Specific action of alpha-amanitin on mammalian RNA polymerase protein

Determination of protein-bound α-amanitin in mouse plasma: A potential new indicator of poisoning with the mushroom toxin α-amanitin

Approximately 70%∼90% of mushroom poisoning deaths are caused by the class of mushroom toxins known as amatoxins. However, the rapid elimination of amatoxins from plasma within 48 h after mushroom ingestion limits the practical value of plasma amatoxin analysis as a diagnostic indicator of Amanita mushroom poisoning. To increase the positive detection rate and extend the detection window of amatoxin poisoning, we developed a new method to detect protein-bound α-amanitin based on the hypothesis that RNAP II-bound α-amanitin released from the tissue into the plasma could be degraded by trypsin hydrolysis and then detected by conventional liquid chromatography-mass spectrometry (LC‒MS). Toxicokinetic studies on mice intraperitoneally injected with 0.33 mg/kg α-amanitin were conducted to obtain and compare the concentration trends, detection rates, and detection windows of both free α-amanitin and protein-bound α-amanitin. By comparing detection results with and without trypsin hydrolysis in the liver and plasma of α-amanitin-poisoned mice, we verified the credibility of this method and the existence of protein-bound α-amanitin in plasma. Under the optimized trypsin hydrolysis conditions, we obtained a time-dependent trend of protein-bound α-amanitin in mouse plasma at 1-12 days postexposure. In contrast to the short detection window (0-4 h) of free α-amanitin in mouse plasma, the detection window of protein-bound α-amanitin was extended to 10 days postexposure, with a total detection rate of 53.33%, ranging from the limit of detection to 23.94 μg/L. In conclusion, protein-bound α-amanitin had a higher positive detection rate and a longer detection window than free α-amanitin in mice.

Effects of Transcription-Dependent Physical Perturbations on the Chromosome Dynamics in Living Cells

Recent studies with single-particle tracking in live cells have revealed that chromatin dynamics are directly affected by transcription. However, how transcription alters the chromatin movements followed by changes in the physical properties of chromatin has not been elucidated. Here, we measured diffusion characteristics of chromatin by targeting telomeric DNA repeats with CRISPR-labeling. We found that transcription inhibitors that directly block transcription factors globally increased the movements of chromatin, while the other inhibitor that blocks transcription by DNA intercalating showed an opposite effect. We hypothesized that the increased mobility of chromatin by transcription inhibition and the decreased chromatin movement by a DNA intercalating inhibitor is due to alterations in chromatin rigidity. We also tested how volume confinement of nuclear space affects chromatin movements. We observed decreased chromatin movements under osmotic pressure and with overexpressed chromatin architectural proteins that compact chromatin.

Liver transcriptome analyses of acute poisoning and recovery of male ICR mice exposed to the mushroom toxin α-amanitin

Approximately 70-90% of mushroom poisoning deaths are caused by α-amanitin-induced liver injury resulting from RNA polymerase II (RNAP II) inhibition. Liver regeneration ability may contribute greatly to individual survival after α-amanitin poisoning. However, it is unclear what cellular pathways are activated to stimulate regeneration. We conducted dose-effect and time-effect studies in mice that were intraperitoneally injected with 0.33-0.66 mg/kg α-amanitin to establish a poisoning model. The liver/body weight ratio, serological indices, and pathology were evaluated to characterize the liver injury. In the time-effect study, the liver transcriptome was analyzed to explore the mRNA changes resulting from RNAP II inhibition and the underlying pathways associated with recovery. Based on the two animal studies, we established a poisoning model with three sequential liver states: early injury, regulation, and recovery. The mRNA changes reflected by the differentially expressed genes (DEGs) in the transcriptome could be used to illustrate the inhibition of RNAP II by α-amanitin. DEGs at four key time points were well matched with the three liver states, including 8-h downregulated genes in the early injury state, 16-h and 72-h upregulated genes in the regulation state, and 96-h upregulated/downregulated genes in the recovery state. By clustering analysis, the mTOR signaling pathway was screened out as the most promising potential pathway promoting recovery. The results of our investigations of the pathways and events downstream of the mTOR pathway indicated that the activation of mTOR probably contributes crucially to liver regeneration, which could be a promising basis for drug development.

High-Content Imaging Approaches to Quantitate Stress-Induced Changes in Nucleolar Morphology

The nucleolus is a dynamic subnuclear compartment that has a number of different functions, but its primary role is to coordinate the production and assembly of ribosomes. For well over 100 years, pathologists have used changes in nucleolar number and size to stage diseases such as cancer. New information about the nucleolus' broader role within the cell is leading to the development of drugs which directly target its structure as therapies for disease. Traditionally, it has been difficult to develop high-throughput image analysis pipelines to measure nucleolar changes due to the broad range of morphologies observed. In this study, we describe a simple high-content image analysis algorithm using Harmony software (PerkinElmer), with a PhenoLOGIC™ machine-learning component, that can measure and classify three different nucleolar morphologies based on nucleolin and fibrillarin staining ("normal," "peri-nucleolar rings" and "dispersed"). We have utilized this algorithm to determine the changes in these classes of nucleolar morphologies over time with drugs known to alter nucleolar structure. This approach could be further adapted to include other parameters required for the identification of new therapies that directly target the nucleolus.

Detection of antisense protein (ASP) RNA transcripts in individuals infected with human immunodeficiency virus type 1 (HIV-1)

The detection of antisense RNA is hampered by reverse transcription (RT) non-specific priming, due to the ability of RNA secondary structures to prime RT in the absence of specific primers. The detection of antisense RNA by conventional RT-PCR does not allow assessment of the polarity of the initial RNA template, causing the amplification of non-specific cDNAs. In this study we have developed a modified protocol for the detection of human immunodeficiency virus type 1 (HIV-1) antisense protein (ASP) RNA. Using this approach, we have identified ASP transcripts in CD4+ T cells isolated from five HIV-infected individuals, either untreated or under suppressive therapy. We show that ASP RNA can be detected in stimulated CD4+ T cells from both groups of patients, but not in unstimulated cells. We also show that in untreated patients, the patterns of expression of ASP and env are very similar, with the levels of ASP RNA being markedly lower than those of env. Treatment of cells from one viraemic patient with α-amanitin greatly reduces the rate of ASP RNA synthesis, suggesting that it is associated with RNA polymerase II, the central enzyme in the transcription of protein-coding genes. Our data represent the first nucleotide sequences obtained in patients for ASP, demonstrating that its transcription indeed occurs in those HIV-1 lineages in which the ASP open reading frame is present.

Patterns of cell lineage, movement, and migration from germ layer specification to gastrulation in the amphipod crustacean Parhyale hawaiensis

The acquisition of specific cell fates throughout embryonic development is one of the core problems in developmental and evolutionary biology. In the amphipod Parhyale hawaiensis all three germ layers and the germ line are determined by the eight-cell stage. Despite this early fate determination, multiple cell types can be replaced following ablation of their founder cells, showing that this embryo also has significant regulative properties. Here we present a cellular-level resolution lineage analysis for P. hawaiensis embryos between fertilization and gastrulation, including analysis of cleavage patterns, division times, and clonal behaviors. We compare these cellular behaviors in wild type embryos with those in embryos where specific founder cells have been ablated, or where zygotic transcription has been inhibited. We observe that when germ line, endoderm or mesoderm founder cells are ablated, the remaining cells do not alter their cleavage or migration behaviors before the onset of gastrulation. In the absence of zygotic transcription, ingression movements proceed normally, but epibolic movements are disrupted. This indicates that the embryo's regulative response to germ layer founder loss, in the form of altered cell behavior, is realized in the ~32h between gastrulation and early germ band elongation, and is likely to require zygotic reprogramming rather than alternative deployment of maternally supplied determinants. Combining these data with the observations of previous studies, we propose a framework to elucidate the molecular mechanisms that regulate the determinative and regulative properties of the P. hawaiensis embryo.

Triptolide is an inhibitor of RNA polymerase I and II-dependent transcription leading predominantly to down-regulation of short-lived mRNA

Triptolide, a natural product extracted from the Chinese plant Tripterygium wilfordii, possesses antitumor properties. Despite numerous reports showing the proapoptotic capacity and the inhibition of NF-kappaB-mediated transcription by triptolide, the identity of its cellular target is still unknown. To clarify its mechanism of action, we further investigated the effect of triptolide on RNA synthesis in the human non-small cell lung cancer cell line A549. Triptolide inhibited both total RNA and mRNA de novo synthesis, with the primary action being on the latter pool. We used 44K human pan-genomic DNA microarrays and identified the genes primarily affected by a short treatment with triptolide. Among the modulated genes, up to 98% are down-regulated, encompassing a large array of oncogenes including transcription factors and cell cycle regulators. We next observed that triptolide induced a rapid depletion of RPB1, the RNA polymerase II main subunit that is considered a hallmark of a transcription elongation blockage. However, we also show that triptolide does not directly interact with the RNA polymerase II complex nor does it damage DNA. We thus conclude that triptolide is an original pharmacologic inhibitor of RNA polymerase activity, affecting indirectly the transcription machinery, leading to a rapid depletion of short-lived mRNA, including transcription factors, cell cycle regulators such as CDC25A, and the oncogenes MYC and Src. Overall, the data shed light on the effect of triptolide on transcription, along with its novel potential applications in cancers, including acute myeloid leukemia, which is in part driven by the aforementioned oncogenic factors.

Plasmodium falciparum: Preinitiation complex occupancy of active and inactive promoters during erythrocytic stage

Over 80% of Plasmodium falciparum genes are developmentally regulated during the parasite's life cycle with most genes expressed in a "just in time" fashion. However, the molecular mechanisms of gene regulation are still poorly understood. Analysis of P. falciparum genome shows that the parasite appears to encode relatively few transcription factors homologous to those in other eukaryotes. We used Chromatin immunoprecipitation (ChIP) to study interaction of PfTBP and PfTFIIE with stage specific Plasmodium promoters. Our results indicate that PfTBP and PfTFIIE are bound to their cognate sequence in active and inactive erythrocytic-expressed promoters. In addition, TF occupancy appears to extend beyond the promoter regions, since PfTBP interaction with the coding and 3' end regions was also detected. No PfTBP or PfTFIIE interaction was detected on csp and pfs25 genes which are not active during the erythrocytic asexual stage. Furthermore, PfTBP and PfTFIIE binding did not appear to correlate with histone 3 and/or 4 acetylation, suggesting that histone acetylation may not be a prerequisite for PfTBP or PfTFIIE promoter interaction. Based on our observations we concluded that the PfTBP/PfTFIIE-containing preinitiation complex (PIC) would be preassembled on promoters of all erythrocytic-expressed genes in a fashion independent of histone acetylation, providing support for the "poised" model. Contrary to the classical model of eukaryotic gene regulation, PIC interaction with Plasmodium promoters occurred independent of transcriptional activity and to the notion that chromatin acetylation leads to PIC assembly.

An RNA polymerase inhibitor, cyclothiazomycin B1, and its isomer

Novel cyclic thiopeptides, cyclothiazomycins B1 (1) and B2 (2), were isolated from Streptomyces sp. A307 as potent hyphal swelling inducing substances. They are stable in the solid state but slowly isomerize with one another in solution. Degradation experiments and spectroscopic analyses disclosed that they comprise unique tricyclic structures each containing a dehydroalanine, and two dehydrohomoalanine residues, along with three thiazolines, three thiazoles, and a trisubstituted pyridine. Cyclothiazomycin B1 (1) is expected to be a powerful tool for DNA-RNA transcription studies, because this cyclopeptide inhibits DNA-dependent RNA synthesis by bacteriophage RNA polymerases.

Transition from non-motile behaviour to directed migration during early PGC development in zebrafish

The migration of zebrafish primordial germ cells (PGCs) is directed by SDF-1a and serves as a model for long-range chemokine-guided cell migration. Whereas the development and migration of zebrafish PGCs have been studied in great detail starting at mid-gastrulation stages when the cells exhibit guided active migration [7-8 hours post fertilization (hpf)], earlier stages have not yet been examined. Here we show that the PGCs acquire competence to respond to the chemokine following discrete maturation steps. Using the promoter of the novel gene askopos and RNA elements of nanos1 to drive GFP expression in PGCs, we found that immediately after their specification (about 3 hpf) PGCs exhibit simple cell shape. This stage is followed by a phase at which the cells assume complex morphology yet they neither change their position nor do they respond to SDF-1a. During the third phase, a transition into a ;migratory stage' occurs as PGCs become responsive to directional cues provided by somatic cells secreting the chemokine SDF-1a. This transition depends on zygotic transcription and on the function of the RNA-binding protein Dead end and is correlated with down regulation of the cell adhesion molecule E-cadherin. These distinctive morphological and molecular alterations could represent a general occurrence in similar processes critical for development and disease.

Down-regulation of nuclear protein ICBP90 by p53/p21Cip1/WAF1-dependent DNA-damage checkpoint signals contributes to cell cycle arrest at G1/S transition

Checkpoints, which monitor DNA damage and regulate cell cycle progression, ensure genomic integrity and prevent the propagation of transformed cells. DNA damage activates the p53-dependent checkpoint pathway that induces expression of p21Cip1/WAF1, resulting in cell cycle arrest at G1/S transition by inhibition of cdk activity and DNA replication. ICBP90 was identified as a nuclear protein that binds to the TopoII alpha gene promoter and is speculated to be involved in DNA replication. ICBP90 expression is cell cycle regulated in normal cells but stably high throughout cell cycle in various cancer cell lines. We here demonstrate that ICBP90 expression is down-regulated by the p53/p21Cip1/WAF1-dependent DNA damage checkpoint signals. The reduction of ICBP90 appeared to be caused by both transcriptional suppression and protein degradation. Adenoviral expression of p21Cip1/WAF1 directly led to ICBP90 reduction in p53-/- HCT116 cells without DNA damage. Furthermore, ICPB90 depletion by RNA interference significantly blocked G1/S transition after DNA damage in HeLa cells. The down-regulation of ICBP90 is an important mechanism for cell cycle arrest at G1/S transition, which is induced by the activation of a p53/p21Cip1/WAF1-dependent DNA-damage checkpoint. Deregulation of ICBP90 may impair the control of G1/S transition during checkpoint activation and lead to genomic instability.

The tax protein of human T-cell leukemia virus type 1 mediates the transactivation of the c-sis/platelet-derived growth factor-B promoter through interactions with the zinc finger transcription factors Sp1 and NGFI-A/Egr-1

Transcriptional up-regulation of the c-sis/platelet-derived growth factor-B (PDGF-B) proto-oncogene by the Tax protein of human T-cell leukemia virus type 1 has been implicated as one possible mechanism of cellular transformation by human T-cell leukemia virus type 1. In previous work, we identified an essential site in the c-sis/PDGF-B promoter, Tax-responsive element 1 (TRE1), necessary for transactivation by Tax. We also identified Sp1, Sp3, and NGFI-A/Egr-1 as the primary nuclear transcription factors binding to TRE1 which mediate Tax responsiveness. In the present work, we have investigated the mechanism(s) whereby Tax transactivates the c-sis/PDGF-B proto-oncogene. In vitro transcription assays showed that Tax was able to significantly increase the transcriptional activity of a template containing the -257 to +74 region of the c-sis/PDGF-B promoter. Electrophoretic mobility shift assay analysis showed that Tax increased the DNA binding activity of both Sp1 and NGFI-A/Egr-1 using a TRE1 probe. Analysis of Tax mutants showed that two mutants, IEXC29S and IEXL320G, were unable to significantly transactivate the c-sis/PDGF-B promoter. Finally, co-immunoprecipitation analysis revealed that Tax is able to stably bind to both Sp1 and NGFI-A/Egr-1. Interestingly, co-immunoprecipitation analysis also revealed that Tax mutant IEXC29S is unable to interact with NGFI-A/Egr-1, whereas Tax mutant IEXL320G is able to interact with NGFI-A/Egr-1.

In vitro transcription of human T-cell leukemia virus type 1 is RNA polymerase II dependent

The HTLV-1 promoter directs RNA polymerase II transcription of viral genomic RNA in vivo. However, it has been reported that in vitro, a unique RNA polymerase, with characteristics of RNA polymerases II and III, is capable of HTLV-1 transcription (G. Piras, F. Kashanchi, M. F. Radonovich, J. F. Duvall, and J. N. Brady, J. Virol. 68:6170-6179, 1994). To further characterize the polymerase involved in HTLV-1 transcription in vitro, runoff transcription assays were performed with a variety of extracts and RNA polymerase inhibitors. Under all in vitro reaction conditions tested, RNA polymerase II appeared to be the only polymerase capable of correct transcriptional initiation from the HTLV-1 promoter. Synthesis of the specific HTLV-1 RNA transcript showed sensitivities to the RNA polymerase inhibitors tagetitoxin and alpha-amanitin that are consistent with RNA polymerase II transcription. Together, these data indicate that in vitro, as in vivo, the HTLV-1 promoter directs transcription by RNA polymerase II.

Amanitin greatly reduces the rate of transcription by RNA polymerase II ternary complexes but fails to inhibit some transcript cleavage modes

The toxin alpha-amanitin is frequently employed to completely block RNA synthesis by RNA polymerase II. However, we find that polymerase II ternary transcription complexes stalled by the absence of NTPs resume RNA synthesis when NTPs and amanitin are added. Chain elongation with amanitin can continue for hours at approximately 1% of the normal rate. Amanitin also greatly slows pyrophosphorolysis by elongation-competent complexes. Complexes which are arrested (that is, which have paused in transcription for long periods in the presence of excess NTPs) are essentially incapable of resuming transcription in the presence of alpha-amanitin. Complexes traversing sequences that can provoke arrest are much more likely to stop transcription in the presence of the toxin. The substitution of IMP for GMP at the 3' end of the nascent RNA greatly increases the sensitivity of stalled transcription complexes to amanitin. Neither arrested nor stalled complexes display detectable SII-mediated transcript cleavage following amanitin treatment. However, arrested complexes possess a low level, intrinsic transcript cleavage activity which is completely amanitin-resistant; furthermore, pyrophosphorolytic transcript cleavage in arrested complexes is not affected by amanitin.

Action of alpha-amanitin during pyrophosphorolysis and elongation by RNA polymerase II

Using defined elongation complexes formed on dC-tailed templates with Drosophila RNA polymerase II, we have examined elongation, pyrophosphorolysis, and DmS-II-mediated transcript cleavage and the inhibitory effect of alpha-amanitin on these processes. Analysis of pyrophosphorolysis on soluble or immobilized and templates confirmed that NTPs are liberated instead of dinucleotides that are released during DmS-II-mediated transcript cleavage. 10 microgram/ml alpha-amanitin completely inhibited DmS-II-mediated transcript cleavage but allowed extended pyrophosphorolysis and nucleotide addition to occur. alpha-Amanitin dramatically decreased the Vmax for nucleotide addition but only slightly affected the Km for nucleotides. Although the processes ae mechanistically distinct, both pyrophosphorolysis and DmS-II-mediated transcript cleavage frequently resulted in similar patterns of shortened transcript. Since polymerase molecules encounter similar kinetic barriers during both processes, it is possible that there is a common step in the reverse movement of the polymerase.

Transcription of the human T-cell lymphotropic virus type I promoter by an alpha-amanitin-resistant polymerase

The human T-lymphotropic virus type I (HTLV-I) promoter contains the structural features of a typical RNA polymerase II (pol II) template. The promoter contains a TATA box 30 bp upstream of the transcription initiation site and binding sites for several pol II transcription factors, and long poly(A)+ RNA is synthesized from the integrated HTLV-I proviral DNA in vivo. Consistent with these characteristics, HTLV-I transcription activity was reconstituted in vitro by using TATA-binding protein, TFIIA, recombinant TFIIB, TFIIE, and TFIIF, TFIIH, and pol II. Transcription of the HTLV-I promoter in the reconstituted system requires RNA pol II. In HeLa whole cell extracts, however, the HTLV-I long terminal repeat also contains an overlapping transcription unit (OTU). HTLV-I OTU transcription is initiated at the same nucleotide site as the RNA isolated from the HTLV-I-infected cell line MT-2 but was not inhibited by the presence of alpha-amanitin at concentrations which inhibited the adenovirus major late pol II promoter (6 micrograms/ml). HTLV-I transcription was inhibited when higher concentrations of alpha-amanitin (60 micrograms/ml) were used, in the range of a typical pol III promoter (VA-I). Neutralization and depletion experiments with three distinct pol II antibodies demonstrate that RNA pol II is not required for HTLV-I OTU transcription. Antibodies to basal transcription factors TATA-binding protein and TFIIB, but not TFIIIC, inhibited HTLV-I OTU transcription. These observations suggest that the HTLV-I long terminal repeat contains overlapping promoters, a typical pol II promoter and a unique pol III promoter which requires a distinct set of transcription factors.

Clinical symptomatology and management of mushroom poisoning

Among poisonous mushrooms, a small number may cause serious intoxication and even fatalities in man. Humans may become symptomatic after a mushroom meal for rather different reasons: (1) ingestion of mushrooms containing toxins, (2) large amounts of mushrooms may be hard to digest, (3) immunological reactions to mushroom-derived antigens, (4) ingestion of mushrooms causing ethanol intolerance, and (5) vegetative symptoms may occur whenever a patient realizes that there might be a possibility of ingestion of a toxic mushroom after a mushroom meal. Based on the classes of toxins and their clinical symptoms, seven different types of mushroom poisoning can be distinguished: (1) phalloides, (2) orellanus, (3) gyromitra, (4) muscarine, (5) pantherina, (6) psilocybin, and (7) gastrointestinal mushroom syndrome. Two other entities of adverse reactions to mushrooms are (8) coprinus and (9) paxillus syndrome. Phalloides, orellanus, gyromitra and paxillus syndrome may lead to serious poisoning, which generally requires treatment of the patient in an intensive care unit. Diagnosis of mushroom poisoning is primarily based on anamnestic data, identification of mushrooms from leftovers of the mushroom meal, spore analysis, and/or chemical analysis. Therapeutic strategies include primary detoxification by induced emesis, gastric lavage and activated charcoal, secondary detoxification, symptomatic treatment and rarely specific antidotes. Owing to progressing fulminant hepatic failure, lethality associated with phalloides syndrome is still high (5-20%). Basic treatment includes administration of silibinin and penicillin G, although controlled studies on its therapeutic efficacy are still lacking. In serious phalloides syndrome, orthotopic liver transplantation has to be considered. Fortunately, the prognosis in most other mushroom poisonings is excellent.

Localization of an alpha-amanitin resistance mutation in the gene encoding the largest subunit of mouse RNA polymerase II

RNA polymerase II is inhibited by the mushroom toxin alpha-amanitin. A mouse BALB/c 3T3 cell line was selected for resistance to alpha-amanitin and characterized in detail. This cell line, designated A21, was heterozygous, possessing both amanitin-sensitive and -resistant forms of RNA polymerase II; the mutant form was 500 times more resistant to alpha-amanitin than the sensitive form. By using the wild-type mouse RNA polymerase II largest subunit (RPII215) gene (J.A. Ahearn, M.S. Bartolomei, M. L. West, and J. L. Corden, submitted for publication) as the probe, RPII215 genes were isolated from an A21 genomic DNA library. The mutant allele was identified by its ability to transfer amanitin resistance in a transfection assay. Genomic reconstructions between mutant and wild-type alleles localized the mutation to a 450-base-pair fragment that included parts of exons 14 and 15. This fragment was sequenced and compared with the wild-type sequence; a single AT-to-GC transition was detected at nucleotide 6819, corresponding to an asparagine-to-aspartate substitution at amino acid 793 of the predicted protein sequence. Knowledge of the position of the A21 mutation should facilitate the study of the mechanism of alpha-amanitin resistance. Furthermore, the A21 gene will be useful for studying the phenotype of site-directed mutations in the RPII215 gene.

Localization of an alpha-amanitin resistance mutation in the gene encoding the largest subunit of mouse RNA polymerase II

RNA polymerase II is inhibited by the mushroom toxin alpha-amanitin. A mouse BALB/c 3T3 cell line was selected for resistance to alpha-amanitin and characterized in detail. This cell line, designated A21, was heterozygous, possessing both amanitin-sensitive and -resistant forms of RNA polymerase II; the mutant form was 500 times more resistant to alpha-amanitin than the sensitive form. By using the wild-type mouse RNA polymerase II largest subunit (RPII215) gene (J.A. Ahearn, M.S. Bartolomei, M. L. West, and J. L. Corden, submitted for publication) as the probe, RPII215 genes were isolated from an A21 genomic DNA library. The mutant allele was identified by its ability to transfer amanitin resistance in a transfection assay. Genomic reconstructions between mutant and wild-type alleles localized the mutation to a 450-base-pair fragment that included parts of exons 14 and 15. This fragment was sequenced and compared with the wild-type sequence; a single AT-to-GC transition was detected at nucleotide 6819, corresponding to an asparagine-to-aspartate substitution at amino acid 793 of the predicted protein sequence. Knowledge of the position of the A21 mutation should facilitate the study of the mechanism of alpha-amanitin resistance. Furthermore, the A21 gene will be useful for studying the phenotype of site-directed mutations in the RPII215 gene.

In vitro and in vivo analysis of the control of dihydrofolate reductase gene transcription in serum-stimulated mouse fibroblasts

We have studied the rate of transcription of the gene for dihydrofolate reductase (DHFR) in mouse 3T6 fibroblasts during serum-induced transitions between the resting (G0) and growing states. As a model system, we have used a methotrexate-resistant 3T6 cell line that overproduces DHFR and its mRNA about 300-fold, yet regulates the expression of the DHFR gene in the same manner as normal 3T6 cells. In previous studies, we showed that the rate of production of cytoplasmic DHFR mRNA relative to total mRNA is about 4 times lower in resting than in exponentially growing cells. The rate increases to the growing value by about 15 hr following serum stimulation of the resting cells. This increase appeared to be controlled by regulating the rate of synthesis of DHFR hnRNA. In this study, we analyze the transcription of the DHFR gene in more detail. We use a variety of labeling times and RNA extraction procedures to measure the rate of synthesis of DHFR hnRNA relative to total hnRNA in pulse-labeled cells or in nuclei isolated from cells at various times following serum stimulation. The amount of labeled DHFR RNA is determined by DNA-excess filter hybridization. In all cases, the relative rate of synthesis of DHFR hnRNA increases at the same time, and to the same extent, as the rate of production of DHFR mRNA, suggesting that the increase in DHFR mRNA production is due to a corresponding increase in the rate of transcription of the DHFR gene. The increase in DHFR gene transcription is not blocked by cytosine arabinoside, showing that the increase does not depend on gene duplication. In isolated nuclei, DHFR RNA synthesis is inhibited by alpha-amanitin (1 microgram/ml), indicating that the DHFR gene is transcribed by RNA polymerase II. Others have shown that when stationary phase cells are stimulated to proliferate, the increase in DHFR mRNA content is controlled primarily at the post-transcriptional level. Therefore, it appears that the rate of production of DHFR mRNA is controlled by different biochemical mechanisms when cells are in different physiological states.

alpha-Amanitin and 5-fluorouridine inhibition of serum-stimulated DNA synthesis in quiescent AKR-2B mouse embryo cells

AKR-2B mouse embryo cells undergoing the serum-stimulated transition from a quiescent to a proliferating state exhibit an increase in the rate of hnRNA synthesis which appears to be mediated through an increase in the actual number of RNA polymerase II molecules. alpha-Amanitin, administered early in the prereplication interval following stimulation, effectively inhibits hnRNA synthesis, polysomal mRNA accumulation, polyribosome formation, and subsequent DNA synthesis, and cell division. Unexpectedly, alpha-amanitin treatment also produces almost complete inhibition of the synthesis of 45S rRNA precursor and the increase in accumulation of cytoplasmic rRNA following serum stimulation. In order to determine whether the inhibition of new ribosomal synthesis might in itself be sufficient to prevent serum-stimulated DNA synthesis, the effects of 5-fluorouridine (5-FU), a specific inhibitor of 45S rRNA processing, were investigated. If added within eight hours following serum stimulation, 5-FU was found to completely inhibit subsequent DNA synthesis. These results suggest that quiescent AKR-2B cells do not contain a sufficient excess of ribosomes to support the synthesis of proteins which are required for DNA synthesis in response to serum growth factors. Furthermore, an early polymerase II mediated synthesis of mRNA(s) coding for some factor(s) necessary for ribosomal gene transcription may be an essential step in the serum-stimulated synthesis of new ribosomes.

Amatoxins, phallotoxins, phallolysin, and antamanide: the biologically active components of poisonous Amanita mushrooms

This review gives a comprehensive account of the molecular toxicology of the bicyclic peptides obtained from the poisonous mushrooms of the genus Amanita. The discussion of the biochemical events will be preceded by a consideration of the chemistry of the toxic peptides. The structural features essential for biological activities of both the amatoxins and the phallotoxins will be discussed, also including the most important analytical data. Similar consideration will be given to antamanide, a cyclic peptide, which counteracts phalloidin. In addition, the phallolysins, three cytolytic proteins from Amanita phalloides will be discussed. The report on the biological activity of the amatoxins will deal with the sensitivity of the different RNA-polymerases towards the toxins and with their action on various cell types. Consideration will also be given to systems in which alpha-amanitin was used and can be used as a molecular tool; in the past, many investigators used the inhibitor in molecular biology, genetics, and even in physiological research. As for the phallotoxins, discussion of the affinity of these toxins for actin is provied. Further discussion attempts to understand the course of intoxication by filling in the gap between the first molecular event, formation of microfilaments, and the various lesions in hepatocytes during the intoxication.

[Effect of silymarin on the total dry mass of hepatocytes inacute poisoning by phalloidin and alpha-amanitine (author's transl)]

The effects of silymarin on the total dry mass and class pattern of rat hepatocytes have been studied during acute poisoning by phalloidin and alpha-amanitine. Phalloidin (2/5 of the LD50) after 3 h causes a marked change in the hepatocyte class pattern due to a displacement of a high percentage of cells in the intervals among classes, while the cell dry mass increases slightly. alpha-Amanitine (1/4 or 1/2 of the LD50) after 3 h causes a decrease in the number of classes of hepatocytes due to a disappearance of the heavier ones, a displacement of cells in the intervals among classes, an appearance of very light cells, and a decrease by about 25% in the mean dry mass of the hepatocytes. Silymarin, administered 30 min before poisoning, prevents all the changes due to 2/5 of the LD50 of phalloidin and to 1/4 of the LD50 of alpha-amanitine, and strongly reduces the effects of 1/2 of the LD50 of alpha-amanitine. The effects of alpha-amanitine and phalloidin and the protective action of silymarin on the dry mass and class pattern of hepatocytes are discussed.

Nuclear poly(A) polymerase from rat liver and a hepatoma. Comparison of properties, molecular weights and amino acid compositions

Poly(A) polymerase was extracted from isolated nuclei of rat liver and a rapidly growing solid tumor (Morris hepatoma 3924A). The enzyme from each tissue was purified by successive chromatography on DEAE-Sephadex, phosphoecllulose, hydroxyapatite and QAE-Sephadex. Purified enzyme from both liver and tumor was essentially homogeneous as judged by polyacrylamide gel electrophoresis. Under nondenaturing conditions, enzyme activity corresponded to visible protein and, upon denaturation, a single polypeptide was detected. The enzymes had absolute requirements for Mn2+ as the divalent ion, ATP as the substrate and an oligonucleotide or polynucleotide as the primer. Both enzymes were inhibited by sodium pyrophosphate, N-ethylmaleimide, Rose Bengal, cordycepin 5'-triphosphate and several rifamycin derivatives. The reactions were unaffected by potassium phosphate, alpha-amanitin and pancreatic ribonuclease. However, the liver and hepatoma enzymes differed from each other with respect to apparent Km, primer saturation levels and sensitivity to pH changes. The most striking differences between the enzymes were in their calculated molecular weights (liver, 48000; hepatoma, 60000) and amino acid compositions. Finally, the level of the hepatoma enzyme relative to that of the liver enzyme was at least 1.5-fold higher when expressed per mg DNA.

In vitro transcription by isolated nuclei of Rhynchosciara americana salivary glands. Characteristics of incorporation and inhibition by alpha-amanitin

A method for the isolation of polytene nuclei from salivary glands cells of the Diptera Rhynchosciara americana is described. The stage-specific morphological pattern of the chromosome is maintained during the isolation. The isolated nuclei show two distinct RNA polymerase activities, namely I and II, characterized on the basis of ionic requirements and alpha-amanitin sensitivity. Studies of the product under the incubation conditions show that the system allows the synthesis of high-molecular weight RNA, beside a low molecular weight peak which may comprise pre-4S and 5S RNAs. Autoradiographic studies carried out in the presence or absence of the toxin alpha-amanitin showed that micronucleoli contain products of RNA polymerase type I activity (ribosomal RNA) and that the DNA puffs are engaged in alpha-amanitin sensitive RNA synthesis and thus are sites of polymerase type II activity.

Growth hormone and drug metabolism. Acute effects on nuclear ribonucleic acid polymerase activity and chromatin

Adult male rats, subjected either to sham operation or to hypophysectomy and adrenalectomy were maintained for 10 days before treatment with growth hormone. Results of the acute effects of growth hormone on the rat liver nuclear RNA polymerase I (nucleolar) and II (nucleoplasmic) activities as well as the chromatin template capacity were then studied and compared with the growth-hormone effects on the drug metabolism described in the preceding paper (Wilson & Spelsberg, 1976). 2. Conditions for isolation and storage of nuclei for maintenance of optimal polymerase activities are described. It is verified that the assays for polymerase activities require a DNA template, all four nucleoside triphosphates, and a bivalent cation, and that the acid-insoluble radioactive product represents RNA. Proof is presented that under high-salt conditions DNA-like RNA (polymerase II) is synthesized, and that under low-salt conditions in the presence of alpha-amanitin, rRNA (polymerase I) is synthesized. 3. In the livers of hypophysectomized/adrenalectomized rats, growth hormone increases the activity of both RNA polymerase enzymes and the chromatin template capacity within 1h after treatment. The effects last for 12h in the case of polymerase II but for only 6h in the case of polymerase I. Sham-operated rats respond to growth hormone in a manner somewhat similar to that shown by hypophysectomized/adrenalectomized rats. These results, which demonstrate an enhancement of RNA polymerase I activity in response to growth hormone, support those from other laboratories. 4. Growth-hormone enhancement of the chromatin template capacity in the liver of hypophysectomized/adrenalectomized rats contrasts with previous reports. The growth-hormone-induced de-repression of the chromatin DNA could represent the basis of the growth-hormone-induced enhancement of RNA polymerase II activity in the hypophysectomized/adrenalectomized rats, although some effect of growth-hormone on the polymerase enzymes is still suggested.

Mitochondrial poly(A) polymerase from a poorly differentiated hepatoma: purification and characteristics

Poly(A) polymerase (EC 2.7.7.19) solubilized from mitochondria of a poorly differentiated rat tumor, Morris hepatoma 3924A, was purified more than 1000-fold by successive column chromatography on phosphocellulose, DEAE-Sephadex, and hydroxylapatite. Purified enzyme catalyzed the incorporation of ATP into poly(A) only upon addition of an exogenous primer. Of several primers tested, synthetic poly(A) was the most effective. The enzyme utilized mitochondrial RNA as a primer at least five times as efficiently as nuclear RNA. The enzyme required Mn2+, and had a pH optimum of 7.8-8.2. The enzyme utilized ATP exclusively as a substrate; the calculated K-m for ATP was 28 muM. The polymerization reaction was not inhibited by RNase, ethidium bromide, distamycin, or alpha-amanitin. The reaction was sensitive to O-n-octyloxime of 3-formylrifamycin SV (AF/013). As estimated from glycerol gradient centrifugation and acrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, the molecular weight of the enzyme was 60,000. The product was covalently linked to the polynucleotide primer and the average length of the poly(A) formed was 600 nucleotides.

Purification and subunit analysis of wheat-germ ribonucleic acid polymerase II

A procedure is described for the purification of the alpha-amanitin-sensitive DNA-dependent RNA polymerase [EC 2.7.7.6] from wheat germ. Solubilization of the enzyme activity was achieved by sonication of a crude extract in a high-salt buffer. Purification involved precipitation with protamine sulphate and (NH(4))(2)SO(4), chromatography on DEAE-cellulose and phosphocellulose, and sucrose gradient centrifugation. Under denaturing conditions the enzyme dissociated into five polypeptides with molecular weights and molar ratios of 220000 (0.9), 170000 (0.1), 140000 (1.0), 45000 (0.2), and 40000 (0.4). Approx. 1mg of purified RNA polymerase was obtained as a routine from 100g of starting material.

Role of RNA in induction of hepatic microsomal mixed function oxidases

Induction of hepatic microsomal cytochrome P-450 and ethylmorphine N-demethylase activity by phenobarbital requires de novo synthesis of mRNA. Inhibition of RNA synthesis by alpha-amanitin given up to 8 hr after phenobarbital administration substantially inhibits this induction. However, beyond 8 hr after phenobarbital administration, RNA synthesis is not required for induction of these hepatic microsomal systems. Thus, mRNAs for cytochrome P-450 and ethylmorphine N-demethylase appear to be stable. Furthermore, these experiments reveal that the lag period for RNA synthesis approximates the length of the lag period for induction of the hepatic microsomal enzyme systems.