Modification of the template capacity of chick-oviduct chromatin for form-B RNA polymerase by estradiol

The control of ribosomal RNA transcription in lymphocytes. Evidence that the rate of chain elongation is the limiting factor

During activation of lymphocytes by phytohaemagglutinin, the nuclear activity of RNA polymerase I increases with no proportional increase in the amount of catalytic efficiency of the enzyme in the cell. The mechanism by which rRNA transcription in lymphocytes is modified by phytohaemagglutinin stimulation was investigated. The following results were obtained. (a) In resting lymphocytes all RNA polymerase II molecules are bound to the template while a pool of excess free RNA polymerase I, not engaged in transcription, can be detected by its ability to transcribe added poly[d(A-T)]. (b) Although the free RNA polymerase I activity increases twofold to threefold during phytohaemagglutinin stimulation, there is no evidence that the free enzymes ever become engaged in transcription. (c) Most of the RNA chains in elongation in nuclei from resting lymphocytes are being elongated by class II RNA polymerase and their rate of elongation is much higher than that of other RNA species. (d) The same number of pre-rRNA chains are in the process of being elongated in nuclei from resting and stimulated lymphocytes. However, the rate of elongation of pre-rRNA, which is slow relative to the average in resting lymphocytes, increases twofold to threefold within 6 h of phytohaemagglutinin stimulation and rises to sixfold by 19 h. These results suggest that the control of rRNA transcription in phytohaemagglutinin-stimulated lymphocytes lies in the elongation step of transcription rather than in initiation, and that little or no additional rRNA template is transcribed in phytohaemagglutinin-stimulated lymphocytes.

Aging research: current and future

The key events that lead to organismic senescence appear to occur in cells that have been genetically programmed to arrest their own proliferation. After arrest, a variety of subcellular events occur, among the more important of which are the accumulation of waste product pigments, an increase in the fraction of inactive enzymes, and (probably of central importance) the loss of key genes, specifically those that code for rRNA. An understanding of the senescence of the epidermis and dermis would almost certainly provide answers to related problems in those organ systems whose sudden or slow failure leads to individual death.

RNA synthesis by nuclei and nucleoli from ischemic liver cells

Nuclei and nucleoli were isolated from rat livers subjected to an interruption of the blood supply for periods of different duration, as well as after restoration of the blood supply. They were assayed for RNA synthesis under conditions of diverse ionic strengths, and in the presence of an exogenous template, such as poly d (A-T), and actinomycin to inactivate the endogenous template; alpha-amanitin was made used of to distinguish polymerase I and polymerase II dependent RNA synthesis. Nuclei and nucleoli from ischemic livers showed a severe impairment of RNA synthesis, which is likely to be due to decreased initiation frequency of the engaged polymerases, while free polymerases were essentially unchanged. Both form I and II polymerase were equally involved. After restoration of the blood supply RNA synthesis recovered with an overshooting well above normal levels of activity, lasting for at least 24 hours. Increased RNA synthesis was not followed by thymidine incorporation into DNA.

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.

Estrogen withdrawal in chick oviduct. Characterization of RNA synthesized in isolated nuclei using a mercurated precursor

1. As a prerequisite for analyzing the effect of estrogen on transcription in chick oviduct, we describe suitable methods for the synthesis (under conditions restricting reinitiation), and isolation of RNA transcripts from oviduct nuclei in vitro, utilizing mercurated UTP (Hg-UTP) as an RNA precursor and chromatography on sulphydryl-Sepharose (SH-Sepharose) to recover mercurated RNA (Hg-RNA). The techniques described include treatment of Hg-RNA with p-hydroxymercuribenzoate, to improve the efficiency of binding to SH-Sepharose, and elution of Hg-RNA from SH-Sepharose after treatment with 60% formamide at 90 degrees C, to eliminate contamination by aggregated nucleic acid. 2. RNA synthesized by endogenous form B RNA polymerase (using either UTP or Hg-UTP as precursor) was recovered in nuclear lysates in the form of 30--85-S heterogeneous RNA . protein complexes, and after removal of protein, was 10--12 S in size. 3. The nature of RNA transcripts synthesized in vitro was examined by hybridization. More than 90% of the RNA was complementary to "unique" DNA sequences, and 50--60% of the hybridized RNA could be competed with homologous, steady-state nuclear RNA, indicating a significant degree with homologous, steady-state nuclear RNA, indicating a significant degree of homology between in vitro transcripts and in vivo RNA. The level of homology was similar whether RNA synthesis was performed in low salt, or in high salt in the presence of heparin. Possible reasons for only partial competition in these experiments are discussed. 4. Withdrawal of estrogen from chicks leads to a 50% reduction in endogenous RNA polymerase activities in nuclei within 48 h. Similar levels of competition with Hg-RNA transcripts for "unique" DNA were obtained using oviduct nuclear RNAs isolated before or after estrogen withdrawal, and even with liver nuclear RNA. Thus, in oviduct, those sequences present in primary transcripts, and analyzed under our experimental conditions, are present in different hormonal states and also in other chick tissues.

Developmental restrictions on hormone modulated gene transcription. II. Hormone induced interactions of RNA polymerase with chromatin

Chromatin-bound and soluble RNA polymerase subspecies have been isolated and fractionated by isoelectric focusing at various times (0, 6, 12 and 18 h) following auxin treatment of 4 day (responsive) and 8 day (unresponsive) soybean hypocotyls. Young 4 day seedlings displayed two well defined phases of auxin induced gene transcription. Phase I (6 h) evidenced the selective dissociation of many RNA polymerase subspecies from the chromatin complex which was accompanied by the retention of three class II enzymes. Phase II occurred after 12 h of treatment when the dissociated enzymes including some species which were soluble in the 0 h controls became re-associated with chromatin. These induced RNA polymerases may be responsible for the synthesis of auxin induced RNAs. In contrast, the unresponsive 8 day hypocotyl did not display two phases of auxin induction. Phase one, the dissociation of the chromatin bound enzymes, occurred at 12 h (compared to 6 h for the 4 day seedling) and was not followed by the later translocation of any soluble enzymes towards the chromatin complex. The results support earlier findings suggesting that the developmental "phasing out" of RNA polymerase subspecies limits the hormone induced growth response of this tissue and thus is regarded as an off switch for the transcription of such hormone controlled gene sequences.

On the activity of RNA polymerase B in lysates from Ehrlich ascites cells

Transcription by endogenous RNA polymerase B in lysates of Ehrlich ascites cells was investigated. The enzyme exhibits two salt optima at 0.025 M and at 0.3 M (NH4)2SO4 respectively. Preincubation of the cells with the nucleoside analogue 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole results in an inactivation of the polymerase molecules active under condition of low salt. This indicates two functional states of the enzyme in vivo. Initiations of RNA chains by polymerase B do not occur in vitro as judged by the incorporation of [beta-32P]GTP. Thus the two functional states seem to be both elongating polymerase molecules. Polymerase B does not occur in the lysates in a state ready to initiate on an exogenous template, in contrast to polymerase A and C which do occur in free form. Pretreatment with dichlororibofuranosylbenzimidazole in vivo does not result in an accumulation of free polymerase B.

Progesterone receptors of chick oviduct

The chick oviduct progesterone receptor has been purified to homogeneity by affinity chromatography and its molecular action studied in vitro. The native receptor is a 200,000 MW dimer of two dissimilar 4S subunits with different intranuclear function. The receptors directly regulate RNA chain initiation sites in oviduct chromatin by interactions involving target tissue nuclear acceptor sites. There is a 1:1 correspondence between receptor "acceptor" sites and RNA sites. Only the dimer form of the receptor is active in vitro on chromatin templates. The study suggests a novel model for hormone action which can be tested directly in this system.

RNA polymerase activities in isolated nuclei of guinea pig seminal vesicle epithelium: influence of castration and androgen administration

Nuclei from seminal vesicle epithelium of adult guinea pigs were isolated in hypertonic sucrose solution. The incorporation of [3H]UTP by the isolated nuclei into acid-precipitable products was studied. Incorporation required ATP, GTP, CTP, UTP, and Mg+2. It was inhibited by addition of actinomycin D, deoxyribonuclease, or pyrophosphate to the reaction mixture. Thus, incorporation of [3H]UTP by isolated nuclei had the same characteristics that have been demonstrated for the reactions catalyzed by nuclear RNA polymerases. Using alpha-amanitin as a metabolic tool, we established concentrations of (NH4)2SO4. Mg+2, and nucleotides that give maximum assayable activities of nuclear RNA polymerases I and II. When the activities of polymerases I and II were measured in isolated seminal vesicle nuclei of guinea pigs that had been castrated 4 days earlier, a marked decrease in activities was found relative to control values (nuclei from intact animals). No further decrease was found 8 days after castration. Diminished accessibility to the nuclear DNA template and a decrease in the concentration of RNA polymerase molecules seemed to be responsible for the observed effects of castration on activities of RNA polymerases. An increase in ribonuclease activity did not seem to be responsible for the effects of castration. Activities of the enzymes did not change 2, 3, or 4 hours after intraperitoneal injection (2 mg/kg body weight) of each of five different androgens. Similarly, a single intraperitoneal injection of testosterone did not restore enzyme activity of polymerade I or II at any time during the first 24-hour period after hormone administration.

Structural features of encephalomyocarditis virus RNA from analysis of reverse transcription products

The presence in encephalomyocarditis (EMC) virus RNA of homonucleotide tracts 10 nucleotides or more in length has been investigated by testing the ability of homo-oligodeoxynucleotides to prime DNA synthesis in the reverse transcriptase from avian myeloblastosis virus. Neither (dC)10 nor (dA)10 promoted incorporation of [3H]deoxynucleotides into acid-insoluble material but (dG)10 and (dT)12-18 were effective primers and produced DNA products approximately 2000 nucleotides in length. We conclude that there are single-stranded oligo(rC) and oligo(rA) tracts in native EMC virus RNA at 37 degrees C. Kinetic analysis indicated that oligo(dT) priming is similar to priming on ovalbumin mRNA and that it gives rise to only one DNA product per template molecule. Oligo(dG) priming appears to be complicated by self-aggregation of the primer. Oligo(dT)-primed and oligo(dG)-primed DNA have both been separated on alkaline-sucrose gradients into two peaks of which only the 'heavier' will hybridise to EMC virus RNA. Competitive hybridisation experiments indicate that the 'heavy' oligo(dT)-primed and oligo(dG)-primed DNA fractions hybridise to overlapping sequences of EMC virus RNA and place the priming regions of EMC virus RNA approximately 500 nucleotides apart during reverse transcription.

Poly ADP-ribosylation of DNA-dependent RNA polymerase I from quail oviduct. Dependence on progesterone stimulation

Progesterone causes in goblet cells of oviducts of estrogen hormone-stimulated immature quails selectively gene activation without affecting DNA synthesis. This biological model has been used to study the influence of poly ADP-ribosylation during the processes of DNA transcription. Administration of progesterone in vivo causes an increase of the activity of RNA polymerase I and II in isolated nuclei. This increase is accompanied by a marked decrease of the specific activity of poly (ADP-Rib) polymerase. After in vitro ADP-ribosylation of nuclear proteins the template capacity of chromatin for ""exogenous'' RNA synthesis (with E. coli DNA-dependent RNA polymerases) as well as for ""endogenous'' RNA synthesis with DNA dependent RNA polymerases II is not affected, whereas the data presented seem to indicate that the capacity for RNA synthesis mediated by ""endogenous'' DNA-dependent RNA polymerase I might be inhibited after ADP-ribosylation. Evidence is presented to show that a considerable amount of poly (ADP-Rib), synthesized by poly (ADP-Rib) polymerase in isolated nuclei, is linked with RNA polymerase I. The rate of synthesis of poly (ADP-Rib) is dependent on the incubation temperature (optimum at 25 degrees C) and it can be inhibited by the specific inhibitors of poly (ADP-Rib) polymerase nicotineamide, thymidine and formycin B. Poly (ADP-Rib) is probably associated with RNA polymerase I through a covalent linkage. ADP-ribosylated RNA polymerase I has been purified 550 fold with respect to the nuclear extract corresponding to a 4,000 fold purification from the whole cell homogenate. The ratio between poly (ADP-Rib), formed during preincubation of nuclei with NAD, and RNA polymerase I remains almost constant during the purification procedures. The extent of ADP-ribosylation of RNA polymerase I decreases during gene expression. Thus we conclude that poly ADP-ribosylation of this enzyme is one of the regulatory mechanisms by which specificity of DNA transcription is achieved.

On the mechanism of lanthanide-induced liver toxicity

The intravenous injection of the lighter lanthanide ions Pr(III), Nd(III), and Sm(III) in doses of 35 mumoles/kg inhibits, and isoosmolar doses of the heavier lanthanide ions Gd(III), Dy(III), and Er(III) stimulate rat liver nuclear in vitro RNA synthesis catalyzed by RNA polymerase B 24 h after their application, while nuclear RNA synthesis, catalyzed by RNA polymerase A, was inhibited by the same isoosmolar doses of Pr(III), Nd(III) and not influenced by Sm(III), Gd(III), Dy(III), or Er(III). The effect of in vivo applied Pr(III) and Nd(III) on rat liver in vitro nuclear RNA synthesis shows a similar time and dose-dependent pattern. The decreased rat liver nuclear in vitro RNA synthesis 24 h after intravenous injection of Pr(III) as well as after Nd(III) was accompanied by a decreased nuclear in vitro 3H-acetate uptake by the chromatin-bound histone fractions, F 2a2, F 3, and F 2al. At the same time after the Pr(III) injection, the capacity and number of initiation sites of the rat liver nuclear template for homologous nuclear RNA polymerases, prepared from control rat liver nuclei, was lower than the corresponding control template. A decreased activity of endogenous free nuclear RNA polymerases, as determined with the aim of the synthetic poly(dA-dT) template 24 h after Pr(III), may further contribute to the decreased nuclear RNA synthesis. The results indicate a primary ionic size-correlated interference of lanthanides with the nuclear control mechanisms of RNA synthesis.

Effects of estrogen and progesterone on transcription, chromatin and ovalbumin gene expression in the chick oviduct

The effects of estrogen, progesterone and estrogen + progesterone combined on nuclear transcriptional processes in oviducts of immature chicks, previously withdrawn from estrogen, are reported. The responses to the steroids of the endogenous nuclear RNA polymerase activities, both nucleolar (I) and nucleoplasmic (II), the chromatin compositions and template capacities, and the appearance of ovalbumin messenger RNA (mRNA) are compared. When immature chicks (previously treated at 14 days with estrogen) are withdrawn from estrogen treatment, there is a gradual reduction in both polymerase activities. Diurnal variations in polymerase II activties in the oviduct of withdrawn chicks required that subsequent experiments include time-matched controls. The hormones alter RNA polymerase II and II activities in vivo as assayed in isolated nuclei. Progesterone represses the polymerase I and II activities, while estrogen alone and estrogen + progesterone enhance both polymerase activities immediately after injection. Diethylstillbestrol, a synthetic estrogen, causes changes similar to those of estrogen. The effects of these steroids on the polymerases are detected within 15 min of hormone injection. Changes in the capacities of chromatins to serve as template for RNA synthesis in general correlated with changes in polymerase II activities. Interestingly, in the case of estrogen treatment, the acidic chromatin protein (but not histone) levels fluctuate positively with the template capacities of the chromatin. An antagonism between estrogen and progesterone is observed in the responses of both RNA polymerases I and II activities as well as in the chromatin template capacity. Levels of messenger RNA coding for ovalbumin, as detected by hybridization with labeled complementary DNA, increase in oviducts of withdrawn chicks within 2--3 of the injection of estrogen, progesterone or estrogen + progesterone. This rapid accumulation of ovalbumin mRNA is not accompanied in each case by a similar increase in polymerase II activity or chromatin template capacity.

RNA synthesis in Yoshida ascites tumor cells after ultraviolet and gamma irradiation

The residual RNA synthesis in Yoshida ascites tumor cells following exposure to ultraviolet radiation was insensitive to rifampicin AF/013, an inhibitor of initiation of RNA synthesis in eukaryotic cells. On the other hand, residual RNA synthesis in the ascites tumor cells after gamma-irradiation was inhibited by rifampicin AF/013. In vitro synthesis of RNA on ultraviolet-irradiated ascites chromatin likewise was resistant to rifampicin AF/013. Ultraviolet irradiation was found to decrease the number of RNA chain initiation sites on ascites chromatin considerably whereas the number of initiation sites on ascites chromatin increased considerably after gamma irradiation. These results indicate that ultraviolet irradiation may preferentially affect the DNA segments participating in initiation of RNA synthesis.

Testicular chromatin activation in hypophysectomized rats

Incorporation of labeled thymidine into testicular DNA of hypophysectomized rats began to increase after the administration of testosterone propionate and choriogenic gonadotrophin. While the thymidine incorporation reached maximum in 4 days, the DNA polymerase activity did not culminate until 8 days after the initiation of hormone treatment. The high molecular weight (6--8 S), presumably cytoplasmic DNA polymerase accounted almost entirely for this increase. Administration of testosterone propionate and chorionic gonadotrophin to hypophysectomized rats results in an increase of testicular RNA polymerase and chromatin templating activity. Chain elongation and initiation studies revealed that the increased templating capacity of androgen-stimulated testicular chromatin was almost entirely caused by the increase in the number of initiation sites. While the nuclear polymerase I responded relatively rapidly to hormone stimulation and reached a prominent maximum in about three days, the activity of polymerase II was more sluggish and not as prominent. The in vivo incorporation of ortho[32P]phosphate into chromosomal phosphoproteins occurred early during the androgen treatment and reached a maximum in about 20 h. The protein phosphokinase activity peaked later, approx. 72 h after the first administration of hormones.

Quantitation of elongating form A and B RNA polymerases in chick oviduct nuclei and effects of estradiol

The number of elongating form A and B RNA polymerases in chick oviduct nuclei was estimated by measuring incorportaion of 3H-UTP into 3' termini of nascent RNA chains in the presence of heparin to inhibit initiation, and quantitation labeled uridine released from these termini after alkaline hydrolysis. The method corrects for conversion of UMP to uridine (U) during manipulations and for production of 3' termini by ribonucleases and phosphatases. The results indicate that a large fraction of RNA polymerases elongating in vivo is retained in isolated nuclei: per diploid genome, approximately 1 x104 form B and 2 x103 form A enzymes are present. These levels are sufficient to maintain normal in vivo rates of mRNA and rRNA synthesis, but the average density of packing of polymerases on DNA is considerably less than the maximum density predicted by Miller and Bakken (1972), suggesting that initiation of polymerases of DNA is a limiting factor in the control of transcription. Rates of elongation of polymerases in vitro are severely impaired, indicative of a loss of elongation factors during nuclear isolation. After 6 hr of estradiol treatment in vivo, the level of form A enzymes in ovoduct nuclei increases to 5 x 103; little change in the number of form B enzymes is seen.

Poly(A) polymerase in quail oviduct. Changes during estrogen induction

A nuclear poly(A) polymerase has been isolated from oviducts of immature quails. It could be purified 4300-fold. The enzyme depends specifically on ATP as substrate and requires Mg2+. The most effective primer for the enzyme is a polynucleotide, isolated from oviduct tissue. A poly(A) sequence to a maximum of 60 AMP residues is covalently linked per primer molecule. The poly(A)-rich product of the enzymatic reaction can be annealed to oligo(dT)-cellulose. The purest fraction does not contain any detectable poly(A)-degrading enzyme activity. Only very low activities of RNA polymerase are present. The poly(A polymerase activity in the assay with ATP is reduced by the ATP analogue, beta, lambda-ATP-methylene-diphosphonate. Both K-m and V are lowered. The ATP analogue is incorporated to a smaller extent into the poly(A) sequence, synthesized by the enzyme. Several other analogues of adenine, adenine nucleosides and adenine nucleotides are without effect on the enzymatic reaction. By these properties poly(A) polymerase can be distinguished from RNA polymerases form I and form II, isolated from the same tissue. Actinomycin D and alpha-amanitin failed to inhibit poly(A) polymerase activity. The activity of poly(A) polymerase has been determined during primary stimulation with the estrogen analogue diethylstilbestrol (daily injection for 5 days), after withdrawal of the hormone for 17 days and after secondary stimulation with the hormone analogue. The enzyme activity does not change during primary stimulation, withdrawal of the hormone or secondary stimulation. However the activity of a poly(A) degrading enzyme, localized in the nucleus, is reduced in oviducts from hormone-treated quails.

The effect of spermine on transcription of mammalian chromatin by mammalian deoxyribonucleic acid-dependent ribonucleic acid polymerase

Isolated rat liver nuclei demonstrate an increased ability to synthesize RNA in the presence of either spermine or spermidine. Spermidine has more effect on the low-salt alpha-amanitin-insensitive reaction, and spermine has more effect on the high-salt alpha-amanitin-sensitive reaction. Spermine is effective at concentrations of 0.1 mM and 1 muM, showing a biphasic effect. The RNA polymerase activity associated with nuclear chromatin is increased in the presence of spermine only at a concentration of 0.1 mM. Aso the transcription of deproteinized liver DNA by liver form-B polymerase or Escherichia coli enzyme is more efficient in the presence of 0.1 mM-spermine. Only when liver chromatin is transcribed by its homologous enzyme (and not by E. coli enzyme) is spermine active at both 0.1mM and 1 muM as in purified nuclei. The lower concentration of spermine (1 muM) is able to affect chromatin transcription by increasing the affinity of chromatin for the enzyme. Our findings suggest a regulatory role of spermine at the level of genome transcription.

Modification of the template capacity of liver chromatin for form-B ribonucleic acid polymerase by food intake in rats under controlled feeding schedules

Nuclei from liver of rats accustomed to eating during the first 8h of a daily 12h dark period demonstrate an increased capacity to synthesize RNA 6H after the beginning of the feeding period. 2. This increase is accompanied by a higher yield of extractable form-B DNA-dependent RNA polymerase activity. 3. The endogenous RNA polymerase activity associated with nuclear chromatin is also stimulated by food intake. Both purified and chromatin-associated form-B enzyme activities exhibit different ionic strength requirements after food intake. 4. The sensitivity of exogenous (added) form-B-enzyme to changes in ionic strength changes after feeding when chromatin is used as template. 5. Chromatin extracted from the liver of fed rats is a better template for form-B-enzyme than chromatin extracted from starved rats.

Changes in the number of binding sites for ribonucleic acid polymerase in chromatin of WI-38 fibroblasts stimulated to proliferate

1. When WI-38 human diploid fibroblasts form confluent monolayers, DNA synthesis and cell division almost completely cease. A change of medium causes these density-inhibited cells to proliferate and within 1h after the application of the stimulus there is an increase in template activity of the chromatin isolated from stimulated cells. 2. The number of binding sites for Escherichia coli RNA polymerase was determined on chromatin from WI-38 cells by two different methods, i.e. incorporation of [(3)H]UTP into RNA in the absence of reinitiation, and incorporation of [gamma-(32)P]GTP into chain termini. 3. Both methods indicate that the capacity of chromatin to bind E. coli RNA polymerase is increased in WI-38 cells stimulated to proliferate. 4. The increase in the number of binding sites for E. coli RNA polymerase parallels the increase in chromatin template activity and suggests that the latter reflects an increase in the number of initiation sites, rather than an increase in the rate of transcription.

Increased RNA synthesis in nuclear monolayers of WI-38 cells stimulated to proliferate

Nuclear monolayers of WI-38 cells prepared by the method of Tsai and Green were used to determine RNA synthesis in isolated nuclei in situ. In nuclear monolayers, incorporation of [(3)H]UTP into RNA is dependent on the presence of the other three nucleotide triphosphate and is abolished by actinomycin D. The extent of RNA synthesis under these conditions was measured in density-inhibited WI-38 human diploid fibroblasts at various intervals after cell proliferation was stimulated by a change of medium.RNA synthesis increases 15 min after the nutritional change and reaches a peak at 18 hr, which is also the peak of DNA synthesis. Thereafter RNA synthesis declines. Essentially similar results are obtained whether the endogenous RNA polymerase or a bacterial polymerase is used. Replacement of the stimulating medium by conditioned medium stops the increase in RNA synthesis that occurs in cultures subject to continuous stimulation. Finally, RNA synthesis in nuclear monolayers, using the endogenous RNA polymerase, occurs by chain elongation only, while re-initiation occurs with the bacterial RNA polymerase.