Positive and negative functions of the SAGA complex mediated through interaction of Spt8 with TBP and the N-terminal domain of TFIIA

A CUG codon-adapted anchor-away toolkit for functional analysis of genes in Candida albicans

Promoter shutoff of essential genes in the diploid Candida albicans has often been insufficient to create tight, conditional null alleles due to leaky expression and has been a stumbling block in pathogenesis research. Moreover, homozygous deletion of non-essential genes has often been problematic due to the frequent aneuploidy in the mutant strains. Rapid, conditional depletion of essential genes by the anchor-away strategy has been successfully employed in Saccharomyces cerevisiae and other model organisms. Here, rapamycin mediates the dimerization of human FK506-binding protein (FKBP12) and FKBP12-rapamycin-binding (FRB) domain-containing target protein, resulting in relocalization to altered sub-cellular locations. In this work, we used the ribosomal protein Rpl13 as the anchor and took two nuclear proteins as targets to construct a set of mutants in a proof-of-principle approach. We first constructed a rapamycin-resistant C. albicans strain by introducing a dominant mutation in the CaTOR1 gene and a homozygous deletion of RBP1, the ortholog of FKBP12, a primary target of rapamycin. The FKBP12 and the FRB coding sequences were then CUG codon-adapted for C. albicans by site-directed mutagenesis. Anchor-away strains expressing the essential TBP1 gene or the non-essential SPT8 gene as FRB fusions were constructed. We found that rapamycin caused rapid cessation of growth of the TBP-AA strain within 15 minutes and the SPT8-AA strain phenocopied the constitutive filamentous phenotype of the spt8Δ/spt8Δ mutant. Thus, the anchor-away toolbox for C. albicans developed here can be employed for genome-wide analysis to identify gene function in a rapid and reliable manner, further accelerating anti-fungal drug development in C. albicans.

IMPORTANCE

Molecular genetic studies thus far have identified ~27% open-reading frames as being essential for the vegetative growth of Candida albicans in rich medium out of a total 6,198 haploid set of open reading frames. However, a major limitation has been to construct rapid conditional alleles of essential C. albicans genes with near quantitative depletion of encoded proteins. Here, we have developed a toolbox for rapid and conditional depletion of genes that would aid studies of gene function of both essential and non-essential genes.

Increased histone acetylation is the signature of repressed state on the genes transcribed by RNA polymerase III

Several covalent modifications are found associated with the transcriptionally active chromatin regions constituted by the genes transcribed by RNA polymerase (pol) II. Pol III-transcribed genes code for the small, stable RNA species, which participate in many cellular processes, essential for survival. Pol III transcription is repressed under most of the stress conditions by its negative regulator Maf1. We found that most of the histone acetylations increase with starvation-induced repression on several genes transcribed by the yeast pol III. On one of these genes, SNR6 (coding for the U6snRNA), a strongly positioned nucleosome in the gene upstream region plays regulatory role under repression. On this nucleosome, the changes in H3K9 and H3K14 acetylations show different dynamics. During repression, acetylation levels on H3K9 show steady increase whereas H3K14 acetylation increases with a peak at 40 min after which levels reduce. Both the levels settle by 2 hr to a level higher than the active state, which revert to normal levels with nutrient repletion. The increase in H3 acetylations is seen in the mutants reported to show reduced SNR6 transcription but not in the maf1Δ cells. This increase on a regulatory nucleosome may be part of the signaling mechanisms, which prepare cells for the stress-related quick repression as well as reactivation. The contrasting association of the histone acetylations with pol II and pol III transcription may be an important consideration to make in research studies focused on drug developments targeting histone modifications.

The Saccharomyces cerevisiae acetyltransferase Gcn5 exerts antagonistic pleiotropic effects on chronological ageing

Compared to replicative lifespan, epigenetic regulation of chronological lifespan (CLS) is less well understood in yeast. Here, by screening all the viable mutants of histone acetyltransferase (HAT) and histone deacetylase (HDAC), we demonstrate that Gcn5, functioning in the HAT module of the SAGA/SLIK complex, exhibits an epistatic relationship with the HDAC Hda1 to control the expression of starvation-induced stress response and respiratory cell growth. Surprisingly, the gcn5Δ mutants lose their colony-forming potential early in the stationary phase but display a longer maximum CLS than their WT counterparts, suggesting the contradictory roles of Gcn5 in lifespan regulation. Integrative analyses of the transcriptome, metabolome and ChIP assays reveal that Gcn5 is necessary for the activation of two regulons upon glucose starvation: the Msn2/4-/Gis1-dependent stress response and the Cat8-/Adr1-mediated metabolic reprogramming, to enable pro-longevity characteristics, including redox homeostasis, stress resistance and maximal storage of carbohydrates. The activation of Cat8-/Adr1-dependent regulon also promotes the pyruvate dehydrogenase (PDH) bypass, leading to acetyl-CoA synthesis, global and targeted H3K9 acetylation. Global H3K9 acetylation levels mediated by Gcn5 and Hda1 during the transition into stationary phase are positively correlated with senescent cell populations accumulated in the aged cell cultures. These data suggest that Gcn5 lies in the centre of a feed-forward loop between histone acetylation and starvation-induced gene expression, enabling stress resistance and homeostasis but also promoting chronological ageing concomitantly.

Involvement of the SAGA and TFIID coactivator complexes in transcriptional dysregulation caused by the separation of core and tail Mediator modules

Regulation of RNA polymerase II transcription requires the concerted efforts of several multisubunit coactivator complexes, which interact with the RNA polymerase II preinitiation complex to stimulate transcription. We previously showed that separation of the Mediator core from Mediator's tail module results in modest overactivation of genes annotated as highly dependent on TFIID for expression. However, it is unclear if other coactivators are involved in this phenomenon. Here, we show that the overactivation of certain genes by Mediator core/tail separation is blunted by disruption of the Spt-Ada-Gcn5-Acetyl transferase complex through the removal of its structural Spt20 subunit, though this downregulation does not appear to completely depend on reduced Spt-Ada-Gcn5-Acetyl transferase association with the genome. Consistent with the enrichment of TFIID-dependent genes among genes overactivated by Mediator core/tail separation, depletion of the essential TFIID subunit Taf13 suppressed the overactivation of these genes when Med16 was simultaneously removed. As with Spt-Ada-Gcn5-Acetyl transferase, this effect did not appear to be fully dependent on the reduced genomic association of TFIID. Given that the observed changes in gene expression could not be clearly linked to alterations in Spt-Ada-Gcn5-Acetyl transferase or TFIID occupancy, our data may suggest that the Mediator core/tail connection is important for the modulation of Spt-Ada-Gcn5-Acetyl transferase and/or TFIID conformation and/or function at target genes.

SAGA Complex Subunits in Candida albicans Differentially Regulate Filamentation, Invasiveness, and Biofilm Formation

SAGA (Spt-Ada-Gcn5-acetyltransferase) is a highly conserved, multiprotein co-activator complex that consists of five distinct modules. It has two enzymatic functions, a histone acetyltransferase (HAT) and a deubiquitinase (DUB) and plays a central role in processes such as transcription initiation, elongation, protein stability, and telomere maintenance. We analyzed conditional and null mutants of the SAGA complex module components in the fungal pathogen Candida albicans; Ngg1, (the HAT module); Ubp8, (the DUB module); Tra1, (the recruitment module), Spt7, (the architecture module) and Spt8, (the TBP interaction unit), and assessed their roles in a variety of cellular processes. We observed that spt7Δ/Δ and spt8Δ/Δ strains have a filamentous phenotype, and both are highly invasive in yeast growing conditions as compared to the wild type, while ngg1Δ/Δ and ubp8Δ/Δ are in yeast-locked state and non-invasive in both YPD media and filamentous induced conditions compared to wild type. RNA-sequencing-based transcriptional profiling of SAGA mutants reveals upregulation of hyphal specific genes in spt7Δ/Δ and spt8Δ/Δ strains and downregulation of ergosterol metabolism pathway. As well, spt7Δ/Δ and spt8Δ/Δ confer susceptibility to antifungal drugs, to acidic and alkaline pH, to high temperature, and to osmotic, oxidative, cell wall, and DNA damage stresses, indicating that these proteins are important for genotoxic and cellular stress responses. Despite having similar morphological phenotypes (constitutively filamentous and invasive) spt7 and spt8 mutants displayed variation in nuclear distribution where spt7Δ/Δ cells were frequently binucleate and spt8Δ/Δ cells were consistently mononucleate. We also observed that spt7Δ/Δ and spt8Δ/Δ mutants were quickly engulfed by macrophages compared to ngg1Δ/Δ and ubp8Δ/Δ strains. All these findings suggest that the SAGA complex modules can have contrasting functions where loss of Spt7 or Spt8 enhances filamentation and invasiveness while loss of Ngg1 or Ubp8 blocks these processes.

The SAGA chromatin-modifying complex: the sum of its parts is greater than the whole

In this review, Soffers and Workman discuss the initial discovery of the canonical SAGA complex, the subsequent studies that have shaped our view on the internal organization of its subunits into modules, and the latest structural work that visualizes the modules and provides insights into their function.

There are many large protein complexes involved in transcription in a chromatin context. However, recent studies on the SAGA coactivator complex are generating new paradigms for how the components of these complexes function, both independently and in concert. This review highlights the initial discovery of the canonical SAGA complex 23 years ago, our evolving understanding of its modular structure and the relevance of its modular nature for its coactivator function in gene regulation.

SAGA and SAGA-like SLIK transcriptional coactivators are structurally and biochemically equivalent

The SAGA-like complex SLIK is a modified version of the Spt-Ada-Gcn5-Acetyltransferase (SAGA) complex. SLIK is formed through C-terminal truncation of the Spt7 SAGA subunit, causing loss of Spt8, one of the subunits that interacts with the TATA-binding protein (TBP). SLIK and SAGA are both coactivators of RNA polymerase II transcription in yeast, and both SAGA and SLIK perform chromatin modifications. The two complexes have been speculated to uniquely contribute to transcriptional regulation, but their respective contributions are not clear. To investigate, we assayed the chromatin modifying functions of SAGA and SLIK, revealing identical kinetics on minimal substrates in vitro. We also examined the binding of SAGA and SLIK to TBP and concluded that interestingly, both protein complexes have similar affinity for TBP. Additionally, despite the loss of Spt8 and C-terminus of Spt7 in SLIK, TBP prebound to SLIK is not released in the presence of TATA-box DNA, just like TBP prebound to SAGA. Furthermore, we determined a low-resolution cryo-EM structure of SLIK, revealing a modular architecture identical to SAGA. Finally, we performed a comprehensive study of DNA-binding properties of both coactivators. Purified SAGA and SLIK both associate with ssDNA and dsDNA with high affinity (K_D = 10–17 nM), and the binding is sequence-independent. In conclusion, our study shows that the cleavage of Spt7 and the absence of the Spt8 subunit in SLIK neither drive any major conformational differences in its structure compared with SAGA, nor significantly affect HAT, DUB, or DNA-binding activities in vitro.

The Spt7 subunit of the SAGA complex is required for the regulation of lifespan in both dividing and nondividing yeast cells

Spt7 belongs to the suppressor of Ty (SPT) module of the Spt-Ada-Gcn5-acetyltransferase (SAGA) complex and is known as the yeast ortholog of human STAF65γ. Spt7 lacks intrinsic enzymatic activity but is responsible for the integrity and proper assembly of the SAGA complex. Here, we determined the role of the SAGA Spt7 subunit in cellular aging. We found that Spt7 was indispensable for a normal lifespan in both dividing and nondividing yeast cells. In the quiescent state of cells, Spt7 was required for the control of overall mRNA levels. In mitotically active cells, deletion of the SPT module had little effect on the recombination rate within heterochromatic ribosomal DNA (rDNA) loci, but loss of Spt7 profoundly elevated the plasmid-based DNA recombination frequency. Consistently, loss of Spt7 increased spontaneous Rad52 foci by approximately two-fold upon entry into S phase. These results provide evidence that Spt7 contributes to the regulation of the normal replicative lifespan (RLS) and chronological lifespan (CLS), possibly by controlling the DNA recombination rate and overall mRNA expression. We propose that the regulation of SAGA complex integrity by Spt7 might be involved in the conserved regulatory pathway for lifespan regulation in eukaryotes.

The biochemical and genetic discovery of the SAGA complex

One of the major advances in our understanding of gene regulation in eukaryotes was the discovery of factors that regulate transcription by controlling chromatin structure. Prominent among these discoveries was the demonstration that Gcn5 is a histone acetyltransferase, establishing a direct connection between transcriptional activation and histone acetylation. This breakthrough was soon followed by the purification of a protein complex that contains Gcn5, the SAGA complex. In this article, we review the early genetic and biochemical experiments that led to the discovery of SAGA and the elucidation of its multiple activities.

Structure of SAGA and mechanism of TBP deposition on gene promoters

SAGA (Spt-Ada-Gcn5-acetyltransferase) is a 19-subunit complex that stimulates transcription via two chromatin-modifying enzymatic modules and by delivering the TATA box binding protein (TBP) to nucleate the pre-initiation complex on DNA, a pivotal event in the expression of protein-encoding genes¹. Here we present the structure of yeast SAGA with bound TBP. The core of the complex is resolved at 3.5 Å resolution (0.143 Fourier shell correlation). The structure reveals the intricate network of interactions that coordinate the different functional domains of SAGA and resolves an octamer of histone-fold domains at the core of SAGA. This deformed octamer deviates considerably from the symmetrical analogue in the nucleosome and is precisely tuned to establish a peripheral site for TBP, where steric hindrance represses binding of spurious DNA. Complementary biochemical analysis points to a mechanism for TBP delivery and release from SAGA that requires transcription factor IIA and whose efficiency correlates with the affinity of DNA to TBP. We provide the foundations for understanding the specific delivery of TBP to gene promoters and the multiple roles of SAGA in regulating gene expression.

Structure of the transcription coactivator SAGA

Gene transcription by RNA polymerase II is regulated by activator proteins that recruit the coactivator complexes SAGA (Spt-Ada-Gcn5-acetyltransferase)1,2 and transcription factor IID (TFIID)2-4. SAGA is required for all regulated transcription5 and is conserved among eukaryotes6. SAGA contains four modules7-9: the activator-binding Tra1 module, the core module, the histone acetyltransferase (HAT) module and the histone deubiquitination (DUB) module. Previous studies provided partial structures10-14, but the structure of the central core module is unknown. Here we present the cryo-electron microscopy structure of SAGA from the yeast Saccharomyces cerevisiae and resolve the core module at 3.3 Å resolution. The core module consists of subunits Taf5, Sgf73 and Spt20, and a histone octamer-like fold. The octamer-like fold comprises the heterodimers Taf6-Taf9, Taf10-Spt7 and Taf12-Ada1, and two histone-fold domains in Spt3. Spt3 and the adjacent subunit Spt8 interact with the TATA box-binding protein (TBP)2,7,15-17. The octamer-like fold and its TBP-interacting region are similar in TFIID, whereas Taf5 and the Taf6 HEAT domain adopt distinct conformations. Taf12 and Spt20 form flexible connections to the Tra1 module, whereas Sgf73 tethers the DUB module. Binding of a nucleosome to SAGA displaces the HAT and DUB modules from the core-module surface, allowing the DUB module to bind one face of an ubiquitinated nucleosome.

Gcn5-mediated acetylation at MBF-regulated promoters induces the G1/S transcriptional wave

In fission yeast, MBF-dependent transcription is inactivated at the end of S phase through a negative feedback loop that involves the co-repressors, Yox1 and Nrm1. Although this repression system is well known, the molecular mechanisms involved in MBF activation remain largely unknown. Compacted chromatin constitutes a barrier to activators accessing promoters. Here, we show that chromatin regulation plays a key role in activating MBF-dependent transcription. Gcn5, a part of the SAGA complex, binds to MBF-regulated promoters through the MBF co-activator Rep2 in a cell cycle-dependent manner and in a reverse correlation to the binding of the MBF co-repressors, Nrm1 or Yox1. We propose that the co-repressors function as physical barriers to SAGA recruitment onto MBF promoters. We also show that Gcn5 acetylates specific lysine residues on histone H3 in a cell cycle-regulated manner. Furthermore, either in a gcn5 mutant or in a strain in which histone H3 is kept in an unacetylated form, MBF-dependent transcription is downregulated. In summary, Gcn5 is required for the full activation and correct timing of MBF-regulated gene transcription.

A synthetic non-histone substrate to study substrate targeting by the Gcn5 HAT and sirtuin HDACs

Gcn5 and sirtuins are highly conserved histone acetyltransferase (HAT) and histone deacetylase (HDAC) enzymes that were first characterized as regulators of gene expression. Although histone tails are important substrates of these enzymes, they also target many nonhistone proteins that function in diverse biological processes. However, the mechanisms used by these enzymes to choose their nonhistone substrates are unknown. Previously, we used SILAC-based MS to identify novel nonhistone substrates of Gcn5 and sirtuins in yeast and found a shared target consensus sequence. Here, we use a synthetic biology approach to demonstrate that this consensus sequence can direct acetylation and deacetylation targeting by these enzymes in vivo Remarkably, fusion of the sequence to a nonsubstrate confers de novo acetylation that is regulated by both Gcn5 and sirtuins. We exploit this synthetic fusion substrate as a tool to define subunits of the Gcn5-containing SAGA and ADA complexes required for nonhistone protein acetylation. In particular, we find a key role for the Ada2 and Ada3 subunits in regulating acetylations on our fusion substrate. In contrast, other subunits tested were largely dispensable, including those required for SAGA stability. In an extended analysis, defects in proteome-wide acetylation observed in ada3Δ mutants mirror those in ada2Δ mutants. Altogether, our work argues that nonhistone protein acetylation by Gcn5 is determined in part by specific amino acids surrounding target lysines but that even optimal sequences require both Ada2 and Ada3 for robust acetylation. The synthetic fusion substrate we describe can serve as a tool to further dissect the regulation of both Gcn5 and sirtuin activities in vivo.

Promotion of Cell Viability and Histone Gene Expression by the Acetyltransferase Gcn5 and the Protein Phosphatase PP2A in Saccharomyces cerevisiae

Histone modifications direct chromatin-templated events in the genome and regulate access to DNA sequence information. There are multiple types of modifications, and a common feature is their dynamic nature. An essential step for understanding their regulation, therefore, lies in characterizing the enzymes responsible for adding and removing histone modifications. Starting with a dosage-suppressor screen in Saccharomyces cerevisiae, we have discovered a functional interaction between the acetyltransferase Gcn5 and the protein phosphatase 2A (PP2A) complex, two factors that regulate post-translational modifications. We find that RTS1, one of two genes encoding PP2A regulatory subunits, is a robust and specific high-copy suppressor of temperature sensitivity of gcn5∆ and a subset of other gcn5∆ phenotypes. Conversely, loss of both PP2A(Rts1) and Gcn5 function in the SAGA and SLIK/SALSA complexes is lethal. RTS1 does not restore global transcriptional defects in gcn5∆; however, histone gene expression is restored, suggesting that the mechanism of RTS1 rescue includes restoration of specific cell cycle transcripts. Pointing to new mechanisms of acetylation-phosphorylation cross-talk, RTS1 high-copy rescue of gcn5∆ growth requires two residues of H2B that are phosphorylated in human cells. These data highlight the potential significance of dynamic phosphorylation and dephosphorylation of these deeply conserved histone residues for cell viability.

Affinity and competition for TBP are molecular determinants of gene expression noise

Cell-to-cell variation in gene expression levels (noise) generates phenotypic diversity and is an important phenomenon in evolution, development and disease. TATA-box binding protein (TBP) is an essential factor that is required at virtually every eukaryotic promoter to initiate transcription. While the presence of a TATA-box motif in the promoter has been strongly linked with noise, the molecular mechanism driving this relationship is less well understood. Through an integrated analysis of multiple large-scale data sets, computer simulation and experimental validation in yeast, we provide molecular insights into how noise arises as an emergent property of variable binding affinity of TBP for different promoter sequences, competition between interaction partners to bind the same surface on TBP (to either promote or disrupt transcription initiation) and variable residence times of TBP complexes at a promoter. These determinants may be fine-tuned under different conditions and during evolution to modulate eukaryotic gene expression noise.

Mapping the deubiquitination module within the SAGA complex

The molecular organization of the yeast transcriptional coactivator Spt-Ada-Gcn5 acetyltransferase (SAGA) was analyzed by single-particle electron microscopy. Complete or partial deletion of the Sgf73 subunit disconnects the deubiquitination (DUB) module from SAGA and favors in our conditions the cleavage of the C-terminal ends of the Spt7 subunit and the loss of the Spt8 subunit. The structural comparison of the wild-type SAGA with two deletion mutants positioned the DUB module and enabled the fitting of the available atomic models. The localization of the DUB module close to Gcn5 defines a chromatin-binding interface within SAGA, which could be demonstrated by the binding of nucleosome core particles. The TATA-box binding protein (TBP)-interacting subunit Spt8 was found to be located close to the DUB but in a different domain than Spt3, also known to contact TBP. A flexible protein arm brings both subunits close enough to interact simultaneously with TBP.

Architecture of the Saccharomyces cerevisiae SAGA transcription coactivator complex

The conserved transcription coactivator SAGA is comprised of several modules that are involved in activator binding, TBP binding, histone acetylation (HAT) and deubiquitination (DUB). Crosslinking and mass spectrometry, together with genetic and biochemical analyses, were used to determine the molecular architecture of the SAGA-TBP complex. We find that the SAGA Taf and Taf-like subunits form a TFIID-like core complex at the center of SAGA that makes extensive interactions with all other SAGA modules. SAGA-TBP binding involves a network of interactions between subunits Spt3, Spt8, Spt20, and Spt7. The HAT and DUB modules are in close proximity, and the DUB module modestly stimulates HAT function. The large activator-binding subunit Tra1 primarily connects to the TFIID-like core via its FAT domain. These combined results were used to derive a model for the arrangement of the SAGA subunits and its interactions with TBP. Our results provide new insight into SAGA function in gene regulation, its structural similarity with TFIID, and functional interactions between the SAGA modules.

Sus1p facilitates pre-initiation complex formation at the SAGA-regulated genes independently of histone H2B de-ubiquitylation

Sus1p is a common component of transcriptional co-activator, SAGA (Spt-Ada-Gcn5-Acetyltransferase), and mRNA export complex, TREX-2 (Transcription-export 2), and is involved in promoting transcription and mRNA export. However, it is not clearly understood how Sus1p promotes transcription. Here, we show that Sus1p is predominantly recruited to the upstream activating sequence of a SAGA-dependent gene, GAL1, under transcriptionally active conditions as a component of SAGA to promote the formation of pre-initiation complex (PIC) at the core promoter and, consequently, transcriptional initiation. Likewise, Sus1p promotes the PIC formation at other SAGA-dependent genes and hence transcriptional initiation. Such function of Sus1p in promoting PIC formation and transcriptional initiation is not mediated via its role in regulation of SAGA's histone H2B de-ubiquitylation activity. However, Sus1p's function in regulation of histone H2B ubiquitylation is associated with transcriptional elongation, DNA repair and replication. Collectively, our results support that Sus1p promotes PIC formation (and hence transcriptional initiation) at the SAGA-regulated genes independently of histone H2B de-ubiquitylation and further controls transcriptional elongation, DNA repair and replication via orchestration of histone H2B ubiquitylation, thus providing distinct functional insights of Sus1p in regulation of DNA transacting processes.

Direct TFIIA-TFIID protein contacts drive budding yeast ribosomal protein gene transcription

We have previously shown that yeast TFIID provides coactivator function on the promoters of ribosomal protein-encoding genes (RPGs) by making direct contact with the transactivator repressor activator protein 1 (Rap1). Further, our structural studies of assemblies generated with purified Rap1, TFIID, and TFIIA on RPG enhancer-promoter DNA indicate that Rap1-TFIID interaction induces dramatic conformational rearrangements of enhancer-promoter DNA and TFIID-bound TFIIA. These data indicate a previously unknown yet critical role for yeast TFIIA in the integration of activator-TFIID contacts with promoter conformation and downstream preinitiation complex formation and/or function. Here we describe the use of systematic mutagenesis to define how specific TFIIA contacts contribute to these processes. We have verified that TFIIA is required for RPG transcription in vivo and in vitro, consistent with the existence of a critical Rap1-TFIIA-TFIID interaction network. We also identified essential points of contact for TFIIA and Rap1 within the Rap1 binding domain of the Taf4 subunit of TFIID. These data suggest a mechanism for how interactions between TFIID, TFIIA, and Rap1 contribute to the high rate of transcription initiation seen on RPGs in vivo.

Mediator and SAGA have distinct roles in Pol II preinitiation complex assembly and function

A key feature of RNA polymerase II (Pol II) preinitiation complexes (PICs) is their ability to coordinate transcription initiation with chromatin modification and remodeling. To understand how this coordination is achieved, we employed extensive proteomic and mechanistic analyses to study the composition and assembly of PICs in HeLa cell and mouse embryonic stem cell (ESC) nuclear extracts. Strikingly, most of the machinery that is necessary for transcription initiation on chromatin is part of the PIC. The PIC is nearly identical between ESCs and HeLa cells and contains two major coactivator complexes: Mediator and SAGA. Genome-wide analysis of Mediator reveals that it has a close correlation with Pol II, TATA-binding protein, and messenger RNA levels and thus may play a major role in PIC assembly. Moreover, Mediator coordinates assembly of the Pol II initiation factors and chromatin machinery into a PIC in vitro, whereas SAGA acts after PIC assembly to allow transcription on chromatin.

The NSL complex regulates housekeeping genes in Drosophila

MOF is the major histone H4 lysine 16-specific (H4K16) acetyltransferase in mammals and Drosophila. In flies, it is involved in the regulation of X-chromosomal and autosomal genes as part of the MSL and the NSL complexes, respectively. While the function of the MSL complex as a dosage compensation regulator is fairly well understood, the role of the NSL complex in gene regulation is still poorly characterized. Here we report a comprehensive ChIP-seq analysis of four NSL complex members (NSL1, NSL3, MBD-R2, and MCRS2) throughout the Drosophila melanogaster genome. Strikingly, the majority (85.5%) of NSL-bound genes are constitutively expressed across different cell types. We find that an increased abundance of the histone modifications H4K16ac, H3K4me2, H3K4me3, and H3K9ac in gene promoter regions is characteristic of NSL-targeted genes. Furthermore, we show that these genes have a well-defined nucleosome free region and broad transcription initiation patterns. Finally, by performing ChIP-seq analyses of RNA polymerase II (Pol II) in NSL1- and NSL3-depleted cells, we demonstrate that both NSL proteins are required for efficient recruitment of Pol II to NSL target gene promoters. The observed Pol II reduction coincides with compromised binding of TBP and TFIIB to target promoters, indicating that the NSL complex is required for optimal recruitment of the pre-initiation complex on target genes. Moreover, genes that undergo the most dramatic loss of Pol II upon NSL knockdowns tend to be enriched in DNA Replication-related Element (DRE). Taken together, our findings show that the MOF-containing NSL complex acts as a major regulator of housekeeping genes in flies by modulating initiation of Pol II transcription.

Transcriptional regulation in Saccharomyces cerevisiae: transcription factor regulation and function, mechanisms of initiation, and roles of activators and coactivators

Here we review recent advances in understanding the regulation of mRNA synthesis in Saccharomyces cerevisiae. Many fundamental gene regulatory mechanisms have been conserved in all eukaryotes, and budding yeast has been at the forefront in the discovery and dissection of these conserved mechanisms. Topics covered include upstream activation sequence and promoter structure, transcription factor classification, and examples of regulated transcription factor activity. We also examine advances in understanding the RNA polymerase II transcription machinery, conserved coactivator complexes, transcription activation domains, and the cooperation of these factors in gene regulatory mechanisms.

Identification of Pep4p as the protease responsible for formation of the SAGA-related SLIK protein complex

The Saccharomyces cerevisiae Spt-Ada-Gcn5 acetyltransferase (SAGA) protein complex is a coactivator for transcription by RNA polymerase II and has various activities, including acetylation and deubuiqitination of histones and recruitment of TATA-binding protein to promoters. The Spt7p subunit is subject to proteolytic cleavage at its C terminus resulting in removal of the Spt8p-binding domain and generation of the SAGA-related SALSA/SAGA-like (SLIK) protein complex. Here, we report identification of the protease responsible for this cleavage. Screening of a protease knock-out collection revealed PEP4 to be required for cleavage of Spt7p within SAGA in vitro. Endogenous formation of truncated Spt7p was abolished in cells lacking PEP4. Purified Pep4p but not catalytic dead mutant Pep4p or unrelated Prc1p protease specifically cleaved Spt7p within SAGA into SLIK-related Spt7p. Interestingly, SAGA lacking Spt8p was more sensitive to Pep4p-mediated truncation of Spt7p, suggesting that Spt8p counteracted its own release from SAGA. Strains mimicking constitutive SLIK formation showed increased resistance to rapamycin treatment, suggesting a role for SLIK in regulating cellular responses to nutrient stress.

In-depth profiling of post-translational modifications on the related transcription factor complexes TFIID and SAGA

The basal transcription factor TFIID and the chromatin-modifying complex SAGA, which have several subunits in common, are crucial for transcription regulation. Here, we describe an in-depth profiling of post-translational modifications (PTMs) on both TFIID and SAGA from yeast. We took a multipronged approach using high-resolution mass spectrometry (LC-MS) in combination with the proteases Trypsin, Chymotrypsin and Glu-C. The cumulative peptide identification data, at a false discovery rate <1%, allowed us to cover most TFIID and SAGA subunit sequences to near completion. Additionally, for TFIID/SAGA subunits, we identified 118/102 unique phosphorylated and 54/61 unique lysine acetylated sites. Especially, several lysine residues on the SAGA subunits Spt7p and Sgf73p were found to be acetylated. Using a spectral counting approach, we found that the shared subunit TAF5p is phosphorylated to a significant greater extent in SAGA than in TFIID. Finally, we were able to map for the first time the cleavage site in Spt7p that is related to formation of the SAGA-like complex SLIK/SALSA. In general, our combination of tandem affinity enrichment, digestion with different proteases, extensive prefractionation and high-resolution LC-MS identifies a large number of PTMs of TFIID and SAGA/SLIK that might aid in future functional studies on these transcription factors.

A novel histone fold domain-containing protein that replaces TAF6 in Drosophila SAGA is required for SAGA-dependent gene expression

The histone acetyltransferase complex SAGA is well characterized as a coactivator complex in yeast. In this study of Drosophila SAGA (dSAGA), we describe three novel components that include an ortholog of Spt20, a potential ortholog of Sgf73/ATXN7, and a novel histone fold protein, SAF6 (SAGA factor-like TAF6). SAF6, which binds directly to TAF9, functions analogously in dSAGA to TAF6/TAF6L in the yeast and human SAGA complexes, respectively. Moreover, TAF6 in flies is restricted to TFIID. Mutations in saf6 disrupt SAGA-regulated gene expression without disrupting acetylated or ubiquitinated histone levels. Thus, SAF6 is essential for SAGA coactivator function independent of the enzymatic activities of the complex.

Snf1p regulates Gcn5p transcriptional activity by antagonizing Spt3p

The budding yeast Gcn5p is a prototypic histone acetyltransferase controlling transcription of diverse genes. Here we show that Gcn5p is itself regulated by Snf1p and Spt3p. Snf1p likely controls Gcn5p via direct interaction. Mutating four residues in the Gcn5p catalytic domain, T203, S204, T211, and Y212 (TSTY), phenocopies snf1 null cells, including Gcn5p hypophosphorylation, hypoacetylation at the HIS3 promoter, and transcriptional defects of the HIS3 gene. However, overexpressing Snf1p suppresses the above phenotypes associated with the phosphodeficient TSTY mutant, suggesting that it is the interaction with Snf1p important for Gcn5p to activate HIS3. A likely mechanism by which Snf1p potentiates Gcn5p function is to antagonize Spt3p, because the HIS3 expression defects caused by snf1 knockout, or by the TSTY gcn5 mutations, can be suppressed by deleting SPT3. In vitro, Spt3p binds Gcn5p, but the interaction is drastically enhanced by the TSTY mutations, indicating that a stabilized Spt3p-Gcn5p interaction may be an underlying cause for the aforementioned HIS3 transcriptional defects. These results suggest that Gcn5p is a target regulated by the competing actions of Snf1p and Spt3p.

The involvement of the Schizosaccharomyces pombe sep9/spt8 gene in the regulation of septum cleavage

Schizosaccharomyces cells divide by medial septation, followed by enzymatic degradation of parts of the septum (septum cleavage) to allow the sister cells to separate from each other. In a previous study we found that the cell separation mutant sep9-307 was defective in a gene that encodes a protein highly similar in sequence to the Saccharomyces cerevisiae protein Spt8, a subunit of the SAGA complex. Here, we show that the sep9-307 mutation causes a frameshift in translation. The deletion of sep9(+) is not lethal but abolishes normal septum cleavage by reducing the activity of ace2(+), a gene coding for a transcription factor of numerous genes producing proteins for septum cleavage. Indirect evidence indicates that Sep9 might also act directly in the transcription of certain target genes (e.g. eng1(+)) of this regulator. sep9-307 is synthetically lethal with mutations in the cell separation genes sep11/med18(+) and sep15/med8(+), which encode subunits of the general transcription factor mediator. Heterologous expression of SPT8 and the putative Schizosaccharomyces japonicus sep9(+) orthologue cannot substitute for sep9(+). Both Spt8 and the fission yeast proteins have highly acidic (74-76 amino-acid long) N-terminal regions with no sequence conservation.

The S. pombe SAGA complex controls the switch from proliferation to sexual differentiation through the opposing roles of its subunits Gcn5 and Spt8

The SAGA complex is a conserved multifunctional coactivator known to play broad roles in eukaryotic transcription. To gain new insights into its functions, we performed biochemical and genetic analyses of SAGA in the fission yeast, Schizosaccharomyces pombe. Purification of the S. pombe SAGA complex showed that its subunit composition is identical to that of Saccharomyces cerevisiae. Analysis of S. pombe SAGA mutants revealed that SAGA has two opposing roles regulating sexual differentiation. First, in nutrient-rich conditions, the SAGA histone acetyltransferase Gcn5 represses ste11(+), which encodes the master regulator of the mating pathway. In contrast, the SAGA subunit Spt8 is required for the induction of ste11(+) upon nutrient starvation. Chromatin immunoprecipitation experiments suggest that these regulatory effects are direct, as SAGA is physically associated with the ste11(+) promoter independent of nutrient levels. Genetic tests suggest that nutrient levels do cause a switch in SAGA function, as spt8Delta suppresses gcn5Delta with respect to ste11(+) derepression in rich medium, whereas the opposite relationship, gcn5Delta suppression of spt8Delta, occurs during starvation. Thus, SAGA plays distinct roles in the control of the switch from proliferation to differentiation in S. pombe through the dynamic and opposing activities of Gcn5 and Spt8.

Site-specific cross-linking of TBP in vivo and in vitro reveals a direct functional interaction with the SAGA subunit Spt3

The TATA-binding protein (TBP) is critical for transcription by all three nuclear RNA polymerases. In order to identify factors that interact with TBP, the nonnatural photoreactive amino acid rho-benzoyl-phenylalanine (BPA) was substituted onto the surface of Saccharomyces cerevisiae TBP in vivo. Cross-linking of these TBP derivatives in isolated transcription preinitiation complexes or in living cells reveals physical interactions of TBP with transcriptional coregulator subunits and with the general transcription factor TFIIA. Importantly, the results show a direct interaction between TBP and the SAGA coactivator subunits Spt3 and Spt8. Mutations on the Spt3-interacting surface of TBP significantly reduce the interaction of TBP with SAGA, show a corresponding decrease in transcription activation, and fail to recruit TBP to a SAGA-dependent promoter, demonstrating that the direct interaction of these factors is important for activated transcription. These results prove a key prediction of the model for stimulation of transcription at SAGA-dependent genes via Spt3. Our cross-linking data also significantly extend the known surfaces of TBP that directly interact with the transcriptional regulator Mot1 and the general transcription factor TFIIA.

Nonradioactive, ultrasensitive site-specific protein-protein photocrosslinking: interactions of alpha-helix 2 of TATA-binding protein with general transcription factor TFIIA and transcriptional repressor NC2

We have developed an approach that enables nonradioactive, ultrasensitive (attamole sensitivity) site-specific protein–protein photocrosslinking, and we have applied the approach to the analysis of interactions of α-helix 2 (H2) of human TATA-element binding protein (TBP) with general transcription factor TFIIA and transcriptional repressor NC2. We have found that TBP H2 can be crosslinked to TFIIA in the TFIIA–TBP–DNA complex and in higher order transcription–initiation complexes, and we have mapped the crosslink to the ‘connector’ region of the TFIIA α/β subunit (TFIIAα/β). We further have found that TBP H2 can be crosslinked to NC2 in the NC2–TBP–DNA complex, and we have mapped the crosslink to the C-terminal ‘tail’ of the NC2 α-subunit (NC2α). Interactions of TBP H2 with the TFIIAα/β connector and the NC2α C-terminal tail were not observed in crystal structures of TFIIA–TBP–DNA and NC2–TBP–DNA complexes, since relevant segments of TFIIA and NC2 were not present in truncated TFIIA and NC2 derivatives used for crystallization. We propose that interactions of TBP H2 with the TFIIAα/β connector and the NC2α C-terminal tail provide an explanation for genetic results suggesting importance of TBP H2 in TBP–TFIIA interactions and TBP–NC2 interactions, and provide an explanation—steric exclusion—for competition between TFIIA and NC2.

Dissection of coactivator requirement at RNR3 reveals unexpected contributions from TFIID and SAGA

The gene encoding ribonucleotide reductase 3 (RNR3) is strongly induced in response to DNA damage. Its expression is strictly dependent upon the TAF(II) subunits of TFIID, which are required for the recruitment of SWI/SNF and nucleosome remodeling. However, full activation of RNR3 also requires GCN5, the catalytic subunit of the SAGA histone acetyltransferase complex. Thus, RNR3 is dependent upon both TFIID and SAGA, two complexes that deliver TATA-binding protein (TBP) to promoters. Furthermore, unlike the majority of TFIID-dominated genes, RNR3 contains a consensus TATA-box, a feature of SAGA-regulated core promoters. Although a large fraction of the genome can be characterized as either TFIID- or SAGA-dominant, it is expected that many genes utilize both. The mechanism of activation and the relative contributions of SAGA and TFIID at genes regulated by both complexes have not been examined. Here we delineated the role of SAGA in the regulation of RNR3 and contrast it to that of TFIID. We find that SAGA components fulfill distinct functions in the regulation of RNR3. The core promoter of RNR3 is SAGA-dependent, and we provide evidence that SAGA, not TAF(II)s within TFIID, are largely responsible for TBP recruitment. This taken together with our previous work provides evidence that SAGA recruits TBP, whereas TFIID mediates chromatin remodeling. Thus, we described an unexpected shift in the division of labor between these two complexes and provide the first characterization of a gene that requires both SAGA and TFIID.

Structural changes in TAF4b-TFIID correlate with promoter selectivity

Proper ovarian development requires the cell type-specific transcription factor TAF4b, a subunit of the core promoter recognition complex TFIID. We present the 35 A structure of a cell type-specific core promoter recognition complex containing TAF4b and TAF4 (4b/4-IID), which is responsible for directing transcriptional synergy between c-Jun and Sp1 at a TAF4b target promoter. As a first step toward correlating potential structure/function relationships of the prototypic TFIID versus 4b/4-IID, we have compared their 3D structures by electron microscopy and single-particle reconstruction. These studies reveal that TAF4b incorporation into TFIID induces an open conformation at the lobe involved in TFIIA and putative activator interactions. Importantly, this open conformation correlates with differential activator-dependent transcription and promoter recognition by 4b/4-IID. By combining functional and structural analysis, we find that distinct localized structural changes in a megadalton macromolecular assembly can significantly alter its activity and lead to a TAF4b-induced reprogramming of promoter specificity.

Characterization of new Spt3 and TATA-binding protein mutants of Saccharomyces cerevisiae: Spt3 TBP allele-specific interactions and bypass of Spt8

The Spt-Ada-Gcn5-acetyltransferase (SAGA) complex of Saccharomyces cerevisiae is a multifunctional coactivator complex that has been shown to regulate transcription by distinct mechanisms. Previous results have shown that the Spt3 and Spt8 components of SAGA regulate initiation of transcription of particular genes by controlling the level of TATA-binding protein (TBP/Spt15) associated with the TATA box. While biochemical evidence exists for direct Spt8-TBP interactions, similar evidence for Spt3-TBP interactions has been lacking. To learn more about Spt3-TBP interactions in vivo, we have isolated a new class of spt3 mutations that cause a dominant-negative phenotype when overexpressed. These mutations all cluster within a conserved region of Spt3. The isolation of extragenic suppressors of one of these spt3 mutations has identified two new spt15 mutations that show allele-specific interactions with spt3 mutations with respect to transcription and the recruitment of TBP to particular promoters. In addition, these new spt15 mutations partially bypass an spt8 null mutation. Finally, we have examined the level of SAGA-TBP physical interaction in these mutants. While most spt3, spt8, and spt15 mutations do not alter SAGA-TBP interactions, one spt3 mutation, spt3-401, causes a greatly increased level of SAGA-TBP physical association. These results, taken together, suggest that a direct Spt3-TBP interaction is required for normal TBP levels at Spt3-dependent promoters in vivo.

The transcriptional repressor activator protein Rap1p is a direct regulator of TATA-binding protein

Essentially all nuclear eukaryotic gene transcription depends upon the function of the transcription factor TATA-binding protein (TBP). Here we show that the abundant, multifunctional DNA binding transcription factor repressor activator protein Rap1p interacts directly with TBP. TBP-Rap1p binding occurs efficiently in vivo at physiological expression levels, and in vitro analyses confirm that this is a direct interaction. The DNA binding domains of the two proteins mediate interaction between TBP and Rap1p. TBP-Rap1p complex formation inhibits TBP binding to TATA promoter DNA. Alterations in either Rap1p or TBP levels modulate mRNA gene transcription in vivo. We propose that Rap1p represents a heretofore unrecognized regulator of TBP.

STAGA recruits Mediator to the MYC oncoprotein to stimulate transcription and cell proliferation

Activation of eukaryotic gene transcription involves the recruitment by DNA-binding activators of multiprotein histone acetyltransferase (HAT) and Mediator complexes. How these coactivator complexes functionally cooperate and the roles of the different subunits/modules remain unclear. Here we report physical interactions between the human HAT complex STAGA (SPT3-TAF9-GCN5-acetylase) and a "core" form of the Mediator complex during transcription activation by the MYC oncoprotein. Knockdown of the STAF65gamma component of STAGA in human cells prevents the stable association of TRRAP and GCN5 with the SPT3 and TAF9 subunits; impairs transcription of MYC-dependent genes, including MYC transactivation of the telomerase reverse transcriptase (TERT) promoter; and inhibits proliferation of MYC-dependent cells. STAF65gamma is required for SPT3/STAGA interaction with core Mediator and for MYC recruitment of SPT3, TAF9, and core Mediator components to the TERT promoter but is dispensable for MYC recruitment of TRRAP, GCN5, and p300 and for acetylation of nucleosomes and loading of TFIID and RNA polymerase II on the promoter. These results suggest a novel STAF65gamma-dependent function of STAGA-type complexes in cell proliferation and transcription activation by MYC postloading of TFIID and RNA polymerase II that involves direct recruitment of core Mediator.

A SAGA-independent function of SPT3 mediates transcriptional deregulation in a mutant of the Ccr4-not complex in Saccharomyces cerevisiae

The conserved multi-subunit Ccr4-Not complex regulates gene expression in diverse ways. In this work, we characterize the suppression of temperature sensitivity associated with a mutation in the gene encoding the scaffold subunit of the Ccr4-Not complex, NOT1, by the deletion of SPT3. We determine that the deletion of SPT3, but not the deletion of genes encoding other subunits of the SAGA complex, globally suppresses transcriptional defects of not1-2. We find that transcriptional activation in not1-2 is associated with increased binding of TFIID and SAGA at promoters of upregulated genes, and this is suppressed by the deletion of SPT3. Interestingly, Spt3p-dependent activation of transcription occurs in not1-2 even if the SAGA complex is disrupted by the deletion of SPT7 that encodes a subunit of SAGA required for its integrity. Consistent with a SAGA-independent function of Spt3p, the deletion of SPT3 displays synthetic phenotypes when combined with a deletion of SPT7. Taken together, our results provide a new view of the Spt3 protein by identifying a SAGA-independent function of this protein that is functionally linked to the Ccr4-Not complex.

The positions of TFIIF and TFIIE in the RNA polymerase II transcription preinitiation complex

We incorporated the non-natural photoreactive amino acid p-benzoyl-L-phenylalanine (Bpa) into the RNA polymerase II (Pol II) surface surrounding the central cleft formed by the Rpb1 and Rpb2 subunits. Photo-cross-linking of preinitiation complexes (PICs) with these Pol II derivatives and hydroxyl-radical cleavage assays revealed that the TFIIF dimerization domain interacts with the Rpb2 lobe and protrusion domains adjacent to Rpb9, while TFIIE cross-links to the Rpb1 clamp domain on the opposite side of the Pol II central cleft. Mutations in the Rpb2 lobe and protrusion domains alter both Pol II-TFIIF binding and the transcription start site, a phenotype associated with mutations in TFIIF, Rpb9 and TFIIB. Together with previous biochemical and structural studies, these findings illuminate the structural organization of the PIC and the network of protein-protein interactions involved in transcription start site selection.

Recessive resistance genes and the Oryza sativa-Xanthomonas oryzae pv. oryzae pathosystem

Though recessive resistance is well-studied in viral systems, little is understood regarding the phenomenon in plant-bacterial interactions. The Oryza sativa-Xanthomonas oryzae pv. orzyae pathosystem provides an excellent opportunity to examine recessive resistance in plant-bacterial interactions, in which nine of 30 documented resistance (R) genes are recessively inherited. Infestations of X. oryzae pv. oryzae, the causal agent of bacterial blight, result in significant crop loss and damage throughout South and Southeast Asia. Two recently cloned novel recessive R genes, xa5 and xa13, have yielded insights to this system. Like their viral counterparts, these bacterial recessive R gene products do not conform to the five commonly described classes of R proteins. New findings suggest that such genes may more aptly be viewed as mutations in dominant susceptibility alleles and may also function in a gene-for-gene manner. In this review, we discuss recent accomplishments in the understanding of recessively inherited R genes in the rice-bacterial blight pathosystem and suggest a new model for the function of recessive resistance in plant-bacterial interactions.

A facelift for the general transcription factor TFIIA

TFIIA was classified as a general transcription factor when it was first identified. Since then it has been debated to what extent it can actually be regarded as "general". The most notable feature of TFIIA is the proteolytical cleavage of the TFIIAalphabeta into a TFIIAalpha and TFIIAbeta moiety which has long remained a mystery. Recent studies have showed that TFIIA is cleaved by Taspase1 which was initially identified as the protease for the proto-oncogene MLL. Cleavage of TFIIA does not appear to serve as a step required for its activation as the uncleaved TFIIA in the Taspase1 knock-outs adequately support bulk transcription. Instead, cleavage of TFIIA seems to affect its turn-over and may be a part of an intricate degradation mechanism that allows fine-tuning of cellular levels of TFIIA. Cleavage might also be responsible for switching transcription program as the uncleaved and cleaved TFIIA might have distinct promoter specificity during development and differentiation. This review will focus on functional characteristics of TFIIA and discuss novel insights in the role of this elusive transcription factor.

The general transcription machinery and general cofactors

In eukaryotes, the core promoter serves as a platform for the assembly of transcription preinitiation complex (PIC) that includes TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH, and RNA polymerase II (pol II), which function collectively to specify the transcription start site. PIC formation usually begins with TFIID binding to the TATA box, initiator, and/or downstream promoter element (DPE) found in most core promoters, followed by the entry of other general transcription factors (GTFs) and pol II through either a sequential assembly or a preassembled pol II holoenzyme pathway. Formation of this promoter-bound complex is sufficient for a basal level of transcription. However, for activator-dependent (or regulated) transcription, general cofactors are often required to transmit regulatory signals between gene-specific activators and the general transcription machinery. Three classes of general cofactors, including TBP-associated factors (TAFs), Mediator, and upstream stimulatory activity (USA)-derived positive cofactors (PC1/PARP-1, PC2, PC3/DNA topoisomerase I, and PC4) and negative cofactor 1 (NC1/HMGB1), normally function independently or in combination to fine-tune the promoter activity in a gene-specific or cell-type-specific manner. In addition, other cofactors, such as TAF1, BTAF1, and negative cofactor 2 (NC2), can also modulate TBP or TFIID binding to the core promoter. In general, these cofactors are capable of repressing basal transcription when activators are absent and stimulating transcription in the presence of activators. Here we review the roles of these cofactors and GTFs, as well as TBP-related factors (TRFs), TAF-containing complexes (TFTC, SAGA, SLIK/SALSA, STAGA, and PRC1) and TAF variants, in pol II-mediated transcription, with emphasis on the events occurring after the chromatin has been remodeled but prior to the formation of the first phosphodiester bond.

SAGA binds TBP via its Spt8 subunit in competition with DNA: implications for TBP recruitment

In yeast, the multisubunit SAGA (Spt-Ada-Gcn5-acetyltransferase) complex acts as a coactivator to recruit the TATA-binding protein (TBP) to the TATA box, a critical step in eukaryotic gene regulation. However, it is unclear which SAGA subunits are responsible for SAGA's direct interactions with TBP and precisely how SAGA recruits TBP to the promoter. We have used chemical crosslinking to identify Spt8 and Ada1 as potential SAGA subunits that interact with TBP, and we find that both Spt8 and SAGA bind directly to TBP monomer in competition with TBP dimer. We further find that Spt8 and SAGA compete with DNA to bind TBP rather than forming a triple complex. Our results suggest a handoff model for SAGA recruitment of TBP: instead of binding together with TBP at the TATA box, activator-recruited SAGA transfers TBP to the TATA box. This simple model can explain SAGA's observed ability to both activate and repress transcription.

A DNA-tethered cleavage probe reveals the path for promoter DNA in the yeast preinitiation complex

To directly map the position of promoter DNA within the RNA polymerase II (Pol II) transcription preinitiation complex (PIC), FeBABE was tethered to specific sites within the HIS4 promoter and used to map exposed surfaces of Pol II and the general transcription factors in proximity to DNA. Our results distinguish between previously proposed models for PIC structure and demonstrate that downstream promoter DNA is positioned over the central cleft of Pol II, with DNA upstream of TATA extending toward the Pol II subunit Rpb3. Also mapped were segments of TFIIB, TFIIE, TFIIF and TFIIH in proximity to promoter DNA. DNA downstream of the transcription bubble maps to a path between the two helicase subdomains of the TFIIH subunit Rad25 (also called XPB). Together, our results show how the general factors and Pol II converge on promoter DNA within the PIC.

The proteasome: a utility tool for transcription?

A growing body of evidence demonstrates that the components of the proteasome are involved directly and mechanistically in the regulation of gene transcription. Proteolytic activities of the proteasome are important for establishing limits for transcription, for promoting the exchange of transcription factors on chromatin, and possibly for facilitating multiple rounds of transcription initiation. Non-proteolytic activities of the proteasome are important for co-activator recruitment, transcriptional elongation, and histone modification. Here, we discuss different ways in which the proteasome can influence transcription, and argue that its unique combination of biological activities makes it ideally suited to act at multiple stages in the transcription process.

Regulation of an intergenic transcript controls adjacent gene transcription in Saccharomyces cerevisiae

Recent studies have revealed that transcription of noncoding, intergenic DNA is abundant among eukaryotes. However, the functions of this transcription are poorly understood. We have previously shown that in Saccharomyces cerevisiae, expression of an intergenic transcript, SRG1, represses the transcription of the adjacent gene, SER3, by transcription interference. We now show that SRG1 transcription is regulated by serine, thereby conferring regulation of SER3, a serine biosynthetic gene. This regulation requires Cha4, a serine-dependent activator that binds to the SRG1 promoter and is required for SRG1 induction in the presence of serine. Furthermore, two coactivator complexes, SAGA and Swi/Snf, are also directly required for activation of SRG1 and transcription interference of SER3. Taken together, our results elucidate a physiological role for intergenic transcription in the regulation of SER3. Moreover, our results demonstrate a mechanism by which intergenic transcription allows activators to act indirectly as repressors.

Targets of the Gal4 transcription activator in functional transcription complexes

Although biochemical and genetic methods have detected many activator-transcription factor interactions, the direct functional targets of most activators remain undetermined. For this study, photo-cross-linkers positioned within the Gal4 C-terminal acidic activating region were used to identify polypeptides in close physical proximity to Gal4 during transcription activation in vitro. Of six specifically cross-linked polypeptides, three (Tra1, Taf12, and Gal11) are subunits of four complexes (SAGA, Mediator, NuA4, and TFIID) known to play a role in gene regulation. These cross-linking targets had differential effects on activation. SAGA was critical for activation by Gal4, Gal11 contributed modestly to activation, and TFIID and NuA4 were not important for activation under our conditions. Tra1, Taf12, and Gal11 have also been identified as cross-linking targets of the Gcn4 acidic central activating region. Our results demonstrate that two unrelated acidic activators converge on the same set of functional targets.

The Proteasome Regulatory Particle Alters the SAGA Coactivator to Enhance Its Interactions with Transcriptional Activators

Promoter recruitment of the Saccharomyces cerevisiae SAGA histone acetyltransferase complex is required for RNA polymerase II-dependent transcription of several genes. SAGA is targeted to promoters through interactions with sequence-specific DNA binding transcriptional activators and facilitates preinitiation-complex assembly and transcription. Here, we show that the 19S proteasome regulatory particle (19S RP) alters SAGA to stimulate its interaction with transcriptional activators. The ATPase components of the 19S RP are required for stimulation of SAGA/activator interactions and enhance SAGA recruitment to promoters. Proteasomal ATPases genetically interact with SAGA, and their inhibition reduces global histone H3 acetylation levels and SAGA recruitment to target promoters in vivo. These results indicate that the 19S RP modulates SAGA complex using its ATPase components, thereby facilitating subsequent transcription events at promoters.

The yeast FACT complex has a role in transcriptional initiation

A crucial step in eukaryotic transcriptional initiation is recognition of the promoter TATA by the TATA-binding protein (TBP), which then allows TFIIA and TFIIB to be recruited. However, nucleosomes block the interaction between TBP and DNA. We show that the yeast FACT complex (yFACT) promotes TBP binding to a TATA box in chromatin both in vivo and in vitro. The SPT16 gene encodes a subunit of yFACT, and we show that certain spt16 mutations are synthetically lethal with TBP mutants. Some of these genetic defects can be suppressed by TFIIA overexpression, strongly suggesting a role for yFACT in TBP-TFIIA complex formation in vivo. Mutations in the TOA2 subunit of TFIIA that disrupt TBP-TFIIA complex formation in vitro are also synthetically lethal with spt16. In some cases this spt16 toa2 lethality is suppressed by overexpression of TBP or the Nhp6 architectural transcription factor that is also a component of yFACT. The Spt3 protein in the SAGA complex has been shown to regulate TBP binding at certain promoters, and we show that some spt16 phenotypes can be suppressed by spt3 mutations. Chromatin immunoprecipitations show TBP binding to promoters is reduced in single spt16 and spt3 mutants but increases in the spt16 spt3 double mutant, reflecting the mutual suppression seen in the genetic assays. Finally, in vitro studies show that yFACT promotes TBP binding to a TATA sequence within a reconstituted nucleosome in a TFIIA-dependent manner. Thus, yFACT functions in establishing transcription initiation complexes in addition to the previously described role in elongation.

Differential requirement of SAGA subunits for Mot1p and Taf1p recruitment in gene activation

Transcription activation in yeast (Saccharomyces cerevisiae) involves ordered recruitment of transcription factor complexes, such as TFIID, SAGA, and Mot1p. Previously, we showed that both Mot1p and Taf1p are recruited to the HXT2 and HXT4 genes, which encode hexose transporter proteins. Here, we show that SAGA also binds to the HXT2 and HXT4 promoters and plays a pivotal role in the recruitment of Mot1p and Taf1p. The deletion of either SPT3 or SPT8 reduces Mot1p binding to HXT2 and HXT4. Surprisingly, the deletion of GCN5 reduces Taf1p binding to both promoters. When GCN5 is deleted in spt3Delta or spt8Delta strains, neither Mot1p nor Taf1p binds, and this results in a diminished recruitment of TATA binding protein and polymerase II to the HXT4 but not the HXT2 promoter. This is reflected by the SAGA-dependent expression of HXT4. In contrast, SAGA-independent induction of HXT2 suggests a functional redundancy with other factors. A functional interplay of different SAGA subunits with Mot1p and Taf1p was supported by phenotypic analysis of MOT1 SAGA or TAF1/SAGA double mutant strains, which revealed novel genetic interactions between MOT1 and SPT8 and between TAF1 and GCN5. In conclusion, our data demonstrate functional links between SAGA, Mot1p, and TFIID in HXT gene regulation.