A SWI/SNF- and INO80-dependent nucleosome movement at the INO1 promoter

Transcriptional activation domains interact with ATPase subunits of yeast chromatin remodelling complexes SWI/SNF, RSC and INO80

Chromatin remodelling complexes (CRC) are ATP-dependent molecular machines important for the dynamic organization of nucleosomes along eukaryotic DNA. CRCs SWI/SNF, RSC and INO80 can move positioned nucleosomes in promoter DNA, leading to nucleosome-depleted regions which facilitate access of general transcription factors. This function is strongly supported by transcriptional activators being able to interact with subunits of various CRCs. In this work we show that SWI/SNF subunits Swi1, Swi2, Snf5 and Snf6 can bind to activation domains of Ino2 required for expression of phospholipid biosynthetic genes in yeast. We identify an activator binding domain (ABD) of ATPase Swi2 and show that this ABD is functionally dispensable, presumably because ABDs of other SWI/SNF subunits can compensate for the loss. In contrast, mutational characterization of the ABD of the Swi2-related ATPase Sth1 revealed that some conserved basic and hydrophobic amino acids within this domain are essential for the function of Sth1. While ABDs of Swi2 and Sth1 define separate functional protein domains, mapping of an ABD within ATPase Ino80 showed co-localization with its HSA domain also required for binding actin-related proteins. Comparative interaction studies finally demonstrated that several unrelated activators each exhibit a specific binding pattern with ABDs of Swi2, Sth1 and Ino80.

Yeast chromatin remodeling complexes and their roles in transcription

The nucleosome is a small unit of chromatin, which is dynamic in eukaryotes. Chromatin conformation and post-translational modifications affect nucleosome dynamics under certain conditions, playing an important role in the epigenetic regulation of transcription, replication and reprogramming. The Snf2 remodeling family is one of the crucial remodeling complexes that tightly regulate chromatin structure and affect nucleosome dynamics. This family alters nucleosome positioning, exchanges histone variants, and assembles and disassembles nucleosomes at certain locations. Moreover, the Snf2 family, in conjunction with other co-factors, regulates gene expression in Saccharomyces cerevisiae. Here we first review recent findings on the Snf2 family remodeling complexes and then use some examples to illustrate the cooperation between different members of Snf2 family, and the cooperation between Snf2 family and other co-factors in gene regulation especially during transcription initiation.

Accumulation of unacetylatable Snf2p at the INO1 promoter is detrimental to remodeler recycling supply for CUP1 induction

Chromatin structure plays a decisive role in gene regulation through the actions of transcriptional activators, coactivators, and epigenetic machinery. These trans-acting factors contribute to gene expression through their interactions with chromatin structure. In yeast INO1 activation, transcriptional activators and coactivators have been defined through intense study but the mechanistic links within these trans-acting factors and their functional implications are not yet fully understood. In this study, we examined the crosstalk within transcriptional coactivators with regard to the implications of Snf2p acetylation during INO1 activation. Through various biochemical analysis, we demonstrated that both Snf2p and Ino80p chromatin remodelers accumulate at the INO1 promoter in the absence of Snf2p acetylation during induction. Furthermore, nucleosome density and histone acetylation patterns remained unaffected by Snf2p acetylation status. We also showed that cells experience increased sensitivity to copper toxicity when remodelers accumulate at the INO1 promoter due to the decreased CUP1 expression. Therefore, our data provide evidence for crosstalk within transcriptional co-activators during INO1 activation. In light of these findings, we propose a model in which acetylation-driven chromatin remodeler recycling allows for efficient regulation of genes that are dependent upon limited co-activators.

Regulation of Inositol Biosynthesis: Balancing Health and Pathophysiology

Inositol is the precursor for all inositol compounds and is essential for viability of eukaryotic cells. Numerous cellular processes and signaling functions are dependent on inositol compounds, and perturbation of their synthesis leads to a wide range of human diseases. Although considerable research has been directed at understanding the function of inositol compounds, especially phosphoinositides and inositol phosphates, a focus on regulatory and homeostatic mechanisms controlling inositol biosynthesis has been largely neglected. Consequently, little is known about how synthesis of inositol is regulated in human cells. Identifying physiological regulators of inositol synthesis and elucidating the molecular mechanisms that regulate inositol synthesis will contribute fundamental insight into cellular processes that are mediated by inositol compounds and will provide a foundation to understand numerous disease processes that result from perturbation of inositol homeostasis. In addition, elucidating the mechanisms of action of inositol-depleting drugs may suggest new strategies for the design of second-generation pharmaceuticals to treat psychiatric disorders and other illnesses.

Widespread Impact of Chromosomal Inversions on Gene Expression Uncovers Robustness via Phenotypic Buffering

The nonrandom gene organization in eukaryotes plays a significant role in genome evolution and function. Chromosomal structural changes impact meiotic fitness and, in several organisms, are associated with speciation and rapid adaptation to different environments. Small sized chromosomal inversions, encompassing few genes, are pervasive in Saccharomyces “sensu stricto” species, while larger inversions are less common in yeasts compared with higher eukaryotes. To explore the effect of gene order on phenotype, reproductive isolation, and gene expression, we engineered 16 Saccharomyces cerevisiae strains carrying all possible paracentric and pericentric inversions between Ty1 elements, a natural substrate for rearrangements. We found that 4 inversions were lethal, while the other 12 did not show any fitness advantage or disadvantage in rich and minimal media. At meiosis, only a weak negative correlation with fitness was seen with the size of the inverted region. However, significantly lower fertility was seen in heterozygote invertant strains carrying recombination hotspots within the breakpoints. Altered transcription was observed throughout the genome rather than being overrepresented within the inversions. In spite of the large difference in gene expression in the inverted strains, mitotic fitness was not impaired in the majority of the 94 conditions tested, indicating that the robustness of the expression network buffers the deleterious effects of structural changes in several environments. Overall, our results support the notion that transcriptional changes may compensate for Ty-mediated rearrangements resulting in the maintenance of a constant phenotype, and suggest that large inversions in yeast are unlikely to be a selectable trait during vegetative growth.

The transcriptional activator Ino2p dissociates from the yeast INM1 promoter in induction

Mood stabilizers lithium and valproates are widely used in the treatment of bipolar disorder. It has been shown that these drugs can affect the inositol monophosphatase activity and thus the inositol de novo biosynthesis. However, the molecular mechanism of this action has thus far been vague. As such, characterizing the regulation of the gene encoding inositol monophosphatase at the molecular level can help to understand the bipolar disorder. As the model organism, the inositol monophosphatase is encoded by INM1 in Saccharomyces cerevisiae. In this study, we showed, using real-time reverse transcriptase polymerase chain reaction analysis, that INM1 is expressed in the presence of inositol, suggesting that the presence of inositol is required for INM1 transcriptional activation. We also demonstrated, using chromatin immunoprecipitation, that Ino2p is present at the promoter under uninduced conditions. Upon induction, Ino2p dissociates from the INM1 promoter. Furthermore, chromatin remodelers Ino80p and Snf2p are recruited to INM1 promoter upon induction as well as histone acetylases Gcn5p and Esa1p. Altogether, we have provided the evidence which describes how the transcriptional activator and coactivators participate in INM1 activation.

Co-dependent recruitment of Ino80p and Snf2p is required for yeast CUP1 activation

In yeast, Ace1p-dependent induction of CUP1 is responsible for protecting cells from copper toxicity. Although the mechanism of yeast CUP1 induction has been studied intensively, it is still uncertain which chromatin remodelers are involved in CUP1 transcriptional activation. Here, we show that yeast cells are inviable in the presence of copper when either chromatin remodeler, Ino80p or Snf2p, is not present. This inviability is due to the lack of CUP1 expression in ino80Δ and snf2Δ cells. Subsequently, we observe that both Ino80p and Snf2p are present at the promoter and they are responsible for recruiting chromatin remodeling activity to the CUP1 promoter under induced conditions. These results suggest that they directly participate in CUP1 transcriptional activation. Furthermore, the codependent recruitment of both INO80 and SWI/SNF depends on the presence of the transcriptional activator, Ace1p. We also demonstrate that both remodelers are required to recruit RNA polymerase II and targeted histone acetylation, indicating that remodelers are recruited to the CUP1 promoter before RNA polymerase II and histone acetylases. These observations provide evidence for the mechanism of CUP1 induction. As such, we propose a model that describes novel insight into the order of events in CUP1 activation.

A Gro/TLE-NuRD corepressor complex facilitates Tbx20-dependent transcriptional repression

The cardiac transcription factor Tbx20 has a critical role in the proper morphogenetic development of the vertebrate heart, and its misregulation has been implicated in human congenital heart disease. Although it is established that Tbx20 exerts its function in the embryonic heart through positive and negative regulation of distinct gene programs, it is unclear how Tbx20 mediates proper transcriptional regulation of its target genes. Here, using a combinatorial proteomic and bioinformatic approach, we present the first characterization of Tbx20 transcriptional protein complexes. We have systematically investigated Tbx20 protein-protein interactions by immunoaffinity purification of tagged Tbx20 followed by proteomic analysis using GeLC-MS/MS, gene ontology classification, and functional network analysis. We demonstrate that Tbx20 is associated with a chromatin remodeling network composed of TLE/Groucho corepressors, members of the Nucleosome Remodeling and Deacetylase (NuRD) complex, the chromatin remodeling ATPases RUVBL1/RUVBL2, and the T-box repressor Tbx18. We determined that the interaction with TLE corepressors is mediated via an eh1 binding motif in Tbx20. Moreover, we demonstrated that ablation of this motif results in a failure to properly assemble the repression network and disrupts Tbx20 function in vivo. Importantly, we validated Tbx20-TLE interactions in the mouse embryonic heart, and identified developmental genes regulated by Tbx20-TLE binding, thereby confirming a primary role for a Tbx20-TLE repressor complex in embryonic heart development. Together, these studies suggest a model in which Tbx20 associates with a Gro/TLE-NuRD repressor complex to prevent inappropriate gene activation within the forming heart.

Neuroprotective role of taurine during aging

Aging of the brain is characterized by several neurochemical modifications involving structural proteins, neurotransmitters, neuropeptides and related receptors. Alterations of neurochemical indices of synaptic function are indicators of age-related impairment of central functions, such as locomotion, memory and sensory performances. Several studies demonstrate that ionotropic GABA receptors, glutamate decarboxylase (GAD), and somatostatinergic subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Additionally, levels of several neuropeptides co-expressed with GAD decrease during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in GABA inhibitory neurotransmission. In this study, we showed that chronic supplementation of taurine to aged mice significantly ameliorated the age-dependent decline in spatial memory acquisition and retention. We also demonstrated that concomitant with the amelioration in cognitive function, taurine caused significant alterations in the GABAergic and somatostatinergic system. These changes included (1) increased levels of the neurotransmitters GABA and glutamate, (2) increased expression of both isoforms of GAD (65 and 67) and the neuropeptide somatostatin, (3) decreased hippocampal expression of the β3 subunits of the GABAA receptor, (4) increased expression in the number of somatostatin-positive neurons, (5) increased amplitude and duration of population spikes recorded from CA1 in response to Schaefer collateral stimulation and (6) enhanced paired pulse facilitation in the hippocampus. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally occurring in the aging brain, suggesting a protective role of taurine in this process. An increased understanding of age-related neurochemical changes in the GABAergic system will be important in elucidating the underpinnings of the functional changes of aging. Taurine supplementation might help forestall the age-related decline in cognitive functions through interaction with the GABAergic system.

SCS3 and YFT2 link transcription of phospholipid biosynthetic genes to ER stress and the UPR

The ability to store nutrients in lipid droplets (LDs) is an ancient function that provides the primary source of metabolic energy during periods of nutrient insufficiency and between meals. The Fat storage-Inducing Transmembrane (FIT) proteins are conserved ER–resident proteins that facilitate fat storage by partitioning energy-rich triglycerides into LDs. FIT2, the ancient ortholog of the FIT gene family first identified in mammals has two homologs in Saccharomyces cerevisiae (SCS3 and YFT2) and other fungi of the Saccharomycotina lineage. Despite the coevolution of these genes for more than 170 million years and their divergence from higher eukaryotes, SCS3, YFT2, and the human FIT2 gene retain some common functions: expression of the yeast genes in a human embryonic kidney cell line promotes LD formation, and expression of human FIT2 in yeast rescues the inositol auxotrophy and chemical and genetic phenotypes of strains lacking SCS3. To better understand the function of SCS3 and YFT2, we investigated the chemical sensitivities of strains deleted for either or both genes and identified synthetic genetic interactions against the viable yeast gene-deletion collection. We show that SCS3 and YFT2 have shared and unique functions that connect major biosynthetic processes critical for cell growth. These include lipid metabolism, vesicular trafficking, transcription of phospholipid biosynthetic genes, and protein synthesis. The genetic data indicate that optimal strain fitness requires a balance between phospholipid synthesis and protein synthesis and that deletion of SCS3 and YFT2 impacts a regulatory mechanism that coordinates these processes. Part of this mechanism involves a role for SCS3 in communicating changes in the ER (e.g. due to low inositol) to Opi1-regulated transcription of phospholipid biosynthetic genes. We conclude that SCS3 and YFT2 are required for normal ER membrane biosynthesis in response to perturbations in lipid metabolism and ER stress.

The ability to form lipid droplets is a conserved property of eukaryotic cells that allows the storage of excess metabolic energy in a form that can be readily accessed. In adipose tissue, the storage of excess calories in lipid droplets normally protects other tissues from lipotoxicity and insulin resistance, but this protection is lost with chronic over-nutrition. The FAT storage-inducing transmembrane (FIT) proteins were recently identified as a conserved family of proteins that reside in the lipid bilayer of the endoplasmic reticulum and are implicated in lipid droplet formation. In this work we show that specific functions of the FIT proteins are conserved between yeast and humans and that SCS3 and YFT2, the yeast homologs of mammalian FIT2, are part of a large genetic interaction network connecting lipid metabolism, vesicle trafficking, transcription, and protein synthesis. From these interactions we determined that yeast strains lacking SCS3 and YFT2 are defective in their response to chronic ER stress and cannot induce the unfolded protein response pathway or transcription of phospholipid biosynthetic genes in low inositol. Our findings suggest that the mammalian FIT genes may play an important role in ER stress pathways, which are linked to obesity and type 2 diabetes.

Chromatin repositioning activity and transcription machinery are both recruited by Ace1p in yeast CUP1 activation

The relationship among transcriptional activators, nucleosome repositioning activity and transcription machinery at the yeast CUP1 gene was addressed. CUP1 encodes a cysteine-rich, copper-binding metallothionein that protects cells against copper toxicity through its ability to sequester copper. The induction of CUP1 requires the presence of Ace1p and the binding of Ace1p at the CUP1 promoter during activation provides evidence that Ace1p is directly involved in CUP1 induction. Furthermore, transcriptional activation of CUP1 resulted in nucleosome repositioning at the CUP1 promoter and sequences further downstream in the coding region, suggesting a gene-wide chromatin remodeling activity. Such remodeling activity depends on the presence of transcription activator Ace1p. The recruitment of RNA polymerase II also requires the presence of Ace1p. Therefore, these observations provide insight into the molecular mechanism of CUP1 activation.

INO1 induction requires chromatin remodelers Ino80p and Snf2p but not the histone acetylases

Transcriptional co-activators contribute to gene expression through different mechanisms. We used various biochemical tools available for Saccharomyces cerevisiae to examine the mechanism of INO1 expression. INO1 encodes inositol-3-phosphate synthase, which catalyzes the rate-limiting step in the synthesis of inositol, a key player in phospholipid biosynthesis. Herein, we had demonstrated that the recruitment of histone acetylases Gcn5p and Esa1p mainly relied on the presence of transcriptional activator Ino2p during INO1 activation. However, the presence of the chromatin remodelers, Ino80p and Snf2p, may contribute to the additive effect of Gcn5p recruitment. We also showed that the recruitment of chromatin remodelers, Ino80p and Snf2p, is independent of the presence of histone acetylases. Furthermore, INO1 expression can be activated exclusively by the activator and chromatin remodelers, suggesting a dispensable role of histone acetylases in INO1 induction. Therefore, our data provide a mechanism for cross talk within transcriptional co-activators during INO1 activation.

Interplay of dynamic transcription and chromatin remodeling: lessons from yeast

Regulation of transcription involves dynamic rearrangements of chromatin structure. The budding yeast Saccharomyces cerevisiae has a variety of highly conserved factors necessary for these reconstructions. Chromatin remodelers, histone modifiers and histone chaperones directly associate to promoters and open reading frames of exposed genes and facilitate activation and repression of transcription. We compare two distinct patterns of induced transcription: Sustained transcribed genes switch to an activated state where they remain as long as the induction signal is present. In contrast, single pulsed transcribed genes show a quick and strong induction pulse resulting in high transcript levels followed by adaptation and repression to basal levels. We discuss intensively studied promoters and coding regions from both groups for their co-factor requirements during transcription. Interplay between chromatin restructuring factors and dynamic transcription is highly variable and locus dependent.

The INO80 ATP-dependent chromatin remodeling complex is a nucleosome spacing factor

The mobilization of nucleosomes by the ATP-dependent remodeler INO80 is quite different from another remodeler (SWI/SNF) that is also involved in gene activation. Unlike that recently shown for SWI/SNF, INO80 is unable to disassemble nucleosomes when remodeling short nucleosomal arrays. Instead, INO80 more closely resembles, although with notable exceptions, the nucleosome spacing activity of ISW2 and ISW1a, which are generally involved in transcription repression. INO80 required a minimum of 33 to 43 bp of extranucleosomal DNA for mobilizing nucleosomes, with 70 bp being optimal. INO80 prefers to move mononucleosomes to the center of DNA, like ISW2 and ISW1a, but does so with higher precision. Unlike ISW2/1a, INO80 does not require the H4 tail for nucleosome mobilization; instead, the H2A histone tail negatively regulates nucleosome movement by INO80. INO80 moved arrays of two or three nucleosomes with 50 or 79 bp of linker DNA closer together, with a final length of ∼30 bp of linker DNA or a repeat length of ∼177 bp. A minimum length of >30 bp of linker DNA was required for nucleosome movement and spacing by INO80 in arrays.

Derepression of INO1 transcription requires cooperation between the Ino2p-Ino4p heterodimer and Cbf1p and recruitment of the ISW2 chromatin-remodeling complex

The Saccharomyces cerevisiae INO1 gene encodes the structural enzyme inositol-3-phosphate synthase for the synthesis de novo of inositol and inositol-containing phospholipids. The transcription of INO1 is completely derepressed in the absence of inositol and choline (I(-) C(-)). Derepression requires the binding of the Ino2p-Ino4p basic helix-loop-helix (bHLH) heterodimer to the UAS(INO) promoter element. We report here the requirement of a third bHLH protein, centromere-binding factor 1 (Cbf1p), for the complete derepression of INO1 transcription. We found that Cbf1p regulates INO1 transcription by binding to sites distal to the INO1 promoter and encompassing the upstream SNA3 open reading frame (ORF) and promoter. The binding of Cbf1p requires Ino2p-Ino4p binding to the UAS(INO) sites in the INO1 promoter and vice versa, suggesting a cooperative mechanism. Furthermore, Cbf1p binding to the upstream sites was required for the binding of the ISW2 chromatin-remodeling complex to the Ino2p-Ino4p-binding sites on the INO1 promoter. Consistent with this, ISW2 was also required for the complete derepression of INO1 transcription.

Genome-wide association data reveal a global map of genetic interactions among protein complexes

This work demonstrates how gene association studies can be analyzed to map a global landscape of genetic interactions among protein complexes and pathways. Despite the immense potential of gene association studies, they have been challenging to analyze because most traits are complex, involving the combined effect of mutations at many different genes. Due to lack of statistical power, only the strongest single markers are typically identified. Here, we present an integrative approach that greatly increases power through marker clustering and projection of marker interactions within and across protein complexes. Applied to a recent gene association study in yeast, this approach identifies 2,023 genetic interactions which map to 208 functional interactions among protein complexes. We show that such interactions are analogous to interactions derived through reverse genetic screens and that they provide coverage in areas not yet tested by reverse genetic analysis. This work has the potential to transform gene association studies, by elevating the analysis from the level of individual markers to global maps of genetic interactions. As proof of principle, we use synthetic genetic screens to confirm numerous novel genetic interactions for the INO80 chromatin remodeling complex.

Fission yeast Iec1-ino80-mediated nucleosome eviction regulates nucleotide and phosphate metabolism

Ino80 is an ATP-dependent nucleosome-remodeling enzyme involved in transcription, replication, and the DNA damage response. Here, we characterize the fission yeast Ino80 and find that it is essential for cell viability. We show that the Ino80 complex from fission yeast mediates ATP-dependent nucleosome remodeling in vitro. The purification of the Ino80-associated complex identified a highly conserved complex and the presence of a novel zinc finger protein with similarities to the mammalian transcriptional regulator Yin Yang 1 (YY1) and other members of the GLI-Krüppel family of proteins. Deletion of this Iec1 protein or the Ino80 complex subunit arp8, ies6, or ies2 causes defects in DNA damage repair, the response to replication stress, and nucleotide metabolism. We show that Iec1 is important for the correct expression of genes involved in nucleotide metabolism, including the ribonucleotide reductase subunit cdc22 and phosphate- and adenine-responsive genes. We find that Ino80 is recruited to a large number of promoter regions on phosphate starvation, including those of phosphate- and adenine-responsive genes that depend on Iec1 for correct expression. Iec1 is required for the binding of Ino80 to target genes and subsequent histone loss at the promoter and throughout the body of these genes on phosphate starvation. This suggests that the Iec1-Ino80 complex promotes transcription through nucleosome eviction.

NTE1-encoded phosphatidylcholine phospholipase b regulates transcription of phospholipid biosynthetic genes

The Saccharomyces cerevisiae NTE1 gene encodes an evolutionarily conserved phospholipase B localized to the endoplasmic reticulum (ER) that degrades phosphatidylcholine (PC) generating glycerophosphocholine and free fatty acids. We show here that the activity of NTE1-encoded phospholipase B (Nte1p) prevents the attenuation of transcription of genes encoding enzymes involved in phospholipid synthesis in response to increased rates of PC synthesis by affecting the nuclear localization of the transcriptional repressor Opi1p. Nte1p activity becomes necessary for cells growing in inositol-free media under conditions of high rates of PC synthesis elicited by the presence of choline at 37 degrees C. The specific choline transporter encoded by the HNM1 gene is necessary for the burst of PC synthesis observed at 37 degrees C as follows: (i) Nte1p is dispensable in an hnm1Delta strain under these conditions, and (ii) there is a 3-fold increase in the rate of choline transport via the Hnm1p choline transporter upon a shift to 37 degrees C. Overexpression of NTE1 alleviated the inositol auxotrophy of a plethora of mutants, including scs2Delta, scs3Delta, ire1Delta, and hac1Delta among others. Overexpression of NTE1 sustained phospholipid synthesis gene transcription under conditions that normally repress transcription. This effect was also observed in a strain defective in the activation of free fatty acids for phosphatidic acid synthesis. No changes in the levels of phosphatidic acid were detected under conditions of altered expression of NTE1. Consistent with a synthetic impairment between challenged ER function and inositol deprivation, increased expression of NTE1 improved the growth of cells exposed to tunicamycin in the absence of inositol. We describe a new role for Nte1p toward membrane homeostasis regulating phospholipid synthesis gene transcription. We propose that Nte1p activity, by controlling PC abundance at the ER, affects lateral membrane packing and that this parameter, in turn, impacts the repressing transcriptional activity of Opi1p, the main regulator of phospholipid synthesis gene transcription.

The INO80 chromatin remodeling complex in transcription, replication and repair

The Ino80 ATPase is a member of the SNF2 family of ATPases and functions as an integral component of a multisubunit ATP-dependent chromatin remodeling complex. Although INO80 complexes from yeast and higher eukaryotes share a common core of conserved subunits, the complexes have diverged substantially during evolution and have acquired new subunits with apparently species-specific functions. Recent studies have shown that the INO80 complex contributes to a wide variety of chromatin-dependent nuclear transactions, including transcription, DNA repair and DNA replication.

Activator-dependent recruitment of SWI/SNF and INO80 during INO1 activation

Transcriptional activation of yeast INO1 requires SWI/SNF and INO80 for nucleosome disruption at the promoter. However, the cooperative interplay among remodelers and their recruitment dynamics in activation have thus far been vague. Here, we showed, using chromatin immunoprecipitation, that both SWI/SNF and INO80 are present at the promoter and are restricted to the promoter, indicating that they directly participate in localized INO1 chromatin remodeling. Furthermore, both SWI/SNF and INO80 are absent at the INO1 promoter in ino2Delta cells, suggesting that these are activator-dependent remodelers. We have also found that the presence of INO80 is required for SWI/SNF recruitment, indicating that INO80 arrives first at the promoter followed by SWI/SNF. In light of these findings, we proposed a model which describes the order of events in INO1 activation.