Interplay between positive and negative elongation factors: drawing a new view of DRB

Influenza Virus RNA-Dependent RNA Polymerase and the Host Transcriptional Apparatus

Influenza virus RNA-dependent RNA polymerase (FluPol) transcribes the viral RNA genome in the infected cell nucleus. In the 1970s, researchers showed that viral transcription depends on host RNA polymerase II (RNAP II) activity and subsequently that FluPol snatches capped oligomers from nascent RNAP II transcripts to prime its own transcription. Exactly how this occurs remains elusive. Here, we review recent advances in the mechanistic understanding of FluPol transcription and early events in RNAP II transcription that are relevant to cap-snatching. We describe the known direct interactions between FluPol and the RNAP II C-terminal domain and summarize the transcription-related host factors that have been found to interact with FluPol. We also discuss open questions regarding how FluPol may be targeted to actively transcribing RNAP II and the exact context and timing of cap-snatching, which is presumed to occur after cap completion but before the cap is sequestered by the nuclear cap-binding complex.

A 50 year history of technologies that drove discovery in eukaryotic transcription regulation

Transcription regulation is critical to organism development and homeostasis. Control of expression of the 20,000 genes in human cells requires many hundreds of proteins acting through sophisticated multistep mechanisms. In this Historical Perspective, I highlight the progress that has been made in elucidating eukaryotic transcriptional mechanisms through an array of disciplines and approaches, and how this concerted effort has been driven by the development of new technologies.

Study of RNA Polymerase II Clustering inside Live-Cell Nuclei Using Bayesian Nanoscopy

Nanoscale spatiotemporal clustering of RNA polymerase II (Pol II) plays an important role in transcription regulation. However, dynamics of individual Pol II clusters in live-cell nuclei has not been measured directly, prohibiting in-depth understanding of their working mechanisms. In this work, we studied the dynamics of Pol II clustering using Bayesian nanoscopy in live mammalian cell nuclei. With 50 nm spatial resolution and 4 s temporal resolution, Bayesian nanoscopy allows direct observation of the assembly and disassembly dynamics of individual Pol II clusters. The results not only provide quantifications of Pol II clusters but also shed light on the understanding of cluster formation and regulation. Our study suggests that transcription factories form on-demand and recruit Pol II molecules in their pre-elongation phase. The assembly and disassembly of individual Pol II clusters take place asynchronously. Overall, the methods developed herein are also applicable to studying a wide realm of real-time nanometer-scale nuclear processes in live cells.

Activation of a novel ubiquitin-independent proteasome pathway when RNA polymerase II encounters a protein roadblock

Topoisomerase IIβ (Top2β)-DNA cleavage complexes are known to arrest elongating RNA polymerase II (RNAPII), triggering a proteasomal degradation of the RNAPII large subunit (RNAPII LS) and Top2β itself as a prelude to DNA repair. Here, we demonstrate that the degradation of Top2β occurs through a novel ubiquitin-independent mechanism that requires only 19S AAA ATPases and 20S proteasome. Our results suggest that 19S AAA ATPases play a dual role in sensing the Top2β cleavage complex and coordinating its degradation by 20S proteasome when RNAPII is persistently stalled by the Top2β protein roadblock. Clarification of this transcription-associated proteasome pathway could shed light on a general role of 19S AAA ATPases in processing tight protein-DNA complexes during transcription elongation.

Negative elongation factor-mediated suppression of RNA polymerase II elongation of Kaposi's sarcoma-associated herpesvirus lytic gene expression

Genome-wide chromatin immunoprecipitation assays indicate that the promoter-proximal pausing of RNA polymerase II (RNAPII) is an important postinitiation step for gene regulation. During latent infection, the majority of Kaposi's sarcoma-associated herpesvirus (KSHV) genes is silenced via repressive histone marks on their promoters. Despite the absence of their expression during latency, however, several lytic promoters are enriched with activating histone marks, suggesting that mechanisms other than heterochromatin-mediated suppression contribute to preventing lytic gene expression. Here, we show that the RNAPII-mediated transcription of the KSHV OriLytL, K5, K6, and K7 (OriLytL-K7) lytic genes is paused at the elongation step during latency. Specifically, the RNAPII-mediated transcription is stalled by the host's negative elongation factor (NELF) at the promoter regions of OriLytL-K7 lytic genes during latency, leading to the hyperphosphorylation of the serine 5 residue and the hypophosphorylation of the serine 2 of the C-terminal domain of the RNAPII large subunit, a hallmark of stalled RNAPII. Consequently, depletion of NELF expression induced transition of stalled RNAPII into a productive transcription elongation at the promoter-proximal regions of OriLytL-K7 lytic genes, leading to their RTA-independent expression. Using an RTA-deficient recombinant KSHV, we also showed that expression of the K5, K6, and K7 lytic genes was highly inducible upon external stimuli compared to other lytic genes that lack RNAPII on their promoters during latency. These results indicate that the transcription elongation of KSHV OriLytL-K7 lytic genes is inhibited by NELF during latency, but can also be promptly reactivated in an RTA-independent manner upon external stimuli.

Transcriptional and epigenetic regulation of B cell development

B cell development starts in the bone marrow where hematopoietic stem cells (HSCs) progress through sequential developmental stages, as it differentiates into a naïve B cell expressing surface immunoglobulin. In the periphery, B cells that encounter antigen can further differentiate into antibody-secreting plasma cells. In this review, we focus on two factors, E47 and ELL2, which play important roles in the regulation of B cell development in the bone marrow and differentiation of mature B cells into plasma cells in the periphery, respectively. First, E47 activity is required for B cell development in the bone marrow. In addition, we have identified a cell-intrinsic role for E47 in regulating efficient self-renewal and long-term multilineage bone marrow reconstitution potential of HSCs. Second, we explored the role of transcription elongation factors in the super elongation complex (SEC), including ELL2 (eleven-nineteen lysine-rich leukemia factor) in driving poly(A) site choice and plasma cell development. We found that elongation factors impel high levels of IgH mRNA production and alternative processing at the promoter proximal, secretory-specific (sec) poly(A) site in plasma cells by enhancing RNA polymerase II modifications and downstream events. The sec poly(A) site, essentially hidden in B cells, is found by SEC factors in plasma cells.

Conserved proximal promoter elements control repulsive guidance molecule c/hemojuvelin (Hfe2) gene transcription in skeletal muscle

Repulsive guidance molecule c (RGMc; gene symbol: Hfe2) plays a critical role in iron metabolism. Inactivating mutations cause juvenile hemochromatosis, a severe iron overload disorder. Understanding mechanisms controlling RGMc biosynthesis has been hampered by minimal information about the RGMc gene. Here we define the structure, examine the evolution, and establish mechanisms of regulation of the mouse RGMc gene. RGMc is a 4-exon gene that undergoes alternative RNA splicing to yield 3 mRNAs with 5' different untranslated regions. Gene transcription is induced during myoblast differentiation, producing all 3 mRNAs. We identify 3 critical promoter elements responsible for transcriptional activation in skeletal muscle, comprising paired E-boxes, a putative Stat and/or Ets element, and a MEF2 site, and muscle transcription factors myogenin and MEF2C stimulate RGMc promoter function in non-muscle cells. As these elements are conserved in RGMc genes from multiple species, our results suggest that RGMc has been a muscle-enriched gene throughout its evolutionary history.

Long-term habituation of the gill-withdrawal reflex in aplysia requires gene transcription, calcineurin and L-type voltage-gated calcium channels

Although habituation is possibly the simplest form of learning, we still do not fully understand the neurobiological basis of habituation in any organism. To advance the goal of a comprehensive understanding of habituation, we have studied long-term habituation (LTH) of the gill-withdrawal reflex (GWR) in the marine snail Aplysia californica. Previously, we showed that habituation of the GWR in a reduced preparation lasts for up to 12 h, and depends on protein synthesis, as well as activation of protein phosphatases 1 and 2A and postsynaptic glutamate receptors. Here, we have used the reduced preparation to further analyze the mechanisms of LTH in Aplysia. We found that LTH of the GWR depends on RNA synthesis because it was blocked by both the irreversible transcriptional inhibitor actinomycin-D and the reversible transcriptional inhibitor, 5,6-dichlorobenzimidazole riboside (DRB). In addition, LTH requires activation of protein phosphatase 2B (calcineurin), because it was disrupted by ascomycin. Finally, LTH was blocked by nitrendipine, which indicates that activation of l-type voltage-gated Ca²⁺ channels is required for this form of learning. Together with our previous results, the present results indicate that exclusively presynaptic mechanisms, although possibly sufficient for short-term habituation, are insufficient for LTH. Rather, LTH must involve postsynaptic, as well as presynaptic, mechanisms.

Cyclin T1/CDK9 interacts with influenza A virus polymerase and facilitates its association with cellular RNA polymerase II

Influenza virus RNA-dependent RNA polymerase scavenges the 5' cap from host pre-mRNA to prime viral transcription initiation. It is also well established that viral RNA-dependent RNA polymerase (vRNP) associates with cellular RNA polymerase II (Pol II), on which viral replication depends. Here we report that cyclin T1/CDK9 can interact with influenza virus polymerase and facilitate its association with cellular Pol II. The immunodepletion of cyclin T1/CDK9 totally abolished the association of vRNP with the C-terminal domain (CTD) Ser-2-phosphorylated form of RNA polymerase II. Further studies showed that overexpression of cyclin T1/CDK9 increased the transcription activity of vRNP, while knockdown of cyclin T1/CDK9 impaired viral replication. Our results suggest that cyclin T1/CDK9 serves as an adapter to mediate the interaction of vRNP and RNA Pol II and promote viral transcription.

Cell cycle-dependent accumulation of histone H3.3 and euchromatic histone modifications in pericentromeric heterochromatin in response to a decrease in DNA methylation levels

In mammals, DNA methylation is an important epigenetic mark that is associated with gene silencing, particularly in constitutive heterochromatin. However, the effect of DNA methylation on other epigenetic properties of chromatin is controversial. In this study, we show that inhibition of DNA methylation in mouse fibroblast cells affects histone modification and the subnuclear localization of histone H3.3 in a cell cycle-dependent manner. Using a DNA methyltransferase (Dnmt) inhibitor 5-aza-2'-deoxycytidine (5-aza-dC), we found that reduced levels of DNA methylation were associated with the activation of transcription from centromeric and pericentromeric satellite repeats. The de-repressed pericentromeric chromatin was enriched in euchromatic histone modifications such as acetylation of histone H4, and di- and tri-methylation of lysine 4 on histone H3. Spatio-temporal analysis showed that the accumulation of these euchromatic histone modifications occurred during the second S phase following 5-aza-dC treatment, corresponding precisely with a shift in replication timing of the pericentromeric satellite repeats from middle/late S phase to early S phase. Moreover, we found that histone H3.3 was deposited on the pericentromeric heterochromatin prior to the accumulation of the euchromatic histone modifications. These results suggest that DNA CpG methylation is essential for the proper organization of pericentromeric heterochromatin in differentiated mouse cells.

Long intronic GAA repeats causing Friedreich ataxia impede transcription elongation

Friedreich ataxia is a degenerative disease caused by deficiency of the protein frataxin (FXN). An intronic expansion of GAA triplets in the FXN-encoding gene, FXN, causes gene silencing and thus reduced FXN protein levels. Although it is widely assumed that GAA repeats block transcription via the assembly of an inaccessible chromatin structure marked by methylated H3K9, direct proof for this is lacking. In this study, we analysed different histone modification patterns along the human FXN gene in FRDA patient-derived lymphoblastoid cell lines. We show that FXN mRNA synthesis, but not turnover rates are affected by an expanded GAA repeat tract. Importantly, rather than preventing transcription initiation, long GAA repeat tracts affect transcription at the elongation step and this can occur independently of H3K9 methylation. Our data demonstrate that finding novel strategies to overcome the transcription elongation problem may develop into promising new treatments for FRDA.

8-Amino-adenosine inhibits multiple mechanisms of transcription

Roscovitine and flavopiridol suppress cyclin-dependent kinase 7 (CDK7) and CDK9 activity resulting in transcription inhibition, thus providing an alternative mechanism to traditional genotoxic chemotherapy. These agents have been effective in slow or nonreplicative cell types. 8-Amino-adenosine is a transcription inhibitor that has proved very effective in multiple myeloma cell lines and primary indolent leukemia cells. The objective of the current work was to define mechanisms of action that lead to transcription inhibition by 8-amino-adenosine. 8-Amino-adenosine is metabolized into the active triphosphate (8-amino-ATP) in cells. This accumulation resulted in a simultaneous decrease of intracellular ATP and RNA synthesis. When the effects of established ATP synthesis inhibitors and transcription inhibitors on intracellular ATP concentrations and RNA synthesis were studied, there was a strong correlation between ATP decline and RNA synthesis. This correlation substantiated the hypothesis that the loss of ATP in 8-amino-adenosine-treated cells contributes to the decrease in transcription due to the lack of substrate needed for mRNA body and polyadenylation tail synthesis. RNA polymerase II COOH terminal domain phosphorylation declined sharply in 8-amino-adenosine-treated cells, which may have been due to the lack of an ATP phosphate donor or competitive inhibition with 8-amino-ATP at CDK7 and CDK9. Furthermore, 8-amino-ATP was incorporated into nascent RNA in a dose-dependent manner at the 3'-end resulting in transcription termination. Finally, in vitro transcription assays showed that 8-amino-ATP competes with ATP for incorporation into mRNA. Collectively, we have concluded that 8-amino-adenosine elicits effects on multiple mechanisms of transcription, providing a new class of transcription inhibitors.

Chapter 3. Replication of the hepatitis delta virus RNA genome

Hepatitis delta virus (HDV) is a subviral agent dependent upon hepatitis B virus (HBV). HDV uses the envelope proteins of HBV to achieve assembly and infection of target cells. Otherwise, the replication of the RNA genome of HDV is totally different from that of its helper virus, and involves redirection of host polymerase activity. This chapter is concerned with recent developments in our understanding of the genome replication process.

Stalled Hox promoters as chromosomal boundaries

Many developmental control genes contain stalled RNA Polymerase II (Pol II) in the early Drosophila embryo, including four of the eight Hox genes. Here, we present evidence that the stalled Hox promoters possess an intrinsic insulator activity. The enhancer-blocking activities of these promoters are dependent on general transcription factors that inhibit Pol II elongation, including components of the DSIF and NELF complexes. The activities of conventional insulators are also impaired in embryos containing reduced levels of DSIF and NELF. Thus, promoter-proximal stalling factors might help promote insulator-promoter interactions. We propose that stalled promoters help organize gene complexes within chromosomal loop domains.

Mitogen-activated protein (MAP) kinase/MAP kinase phosphatase regulation: roles in cell growth, death, and cancer

Mitogen-activated protein kinase dual-specificity phosphatase-1 (also called MKP-1, DUSP1, ERP, CL100, HVH1, PTPN10, and 3CH134) is a member of the threonine-tyrosine dual-specificity phosphatases, one of more than 100 protein tyrosine phosphatases. It was first identified approximately 20 years ago, and since that time extensive investigations into both mkp-1 mRNA and protein regulation and function in different cells, tissues, and organs have been conducted. However, no general review on the topic of MKP-1 exists. As the subject matter pertaining to MKP-1 encompasses many branches of the biomedical field, we focus on the role of this protein in cancer development and progression, highlighting the potential role of the mitogen-activated protein kinase (MAPK) family. Section II of this article elucidates the MAPK family cross-talk. Section III reviews the structure of the mkp-1 encoding gene, and the known mechanisms regulating the expression and activity of the protein. Section IV is an overview of the MAPK-specific dual-specificity phosphatases and their role in cancer. In sections V and VI, mkp-1 mRNA and protein are examined in relation to cancer biology, therapeutics, and clinical studies, including a discussion of the potential role of the MAPK family. We conclude by proposing an integrated scheme for MKP-1 and MAPK in cancer.

Negative elongation factor NELF controls transcription of immediate early genes in a stimulus-specific manner

The transcription rate of immediate early genes (IEGs) is controlled directly by transcription elongation factors at the transcription elongation step. Negative elongation factor (NELF) and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) sensitivity-inducing factor (DSIF) stall RNA polymerase II (pol II) soon after transcription initiation. Upon induction of IEG transcription, DSIF is converted into an accelerator for pol II elongation. To address whether and how NELF as well as DSIF controls overall IEG transcription, its expression was reduced using stable RNA interference in GH4C1 cells. NELF knock-down reduced thyrotropin-releasing hormone (TRH)-induced transcription of the IEGs c-fos, MKP-1, and junB. In contrast, epidermal growth factor (EGF)-induced transcription of these IEGs was unaltered or even slightly increased by NELF knock-down. Thus, stable knock-down of NELF affects IEG transcription stimulation-specifically. Conversely, DSIF knock-down reduced both TRH- and EGF-induced transcription of the three IEGs. Interestingly, TRH-induced activation of the MAP kinase pathway, a pathway essential for transcription of the three IEGs, was down-regulated by NELF knock-down. Thus, stable knock-down of NELF, by modulating intracellular signaling pathways, caused stimulation-specific loss of IEG transcription. These observations indicate that NELF controls overall IEG transcription via multiple mechanisms both directly and indirectly.

Subnuclear compartmentalization and function of actin and nuclear myosin I in plants

Actins are highly conserved proteins that serve as the basic building blocks of cytoskeletal microfilaments. In animal cells, specific nuclear actin adopts unconventional conformations that are involved in multiple nuclear functions and that associate with nuclear actin binding proteins. However, there is practically no information available about nuclear actin in plants. Indeed, actin has not been detected in the nuclear proteomes of many plants, and orthologs of the main structural nuclear actin-binding proteins have yet to be identified. Here, we have investigated the characteristics, intranuclear compartmentalization, and function of actin in isolated Allium cepa nuclei as well as that of its motor protein nuclear myosin I (NMI). Using conformation-specific antibodies for nuclear actin isoforms, ss-actin, and NMI, the distribution of these proteins was studied in Western blots and by immunocytochemistry. Moreover, the participation of nuclear actin in transcription was analyzed in run on in situ assays and inhibition of RNA polymerases I and II. We show that actin isoforms with distinct solubilities are present in onion nuclei with a consistent subnuclear compartmentalization. Actin and NMI are highly enriched in foci that are similar to transcription foci, although actin is also distributed diffusely in the nucleus and nucleolus as well as accumulating in a subset of the Cajal bodies. Immunogold labeling identified both proteins in the nuclear transcription subdomains and in other subnuclear compartments. In addition, actin and NMI were diffusely distributed in the nuclear matrix.

A cyclin-dependent protein kinase, CDKC2, colocalizes with and modulates the distribution of spliceosomal components in Arabidopsis

Cyclin-dependent kinases (CDKs) play key regulatory roles in diverse cellular functions, including cell-cycle progression, transcription and translation. In plants, CDKs have been classified into several groups, named A through to G, but the functions of most are poorly characterized. CDKCs are known to phosphorylate the C-terminal domain (CTD) of RNA polymerase II (RNAP II), and therefore the CDKC-cyclinT (CycT) complex may have a role similar to the animal CDK9-CycT complex of the positive transcription elongation factor b (P-TEFb). However, we found that the predicted structure of the Arabidopsis CDKC2 protein is more similar to the mammalian cdc2-related kinase, CRK7, than to CDK9. CRK7 is proposed to link transcription with splicing, and CDKC2 contains all the structural features of CRK7 that make the latter distinct from CDK9. Consistent with this, we show that GFP-CDKC2 fusion proteins co-localize with spliceosomal components, that the expression of CDKC2 modifies the location of these components, and that co-localization was dependent on the transcriptional status of the cells and on CDKC2-kinase activity. We propose, therefore, that the Arabidopsis CDKC2 combines the functions of both CRK7 and CDK9, and could also couple splicing with transcription.

Up-regulation of P-TEFb by the MEK1-extracellular signal-regulated kinase signaling pathway contributes to stimulated transcription elongation of immediate early genes in neuroendocrine cells

The positive elongation factor P-TEFb appears to function as a crucial C-terminal-domain (CTD) kinase for RNA polymerase II (Pol II) transcribing immediate early genes (IEGs) in neuroendocrine GH4C1 cells. Chromatin immunoprecipitation indicated that in resting cells Pol II occupied the promoter-proximal regions of the c-fos and junB genes, together with the negative elongation factors DSIF and NELF. Thyrotropin-releasing hormone (TRH)-induced recruitment of positive transcription elongation factor b (P-TEFb) abolished the pausing of Pol II and enhanced phosphorylation of CTD serine 2, resulting in transcription elongation. In addition, P-TEFb was essential for splicing and 3'-end processing of IEG transcripts. Importantly, the MEK1-extracellular signal-regulated kinase (ERK) signaling pathway activated by TRH up-regulated nuclear CDK9 and CDK9/cyclinT1 dimers (i.e., P-TEFb), facilitating the recruitment of P-TEFb to c-fos and other IEGs. Thus, in addition to established gene transcription control via promoter response elements, the MEK1-ERK signaling pathway controls transcription elongation by Pol II via the up-regulation of nuclear CDK9 integrated into P-TEFb.

The regulation of HIV-1 transcription: molecular targets for chemotherapeutic intervention

The regulation of transcription of the human immunodeficiency virus (HIV) is a complex event that requires the cooperative action of both viral and cellular components. In latently infected resting CD4⁺ T cells HIV‐1 transcription seems to be repressed by deacetylation events mediated by histone deacetylases (HDACs). Upon reactivation of HIV‐1 from latency, HDACs are displaced in response to the recruitment of histone acetyltransferases (HATs) by NF‐κB or the viral transcriptional activator Tat and result in multiple acetylation events. Following chromatin remodeling of the viral promoter region, transcription is initiated and leads to the formation of the TAR element. The complex of Tat with p‐TEFb then binds the loop structures of TAR RNA thereby positioning CDK9 to phosphorylate the cellular RNA polymerase II. The Tat‐TAR‐dependent phosphorylation of RNA polymerase II plays an important role in transcriptional elongation as well as in other post‐transcriptional events. As such, targeting of Tat protein (and/or cellular cofactors) provide an interesting perspective for therapeutic intervention in the HIV replicative cycle and may afford lifetime control of the HIV infection. © 2006 Wiley Periodicals, Inc. Med Res Rev, 26, No. 5, 595–625, 2006

In vivo dynamics of RNA polymerase II transcription

We imaged transcription in living cells using a locus-specific reporter system, which allowed precise, single-cell kinetic measurements of promoter binding, initiation and elongation. Photobleaching of fluorescent RNA polymerase II revealed several kinetically distinct populations of the enzyme interacting with a specific gene. Photobleaching and photoactivation of fluorescent MS2 proteins used to label nascent messenger RNAs provided sensitive elongation measurements. A mechanistic kinetic model that fits our data was validated using specific inhibitors. Polymerases elongated at 4.3 kilobases min(-1), much faster than previously documented, and entered a paused state for unexpectedly long times. Transcription onset was inefficient, with only 1% of polymerase-gene interactions leading to completion of an mRNA. Our systems approach, quantifying both polymerase and mRNA kinetics on a defined DNA template in vivo with high temporal resolution, opens new avenues for studying regulation of transcriptional processes in vivo.

Gene-specific recruitment of positive and negative elongation factors during stimulated transcription of the MKP-1 gene in neuroendocrine cells

MAP kinase phosphatase-1 (MKP-1) controls nuclear MAP kinase activity with important consequences on cell growth or apoptosis. MKP-1 transcription is initiated constitutively but elongation is blocked within exon 1. It is unclear how induction of MKP-1 is controlled. Here, we report that the transcriptional elongation factors P-TEFb, DSIF and NELF regulate MKP-1 transcription in the pituitary GH4C1 cell line. Prior to stimulation, DSIF, NELF and RNA polymerase II (pol II) associate with the promoter-proximal region of the MKP-1 gene upstream of the elongation block site. Thyrotropin-releasing hormone (TRH) leads to recruitment of P-TEFb along the whole gene and a marked increase of DSIF and pol II downstream of the elongation block site, whereas NELF remains confined to the promoter-proximal region. 5,6-Dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) an inhibitor of P-TEFb eliminated TRH stimulation of MKP-1 transcription. DRB specifically inhibited TRH-induced recruitment of DSIF and P-TEFb to the MKP-1 gene. Furthermore, DRB treatment eliminated TRH-induced progression along the MKP-1 gene of pol II phosphorylated on Ser-2 of its CTD. These results indicate that P-TEFb is essential for gene-specific stimulated transcriptional elongation in mammalian cells via mechanisms which involve the activation of the DSIF–NELF complex and Ser-2 phosphorylation of pol II.

Nuclear export of influenza A virus mRNAs requires ongoing RNA polymerase II activity

Influenza A virus transcribes its segmented negative sense RNA genome in the nuclei of infected cells in a process long known to require host RNA polymerase II (RNAP-II). RNA polymerase II synthesizes pre-mRNAs whose 5'-cap structures are scavenged by the viral RNA-dependent RNA polymerase during synthesis of viral mRNAs. Drugs that inhibit RNAP-II therefore block viral replication, but not necessarily solely by denying the viral polymerase a source of cap-donor molecules. We show here that 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole (DRB), a compound that prevents processive transcription by RNAP-II, inhibits expression of the viral HA, M1 and NS1 genes at the post-transcriptional level. Abundant quantities of functionally and structurally intact viral mRNAs are made in the presence of DRB but with the exception of NP and NS2 mRNAs, are not efficiently translated. Taking M1 and NP mRNAs as representatives of DRB-sensitive and insensitive mRNAs, respectively, we found that the block to translation operates at the level of nuclear export. Furthermore, removal of DRB reversed this block unless a variety of chemically and mechanistically distinct RNAP-II inhibitors were added instead. We conclude that influenza A virus replication requires RNAP-II activity not just to provide capped mRNA substrates but also to facilitate nuclear export of selected viral mRNAs.

Influenza virus inhibits RNA polymerase II elongation

The influenza virus RNA-dependent RNA polymerase interacts with the serine-5 phosphorylated carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II (Pol II). It was proposed that this interaction allows the viral RNA polymerase to gain access to host mRNA-derived capped RNA fragments required as primers for the initiation of viral mRNA synthesis. Here, we show, using a chromatin immunoprecipitation (ChIP) analysis, that similar amounts of Pol II associate with Pol II promoter DNAs in influenza virus-infected and mock-infected cells. However, there is a statistically significant reduction in Pol II densities in the coding region of Pol II genes in infected cells. Thus, influenza virus specifically interferes with Pol II elongation, but not Pol II initiation. We propose that influenza virus RNA polymerase, by binding to the CTD of initiating Pol II and subsequent cleavage of the capped 5' end of the nascent transcript, triggers premature Pol II termination.

Action of inhibitors on accumulation of processed hepatitis delta virus RNAs

Hepatitis delta virus (HDV) replication involves processing and accumulation of three RNA species: the genome, its exact complement (the antigenome), and a polyadenylated mRNA that acts as a template for the small delta antigen (deltaAg), the only protein of HDV and essential for genome replication. In a recently reported experimental system, addition of tetracycline induced synthesis of a DNA-directed source of deltaAg, producing within 24 h a significant increase in accumulation of newly transcribed and processed HDV RNAs. This induction was used here to study the action of various inhibitors on accumulation. For example, potent and HDV-specific inhibition, in the absence of detected host toxicity, could be obtained with ribavirin, mycophenolic acid, and viramidine. An interpretation is that these inhibitors reduced the available GTP pool, leading to a specific inhibition of the synthesis and accumulation of HDV RNA-directed RNA species. In contrast, no inhibition was observed with L-FMAU (2'-fluoro-5-methyl-beta-L-arabinofuranosyl-uridine), alpha interferon, or pegylated alpha interferon. After modifications to the experimental system, it was also possible to examine the effects of three known host RNA polymerase inhibitors on HDV genome replication: amanitin, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), and actinomycin. Of most interest, amanitin at low doses blocked accumulation of HDV RNA-directed mRNA but had less effect on HDV genomic and antigenomic RNAs. Additional experiments indicated that this apparent resistance to amanitin inhibition of genomic and antigenomic RNA relative to mRNA may not reflect a difference in the transcribing polymerase but rather relative differences in the processing and stabilization of nascent RNA transcripts.

Arsenite Regulates Cystic Fibrosis Transmem­brane Conductance Regulator and P-glycoprotein: Evidence of Pathway Independence

In the past, people have argued for and against the theory of reciprocal regulation of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and P-glycoprotein (Pgp). Data have indicated that this may occur in vitro during drug-induced selection of cells, and in vivo during development. Much of this debate has been caused by a severe lack of mechanistic details involved in such regulation. Our past data indicate that certain Pgp modulators can affect CFTR expression and function. The goal of this study was to investigate the effects of trivalent arsenic (arsenite), a known transcriptional activator of Pgp, on CFTR expression. In vitro analyses in T-84 cells that express basal levels of Pgp and CFTR were conducted using a variety of molecular techniques. Expressions of both genes were altered following treatment with arsenite in a dose- and time-dependent fashion. CFTR expression was suppressed almost three-fold by arsenite, along with a concomitant increase in P-glycoprotein expression. We also report that a member of the MAPK-family, the ERK-mediated signaling cascade is implicated in suppression of CFTR expression following treatment with arsenite. However, this particular pathway is not involved in regulation of P-glycoprotein expression in T-84 cells following treatment with arsenite. Thus, the regulatory pathways that control functional expression of CFTR and P-glycoprotein following arsenite treatment in T-84 cells are distinct and independent.

CTD-dependent dismantling of the RNA polymerase II elongation complex by the pre-mRNA 3'-end processing factor, Pcf11

Pcf11 is one of numerous proteins involved in pre-mRNA 3'-end processing and transcription termination. Using elongation complexes (ECs) formed from purified yeast RNA polymerase II (Pol II), we show that a 140-amino acid polypeptide from yeast Pcf11 is capable of dismantling the EC in vitro. This action depends on the C-terminal domain (CTD) of the largest subunit of Pol II and the CTD-interaction domain (CID) of Pcf11. Our experiments reveal a novel termination mechanism whereby Pcf11 bridges the CTD to the nascent transcript and causes dissociation of both Pol II and the nascent transcript from the DNA in the absence of nucleotide hydrolysis. We posit that conformational changes in the CTD are transduced through Pcf11 to the nascent transcript to cause termination.

Identification of amino acids that promote specific and rigid TAR RNA-tat protein complex formation

The Tat protein and the transactivation responsive (TAR) RNA form an essential complex in the HIV lifecycle, and mutations in the basic region of the Tat protein alter this RNA-protein molecular recognition. Here, EPR spectroscopy was used to identify amino acids, flanking an essential arginine of the Tat protein, which contribute to specific and rigid TAR-Tat complex formation by monitoring changes in the mobility of nitroxide spin-labeled TAR RNA nucleotides upon binding. Arginine to lysine N-terminal mutations did not affect TAR RNA interfacial dynamics. In contrast, C-terminal point mutations, R56 in particular, affected the mobility of nucleotides U23 and U38, which are involved in a base-triple interaction in the complex. This report highlights the role of dynamics in specific molecular complex formation and demonstrates the ability of EPR spectroscopy to study interfacial dynamics of macromolecular complexes.

A structural perspective of CTD function

The C-terminal domain (CTD) of RNA polymerase II (Pol II) integrates nuclear events by binding proteins involved in mRNA biogenesis. CTD-binding proteins recognize a specific CTD phosphorylation pattern, which changes during the transcription cycle, due to the action of CTD-modifying enzymes. Structural and functional studies of CTD-binding and -modifying proteins now reveal some of the mechanisms underlying CTD function. Proteins recognize CTD phosphorylation patterns either directly, by contacting phosphorylated residues, or indirectly, without contact to the phosphate. The catalytic mechanisms of CTD kinases and phosphatases are known, but the basis for CTD specificity of these enzymes remains to be understood.

Elongation by RNA polymerase II: the short and long of it

Appreciable advances into the process of transcript elongation by RNA polymerase II (RNAP II) have identified this stage as a dynamic and highly regulated step of the transcription cycle. Here, we discuss the many factors that regulate the elongation stage of transcription. Our discussion includes the classical elongation factors that modulate the activity of RNAP II, and the more recently identified factors that facilitate elongation on chromatin templates. Additionally, we discuss the factors that associate with RNAP II, but do not modulate its catalytic activity. Elongation is highlighted as a central process that coordinates multiple stages in mRNA biogenesis and maturation.

Tissue-specific expression and dynamic organization of SR splicing factors in Arabidopsis

The organization of the pre-mRNA splicing machinery has been extensively studied in mammalian and yeast cells and far less is known in living plant cells and different cell types of an intact organism. Here, we report on the expression, organization, and dynamics of pre-mRNA splicing factors (SR33, SR1/atSRp34, and atSRp30) under control of their endogenous promoters in Arabidopsis. Distinct tissue-specific expression patterns were observed, and differences in the distribution of these proteins within nuclei of different cell types were identified. These factors localized in a cell type-dependent speckled pattern as well as being diffusely distributed throughout the nucleoplasm. Electron microscopic analysis has revealed that these speckles correspond to interchromatin granule clusters. Time-lapse microscopy revealed that speckles move within a constrained nuclear space, and their organization is altered during the cell cycle. Fluorescence recovery after photobleaching analysis revealed a rapid exchange rate of splicing factors in nuclear speckles. The dynamic organization of plant speckles is closely related to the transcriptional activity of the cells. The organization and dynamic behavior of speckles in Arabidopsis cell nuclei provides significant insight into understanding the functional compartmentalization of the nucleus and its relationship to chromatin organization within various cell types of a single organism.

NELF and DSIF cause promoter proximal pausing on the hsp70 promoter in Drosophila

NELF and DSIF collaborate to inhibit elongation by RNA polymerase IIa in extracts from human cells. A multifaceted approach was taken to investigate the potential role of these factors in promoter proximal pausing on the hsp70 gene in Drosophila. Immunodepletion of DSIF from a Drosophila nuclear extract reduced the level of polymerase that paused in the promoter proximal region of hsp70. Depletion of one NELF subunit in salivary glands using RNA interference also reduced the level of paused polymerase. In vivo protein-DNA cross-linking showed that NELF and DSIF associate with the promoter region before heat shock. Immunofluorescence analysis of polytene chromosomes corroborated the cross-linking result and showed that NELF, DSIF, and RNA polymerase IIa colocalize at the hsp70 genes, small heat shock genes, and many other chromosomal locations. Finally, following heat shock induction, DSIF and polymerase but not NELF were strongly recruited to chromosomal puffs harboring the hsp70 genes. We propose that NELF and DSIF cause polymerase to pause in the promoter proximal region of hsp70. The transcriptional activator, HSF, might cause NELF to dissociate from the elongation complex. DSIF continues to associate with the elongation complex and could serve a positive role in elongation.

Interplay between transcriptional and post-transcriptional regulation of Cyp2a5 expression

The cytochrome P450 (Cyp) 2a5 gene can be upregulated transcriptionally or by mRNA stabilization. The heterogeneous nuclear ribonucleoprotein (hnRNP) A1 interacting with the CYP2A5 mRNA has been shown to be a key post-transcriptional regulator of the Cyp2a5 gene. The aim of this study was to investigate if the transcriptional and post-transcriptional steps of Cyp2a5 expression are linked. This was done by modifying the transcription rate with transcriptional inducers (phenobarbital and cyclic AMP) and inhibitors (actinomycin D and 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole) and analyzing the effects upon post-transcriptional events. We found that inhibition of transcription led to relocalization of hnRNP A1 from the nucleus to the cytoplasm, to its strongly increased binding to the cytoplasmic CYP2A5 mRNA and to CYP2A5 mRNA stabilization. In contrast, stimulated transcription resulted in increased binding of nuclear hnRNP A1 to the Cyp2a5 promoter, and overexpression of hnRNP A1 led to stimulated transcription of a Cyp2a5 promoter-driven luciferase recombinant. This strongly suggests that the transcriptional and post-transcriptional stages of Cyp2a5 expression are interrelated and that the nucleocytoplasmic shuttling hnRNP A1 may coordinate these different steps.

Sm and U2B" proteins redistribute to different nuclear domains in dormant and proliferating onion cells

Monoclonal antibodies against the spliceosomal proteins Sm and U2B", and against p105, a protein component of interchromatin granules, were used to investigate the nuclear distribution of the splicing factors in Allium cepa L. meristematic cells. Confocal microscopy showed that in steady-state proliferating cells, the spliceosomal components were distributed into two nuclear domains: (i) a diffuse nucleoplasmic network similar to that formed by interchromatin granules and (ii) numerous Cajal bodies. These domains were the counterpart of the perichromatin fibrils and granules, interchromatin granules and Cajal bodies observed by electron microscopy after EDTA and bismuth oxynitrate stainings. Dormant cells showed a nuclear distribution of the proteins in small Cajal bodies and numerous micro-speckles, correlated with the distribution of ribonucleoproteins (RNPs) observed by electron microscopy. The spliceosomal proteins relocated to the diffuse nucleoplasmic network and Cajal bodies when the cells were released from dormancy by water soaking and they re-started their proliferative activity. Inhibition of RNA synthesis by 5,6-dichloro-1-beta- d-ribofuranosylbenzimidazole (DRB) treatment in proliferating cells demonstrated that the micro-speckles were not the morphological expression of a transcription block. Fractionation and confocal microscopy studies showed a differential association of the splicing factors with the nuclear matrix depending not only on the protein, but also on nuclear activity. Our results suggest a reversible relocation of the spliceosomal proteins between different sub-nuclear domains in physiological conditions. We report here an unusual nuclear domain in dormant nuclei, the micro-speckles, corresponding to storage sites for RNPs, which were rapidly mobilised after water imbibition.

The transcription cycle of RNA polymerase II in living cells

RNA polymerase II transcribes most eukaryotic genes. Its catalytic subunit was tagged with green fluorescent protein and expressed in Chinese hamster cells bearing a mutation in the same subunit; it complemented the defect and so was functional. Photobleaching revealed two kinetic fractions of polymerase in living nuclei: approximately 75% moved rapidly, but approximately 25% was transiently immobile (association t1/2 approximately 20 min) and transcriptionally active, as incubation with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole eliminated it. No immobile but inactive fraction was detected, providing little support for the existence of a stable holoenzyme, or the slow stepwise assembly of a preinitiation complex on promoters or the nuclear substructure. Actinomycin D decreased the rapidly moving fraction, suggesting that engaged polymerases stall at intercalated molecules while others initiate. When wild-type cells containing only the endogenous enzyme were incubated with [3H]uridine, nascent transcripts became saturated with tritium with similar kinetics (t1/2 approximately 14 min). These data are consistent with a polymerase being mobile for one half to five sixths of a transcription cycle, and rapid assembly into the preinitiation complex. Then, most expressed transcription units would spend significant times unassociated with engaged polymerases.

HIV and hepatitis delta virus: evolution takes different paths to relieve blocks in transcriptional elongation

The elongation step of transcription by RNA polymerase II (RNAPII) is controlled both positively and negatively by over a dozen cellular proteins. Recent findings suggest that two distinct viruses, human immunodeficiency virus type 1 and hepatitis delta virus, encode proteins that facilitate viral replication and transcription by targeting the same cellular transcription elongation machinery.

MCEF, the newest member of the AF4 family of transcription factors involved in leukemia, is a positive transcription elongation factor-b-associated protein

Positive transcription elongation factor-b (P-TEFb) contains CDK9 and cyclin T(1). P-TEFb was affinity purified from a stably transfected cell line that expresses epitope-tagged CDK9, and proteins that appeared to be specifically bound were sequenced. In addition to CDK9, previously identified isoforms of cyclin T (including T(1), T(2A) and T(2B)), HSP90 and CDC37, this analysis identified a novel protein named MCEF. Cloning of its cognate cDNA revealed that MCEF is the newest member of the AF4 family of transcription factors involved in acute lymphoblastic leukemia. MCEF RNA was expressed in all human tissues examined, and antisera directed against recombinant MCEF specifically immunoprecipitated P-TEFb. Ectopic expression of MCEF did not activate HIV-1 replication, and tethering of MCEF to a promoter did not activate transcription.

Interaction between cyclin T1 and SCF(SKP2) targets CDK9 for ubiquitination and degradation by the proteasome

CDK9 paired with cyclin T1 forms the human P-TEFb complex and stimulates productive transcription through phosphorylation of the RNA polymerase II C-terminal domain. Here we report that CDK9 is ubiquitinated and degraded by the proteasome whereas cyclin T1 is stable. SCF(SKP2) was recruited to CDK9/cyclin T1 via cyclin T1 in an interaction requiring its PEST domain. CDK9 ubiquitination was modulated by cyclin T1 and p45(SKP2). CDK9 accumulated in p45(SKP2-/-) cells, and its expression during the cell cycle was periodic. The transcriptional activity of CDK9/cyclin T1 on the class II major histocompatibility complex promoter could be regulated by CDK9 degradation in vivo. We propose a novel mechanism whereby recruitment of SCF(SKP2) is mediated by cyclin T1 while ubiquitination occurs exclusively on CDK9.

Ultraviolet radiation alters the phosphorylation of RNA polymerase II large subunit and accelerates its proteasome-dependent degradation

It has been shown that ultraviolet (UV) radiation induces the ubiquitination of the large subunit of RNA polymerase II (RNAP II-LS) as well as its proteasomal degradation. Studies in mammalian cells have indicated that highly phosphorylated forms of RNAP II-LS are preferentially ubiquitinated, but studies in Saccharomyces cerevisiae have provided evidence that unphosphorylated RNAP II-LS is an equally suitable substrate. In the present study, an antibody (ARNA-3) that recognizes all forms of RNAP II-LS, regardless of the phosphorylation status of its C-terminal domain (CTD), was utilized to evaluate the degradation of total cellular RNAP II-LS in human fibroblasts under basal conditions or after UV-C (10J/m(2)) irradiation. It was found that UV radiation rapidly shifted the phosphorylation profile of RNAP II-LS from a mixture of dephosphorylated and phosphorylated forms to entirely more phosphorylated forms. This shift in phosphorylation status was not blocked by pharmacologic inhibition of either the ERK or p38 pathways, both of which have been implicated in the cellular UV response. In addition to shifting the phosphorylation profile, UV radiation led to net degradation of total RNAP II-LS. UV-induced degradation of RNAP II-LS was also greatly reduced in the presence of the transcriptional and CTD kinase inhibitor DRB. Using a panel of protease inhibitors, it was shown that the bulk of UV-induced degradation is proteasome-dependent. However, the UV-induced loss of hypophosphorylated RNAP II-LS was proteasome-independent. Lastly, UV radiation induced a similar shift to all hyperphosphorylated RNAP II-LS in Cockayne syndrome (CS) cells of complementation groups A or B (CSA or CSB) when compared to appropriate controls. The UV-induced degradation rates of RNAP II-LS were not significantly altered when comparing CSA or CSB to repair competent control cells. The implications for the cellular UV response are discussed.

CPT-11 alters the circadian rhythm of dihydropyrimidine dehydrogenase mRNA in mouse liver

Combination chemotherapy consisting of 5‐fluorouracil (5‐FU) and 7‐ethyl‐10‐[4‐(l‐piperidino)‐l‐piperidino]carboxycamptothecin (CPT‐11) is a promising regimen for gastrointestinal cancer. The circadian‐dependent efficacy and toxicity of 5‐FU are related to the circadian variation in the activity of dihydropyrimidine dehydrogenase (DPD), which is a rate‐limiting enzyme in the pyrimidine catabolic pathway. To optimize the schedule of the CPT‐11 plus 5‐FU combination, we investigated the effect of CPT‐11 on the circadian rhythm of DPD in vivo. In control mice, the DPD mRNA level in the liver was significantly higher at 14:00 than that at 02:00. After intravenous administration of CPT‐11 (30 mg/kg) at 20:00, the circadian rhythm of the DPD mRNA level in the liver was no longer observed 18 h later (14:00), but it was unaffected 6 and 18 h later (at 14:00 and 02:00) when CPT‐11 was given at 08:00. In addition, a dose‐dependent lengthening of the period of the circadian rhythm of DPD was observed for 42 h after intravenous injection of CPT‐11 at 20:00. The levels of DPD protein and activity at 21 h after administration of CPT‐11 (at 17:00) were significantly higher than at 9 h (at 05:00). These results suggest that CPT‐11 may influence the circadian rhythm of DPD at the transcriptional level. Modulation of the circadian rhythm of DPD by CPT‐11 may be a factor in optimizing the combination of 5‐FU and CPT‐11.

Stimulation of RNA polymerase II elongation by hepatitis delta antigen

Transcription elongation by RNA polymerase II (RNAPII) is negatively regulated by the human factors DRB-sensitivity inducing factor (DSIF) and negative elongation factor (NELF). A 66-kilodalton subunit of NELF (NELF-A) shows limited sequence similarity to hepatitis delta antigen (HDAg), the viral protein required for replication of hepatitis delta virus (HDV). The host RNAPII has been implicated in HDV replication, but the detailed mechanism and the role of HDAg in this process are not understood. We show that HDAg binds RNAPII directly and stimulates transcription by displacing NELF and promoting RNAPII elongation. These results suggest that HDAg may regulate RNAPII elongation during both cellular messenger RNA synthesis and HDV RNA replication.

UV light-induced degradation of RNA polymerase II is dependent on the Cockayne's syndrome A and B proteins but not p53 or MLH1

It has been hypothesized that the degradation of the largest subunit of RNA polymerase II (polIILS) is required for transcription-coupled repair (TCR) of UV light-induced transcription-blocking lesions. In this study we further investigated the mechanism of UV-induced degradation of polIILS using cell lines with specific defects in TCR or in the recovery of RNA synthesis. It was found that the hypophosphorylated IIa form of polIILS rapidly decreased following UV-irradiation in all cell lines tested. Inhibition of proteasome activity resulted in an increase of the hyperphosphorylated IIo form of polIILS in UV-irradiated cells, while inhibition of CTD-kinases resulted in the retention of the IIa form. In UV-irradiated Cockayne's syndrome cells, which are defective in TCR, the levels of the IIo form increased in a similar manner as when proteasome inhibitors were added to UV-irradiated normal cells. In contrast, TCR-deficient HCT116 cells, which lack the mismatch repair protein MLH1, showed proficient degradation of polIILS as did cells with deficiencies in the recovery of RNA synthesis following UV-irradiation due to defective p53. Furthermore, we found that proteasome function was important for the recovery of mRNA synthesis even in TCR-deficient HCT116 cells. Our results suggest that proteasome-mediated degradation of polIILS is preceded by phosphorylation of the C-terminal domain of polIILS and requires the CS-A and CS-B but not MLH1 or p53 proteins. Furthermore, our results suggest that following UV-irradiation, the degradation of polIILS is required for the efficient recovery of mRNA synthesis but not for TCR per se.

Juglone, an inhibitor of the peptidyl-prolyl isomerase Pin1, also directly blocks transcription

The C-terminal domain (CTD) of the large subunit of RNA polymerase II plays a role in transcription and RNA processing. Yeast ESS1, a peptidyl-prolyl cis/trans isomerase, is involved in RNA processing and can associate with the CTD. Using several types of assays we could not find any evidence of an effect of Pin1, the human homolog of ESS1, on transcription by RNA polymerase II in vitro or on the expression of a reporter gene in vivo. However, an inhibitor of Pin1, 5-hydroxy-1,4-naphthoquinone (juglone), blocked transcription by RNA polymerase II. Unlike N-ethylmaleimide, which inhibited all phases of transcription by RNA polymerase II, juglone disrupted the formation of functional preinitiation complexes by modifying sulfhydryl groups but did not have any significant effect on either initiation or elongation. Both RNA polymerases I and III, but not T7 RNA polymerase, were inhibited by juglone. The primary target of juglone has not been unambiguously identified, although a site on the polymerase itself is suggested by inhibition of RNA polymerase II during factor-independent transcription of single-stranded DNA. Because of its unique inhibitory properties juglone should prove useful in studying transcription in vitro.

A regulator of transcriptional elongation controls vertebrate neuronal development

The development of distinct vertebrate neurons is defined by the unique profiles of genes that neurons express. It is accepted that neural genes are regulated at the point of transcription initiation, but the role of messenger RNA elongation in neural gene regulation has not been examined. Here we describe the mutant foggy, identified in a genetic screen for mutations that affect neuronal development in zebrafish, that displayed a reduction of dopamine-containing neurons and a corresponding surplus of serotonin-containing neurons in the hypothalamus. Positional cloning disclosed that Foggy is a brain-enriched nuclear protein that is structurally related to the transcription elongation factor Spt5 (refs 5-12). Foggy is not part of the basic transcription apparatus but a phosphorylation-dependent, dual regulator of transcription elongation. The mutation disrupts its repressive but not its stimulatory activity. Our results provide molecular, genetic and biochemical evidence that negative regulators of transcription elongation control key aspects of neuronal development.

Spt5 and spt6 are associated with active transcription and have characteristics of general elongation factors in D. melanogaster

The Spt4, Spt5, and Spt6 proteins are conserved throughout eukaryotes and are believed to play critical and related roles in transcription. They have a positive role in transcription elongation in Saccharomyces cerevisiae and in the activation of transcription by the HIV Tat protein in human cells. In contrast, a complex of Spt4 and Spt5 is required in vitro for the inhibition of RNA polymerase II (Pol II) elongation by the drug DRB, suggesting also a negative role in vivo. To learn more about the function of the Spt4/Spt5 complex and Spt6 in vivo, we have identified Drosophila homologs of Spt5 and Spt6 and characterized their localization on Drosophila polytene chromosomes. We find that Spt5 and Spt6 localize extensively with the phosphorylated, actively elongating form of Pol II, to transcriptionally active sites during salivary gland development and upon heat shock. Furthermore, Spt5 and Spt6 do not colocalize widely with the unphosphorylated, nonelongating form of Pol II. These results strongly suggest that Spt5 and Spt6 play closely related roles associated with active transcription in vivo.

CDK9 autophosphorylation regulates high-affinity binding of the human immunodeficiency virus type 1 tat-P-TEFb complex to TAR RNA

Human immunodeficiency virus type 1 (HIV-1) Tat interacts with cyclin T1 (CycT1), a regulatory partner of CDK9 in the positive transcription elongation factor (P-TEFb) complex, and binds cooperatively with CycT1 to TAR RNA to recruit P-TEFb and promote transcription elongation. We show here that Tat also stimulates phosphorylation of affinity-purified core RNA polymerase II and glutathione S-transferase-C-terminal-domain substrates by CycT1-CDK9, but not CycH-CDK7, in vitro. Interestingly, incubation of recombinant Tat-P-TEFb complexes with ATP enhanced binding to TAR RNA dramatically, and the C-terminal half of CycT1 masked binding of Tat to TAR RNA in the absence of ATP. ATP incubation lead to autophosphorylation of CDK9 at multiple C-terminal Ser and Thr residues, and full-length CycT1 (amino acids 728) [CycT1(1-728)], but not truncated CycT1(1-303), was also phosphorylated by CDK9. P-TEFb complexes containing a catalytically inactive CDK9 mutant (D167N) bound TAR RNA weakly and independently of ATP, as did a C-terminal truncated CDK9 mutant that was catalytically active but unable to undergo autophosphorylation. Analysis of different Tat proteins revealed that the 101-amino-acid SF2 HIV-1 Tat was unable to bind TAR with CycT1(1-303) in the absence of phosphorylated CDK9, whereas unphosphorylated CDK9 strongly blocked binding of HIV-2 Tat to TAR RNA in a manner that was reversed upon autophosphorylation. Replacement of CDK9 phosphorylation sites with negatively charged residues restored binding of CycT1(1-303)-D167N-Tat, and rendered D167N a more potent inhibitor of transcription in vitro. Taken together, these results demonstrate that CDK9 phosphorylation is required for high-affinity binding of Tat-P-TEFb to TAR RNA and that the state of P-TEFb phosphorylation may regulate Tat transactivation in vivo.

Conditional expression of RNA polymerase II in mammalian cells. Deletion of the carboxyl-terminal domain of the large subunit affects early steps in transcription

The carboxyl-terminal domain (CTD) of the large subunit of mammalian RNA polymerase II contains 52 repeats of a heptapeptide that is the target of a variety of kinases. The hyperphosphorylated CTD recruits important factors for mRNA capping, splicing, and 3'-processing. The role of the CTD for the transcription process in vivo, however, is not yet clear. We have conditionally expressed an alpha-amanitin-resistant large subunit with an almost entirely deleted CTD (LS*Delta5) in B-cells. These cells have a defect in global transcription of cellular genes in the presence of alpha-amanitin. Moreover, pol II harboring LS*Delta5 failed to transcribe up to the promoter-proximal pause sites in the hsp70A and c-fos gene promoters. The results indicate that the CTD is already required for steps that occur before promoter-proximal pausing and maturation of mRNA.

Rapid induction of nuclear transcripts and inhibition of intron decay in response to the polymerase II inhibitor DRB

The transcriptional inhibitor 5, 6-dichloro-1-beta-d-ribofuranosylbenzimidazole (DRB) is an adenosine analog that has been shown to cause premature transcriptional termination and thus has been a useful tool to identify factors important for transcriptional elongation. Here, we establish an efficient system for studying DRB-sensitive steps of transcriptional elongation. In addition, we establish two novel effects of DRB not previously reported: intron stabilization and the induction of long transcripts by a mechanism other than premature termination. We found that DRB had a biphasic effect on T-cell receptor-beta (TCRbeta) transcripts driven by a tetracycline (tet)-responsive promoter in transfected HeLa cells. In the first phase, DRB caused a rapid decrease (within five minutes) of pre-mRNA and its spliced intron (IVS1(Cbeta1)), consistent with the known ability of DRB to inhibit transcription. In the second phase (which began ten minutes to two hours after treatment, depending on the dose), DRB dramatically increased the levels of IVS1(Cbeta1)-containing transcripts by a mechanism requiring de novo RNA synthesis. DRB induced the appearance of short 0.4 to 0.8 kb TCRbeta transcripts in vivo, indicating DRB enhances premature transcriptional termination. A approximately 475 nt prematurely terminated transcript (PT) was characterized that terminated at an internal poly(A) tract in the intron IVS1(Cbeta1). We identified three other effects of DRB. First, we observed that DRB induced the appearance of heterodisperse TCRbeta transcripts that were too long ( approximately 1 kb to >8 kb) to result from the type of premature termination events previously described. Their production was not promoter-specific, as we found that long transcripts were induced by DRB from both the tet-responsive and beta-actin promoters. Second, DRB upregulated full-length normal-sized c-myc mRNA, which provided further evidence that DRB has effects besides regulation of premature termination. Third, DRB stabilized lariat forms of the intron IVS1(Cbeta1), indicating that DRB exerts post-transcriptional actions. We propose that our model system will be useful for elucidating the factors that regulate RNA decay and transcriptional elongation in vivo.

Microbial pathogens and apoptotic anticancer therapy

In spite of tremendous efforts to control cancer, the mortality associated with this disease has been increasing in developed countries in the recent decades. Inadequate efficiency of existing therapeutic regimens and the rise of multi-drug resistant cancer cells are the main factors which require a broadening of investigations into novel anticancer strategies. Enhancement of apoptosis in tumours has been suggested recently as a new anticancer strategy. It targets the disruption of equilibrium between cell proliferation and cell death in tumours and suggests to restore it through the use of pharmacological agents or genetic approaches. Apoptotic therapy has attracted many groups of investigators and several companies have entered the race to develop the first generation of apoptotic anticancer agents. The review discusses the role that pathogenic microorganisms may have as the source of agents for apoptotic therapy.

Domains in the SPT5 protein that modulate its transcriptional regulatory properties

SPT5 and its binding partner SPT4 regulate transcriptional elongation by RNA polymerase II. SPT4 and SPT5 are involved in both 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB)-mediated transcriptional inhibition and the activation of transcriptional elongation by the human immunodeficiency virus type 1 (HIV-1) Tat protein. Recent data suggest that P-TEFb, which is composed of CDK9 and cyclin T1, is also critical in regulating transcriptional elongation by SPT4 and SPT5. In this study, we analyze the domains of SPT5 that regulate transcriptional elongation in the presence of either DRB or the HIV-1 Tat protein. We demonstrate that SPT5 domains that bind SPT4 and RNA polymerase II, in addition to a region in the C terminus of SPT5 that contains multiple heptad repeats and is designated CTR1, are critical for in vitro transcriptional repression by DRB and activation by the Tat protein. Furthermore, the SPT5 CTR1 domain is a substrate for P-TEFb phosphorylation. These results suggest that C-terminal repeats in SPT5, like those in the RNA polymerase II C-terminal domain, are sites for P-TEFb phosphorylation and function in modulating its transcriptional elongation properties.