Transcription coupled repair of 8-oxoguanine in murine cells: The Ogg1 protein is required for repair in nontranscribed sequences but not in transcribed sequences

To assess the role of the Ogg1 DNA glycosylase in the transcription-coupled repair (TCR) of the mutagenic lesion, 7,8-dihydro-8oxoguanine (8-OxoG), we have investigated the removal of this lesion in wild-type and ogg1−/− null mouse embryo fibroblast (MEF) cell lines. We used nonreplicating plasmids containing a single 8-OxoG·C base pair in a different assay that allowed us to study the removal of 8-OxoG located in a transcribed sequence (TS) or in a nontranscribed sequence (NTS). The results show that the removal of 8-OxoG in a wild-type MEF cell line is faster in the TS than in the NTS, indicating TCR of 8-OxoG in murine cells. In the homozygous ogg1−/− MEF cell line, 8-OxoG was not removed from the NTS whereas there was still efficient 8-OxoG repair in the TS. Expression of the mouse Ogg1 protein in the homozygous ogg1−/− cell line restored the ability to remove 8-OxoG in the NTS. Therefore, we have demonstrated that Ogg1 is essential for the repair of 8-OxoG in the NTS but is not required in the TS. These results indicate the existence of an Ogg1-independent pathway for the TCR of 8-OxoG in vivo.

Dynamic regulation of neuronal NO synthase transcription by calcium influx through a CREB family transcription factor-dependent mechanism

Neuronal nitric oxide (NO) synthase (nNOS) is dynamically regulated in response to a variety of physiologic and pathologic stimuli. Although the dynamic regulation of nNOS is well established, the molecular mechanisms by which such diverse stimuli regulate nNOS expression have not yet been identified. We describe experiments demonstrating that Ca2+ entry through voltage-sensitive Ca2+ channels regulates nNOS expression through alternate promoter usage in cortical neurons and that nNOS exon 2 contains the regulatory sequences that respond to Ca2+. Deletion and mutational analysis of the nNOS exon 2 promoter reveals two critical cAMP/Ca2+ response elements (CREs) that are immediately upstream of the transcription start site. CREB binds to the CREs within the nNOS gene. Mutation of the nNOS CREs as well as blockade of CREB function results in a dramatic loss of nNOS transcription. These findings suggest that nNOS is a Ca2+-regulated gene through the interactions of CREB on the CREs within the nNOS exon 2 promoter and that these interactions are likely to be centrally involved in the regulation of nNOS in response to neuronal injury and activity-dependent plasticity.

UV-induced inhibition of transcription involves repression of transcription initiation and phosphorylation of RNA polymerase II

Cells from patients with Cockayne syndrome (CS) are hypersensitive to DNA-damaging agents and are unable to restore damage-inhibited RNA synthesis. On the basis of repair kinetics of different types of lesions in transcriptionally active genes, we hypothesized previously that impaired transcription in CS cells is a consequence of defective transcription initiation after DNA damage induction. Here, we investigated the effect of UV irradiation on transcription by using an in vitro transcription system that allowed uncoupling of initiation from elongation events. Nuclear extracts prepared from UV-irradiated or mock-treated normal human and CS cells were assayed for transcription activity on an undamaged β-globin template. Transcription activity in nuclear extracts closely mimicked kinetics of transcription in intact cells: extracts from normal cells prepared 1 h after UV exposure showed a strongly reduced activity, whereas transcription activity was fully restored in extracts prepared 6 h after treatment. Extracts from CS cells exhibited reduced transcription activity at any time after UV exposure. Reduced transcription activity in extracts coincided with a strong reduction of RNA polymerase II (RNAPII) containing hypophosphorylated C-terminal domain, the form of RNAPII known to be recruited to the initiation complex. These results suggest that inhibition of transcription after UV irradiation is at least partially caused by repression of transcription initiation and not solely by blocked elongation at sites of lesions. Generation of hypophosphorylated RNAPII after DNA damage appears to play a crucial role in restoration of transcription. CS proteins may be required for this process in a yet unknown way.

Regulation of vascular endothelial growth factor (VEGF) gene transcription by estrogen receptors α and β

Vascular endothelial growth factor (VEGF) mediates angiogenic activity in a variety of estrogen target tissues. To determine whether estrogen has a direct transcriptional effect on VEGF gene expression, we developed a model system by transiently transfecting human VEGF promoter-luciferase reporter constructs into primary human endometrial cells and into Ishikawa cells, derived from a well-differentiated human endometrial adenocarcinoma. In primary endometrial epithelial cells, treatment with 17β-estradiol (E2) resulted in a 3.8-fold increase in luciferase activity, whereas a 3.2-fold induction was demonstrated for stromal cells. Our Ishikawa cells had less than 100 functional estrogen receptors (ER)/cell and were therefore cotransfected with expression vectors encoding either the α- or the β-form of the human ER. In cells cotransfected with ERα, E2 induced 3.2-fold induction in VEGF-promoter luciferase activity. A 2.3-fold increase was observed in cells cotransfected with ERβ. Through specific deletions, the E2 response was restricted to a single 385-bp PvuII-SstI fragment in the 5′ flanking DNA. Cotransfection of this upstream region with a DNA binding domain ER mutant, or site-directed mutagenesis of a variant ERE within this fragment, resulted in the loss of the E2 response. Electromobility shift assays demonstrated that this same ERE sequence specifically binds estradiol-ER complexes. These studies demonstrate that E2-regulated VEGF gene transcription requires a variant ERE located 1.5 kb upstream from the transcriptional start site. Site-directed mutagenesis of this ERE abrogated E2-induced VEGF gene expression.

A cellular defense pathway regulating transcription through poly(ADP-ribosyl)ation in response to DNA damage

DNA damage is known to trigger key cellular defense pathways such as those involved in DNA repair. Here we provide evidence for a previously unrecognized pathway regulating transcription in response to DNA damage and show that this regulation is mediated by the abundant nuclear enzyme poly(ADP-ribose) polymerase. We found that poly(ADP-ribose) polymerase reduced the rate of transcription elongation by RNA polymerase II, suggesting that poly(ADP-ribose) polymerase negatively regulates transcription, possibly through the formation of poly(ADP-ribose) polymerase–RNA complexes. In damaged cells, poly(ADP-ribose) polymerase binds to DNA breaks and automodifies itself in the presence of NAD+, resulting in poly(ADP-ribose) polymerase inactivation. We found that automodification of poly(ADP-ribose) polymerase in response to DNA damage resulted in the up-regulation of transcription, presumably because automodified poly(ADP-ribose) polymerase molecules were released from transcripts, thereby relieving the block on transcription. Because agents that damage DNA damage RNA as well, up-regulation of RNA synthesis in response to DNA damage may provide cells with a mechanism to compensate for the loss of damaged transcripts and may be critical for cell survival after exposure to DNA-damaging agents.

CIITA stimulation of transcription factor binding to major histocompatibility complex class II and associated promoters in vivo

CIITA is a master transactivator of the major histocompatibility complex class II genes, which are involved in antigen presentation. Defects in CIITA result in fatal immunodeficiencies. CIITA activation is also the control point for the induction of major histocompatibility complex class II and associated genes by interferon-γ, but CIITA does not bind directly to DNA. Expression of CIITA in G3A cells, which lack endogenous CIITA, followed by in vivo genomic footprinting, now reveals that CIITA is required for the assembly of transcription factor complexes on the promoters of this gene family, including DRA, Ii, and DMB. CIITA-dependent promoter assembly occurs in interferon-γ-inducible cell types, but not in B lymphocytes. Dissection of the CIITA protein indicates that transactivation and promoter loading are inseparable and reveal a requirement for a GTP binding motif. These findings suggest that CIITA may be a new class of transactivator.

The incompatible interaction between Phytophthora parasitica var. nicotianae race 0 and tobacco is suppressed in transgenic plants expressing antisense lipoxygenase sequences

Nicotiana tabacum 46-8 cultivar displays an incompatible interaction with race 0 of Phytophthora parasitica var. nicotianae (Ppn), a fungal pathogen of most tobacco cultivars. At the plant level, incompatibility is characterized by the induction of lipoxygenase (LOX, EC = 1.13.11.12) activity and localized hypersensitive cell death before defense gene activation. To evaluate the involvement of LOX in the onset of plant defense, tobacco 46-8 plants were genetically engineered using full-length or partial-length antisense (AS) tobacco LOX cDNA constructs. AS expression strongly reduced elicitor- and pathogen-induced LOX activity. Eight independent AS-LOX lines were selected and assayed for their response to Ppn. After root or stem inoculation with race 0, all AS-LOX lines but one displayed a compatible phenotype whereas control transformed plants, not containing the AS-LOX cassette, showed the typical incompatible reaction. The presence of the fungus in transgenic lines was demonstrated by PCR amplification of a Ppn-specific genomic sequence. A linear relationship was found between the extent of LOX suppression and the size of the lesion caused by the fungus. The AS-LOX plants also showed enhanced susceptibility toward the compatible fungus Rhizoctonia solani. The results demonstrate the strong involvement of LOX in the establishment of incompatibility in plant–microorganism interactions, consistent with its role in the defense of host plants.

Altering the anaerobic transcription factor FNR confers a hemolytic phenotype on Escherichia coli K12

The recent outbreaks of Escherichia coli 0157-associated food poisoning have focused attention on the virulence determinants of E. coli. Here, it is reported that single base substitutions in the fnr gene encoding the oxygen-responsive transcription regulator FNR (fumarate and nitrate reduction regulator) are sufficient to confer a hemolytic phenotype on E. coli K12, the widely used laboratory strain. The mechanism involves enhancing the expression of a normally dormant hemolysin gene (hlyE) located in the E. coli chromosome. The mutations direct single amino acid substitutions in the activating regions (AR1 and AR3) of FNR that contact RNA polymerase. It is concluded that altering a resident transcription regulator, or acquisition of a competent heterologous regulator, could generate a pool of hemolytic, and therefore more virulent, strains of E. coli in nature.

ETO, fusion partner in t(8;21) acute myeloid leukemia, represses transcription by interaction with the human N-CoR/mSin3/HDAC1 complex

The t(8;21) translocation between two genes known as AML1 and ETO is seen in approximately 12–15% of all acute myeloid leukemia (AML) and is the second-most-frequently observed nonrandom genetic alteration associated with AML. AML1 up-regulates a number of target genes critical to normal hematopoiesis, whereas the AML1/ETO fusion interferes with this trans-activation. We discovered that the fusion partner ETO binds to the human homolog of the murine nuclear receptor corepressor (N-CoR). The interaction is mediated by two unusual zinc finger motifs present at the carboxyl terminus of ETO. Human N-CoR (HuN-CoR), which we cloned and sequenced in its entirety, encodes a 2,440-amino acid polypeptide and has a central domain that binds ETO. N-CoR, mammalian Sin3 (mSin3A and B), and histone deacetylase 1 (HDAC1) form a complex that alters chromatin structure and mediates transcriptional repression by nuclear receptors and by a number of oncoregulatory proteins. We found that ETO, through its interaction with the N-CoR/mSin3/HDAC1 complex, is also a potent repressor of transcription. This observation provides a mechanism for how the AML1/ETO fusion may inhibit expression of AML1-responsive target genes and disturb normal hematopoiesis.

β-Adrenergic receptor-induced activation of nerve growth factor gene transcription in rat cerebral cortex involves CCAAT/enhancer-binding protein δ

Stimulation of β-adrenergic receptors (BAR) by clenbuterol (CLE) increases nerve growth factor (NGF) biosynthesis in the rat cerebral cortex but not in other regions of the brain. We have explored the transcription mechanisms that may account for the cortex-specific activation of the NGF gene. Although the NGF promoter contains an AP-1 element, AP-1-binding activity in the cerebral cortex was not induced by CLE, suggesting that other transcription factors govern the brain area-specific induction of NGF. Because BAR activation increases cAMP levels, we examined the role of CCAAT/enhancer-binding proteins (C/EBP), some of which are known to be cAMP-inducible. In C6–2B glioma cells, whose NGF expression is induced by BAR agonists, (i) CLE increased C/EBPδ-binding activity, (ii) NGF mRNA levels were increased by overexpressing C/EBPδ, and (iii) C/EBPδ increased the activity of an NGF promoter–reporter construct. Moreover, DNase footprinting and deletion analyses identified a C/EBPδ site in the proximal region of the NGF promoter. C/EBPδ appears to be responsible for the BAR-mediated activation of the NGF gene in vivo, since CLE elicited a time-dependent increase in C/EBPδ-binding activity in the cerebral cortex only. Our data suggest that, while AP-1 may regulate basal levels of NGF expression, C/EBPδ is a critical component determining the area-specific expression of NGF in response to BAR stimulation.

Temperature, template topology, and factor requirements of archaeal transcription

Although Archaea are prokaryotic and resemble Bacteria morphologically, their transcription apparatus is remarkably similar to those of eukaryotic cell nuclei. Because some Archaea exist in environments with temperatures of around 100°C, they are likely to have evolved unique strategies for transcriptional control. Here, we investigate the effects of temperature and DNA template topology in a thermophilic archaeal transcription system. Significantly, and in marked contrast with characterized eucaryal systems, archaeal DNA template topology has negligible effect on transcription levels at physiological temperatures using highly purified polymerase and recombinant transcription factors. Furthermore, archaeal transcription does not require hydrolysis of the β-γ phosphoanhydride bond of ATP. However, at lower temperatures, negatively supercoiled templates are transcribed more highly than those that are positively supercoiled. Notably, the block to transcription on positively supercoiled templates at lowered temperatures is at the level of polymerase binding and promoter opening. These data imply that Archaea do not possess a functional homologue of transcription factor TFIIH, and that for the promoters studied, transcription is mediated by TATA box-binding protein, transcription factor TFB, and RNA polymerase alone. Furthermore, they suggest that the reduction of plasmid linking number by hyperthermophilic Archaea in vivo in response to cold shock is a mechanism to maintain gene expression under these adverse circumstances.

Stimulation of bacteriophage T4 middle transcription by the T4 proteins MotA and AsiA occurs at two distinct steps in the transcription cycle

The bacteriophage T4 encodes proteins that are responsible for tightly regulating mRNA synthesis throughout phage development in Escherichia coli. The three classes of T4 promoters (early, middle, and late) are utilized sequentially by the host RNA polymerase as a result of phage-induced modifications. One such modification is the tight binding of the T4 AsiA protein to the σ70 subunit of the RNA polymerase. This interaction is pivotal for the transition between T4 early and middle transcription, since it both inhibits recognition of host and T4 early promoters and stimulates T4 middle mode synthesis. The activation of T4 middle transcription also requires the T4 MotA protein, bound specifically to its recognition sequence, the “Mot box,” which is centered at position −30 of these promoters. Accordingly, the two T4 proteins working in concert are sufficient to effectively switch the transcription specificity of the RNA polymerase holoenzyme. Herein, we investigate the mechanism of transcription activation and report that, while the presence of MotA and AsiA increases the initial recruitment of RNA polymerase to a T4 middle promoter, it does not alter the intrinsic stability of the discrete complexes formed. In addition, we have characterized the RNA polymerase-promoter species by UV laser footprinting and followed their evolution from open into initiating complexes. These data, combined with in vitro transcription assays, indicate that AsiA and MotA facilitate promoter escape, thereby stimulating the production of full-length transcripts.

Formation of the preprimosome protects λ O from RNA transcription-dependent proteolysis by ClpP/ClpX

Using the bacteriophage λ DNA replication system, composed entirely of purified proteins, we have tested the accessibility of the short-lived λ O protein to the ClpP/ClpX protease during the various stages of λ DNA replication. We find that binding of λ O protein to its oriλ DNA sequence, leading to the so-called “O-some” formation, largely inhibits its degradation. On the contrary, under conditions permissive for transcription, the λ O protein bound to the oriλ sequence becomes largely accessible to ClpP/ClpX-mediated proteolysis. However, when the λ O protein is part of the larger oriλ:O⋅P⋅DnaB preprimosomal complex, transcription does not significantly increase ClpP/ClpX-dependent λ O degradation. These results show that transcription can stimulate proteolysis of a protein that is required for the initiation of DNA replication.

Differential regulation of transcription: Repression by unactivated mitogen-activated protein kinase Kss1 requires the Dig1 and Dig2 proteins

Kss1, a yeast mitogen-activated protein kinase (MAPK), in its unphosphorylated (unactivated) state binds directly to and represses Ste12, a transcription factor necessary for expression of genes whose promoters contain filamentous response elements (FREs) and genes whose promoters contain pheromone response elements (PREs). Herein we show that two nuclear proteins, Dig1 and Dig2, are required cofactors in Kss1-imposed repression. Dig1 and Dig2 cooperate with Kss1 to repress Ste12 action at FREs and regulate invasive growth in a naturally invasive strain. Kss1-imposed Dig-dependent repression of Ste12 also occurs at PREs. However, maintenance of repression at PREs is more dependent on Dig1 and/or Dig2 and less dependent on Kss1 than repression at FREs. In addition, derepression at PREs is more dependent on MAPK-mediated phosphorylation than is derepression at FREs. Differential utilization of two types of MAPK-mediated regulation (binding-imposed repression and phosphorylation-dependent activation), in combination with distinct Ste12-containing complexes, contributes to the mechanisms by which separate extracellular stimuli that use the same MAPK cascade can elicit two different transcriptional responses.

Histones H3 and H4 are components of upstream activation factor required for the high-level transcription of yeast rDNA by RNA polymerase I

RNA polymerase I (Pol I) transcription in the yeast Saccharomyces cerevisiae is greatly stimulated in vivo and in vitro by the multiprotein complex, upstream activation factor (UAF). UAF binds tightly to the upstream element of the rDNA promoter, such that once bound (in vitro), UAF does not readily exchange onto a competing template. Of the polypeptides previously identified in purified UAF, three are encoded by genes required for Pol I transcription in vivo: RRN5, RRN9, and RRN10. Two others, p30 and p18, have remained uncharacterized. We report here that the N-terminal amino acid sequence, its mobility in gel electrophoresis, and the immunoreactivity of p18 shows that it is histone H3. In addition, histone H4 was found in UAF, and myc-tagged histone H4 could be used to affinity-purify UAF. Histones H2A and H2B were not detectable in UAF. These results suggest that histones H3 and H4 probably account for the strong binding of UAF to DNA and may offer a means by which general nuclear regulatory signals could be transmitted to Pol I.