo, and were shown to bind polymerase in vitro. A third line of evidence has come recently from studies in mammalian transcription systems. Mammalian counterparts of yeast Mediator were shown to interact with transcriptional activator proteins and to play an essential role in transcriptional regulation. Mediator evidently integrates and transduces positive and negative regulatory information from enhancers and operators to promoters. It functions directly through RNA polymerase II, modulating its activity in promoter-dependent transcription. Details of the Mediator mechanism remain obscure. Additional outstanding questions include the patterns of promoter-specificity of the various Mediator subunits, the possible cell-type-specificity of Mediator subunit composition, and the full structures of both free Mediator and RNA polymerase II holoenzyme.

Imaging and mapping protein-binding sites on DNA regulatory regions with atomic force microscopy

Regulation of gene expression is fundamental in biological systems. A systematic search for protein binding sites in gene promoters has been done in recent years. Biochemical techniques are easy and reliable when analysing protein interactions with short pieces of DNA, but are difficult and tedious when long pieces of DNA have to be analysed. Here we propose AFM as a reliable and easy technique for identifying protein interaction sites in long DNA molecules like gene promoters. We support this idea using a well-known model: the interaction of the Pho4 protein with the PHO5 gene promoter. We have also applied the technique to demonstrate that Mig1 protein binds to two motifs in the promoter of HXK2 gene. Our results allow us to define Mig1p as a new factor probably contributing to the carbon source-dependent transcription regulation of HXK2 gene.

Isolation of the MIG1 gene from Candida albicans and effects of its disruption on catabolite repression

We have cloned a Candida albicans gene (CaMIG1) that encodes a protein homologous to the DNA-binding protein Mig1 from Saccharomyces cerevisiae (ScMig1). The C. albicans Mig1 protein (CaMig1) differs from ScMig1, in that, among other things, it lacks a putative phosphorylation site for Snf1 and presents several long stretches rich in glutamine or in asparagine, serine, and threonine and has the effector domain located at some distance (50 amino acids) from the carboxy terminus. Expression of CaMIG1 was low and was similar in glucose-, sucrose-, or ethanol-containing media. Disruption of the two CaMIG1 genomic copies had no effect in filamentation or infectivity. Levels of a glucose-repressible alpha-glucosidase, implicated in both sucrose and maltose utilization, were similar in wild-type or mig1/mig1 cells. Disruption of CaMIG1 had also no effect on the expression of the glucose-repressed gene CaGAL1. CaMIG1 was functional in S. cerevisiae, as judged by its ability to suppress the phenotypes produced by mig1 or tps1 mutations. In addition, CaMig1 formed specific complexes with the URS1 region of the S. cerevisiae FBP1 gene. The existence of a possible functional analogue of CaMIG1 in C. albicans was suggested by the results of band shift experiments.

Analysis by atomic force microscopy of Med8 binding to cis-acting regulatory elements of the SUC2 and HXK2 genes of saccharomyces cerevisiae

Med8 protein is a regulator that specifically binds to upstream activating sequences (UASs) of SUC2 promoter, to downstream repressing sequences (DRSs) of the HXK2 gene and to the carboxy-terminal domain of the RNA polymerase II. Atomic force microscopy has allowed for direct visualization of Med8 interactions with a 305 bp fragment of SUC2 promoter and with a 676 bp fragment of HXK2 gene, containing respectively the UASs and DRSs regulatory regions. This approach has provided complementary information about the position and the structure of the DNA-protein complexes. Med8 binding to DNA results in total covering of one of the two existing 7 bp motives (consensus, (A/C)(A/G)GAAAT) in the studied DNA fragments. No preference for binding either of the two UASs of SUC2 promoter as well as for the two DRSs of HXK2 gene has been found. We also discuss whether this protein works as dimer or as a monomer.