Phosphorylation of rat liver ribonucleic acid polymerase I by nuclear protein kinases

Deficient brain RNA polymerase and altered nucleolar structure persists until day 8 after perinatal asphyxia of the rat

RNA polymerases (POL) are integral constituents of the protein synthesis machinery, with POL I and POL III coding for ribosomal RNA and POL II coding for protein. POL I is located in the nucleolus and transcribes class I genes, those that code for large ribosomal RNA. It has been reported that the POL system is seriously affected in perinatal asphyxia (PA) immediately after birth. Because POL I is necessary for protein synthesis and brain protein synthesis was shown to be deranged after hypoxic-ischemic conditions, we aimed to study whether POL derangement persists in a simple, well-documented animal model of graded global PA at the activity, mRNA, protein, and morphologic level until 8 d after the asphyctic insult. Nuclear POL I activity was determined according to a radiochemical method; mRNA steady state and protein levels of RPA4O-an essential subunit of POL I and III-were evaluated by blotting methods; and the POL I subunit polymerase activating factor-53 was evaluated using immunohistochemistry. Silver staining and transmission electron microscopy were used to examine the nucleolus. At the eighth day after PA, nuclear POL I decreased with the length of the asphyctic period, whereas mRNA and protein levels for RPA4O were unchanged. The subunit polymerase activating factor-53, however, was unambiguously reduced in several brain regions. Dramatic changes of nucleolar morphology were observed, the main finding being nucleolar disintegration at the electron microscopy level. We suggest that severe acidosis and/or deficient protein kinase C in the brain during the asphyctic period may be responsible for disintegration of the nucleolus as well as for decreased POL activity persisting until the eighth day after PA. The biologic effect may be that PA causes impaired RNA and protein synthesis, which has been already observed in hypoxic-ischemic states.

Brain RNA polymerase and nucleolar structure in perinatal asphyxia of the rat

Ribosomes are integral constitutens of the protein synthesis machinery. Polymerase I (POL I) is located in the nucleolus and transcribes the large ribosomal genes. POL I activity is decreased in ischemia but nothing is known so far on POL I in perinatal asphyxia. We investigated the involvement of POL I in a well-documented model of graded systemic asphyxia at the level of activity, mRNA, protein, and morphology. Caeserean section was performed at the 21st day of gestation. Rat pups still in the uterus horns were immerged in a water bath for asphyctic periods from 5-20 min. Brain was taken for measurement of pH, nuclear POL I activity, and mRNA steady state, and protein levels of RPA40, an essential subunit of POL I and III. Silver staining and transmission electron microscopy with morphometry when appropriate were used to examine the nucleolus. Brain pH and nuclear POL I activity decreased with the length of the asphyctic period while POL-I mRNA and protein levels were unchanged. Accompanying the decrease in brain pH we found significant changes of nucleolar structure in the course of perinatal asphyxia at the light and electron microscopic level. As early as ten min following the asphyctic insult, morphological disintegration of the nucleolus was observed. The changes became more dramatic with longer duration of perinatal asphyxia. We conclude that severe acidosis may be responsible for decreased POL activity and for disintegration of nucleoli in neurons. This condition may lower the ribosome content in neonatal neurons and impair protein synthesis.

Activity of DNA and RNA polymerases in resurfacing rabbit corneal epithelium

Activity of RNA polymerases I, II and III (distinguished using alpha-amanitin) and activity of DNA polymerases alpha and beta (distinguished using N-ethylmaleimide) were assayed for varying intervals and at varying substrate (UTP or dTTP) concentrations in the purified nuclear fraction from corneal epithelium of carbamylcholine-treated and control eyes of rabbits with resurfacing acid burn defects. Incorporation was linear with time for all enzymes up to 30 min. In 10 min assays at varying substrate concentrations, all polymerases from carbamylcholine-treated eyes had significantly elevated Vmax compared to corresponding control enzymes. The drug also increased apparent affinity of RNA polymerase II for UTP and apparent affinity of DNA polymerases alpha and beta for dTTP. Results are discussed in relation to potential mechanisms by which effects of carbamylcholine on polymerase activity may be mediated.

Rat liver nuclear protein kinases NI and NII. Purification, subunit composition, substrate specificity, possible levels of regulation

Rat liver nuclear protein kinases NI and NII have been purified to homogeneity by an improved method. This method includes a casein-phosvitin-Sepharose column step, which separates the enzymes from the other chromosomal non-histone proteins, and a gel filtration at high ionic strength in the presence of a high concentration of protease inhibitors to separate the two enzymes from each other. NI has an apparent molecular mass of approximately 50 kDa and is composed of a single subunit. NII has an apparent molecular mass of 133 kDa and is composed of two subunits of identical molecular mass. The V and the Km of the two enzymes were determined for several substrates. Both enzymes phosphorylate chromosomal non-histone proteins with partly different specificities as shown by two-dimensional electrophoreses. When incubated in the absence of protease inhibitors, the enzymes were degraded into discrete polypeptides. Autophosphorylation of a polypeptide derived from NII was observed after incubation of the enzyme with ATP. This phosphorylation stimulated the enzyme activity. Several chromosomal proteins coeluted with NII from the casein-phosvitin-Sepharose column. They remained associated with the enzyme in sucrose gradients, during gel filtration performed at physiological ionic strength, and are dissociated at high ionic strength. These proteins were highly phosphorylated when the protein-NII complex was incubated with ATP.

Phosphatidylinositol-dependent activation of DNA polymerase alpha

DNA polymerase alpha was activated in vitro by cAMP-independent, phospholipid-dependent, protein kinase catalytic subunit. Of the phospholipids examined, phosphatidylinositol showed the greatest potential for interaction with protein kinase and ATP to activate DNA polymerase alpha in vitro. DNA polymerase alpha was directly activated by phosphorylated phosphatidylinositol in the absence of protein kinase and ATP. Activation of DNA polymerase alpha as a function of phosphorylation was demonstrated using 32P-ATP as the phosphate donor. In vitro treatment of the enzyme with phosphatidylinositol produced Linweaver-Burk plots showing noncompetitive kinetics of enzyme activation, suggesting that activation occurs prior to binding of the enzyme to DNA template/primer. These data indicate that DNA polymerase alpha may be activated in vitro in the presence of protein kinase, ATP, and phosphatidylinositol, and suggest that phosphorylation of the enzyme may constitute an intracellular mechanism of enzyme activation.

Purification of nuclear cAMP-independent protein kinases from rat ventral prostate

Two nuclear cAMP-independent protein kinases (designated PK-N1 and PK-N2) were purified from rat ventral-prostate and liver. The yield of enzyme units was 4-5% and 7-9% for each enzyme from the prostatic nuclei and liver nuclei, respectively. The average fold purification for prostatic nuclear protein kinase N1 and N2 was 1360 and 1833, respectively. The respective average specific activity of the two enzymes towards casein was 81,585 and 110,000 nmol 32P incorporated/hr/mg of enzyme. Protein kinase N1 comprised one polypeptide of Mr 35,000 which underwent phosphorylation in the presence of Mg2+ + ATP. Protein kinase N2 comprised two polypeptides Mr 40,000 and 30,000 of which only the Mr 30,000 polypeptide was autophosphorylated. Both enzymes were active towards casein, phosvitin, dephosphophosvitin, spermine-binding protein, and non-histone proteins in vitro. Little activity was detected towards histones. Both enzymes were stimulated by 150-200 mM NaCl. MgCl2 requirement varied with the protein substrate but was between 2-4 mM for both enzymes. With dephosphophosvitin as substrate, the apparent Km for ATP for N1 protein kinase was 0.01 mM. GTP did not replace ATP in this reaction. Protein kinase N2 was active in the presence of ATP or GTP. The apparent Km was 0.01 mM for ATP, but 0.1 mM for GTP.

Cyclic nucleotides in muscarinic regulation of DNA and RNA polymerase activity in cultured corneal epithelial cells of the rabbit

DNA and RNA polymerase activities in the purified nuclear fraction from cultured rabbit corneal epithelial cells were assayed over a range of substrate (labeled dTTP or UTP) concentrations using calf thymus DNA as template. Effects of carbamylcholine on polymerase activities were evaluated over a range of drug concentrations including those saturating muscarinic receptors. Carbamylcholine significantly (p less than 0.001) enhanced activity of both polymerases, both in nuclei incubated with the drug during assay and in nuclei from carbamylcholine-treated cells. Drug effects were blocked by atropine. Regression analysis of Hill plots for variation of polymerase activity with carbamylcholine concentration indicated half-maximal activity of both polymerases at approximately 1 microM carbamylcholine. Mechanisms by which carbamylcholine may alter polymerase activities are discussed in relation to effects of the drug on nuclear enzymes of cyclic nucleotide metabolism and on cyclic nucleotide-dependent protein phosphorylation.

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.

Differential effects of polyamines on rat thyroid protein kinase activities

Ornithine decarboxylase, the rate-limiting enzyme in polyamine biosynthesis, has been shown to be regulated in thyroid by thyrotropin both in vivo and in vitro. Little, however, is known of the role of polyamines in thyroid cell function. Since studies in other tissues suggest that polyamines may influence protein phosphorylation, we studied the effect of the polyamines on various protein kinase activities in rat thyroid. Putrescine, spermidine, and spermine inhibit cyclic-AMP-dependent histone H1 kinase activity when measured in the cytosol fraction of rat thyroid; this effect is largely reproduced by NaCl concentrations of equivalent ionic strength. Both spermidine and spermine effect a 1.6-2.4-fold increase in cytosolic cyclic-AMP-independent (messenger-independent) casein kinase activity; stimulation by both polyamines is maximal at 5mM. A similar profile of stimulation is observed for messenger-independent casein kinase activity in crude nuclear preparations. Sodium chloride fails to stimulate both cytosolic and nuclear messenger-independent casein kinase activities at ionic strength equivalent to the spermine concentrations used. Spermine, but not putrescine, spermidine, or sodium chloride, inhibits calcium/phospholipid-dependent protein kinase C activity in cytosol extracts partially purified by DEAE chromatography. These findings suggest that regulation of protein kinase(s) by polyamines may represent a proximal locus (i) of action of thyrotropin-regulated ornithine decarboxylase activity in thyroid.

Alterations in hepatic protein kinase activity induced by triiodothyronine

Quantitative and qualitative alterations in protein kinase (PK) activity in preparations of rat liver from hypothyroid (H) and T3-treated hypothyroid animals (T) were analyzed in comparison to enzyme from normal (N) animals. Qualitative variations in type of protein kinase were assessed by chromatography of cytosol preparations on DE-52 cellulose columns. Cytosol kinases resolved into a small fraction I containing a catalytic subunit, fraction II containing a type 1 holoenzyme, fraction III containing a cyclic-AMP independent PK, and fraction IV containing a type 2 holoenzyme. A cytosolic kinase active with casein as substrate was also identified. In H the type 1 holoenzyme was reduced in comparison to N. There were no other qualitative changes. Nuclear PKs were extracted with solvents containing 0.3 M KCl. Qualitative changes were evaluated by chromatography on phosphocellulose columns, but available methodology did not give reproducible evidence of changes in individual PKs. H had significantly more cytosolic PK active with protamine substrate than did normal animals, and by administration of T3, the H level was reduced progressively over 48 h to the N level. The activation ratio of total PK in cytosol was higher in H, and was also reduced to the N level by T3 administration. This suggests a higher steady state level of cyclic-AMP may be present in H rat liver cytosol. Cytosolic protein kinase reactive with casein as substrate increased gradually over 48 h after T3 administration from the H to the higher N level. It was significantly elevated to 110% of the H value by 48 h after T3 administration. The behavior of nuclear PK was entirely different. Nuclear PK reactive with protamine as substrate was increased within 1-1/2 h after T3 administration, reaching a peak of 130% of the control value at 5 h and returning to the normal level by 48 h. In contrast, nuclear PK reactive with casein as substrate also increased by 1-1/2 h after T3 administration and remained elevated at 110-115% of the control value throughout 48 h after T3 administration. The early changes in nuclear PK activity were prevented by administration of cycloheximide or alpha-amanitin. The observed changes in cytosolic PK, including the increment induced by T3 in type 1 enzyme, reduction in

Polyamine-mediated protein phosphorylations: a possible target for intracellular polyamine action

Polyamines are well-known ubiquitous components of living cells. Although these polycations have been implicated in the regulation of major cellular functions such as DNA, RNA and protein synthesis occurring during cellular proliferation and/or differentiation processes, their mechanism of action at the molecular level has remained obscure. On the other hand, protein phosphorylation has emerged as a regulatory process of prime importance in cellular regulation. Data have recently been presented suggesting that polyamines may express at least part of their biological action through an effect upon selective protein phosphorylation systems. Two types of polyamine-sensitive protein kinases have been characterized in the last few years. The best known in molecular terms is the widespread casein kinase G (also termed casein kinase II), which represents a multifunctional protein kinase, at present classified as a messenger-independent activity. The other is a polyamine-dependent nuclear ornithine decarboxylase kinase characterized in Physarum polycephalum and several mammalian tissues. Both protein kinases are activated by polyamines in vitro at concentrations compatible with a physiological role, by a mechanism which most likely also involves an effect through the protein substrate conformation. Preliminary evidence suggests that both kinases may be implicated in the regulation of DNA-dependent RNA polymerase activities, although several other potential substrates have been suggested for casein kinase G. Another suggestion is that these kinases may also participate in the post-translational regulation of ornithine decarboxylase, the rate-limiting step in the polyamine biosynthetic pathway. A novel class of protein kinase activities may thus be defined as polyamine-mediated phosphorylation systems for which polyamines may function as intracellular messenger. Although their biological significance remains to be fully established, especially with regard to the definition of their specific intracellular target(s) and subsequent biological functions, these systems will be interesting to consider in future studies aimed at understanding the role of polyamines in cell regulation.

On the phosphorylation of yeast RNA polymerases A and B

In exponentially growing cells, RNA polymerase B is exclusively form BI enzyme with several phosphorylated subunits: B220, B23 and possibly B44.5. In RNA polymerase A an average of fifteen phosphate groups are distributed on the five phosphorylated subunits: A190 (6), A43 (4), A34.5 (2), A23 (1-2) and A19 (1-2). Phosphorylation of enzyme A by a yeast protein kinase in vitro adds less than 1 mol phosphate/mol enzyme but occurs essentially at the physiological sites, as shown by a comparison of the peptide patterns obtained by limited proteolysis of subunits 32P-labelled in vivo and in vitro. No evidence was found in favor of a modulation of RNA polymerase activity in vitro or in vivo via phosphorylation.

RNA polymerase I in hepatoma 3924A: Mechanism of enhanced activity relative to liver

Two major cyclic nucleotide-independent protein kinases, NI and NII, have been identified in Morris hepatoma 3924A and rat liver. When expressed per unit DNA, the activities of protein kinase NI and NII were 1.3 and 12 times greater, respectively, in the hepatoma than in liver. Protein kinase NII, but not NI, was capable of phosphorylating and activating the DNA-dependent RNA polymerases I and II. Phosphorylation of RNA polymerase I was accompanied by an increase in average size of the RNA synthesized in vitro, whereas phosphorylation of RNA polymerase II was concomitant with an elevation in the number of RNA chains initiated. RNA polymerase I polypeptides of Mr 120,000, 65,000 and 25,000 were phosphorylated by protein kinase NII; RNA polymerase II polypeptides of Mr 214,000, 140,000 and 21,000 were modified by this kinase. In contrast to the purified hepatoma enzyme, RNA polymerase I activity in nuclear lysates was not affected by addition of protein kinase NII. In vitro phosphorylation of the tumor lysate followed by immunoprecipitation of RNA polymerase I polypeptides indicated little or no phosphate transfer to the 65,000 Mr polypeptide of the enzyme. These data suggested that the tumor enzyme, particularly the 65,000 Mr polypeptide, was highly phosphorylated in vivo, but becomes dephosphorylated during purification. Unlike the tumor enzyme, RNA polymerase I in the liver lysate responded to protein kinase addition; phosphorylation of the liver polymerase I polypeptides of Mr 120,000, 65,000 and 25,000 was observed. These observations indicate that the liver enzyme is not completely phosphorylated (activated) in vivo and that the relatively rapid rate of ribosomal RNA synthesis in the rapidly growing hepatoma may result, at least in part, from a polymerase I which is maximally phosphorylated.

Biochemical characterization of a specific phosphate acceptor of nuclear cyclic AMP-independent protein kinase

The regulatory mechanism of transcription involved in the phosphorylation of a 13 kDa non-histone chromatin protein from calf thymus, which is the most effective phosphate acceptor for cyclic AMP-independent protein kinase purified from the nuclei of mouse spleen cells, by the kinase has been studied in vitro. An analytical study of the circular dichroism (CD) spectra of the 13 kDa protein under different conditions showed that it underwent a major conformational change when incubated with DNA. The presented data suggest that the DNA-induced conformational change may result in a great increase of the 13 kDa protein phosphorylation by the kinase in vitro. Mg2+ (8-10 mM) enhanced the binding of the protein to DNA. Furthermore, the phosphorylated 13 kDa protein stimulated elongation of RNA synthesis by RNA polymerase II from calf thymus. However, neither the 13 kDa protein nor the phosphorylated 13 kDa protein had any affect on DNA synthesis. The available evidence suggests that the 13 kDa protein may play a role in the regulation of transcription through its phosphorylation by the kinase in vitro.

DNA-dependent RNA polymerases from bovine thyroid: catalytic properties and template specificities

1. DNA-dependent RNA polymerases I and II have been purified starting from bovine thyroid nuclei yielding a purification factor of 230 for the RNA polymerase I and a purification factor 3212 for RNA polymerase II. RNA polymerase II was further characterized by gel electrophoresis and amino-acid analysis. 2. Kinetics and optimal assay conditions for both RNA polymerases were studied. 3. The template efficiency of a number of DNA preparations was investigated. 4. Rifamycin AF 013 and heparin act as initiation inhibitors. 5. Polyamines were shown to enhance the rate of chain elongation.

The role of cyclic 3',5'-AMP in the regulation of aminoacyl-tRNA synthetase activities in mouse uterus and liver following 17beta-oestradiol treatment. Activation of a phosphoaminoacyl-tRNA synthetase phosphatase by phosphorylation with cyclic 3',5'-AMP dependent protein kinase

The changes in the activities of 17 aminoacyl-tRNA synthetases induced by phosphorylation [1] were reversed by the action of cyclic AMP in preparations from both uterus and liver. Cyclic AMP also inhibited the phosphorylation of aminoacyl-tRNA synthetase protein by endogenous non-cyclic AMP-dependent protein kinase and [gamma-32P]ATP. The effect was not due to a stimulation of phosphoaminoacyl-tRNA synthetase phosphatase or to an influence of cyclic AMP on aminoacyl-tRNA synthetases. The activity of phosphoaminoacyl-tRNA synthetase phosphatase was increased by treatment with endogenous cyclic AMP-dependent protein kinase, ATP and cyclic AMP. Affinity chromatography of the 32P-labeled phosphorylated phosphosynthetase phosphatase protein followed by gel electrophoresis showed that the activated phosphatase was phosphorylated. In the uterus, the changes in 17 aminoacyl-tRNA synthetase activities observed 5 min after dibutyryl cyclic AMP administration to ovariectomized mice were similar to those observed after 17beta-oestradiol treatment, whereas in the liver the changes in these activities were the opposite to those found after treatment with 17beta-oestradiol. A mechanism for the regulation of the 17 aminoacyl-tRNA synthetase activities is proposed, which suggests that the synthetase activities inhibited (group I) or stimulated (group II) by phosphorylation with a non-cyclic AMP-dependent aminoacyl-tRNA synthetase kinase are reactivated (group I) or inhibited (group II), respectively, by the action of a cyclic AMP-dependent phosphatase kinase through the increased activity of phosphorylated phosphoaminoacyl-tRNA synthetase phosphatase.

Isolation and characterization of RNA polymerase B from the larval fat body of the tobacco hornworm, Manduca sexta

DNA-dependent RNA polymerase B has been extensively purified from the larval fat body of the tobacco hornworm (Manduca sexta) by employing chromatography on ion-exchange columns of DEAE-Sephadex, DEAE-cellulose and phosphocellulose and centrifugation on glycerol gradients. The isolated enzyme after electrophoresis on acrylamide gels shows one main band and one minor band, both having enzyme activity sensitive to alpha-amanitin. The catalytic and physicochemical properties of the enzyme are similar to those of other eucaryotic B-type RNA polymerases. The enzyme has an apparent molecular weight of 530000, is inhibited 50% by alpha-amanitin at 0.04 microgram/ml and shows maximum activity on denatured DNA at 5 mM Mn2+ and 100 mM ammonium sulfate. An antibody was obtained that cross-reacts with the pure enzyme and forms a precipitin line. This antibody does not cross react with either Escherichia coli RNA polymerase or with wheat germ RNA polymerase but does react with one of the B polymerases isolated from wing tissue of the silkmoth, Antheraea pernyi.

Purification and properties of a cyclic AMP-independent protein kinase from calf thymus nuclei

A phosphoprotein kinase (ATP : protein phosphotransferase, EC 2.7.1.37) from calf thymus nuclei was purified by DEAE-cellulose chromatography, hydroxyapatite, and Sepharose 6B gel filtration. The enzyme is a cyclic AMP-independent protein kinase by the following criteria: (a) the protein kinase did not bind cyclic AMP; (b) no inhibition of activity was obtained with the heat-stable protein kinase inhibitor from rabbit skeletal muscle; (c) the regulatory subunit of cyclic AMP-dependent protein kinase had no effect on activity; and (d) no inhibition was obtained with antibody to cyclic AMP-dependent protein kinase. The nuclear cyclic AMP-independent protein kinase readily phosphorylated protamine on serine and to a lesser extent on threonine. Homologous nucleoplasmic RNA polymerase (EC 2.7.7.6) is a better substrate than arginine-rich histone, phosvitin or casein. Physical characteristics of the enzyme are described.

Phosphorylation of calf thymus RNA polymerase II by nuclear cyclic 3',5'-AMP-independent protein kinase

Nucleoplasmic RNA polymerase II (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) from calfthymus is phosphorylated by homologous cyclic AMP-independent protein kinase (ATP:protein phosphotransferase, EC 2.7.1.37). Polyacrylamide gel electrophoresis of the 32P-labeled RNA polymerase II under non-denaturing conditions revealed that both forms of the enzyme were phosphorylated. Polyacrylamide gel electrophoresis of the 32P-labeled RNA polymerase II under denaturing conditions showed that the 25 000 dalton subunit was the phosphate acceptor subunit. Partial acid hydrolysis of the 32P-labeled RNA polymerase II followed by ion-exchange chromatography revealed serine and threonine as the [32P]phosphate acceptor amino acids. Phosphorylation of the RNA polymerase II was accompanied by a stimulation of enzymatic activity and was dependent upon the presence of ATP.

Adenosine 3',5'-monophosphate-dependent protein kinases of myocardial non-histone nuclear proteins

Myocardial acidic non-histone nuclear proteins (NHPs) contain endogenous protein kinase activity. Phosphocellulose chromatography of purified NHPs identifies nine separate peaks of protein kinases which can phosphorylate both endogenous and exogenous substrates to a variable degree; endogenous NHPs are the best substrates. Cyclic AMP-stimulated protein kinase induced phosphorylation of endogenous and exogenous substrates; the extent of this stimulation varied according to the protein kinase fraction and substrate used. Cyclic AMP also enhanced NHP-induced stimulation of RNA polymerase activity. This enhancement was dependent on protein kinase-induced phosphorylation of NHPs since it was prevented by alkaline phosphatase pretreatment. It is concluded that nuclear protein kinases regulate myocardial RNA synthesis by enhancing phosphorylation of NHPs and that this regulation is under cyclic AMP control.

Transcription of specific genes in isolated nuclei by exogenous RNA polymerases

Mouse plasmacytoma (MOPC) 460 cells contain two chromatographic forms of RNA polymerase III (IIIA and IIIB) in addition to the major class I and II RNA polymerases. Nuclei isolated from these cells actively synthesize RNA. Among the discrete transcription products observed are the 5S and 4.5S RNAs and additional low molecular weight RNA species (approximately 5.8S, 6.3S, and 6.6S in size). The 4.5S RNAs appear to be tRNA precursors since they can be converted in vitro to 4S RNAs. Studies with alpha-amanitin have shown that the synthesis of these discrete RNA species, and other uncharacterized transcripts somewhat larger in size, is mediated by an endogenous RNA polymerase III activity(ies). Nuclear RNA synthesis is stimulated by exogenous purified RNA polymerases. Exogenous MOPC class III RNA polymerases stimulate the synthesis of each of the distinct low molecular weight species (including 5S and 4.5S RNAs) about 3-6 fold. The hybridization of nuclear transcripts to purified 5S genes (5S DNA) confirms that exogenous class III RNA polymerases stimulate (approximately 4 fold) the synthesis of ribosomal 5S RNA. The 5S RNA genes in nuclei are transcribed asymmetrically by both the endogenous and the exogenous class III enzymes. Exogenous RNA polymerase III from Xenopus laevis ovaries stimulates 4.5S and 5S RNA synthesis in MOPC nuclei as effectively as do the MOPC class III RNA polymerases. However, exogenous MOPC class I and II RNA polymerases do not stimulate 4.5S and 5S RNA synthesis, suggesting that this effect is specific for the structurally similar class III RNA polymerases.