Cdk7 is required for full activation of Drosophila heat shock genes and RNA polymerase II phosphorylation in vivo

The gene AccCyclin H mitigates oxidative stress by influencing trehalose metabolism in Apis cerana cerana

BACKGROUND

Environmental stress can induce oxidative stress in Apis cerana cerana, leading to cellular oxidative damage, reduced vitality, and even death. Currently, owing to an incomplete understanding of the molecular mechanisms by which A. cerana cerana resists oxidative damage, there is no available method to mitigate the risk of this type of damage. Cyclin plays an important role in cell stress resistance. The aim of this study was to explore the in vivo protection of cyclin H against oxidative damage induced by abiotic stress in A. cerana cerana and clarify the mechanism of action. We isolated and identified the AccCyclin H gene in A. cerana cerana and analysed its responses to different exogenous stresses.

RESULTS

The results showed that different oxidative stressors can induce or inhibit the expression of AccCyclin H. After RNA-interference-mediated AccCyclin H silencing, the activity of antioxidant-related genes and related enzymes was inhibited, and trehalose metabolism was reduced. AccCyclin H gene silencing reduced A. cerana cerana high-temperature tolerance. Exogenous trehalose supplementation enhanced the total antioxidant capacity of A. cerana cerana, reduced the accumulation of oxidants, and improved the viability of A. cerana cerana under high-temperature stress.

CONCLUSION

Our findings suggest that trehalose can alleviate adverse stress and that AccCyclin H may participate in oxidative stress reactions by regulating trehalose metabolism. © 2023 Society of Chemical Industry.

Oncogenic CDK13 mutations impede nuclear RNA surveillance

RNA surveillance pathways detect and degrade defective transcripts to ensure RNA fidelity. We found that disrupted nuclear RNA surveillance is oncogenic. Cyclin-dependent kinase 13 (CDK13) is mutated in melanoma, and patient-mutated CDK13 accelerates zebrafish melanoma. CDK13 mutation causes aberrant RNA stabilization. CDK13 is required for ZC3H14 phosphorylation, which is necessary and sufficient to promote nuclear RNA degradation. Mutant CDK13 fails to activate nuclear RNA surveillance, causing aberrant protein-coding transcripts to be stabilized and translated. Forced aberrant RNA expression accelerates melanoma in zebrafish. We found recurrent mutations in genes encoding nuclear RNA surveillance components in many malignancies, establishing nuclear RNA surveillance as a tumor-suppressive pathway. Activating nuclear RNA surveillance is crucial to avoid accumulation of aberrant RNAs and their ensuing consequences in development and disease.

Enhancing HIV-1 latency reversal through regulating the elongating RNA Pol II pause-release by a small-molecule disruptor of PAF1C

The polymerase-associated factor 1 complex (PAF1C) is a key, post-initiation transcriptional regulator of both promoter-proximal pausing and productive elongation catalyzed by RNA Pol II and is also involved in transcriptional repression of viral gene expression during human immunodeficiency virus-1 (HIV-1) latency. Using a molecular docking-based compound screen in silico and global sequencing-based candidate evaluation in vivo, we identified a first-in-class, small-molecule inhibitor of PAF1C (iPAF1C) that disrupts PAF1 chromatin occupancy and induces global release of promoter-proximal paused RNA Pol II into gene bodies. Transcriptomic analysis revealed that iPAF1C treatment mimics acute PAF1 subunit depletion and impairs RNA Pol II pausing at heat shock-down-regulated genes. Furthermore, iPAF1C enhances the activity of diverse HIV-1 latency reversal agents both in cell line latency models and in primary cells from persons living with HIV-1. In sum, this study demonstrates that efficient disruption of PAF1C by a first-in-class, small-molecule inhibitor may have therapeutic potential for improving current HIV-1 latency reversal strategies.

The Long Road to Understanding RNAPII Transcription Initiation and Related Syndromes

In eukaryotes, transcription of protein-coding genes requires the assembly at core promoters of a large preinitiation machinery containing RNA polymerase II (RNAPII) and general transcription factors (GTFs). Transcription is potentiated by regulatory elements called enhancers, which are recognized by specific DNA-binding transcription factors that recruit cofactors and convey, following chromatin remodeling, the activating cues to the preinitiation complex. This review summarizes nearly five decades of work on transcription initiation by describing the sequential recruitment of diverse molecular players including the GTFs, the Mediator complex, and DNA repair factors that support RNAPII to enable RNA synthesis. The elucidation of the transcription initiation mechanism has greatly benefited from the study of altered transcription components associated with human diseases that could be considered transcription syndromes.

The CDK7 inhibitor THZ1 alters RNA polymerase dynamics at the 5' and 3' ends of genes

The t(8;21) is one of the most frequent chromosomal translocations associated with acute myeloid leukemia (AML). We found that t(8;21) AML were extremely sensitive to THZ1, which triggered apoptosis after only 4 h. We used precision nuclear run-on transcription sequencing (PROseq) to define the global effects of THZ1 and other CDK inhibitors on RNA polymerase II dynamics. Inhibition of CDK7 using THZ1 caused wide-spread loss of promoter-proximal paused RNA polymerase. This loss of 5′ pausing was associated with accumulation of polymerases in the body of a large number of genes. However, there were modest effects on genes regulated by ‘super-enhancers’. At the 3′ ends of genes, treatment with THZ1 suppressed RNA polymerase ‘read through’ at the end of the last exon, which resembled a phenotype associated with a mutant RNA polymerase with slower elongation rates. Consistent with this hypothesis, polyA site-sequencing (PolyA-seq) did not detect differences in poly A sites after THZ1 treatment. PROseq analysis after short treatments with THZ1 suggested that these 3′ effects were due to altered CDK7 activity at the 5′ end of long genes, and were likely to be due to slower rates of elongation.

mRNA Cap Methyltransferase, RNMT-RAM, Promotes RNA Pol II-Dependent Transcription

mRNA cap addition occurs early during RNA Pol II-dependent transcription, facilitating pre-mRNA processing and translation. We report that the mammalian mRNA cap methyltransferase, RNMT-RAM, promotes RNA Pol II transcription independent of mRNA capping and translation. In cells, sublethal suppression of RNMT-RAM reduces RNA Pol II occupancy, net mRNA synthesis, and pre-mRNA levels. Conversely, expression of RNMT-RAM increases transcription independent of cap methyltransferase activity. In isolated nuclei, recombinant RNMT-RAM stimulates transcriptional output; this requires the RAM RNA binding domain. RNMT-RAM interacts with nascent transcripts along their entire length and with transcription-associated factors including the RNA Pol II subunits SPT4, SPT6, and PAFc. Suppression of RNMT-RAM inhibits transcriptional markers including histone H2BK120 ubiquitination, H3K4 and H3K36 methylation, RNA Pol II CTD S5 and S2 phosphorylation, and PAFc recruitment. These findings suggest that multiple interactions among RNMT-RAM, RNA Pol II factors, and RNA along the transcription unit stimulate transcription.

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mRNA cap methyltransferase, RNMT-RAM, promotes RNA Pol II-dependent transcription

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RNMT-RAM-dependent transcription is independent of mRNA cap methylation

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RNMT-RAM binds to the entire length of pre-mRNA and to transcription-associated proteins

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Significant loss of RNA Pol II gene occupancy is observed on RNMT-RAM suppression

The pause-initiation limit restricts transcription activation in human cells

Eukaryotic gene transcription is often controlled at the level of RNA polymerase II (Pol II) pausing in the promoter-proximal region. Pausing Pol II limits the frequency of transcription initiation ('pause-initiation limit'), predicting that the pause duration must be decreased for transcriptional activation. To test this prediction, we conduct a genome-wide kinetic analysis of the heat shock response in human cells. We show that the pause-initiation limit restricts transcriptional activation at most genes. Gene activation generally requires the activity of the P-TEFb kinase CDK9, which decreases the duration of Pol II pausing and thereby enables an increase in the productive initiation frequency. The transcription of enhancer elements is generally not pause limited and can be activated without CDK9 activity. Our results define the kinetics of Pol II transcriptional regulation in human cells at all gene classes during a natural transcription response.

Perturbing Enhancer Activity in Cancer Therapy

Tight regulation of gene transcription is essential for normal development, tissue homeostasis, and disease-free survival. Enhancers are distal regulatory elements in the genome that provide specificity to gene expression programs and are frequently misregulated in cancer. Recent studies examined various enhancer-driven malignant dependencies and identified different approaches to specifically target these programs. In this review, we describe numerous features that make enhancers good transcriptional targets in cancer therapy and discuss different approaches to overcome enhancer perturbation. Interestingly, a number of approved therapeutic agents, such as cyclosporine, steroid hormones, and thiazolidinediones, actually function by affecting enhancer landscapes by directly targeting very specific transcription factor programs. More recently, a broader approach to targeting deregulated enhancer programs has been achieved via Bromodomain and Extraterminal (BET) inhibition or perturbation of transcription-related cyclin-dependent kinases (CDK). One challenge to enhancer-targeted therapy is proper patient stratification. We suggest that monitoring of enhancer RNA (eRNA) expression may serve as a unique biomarker of enhancer activity that can help to predict and monitor responsiveness to enhancer-targeted therapies. A more thorough investigation of cancer-specific enhancers and the underlying mechanisms of deregulation will pave the road for an effective utilization of enhancer modulators in a precision oncology approach to cancer treatment.

TFIIE orchestrates the recruitment of the TFIIH kinase module at promoter before release during transcription

In eukaryotes, the general transcription factors TFIIE and TFIIH assemble at the transcription start site with RNA Polymerase II. However, the mechanism by which these transcription factors incorporate the preinitiation complex and coordinate their action during RNA polymerase II transcription remains elusive. Here we show that the TFIIEα and TFIIEβ subunits anchor the TFIIH kinase module (CAK) within the preinitiation complex. In addition, we show that while RNA polymerase II phosphorylation and DNA opening occur, CAK and TFIIEα are released from the promoter. This dissociation is impeded by either ATP-γS or CDK7 inhibitor THZ1, but still occurs when XPB activity is abrogated. Finally, we show that the Core-TFIIH and TFIIEβ are subsequently removed, while elongation factors such as DSIF are recruited. Remarkably, these early transcriptional events are affected by TFIIE and TFIIH mutations associated with the developmental disorder, trichothiodystrophy.

The general transcription factors TFIIE and TFIIH assemble at the transcription start site with RNA Polymerase II. Here the authors provide evidence that the TFIIEα and TFIIEβ subunits anchor the TFIIH kinase module within the preinitiation complex before their release during transcription.

Argonaute2 attenuates active transcription by limiting RNA Polymerase II elongation in Drosophila melanogaster

Increasing lines of evidence support that Argonaute2 (AGO2) harbors several nuclear functions in metazoa. In particular, Drosophila AGO2 modulates transcription of developmentally regulated genes; however, the molecular mechanisms behind AGO2 recruitment into chromatin and its function in transcription have not been deeply explored. In this study, we show that Drosophila AGO2 chromatin association depends on active transcription. In order to gain insight into how AGO2 controls transcription, we performed differential ChIP-seq analysis for RNA Polymerase II (Pol II) upon depletion of AGO2. Remarkably, we find specific accumulation of the elongating but not initiating form of Pol II after AGO2 knockdown, suggesting that AGO2 impairs transcription elongation. Finally, AGO2 also affects Negative Elongation Factor (NELF) chromatin association but not the Cyclin Dependent Kinase 9 (CDK9). Altogether, these results provide key insights into the molecular role of AGO2 in attenuating elongation of certain actively transcribed genes.

Is TFIIH the new Achilles heel of cancer cells?

TFIIH is a 10-subunit complex involved in transcription and DNA repair. It contains several enzymatic activities including a ATP-dependent DNA translocase in XPB and a cyclin-dependent kinase in CDK7. Recently the discovery of several XPB and CDK7 inhibitors with specific impact on the transcriptional addiction of many tumors pinpointed these activities as potential target in cancer chemotherapy. Unexpectedly a basal transcription factor involved in global mRNA expression now emerges a one of the most clinically promising Achilles heels of cancerous cells. These inhibitors also proved to be useful tools to unveil new functions of TFIIH in gene expression.

Microprocessor Recruitment to Elongating RNA Polymerase II Is Required for Differential Expression of MicroRNAs

The cellular abundance of mature microRNAs (miRNAs) is dictated by the efficiency of nuclear processing of primary miRNA transcripts (pri-miRNAs) into pre-miRNA intermediates. The Microprocessor complex of Drosha and DGCR8 carries this out, but it has been unclear what controls Microprocessor's differential processing of various pri-miRNAs. Here, we show that Drosophila DGCR8 (Pasha) directly associates with the C-terminal domain of the RNA polymerase II elongation complex when it is phosphorylated by the Cdk9 kinase (pTEFb). When association is blocked by loss of Cdk9 activity, a global change in pri-miRNA processing is detected. Processing of pri-miRNAs with a UGU sequence motif in their apical junction domain increases, while processing of pri-miRNAs lacking this motif decreases. Therefore, phosphorylation of RNA polymerase II recruits Microprocessor for co-transcriptional processing of non-UGU pri-miRNAs that would otherwise be poorly processed. In contrast, UGU-positive pri-miRNAs are robustly processed by Microprocessor independent of RNA polymerase association.

Functional interplay between MSL1 and CDK7 controls RNA polymerase II Ser5 phosphorylation

Proper gene expression requires coordinated interplay among transcriptional coactivators, transcription factors and the general transcription machinery. We report here that MSL1, a central component of the dosage compensation complex in Drosophila melanogaster and Drosophila virilis, displays evolutionarily conserved sex-independent binding to promoters. Genetic and biochemical analyses reveal a functional interaction of MSL1 with CDK7, a subunit of the Cdk-activating kinase (CAK) complex of the general transcription factor TFIIH. Importantly, MSL1 depletion leads to decreased phosphorylation of Ser5 of RNA polymerase II. In addition, we demonstrate that MSL1 is a phosphoprotein, and transgenic flies expressing MSL1 phosphomutants show mislocalization of the histone acetyltransferase MOF and histone H4 K16 acetylation, thus ultimately causing male lethality due to a failure of dosage compensation. We propose that, by virtue of its interaction with components of the general transcription machinery, MSL1 exists in different phosphorylation states, thereby modulating transcription in flies.

The Drosophila HNF4 nuclear receptor promotes glucose-stimulated insulin secretion and mitochondrial function in adults

Although mutations in HNF4A were identified as the cause of Maturity Onset Diabetes of the Young 1 (MODY1) two decades ago, the mechanisms by which this nuclear receptor regulates glucose homeostasis remain unclear. Here we report that loss of Drosophila HNF4 recapitulates hallmark symptoms of MODY1, including adult-onset hyperglycemia, glucose intolerance and impaired glucose-stimulated insulin secretion (GSIS). These defects are linked to a role for dHNF4 in promoting mitochondrial function as well as the expression of Hex-C, a homolog of the MODY2 gene Glucokinase. dHNF4 is required in the fat body and insulin-producing cells to maintain glucose homeostasis by supporting a developmental switch toward oxidative phosphorylation and GSIS at the transition to adulthood. These findings establish an animal model for MODY1 and define a developmental reprogramming of metabolism to support the energetic needs of the mature animal.

THZ1 Reveals Roles for Cdk7 in Co-transcriptional Capping and Pausing

The Cdk7 subunit of TFIIH phosphorylates RNA polymerase II (Pol II) during initiation, and, while recent studies show that inhibition of human Cdk7 negatively influences transcription, the mechanisms involved are unclear. Using in vitro transcription with nuclear extract, we demonstrate that THZ1, a covalent Cdk7 inhibitor, causes defects in Pol II phosphorylation, co-transcriptional capping, promoter proximal pausing, and productive elongation. THZ1 does not affect initiation but blocks essentially all Pol II large subunit C-terminal domain (CTD) phosphorylation. We found that guanylylation of nascent RNAs is length dependent and modulated by a THZ1-sensitive factor present in nuclear extract. THZ1 impacts pausing through a capping-independent block of DSIF and NELF loading. The P-TEFb-dependent transition into productive elongation was also inhibited by THZ1, likely due to loss of DSIF. Capping and pausing were also reduced in THZ1-treated cells. Our results provide mechanistic insights into THZ1 action and how Cdk7 broadly influences transcription and capping.

Functional role of RNA polymerase II and P70 S6 kinase in KCl withdrawal-induced cerebellar granule neuron apoptosis

KCl withdrawal-induced apoptosis in cerebellar granule neurons is associated with aberrant cell cycle activation, and treatment with cyclin-dependent kinase (Cdk) inhibitors protects cells from undergoing apoptosis. Because the Cdk inhibitor flavopiridol is known to inhibit RNA polymerase II (Pol II)-dependent transcription elongation by inhibiting the positive transcription elongation factor b (P-TEFb, a complex of CDK9 and cyclin T), we examined whether inhibition of RNA Pol II protects neurons from apoptosis. Treatment of neurons with 5, 6-dichloro-1-β-D-ribobenzimidazole (DRB), an RNA Pol II-dependent transcription elongation inhibitor, and flavopiridol inhibited phosphorylation and activation of Pol II and protected neurons from undergoing apoptosis. In addition to Pol II, neurons subjected to KCl withdrawal showed increased phosphorylation and activation of p70 S6 kinase, which was inhibited by both DRB and flavopiridol. Immunostaining analysis of the neurons deprived of KCl showed increased nuclear levels of phospho-p70 S6 kinase, and neurons protected with DRB and flavopiridol showed accumulation of the kinase into large spliceosome assembly factor-positive speckle domains within the nuclei. The formation of these foci corresponded with cell survival, and removal of the inhibitors resulted in dispersal of the speckles into smaller foci with subsequent apoptosis induction. Because p70 S6 kinase is known to induce translation of mRNAs containing a 5'-terminal oligopyrimidine tract, our data suggest that transcription and translation of this subset of mRNAs may contribute to KCl withdrawal-induced apoptosis in neurons.

The metazoan-specific mediator subunit 26 (Med26) is essential for viability and is found at both active genes and pericentric heterochromatin in Drosophila melanogaster

Human MED26 was originally purified in the cofactor required for the Sp1 activation complex (CRSP) as a 70-kDa component named CRSP70. This polypeptide was specific to metazoans and the “small” form of the Mediator complex. We report here that a Drosophila melanogaster homologue of MED26 similarly interacts with other components of the core Drosophila Mediator complex but not with the kinase module and is recruited to genes upon activation. Using a null allele of Med26, we show that Med26 is required for organismal viability but not for cell proliferation or survival. Clones lacking Med26 in the wing disc lead to loss of the adult wing margin and reduced expression of genes involved in wing margin formation. Surprisingly, when polytene chromosomes from the salivary gland were examined using antibodies to Med26, it was apparent that a fraction of the protein was associated with the chromocenter, which contains pericentric heterochromatin. This staining colocalizes with heterochromatin protein 1 (HP1). Immunoprecipitation experiments show that Med26 interacts with HP1. The interaction is mediated through the chromoshadow domain of HP1 and through the conserved motif in the carboxy terminus of the Med26 protein. This work is the first characterization of the metazoan-specific Mediator subunit in an animal model.

Microarray analysis of cell cycle gene expression in adult human corneal endothelial cells

Corneal endothelial cells (ECs) form a monolayer that controls the hydration of the cornea and thus its transparency. Their almost nil proliferative status in humans is responsible, in several frequent diseases, for cell pool attrition that leads to irreversible corneal clouding. To screen for candidate genes involved in cell cycle arrest, we studied human ECs subjected to various environments thought to induce different proliferative profiles compared to ECs in vivo. Donor corneas (a few hours after death), organ-cultured (OC) corneas, in vitro confluent and non-confluent primary cultures, and an immortalized EC line were compared to healthy ECs retrieved in the first minutes of corneal grafts. Transcriptional profiles were compared using a cDNA array of 112 key genes of the cell cycle and analysed using Gene Ontology classification; cluster analysis and gene map presentation of the cell cycle regulation pathway were performed by GenMAPP. Results were validated using qRT-PCR on 11 selected genes. We found several transcripts of proteins implicated in cell cycle arrest and not previously reported in human ECs. Early G1-phase arrest effectors and multiple DNA damage-induced cell cycle arrest-associated transcripts were found in vivo and over-represented in OC and in vitro ECs. Though highly proliferative, immortalized ECs also exhibited overexpression of transcripts implicated in cell cycle arrest. These new effectors likely explain the stress-induced premature senescence that characterizes human adult ECs. They are potential targets for triggering and controlling EC proliferation with a view to increasing the cell pool of stored corneas or facilitating mass EC culture for bioengineered endothelial grafts.

A novel CDK9 inhibitor shows potent antitumor efficacy in preclinical hematologic tumor models

DNA-dependent RNA polymerase II (RNAP II) largest subunit RPB1 C-terminal domain (CTD) kinases, including CDK9, are serine/threonine kinases known to regulate transcriptional initiation and elongation by phosphorylating Ser 2, 5, and 7 residues on CTD. Given the reported dysregulation of these kinases in some cancers, we asked whether inhibiting CDK9 may induce stress response and preferentially kill tumor cells. Herein, we describe a potent CDK9 inhibitor, LY2857785, that significantly reduces RNAP II CTD phosphorylation and dramatically decreases MCL1 protein levels to result in apoptosis in a variety of leukemia and solid tumor cell lines. This molecule inhibits the growth of a broad panel of cancer cell lines, and is particularly efficacious in leukemia cells, including orthotopic leukemia preclinical models as well as in ex vivo acute myeloid leukemia and chronic lymphocytic leukemia patient tumor samples. Thus, inhibition of CDK9 may represent an interesting approach as a cancer therapeutic target, especially in hematologic malignancies.

CDK7 regulates the mitochondrial localization of a tail-anchored proapoptotic protein, Hid

The mitochondrial outer membrane is a major site of apoptosis regulation across phyla. Human and C. elegans Bcl-2 family proteins and Drosophila Hid require the C-terminal tail-anchored (TA) sequence in order to insert into the mitochondrial membrane, but it remains unclear whether cytosolic proteins actively regulate the mitochondrial localization of these proteins. Here, we report that the cdk7 complex regulates the mitochondrial localization of Hid and its ability to induce apoptosis. We identified cdk7 through an in vivo RNAi screen of genes required for cell death. Although CDK7 is best known for its role in transcription and cell-cycle progression, a hypomorphic cdk7 mutant suppressed apoptosis without impairing these other known functions. In this cdk7 mutant background, Hid failed to localize to the mitochondria and failed to bind to recombinant inhibitors of apoptosis (IAPs). These findings indicate that apoptosis is promoted by a newly identified function of CDK7, which couples the mitochondrial localization and IAP binding of Hid.

Cyclin-dependent kinase control of the initiation-to-elongation switch of RNA polymerase II

Promoter-proximal pausing by RNA polymerase II (Pol II) ensures both gene-specific regulation and RNA quality control. Structural considerations suggested initiation factor eviction would be required for elongation factor engagement and pausing of transcription complexes. Here we show that selective inhibition of Cdk7—part of TFIIH—increases TFIIE retention, prevents DRB-sensitivity inducing factor (DSIF) recruitment and attenuates pausing in human cells. Pause release depends on Cdk9—cyclin T1 (P-TEFb); Cdk7 is also required for Cdk9-activating phosphorylation and Cdk9-dependent downstream events—Pol II carboxyl-terminal domain Ser2 phosphorylation and histone H2B ubiquitylation—in vivo. Cdk7 inhibition, moreover, impairs Pol II transcript 3′-end formation. Cdk7 thus acts through TFIIE and DSIF to establish and through P-TEFb to relieve barriers to elongation: incoherent feedforward that might create a window to recruit RNA-processing machinery. Therefore, cyclin-dependent kinases govern Pol II handoff from initiation to elongation factors and co-transcriptional RNA maturation.

B2 RNA represses TFIIH phosphorylation of RNA polymerase II

Mouse B2 RNA represses RNA polymerase II (Pol II) transcription during the cellular heat shock response. B2 RNA binds Pol II, enters complexes at promoters, and keeps the polymerase from engaging the DNA. Here we show that phosphorylation of Ser5 residues in the Pol II carboxy terminal domain (CTD) decreases after heat shock at the promoter of the repressed actin gene in mouse cells, despite the continued presence of Cdk7 and cyclin H. Biochemical assays revealed that B2 RNA, when present with Pol II in promoter-bound complexes, specifically represses CTD phosphorylation by TFIIH.

TFIIH: when transcription met DNA repair

The transcription initiation factor TFIIH is a remarkable protein complex that has a fundamental role in the transcription of protein-coding genes as well as during the DNA nucleotide excision repair pathway. The detailed understanding of how TFIIH functions to coordinate these two processes is also providing an explanation for the phenotypes observed in patients who bear mutations in some of the TFIIH subunits. In this way, studies of TFIIH have revealed tight molecular connections between transcription and DNA repair and have helped to define the concept of 'transcription diseases'.

Identical functional organization of nonpolytene and polytene chromosomes in Drosophila melanogaster

Salivary gland polytene chromosomes demonstrate banding pattern, genetic meaning of which is an enigma for decades. Till now it is not known how to mark the band/interband borders on physical map of DNA and structures of polytene chromosomes are not characterized in molecular and genetic terms. It is not known either similar banding pattern exists in chromosomes of regular diploid mitotically dividing nonpolytene cells. Using the newly developed approach permitting to identify the interband material and localization data of interband-specific proteins from modENCODE and other genome-wide projects, we identify physical limits of bands and interbands in small cytological region 9F13-10B3 of the X chromosome in D. melanogaster, as well as characterize their general molecular features. Our results suggests that the polytene and interphase cell line chromosomes have practically the same patterns of bands and interbands reflecting, probably, the basic principle of interphase chromosome organization. Two types of bands have been described in chromosomes, early and late-replicating, which differ in many aspects of their protein and genetic content. As appeared, origin recognition complexes are located almost totally in the interbands of chromosomes.

RNA polymerase II kinetics in polo polyadenylation signal selection

Regulated alternative polyadenylation is an important feature of gene expression, but how gene transcription rate affects this process remains to be investigated. polo is a cell-cycle gene that uses two poly(A) signals in the 3' untranslated region (UTR) to produce alternative messenger RNAs that differ in their 3'UTR length. Using a mutant Drosophila strain that has a lower transcriptional elongation rate, we show that transcription kinetics can determine alternative poly(A) site selection. The physiological consequences of incorrect polo poly(A) site choice are of vital importance; transgenic flies lacking the distal poly(A) signal cannot produce the longer transcript and die at the pupa stage due to a failure in the proliferation of the precursor cells of the abdomen, the histoblasts. This is due to the low translation efficiency of the shorter transcript produced by proximal poly(A) site usage. Our results show that correct polo poly(A) site selection functions to provide the correct levels of protein expression necessary for histoblast proliferation, and that the kinetics of RNA polymerase II have an important role in the mechanism of alternative polyadenylation.

Requirement of TFIIH kinase subunit Mat1 for RNA Pol II C-terminal domain Ser5 phosphorylation, transcription and mRNA turnover

The relevance of serine 5 phosphorylation of RNA polymerase II carboxy-terminal domain during initiation has been difficult to determine in mammalian cells as no general in vivo Ser5 kinase has been identified. Here, we demonstrate that deletion of the TFIIH kinase subunit Mat1 in mouse fibroblasts leads to dramatically reduced Pol II Ser5 phosphorylation. This is associated with defective capping and reduced Ser2 phosphorylation, decreased Pol II progression into elongation and severely attenuated transcription detected through analysis of nascent mRNAs, establishing a general requirement for mammalian Mat1 in transcription. Surprisingly, the general defect in Pol II transcription in Mat1^−/− fibroblasts is not reflected in the majority of steady-state mRNAs. This indicates widespread stabilization of mRNAs and points to the existence of a regulatory mechanism to stabilize mRNAs following transcriptional attenuation, thus revealing a potential caveat in similar studies limited to analysis of steady-state mRNAs.

Promoting the promoter

Recent evolutionary studies clearly indicate that evolution is mainly driven by changes in the complex mechanisms of gene regulation and not solely by polymorphism in protein-encoding genes themselves. After a short description of the cis-regulatory mechanism, we intend in this review to argue that by applying newly available technologies and by merging research areas such as evolutionary and developmental biology, population genetics, ecology and molecular cell biology it is now possible to study evolution in an integrative way. We contend that, by analysing the effects of promoter sequence variation on phenotypic diversity in natural populations, we will soon be able to break the barrier between the study of extant genetic variability and the study of major developmental changes. This will lead to an integrative view of evolution at different scales. Because of their sessile nature and their continuous development, plants must permanently regulate their gene expression to react to their environment, and can, therefore, be considered as a remarkable model for these types of studies.

Cyclin T1 overexpression induces malignant transformation and tumor growth

Human PTE Fb is a protein kinase composed by CDK9 and Cyclin T that controls the elongation phase of RNA Pol II. This complex also affects the activation and differentiation program of lymphoid cells. In this study we found that several head and neck tumor cell lines overexpress PTE Fb. We also established that Cyclin T1 is able to induce transformation in vitro, as we determined by foci and colony formation assays. Nu/nu mice s.c. injected with stable transfected Cyclin T1 cells (NIH 3T3 Cyclin T1) developed tumors faster than animals injected with control cells (NIH 3T3 beta-gal). In vitro, NIH 3T3 Cyclin T1 cells show increased proliferation and CDK4-Rb phosphorylation. Even more, silencing E2F1 expression (shRNA E2F1) in NIH 3T3 cells resulted in a dramatic inhibition of Cyclin T1-induced foci. All these data demonstrate for the first time the Cyclin T1 oncogenic function and suggest a role for this protein in controlling cell cycle probably via Rb/E2F1 pathway.

On the traces of XPD: cell cycle matters - untangling the genotype-phenotype relationship of XPD mutations

Mutations in the human gene coding for XPD lead to segmental progeria - the premature appearance of some of the phenotypes normally associated with aging - which may or may not be accompanied by increased cancer incidence. XPD is required for at least three different critical cellular functions: in addition to participating in the process of nucleotide excision repair (NER), which removes bulky DNA lesions, XPD also regulates transcription as part of the general transcription factor IIH (TFIIH) and controls cell cycle progression through its interaction with CAK, a pivotal activator of cyclin dependent kinases (CDKs). The study of inherited XPD disorders offers the opportunity to gain insights into the coordination of important cellular events and may shed light on the mechanisms that regulate the delicate equilibrium between cell proliferation and functional senescence, which is notably altered during physiological aging and in cancer.

The phenotypic manifestations in the different XPD disorders are the sum of disturbances in the vital processes carried out by TFIIH and CAK. In addition, further TFIIH- and CAK-independent cellular activities of XPD may also play a role. This, added to the complex feedback networks that are in place to guarantee the coordination between cell cycle, DNA repair and transcription, complicates the interpretation of clinical observations. While results obtained from patient cell isolates as well as from murine models have been elementary in revealing such complexity, the Drosophila embryo has proven useful to analyze the role of XPD as a cell cycle regulator independently from its other cellular functions. Together with data from the biochemical and structural analysis of XPD and of the TFIIH complex these results combine into a new picture of the XPD activities that provides ground for a better understanding of the patophysiology of XPD diseases and for future development of diagnostic and therapeutic tools.

Drosophila Xpd regulates Cdk7 localization, mitotic kinase activity, spindle dynamics, and chromosome segregation

The trimeric CAK complex functions in cell cycle control by phosphorylating and activating Cdks while TFIIH-linked CAK functions in transcription. CAK also associates into a tetramer with Xpd, and our analysis of young Drosophila embryos that do not require transcription now suggests a cell cycle function for this interaction. xpd is essential for the coordination and rapid progression of the mitotic divisions during the late nuclear division cycles. Lack of Xpd also causes defects in the dynamics of the mitotic spindle and chromosomal instability as seen in the failure to segregate chromosomes properly during ana- and telophase. These defects appear to be also nucleotide excision repair (NER)–independent. In the absence of Xpd, misrouted spindle microtubules attach to chromosomes of neighboring mitotic figures, removing them from their normal location and causing multipolar spindles and aneuploidy. Lack of Xpd also causes changes in the dynamics of subcellular and temporal distribution of the CAK component Cdk7 and local mitotic kinase activity. xpd thus functions normally to re-localize Cdk7(CAK) to different subcellular compartments, apparently removing it from its cell cycle substrate, the mitotic Cdk. This work proves that the multitask protein Xpd also plays an essential role in cell cycle regulation that appears to be independent of transcription or NER. Xpd dynamically localizes Cdk7/CAK to and away from subcellular substrates, thereby controlling local mitotic kinase activity. Possibly through this activity, xpd controls spindle dynamics and chromosome segregation in our model system. This novel role of xpd should also lead to new insights into the understanding of the neurological and cancer aspects of the human XPD disease phenotypes.

Mutations in human xpd cause three different syndromes—XP (xeroderma pigmentosum), TTD (trichothiodystrophy), and CS (Cockayne syndrome)—and various different phenotypes, such as sun-induced hyperpigmentation of the skin, cutaneous abnormalities, neuronal degeneration, and developmental retardation. In addition, while some mutations cause a highly elevated cancer risk, others do not. The multitask protein Xpd functions in transcription, nucleotide excision repair (NER), and in cell cycle regulation. In a situation where transcription is not required and NER not induced, we specifically analyzed the cell cycle function of Xpd in Drosophila. In this situation Xpd locally controls the dynamic localization of Cdk7, the catalytic subunit of the Cdk activating kinase (CAK) to and away from its cellular targets, thereby regulating mitotic kinase activity and mitotic exit. Xpd also controls spindle dynamics to prevent formation of multipolar and promiscuous spindles and aneuploidy. Through multitask proteins like Xpd and Cdk7 cells regulate different cellular pathways in a coordinated fashion. In addition to the basic research relevance, the newly gained knowledge about the cell cycle function of Xpd and its control of spindle dynamics is also relevant for human xpd patients because it shows a possible pathway that could lead to highly increased cancer risk and neurological defects.

Defining mechanisms that regulate RNA polymerase II transcription in vivo

In the eukaryotic genome, the thousands of genes that encode messenger RNA are transcribed by a molecular machine called RNA polymerase II. Analysing the distribution and status of RNA polymerase II across a genome has provided crucial insights into the long-standing mysteries of transcription and its regulation. These studies identify points in the transcription cycle where RNA polymerase II accumulates after encountering a rate-limiting step. When coupled with genome-wide mapping of transcription factors, these approaches identify key regulatory steps and factors and, importantly, provide an understanding of the mechanistic generalities, as well as the rich diversities, of gene regulation.

TFIIH-associated Cdk7 kinase functions in phosphorylation of C-terminal domain Ser7 residues, promoter-proximal pausing, and termination by RNA polymerase II

The function of human TFIIH-associated Cdk7 in RNA polymerase II (Pol II) transcription and C-terminal domain (CTD) phosphorylation was investigated in analogue-sensitive Cdk7(as/as) mutant cells where the kinase can be inhibited without disrupting TFIIH. We show that both Cdk7 and Cdk9/PTEFb contribute to phosphorylation of Pol II CTD Ser5 residues on transcribed genes. Cdk7 is also a major kinase of CTD Ser7 on Pol II at the c-fos and U snRNA genes. Furthermore, TFIIH and recombinant Cdk7-CycH-Mat1 as well as recombinant Cdk9-CycT1 phosphorylated CTD Ser7 and Ser5 residues in vitro. Inhibition of Cdk7 in vivo suppressed the amount of Pol II accumulated at 5' ends on several genes including c-myc, p21, and glyceraldehyde-3-phosphate dehydrogenase genes, indicating reduced promoter-proximal pausing or polymerase "leaking" into the gene. Consistent with a 5' pausing defect, Cdk7 inhibition reduced recruitment of the negative elongation factor NELF at start sites. A role of Cdk7 in regulating elongation is further suggested by enhanced histone H4 acetylation and diminished histone H4 trimethylation on lysine 36-two marks of elongation-within genes when the kinase was inhibited. Consistent with a new role for TFIIH at 3' ends, it was detected within genes and 3'-flanking regions, and Cdk7 inhibition delayed pausing and transcription termination.

The Lawc protein is required for proper transcription by RNA polymerase II in Drosophila

Genetic analysis of the Drosophila leg-arista-wing complex (lawc) gene suggests a role for the Lawc protein in chromatin-related processes based on its classification as a trxG gene but the molecular mechanisms of its function remain elusive. We have found that Lawc is a small, cysteine-rich protein that is present in most of the interbands of polytene chromosomes. In agreement with this observation, Lawc co-localizes with RNA polymerase IIo (Pol IIo) and it is recruited to transcribed loci after elongation by Pol IIo has begun. Lawc interacts with the nuclear proteasome regulator dREGgamma in a yeast two-hybrid assay and both proteins co-localize on polytene chromosomes. In addition, a mutation in lawc interacts genetically with a mutation in a component of the proteasome. lawc mutants show decreased expression of some genes, while the levels of Pol IIoSer2 increase. We conclude that Lawc is required for proper transcription by RNA polymerase II in a process that involves the nuclear proteasome.

TFIID component TAF7 functionally interacts with both TFIIH and P-TEFb

Transcription consists of a series of highly regulated steps: assembly of the preinitiation complex (PIC) at the promoter, initiation, elongation, and termination. PIC assembly is nucleated by TFIID, a complex composed of the TATA-binding protein (TBP) and a series of TBP-associated factors (TAFs). One component, TAF7, is incorporated in the PIC through its interaction with TFIID but is released from TFIID upon transcription initiation. We now report that TAF7 interacts with the transcription factors, TFIIH and P-TEFb, resulting in the inhibition of their Pol II CTD kinase activities. Importantly, in in vitro transcription reactions, TAF7 inhibits steps after PIC assembly and formation of the first phosphodiester bonds. Further, in vivo TAF7 coelongates with P-TEFb and Pol II downstream of the promoter. We propose a model in which TAF7 contributes to the regulation of the transition from PIC assembly to initiation and elongation.

P-TEFb is critical for the maturation of RNA polymerase II into productive elongation in vivo

Positive transcription elongation factor b (P-TEFb) is the major metazoan RNA polymerase II (Pol II) carboxyl-terminal domain (CTD) Ser2 kinase, and its activity is believed to promote productive elongation and coupled RNA processing. Here, we demonstrate that P-TEFb is critical for the transition of Pol II into a mature transcription elongation complex in vivo. Within 3 min following P-TEFb inhibition, most polymerases were restricted to within 150 bp of the transcription initiation site of the active Drosophila melanogaster Hsp70 gene, and live-cell imaging demonstrated that these polymerases were stably associated. Polymerases already productively elongating at the time of P-TEFb inhibition, however, proceeded with elongation in the absence of active P-TEFb and cleared from the Hsp70 gene. Strikingly, all transcription factors tested (P-TEFb, Spt5, Spt6, and TFIIS) and RNA-processing factor CstF50 exited the body of the gene with kinetics indistinguishable from that of Pol II. An analysis of the phosphorylation state of Pol II upon the inhibition of P-TEFb also revealed no detectable CTD Ser2 phosphatase activity upstream of the Hsp70 polyadenylation site. In the continued presence of P-TEFb inhibitor, Pol II levels across the gene eventually recovered.

Regulation of Transcript Elongation through Cooperative and Ordered Recruitment of Cofactors

We studied the regulation of murine CD80, a gene whose basal transcriptional status was characterized by the presence of a stalled RNA polymerase II complex on the promoter-proximal region. Stimulus-induced activation of productive elongation involved a complex interplay of regulated events that included a synergy between ordered cofactor recruitment. This cascade of recruitments was initiated through the engagement of transcription factor NF-kappaB, leading to the temporal association of histone acetyltransferases and the consequent selective acetylation of a transcription start site downstream nucleosome. This in turn culminated into the nucleosomal association of Brd4-associated P-TEFb, a protein complex containing kinase specific for serine 2 of Rbp 1, the largest subunit of the carboxyl-terminal domain of RNA polymerase II. The consequent phosphorylation of serine 2 residues in CTD by CDK9 in the P-TEFb complex then facilitated escape of polymerase II into the productive elongation phase. Thus, the cooperative mechanisms that integrate between independent pathways characterize regulation of the elongation step of transcription, thereby providing another level at which specificity of gene regulation can be achieved.

Ménage-à-trois 1 is critical for the transcriptional function of PPARgamma coactivator 1

The Cdk7/cyclin H/ménage-à-trois 1 (MAT1) heterotrimer has proposed functions in transcription as the kinase component of basal transcription factor TFIIH and is activated in adult hearts by Gq-, calcineurin-, and biomechanical stress-dependent pathways for hypertrophic growth. Using cardiac-specific Cre, we have ablated MAT1 in myocardium. Despite reduced Cdk7 activity, MAT1-deficient hearts grew normally, but fatal heart failure ensued at 6-8 weeks. By microarray profiling, quantitative RT-PCR, and western blotting at 4 weeks, genes for energy metabolism were found to be suppressed selectively, including targets of peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1). Cardiac metabolic defects were substantiated in isolated perfused hearts and isolated mitochondria. In culture, deleting MAT1 with Cre disrupted PGC-1 function: PGC-1alpha failed to activate PGC-1-responsive promoters and nuclear receptors, GAL4-PGC-1alpha was functionally defective, and PGC-1beta was likewise deficient. PGC-1 bound to both MAT1 and Cdk7 in coprecipitation assays. Thus, we demonstrate a requirement for MAT1 in the operation of PGC-1 coactivators that control cell metabolism.

Diverse phosphoregulatory mechanisms controlling cyclin-dependent kinase-activating kinases in Arabidopsis

For the full activation of cyclin-dependent kinases (CDKs), not only cyclin binding but also phosphorylation of a threonine (Thr) residue within the T-loop is required. This phosphorylation is catalyzed by CDK-activating kinases (CAKs). In Arabidopsis three D-type CDK genes (CDKD;1-CDKD;3) encode vertebrate-type CAK orthologues, of which CDKD;2 exhibits high phosphorylation activity towards the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II. Here, we show that CDKD;2 forms a stable complex with cyclin H and is downregulated by the phosphorylation of the ATP-binding site by WEE1 kinase. A knockout mutant of CDKD;3, which has a higher CDK kinase activity, displayed no defect in plant development. Instead, another type of CAK - CDKF;1 - exhibited significant activity towards CDKA;1 in Arabidopsis root protoplasts, and the activity was dependent on the T-loop phosphorylation of CDKF;1. We propose that two distinct types of CAK, namely CDKF;1 and CDKD;2, play a major role in CDK and CTD phosphorylation, respectively, in Arabidopsis.

TFIIH trafficking and its nuclear assembly during early Drosophila embryo development

We present the first analysis of the dynamics of the transcription DNA-repair factor TFIIH at the onset of transcription in early Drosophila development. TFIIH is composed of ten polypeptides that are part of two complexes - the core and the CAK. We found that the TFIIH core is initially located in the cytoplasm of syncytial blastoderm embryos, and that after mitotic division ten and until the cellular blastoderm stage, the core moves from the cytoplasm to the nucleus. By contrast, the CAK complex is mostly cytoplasmic during cellularization and during gastrulation. However, both components are positioned at promoters of genes that are activated at transcription onset. Later in development, the CAK complex becomes mostly nuclear and co-localizes in most chromosomal regions with the TFIIH core, but not in all sites, suggesting that the CAK complex could have a TFIIH-independent role in transcription of some loci. We also demonstrate that even though the CAK and the core coexist in the early embryo cytoplasm, they do not interact until they are in the nucleus and suggest that the complete assembly of the ten subunits of TFIIH occurs in the nucleus at the mid-blastula transition. In addition, we present evidence that suggests that DNA helicase subunits XPB and XPD are assembled in the core when they are transported into the nucleus and are required for the onset of transcription.

The effects of temperature shock on transcription and replication in Rhynchosciara americana (Diptera: Sciaridae)

The chromosomal response to temperature shock in Rhynchosciara americana is similar to that observed in other Diptera. After a 33 degrees C/90 min or a 36 degrees C/30 min shock the reaction for RNA polymerase II (RpII) is enhanced at five loci. The most prominent of these was identified by in situ hybridization as the site of the hsp70 gene. At 33 degrees C, an accumulation of heat shock factor (HSF) and an increase in the level of RpII was observed at some heat shock loci after 5 min and reached a maximum after 15 min at most loci. The pattern of accumulation of HSF and RpII at individual heat shock loci was similar and their increases were generally coordinated among the loci. RpII gradually decreased at sites active prior to shock, the rate of decrease varying with the site. The B2 DNA puff retained RpII for a significant length of time while the histone locus still contained RpII after a shock of 90 min. With a 36 degrees C/30 min shock, the size of the heat shock puffs and the intensities of HSF and RpII peaked at 1-4 h post stress. The level of HSF declined rapidly after 1 h while the level of RpII remained high for an additional 4 h. The reaction of the DNA puffs to heat shock varied. Usually they did not regress completely and retained traces of RpII. BrdU incorporation continued at both amplifying and non-amplifying bands after shock but on average it appeared depressed for about 24 h post stress.

Recruitment of TFIIH to the HIV LTR is a rate-limiting step in the emergence of HIV from latency

Latently infected cells rapidly initiate HIV transcription after exposure to signals that induce NF-kappaB. To investigate the role of TFIIH during HIV reactivation in vivo, we developed a population of Jurkat cells containing integrated, but transcriptionally silent, HIV proviruses. Surprisingly, the HIV promoter in unactivated Jurkat T cells is partially occupied and carries Mediator containing the CDK8 repressive module, TFIID and RNAP II that is hypophosphorylated and confined to the promoter region. Significantly, the promoter is devoid of TFIIH. Upon stimulation of the cells by TNF-alpha, NF-kappaB and TFIIH are rapidly recruited to the promoter together with additional Mediator and RNAP II, but CDK8 is lost. Detailed time courses show that the levels of TFIIH at the promoter fluctuate in parallel with NF-kappaB recruitment to the promoter. Similarly, recombinant p65 activates HIV transcription in vitro and stimulates phosphorylation of the RNAP II CTD by the CDK7 kinase module of TFIIH. We conclude that the recruitment and activation of TFIIH represents a rate-limiting step for the emergence of HIV from latency.

p8/TTD-A as a repair-specific TFIIH subunit

How subunits of the transcription/repair factor TFIIH cooperate to allow for the removal of DNA lesions or for the transcription of genes is crucial to understand the functioning of this complex. Here, we reveal that p8/TTD-A, the tenth subunit of TFIIH, has a critical role in DNA repair where it triggers DNA opening by stimulating XPB ATPase activity together with the damage recognition factor XPC-hHR23B. Fluorescent antibody labeling shows that such opening is needed for the recruitment of XPA to the site of the damage. By contrast, p8 is dispensable for RNA synthesis and doesn't interfere with the transcriptional function of CAK, although both interact with the XPD subunit. Interestingly, p8 overexpression in TTD-XPD cells counteracts the detrimental effect of XPD mutations by restoring the cellular TFIIH concentration. These findings resolve the primary functions of p8 and unveil how TFIIH components specifically direct the complex toward repair or transcription.

Secrets of a double agent: CDK7 in cell-cycle control and transcription

In metazoans, cyclin-dependent kinase 7 (CDK7) has essential roles in both the cell-division cycle and transcription, as a CDK-activating kinase (CAK) and as a component of the general transcription factor TFIIH, respectively. Controversy over its double duty has been resolved, but questions remain. First, how does CDK7 achieve the dual substrate specificity necessary to perform both roles? Second, is there a deeper connection implied by the dichotomy of CDK7 function, for example similar mechanisms controlling cell division and gene expression, and/or actual coordination of the two processes? Enzymological studies have revealed solutions to the unusual substrate recognition problem, and there is evidence that the distinct functions of CDK7 can be regulated independently. Finally, despite divergence in their wiring, the CAK-CDK networks of budding yeast, fission yeast and metazoans all link transcriptional regulation with operation of the cell-cycle machinery. This connection might help to ensure that mRNAs encoding effectors of cell division are expressed at the right time in the cycle.

The Drosophila dosage compensation complex binds to polytene chromosomes independently of developmental changes in transcription

In Drosophila, the dosage compensation complex (DCC) mediates upregulation of transcription from the single male X chromosome. Despite coating the polytene male X, the DCC pattern looks discontinuous and probably reflects DCC dynamic associations with genes active at a given moment of development in a salivary gland. To test this hypothesis, we compared binding patterns of the DCC and of the elongating form of RNA polymerase II (PolIIo). We found that, unlike PolIIo, the DCC demonstrates a stable banded pattern throughout larval development and escapes binding to a subset of transcriptionally active areas, including developmental puffs. Moreover, these proteins are not completely colocalized at the electron microscopy level. These data combined imply that simple recognition of PolII machinery or of general features of active chromatin is either insufficient or not involved in DCC recruitment to its targets. We propose that DCC-mediated site-specific upregulation of transcription is not the fate of all active X-linked genes in males. Additionally, we found that DCC subunit MLE associates dynamically with developmental and heat-shock-induced puffs and, surprisingly, with those developing within DCC-devoid regions of the male X, thus resembling the PolIIo pattern. These data imply that, independently of other MSL proteins, the RNA-helicase MLE might participate in general transcriptional regulation or RNA processing.

TAF7: a possible transcription initiation check-point regulator

Transcription consists of a series of highly regulated steps: assembly of a preinitiation complex (PIC) at the promoter nucleated by TFIID, followed by initiation, elongation, and termination. The present study has focused on the role of the TFIID component, TAF7, in regulating transcription initiation. In TFIID, TAF7 binds to TAF1 and inhibits its intrinsic acetyl transferase activity. We now report that although TAF7 remains bound to TAF1 and associated with TFIID during the formation of the PIC, TAF7 dissociates from the PIC upon transcription initiation. Entry of polymerase II into the assembling PIC is associated with TAF1 and TAF7 phosphorylation, coincident with TAF7 release. We propose that the TFIID composition is dynamic and that TAF7 functions as a check-point regulator suppressing premature transcription initiation until PIC assembly is complete.

Connections between mRNA 3' end processing and transcription termination

Discoveries within the last few years have revealed that the multiple steps in gene expression are remarkably integrated. There have recently been several advances in deciphering how mRNA 3' end processing is linked with transcription elongation and termination. It has been known for quite a long time that transcription termination is somehow intertwined with polyadenylation, but it is still unclear exactly how these two processes influence each other. Some recent reports are shedding light on these connections.

An alliance between Ras GTPase-activating protein, filamin C, and Ras GTPase-activating protein SH3 domain-binding protein regulates myocyte growth

We have previously reported that Ras GTPase-activating protein (RasGAP) is involved in a pathway that regulates total cellular mRNA and protein synthesis in cardiac myocytes. A yeast two-hybrid screen resulted in identification of filamin C (FLN-C) as one of its targets. Knockdown of RasGAP or FLN-C, or severing their interaction, resulted in down-regulation of the RNA polymerase II kinase, cyclin-dependent kinase 7 (Cdk7). This appeared to be provoked by the release of cdk7 mRNA from RasGAP SH3 domain-binding protein, G3BP, and its subsequent degradation. In parallel, myocyte growth was also inhibited. On the other hand, overexpression of RasGAP induced a Cdk7- and FLN-C-dependent growth. Thus, we propose that the physical interaction between RasGAP and FLN-C facilitates an interaction between G3BP and cdk7 mRNA. This results in stabilization of cdk7 mRNA, an increase in its protein, which is required for cell growth.