Glucocorticoid inhibition of initiation of transcription of the DNA encoding rRNA (rDNA) in lymphosarcoma P1798 cells

DNA binding by the ribosomal DNA transcription factor rrn3 is essential for ribosomal DNA transcription

The human homologue of yeast Rrn3 is an RNA polymerase I-associated transcription factor that is essential for ribosomal DNA (rDNA) transcription. The generally accepted model is that Rrn3 functions as a bridge between RNA polymerase I and the transcription factors bound to the committed template. In this model Rrn3 would mediate an interaction between the mammalian Rrn3-polymerase I complex and SL1, the rDNA transcription factor that binds to the core promoter element of the rDNA. In the course of studying the role of Rrn3 in recruitment, we found that Rrn3 was in fact a DNA-binding protein. Analysis of the sequence of Rrn3 identified a domain with sequence similarity to the DNA binding domain of heat shock transcription factor 2. Randomization, or deletion, of the amino acids in this region in Rrn3, amino acids 382-400, abrogated its ability to bind DNA, indicating that this domain was an important contributor to DNA binding by Rrn3. Control experiments demonstrated that these mutant Rrn3 constructs were capable of interacting with both rpa43 and SL1, two other activities demonstrated to be essential for Rrn3 function. However, neither of these Rrn3 mutants was capable of functioning in transcription in vitro. Moreover, although wild-type human Rrn3 complemented a yeast rrn3-ts mutant, the DNA-binding site mutant did not. These results demonstrate that DNA binding by Rrn3 is essential for transcription by RNA polymerase I.

Mammalian Rrn3 is required for the formation of a transcription competent preinitiation complex containing RNA polymerase I

Mammalian Rrn3, an essential, polymerase-associated protein, is inactivated when cells are treated with cycloheximide, resulting in the inhibition of transcription by RNA polymerase I. Although Rrn3 is essential for transcription, its function in rDNA transcription has not been determined. For example, it is unclear whether Rrn3 is required for initiation or elongation by RNA polymerase I. Rrn3 has been shown to interact with the 43-kDa subunit of RNA polymerase I and with two of the subunits of SL1. In the current model for transcription, Rrn3 functions to recruit RNA polymerase I to the committed complex formed by SL1 and the rDNA promoter. To examine the question as to whether Rrn3 is required for the recruitment of RNA polymerase I to the template, we developed a novel assay similar to chromatin immunoprecipitation assays. We found that RNA polymerase I can be recruited to a template in the absence of active Rrn3. However, that complex will not initiate transcription, even after Rrn3 is added to the reaction. Interestingly, the complex that forms in the presence of active Rrn3 is biochemically distinguishable from that which forms in the absence of active Rrn3. For example, the functional complex is fivefold more resistant to heparin than that which forms in the absence of Rrn3. Our data demonstrate that Rrn3 must be present when the committed template complex is forming for transcription to occur.

Rrn3 becomes inactivated in the process of ribosomal DNA transcription

The human homologue of yeast Rrn3, a 72-kDa protein, is essential for ribosomal DNA (rDNA) transcription. Although the importance of Rrn3 function in rDNA transcription is well established, its mechanism of action has not been determined. It has been suggested that the phosphorylation of either yeast RNA polymerase I or mammalian Rrn3 regulates the formation of RNA polymerase I.Rrn3 complexes that can interact with the committed template. These and other reported differences would have implications with respect to the mechanism by which Rrn3 functions in transcription. For example, in the yeast rDNA transcription system, Rrn3 might function catalytically, but in the mammalian system it might function stoichiometrically. Thus, we examined the question as to whether Rrn3 functions catalytically or stoichiometrically. We report that mammalian Rrn3 becomes the limiting factor as transcription reactions proceed. Moreover, we demonstrate that Rrn3 is inactivated during the transcription reactions. For example, Rrn3 isolated from a reaction that had undergone transcription cannot activate transcription in a subsequent reaction. We also show that this inactivated Rrn3 not only dissociates from RNA polymerase I, but is not capable of forming a stable complex with RNA polymerase I. Our results indicate that Rrn3 functions stoichiometrically in rDNA transcription and that its ability to associate with RNA polymerase I is lost upon transcription.

Rrn3 phosphorylation is a regulatory checkpoint for ribosome biogenesis

Cycloheximide inhibits ribosomal DNA (rDNA) transcription in vivo. The mouse homologue of yeast Rrn3, a polymerase-associated transcription initiation factor, can complement extracts from cycloheximide-treated mammalian cells. Cycloheximide inhibits the phosphorylation of Rrn3 and causes its dissociation from RNA polymerase I. Rrn3 interacts with the rpa43 subunit of RNA polymerase I, and treatment with cycloheximide inhibits the formation of a Rrn3.rpa43 complex in vivo. Rrn3 produced in Sf9 cells but not in bacteria interacts with rpa43 in vitro, and such interaction is dependent upon the phosphorylation state of Rrn3. Significantly, neither dephosphorylated Rrn3 nor Rrn3 produced in Escherichia coli can restore transcription by extracts from cycloheximide-treated cells. These results suggest that the phosphorylation state of Rrn3 regulates rDNA transcription by determining the steady-state concentration of the Rrn3.RNA polymerase I complex within the nucleolus.

RNA polymerase I transcription in confluent cells: Rb downregulates rDNA transcription during confluence-induced cell cycle arrest

When 3T6 cells are confluent, they withdraw from the cell cycle. Concomitant with cell cycle arrest a significant reduction in RNA polymerase I transcription (80% decrease at 100% confluence) is observed. In the present study, we examined mechanism(s) through which transcription of the ribosomal genes is coupled to cell cycle arrest induced by cell density. Interestingly with an increase in cell density (from 3 - 43% confluence), a significant accumulation in the cellular content of hyperphosphorylated Rb was observed. As cell density increased further, the hypophosphorylated form of Rb became predominant and accumulated in the nucleoli. Co-immunoprecipitation experiments demonstrated there was also a significant rise in the amount of hypophosphorylated Rb associated with the rDNA transcription factor UBF. This increased interaction between Rb and UBF correlated with the reduced rate of rDNA transcription. Furthermore, overexpression of recombinant Rb inhibited UBF-dependent activation of transcription from a cotransfected rDNA reporter in either confluent or exponential cells. The amounts or activities of the rDNA transcription components we examined did not significantly change with cell cycle arrest. Although the content of PAF53, a polymerase associated factor, was altered marginally (decreased 38%), the time course and magnitude of the decrease did not correlate with the reduced rate of rDNA transcription. The results presented support a model wherein regulation of the binding of UBF to Rb and, perhaps the cellular content of PAF53, are components of the mechanism through which cell cycle and rDNA transcription are linked. Oncogene (2000) 19, 3487 - 3497

Heat shock selectively inhibits ribosomal RNA gene transcription and down-regulates E1BF/Ku in mouse lymphosarcoma cells

The effect of heat shock on RNA polymerase I (pol I)-directed transcription of the rRNA gene was studied in S-100 extract derived from mouse lymphosarcoma cells, and by in vivo labelling of rRNA. Exposure of cells to 42 degrees C for 2 h resulted in complete inhibition of rRNA synthesis in vivo. Pol I transcription was inhibited by 50% within 2 h of heat shock and was abolished after 3 h exposure at 42 degrees C. Under this condition, the core-promoter-binding activity of the factor (CPBF) that modulates pol I transcription was unaffected. In contrast, the promoter-binding activity of enhancer-1-binding factor, a protein related to the Ku autoantigen, which is involved in pol I transcription initiation, was reduced by 50 and 90% after 2 and 3 h of heat shock respectively. Western-blot analysis with antibodies specific for the two subunits of Ku protein showed the absence of p72 subunit after 3 h of heat shock. Under this condition, pol II transcription from the adenovirus major late promoter and pol III transcription of 5 S RNA gene remained unaffected. Mixing experiments ruled out the possibility that the inhibition of transcription was due to activation of nucleases or other inhibitors. This is the first report to show selective down-regulation of pol I transcription in vitro by heat shock and of the potential involvement of a pol I transcription factor in this process.

Enhancer 1 binding factor (E1BF), a Ku-related protein, is a growth-regulated RNA polymerase I transcription factor: association of a repressor activity with purified E1BF from serum-deprived cells

Previous studies from this laboratory have demonstrated that the enhancer 1 binding factor (E1BF), a Ku-related protein, purified from the serum-enriched cells functions as a positive factor in an RNA polymerase (pol I) transcription system. We have now shown that E1BF purified from the serum-deprived cells (E1BFs) can inhibit rDNA transcription completely in a fractionated extract from the cells grown in serum-enriched medium. The suppression of transcription was overcome by the addition of control E1BF (E1BFc). Immunoprecipitation of purified E1BFs by the anti-Ku monoclonal antibody and addition of the supernatant to the transcription reaction mixture prevented the inhibition significantly, whereas immunoprecipitation with the control mouse IgG did not restore the transcription. The transcriptional repressor activity associated with the final DNA affinity column fractions copurified with E1BF. Neither the amount of E1BF nor its promoter binding activity was altered following serum depletion. E1BFs selectively inhibited the initiation of rDNA transcription. The inhibitory activity of E1BFs was not due to a nonspecific RNase activity. These data suggest that E1BF is post-translationally modified following serum starvation of cells, and that the repressor activity of E1BFs is largely responsible for the down-regulation of pol I transcription in serum-deprived cells.

Factor C*, the specific initiation component of the mouse RNA polymerase I holoenzyme, is inactivated early in the transcription process

Factor C* is the component of the RNA polymerase I holoenzyme (factor C) that allows specific transcriptional initiation on a factor D (SL1)- and UBF-activated rRNA gene promoter. The in vitro transcriptional capacity of a preincubated rDNA promoter complex becomes exhausted very rapidly upon initiation of transcription. This is due to the rapid depletion of C* activity. In contrast, C* activity is not unstable in the absence of transcription, even in the presence of nucleoside triphosphates (NTPs). By using 3'dNTPs to specifically halt elongation, C* is seen to remain active through transcription complex assembly, initiation, and the first approximately 37 nucleotides of elongation, but it is inactivated before synthesis proceeds beyond approximately 40 nucleotides. When elongation is halted before this critical distance, the C* remains active and on that template complex, greatly extending the kinetics of transcription and generating manyfold more transcripts than would have been synthesized if elongation had proceeded past the critical distance where C* is inactivated. In complementary in vivo analysis under conditions where C* activity is not replenished, C* activity becomes depleted from cells, but this also occurs only when there is ongoing rDNA transcription. Thus, both in vitro and in vivo, the specific initiation-conferring component of the RNA polymerase I holoenzyme is used stoichiometrically in the transcription process.

Factor C*, the specific initiation component of the mouse RNA polymerase I holoenzyme, is inactivated early in the transcription process

Factor C* is the component of the RNA polymerase I holoenzyme (factor C) that allows specific transcriptional initiation on a factor D (SL1)- and UBF-activated rRNA gene promoter. The in vitro transcriptional capacity of a preincubated rDNA promoter complex becomes exhausted very rapidly upon initiation of transcription. This is due to the rapid depletion of C* activity. In contrast, C* activity is not unstable in the absence of transcription, even in the presence of nucleoside triphosphates (NTPs). By using 3'dNTPs to specifically halt elongation, C* is seen to remain active through transcription complex assembly, initiation, and the first approximately 37 nucleotides of elongation, but it is inactivated before synthesis proceeds beyond approximately 40 nucleotides. When elongation is halted before this critical distance, the C* remains active and on that template complex, greatly extending the kinetics of transcription and generating manyfold more transcripts than would have been synthesized if elongation had proceeded past the critical distance where C* is inactivated. In complementary in vivo analysis under conditions where C* activity is not replenished, C* activity becomes depleted from cells, but this also occurs only when there is ongoing rDNA transcription. Thus, both in vitro and in vivo, the specific initiation-conferring component of the RNA polymerase I holoenzyme is used stoichiometrically in the transcription process.

Function of the growth-regulated transcription initiation factor TIF-IA in initiation complex formation at the murine ribosomal gene promoter

Alterations in the rate of cell proliferation are accompanied by changes in the transcription of rRNA genes. In mammals, this growth-dependent regulation of transcription of genes coding for rRNA (rDNA) is due to reduction of the amount or activity of an essential transcription factor, called TIF-IA. Extracts prepared from quiescent cells lack this factor activity and, therefore, are transcriptionally inactive. We have purified TIF-IA from exponentially growing cells and have shown that it is a polypeptide with a molecular mass of 75 kDa which exists as a monomer in solution. Using a reconstituted transcription system consisting of purified transcription factors, we demonstrate that TIF-IA is a bona fide transcription initiation factor which interacts with RNA polymerase I. Preinitiation complexes can be assembled in the absence of TIF-IA, but formation of the first phosphodiester bonds of nascent rRNA is precluded. After initiation, TIF-IA is liberated from the initiation complex and facilitates transcription from templates bearing preinitiation complexes which lack TIF-IA. Despite the pronounced species specificity of class I gene transcription, this growth-dependent factor has been identified not only in mouse but also in human cells. Murine TIF-IA complements extracts from both growth-inhibited mouse and human cells. The analogous human activity appears to be similar or identical to that of TIF-IA. Therefore, despite the fact that the RNA polymerase transcription system has evolved sufficiently rapidly that an rDNA promoter from one species will not function in another species, the basic mechanisms that adapt ribosome synthesis to cell proliferation have been conserved.

Function of the growth-regulated transcription initiation factor TIF-IA in initiation complex formation at the murine ribosomal gene promoter

Alterations in the rate of cell proliferation are accompanied by changes in the transcription of rRNA genes. In mammals, this growth-dependent regulation of transcription of genes coding for rRNA (rDNA) is due to reduction of the amount or activity of an essential transcription factor, called TIF-IA. Extracts prepared from quiescent cells lack this factor activity and, therefore, are transcriptionally inactive. We have purified TIF-IA from exponentially growing cells and have shown that it is a polypeptide with a molecular mass of 75 kDa which exists as a monomer in solution. Using a reconstituted transcription system consisting of purified transcription factors, we demonstrate that TIF-IA is a bona fide transcription initiation factor which interacts with RNA polymerase I. Preinitiation complexes can be assembled in the absence of TIF-IA, but formation of the first phosphodiester bonds of nascent rRNA is precluded. After initiation, TIF-IA is liberated from the initiation complex and facilitates transcription from templates bearing preinitiation complexes which lack TIF-IA. Despite the pronounced species specificity of class I gene transcription, this growth-dependent factor has been identified not only in mouse but also in human cells. Murine TIF-IA complements extracts from both growth-inhibited mouse and human cells. The analogous human activity appears to be similar or identical to that of TIF-IA. Therefore, despite the fact that the RNA polymerase transcription system has evolved sufficiently rapidly that an rDNA promoter from one species will not function in another species, the basic mechanisms that adapt ribosome synthesis to cell proliferation have been conserved.

Coordinated decreases in rRNA gene transcription factors and rRNA synthesis during muscle cell differentiation

rRNA synthesis decreases significantly during the differentiation of rat L6 myoblasts to myotubes. Nuclear run-on assays demonstrated that the decrease was attributable to decreased rates of rRNA gene transcription. Immunoblot analysis indicated a marked reduction in amounts of the RNA polymerase I transcription factors UBF1 and UBF2 (upstream binding factors 1 and 2, respectively). The levels of these factors dropped in parallel with the down-shift in rRNA gene transcription. The amount of UBF does not fall due to a general decrease in cellular protein, as myosin heavy-chain protein accumulates markedly during this same time. RNA blots of total RNA isolated from myoblasts and differentiating myotubes showed a decrease in the mRNA for UBF, at the same time the mRNA for myogenin was accumulating. The down-shift in UBF mRNA levels preceded the decrease in the protein levels for UBF. There have been reports that the acute response of the rRNA gene transcription system to physiological signals in many systems involves an RNA polymerase I-associated factor. However, our results imply that the regulation of rRNA gene DNA transcription in response to physiological processes, such as differentiation, may involve multiple regulatory pathways.

In vitro transcription of Drosophila rRNA genes shows stimulation by a phorbol ester and serum

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and serum both stimulate rapid increases in the transcription of Drosophila rRNA genes in vivo. Here we report that this stimulation is observed in in vitro transcription assays using nuclear extracts from cells treated with TPA or serum. Experiments in which extracts from TPA- or serum-treated cells were mixed with extracts from cells grown in serum-restricted medium showed that there was an increased RNA polymerase I (Pol I) activity present in the cell extracts from treated cells. We used a series of plasmids that had been deleted in the region 5' to the start site of rRNA transcription to determine which sequences were necessary to support the increased transcription seen in extracts from stimulated cells. DNA templates that contain sequences between -150 and +32 (with +1 as the Pol I transcription start site) show dramatic increases in transcription with TPA- and serum-stimulated cell extracts; however, templates that contain 5' sequences to -60 or -43 show at most one-third of the stimulation level of transcription in nuclear extracts from treated cells in comparison with untreated cell extracts. The 5' deletion to -34 abolishes the stimulation effect and drops the basal-level transcription by 20-fold. These results indicate that the regulation of Pol I transcription in Drosophila cells by serum and TPA requires two DNA elements, sequences from -150 to -60 (upstream control element) and sequences from -43 to -34 (a portion of the core promoter.

In vitro transcription of Drosophila rRNA genes shows stimulation by a phorbol ester and serum

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and serum both stimulate rapid increases in the transcription of Drosophila rRNA genes in vivo. Here we report that this stimulation is observed in in vitro transcription assays using nuclear extracts from cells treated with TPA or serum. Experiments in which extracts from TPA- or serum-treated cells were mixed with extracts from cells grown in serum-restricted medium showed that there was an increased RNA polymerase I (Pol I) activity present in the cell extracts from treated cells. We used a series of plasmids that had been deleted in the region 5' to the start site of rRNA transcription to determine which sequences were necessary to support the increased transcription seen in extracts from stimulated cells. DNA templates that contain sequences between -150 and +32 (with +1 as the Pol I transcription start site) show dramatic increases in transcription with TPA- and serum-stimulated cell extracts; however, templates that contain 5' sequences to -60 or -43 show at most one-third of the stimulation level of transcription in nuclear extracts from treated cells in comparison with untreated cell extracts. The 5' deletion to -34 abolishes the stimulation effect and drops the basal-level transcription by 20-fold. These results indicate that the regulation of Pol I transcription in Drosophila cells by serum and TPA requires two DNA elements, sequences from -150 to -60 (upstream control element) and sequences from -43 to -34 (a portion of the core promoter.

Glucocorticoid receptors in leukemias, lymphomas and myelomas of young and old

In this paper we have briefly reviewed the nature of leukemias and lymphomas in the old and the young. We surveyed in general the ways in which lymphoid cells and other hematologic elements respond to glucocorticoids, mentioning that there may be direct or indirect effects on their growth by these ligands. We have reviewed the current general model for the action of glucocorticoids in all cells, namely the fact that the actions of these steroids are mediated to a large extent through binding with ligand-activated transcription factors, their receptors. The growing wealth of detail about the nature of the interaction of these receptors with regulatory sites in the genome is discussed. Finally, we have described our results with lines of tissue culture cells representing clones from a typical leukemia of the young, and of myeloma, a typical hematologic malignancy of the elderly. Several features of the effects of glucocorticoids on these cells point up areas that would be pertinent to explore in aging and in the relationship of hematologic diseases to survival and response to therapy in the older versus the younger patient.

Infection of HeLa cells with poliovirus results in modification of a complex that binds to the rRNA promoter

In HeLa cells, RNA polymerase I (Pol I)-mediated transcription is severely inhibited soon after infection with poliovirus. We have developed a gel retardation assay to analyze DNA-protein complexes formed at the Pol I promoter. We show here that two complexes (A and C) formed by nuclear extracts from uninfected cells disappear after infection of cells with poliovirus. In contrast, a new, rapidly migrating complex (D) is formed in virus-infected cell extract. This change in the mobility of gel-retarded complexes correlates well with the kinetics of inhibition of rRNA transcription in virus-infected cells. Incubation of nuclear extracts from mock-infected cells with bacterially expressed, purified poliovirus protease 3C results in the disappearance of complexes A and C with concomitant generation of complex D. A partially purified transcription factor fraction derived from uninfected cells that contains complex A is able to restore Pol I transcription when added to virus-infected cell extracts, suggesting that this complex plays an important role in Pol I transcription. These results suggest that poliovirus proteinase 3C may have an important role in the shutoff of Pol I transcription in cells infected with poliovirus.

Glucocorticoid regulation of rRNA synthesis

Glucocorticoids inhibit transcription of the genes encoding rRNA (rDNA) in P1798 lymphoma cells. This is due to a decrease in the amount or activity of a transcription factor, called TFIC. TFIC has been purified to apparent homogeneity and the properties of this protein have been investigated in detail. TFIC is tightly associated with RNA polymerase I. The data indicate that TFIC is a bona fide initiation factor that is required for formation of the first phosphodiester bond of nascent pre-rRNA. Extracts from dexamethasone-treated cells are devoid of this factor and cannot form initiated complexes in vitro.

Inhibition of rRNA synthesis by poliovirus: specific inactivation of transcription factors

Synthesis of rRNA by RNA polymerase I is almost completely inhibited soon after infection of human cells with poliovirus. We show that extracts prepared from poliovirus-infected HeLa cells are severely inhibited in their ability to transcribe from a human rDNA promoter compared with extracts from mock-infected cells. Two lines of evidence presented here suggest that a specific transcriptional activity required for rDNA transcription in vitro is impaired in virus-infected cells. First, fractionation of individual transcriptional components by phosphocellulose chromatography and subsequent reconstitution experiments showed that the specific transcriptional activity of fraction C (0.8 M KCl eluate) from virus-infected cells was reduced three- to fourfold relative to that isolated from mock-infected cells. The activities of other transcription factors needed for in vitro transcription from the rDNA promoter were unaffected. Second, fraction C derived from mock-infected cells specifically restored transcription in extracts prepared from virus-infected cells. Fraction C contained both a nonspecific RNA polymerase I elongation activity and a specific factor activity which was needed for accurate transcription initiation. It is the specific transcriptional activity and not the nonspecific chain elongation activity of fraction C that is affected in cells infected with poliovirus.

Multiple mechanisms for the inhibition of rRNA synthesis during HL-60 leukemia cell differentiation

Treatment of HL-60 human promyelocytic leukemia cells with inducers of granulocytic differentiation produces a depletion of cellular rRNA, with the anthracycline antibiotics aclacinomycin A (ACM) and marcellomycin (MCM) causing a more rapid loss than dimethylsulfoxide (DMSO). This action is associated with a large reduction in RNA synthesis, which precedes any decreases in protein or DNA synthesis, and is specific for rRNA relative to total polyadenylated RNA synthesis. A 70% reduction in rRNA synthesis occurs within 20 minutes of ACM treatment and by 30 hours of DMSO exposure. Relative to the amount of 28S and 18S rRNA in the cells, there is a nearly complete depletion of the amount of 45S rRNA and other large rRNA precursors in cells treated with ACM, MCM, and the intercalating agent actinomycin D. In contrast, DMSO treatment produces a more coordinated decrease in 18S and 28S rRNA and rRNA precursors. The anthracycline antibiotics inhibited the synthesis of 5' proximal and 3' distal regions of the pre-rRNA transcript, while actinomycin D had a relatively sparing effect on the transcription of the 5' external transcribed spacer region of the gene relative to depletion of 3' transcripts. These studies demonstrate that different inducers of HL-60 differentiation have varying sites of action on rRNA synthesis and/or processing, with depletion of cellular rRNA as a common consequence.

Polymerase III transcription factor B activity is reduced in extracts of growth-restricted cells

Extracts of cells that are down-regulated for transcription by RNA polymerase I and RNA polymerase III exhibit a reduced in vitro transcriptional capacity. We have recently demonstrated that the down-regulation of polymerase I transcription in extracts of cycloheximide-treated and stationary-phase cells results from a lack of an activated subform of RNA polymerase I which is essential for rDNA transcription. To examine whether polymerase III transcriptional down-regulation occurs by a similar mechanism, the polymerase III transcription factors were isolated and added singly and in pairs to control cell extracts and to extracts of cells that had reduced polymerase III transcriptional activity due to cycloheximide treatment or growth into stationary phase. These down-regulations result from a specific reduction in TFIIIB; TFIIIC and polymerase III activities remain relatively constant. Thus, although transcription by both polymerase III and polymerase I is substantially decreased in extracts of growth-arrested cells, this regulation is brought about by reduction of different kinds of activities: a component of the polymerase III stable transcription complex in the former case and the activated subform of RNA polymerase I in the latter.

Polymerase III transcription factor B activity is reduced in extracts of growth-restricted cells

Extracts of cells that are down-regulated for transcription by RNA polymerase I and RNA polymerase III exhibit a reduced in vitro transcriptional capacity. We have recently demonstrated that the down-regulation of polymerase I transcription in extracts of cycloheximide-treated and stationary-phase cells results from a lack of an activated subform of RNA polymerase I which is essential for rDNA transcription. To examine whether polymerase III transcriptional down-regulation occurs by a similar mechanism, the polymerase III transcription factors were isolated and added singly and in pairs to control cell extracts and to extracts of cells that had reduced polymerase III transcriptional activity due to cycloheximide treatment or growth into stationary phase. These down-regulations result from a specific reduction in TFIIIB; TFIIIC and polymerase III activities remain relatively constant. Thus, although transcription by both polymerase III and polymerase I is substantially decreased in extracts of growth-arrested cells, this regulation is brought about by reduction of different kinds of activities: a component of the polymerase III stable transcription complex in the former case and the activated subform of RNA polymerase I in the latter.

Glucocorticoids selectively inhibit translation of ribosomal protein mRNAs in P1798 lymphosarcoma cells

When P1798 murine lymphosarcoma cells are exposed to 10(-7) M dexamethasone, there is a dramatic inhibition of rRNA synthesis, which is completely reversible when the hormone is withdrawn. In the present experiments we examined whether dexamethasone treatment causes any alteration in the accumulation or utilization of mRNAs that encode ribosomal proteins (rp mRNAs). No effect on the accumulation of six different rp mRNAs was detected. However, the translation of five of six rp mRNAs was selectively inhibited in the presence of the hormone, as judged by a substantial decrease in ribosomal loading. Normal translation of rp mRNA was resumed within a few hours after hormone withdrawal. In untreated or fully recovered cells, the distribution of rp mRNAs between polyribosomes and free ribonucleoprotein is distinctly bimodal, suggesting that rp mRNAs are subject to a particular form of translational control in which they are either translationally inactive or fully loaded with ribosomes. A possible relationship between this mode of translational control and the selective suppression of rp mRNA translation by glucocorticoids is discussed.

Glucocorticoids selectively inhibit translation of ribosomal protein mRNAs in P1798 lymphosarcoma cells

When P1798 murine lymphosarcoma cells are exposed to 10(-7) M dexamethasone, there is a dramatic inhibition of rRNA synthesis, which is completely reversible when the hormone is withdrawn. In the present experiments we examined whether dexamethasone treatment causes any alteration in the accumulation or utilization of mRNAs that encode ribosomal proteins (rp mRNAs). No effect on the accumulation of six different rp mRNAs was detected. However, the translation of five of six rp mRNAs was selectively inhibited in the presence of the hormone, as judged by a substantial decrease in ribosomal loading. Normal translation of rp mRNA was resumed within a few hours after hormone withdrawal. In untreated or fully recovered cells, the distribution of rp mRNAs between polyribosomes and free ribonucleoprotein is distinctly bimodal, suggesting that rp mRNAs are subject to a particular form of translational control in which they are either translationally inactive or fully loaded with ribosomes. A possible relationship between this mode of translational control and the selective suppression of rp mRNA translation by glucocorticoids is discussed.

Factors and nucleotide sequences that direct ribosomal DNA transcription and their relationship to the stable transcription complex

We have studied the protein components and nucleic acid sequences involved in stably activating the ribosomal DNA (rDNA) template and in directing accurate transcription of mammalian rRNA genes. Two protein components are necessary to catalyze rDNA transcription, and these have been extensively purified. The first, factor D, can stably associate by itself with the rDNA promoter region and is responsible for template commitment. The second component, factor C, which appears to be an activated subset of polymerase I, can stably bind to the factor D-rDNA complex but not to the rDNA in the absence of factor D. A third component which had been previously identified as a rDNA transcription factor is shown to be a RNase inhibitor. Extending our earlier observation that the approximately 150-base-pair mouse rDNA promoter consists of a minimal essential region (residues approximately -35 to approximately +9) and additional upstream stimulatory domains, we now report that each of these promoter domains acts to augment the binding of the polymerase I transcription factors. A minimum core region (residues approximately -35 to approximately -15) is capable of stable complex formation and of binding transcription factor D. Factor C can also bind to this D-core region complex.

Factors and nucleotide sequences that direct ribosomal DNA transcription and their relationship to the stable transcription complex

We have studied the protein components and nucleic acid sequences involved in stably activating the ribosomal DNA (rDNA) template and in directing accurate transcription of mammalian rRNA genes. Two protein components are necessary to catalyze rDNA transcription, and these have been extensively purified. The first, factor D, can stably associate by itself with the rDNA promoter region and is responsible for template commitment. The second component, factor C, which appears to be an activated subset of polymerase I, can stably bind to the factor D-rDNA complex but not to the rDNA in the absence of factor D. A third component which had been previously identified as a rDNA transcription factor is shown to be a RNase inhibitor. Extending our earlier observation that the approximately 150-base-pair mouse rDNA promoter consists of a minimal essential region (residues approximately -35 to approximately +9) and additional upstream stimulatory domains, we now report that each of these promoter domains acts to augment the binding of the polymerase I transcription factors. A minimum core region (residues approximately -35 to approximately -15) is capable of stable complex formation and of binding transcription factor D. Factor C can also bind to this D-core region complex.

Monoamine oxidases A and B are differentially regulated by glucocorticoids and "aging" in human skin fibroblasts

Two forms of monoamine oxidase (MAO A and MAO B) exist which, although similar in a number of properties, can be distinguished on the basis of their substrate specificity, inhibitor sensitivity, kinetic parameters, and protein structure. These properties were used to study the molecular mechanism(s) by which glucocorticoid hormones and "aging," known to alter MAO activity in vivo, regulated the expression of MAO A and MAO B in cultured human skin fibroblasts. The addition of dexamethasone or hydrocortisone to cultures resulted in a dose- and time-dependent increase in total MAO activity, whereas the removal of hormone from cultures resulted in a time-dependent decrease in activity toward control levels. The response to dexamethasone was affected by culture conditions such as serum concentration, feeding frequency, and cellular "age." Cellular aging, in the absence of hormone, also resulted in increased levels of total MAO activity. The effects of hormones and aging on total MAO activity appeared to be selective for MAO A. The 6- to 14-fold increases in total activity were paralleled by similar increases in the activity and amount of active MAO A but less than 2- to 3-fold increases in the activity and amount of MAO B. Altered synthesis or degradation of the active enzyme appeared to account for the effects of hormones, aging, and various culture conditions on MAO activity. Inhibitor sensitivity, substrate affinity, electrophoretic mobility, and molecular turnover number of either form of the enzyme were not altered during dexamethasone treatment or during cellular aging. However, rates of active MAO synthesis were affected by hormone treatment and feeding frequency, rates of active MAO degradation by serum concentration, and rates of active MAO synthesis or degradation by aging. In summary, we have shown that glucocorticoids and cellular aging selectively affect the amount of MAO A at the level of active enzyme synthesis or degradation. Further, our finding that the expression of the two forms of MAO in human fibroblasts can be independently regulated supports the growing evidence that MAO A and MAO B are separate molecular entities.

Footprinting of ribosomal RNA genes by transcription initiation factor and RNA polymerase I

The binding of a species-specific transcription initiation factor (TIF) and purified RNA polymerase I to the promoter region of the 39S ribosomal RNA gene from Acanthamoeba were studied by using DNase I "footprinting." Conditions were chosen such that the footprints obtained could be correlated with the transcriptional activity of the TIF-containing fractions used and that the labeled DNA present would itself serve as a template for transcription. The transcription factor binds upstream from the transcription start site, protecting a region extending from around -14 to -67 on the coding strand, and -12 to -69 on the noncoding strand. The protein that binds to DNA within this region can be competed out by using wild-type promoters but not by using mutants which do not stably bind the factor. RNA polymerase I can form a stable complex in the presence of DNA and transcription factor, allowing footprinting of the complete transcription initiation complex. RNA polymerase I extends the protected region obtained with TIF alone to around +18 on the coding strand, and to +20 on the noncoding strand. This region is not protected by polymerase I in the absence of TIF. The close apposition of the regions protected by TIF and polymerase provides evidence that accurate transcription of the ribosomal gene may be achieved through protein-protein contacts as well as through DNA-protein interactions.

Growth-dependent regulation of rRNA synthesis is mediated by a transcription initiation factor (TIF-IA)

Mouse RNA polymerase I requires at least two chromatographically distinct transcription factors (designated TIF-IA and TIF-IB) to initiate transcription accurately and efficiently in vitro. In this paper we describe the partial purification of TIF-IA by a four-step fractionation procedure. The amount or activity of TIF-IA fluctuates in response to the physiological state of the cells. Extracts from quiescent cells are incapable of specific transcription and do not contain detectable levels of TIF-IA. Transcriptionally inactive extracts can be restored by the addition of TIF-IA preparations that have been highly purified from exponentially growing cells. During the fractionating procedure TIF-IA co-purifies with RNA polymerase I, suggesting that it is functionally associated with the transcribing enzyme. We suggest that only those enzyme molecules that are associated with TIF-IA are capable to interact with TIF-IB and to initiate transcription.

Hormonal regulation of transcription of rDNA: glucocorticoid effects upon initiation and elongation in vitro

Various parameters of transcription of cloned mouse rDNA have been examined using extracts from control P1798 cells and from cells treated 24 h with 0.1 microM dexamethasone. Highly purified RNA polymerase I from either source catalyzes nucleotidyl transfer (elongation) at a rate of approximately 30 nucleotides/sec. Extracts from hormone-treated cells are capable of forming stable, preinitiation complexes. The rates of stable complex formation are the same in extracts from control and hormone-treated cells. Nevertheless, initiation of transcription does not occur in extracts from hormone-treated cells. Initiation in such extracts may be restored by the addition of a partially purified RNA polymerase I initiation factor, designated TFIC. The data indicate that initiation by RNA polymerase I is a multi-step process. The first step involves the formation of stable, preinitiation complexes, as demonstrated by a number of groups. Initiation, per se, requires an additional protein, TFIC. Glucocorticoids and perhaps other mitogenic agents regulate transcription of rDNA by influencing the amount or activity of TFIC.

Hormonal regulation of transcription of rDNA: use of nucleoside thiotriphosphates to measure initiation in isolated nuclei

Nuclei isolated from cultured mouse and rat cell lines initiated pre-rRNA chains at the correct site and with the correct nucleotide specificity (A for mouse, G for rat). Nucleic acid filter hybridization and S1 nuclease mapping were used to analyze the RNA products initiated with nucleoside (beta-S)triphosphates. Initiation of pre-rRNA was completely resistant to alpha-amanitin but was inhibited by either actinomycin D or heparin. Experiments with P1798 mouse lymphoma cells indicated that the antiproliferative effects of glucocorticoids on lymphoid cells includes a reduction in the ability of nuclei to initiate pre-rRNA chains.