Mechanism of action of 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. II. A resistant human cell mutant with an altered transcriptional machinery

Drug-induced permeabilization of S. cerevisiae

Budding yeast are not permeable to many drugs. This unit provides a protocol in which polygodial is used to permeabilize the cell membrane, thereby allowing budding yeast cells to be treated with drugs that otherwise would be ineffective.

Adaptation in the chemotactic guidance of nerve growth cones

Pathfinding by growing axons in the developing nervous system may be guided by gradients of extracellular guidance factors. Analogous to the process of chemotaxis in microorganisms, we found that axonal growth cones of cultured Xenopus spinal neurons exhibit adaptation during chemotactic migration, undergoing consecutive phases of desensitization and resensitization in the presence of increasing basal concentrations of the guidance factor netrin-1 or brain-derived neurotrophic factor. The desensitization is specific to the guidance factor and is accompanied by a reduction of Ca2+ signalling, whereas resensitization requires activation of mitogen-associated protein kinase and local protein synthesis. Such adaptive behaviour allows the growth cone to re-adjust its sensitivity over a wide range of concentrations of the guidance factor, an essential feature for long-range chemotaxis.

TBP dynamics in living human cells: constitutive association of TBP with mitotic chromosomes

The recruitment of TATA binding protein (TBP) to gene promoters is a critical rate-limiting step in transcriptional regulation for all three eukaryotic RNA polymerases. However, little is known regarding the dynamics of TBP in live mammalian cells. In this report, we examined the distribution and dynamic behavior of green fluorescence protein (GFP)-tagged TBP in live HeLa cells using fluorescence recovery after photobleaching (FRAP) analyses. We observed that GFP-TBP associates with condensed chromosomes throughout mitosis without any FRAP. These results suggest that TBP stably associates with the condensed chromosomes during mitosis. In addition, endogenous TBP and TBP-associated factors (TAFs), specific for RNA polymerase II and III transcription, cofractionated with mitotic chromatin, suggesting that TBP is retained as a TBP-TAF complex on transcriptionally silent chromatin throughout mitosis. In interphase cells, GFP-TBP distributes throughout the nucleoplasm and shows a FRAP that is 100-fold slower than the general transcription factor GFP-TFIIB. This difference supports the idea that TBP and, most likely, TBP-TAF complexes, remain promoter- bound for multiple rounds of transcription. Altogether, our observations demonstrate that there are cell cycle specific characteristics in the dynamic behavior of TBP. We propose a novel model in which the association of TBP-TAF complexes with chromatin during mitosis marks genes for rapid transcriptional activation as cells emerge from mitosis.

Cell signaling by the type IV pili of pathogenic Neisseria

Neisseria gonorrhoeae and Neisseria meningitidis are Gram-negative bacterial pathogens that infect human mucosal epithelia. Type IV pilus-mediated adherence of these bacteria is a crucial early event for establishment of infection. In this work, we show that the type IV pili transduce a signal into the eucaryotic host cell. Purified adherent pili, but not pili from a low binding mutant, trigger an increase in the cytosolic free calcium ([Ca2+]i) in target epithelial cells, a signal known to control many cellular responses. The [Ca2+]i increase was blocked by antibodies against CD46, a putative pilus receptor, suggesting a role for this protein in signal transduction. Pilus-mediated attachment was inhibited by depletion of host cell intracellular Ca2+ stores but not by removal of extracellular Ca2+. Further, kinase inhibition studies showed that pilus-mediated adherence is dependent on casein kinase II. In summary, these data reveal a novel function of the type IV pili, namely induction of signal transduction pathways in host cells.

In vivo degradation of RNA polymerase II largest subunit triggered by alpha-amanitin

Alpha-Amanitin is a well-known specific inhibitor of RNA polymerase II (RNAPII) in vitro and in vivo. It is a cyclic octapeptide which binds with high affinity to the largest subunit of RNAPII, RPB1. We have found that in murine fibroblasts exposure to alpha-amanitin triggered degradation of the RPB1 subunit, while other RNAPII subunits, RPB5 and RPB8, remained almost unaffected. Transcriptional inhibition in alpha-amanitin-treated cells was slow and closely followed the disappearance of RPB1. The degradation rate of RPB1 was alpha-amanitin dose dependent and was not a consequence of transcriptional arrest. Alpha-Amanitin-promoted degradation of RPB1 was prevented in cells exposed to actinomycin D, another transcriptional inhibitor. Epitope-tagged recombinant human RPB1 subunits were expressed in mouse fibroblasts. In cells exposed to alpha-amanitin the wild-type recombinant subunit was degraded like the endogenous protein, but a mutated alpha-amanitin-resistant subunit remained unaffected. Hence, alpha-amanitin did not activate a proteolytic system, but instead its binding to mRPB1 likely represented a signal for degradation. Thus, in contrast to other inhibitors, such as actinomycin D or 5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole, which reversibly act on transcription, inhibition by alpha-amanitin cannot be but an irreversible process because of the destruction of RNAPII.

Steady-state and nuclear run-on analyses of transcription in a temperature-sensitive Chinese hamster cell mutant with a defect in RNA metabolism

We have further characterized a temperature-sensitive mutant of Chinese hamster lung fibroblasts in tissue culture with a defect in RNA metabolism. The mutant phenotype is reflected in transcription in crude extracts or in isolated nuclei, when these are made from cells shifted to the nonpermissive temperature; however, differential heat inactivation between mutant and wild-type extracts cannot be demonstrated with cell-free systems. We tentatively conclude that the mutation may affect initiation of transcription which cannot be observed in our in vitro systems. Partially purified RNA polymerase I, II, and III fractions are indistinguishable from wild type. A temperature shift does not affect transcription by RNA polymerase III measured with intact cells or by nuclear run-on experiments. The nuclear run-on and other experiments suggest that RNA polymerase II-dependent transcription is inhibited before RNA polymerase I-dependent transcription. This conclusion is also supported by Northern analyses of selected mRNAs in nonsynchronized and synchronized cells after a shift to the nonpermissive temperature.

Temperature-sensitive Chinese hamster cell mutant with a defect in glycoprotein synthesis: accumulation of the EGF receptor in the endoplasmic reticulum and the role of the glucose-regulated protein GRP78

A temperature-sensitive mutant of Chinese hamster fibroblasts with a defect in glycoprotein synthesis is investigated after transfection and amplification of the gene for the human EGF receptor. We demonstrate that at the nonpermissive temperature a partially glycosylated species of the receptor accumulates in the endoplasmic reticulum. The oligosaccharides present are the high mannose types, since they can be removed completely by treatment with endoglycosidase H. Pulse-chase experiments show that the abnormal species of the receptor cannot be chased to a form that is either resistant to endoglycosidase H, or altered in its mobility on SDS polyacrylamide gels. The abnormal species of the receptor appears within the first hour of a shift to the nonpermissive temperature, and no further changes are observed upon prolonged incubation of cells at 40 degrees C. However, after 3-4 hours immunoprecipitations of the receptor yield another protein, which has properties very similar, if not identical, to the glucose-regulated protein GRP78. The induction of this protein at 40 degrees C can be suppressed completely with an inhibitor of RNA synthesis, without any effect on the glycosylation defect, or on the accumulation of the EGF receptor in the endoplasmic reticulum.

X-rays mutate human lymphoblast cells at genetic loci that should respond only to point mutagens

We have demonstrated that X-rays induce mutations at 4 of 5 genetic loci. 2 of these loci, which code for a mRNA synthesis factor (resistance to 5,6-dichlororibofuranosylbenzimidazole) and tubulin (resistance to podophyllotoxin), are "small-marker" loci, in that they theoretically respond only to mutations which eliminate a toxin-binding site while leaving the major function of the protein intact. Thus mutations induced by X-rays in these two loci are most likely due to base-pair substitution-type alterations. X-Rays did not induce mutations in the Na+/K+ ATPase (resistance to ouabain), another small-marker locus. Two other loci, hypoxanthine guanine phosphoribosyl transferase (resistance to 6-thioguanine) and thymidine kinase (resistance to trifluorothymidine), are "whole-gene" targets in that they theoretically respond to a wide variety of mutagenic changes. X-Rays induced dose-dependent increases in mutant fraction at both of these loci. Ethyl methanesulfonate (EMS), an agent thought to produce mutations primarily through a base-pair substitution mechanism, induced mutations at all genetic loci tested. The pattern of mutations at the small-marker loci induced by EMS was different than that induced by X-rays, suggesting that the specificities of the mutagens and/or of the loci are different.

Physical change in cytoplasmic messenger ribonucleoproteins in cells treated with inhibitors of mRNA transcription

Exposure of intact cells to UV light brings about cross-linking of polyadenylated mRNA to a set of cytoplasmic proteins which are in direct contact with the mRNA in vivo. Substantial amounts of an additional protein of molecular weight 38,000 (38K) become cross-linked to the mRNA when cells are treated with inhibitors of mRNA synthesis (actinomycin D, camptothecin, and 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole) or after infection with vesicular stomatitis virus. Cordycepin, which inhibits polyadenylation but not mRNA synthesis, has no such effect. Inhibitors of protein synthesis and of rRNA synthesis are also without effect on 38K cross-linking to mRNA. The onset of the effect of inhibitors of mRNA synthesis on the UV cross-linkable interaction between mRNA and 38K is rapid and reaches a maximal level in less than 60 min, and it is completely and rapidly reversible. In cells treated with actinomycin D, the amount of 38K which becomes cross-linked to mRNA is proportional to the extent of inhibition of mRNA synthesis. The association of 38K with mRNA during transcriptional arrest does not require protein synthesis because simultaneous treatment with the protein synthesis inhibitor emetine does not interfere with it. The effectors which promote the interaction of 38K with mRNA do not affect the proteins which are in contact with polyadenylated heterogeneous nuclear RNA and do not markedly affect protein synthesis in the cell. The 38K protein can be isolated with the polyribosomal polyadenylated fraction from which it was purified, and monoclonal antibodies against it were prepared. Immunofluorescence microscopy shows mostly cytoplasmic and some nuclear staining. These observations demonstrate that commonly used inhibitors of transcription affect the physical state of messenger ribonucleoproteins in vivo.

Physical change in cytoplasmic messenger ribonucleoproteins in cells treated with inhibitors of mRNA transcription

Exposure of intact cells to UV light brings about cross-linking of polyadenylated mRNA to a set of cytoplasmic proteins which are in direct contact with the mRNA in vivo. Substantial amounts of an additional protein of molecular weight 38,000 (38K) become cross-linked to the mRNA when cells are treated with inhibitors of mRNA synthesis (actinomycin D, camptothecin, and 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole) or after infection with vesicular stomatitis virus. Cordycepin, which inhibits polyadenylation but not mRNA synthesis, has no such effect. Inhibitors of protein synthesis and of rRNA synthesis are also without effect on 38K cross-linking to mRNA. The onset of the effect of inhibitors of mRNA synthesis on the UV cross-linkable interaction between mRNA and 38K is rapid and reaches a maximal level in less than 60 min, and it is completely and rapidly reversible. In cells treated with actinomycin D, the amount of 38K which becomes cross-linked to mRNA is proportional to the extent of inhibition of mRNA synthesis. The association of 38K with mRNA during transcriptional arrest does not require protein synthesis because simultaneous treatment with the protein synthesis inhibitor emetine does not interfere with it. The effectors which promote the interaction of 38K with mRNA do not affect the proteins which are in contact with polyadenylated heterogeneous nuclear RNA and do not markedly affect protein synthesis in the cell. The 38K protein can be isolated with the polyribosomal polyadenylated fraction from which it was purified, and monoclonal antibodies against it were prepared. Immunofluorescence microscopy shows mostly cytoplasmic and some nuclear staining. These observations demonstrate that commonly used inhibitors of transcription affect the physical state of messenger ribonucleoproteins in vivo.

Mechanism of action of DRB. III. Effect on specific in vitro initiation of transcription

5,6-Dichloro-1-beta-D-ribofuranosylbenzimidazole, an adenosine analogue, has been used previously as an inhibitor of heterogeneous nuclear and messenger RNA synthesis. In an in vitro transcriptional system, we have detected inhibition of synthesis of full-length runoff RNAs at concentrations at which in vivo mRNA synthesis is inhibited. By hybridization of RNA synthesized in vitro to single-stranded DNA and gel analysis, we were able to reduce the background of the transcription reaction, detect DRB-induced inhibition of full-length runoff RNAs and DRB-insensitive transcription of short RNAs. To establish further the effect of DRB on initiation of transcription, preincubation experiments with template, whole cell extract and two initial nucleotides of the transcript were performed. Elongation was then measured as discrete-sized RNAs transcribed from the truncated template after addition of the other triphosphates (one of them labeled), in the presence or absence of DRB. An effect on initiation but not on elongation or termination was detected. Fingerprint analysis of these runoff RNAs indicates that the labeling of U in the presence of DRB is uniform throughout the molecule. A model to explain a novel interpretation of the action of DRB is presented.