Nuclear poly(A) polymerase from rat liver and a hepatoma. Comparison of properties, molecular weights and amino acid compositions

Identification and functional characterization of neo-poly(A) polymerase, an RNA processing enzyme overexpressed in human tumors

Poly(A) polymerase (PAP) plays an essential role in polyadenylation of mRNA precursors, and it has long been thought that mammalian cells contain only a single PAP gene. We describe here the unexpected existence of a human PAP, which we call neo-PAP, encoded by a previously uncharacterized gene. cDNA was isolated from a tumor-derived cDNA library encoding an 82.8-kDa protein bearing 71% overall similarity to human PAP. Strikingly, the organization of the two PAP genes is nearly identical, indicating that they arose from a common ancestor. Neo-PAP and PAP were indistinguishable in in vitro assays of both specific and nonspecific polyadenylation and also endonucleolytic cleavage. Neo-PAP produced by transfection was exclusively nuclear, as demonstrated by immunofluorescence microscopy. However, notable sequence divergence between the C-terminal domains of neo-PAP and PAP suggested that the two enzymes might be differentially regulated. While PAP is phosphorylated throughout the cell cycle and hyperphosphorylated during M phase, neo-PAP did not show evidence of phosphorylation on Western blot analysis, which was unexpected in the context of a conserved cyclin recognition motif and multiple potential cyclin-dependent kinase (cdk) phosphorylation sites. Intriguingly, Northern blot analysis demonstrated that each PAP displayed distinct mRNA splice variants, and both PAP mRNAs were significantly overexpressed in human cancer cells compared to expression in normal or virally transformed cells. Neo-PAP may therefore be an important RNA processing enzyme that is regulated by a mechanism distinct from that utilized by PAP.

Poly(A) polymerase from Escherichia coli adenylylates the 3'-hydroxyl residue of nucleosides, nucleoside 5'-phosphates and nucleoside(5')oligophospho(5')nucleosides (NpnN)

The capacity of Escherichia coli poly(A) polymerase to adenylylate the 3'-OH residue of a variety of nucleosides, nucleoside 5'-phosphates and dinucleotides of the type nucleoside(5')oligophospho(5')nucleoside is described here for the first time. Using micromolar concentrations of [alpha-32P]ATP, the following nucleosides/nucleotides were found to be substrates of the reaction: guanosine, AMP, CMP, GMP, IMP, GDP, CTP, dGTP, GTP, XTP, adenosine(5')diphospho(5')adenosine (Ap2A), adenosine (5')triphospho(5')adenosine (Ap3A), adenosine(5')tetraphospho(5')adenosine (Ap4A), adenosine(5')pentaphospho(5')adenosine (Ap5A), guanosine(5')diphospho(5') guanosine (Gp2G), guanosine(5')triphospho(5')guanosine (Gp3G), guanosine(5')tetraphospho(5')guanosine (Gp4G), and guanosine(5')pentaphospho(5')guanosine (Gp5G). The synthesized products were analysed by TLC or HPLC and characterized by their UV spectra, and by treatment with alkaline phosphatase and snake venom phosphodiesterase. The presence of 1 mM GMP inhibited competitively the polyadenylylation of tRNA. We hypothesize that the type of methods used to measure polyadenylation of RNA is the reason why this novel property of E. coli poly(A) polymerase has not been observed previously.

Biochemical and immunological identification and enrichment of poly(A) polymerase from human thymus

Human thymus poly(A) polymerase (EC 2.7.7.19) activity has been investigated using poly(A) and oligo(A) as initiators. All obtained fractions reveal more than one polypeptide as detected by immunoblotting after SDS-PAGE. In addition to the homogeneously purified (Tsiapalis et al., J Biol Chem 250: 4486-4496, 1975 and Wahle, J Biol Chem 266: 3131-3139, 1991), about 60 kDa polypeptide, a larger polypeptide, about 80 kDa, that comigrates in the region of poly(A) polymerase activity was detected, enriched and partially characterized; it appears having similar size with bovine poly(A) polymerase cloned in E. coli. Polyclonal antiserum produced against recombinant bovine poly(A) polymerase reacts more efficiently with the about 80 kDa polypeptide upon immunoblotting, and can precipitate the poly(A) polymerase activity. This enzyme form, from human tissue, is novel in terms of size and may reflect intact or physiological form of poly(A) polymerase in human thymus, and supports and substantiates recent reports on the enzyme from other sources.

Multiple forms of poly(A) polymerases in human cells

We have cloned human poly(A) polymerase (PAP) mRNA as cDNA in Escherichia coli. The primary structure of the mRNA was determined and compared to the bovine PAP mRNA sequence. The two sequences were 97% identical at the nucleotide level, which translated into 99% similarity at the amino acid level. Polypeptides representing recombinant PAP were expressed in E. coli, purified, and used as antigens to generate monoclonal antibodies. Western blot analysis using these monoclonal antibodies as probes revealed three PAPs, having estimated molecular masses of 90, 100, and 106 kDa in HeLa cell extracts. Fractionation of HeLa cells showed that the 90-kDa polypeptide was nuclear while the 100- and 106-kDa species were present in both nuclear and cytoplasmic fractions. The 106-kDa PAP was most likely a phosphorylated derivative of the 100-kDa species. PAP activity was recovered in vitro by using purified recombinant human PAP. Subsequent mutational analysis revealed that both the N- and C-terminal regions of PAP were important for activity and suggested that cleavage and polyadenylylation specificity factor (CPSF) interacted with the C-terminal region of PAP. Interestingly, tentative phosphorylation sites have been identified in this region, suggesting that phosphorylation/dephosphorylation may regulate the interaction between the two polyadenylylation factors PAP and CPSF.

Potential role of poly(A) polymerase in the assembly of polyadenylation-specific RNP complexes

To elucidate the mechanism by which poly(A) polymerase functions in the 3'-end processing of pre-mRNAs, polyadenylation-specific RNP complexes were isolated by sedimentation in sucrose density gradients and the fractions were analyzed for the presence of the enzyme. At early stages of the reaction, the RNP complexes were resolved into distinct peaks which sedimented at approximately 18S and 25S. When reactions were carried out under conditions which support cleavage or polyadenylation, the pre-mRNA was specifically assembled into the larger 25S RNP complexes. Polyclonal antibodies raised against the enzyme purified from a rat hepatoma, which have been shown to inhibit cleavage and polyadenylation (Terns, M., and Jacob, S. T., Mol. Cell. Biol. 9:1435-1444, 1989) also prevented assembly of the 25S polyadenylation-specific RNP complexes. Furthermore, formation of these complexes required the presence of a chromatographic fraction containing poly(A) polymerase. UV cross-linking analysis indicated that the purified enzyme could be readily cross-linked to pre-mRNA but in an apparent sequence-independent manner. Reconstitution studies with the fractionated components showed that formation of the 25S RNP complex required the poly(A) polymerase fraction. Although the enzyme has not been directly localized to the specific complexes, the data presented in this report supports the role of poly(A) polymerase as an essential component of polyadenylation-specific complexes which functions both as a structural and enzymatic constituent.

Tissue and species distribution of liver type and tumor type nuclear poly(A) polymerases

Previous studies in this laboratory have identified two distinct nuclear poly(A) polymerases, a 48 kDA tumor type enzyme and a 36-38 kDA liver type enzyme. To investigate the tissue and species specificity of these enzymes, nuclear extracts were prepared from various rat tissues, pig brain and two human cell lines. These as well as whole cell extract from yeast were probed for the two enzymes by immunoblot analysis using polyclonal anti-tumor poly(A) polymerase antibodies or autoimmune sera which contain antibodies specific for the liver type enzyme. Results indicate that both tumor and liver type enzymes are conserved across species ranging from rat to human. The yeast enzyme does not appear to be immunologically related to the liver or the tumor type poly(A) polymerase. The liver type enzyme appears to be specific for normal tissues whereas the tumor type enzyme is detected only in tissues in a "tumorigenic" state or cell lines originating from tumor tissues.

Dramatic increase in poly(A) synthesis after infection of Molt-3 cells with HIV

Infection of Molt-3 cells with human immunodeficiency virus-1 (HIV-1) was found to cause a rapid increase in extractable poly(A) polymerase activity, while the activity of poly(A) degrading endoribonuclease IV strongly decreased at the same time. The increase in poly(A) polymerase activity seems not to be due to a change in the actual number of enzyme molecules, but rather to posttranslational enzyme modification, most likely caused by phosphorylation by nuclear protein kinase NI or protein kinase C. Both kinases were found to be able to phosphorylate poly(A) polymerase in vitro [homogeneous enzyme as well as poly(A) polymerase in intact nuclei]. Phosphoamino acid analysis revealed an incorporation of phosphate into serine and, to a lower extent, into threonine residues of the enzyme protein; no phosphotyrosine could be detected. In the nucleus, the poly(A) polymerase and the endoribonuclease IV are bound to the nuclear matrix. The phosphorylation related enhancement of nuclear poly(A) polymerase activity could be abolished by addition of the zinc and copper chelator o-phenanthroline, which inhibited zinc-containing purified poly(A) polymerase and destroyed the poly(A) polymerase containing nuclear matrix structure, resulting in a solubilization of the enzyme.

Role of poly(A) polymerase in the cleavage and polyadenylation of mRNA precursor

To determine the role of poly(A) polymerase in 3'-end processing of mRNA, the effect of purified poly(A) polymerase antibodies on endonucleolytic cleavage and polyadenylation was studied in HeLa nuclear extracts, using adenovirus L3 pre-mRNA as the substrate. Both Mg2+- and Mn2+-dependent reactions catalyzing addition of 200 to 250 and 400 to 800 adenylic acid residues, respectively, were inhibited by the antibodies, which suggested that the two reactions were catalyzed by the same enzyme. Anti-poly(A) polymerase antibodies also inhibited the cleavage reaction when the reaction was coupled or chemically uncoupled with polyadenylation. These antibodies also prevented formation of specific complexes between the RNA substrate and components of nuclear extracts during cleavage or polyadenylation, with the concurrent appearance of another, antibody-specific complex. These studies demonstrate that (i) previously characterized poly(A) polymerase is the enzyme responsible for addition of the poly(A) tract at the correct cleavage site and probably for the elongation of poly(A) chains and (ii) the coupling of these two 3'-end processing reactions appears to result from the potential requirement of poly(A) polymerase for the cleavage reaction. The results suggest that the specific endonuclease is associated with poly(A) polymerase in a functional complex.

Role of poly(A) polymerase in the cleavage and polyadenylation of mRNA precursor

To determine the role of poly(A) polymerase in 3'-end processing of mRNA, the effect of purified poly(A) polymerase antibodies on endonucleolytic cleavage and polyadenylation was studied in HeLa nuclear extracts, using adenovirus L3 pre-mRNA as the substrate. Both Mg2+- and Mn2+-dependent reactions catalyzing addition of 200 to 250 and 400 to 800 adenylic acid residues, respectively, were inhibited by the antibodies, which suggested that the two reactions were catalyzed by the same enzyme. Anti-poly(A) polymerase antibodies also inhibited the cleavage reaction when the reaction was coupled or chemically uncoupled with polyadenylation. These antibodies also prevented formation of specific complexes between the RNA substrate and components of nuclear extracts during cleavage or polyadenylation, with the concurrent appearance of another, antibody-specific complex. These studies demonstrate that (i) previously characterized poly(A) polymerase is the enzyme responsible for addition of the poly(A) tract at the correct cleavage site and probably for the elongation of poly(A) chains and (ii) the coupling of these two 3'-end processing reactions appears to result from the potential requirement of poly(A) polymerase for the cleavage reaction. The results suggest that the specific endonuclease is associated with poly(A) polymerase in a functional complex.

3' cleavage and polyadenylation of mRNA precursors in vitro requires a poly(A) polymerase, a cleavage factor, and a snRNP

We have separated and purified three factors from HeLa cell nuclear extracts that together can accurately cleave and polyadenylate pre-mRNAs containing the adenovirus L3 polyadenylation site. One of the factors is a poly(A) polymerase with a molecular weight of approximately 50-60 kd. The second activity is a cleavage factor with a native molecular weight in the range of 70-120 kd. The third component is a factor (cleavage and polyadenylation factor, CPF) that is needed for the cleavage reaction and, in addition, confers specificity to the poly(A) polymerase activity; the native molecular weight of CPF is approximately 200 kd. Poly(A) polymerase together with CPF is sufficient to specifically polyadenylate pre-mRNA substrates that have been precleaved at the poly(A) addition site. In contrast, all three components are required for accurate cleavage and polyadenylation of pre-mRNA substrates. Further purification of CPF by buoyant density centrifugation, ion exchange, and affinity column chromatography or by gel filtration demonstrates that CPF activity resides in a ribonucleoprotein and copurifies with U11 snRNP.

Separation and characterization of a poly(A) polymerase and a cleavage/specificity factor required for pre-mRNA polyadenylation

To study the mechanism and factors required to form the 3' ends of polyadenylated mRNAs, we have fractionated HeLa cell nuclear extracts carrying out the normally coupled cleavage and polyadenylation reactions. Each reaction is catalyzed by a distinct, separable activity. The partially purified cleavage enzyme (at least 360,000 MW) retained the specificity displayed in nuclear extracts, since substitutions in the AAUAAA signal sequence inhibited cleavage. In contrast, the fractionated poly(A) polymerase (300,000 MW) lost all specificity. When fractions containing the cleavage and polyadenylation activities were mixed, the efficiency and specificity of the polyadenylation reaction were restored. Interestingly, the cleavage activity by itself functioned well on only one of four precursor RNAs tested. However, when mixed with the poly(A) polymerase-containing fraction, the cleavage activity processed the four precursors with comparable efficiencies.

Increase in the levels of activity of polyadenylic acid-metabolizing enzymes following phytohaemagglutinin stimulation of human lymphocytes

Increased levels of soluble activity of all three enzymes involved in polyadenylic acid metabolism were measured in PHA-stimulated versus normal lymphocytes. Poly(A)-polymerase and poly(A)-exonuclease values increased significantly (from 25.7 +/- 4.2 (S.E.M.) to 53.5 +/- 10.6 (S.E.M.), and from 334.6 +/- 33.2 (S.E.M.) to 653.2 +/- 53.4 (S.E.M.) respectively), while a moderate increase was observed in poly(A)-endonuclease (from 299.2 +/- 33.8 (S.E.M.) to 403.0 +/- 77.1 (S.E.M.). The above differences persisted after two fractionations of the crude cell extracts by ion exchange chromatography and molecular sieving, and could not be attributed to the competitive action of all three enzymes in the untreated extracts. Fractionation of the extracts of resting and stimulated cells on Sephadex G-75 revealed two molecular forms of poly(A)-polymerase activity.

Antibodies against nuclear poly(A) polymerases in rheumatic autoimmune diseases

Sera from 53 patients, 26 with systemic lupus erythematosus (SLE), 8 with rheumatoid arthritis (RA), 9 with Sjogren's syndrome (SS), and 10 with scleroderma (Scl), were screened for the presence of antibodies against liver-type poly(A) polymerase and tumor-type poly(A) polymerase. Sixty percent of the patients with the above four autoimmune diseases have antibodies directed against liver poly(A) polymerase, whereas sera from 74% of the patients contained anti-hepatoma poly(A) polymerase antibodies. About 25% of the patients produced antibodies exclusively against the tumor poly(A) polymerase. IgG containing anti-liver or anti-tumor poly(A) polymerase antibodies inhibited the activity of the respective enzyme. IgG containing antibodies against liver and tumor enzymes inhibited the activity of both enzymes, whereas IgG from sera that did not react with poly(A) polymerase had no effect on either enzyme. These data demonstrated the specificity of these autoantibodies and confirmed the results of the radioimmunoassay.

Cell-free synthesis of tumor-type poly(A) polymerase

Previous studies in this laboratory suggested that in adult liver, either the gene for the tumor-type poly(A) polymerase is poorly transcribed or the mRNA for this enzyme is largely not expressed. To test these possibilities, total RNA from rat liver and Morris hepatoma 3924A RNA were isolated by using a guanidine thiocyanate method; poly(A+) RNA and poly(A-) RNA were separated by oligo(dT)-cellulose chromatography and used for translation in a rabbit reticulocyte lysate system. After in vitro translation, the products were immunoprecipitated with either purified anti-tumor poly(A) polymerase antibodies or control immunoglobulins. When the polypeptides translated from poly(A+) or poly(A-) hepatoma RNA were precipitated with immune sera, a unique [35S]methionine-labeled 35-kilodalton (kDa) protein was observed. This band was not apparent when control serum was used for the immunoprecipitation. The radiolabeled 35-kDa polypeptide was not evident when the products were incubated with highly purified tumor nuclear poly(A) polymerase prior to immunoprecipitation. Prior incubation of the translation products with bovine serum albumin instead of poly(A) polymerase had no effect on the immunoprecipitation. This 35-kDa protein was not apparent when liver poly(A+) RNA was used to direct translation. These data demonstrate that (a) the tumor enzyme is not synthesized as a precursor, (b) tumor mRNA, but not normal liver mRNA, contains detectable sequences coding for tumor-type poly(A) polymerase, and (c) poly(A) polymerase mRNA also exists as a poly(A-) population.

Immunization of rabbits with purified RNA polymerase I induces a distinct population of antibodies against nucleic acids as well as anti-RNA polymerase I antibodies, both characteristic of systemic lupus erythematosus

Rabbits were immunized with either RNA polymerase I or poly(A) polymerase that had been purified to apparent homogeneity and was devoid of nucleic acids. Sera from rabbits thus immunized were screened for antibodies against nucleic acids. All seven rabbits injected with RNA polymerase I but none of the four rabbits immunized with poly(A) polymerase produced anti-nucleic acid antibodies. Anti-RNA polymerase I antibodies were induced after a single injection of the enzyme. Anti-polynucleotide antibodies were not detectable until after the second immunization. Anti-RNA polymerase I antibodies could be detected with as little as 100 pg of purified RNA polymerase I in the radioimmunoassay. At least 50 ng of poly(A) or 200 ng of DNA was required to detect anti-nucleic acid antibodies. The immunoreactivity of anti-RNA polymerase I antisera was greater with synthetic polynucleotides than with DNA, particularly early in the immunization schedule. Alkaline phosphatase treatment of poly(A) to remove 5' phosphates nearly abolished its antigenicity with respect to the early sera and decreased antibody binding of later sera by 60%. These results indicate that the anti-nucleic acid antibodies produced early were primarily directed against determinants including the 5'-terminal phosphates while antibodies produced later were directed against other sites. The antinucleic acid antibodies and anti-RNA polymerase I antibodies formed two distinct populations that were not immunologically crossreactive. We suggest that after injection, RNA polymerase I becomes associated with the nucleic acids present in blood plasma which renders them immunogenic; thus, association of nucleic acids with autoimmunogenic RNA polymerase I may be one of the mechanisms by which anti-DNA antibodies are induced in systemic lupus erythematosus.

Isolation and characterization of cytoplasmic poly(A) polymerase from cryptobiotic gastrulae of Artemia salina

Poly(A) polymerase has been purified to near homogeneity from the cytoplasm of Artemia salina cryptobiotic gastrulae by ion-exchange chromatography on DEAE-cellulose, DEAE-Sepharose CL-6B and phosphocellulose P11, gel filtration on CL-Sepharose 6B, affinity chromatography on poly(A)-Sepharose 4B and ATP-agarose. The enzyme is fully dependent on exogeneous oligo(riboadenylic acid) and is free of any nuclease or other enzyme activities. In standard assay conditions the enzyme preparation has a specific activity of 5.6 mumol AMP . h-1 . (mg protein)-1. Sodium dodecyl sulphate/polyacrylamide gel electrophoresis reveals the presence of only two proteins with Mr 94 000 and 70 000. The Mr-70 000 protein has been identified as poly(A) polymerase. The enzyme is exclusively activated by Mn2+. Addition of Ca2+, Mg2+, Zn2+, NH4+, K+ or Na+ inhibits the enzymatic reaction. The activity is specific for ATP and competitive inhibition is observed in the presence of other ribonucleoside 5'-triphosphates. AMP incorporation is time-dependent and is increased non-linearly with protein and primer concentration.

The core proteins of 35S hnRNP complexes. Characterization of nine different species

Ribonucleoprotein complexes (hnRNP) containing fragments of heterogeneous nuclear (hn)RNA and sedimenting at 35-40 S were isolated from the nuclei of HeLa S3 cells using the pH 8.0/diffusion technique. These hnRNP complexes are thought to be part of the hnRNA processing apparatus. The major protein components (core proteins) were identified by their constant ratios in native particles and in 35S hnRNP particles reconstituted in vitro. All of the core proteins, with one exception, show an increase in Mr on sodium dodecylsulfate (NaDodSO4)/polyacrylamide gels containing 8 M urea, indicative of secondary structure elements resistant to denaturation by NaDodSO4. The nine core proteins found by us are: A1 [Mr(NaDodSO4) 31 X 10(3)/Mr (urea) 38 X 10(3), apparent isoelectric point, pIapp 9.3], A2 (32.5 X 10(3)/39 X 10(3), 8.4), B1a (35.5 X 10(3)/41 X 10(3), 8.8), B1b (35.5 X 10(3)/44 X 10(3), 8.3), B1c (35.5 X 10(3)/43 X 10(3), 5.7) B2 (37 X 10(3)/42 X 10(3), 9.15), C1 (39 X 10(3)/46 X 10(3), 9.2), C2 (40.5 X 10(3)/45 X 10(3), 5.55) and C3 (38.5 X 10(3)/37 X 10(3), 4.8). Individual proteins were electroeluted from two-dimensional gels and their amino acid composition determined. Difference indices were calculated and show a group of closely related basic proteins (A1, A2, B1a, B1b, B2, C1), two related slightly acidic proteins (B1c, C2) and a distinct acidic member (C3). Two-dimensional analysis of tryptic fragments and one-dimensional separation of peptides after V8 protease treatment support these data. Peptide mapping of the proteins A1 and A2 from bovine and human cells yields identical fragments indicating a high degree of cross-species conservation. An additional protein (D4: 44 X 10(3)/55 X 10(3), greater than 9.5) was found, which preferentially associates with heavier, oligomeric hnRNP structures. Only traces of actin are present in the 35S hnRNP fraction. All core proteins are modified by charge. A large part of the charge isomers arises by phosphorylation, which has been shown by labeling with 32PO4 in vivo and with [gamma-32P]ATP in vitro. In vitro the phosphate transfer is mediated by an endogenous protein kinase associated with the 35S hnRNP complexes. The major core protein A1 exists in two conformeric forms (A1 and A1x) of which only A1x serves as phosphate acceptor in vivo.

An assessment of the evidence for the role of ribonucleoprotein particles in the maturation of eukaryote mRNA

This article has sought to draw together, on the one hand, what is known of mRNA processing and its control and, on the other hand, what is known of the structure and validity of hnRNP and snRNP particles. At the same time, it has attempted to synthesize these two themes into a critical assessment of the evidence which suggests that the particles are intimately involved in processing. It cannot be said that the case is proven. The evidence is compelling but circumstantial. The last few years have seen the development of the first in vitro splicing systems (Weingartner and Keller, 1981; Goldenberg and Raskus, 1981; Kole and Weissman, 1982), the isolation of monoclonal antibodies to defined snRNP (Lerner et al., 1981a; Billings et al., 1982) and hnRNP proteins (Hugle et al., 1982), and the ability to use artificial lipid vesicles to transfer antisera (Lenk et al., 1982) and radioactive snRNA (Gross and Cetron, 1982) into cells. It is to be hoped that further refinements of these and other techniques will allow us to solve this, one of the major outstanding problems of molecular biology.

Antibodies to distinct polypeptides of RNA polymerase I in sera from patients with rheumatic autoimmune disease

Sera from patients with rheumatic autoimmune diseases were screened for antibodies directed against RNA polymerase I by using a solid-phase radioimmunoassay. Significant levels of the antibodies were detected in the sera of all patients with either systemic lupus erythematosus or mixed connective tissue disease and in 78% of the individuals with rheumatoid arthritis. No detectable anti-RNA polymerase I antibodies were found in the sera from healthy subjects. Individuals taking hydralazine, three of whom exhibited symptoms of drug-induced lupus, had barely detectable levels of the antibodies. Immunoglobulins obtained from sera containing anti-RNA polymerase I antibodies, as determined by the radioimmunoassay, could inhibit RNA polymerase I activity in vitro. Sera from patients with systemic lupus erythematosus contained immunoglobulins directed against the polymerase I-associated polypeptide of Mr 65,000 as well as against the polypeptides of Mr 120,000 or Mr 25,000, or both. Sera from individuals with rheumatoid arthritis reacted with the polypeptide of Mr 65,000 only. The antibodies in the sera of patients with mixed connective tissue disease were directed against the Mr 42,000 polypeptide or a combination of the Mr 65,000, 42,000, and 25,000 polypeptides. These data suggest that the production of anti-RNA polymerase I antibodies may be a unique characteristic of individuals with rheumatic autoimmune diseases and that the production of antibodies against specific polypeptides of RNA polymerase I may be indicative of the particular class of disease.

Anti-poly(A) polymerase antibodies in sera of tumor-bearing rats and human cancer patients

Poly(A) polymerase (polynucleotide adenylyltransferase; ATP:polynucleotide adenylyltransferase, EC 2.7.7.19) was covalently linked to diazobenzyloxymethyl-filters and used to screen the sera from a number of tumor-bearing rats and human cancer patients for antibodies to poly(A) polymerase. Sera from rats that had been inoculated with any of several Morris hepatomas or a mammary adenocarcinoma contained immunoglobulins capable of complexing with poly(A) polymerase. No antibodies to the enzyme could be detected in sera from control animals or from those bearing tumors for short periods of time. Antibodies to poly(A) polymerase were also observed in sera from human patients with leukemia, polycythemia vera, and Wilms tumor. The antibodies were not evident in sera from normal volunteers or from patients with nonneoplastic diseases. These included lupus erythematosus, a disorder in which antibodies are produced against an array of nuclear proteins. Immunoglobulins from the serum of one of the human patients were capable of inhibiting poly(A) polymerase activity in vitro, whereas those prepared from the serum of a normal volunteer did not affect enzyme activity. As determined by the diazobenzyloxymethyl-filter technique, the relative concentration of antibodies in the sera of an individual with leukemia (in remission) increased severalfold during a relapse. These data suggest that the presence of antibodies to poly(A) polymerase may be characteristic of sera from cancer patients and that the relative concentration of these antibodies may be indicative of the disease state.

Terminal uridylyl transferase of Vigna unguiculata: purification and characterization of an enzyme catalyzing the addition of a single UMP residue to the 3'-end of an RNA primer

An enzyme which catalyzes the addition of a single UMP residue from UTP to the 3'-end of an RNA primer and which is referred to as terminal uridylyl transferase (TUT) has been extensively purified from the membrane fraction of vigna unguiculata leaves. The purification procedure involved (i) solubilization by cation depletion (ii) DEAE-Sepharose CL-6B column chromatography (iii) affinity chromatography of poly(U)-Sepharose 4B and (iv) glycerol gradient centrifugation. The molecular weight of the native enzyme was approximately 50,000 as determined by velocity sedimentation. Under conditions that were optimal for UMP-incorporation (5 mM Mg2+, low salt, 30 degrees C) TUT displayed a marked specificity for UTP as substrate, was unable to incorporate deoxyribonucleoside triphosphates and required a single-stranded oligo- or polyribonucleotide as primer. When oligoA20, tRNAasp of E. coli or alfalfa mosaic virus RNA 4 were used as primers at various substrate to primer ratio's, the vast majority of the product appeared to consist of primer molecules elongated with a single UMP residue as shown by polyacrylamide gelelectrophoresis and nearest neighbour analysis. We believe TUT to be a novel enzyme which has not been reported before and which may be a feasible tool in RNA sequencing as it enables the specific 3'-terminal labeling of RNA molecules.

Turnover of the poly(A) moiety of mRNA in wheat-germ extract

The synthesis and degradation of poly(A) were examined in wheat germ lysates under conditions used for protein synthesis. The lysate contained both poly(A)-polymerizing and poly(A)-hydrolytic activities. The synthetic activity was dependent on the presence of either poly(A) or polyadenylated mRNA as primer. Concurrent with the synthesis of poly(A), radioactivity was released from labeled poly(A) and from the poly(A) region of mRNA. Both poly(A) synthesis and hydrolysis were independent of Mn2+ and could proceed in the presence of Mg2+, the major divalent metal ion required for protein synthesis. The synthetic activity was favoured at high (1 mM) ATP concentration, whereas the hydrolytic activity was maximal in the absence of ATP or at low (0.1 mM) ATP concentration. These data indicate that the steady-state length of the poly(A) moeity of mRNAs in the cytoplasm may be regulated by the levels of ATP. Translation products of polyadenylated mRNAs isolated from myeloma cells before and after partial deadenylation were separated by dodecylsulfate/polyacrylamide gel electrophoresis. Shortening of the poly(A) moiety did not alter the relative amounts of the peptides synthesized, indicating that this process does not involve a preferential breakdown of certain species of mRNA.

Two distinct poly(A) polymerases isolated from the cytoplasm of Ehrlich ascites tumour cells

The poly(A) polymerases from the cytosol and ribosomal fractions of Ehrlich ascites tumour cells are isolated and partially purified by DEAE-cellulose and phosphocellulose column chromatography. Two distinct enzymes are identified: (a) a cytosol Mn2+-dependent poly(A) polymerase (ATP:RNA adenylyltransferase) and (b) a ribosome-associated enzyme defined tentatively as ATP(UTP): RNA nucleotidyltransferase. The cytosol poly(A) polymerase is strictly Mn2+-dependent (optimum at 1 mM Mn2+) and uses only ATP as substrate, poly(A) is a better primer than ribosomal RNA. The purified enzyme is free of poly(A) hydrolase activity, but degradation of [3H]poly(A) takes place in the presence of inorganic pyrophosphate. Most likely this enzyme is of nuclear origin. The ribosomal enzyme is associated with the ribosomes but it is found also in free state in the cytosol. The purified enzyme uses both ATP and UTP as substrates. The substrate specificity varies depending on ionic conditions: the optimal enzyme activity with ATP as substrate is at 1 mM Mn2+, while that with UTP as substrate is at 10--20 mM Mg2+. The enzymes uses both ribosomal RNA and poly(A) [but not poly(U)] as primers. The purified enzyme is free of poly(A) hydrolase activity.

Characterization of two poly(A) polymerases from cultured hamster fibroblasts

The enzymatic and physiochemical properties of poly(A) polymerases IIA and IIB from cultured hamster fibroblasts were investigated. The enzymes show an absolute requirement for Mn2+ as the divalent ion. Although Mg2+ alone is inactive, maximum activity is observed in the presence of both Mn2+ and Mg2+. An optimal pH of approx. 8 is found for polymerases IIA and IIB. The enzymes, however, differ somewhat in the pH curves as well as in the Mn2+ and Mg2+ concentration curves. Poly(A) polymerases IIA and IIB have an isoelectric point of about 6 and a sedimentation coefficient of 3.5--4 S. The molecular weights, obtained by gel filtration chromatography, are 145 000 and 155 000 for enzymes IIA and IIB, respectively. Poly(A) polymerases IIA and IIB can utilize a variety of natural and synthetic RNAs as well as DNA as primers. Poly(A) polymerase IIA is saturated at much lower concentrations of primer than enzyme IIB. On the other hand, the chain length of the product synthesized by polymerase IIA is independent of the primer concentration, whereas, with polymerase IIB, the length of the product decreases when the concentration of RNA is increased.

Poly(A) polymerase activity during cell cycle and erythropoietic differentiation in erythroleukemic mouse spleen cells

Poly(A) polymerase activity was studied in lysates of cultured murine erythroleukemic cells (Friend cells). Incorporation of ATP into acid-precipitable products is dependendent on the presence of Mn2+ or Mg2+ and of an RNA primer. The reaction is specific for ATP as the substrate (KM=290 290 micron, it is not inhibited by actinomycin D and only slightly interferred with by ethidium bromide. Cordycepin 5'-triphosphate and sodium pyrophosphate inhibit the enzyme activity. The chain length of the products of the reaction is dependent on the primer concentration and reaches up to 30 nucleotides. Poly(A) polymerase activity is low in resting (G1 phase) cells 75 nmol ATP incorporated/h per 10(6) cells) and increases to a level about twice as high in early S phase of the cell cycle. A possible model for regulation of enzyme activity is discussed. Polymerase activity in the early phase of erythropoietic differentiation of the cells induced by butyric acid does not show any difference in comparison to untreated controls. A decrease in enzyme activity to levels characteristic for cells in G1 phase accompanies shutdown of cell growth in the course of the ongoing differentiation. Analysis of the DNA content of the cells revealed that erythropoietic differentiation of Friend cells induced by butyric acid is characterized by arrest of the cells in G1 phase of the cell cycle. Poly(A) polymerase activity in erythroleukemic cells is thus controlled only by the phase of the cell cycle; it is not affected by changes in gene expression during erythroid differentiation.

Selective inhibition of initial polyadenylation in isolated nuclei by low levels of cordycepin 5"-triphosphate

The effect of cordycepin 5'-triphosphate on poly(A) synthesis was investigated in isolated rat hepatic nuclei. Nuclei were incubated in the absence and presence of exogenous primer in order to distinguish the chromatin-associated poly(A) polymerase from the "free" enzyme (Jacob, S.T., Roe, F.J. and Rose, K.M. (1976) Biochem. J. 153, 733--735). The chromatin-bound enzyme, which adds adenylate residues onto the endogenous RNA, was selectively inhibited at low concentrations of cordycepin 5'-triphosphate, 50% inhibition being achieved at 2microng/ml. At least 80 times more inhibitor was required for 50% reduction in the "free" nuclear poly(A) polymerase activity. Inhibition of DNA-dependent RNA synthesis also required higher concentrations of the nucleotide analogue. These data not only offer a mechanism for the selective inhibition of initial polyadenylation of heterogeneous nuclear RNA in vivo by cordycepin, but also provide a satisfactory explanation for the indiscriminate effect of the inhibitor on partially purified or "free" poly(A) and RNA polymerases.