Preinitiation Complex Assembly Research Articles

Single molecule microscopy reveals mechanistic insight into RNA polymerase II preinitiation complex assembly and transcriptional activity.

Transcription by RNA polymerase II (Pol II) is a complex process that requires general transcription factors and Pol II to assemble on DNA into preinitiation complexes that can begin RNA synthesis upon binding of NTPs (nucleoside triphosphate). The pathways by which preinitiation complexes form, and how this impacts transcriptional activity are not completely clear. To address these issues, we developed a single molecule system using TIRF (total internal reflection fluorescence) microscopy and purified human transcription factors, which allows us to visualize transcriptional activity at individual template molecules. We see that stable interactions between polymerase II (Pol II) and a heteroduplex DNA template do not depend on general transcription factors; however, transcriptional activity is highly dependent upon TATA-binding protein, TFIIB and TFIIF. We also found that subsets of general transcription factors and Pol II can form stable complexes that are precursors for functional transcription complexes upon addition of the remaining factors and DNA. Ultimately we found that Pol II, TATA-binding protein, TFIIB and TFIIF can form a quaternary complex in the absence of promoter DNA, indicating that a stable network of interactions exists between these proteins independent of promoter DNA. Single molecule studies can be used to learn how different modes of preinitiation complex assembly impact transcriptional activity.

Cooperation between SAGA and SWI/SNF complexes is required for efficient transcriptional responses regulated by the yeast MAPK Slt2.

The transcriptional response of Saccharomyces cerevisiae to cell wall stress is mainly mediated by the cell wall integrity (CWI) pathway through the MAPK Slt2 and the transcription factor Rlm1. Once activated, Rlm1 interacts with the chromatin remodeling SWI/SNF complex which locally alters nucleosome positioning at the target promoters. Here we show that the SAGA complex plays along with the SWI/SNF complex an important role for eliciting both early induction and sustained gene expression upon stress. Gcn5 co-regulates together with Swi3 the majority of the CWI transcriptional program, except for a group of genes which are only dependent on the SWI/SNF complex. SAGA subunits are recruited to the promoter of CWI-responsive genes in a Slt2, Rlm1 and SWI/SNF-dependent manner. However, Gcn5 mediates acetylation and nucleosome eviction only at the promoters of the SAGA-dependent genes. This process is not essential for pre-initiation transcriptional complex assembly but rather increase the extent of the remodeling mediated by SWI/SNF. As a consequence, H3 eviction and Rlm1 recruitment is completely blocked in a swi3Δ gcn5Δ double mutant. Therefore, SAGA complex, through its histone acetylase activity, cooperates with the SWI/SNF complex for the mandatory nucleosome displacement required for full gene expression through the CWI pathway.

Corrigendum: Structure of promoter-bound TFIID and model of human pre-initiation complex assembly.

Nature 531, 604–609 (2016); doi:10.1038/nature17394 In this Article, author Pablo Chacon should have affiliation 4 (Department of Biological Physical Chemistry, Rocasolano Physical Chemistry Institute, CSIC, Serrano 119, Madrid 28006, Spain), not affiliation 3. This error has been corrected online.

Structure of promoter-bound TFIID and model of human pre-initiation complex assembly.

The general transcription factor IID (TFIID) plays a central role in the initiation of RNA polymerase II-dependent transcription by nucleating pre-initiation complex (PIC) assembly at the core promoter. TFIID comprises the TATA-binding protein (TBP) and 13 TBP-associated factors (TAF1-13), which specifically interact with a variety of core promoter DNA sequences. Here we present the structure of human TFIID in complex with TFIIA and core promoter DNA, determined by single-particle cryo-electron microscopy (cryo-EM) at sub-nanometer resolution. All core promoter elements are contacted by subunits of TFIID, with TAF1 and TAF2 mediating major interactions with the downstream promoter. TFIIA bridges the TBP-TATA complex with lobe B of TFIID. We also present the cryo-EM reconstruction of a fully-assembled human TAF-less PIC. Superposition of common elements between the two structures provides novel insights into the general role of TFIID in promoter recognition, PIC assembly, and transcription initiation.

Hypothetical SNP Markers That Significantly Affect the Affinity of the TATA-Binding Protein to VEGFA, ERBB2, IGF1R, FLT1, KDR, and MET Oncogene Promoters as Chemotherapy Targets

The following hypothesis has been proposed: IF an SNP can significantly increase the expression of an oncogene by increasing the affinity of the TATA-binding protein (TBP) to its promoter, THEN this SNP can also reduce the apparent bioactivity of inhibitors of this oncogene during antitumor chemotherapy and vice versa. In the context of this hypothesis, the previously proposed method (http://beehive.bionet.nsc. ru/cgi-bin/mgs/tatascan/start.pl) was applied to analyze all SNPs found within the [-70; -20] regions (which harbor all proven TBP-binding sites) of the promoters of VEGFA, EGFR, ERBB2, IGF1R, FLT1, KDR, and MET oncogenes according to the human reference genome, hg19. For 83% of these SNPs, their effect on TBP affinity to the oncogene promoters required for assembly of preinitiation complexes was not significant. rs36208385, rs36208384, rs370995111, rs372731987, rs111811434, rs369547510, rs76407893, rs369728300, and rs72001900 can potentially serve as SNP markers to reduce the apparent bioactivity of oncogene inhibitors, while rs141092704, rs184083669, rs145139616, rs200697953, rs187746433, rs199730913, rs377370642, rs114484350, rs374921120, rs146790957, rs376727645, and rs72001900 can be the markers for enhancing this activity.

DTAF10- and dTAF10b-Containing Complexes Are Required for Ecdysone-Driven Larval-Pupal Morphogenesis in Drosophila melanogaster.

In eukaryotes the TFIID complex is required for preinitiation complex assembly which positions RNA polymerase II around transcription start sites. On the other hand, histone acetyltransferase complexes including SAGA and ATAC, modulate transcription at several steps through modification of specific core histone residues. In this study we investigated the function of Drosophila melanogaster proteins TAF10 and TAF10b, which are subunits of dTFIID and dSAGA, respectively. We generated a mutation which eliminated the production of both Drosophila TAF10 orthologues. The simultaneous deletion of both dTaf10 genes impaired the recruitment of the dTFIID subunit dTAF5 to polytene chromosomes, while binding of other TFIID subunits, dTAF1 and RNAPII was not affected. The lack of both dTAF10 proteins resulted in failures in the larval-pupal transition during metamorphosis and in transcriptional reprogramming at this developmental stage. Surprisingly, unlike dSAGA mutations, dATAC subunit mutations resulted in very similar changes in the steady state mRNA levels of approximately 5000 genes as did ablation of both dTaf10 genes, indicating that dTAF10- and/or dTAF10b-containing complexes and dATAC affect similar pathways. Importantly, the phenotype resulting from dTaf10+dTaf10b mutation could be rescued by ectopically added ecdysone, suggesting that dTAF10- and/or dTAF10b-containing complexes are involved in the expression of ecdysone biosynthetic genes. Indeed, in dTaf10+dTaf10b mutants, cytochrome genes, which regulate ecdysone synthesis in the ring gland, were underrepresented. Therefore our data support the idea that the presence of dTAF10 proteins in dTFIID and/or dSAGA is required only at specific developmental steps. We propose that distinct forms of dTFIID and/or dSAGA exist during Drosophila metamorphosis, wherein different TAF compositions serve to target RNAPII at different developmental stages and tissues.

The CD8+ cell non-cytotoxic antiviral response affects RNA polymerase II-mediated human immunodeficiency virus transcription in infected CD4+ cells.

A CD8+ cell non-cytotoxic antiviral response (CNAR), mediated by a CD8+ cell antiviral factor (CAF), is associated with a long-term healthy state in human immunodeficiency virus (HIV) infection. CNAR/CAF reduces viral transcription without a known effect on specific viral sequences in the HIV genome. In studies to define the mechanism involved in the block in viral transcription, we now report that transcription from the HIV-LTR reporter is reduced in infected CD4+ cells upon treatment with CAF. In agreement with this observation, the amount of RNA polymerase II (RNAPII) on the HIV promoter and other viral regions was strongly diminished in HIV-infected CD4+ cells co-cultivated with CNAR-expressing CD8+ cells. These results demonstrate further that CNAR/CAF has a specific role in regulating HIV transcription and a step during the preinitiation complex assembly appears to be sensitive to CNAR/CAF.

Mediator independently orchestrates multiple steps of preinitiation complex assembly in vivo.

Mediator is a large multiprotein complex conserved in all eukaryotes, which has a crucial coregulator function in transcription by RNA polymerase II (Pol II). However, the molecular mechanisms of its action in vivo remain to be understood. Med17 is an essential and central component of the Mediator head module. In this work, we utilised our large collection of conditional temperature-sensitive med17 mutants to investigate Mediator's role in coordinating preinitiation complex (PIC) formation in vivo at the genome level after a transfer to a non-permissive temperature for 45 minutes. The effect of a yeast mutation proposed to be equivalent to the human Med17-L371P responsible for infantile cerebral atrophy was also analyzed. The ChIP-seq results demonstrate that med17 mutations differentially affected the global presence of several PIC components including Mediator, TBP, TFIIH modules and Pol II. Our data show that Mediator stabilizes TFIIK kinase and TFIIH core modules independently, suggesting that the recruitment or the stability of TFIIH modules is regulated independently on yeast genome. We demonstrate that Mediator selectively contributes to TBP recruitment or stabilization to chromatin. This study provides an extensive genome-wide view of Mediator's role in PIC formation, suggesting that Mediator coordinates multiple steps of a PIC assembly pathway.

DNA Topoisomerases Are Required for Preinitiation Complex Assembly during GAL Gene Activation.

To investigate the importance of topoisomerases for transcription of the galactose induced genes, we have studied the expression of GAL1, GAL2, GAL7 and GAL10 in Saccharomyces cerevisiae cells deficient for topoisomerases I and II. We find that topoisomerases are required for transcriptional activation of the GAL genes, but are dispensable for ongoing transcription, eliminating a role of the enzymes in transcriptional elongation. Furthermore, we demonstrate that promoter chromatin remodeling of the GAL genes is unaffected in the topoisomerase deficient strain. However, the cells fail to successfully recruit RNA polymerase II due to an inability of the TATA-binding protein (TBP) to bind to the TATA box in these promoters. We therefore argue that topoisomerases are required for accurate assembly of the preinitiation complex at the promoters of the GAL genes.

Architecture of TFIIIC and its role in RNA polymerase III pre-initiation complex assembly.

In eukaryotes, RNA Polymerase III (Pol III) is specifically responsible for transcribing genes encoding tRNAs and other short non-coding RNAs. The recruitment of Pol III to tRNA-encoding genes requires the transcription factors (TF) IIIB and IIIC. TFIIIC has been described as a conserved, multi-subunit protein complex composed of two subcomplexes, called τA and τB. How these two subcomplexes are linked and how their interaction affects the formation of the Pol III pre-initiation complex (PIC) is poorly understood. Here we use chemical crosslinking mass spectrometry and determine the molecular architecture of TFIIIC. We further report the crystal structure of the essential TPR array from τA subunit τ131 and characterize its interaction with a central region of τB subunit τ138. The identified τ131–τ138 interacting region is essential in vivo and overlaps with TFIIIB-binding sites, revealing a crucial interaction platform for the regulation of tRNA transcription initiation.

A Random Screen Using a Novel Reporter Assay System Reveals a Set of Sequences That Are Preferred as the TATA or TATA-Like Elements in the CYC1 Promoter of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the core promoters of class II genes contain either TATA or TATA-like elements to direct accurate transcriptional initiation. Genome-wide analyses show that the consensus sequence of the TATA element is TATAWAWR (8 bp), whereas TATA-like elements carry one or two mismatches to this consensus. The fact that several functionally distinct TATA sequences have been identified indicates that these elements may function, at least to some extent, in a gene-specific manner. The purpose of the present study was to identify functional TATA sequences enriched in one particular core promoter and compare them with the TATA or TATA-like elements that serve as the pre-initiation complex (PIC) assembly sites on the yeast genome. For this purpose, we conducted a randomized screen of the TATA element in the CYC1 promoter by using a novel reporter assay system and identified several hundreds of unique sequences that were tentatively classified into nine groups. The results indicated that the 7 bp TATA element (i.e., TATAWAD) and several sets of TATA-like sequences are preferred specifically by this promoter. Furthermore, we find that the most frequently isolated TATA-like sequence, i.e., TATTTAAA, is actually utilized as a functional core promoter element for the endogenous genes, e.g., ADE5,7 and ADE6. Collectively, these results indicate that the sequence requirements for the functional TATA or TATA-like elements in one particular core promoter are not as stringent. However, the variation of these sequences differs significantly from that of the PIC assembly sites on the genome, presumably depending on promoter structures and reflecting the gene-specific function of these sequences.

TAF10 Interacts with the GATA1 Transcription Factor and Controls Mouse Erythropoiesis.

The ordered assembly of a functional preinitiation complex (PIC), composed of general transcription factors (GTFs), is a prerequisite for the transcription of protein-coding genes by RNA polymerase II. TFIID, comprised of the TATA binding protein (TBP) and 13 TBP-associated factors (TAFs), is the GTF that is thought to recognize the promoter sequences allowing site-specific PIC assembly. Transcriptional cofactors, such as SAGA, are also necessary for tightly regulated transcription initiation. The contribution of the two TAF10-containing complexes (TFIID, SAGA) to erythropoiesis remains elusive. By ablating TAF10 specifically in erythroid cells in vivo, we observed a differentiation block accompanied by deregulated GATA1 target genes, including Gata1 itself, suggesting functional cross talk between GATA1 and TAF10. Additionally, we analyzed by mass spectrometry the composition of TFIID and SAGA complexes in mouse and human cells and found that their global integrity is maintained, with minor changes, during erythroid cell differentiation and development. In agreement with our functional data, we show that TAF10 interacts directly with GATA1 and that TAF10 is enriched on the GATA1 locus in human fetal erythroid cells. Thus, our findings demonstrate a cross talk between canonical TFIID and SAGA complexes and cell-specific transcription activators during development and differentiation.

Transcription factor interaction with COMPASS-like complex regulates histone H3K4 trimethylation for specific gene expression in plants.

Accumulation of unfolded or misfolded proteins causes endoplasmic reticulum (ER) stress, which activates a set of ER membrane-associated transcription factors for protein homeostasis regulation. Previous genome-wide chromatin immunoprecipitation analysis shows a strong correlation between histone H3K4 trimethylation (H3K4me3) and active gene expression. However, how the histone modification complex is specifically and timely recruited to the active promoters remains unknown. Using ER stress responsive gene expression as a model system, we demonstrate that sequence-specific transcription factors interact with COMPASS-like components and affect H3K4me3 formation at specific target sites in Arabidopsis. Gene profiling analysis reveals that membrane-associated basic leucine zipper (bZIP) transcription factors bZIP28 and bZIP60 regulate most of the ER stress responsive genes. Loss-of-functions of bZIP28 and bZIP60 impair the occupancy of H3K4me3 on promoter regions of ER stress responsive genes. Further, in vitro pull-down assays and in vivo bimolecular fluorescence complementation (BiFC) experiments show that bZIP28 and bZIP60 interact with Ash2 and WDR5a, both of which are core COMPASS-like components. Knockdown expression of either Ash2 or WDR5a decreased the expression of several ER stress responsive genes. The COMPASS-like complex is known to interact with histone methyltransferase to facilitate preinitiation complex (PIC) assembly and generate H3K4me3 during transcription elongation. Thus, our data shows that the ER stress stimulus causes the formation of PIC and deposition of H3K4me3 mark at specific promoters through the interaction between transcription factor and COMPASS-like components.

TAF10 Interacts with GATA1 Transcription Factor and Controls Mouse Erythropoiesis

The ordered assembly of a functional preinitiation complex (PIC), composed of general transcription factors (GTFs) is a prerequisite for the transcription of protein coding genes by RNA polymerase II. TFIID, comprised of the TATA binding protein (TBP) and 13 TBP-associated factors (TAFs), is the GTF that is thought to recognize the promoter sequences allowing site-specific PIC assembly. Transcriptional cofactors, such as SAGA (Spt-Ada-Gcn5-acetyltransferase), are also necessary to have tightly regulated transcription initiation. However, a new era on the role of the GTFs and specifically on the role of TFIID in tissue specific and promoter specific transcriptional regulation has emerged in the light of novel findings regarding the differentiation programs of different cell types1.TAF10 is a subunit of both the TFIID and the SAGA co-activator HAT complexes2. The role of TAF10 is indispensable for early embryonic transcription and mouse development as knockout (KO) embryos die early in gestation between E3.5 and E5.5, around the stage when the supply of maternal protein becomes insufficient3. However, when analyzing TFIID stability and transcription it was noted that not all cells and tissues were equally affected by the loss of TAF10.The contribution of the two TAF10-containing complexes (TFIID, SAGA) to erythropoiesis remains elusive. Ablation of TAF10 specifically in erythroid cells by crossing the TAF10-Lox with the EpoR-Cre mouse led to a differentiation block at around E13.5 with erythroid progenitor cells accumulating at a higher percentage (26% in the KO embryos vs 16% in the WTs at E12.5) at the double positive stage KIT+CD71+ and giving rise to fewer mature TER119+ cells in the fetal liver. At E13.5 embryos were dead with almost no erythroid cells in the fetal liver. Gene expression analysis of the fetal liver cells of the embryos revealed down-regulation of GATA1 expression and its target genes, bh1&bmaj/min globins and KLF1 transcription factor while expression of other genes known to have a role in mouse hematopoiesis remained unaffected (MYB, GATA2, PU.1). In order to get insight to the role of TAF10 during erythropoiesis we analyzed the composition of both TAF10-containing complexes (TFIID and SAGA) by mass spectrometry. We found that their stoichiometry changes slightly but not fundamentally during erythroid differentiation and development (human fetal liver erythroid progenitors, human blood erythroid progenitors and mouse erythroid progenitor cells) and no major rearrangements were generated in the composition of the TFIID as it was reported in other cell differentiation programs (e.g. skeletal differentiation, hepatogenesis). Additionally, we found GATA1 transcription factor only in the fetal liver and not in the adult erythroid cells in the mass spectrometry data of TAF10 immunoprecipitations (IPs), an interaction that we confirmed by reciprocal IP of TAF10 and GATA1 in MEL and mouse fetal liver cells. Most importantly, we checked whether TAF10 binding is enriched on the GATA1 locus in human erythroid cells during the fetal and the adult stage in erythroid proerythroblasts and we found that there is enriched binding of TAF10 in the palindromic GATA1 site in the fetal stage. Our results support a developmental role for TAF10 in GATA1 regulated genes, including GATA1 itself, during erythroid differentiation emphasizing the crosstalk between the transcriptional machinery and activators in erythropoiesis.

NIa-Pro of Papaya ringspot virus interacts with Carica papaya eukaryotic translation initiation factor 3 subunit G (CpeIF3G).

The interaction of papaya eukaryotic translation initiation factor 3 subunit G (CpeIF3G) with Papaya ringspot virus (PRSV) NIa-Pro was validated using a bimolecular fluorescence complementation assay in papaya protoplasts based on the previous yeast two-hybrid assay results. The C-terminal (residues 133-239) fragment of PRSV NIa-Pro and the central domain (residues 59-167) of CpeIF3G were required for effective interaction between NIa-Pro and CpeIF3G as shown by a Sos recruitment yeast two-hybrid system with several deletion mutants of NIa-Pro and CpeIF3G. The central domain of CpeIF3G, which contains a C2HC-type zinc finger motif, is required to bind to other eIFs of the translational machinery. In addition, quantitative real-time reverse transcription PCR assay confirmed that PRSV infection leads to a 2- to 4.5-fold up-regulation of CpeIF3G mRNA in papaya. Plant eIF3G is involved in various stress response by enhancing the translation of resistance-related proteins. It is proposed that the NIa-Pro-CpeIF3G interaction may impair translation preinitiation complex assembly of defense proteins and interfere with host defense.

Regulatory transcription factors can control the process of transcription at the stage of pre-mRNA elongation

Sequence-specific (regulatory) transcription factors are the proteins that recognize certain DNA sequences. They selectively regulate transcription levels of different sets of genes depending on the ontogeny stage, cell type, and external conditions. According to the established opinion, these proteins control the process of transcription at the stage of the RNA Pol II preinitiation complex assembly. However, according to the growing body of evidence, regulatory transcription factors can be also involved in the control of transcription elongation. This review focuses on systematization of such data.

Cellular microRNAs up-regulate transcription via interaction with promoter TATA-box motifs

The TATA box represents one of the most prevalent core promoters where the pre-initiation complexes (PICs) for gene transcription are assembled. This assembly is crucial for transcription initiation and well regulated. Here we show that some cellular microRNAs (miRNAs) are associated with RNA polymerase II (Pol II) and TATA box-binding protein (TBP) in human peripheral blood mononuclear cells (PBMCs). Among them, let-7i sequence specifically binds to the TATA-box motif of interleukin-2 (IL-2) gene and elevates IL-2 mRNA and protein production in CD4+ T-lymphocytes in vitro and in vivo. Through direct interaction with the TATA-box motif, let-7i facilitates the PIC assembly and transcription initiation of IL-2 promoter. Several other cellular miRNAs, such as mir-138, mir-92a or mir-181d, also enhance the promoter activities via binding to the TATA-box motifs of insulin, calcitonin or c-myc, respectively. In agreement with the finding that an HIV-1–encoded miRNA could enhance viral replication through targeting the viral promoter TATA-box motif, our data demonstrate that the interaction with core transcription machinery is a novel mechanism for miRNAs to regulate gene expression.

Epstein-Barr virus late gene transcription depends on the assembly of a virus-specific preinitiation complex.

During their productive cycle, herpesviruses exhibit a strictly regulated temporal cascade of gene expression that has three general stages: immediate early (IE), early (E), and late (L). Promoter complexity differs strikingly between IE/E genes and L genes. IE and E promoters contain cis-regulating sequences upstream of a TATA box, whereas L promoters comprise a unique cis element. In the case of the gammaherpesviruses, this element is usually a TATT motif found in the position where the consensus TATA box of eukaryotic promoters is typically found. Epstein-Barr virus (EBV) encodes a protein, called BcRF1, which has structural homology with the TATA-binding protein and interacts specifically with the TATT box. However, although necessary for the expression of the L genes, BcRF1 is not sufficient, suggesting that other viral proteins are also required. Here, we present the identification and characterization of a viral protein complex necessary and sufficient for the expression of the late viral genes. This viral complex is composed of five different proteins in addition to BcRF1 and interacts with cellular RNA polymerase II. During the viral productive cycle, this complex, which we call the vPIC (for viral preinitiation complex), works in concert with the viral DNA replication machinery to activate expression of the late viral genes. The EBV vPIC components have homologs in beta- and gammaherpesviruses but not in alphaherpesviruses. Our results not only reveal that beta- and gammaherpesviruses encode their own transcription preinitiation complex responsible for the expression of the late viral genes but also indicate the close evolutionary history of these viruses. Control of late gene transcription in DNA viruses is a major unsolved question in virology. In eukaryotes, the first step in transcriptional activation is the formation of a permissive chromatin, which allows assembly of the preinitiation complex (PIC) at the core promoter. Fixation of the TATA box-binding protein (TBP) is a key rate-limiting step in this process. This study provides evidence that EBV encodes a complex composed of six proteins necessary for the expression of the late viral genes. This complex is formed around a viral TBP-like protein and interacts with cellular RNA polymerase II, suggesting that it is directly involved in the assembly of a virus-specific PIC (vPIC).

Functional and biochemical characterization of human eukaryotic translation initiation factor 3 in living cells.

The main role of the translation initiation factor 3 (eIF3) is to orchestrate formation of 43S-48S preinitiation complexes (PICs). Until now, most of our knowledge on eIF3 functional contribution to regulation of gene expression comes from yeast studies. Hence, here we developed several novel in vivo assays to monitor the integrity of the 13-subunit human eIF3 complex, defects in assembly of 43S PICs, efficiency of mRNA recruitment, and postassembly events such as AUG recognition. We knocked down expression of the PCI domain-containing eIF3c and eIF3a subunits and of eIF3j in human HeLa and HEK293 cells and analyzed the functional consequences. Whereas eIF3j downregulation had barely any effect and eIF3a knockdown disintegrated the entire eIF3 complex, eIF3c knockdown produced a separate assembly of the a, b, g, and i subunits (closely resembling the yeast evolutionary conserved eIF3 core), which preserved relatively high 40S binding affinity and an ability to promote mRNA recruitment to 40S subunits and displayed defects in AUG recognition. Both eIF3c and eIF3a knockdowns also severely reduced protein but not mRNA levels of many other eIF3 subunits and indeed shut off translation. We propose that eIF3a and eIF3c control abundance and assembly of the entire eIF3 and thus represent its crucial scaffolding elements critically required for formation of PICs.

P09 : Regulation of mTOR complex 1 in Liver of Freeze Tolerant wood frogs, Rana sylvatica

Many organisms experience seasonal cold environments during their life and need specific adaptations to allow them to survive when the ambient temperature fall below 0 °C. Previous studies showed a high survival rate (∼100%) of wood frog during different conditions of freezing exposure (various time and/or temperature freezing, and developmental stages of the animals). In order to balance ATP synthesis and consumption under the cold condition, optional yet energy expensive processes such as cell growth need to be suppressed or inhibited. The rate-limiting step of mRNA translation is the initiation stage, which largely regulated by the kinase complex mTORC1 (mTOR complex 1). Active mTORC1 phosphorylates the translation inhibitor 4EBP (eIF4E binding protein) and promoter S6K (40S ribosomal protein S6 kinase). Hyperphosphorylation of 4EBP inhibits its interaction with eIF4E, thereby relieving the inhibitory effect on translation pre-initiation complex assembly. Phosphorylated S6K by mTORC1 is able to enhance the RNA helicase activity of eIF4A by phosphorylating eIF4B, to clear the inhibitory secondary structures of mRNA for translation initiation machinery to reach start codon.