Control Mechanism of Ribonucleic Acid Synthesis in Eukaryotes

Selective inactivation of two components of the multiprotein transcription factor TFIIIB in cycloheximide growth-arrested yeast cells

Following protein synthesis inhibition in cycloheximide growth-arrested yeast cells, the rates of tRNA and 5 S RNA synthesis decrease with apparent half-times of about 20 and 10 min, respectively. This effect is mimicked by extracts of treated cells, and the impairment of tRNA gene transcription activity that is observed in vitro parallels the in vivo inactivation of RNA polymerase III transcription. As revealed by experiments in which partially purified class III transcription factors were singly added to extracts of treated cells, only the activity of the multiprotein transcription factor TFIIIB is severely impaired after 3 h of cycloheximide treatment. Similar assays carried out in an in vitro transcription system in which TFIIIB activity was reconstituted by a combination of the TATA box-binding protein (TBP), the 70-kDa component TFIIIB70, plus a partially purified fraction known as B" have shown that the latter two components are both necessary and sufficient to restore control levels of transcription. Their activity, but not TBP activity, is considerably reduced in extracts of treated cells. TFIIIB70 and a component of fraction B" thus appear to be the selective targets of the down-regulation of polymerase III transcription that is brought about by cycloheximide. A substantial depletion of the TFIIIB70 polypeptide was detected by Western immunoblot analysis of extracts derived from cycloheximide growth-arrested cells, indicating that the inactivation of this TFIIIB component results primarily from its enhanced destabilization under conditions of protein synthesis inhibition.

Mitochondrial rRNA-containing petite strains of yeast (Saccharomyces cerevisiae) show a normal nuclear-mitochondrial stringent response

The nuclear-mitochondrial stringent response was examined in isonuclear rho+, 21S rRNA-containing rho-, and rho o strains of S. cerevisiae. By 30 min after nutritional downshift, nuclear rDNA transcription falls to 15% of control levels congruently in all strains, as assayed via whole-cell RNA or by hybrid selection of specific double-labeled transcripts. Both in vivo and in vitro, the mitochondrial stringent response is identical between the rho- strain and its parental rho+ strain, and in both, the kinetics and magnitude of the organellar response mirror those of the nuclear response. The data show that mitochondrial transcription and protein synthesis are not required for stringent regulation of either nuclear or mitochondrial rDNA transcription.

Transcription of ribosomal RNA is differentially controlled in resting and growing BALB/c 3T3 cells

Shortly after serum-deprived BALB/c 3T3 fibroblasts are stimulated to grow in medium containing 10% calf serum, the RNA polymerase I activity in permeabilized cells shows a two-fold increase over the values observed in either serum-deprived or density-inhibited resting cells. Inhibition of protein synthesis by pactamycin or cycloheximide specifically reduces the enhanced RNA polymerase I activity in serum-stimulated cultures without affecting the values in resting cells. On the other hand, inhibition of rRNA processing by the nucleoside analogs 5-fluoruridine and toyocamycin decreases the rate of 45S rRNA transcription in serum-stimulated cells but has no effect on the values found in resting cultures. These data suggest that the regulation of rRNA transcription occurs by two different mechanisms, depending on the growth state of the cell. One mechanism, in serum-stimulated cells, is dependent on a continuous protein synthesis and a correct 45S rRNA processing; the other, in resting cells, is independent of these two parameters.

Eukaryotic RNA polymerases

This review will attempt to cover the present information on the multiple forms of eukaryotic DNA-dependent RNA polymerases, both at the structural and functional level. Nuclear RNA polymerases constitute a group of three large multimeric enzymes, each with a different and complex subunit structure and distinct specificity. The review will include a detailed description of their molecular structure. The current approaches to elucidate subunit function via chemical modification, phosphorylation, enzyme reconstitution, immunological studies, and mutant analysis will be described. In vitro reconstituted systems are available for the accurate transcription of cloned genes coding for rRNA, tRNA, 5 SRNA, and mRNA. These systems will be described with special attention to the cellular factors required for specific transcription. A section on future prospects will address questions concerning the significance of the complex subunit structure of the nuclear enzymes; the organization and regulation of the gene coding for RNA polymerase subunits; the obtention of mutants affected at the level of factors, or RNA polymerases; the mechanism of template recognition by factors and RNA polymerase.

Relaxed mutant of Saccharomyces cerevisiae: proper maturation of ribosomal RNA in absence of protein synthesis

We have characterized a relaxed yeast mutant, S. cerevisiae SY15, isolated by mutagenesis with ethylmethanesulfonate of strain A364A. Starvation for a required amino acid or treatment with cycloheximide blocks protein synthesis in both parental and mutant strains, while the synthesis of total RNA is inhibited by 72% in A364A and 23% in SY15 cells. In the absence of protein synthesis, the transcription of 37S primary precursor to rRNA is not inhibited in the SY15 mutant, and the rRNA transcripts are correctly processed, although at a reduced rate, and are almost free of ribosomal proteins. The relaxed phenotype in yeast is accompanied by alteration in the regulation of rRNA biosynthesis at transcriptional and posttranscriptional levels.

The regulation of RNA synthesis in yeast. V. tRNA charging studies

The stringent control of RNA synthesis in the yeast Saccharomyces cerevisiae may be evoked either by starving for a required amino acid or by inhibiting protein synthesis. The response is non-coordinate in that the synthesis of ribosomal and messenger RNA is depressed whereas that of transfer RNA continues. If protein synthesis is blocked in starved cells then tRNA synthesis is stimulated. In this paper, the relationship between the level of tRNA charging and the transcriptional and translational state of the yeast cell has been examined. When cells are starved for an amino acid the corresponding tRNA species only becomes uncharged. This effect can be counteracted by the addition of protein synthesis inhibitors to the starved cells. In contrast, the same inhibitors provoked the discharge of tRNA in growing (non-starved) yeast. Similar results were obtained when protein synthesis was blocked using a temperature-sensitive mutant. These contrasting effects of translation inhibition on tRNA charging in starved and non-starved cells correlate with the changes that inhibition evoked in the transcriptional state of those cells. The data indicate that tRNA synthesis is under autoregulatory control and that tRNA charging may also play an important role in the regulation of rRNA synthesis.

Further characterization of the effects of 3-methylcholanthrene administration upon hepatic ribonucleic acid polymerase activities

The administration of 3-methylcholanthrene (MC) to rats results in a marked increase in the specific activities of hepatic RNA polymerases I and II. In the present study, we were able to show that this increase was not caused by a shift in the ratio of 'free' to 'template-engaged' RNA polymerase. By means of binding studies with [3H]amatoxin, we were unable to demonstrate any increase in the number of RNA polymerase II molecules in liver after MC administration to the rats. RNA polymerase I was purified in excess of 3000-fold from hepatic nuclei isolated both from control and MC-treated rats. The stimulation in activity was demonstrated at each step in the purification scheme until glycerol sedimentation analysis. Results from cation-exchange chromatography on phosphocellulose indicated that the polycyclic hydrocarbon increased the enzyme activity of RNA polymerase Ib somewhat specifically. Subsequent to glycerol gradient centrifugation, this stimulatory advantage was no longer evident. Reconstitution experiments revealed the presence of a stimulatory component, which was demonstrated in low molecular weight fractions from both control and experimental preparations.

Diabetes and cancer: a postulated relationship

Diabetes leads to periodic fluctuations in levels of nutrients and cyclic nucleotides in plasma and various tissues. Variations in cyclic nucleotide levels, periods of nutrient elevation and surges of insulin combine to stimulate growth. Growth stimulation may be associated with the promotional phase of carcinogenesis. Diabetes should therefore predispose the individual to carcinogenesis and limited epidemiological data available suggest that it does.

Regulation of mitochondrial ribosomal RNA synthesis in yeast. I. In search of a relaxation of stringency

Studies were undertaken to determine if mitochondrial rRNA synthesis in yeast is regulated by general cellular stringent control mechanism. Those variables affecting the relaxation of a cycloheximide-induced stringent response as a result of medium-shift-down or tyrosine limitation include: 1) the stage of cell growth, 2) carbon source, 3) strain differences and, 4) integrity of the cell wall. The extent of phenotypic relaxation decreased or was eliminated entirely in a strain dependent manner as cells entered stationary phase of growth or by growth of cells on galactose or in osmotically stabilized spheroplast cultures. Cytoplasmic and mitochondrial RNA species were extracted from regrowing spheroplast cultures subjected to different experimental regimens and analyzed by electrophoresis on 2.5% polyacrylamide gels. Relative rates of synthesis were determined in pulse experiments and normalized by double-label procedures to longterm label material. Tyrosine starvation was found to inhibit synthesis of the large and small rRNA species of both cytoplasmic and mitochondrial rRNAs to about 5-20% of the control values. Chloramphenicol inhibits mitochondrial and cytoplasmic rRNA synthesis to 60-80% of control; however, chloramphenicol addition does not relax the stringent inhibition of either class of rRNAs. Cycloheximide addition results in 70-80% inhibition of synthesis of both cellular speceis of rRNAs. As noted above, cycloheximide does not relax the stringent response of cytoplasmic rRNA synthesis in spheroplasts, and also does not relax the stringent inhibition of mitochondrial rRNA synthesis. From these studies, we conclude that both cytoplasmic and mitochondrial rRNA synthesis share common control mechanisms related to regulation of protein synthesis by shift-down or amino acid limitation.

Ribosomal RNA transcription in a mutant of Saccharomyces cerevisiae defective in ribosomal protein synthesis

In Saccharomyces cerevisiae, the transcription of ribosomal precursor RNA is severely inhibited in the absence of protein synthesis. However, such transcription is not dependent on the synthesis of ribosomal proteins, nor on the synthesis of mRNA for ribosomal proteins, nor on the processing of ribosomal precursor RNA.

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.

The plasma membrane of Saccharomyces cerevisiae. Isolation and some properties

The isolation of Saccharomyces cerevisiae plasma membrane was carried out after hypotonic lysis of yeast protoplasts treated with concanavalin A by two independent methods: a, at low speed centrifugation and b, at high speed centrifugation in a density gradient. Several techniques (electron microscopic, enzymic, tagging, etc.) were used to ascertain the degree of purification of the plasma membranes obtained. The low speed centrifugation technique as compared with the other method gave a higher yield of plasma membranes with a similar degree of purification. Analysis of the yeast plasma membrane of normally growing cells by sodium dodecyl sulphate polyacrylamide gel electrophoresis showed at least 25 polypeptide bands. Twelve glycoprotein bands were also found, and their apparent molecular weights were determined. Treatment of the protoplasts with cycloheximide resulted in a significant decrease in the carbohydrate and protein content of the plasma membrane. The electrophoretic pattern of the plasma membrane of cycloheximide-treated cells showed a redistribution of the relative amounts of each protein band and a drastic reduction in the number of Schiff-positive bands. The isoelectric point of the most abundant proteins was low (pI 4) or lower than expected from previous data. A large part of the mannosyl transferase activity found in the cell (80%) was associated with the internal membranes, the remaining activity (20%) was located in the plasma membrane preparation. Part of the mannosyl transferase activity of the cells is located at the plasma membrane surface. Invertase (an external mannoprotein) is found in both the plasma and internal membranes, and as the specific activity dropped significantly following cycloheximide treatment of the cells, it is suggested that these membranes systems are the structures for the glycosylation of a precursor invertase and its subsequent release into the periplasmic space. Other transferase found in the plasma membrane preparation transfers glucose residues from UDPglucose to a poly(alpha(1 leads to 4) polymer identified as glycogen.

Purification and properties of RNA polymerase I from Ehrlich ascites cells

DNA-dependent RNA polymerase I (or A) (nucleoside triphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) was purified from Ehrlich ascites cells after solubilization from isolated nuclei. The purification was accomplished by a procedure involving initial precipitation with ammonium sulfate, following by chromatographies on DEAE-Sephadex and phosphocellulose ion exchange resins and gel filtration on Sepharose 6B. A chromatographically homogeneous enzyme was obtained which was purified about 2300-fold relative to nuclear extracts. The specific activity of the most purified enzyme fraction was 230 nmol of [3H]UTP incorporated into RNA per mg of protein in 10 min at 37 degrees C, which is similar to those reported for the highly purified RNA polymerase I from mouse myeloma and calf thymus. The elution position on Sepharose 6B gel filtration indicated a molecular weight of approx. 580 000. Analysis of the purified enzyme by polyacrylamide gel electrophoresis under nondenaturing conditions revealed only one protein band. Certain heterogeneity in the RNA polymerase I fractions was found in the early chromatographic steps, but not in the most purified fractions.

The regulation of RNA synthesis in yeast. I: Starvation experiments

The synthesis of tRNA in yeast is shown to be under separate control to that of rRNA during amino acid and nitrogen starvation. Inhibitors of the elongation and termination steps of protein synthesis were found to stimulate the synthesis of tRNA in starved yeast cells. This effect appeared to be due to the "trickle-charging" of tRNA. Two inhibitors of early steps in the initiation of protein synthesis were found to be unable to stimulate RNA synthesis in starved cells. It is proposed that yeast tRNA synthesis is under autoregulatory control and that the level of tRNA charging and the mRNA-ribosome complex are important components of this control system.

Effect of cycloheximide on RNA synthesis in Chironomus polytene chromosomes

Modifications in the synthesis of salivary gland RNA were induced by treatments with 10 microgram/ml cycloheximide (CHM) on 4th instar larvae of Chironomus pallidivitattus. After 3, 6 and 24 h CHM treatment, RNA was labeled "in vitro", by incubating the salivary glands in a medium containing H3-uridine. The electrophoretical analyses corresponding to the 3 and 6 h treatment showed a stimulation of the non-ribosomal components of the newly synthesized RNA, while preribosomal RNA synthesis appeared depressed. This fact was also confirmed at cytological level, since autoradiograms made after 3 h of CHM treatment showed a reduced H3-uridine label over the nucleolus and an increase of diffuse labeling over the chromosomes. Longer treatments (24 h) causes a considerable inhibition of the synthesis of all RNA species. The role played by protein synthesis inhibition in the aforementioned effects is discussed.--Some of the morphological implications of CHM treatment, such as modifications of the nucleolar structure (nucleolar segregation) are also reported. The use of a squash technique based on glutaraldehyde fixation of the salivary glands, considerably facilitates such studies.

Ornithine decarboxylase may function as an initiation factor for RNA polymerase I

Reparts suggest that the activity of RNA polymerase I is modulated by a labile protein with a hlaf-life of 10 to 20 minutes. Ornithine decarboxylase is the only labile protein (half-life, 10 to 20 minutes) that increases in activity prior to increased RNA polymerase I activity. The addition of a small amount of a highly purified ornithine decarboxylase preparation to an RNA polymerase I assay increases the initial rate of the reaction as well as the time for which the assay is linear. The incorporation patterns of 14C-labeled adenosine triphosphate and 32P-labeled adenosine triphosphate into RNA indicate that the addition of ornithine decarboxylase to the RNA polymerase assay increases the rate of initiation. This report demonstrates a novel way to purify ornithine decarboxylase by RNA polymerase I affinity chromatography and presents data in support of the hypothesis that the labile protein which modulates RNA polymerase I activity is ornithine decarboxylase.

Regulation of RNA polymerase II activity in alpha-amanitin-resistant CHO hybrid cells

CHO hybrid cell lines obtained by fusing cells of wild-type sensitivity to alpha-amanitin with mutant cells containing RNA polymerase II activity resistant to alpha-amanitin have both sensitive (wild-type) and resistant forms of RNA polymerase II. When these hybrids were grown in medium containing alpha-amanitin, the sensitive form of polymerase II was inactivated, and the activity resistant to alpha-amanitin increased proportionally. The total polymerase II activity level therefore remained constant. This regulation of RNA polymerase II activity occurred independently of that of RNA polymerase I and was similar to that observed previously in the alpha-amanitin-resistant rat myoblast mutant clone Ama102 (Somers, Pearson, and Ingles, 1975a). A sensitive radioimmunoassay was developed to quantitate the total mass of RNA polymerase II enzyme. Under conditions of regulation of the enzymatic activity when hybrids grown in alpha-amanitin exhibited a 2-3 fold increase in the activity of the alpha-amanitin-resistant enzyme, no major change in the enzyme mass was detected immunologically. However, quantitation of the alpha-amanitin-inactivated polymerase II of wild-type sensitivity by 3H-amanitin binding indicated that the loss of its enzymic activity was accompanied by a loss of 3H-amanitin binding capacity in the cell lysates. All these results taken together indicate that a mechanism for regulating the intracellular level of RNA polymerase II exists and that it involves changes in the concentration of enzyme.

Control of ribonucleic acid synthesis in eukaryotes. 2. The effect of protein synthesis on the activities of nuclear and total DNA-dependent RNA polymerase in yeast

A thermosensitive conditional yeast mutant (ts-187) which suppresses protein synthesis at the nonpermissive temperature (36 degrees C) also suppresses RNA synthesis. The effect of temperature on the mutant is similar to the addition of cycloheximide--it inhibits the incorporation of labeled precursors into RNA in both whole cells and isolated nuclei. The effect of temperature is selective for the RNA polymerases bound to the nuclear template but not for the total RNA polymerases. Thus, the specific activities and total amounts of RNA polymerase species extracted and assayed with exogenous DNA template are similar in the ts-187 cultured at 23 degrees C and at 36 degrees C. On the contrary, the nuclear polymerases, i.e., RNA synthesis in isolated nuclei, are dramatically inhibited in cells cultured at 36 degrees C. When amino acid starved ts-187 cells are transferred to 36 degrees C, release from the inhibtion of RNA synthesis is observed. As with the addition of cycloheximide, this relaxation is observed in cells but not in isolated nuclei. The parental strain, A364A, which responds by stimulating instead of inhibiting protein synthesis when the temperature is increased to 36 degrees C, also exhibits an inhibition in the incorporation of labeled precursor into RNA as well as reducing RNA synthesis in isolated nuclei. However, these are transitory inhibitions and afterward there is reinitiation of both processes. Reinitiation of RNA synthesis in isolated nuclei is similar to the relaxed phenomenon and it is called "nuclear relaxation". This relaxation can only be obtained if protein synthesis is not inhibited; however, cellular relaxation occurs in the absence of protein synthesis. The repression of the nuclear RNA polymerase activities which starvation and inhibition of protein synthesis produce appears to be due to a restriction in the nuclear DNA template. This notion is supported by the fact that a net diminution of these nuclear enzyme activities is observed in spheroplasts cultured under starving conditions. Studies of the four main ribonucleotide pools indicate that stringency and inhibition of protein synthesis (ts-187 cultured at 36 degrees C) produce an increase in UTP and CTP pools. This is consistent with the concept that stringency and inhibition of protein synthesis affect the rate of utilization rather than the synthesis of these ribonucleotide residues. In the A364A and ts-187 yeast strains, the conversion of uracil but not of uridine into the UTP and CTP is inhibited when there is inhibition of the nuclear RNA polymerases. This indicates that the uracil phosphoribosyltransferase but not the uridine-cytidine kinase is allosterically inhibited by UTP and CTP in yeast. The feedback inhibition in the metabolic pathway of the base explains why relaxation cannot be detected when uracil instead of uridine is used as the labeled RNA precursor.

Control of ribonucleic acid synthesis in eukaryotes. 3. The effect of cycloheximide and edeine on rna synthesis in yeast

The addition of cycloheximide to a thermosensitive conditional yeast mutant (ts-187) before and after transfer to the nonpermissive temperature (36 degrees C) for initiation of protein synthesis produces the uncoupling of the RNA and protein synthetic machineries. Since the drug can produce this relaxation in the presence and absence of protein synthesis, it is concluded that the coupling of protein and RNA synthesis, which a temperature shift produces, is not exclusively related to the inhibition of protein synthesis. Support for this assumption has been obtained using the parental (A364A) strain. Transferring this strain to 36 degrees C produces inhibition of RNA synthesis in the presence of stimulation of protein synthesis. Furthermore, cycloheximide and edeine prevent this inhibtion of RNA synthesis that temperature shift produces. It is, therefore, postulated that this inhibition of RNA synthesis results from the synthesis or activation of a factor(s) elicited by the increase in temperature whose function is to repress the transcriptional apparatus. Cycloheximide or edeine can prevent the function of this repressor-like factor by binding to the factor or by preventing its synthesis. The fact that inhibition of protein synthesis either by cycloheximide action or temperature shift in ts-187 produces inhibition of RNA synthesis in isolated nuclei indicates that, in addition to the aforementioned repressor, other factor(s) having a promoter function may exist. Since a slight inhibition of protein synthesis produces nuclear template restrictions, it is postulated that the promoter-like factor(s) is a polypeptide different from the RNA polymerase and, at least in yeast, has a high turnover.

Amino acid starvation affects the initiation frequency of nucleolar RNA polymerase

The synthesis of ribosomal precursor RNA in mouse ascites nucleoli derived from cells starved for amino acids is compared with the activity of nucleoli from control cells cultivated in the presence of all amino acids. It is shown that deprivation of a single essential amino acid from the culture medium results in a drastic decrease of the RNA-forming capacity of the isolated nucleoli by a factor of 2-3. This switchoff in rRNA synthesis is a very fast process. Half-maximal inactivation occurs after only 30 min. Addition of amino acids to starved cells leads to a rapid recovery, which is reflected by a sharp increase in the RNA polymerase activity of the isolated nucleoli. Studies on the molecular mechanism of this amino acid-mediated control of rRNA synthesis indicate that this effect is not caused by different growth rates of the RNA chains, but rather by an altered initiation frequency of the RNA polymerase in vivo. Whereas in nucleoli derived from cells grown in full medium almost all the polymerase is tightly bound in a transcriptional complex, a high amount of "free" polymerase which becomes active after addition of exogenous template is present in nucleoli from starved cells.

Purification of form AI and AII DNA-dependent RNA polymerases from rat-liver nucleoli using low-ionic-strength extraction conditions

Recent findings have confirmed the role of form A DNA-dependent polymerase activity as that which is responsible for the transcription of the ribosomal RNA-coding genes. Unfortunately, the form A enzymes have proved to be very labile and difficult to work with, especially under high ionic strength conditions. We have, therefore, investigated a method for the purification of the form AI and AII enzymes from rat liver using mild low-ionic-strength conditions. Since preparations from whole nuclei were found to be grossly contaminated with protein having similar properties, the enzymes are extracted from nucleoli. Forms AI and AII are separated on a phosphocellulose column, purified by further ion-exchange chromatography, and by sedimentation through a glycerol gradient. The purified enzymes each migrate as a single band on native polyacrylamide gels and have the expected characteristics of form A RNA polymerase. Sedimentation rates through glycerol gradients indicate that they both have a similar size to that of Escherichia coli RNA polymerase (Mr about 500,000). The purified enzymes are free of DNase and RNase. A method is also described for the purification of form B from the nucleoplasm remaining after isolation of nucleoli. The presence of form C activity was not detected.

Studies on the control of ribosomal RNA synthesis in HeLa cells

In many eucaryotic systems protein synthesis is coupled to ribosomal RNA synthesis such that shut-down of the former causes inhibition of the latter. We have investigated this stringency phenomenon in HeLa cells. The protein synthesis inhibitors cycloheximide and puromycin cause inactivation of both processes but valine starvation totally inhibits only the processing of 45-S RNA. DNA-dependent RNA polymerases from A, B and C (or I, II and III respectively) were extracted, separated partially by DEAE-cellulose chromatography and their activity levels determined. These do not decrease significantly during inhibition of protein synthesis. To find out whether or not form A is bound to its template under these conditions, proteins were removed from chromatin with the detergent sarkosyl. This does not affect bound RNA polymerase. Inhibition of protein synthesis caused up to 50% reduction in endogenous alpha-amanitin-insensitive chromatin-RNA-synthesising activity. This reduced level of activity was not affected by sarkosyl treatment. Levels in normal cells were stimulated. This result indicates that the form A RNA polymerase is not bound to its template when protein synthesis is inhibited.

Adrenocorticotropic hormone stimulation of adrenal RNA polymerase I and III activities. Nucleotide incorporation into internal positions and 3' chain termini

In the presence of 50 mM (NH4)2SO4 and low concentrations of alpha-amanitin (7.7 mug/ml), adrenal nuclei synthesize predominately rRNA as characterized by size and base composition. Approximately 10% of the RNA synthesized under these conditions sediments at 4-5 S; this RNA synthesizing activity is inhibited by high concentrations of alpha-amanitin (231 mug/ml) indicating the presence of RNA polymerase III activity. ACTH administration to guinea pigs results in a twofold increase in adrenal nuclear RNA polymerase I and III activities at 14 hr of hormone treatment. Analysis of the amount of radiolabeled nucleoside triphosphate incorporated in vitro into 3' chain termini and into internal nucleotide positions has been utilized to measure the number of RNA chains and the average chain length synthesized in vitro. Incorporation into 3' chain termini is not changed by ACTH; incorporation into internal nucleotides is doubled in parallel with the increase in RNA polymerase I activity. These results are not due to an altered Km of RNA polymerase I for the four nucleoside triphosphates, nor to differential R Nase or phosphatase activity. These studies suggest that the regulation of RNA polymerase I by ACTH is accomplished in part through an increase in the rate of RNA chain elongation.

Biogenesis of polysomes and transport of messenger RNA in yeast

The study of polysomal formation in the ts-136 thermo-sensitive yeast mutant indicates that ribosomes are assembled with an mRNA, which is in a structure tightly bound to membranes, and are then released into the soluble cytoplasmic fraction. This has been observed by studying the recruitment of ribosomes when glucose is added to glucose-starved cells and when transport of mRNA is permitted by shifting the mutant to the permissive temperature. A soluble cytoplasm and a fraction becoming soluble after sodium deoxycholate treatment of a rapidly sedimenting structure have been characterized. The former contains the majority of polysomes, free 80-S monomers and almost all of the ribosomal subunits. The latter fraction is composed of bound 80-S monomers and polysomes, but lacks ribosomal subunits. Treatment of the rapidly sedimenting structure with pancreatic ribonuclease produces the release of 80-S monomers, with EDTA the release of an equal proportion of both ribosomal subunits, and with sodium deoxycholate the release of 80-S monomers and polysomes. These findings are consistent with the assumption that bound ribosomes are assembled with an mRNA which is tightly bound to this rapidly sedimenting structure, presumably membranes. From the operational viewpoint this fraction is called the "membrane." During the process of polysomal formation ribosomes are recruited more rapidly in the "membranes" than in the soluble cytoplasm. Since "membranes" do not accumulate polysomes and contain only a small fraction of the total amount of ribosomes, the result is consistent with the assumption that either there is a higher turnover of bound versus free polysomes or bound polysomes are the precursors of free polysomes. The latter assumption is more likely since we have shown previously (a) that in yeast, transport is coupled with the translation of bound mRNA, (b) that this mRNA is tightly bound to a structure which sediments very rapidly and becomes soluble only after sodium deoxycholate treatment, and (c) when cycloheximide is added during the recruitment of ribosomes there is accumulation of membrane-bound ribosomes.

DNA-dependent RNA polymerase levels during the response of human peripheral lymphocytes to phytohemagglutinin

The cellular levels of the various RNA polymerases have been monitored in resting human peripheral lymphocytes and in lymphocytes stimulated by phytohemagglutin. Activity was measured in the presence of exogenous templates following solubilization and chromatographic resolution of the different RNA polymerases. Resting lymphocytes contain Class I, II, and III RNA polymerases, although the respective levels of activity are very low compared to the levels in metabolically active cell types. During the PHA-induced transformation of resting lymphocytes, the Class I, II, and III enzyme levels rise dramatically. During four days exposure to PHA, the levels of RNA polymerases I and III (which synthesize, respectively, rRNA and the transfer and 5S RNAs) increase 17 fold, while the level of RNA polymerase II (which synthezies heterogeneous nuclear RNA) increase 8 fold. The possible relationship between enzyme levels and the regulation of gene expression is discussed.