RNA synthesis in isolated nuclei: the use of mercurated nucleotides

Transcriptional regulation of the hepatic glutaminase gene in the streptozotocin-diabetic rat

1. Liver possesses a unique isozyme of phosphate activated glutaminase which is subject to long-term regulation. 2. In the rat streptozotocin-diabetes results in a 4-fold increase in the rate of transcription of the rat hepatic glutaminase gene. 3. This is consistent with previous reports from this laboratory of increases, of similar magnitude, in the relative abundance of hepatic glutaminase mRNA (Smith and Watford (1990) J. Biol. Chem. 265, 10631-10636), and enzyme activity (Watford, et al. (1984) Biochem. J. 224, 207-214). 4. The work establishes that, in contrast to the regulation of renal glutaminase where mRNA stability plays an important role, the predominant site of long-term regulation of hepatic glutaminase is at the level of gene transcription.

Influence of diet on the transcriptional and post-transcriptional regulation of malic enzyme induction in the rat liver

Fasted rats were refed a carbohydrate/protein diet, a carbohydrate diet (without protein) or a protein diet (without carbohydrate) to investigate, using a cDNA cloned in our laboratory, the regulatory mechanisms involved in hepatic malic enzyme induction. In the carbohydrate/protein diet, although the enzyme activity and the mRNA concentration of malic enzyme were increased about 7-fold above the levels in the fasted rat, the rate of transcription was increased only 2-fold. In the carbohydrate diet group the rate of transcription and the concentration of mRNA were increased to the levels in the carbohydrate/protein diet group, whereas the enzyme activity increased only to 60% of those levels. Protein appears to contribute to an increase in the translation of malic enzyme mRNA. In the protein diet group the transcriptional rate was not low, but the mRNA concentration was about half in comparison with the level of the carbohydrate/protein diet group. Further, dietary fat did not reduce the transcriptional activity, but reduced the mRNA concentration and the enzyme activity to half of the basal levels. Therefore, it is suggested that fat stimulates the degradation of the mRNA in liver cytosol, whereas carbohydrate tends to stabilize the mRNA. On the other hand 3,5,3'-triiodothyronine treatment increased the transcriptional activity by 1.5-2-fold above the basal values on all the diets and even on fasting. Thus, it is suggested that 3,5,3'-triiodothyronine increases the transcriptional activity of malic enzyme independently from nutritional regulation, while the nutrients are predominantly involved in the post-transcriptional steps.

Regulation of genes for enzymes involved in fatty acid synthesis

The levels of malic enzyme and fatty acid synthase are increased by feeding and decreased by starvation in liver in vivo and are increased by triiodothyronine and decreased by glucagon in hepatocytes in culture. Cloned malic enzyme and fatty acid synthase cDNAs are being used to analyze regulation of these unique genes. Dietary regulation of both enzymes occurs at pretranslational steps. Increased transcription and increased mRNA stability contribute about equally to a 20-fold increase in malic enzyme mRNA level when starved ducklings are refed. In contrast, a 10-fold increase in the level of fatty acid synthase mRNA is largely accounted for by increased transcription of this gene. In chick-embryo hepatocytes incubated in serum-free medium containing insulin, triiodothyronine causes a greater than 10-fold increase in levels of both malic enzyme and fatty acid synthase mRNAs. Kinetic and inhibitor experiments suggest a protein intermediate in the increases of malic enzyme and fatty acid synthase mRNAs caused by triiodothyronine. For malic enzyme, the stimulation by triiodothyronine is predominantly posttranscriptional. Glucagon decreases the level of malic enzyme mRNA by 90 to 95%, with regulation occurring at a posttranscriptional step. Inhibitor experiments suggest that stimulation of the degradation of malic enzyme mRNA is partially responsible. Glucagon inhibited fatty acid synthase mRNA level by less than 50%; the inhibited step has not been identified. Thus, the coordinated regulation of malic enzyme and fatty acid synthase proteins by nutritional state may involve different hormones regulating at different points. A surprisingly large component of the regulation is posttranscriptional.

Changes of renal mRNA species abundance by ochratoxin A

Ochratoxin A is a nephrotoxin produced by certain species of Aspergillus and Penicillium. We have found previously that renal but not hepatic P-enolpyruvate carboxykinase, and the mRNA for this enzyme, are rapidly decreased in rats and swine fed 0.1 to 1 mg/kg body weight for a few days. In the present study, we isolated kidney mRNA from rats fed ochratoxin A for 2-5 days. By screening a rat kidney cDNA library with [32P]RNA, we have identified several renal mRNAs whose concentration is changed within 2 days by the toxin. The transcription rate of each mRNA was measured in nuclei isolated from kidneys of rats fed ochratoxin A. The incorporation of [32P]UMP into P-enolpyruvate carboxykinase mRNA and the synthesis of other RNAs were not affected. Therefore, the toxin changes mRNA abundance at the post-transcriptional level.

Effect of hormones on transcription of the gene for cytosolic phosphoenolpyruvate carboxykinase (GTP) in rat kidney

The effect of hormones on the transcription rate of cytosolic phosphoenolpyruvate carboxykinase and level of mRNA for this enzyme in the rat kidney has been investigated. In renal nuclei isolated from rats given dibutyryladenosine cyclic 3',5'-phosphate (Bt2cAMP) or 8-bromoadenosine cyclic 3',5'-phosphate (8-Br-cAMP), [32P]UMP incorporation into hybridizable phosphoenolpyruvate carboxykinase mRNA increased severalfold within 1 h. Changes in the concentration of cytosolic phosphoenolpyruvate carboxykinase mRNA, measured by hybridization of [32P]cDNA to poly(A)+ mRNA, paralleled alterations in the transcription rate. Dexamethasone treatment of adrenalectomized rats increased the transcription rate and the level of phosphoenolpyruvate carboxykinase mRNA 3-4-fold after 4 h. Both parameters then declined to control values by 8 h. When dexamethasone (5 mg/kg) and Bt2cAMP (25 mg/kg) were given together, the rate of phosphoenolpyruvate carboxykinase RNA synthesis and the level of cytosolic mRNA were not increased more than those with either drug alone. Transcription of the gene for renal phosphoenolpyruvate carboxykinase was not affected by diabetes or glucose refeeding but was increased 2-fold after 24 h of starvation and reduced by bicarbonate feeding after 2 h. We conclude that glucocorticoids and cAMP change the rate of transcription of the phosphoenolpyruvate carboxykinase gene in rat kidney, leading to changes of similar magnitude in mRNA level and, hence, enzyme activity. The results presented here and in previous work [Lamers, W., Hanson, R. W., & Meisner, H. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 5137] indicate that the transcription rate of the gene for phosphoenolpyruvate carboxykinase in liver and kidney responds to hormones in a tissue-specific manner.

cAMP stimulates transcription of the gene for cytosolic phosphoenolpyruvate carboxykinase in rat liver nuclei

The effects of starvation, glucose refeeding, dibutyryl cAMP, and dexamethasone on expression of the gene for phosphoenolpyruvate carboxykinase (GTP) [GTP:oxaloacetate carboxy-lyase (transphosphorylating), EC 4.1.1.32] from rat liver cytosol was studied by using a cloned cDNA probe. The rate of transcription of the gene for phosphoenolpyruvate carboxykinase in hepatic nuclei isolated from starved rats decreased rapidly after refeeding with glucose. Administration of dibutyryl cAMP to glucose-refed animals increased the rate of phosphoenolpyruvate carboxykinase gene transcription seven-fold within 20 min. Phosphoenolpyruvate carboxykinase mRNA in the cytosol is 2.8 kilobases long whereas liver nuclei contain four precursor RNA species that are up to 6.5 kilobases long. Feeding glucose to starved rats rapidly decreased the sequence abundance of enzyme mRNA in both nuclei and cytosol. However, the decrease in cytosolic phosphoenolpyruvate carboxykinase mRNA was preceded by a transient increase in enzyme mRNA over the first 20 min after glucose refeeding. Administration of dibutyryl cAMP to glucose-refed starved animals increased the concentration of the nuclear RNA precursors of phosphoenolpyruvate carboxykinase five- to eight-fold within 30 min and induced the mRNA for the cytosolic enzyme over a period of 60 min. We conclude that cAMP induces phosphoenolpyruvate carboxykinase mRNA by increasing the rate of gene transcription.

Separation of mercury substituted RNA synthesized in isolated rat liver nuclei

The population of RNA molecules synthesized in isolated rat liver nuclei in vitro in the presence of [3H]CTP and Hg-UTP was successfully fractionated into at least two subfractions containing various proportions of mercury label. Fractionation was achieved either by step-wise chromatography of Hg-RNA on thiopropyl-Sepharose columns or by density gradient centrifugation in metrizamide. The fraction of RNA heavily labeled with Hg-UTP was composed mainly of 4--18S RNA and contained virtually all radioactivity derived from [gamma-32P]ATP or [gamma-32P]GTP. The slightly mercurated RNA fraction consisted mainly of longer RNA molecules (12- greater than 28S) and was not labeled with [gamma-32P]ATP or [gamma-32P]GTP. Labeling with gamma-32P nucleoside triphosphates was sensitive both to rifamycin AF/013 and heparin whereas labeling with [3H]CTP was fully resistant to the inhibitors and showed sensitivity to low doses of alpha-amanitin. We assume that the observed subpopulation of heavily mercurated RNAs consists of RNA molecules initiated in vitro.

Initiation of transcription by RNA polymerase II in permeable, SV40-infected or noninfected, CVI cells; evidence for multiple promoters of SV40 late transcription

CV1 cells were made permeable by treatment with lysolecithin and incubated in a transcription mixture containing ribonucleoside triphosphates including ATP or GTP 32P-labeled either in the alpha or beta position. 5'-terminal cap structures (7mGpgamma pbeta palpha X) on newly synthesized RNA were analyzed by digestion with nuclease P1 or with ribonuclease T2/bacterial alkaline phosphatase. Cap structures obtained after labeling with alpha-32P-GTP show that the 32P is found only adjacent to the 7mG residue (i.e., in the gamma position) and adjacent to the penultimate Gm or G nucleotide (i.e., in the alpha position). Analysis of RNA synthesized in the presence of beta-32P-ATP, however, shows GpppA cap structures which are labeled only in the beta position. In the presence of beta-32-p-GTP, only GpppG structures are labeled; these findings exclude the hypothesis that caps are synthesized from GTP and a monophosphate 5'-terminal RNA molecule. The results imply that the initial transcripts are used for cap formation, which indicates that the large majority (if not all) of capping sites correspond to initiation sites for transcription. In cells infected with wild-types SV40 the distribution of virus-specific caps is similar when labeled either with beta-32P-ATP or with alpha-32P-GTP or with 32p-phosphate. Thus, evidence is presented that heterogeneity of the cap structures in late SV40 is a consequence of independent initiation events and not of processing of a primary transcript followed by capping of the 5' ends generated.

Simian virus 40 transcriptional complexes incorporate mercurated nucleotides into RNA in vitro

Simian virus 40 Sarkosyl transcriptional complexes incorporated mercurated nucleotide precursors into preinitiated RNA chains. Synthesis with mercurated precursors was three- to fivefold slower than with nonmercurated nucleotides (12 to 20 pmol per 10(6) cells per h at 25 degrees C), and 50 to 70% of the RNA product bound specifically to sulfhydryl-agarose. The amount of mercurated CMP incorporated into RNA was measured by specific binding of [35S]cysteine to the mercury residues. More than 90% of the mercurated RNA hybridized to simian virus 40 DNA.

Antibodies to small nuclear RNAs complexed with proteins are produced by patients with systemic lupus erythematosus

Patients with systemic lupus erythematosus often possess antibodies against two nuclear antigens called Sm and RNP (ribonucleoprotein). We have established the molecular identity of these antigens by analyzing immune precipitates of nuclear extracts from mouse Ehrlich ascites cells labeled with (32)P and (35)S. Anti-Sm serum selectively precipitates six small nuclear RNA molecules (snRNAs); anti-RNP serum reacts with only two of these; and a third serum, characterized as mostly anti-RNP, precipitates a subset of three snRNA bands. Three of the six RNAs are identified by fingerprint analysis as the previously characterized and highly abundant nucleoplasmic snRNA species U1a (171 nucleotides), U1b, and U2 (196 nucleotides). The other three RNAs (U4, U5, and U6) likewise are uridine rich and contain modified nucleotides, but they are smaller, with lengths of about 145, 120, and 95 residues, respectively. Each of the six snRNAs is complexed with and apparently antigenic by virtue of association with specific proteins. All three sera precipitate an identical complement of seven different polypeptides ranging in molecular weight from 12,000 to 35,000; these proteins are abundant in nuclear extracts, but are neither histones nor the major polypeptides comprising the 30S heterogeneous nuclear RNP particles of mammalian nuclei. Our data argue that each of the six snRNAs exists in a separate small nuclear ribonucleoprotein (snRNP) complex with a total molecular weight of about 175,000. We find that human antisera also precipitate snRNAs from a wide range of vertebrate species and from arthropods. We discuss the antigenic snRNPs in relation to the published literature on snRNAs and nuclear RNPs and consider possible functions of snRNPs in nuclear processes.

Transcription by eukaryotic RNA polymerases A and B of chromatin assembled in vitro

Chromatin was assembled in vitro from simian virus 40 DNA form I and the calf-thymus four histones H2A, H2B, H3 and H4. Transcription with calf thymus RNA polymerases A and B (I and II) was greatly inhibited. Nucleosomes were found to inhibit both RNA chain initiation and elongation. The inhibition of elongation could be relieved by increasing ionic strength, suggesting that electrostatic interactions between histone octamer and DNA have to be broken for RNA polymerase to transcribe DNA organised into nucleosomes.