Endogenous RNA-directed DNA polymerase activity in uninfected chicken embryos

Reverse transcriptase activity in bovine bone marrow: purification of a 66-kDa enzyme

The presence of reverse transcriptase (RT) activity in a DNA-binding protein complex of the goat bone marrow has been reported earlier from our laboratory. Here we report a procedure for the purification of the enzyme with RT activity from bovine bone marrow and show that the basic function is associated with a approximately 66-kDa protein. This enzyme can use RT specific homopolymers as template and short oligonucleotides as primers, while displaying a Mg(2+)-ion requirement. Eukaryotic RTs have been shown to have endogenous RNAs associated with the enzymes. Evidence is presented here to show that some endogenous RNAs are associated with the RT activity in bovine bone marrow. Even though the enzyme activity appears to be associated with a approximately 66-kDa protein, the results indicate that for a full expression of its activity, the enzyme needs to interact with a 55-kDa protein that co-purifies with the enzyme during ion-exchange chromatography.

Expression of human immunodeficiency virus long terminal repeat-coupled genes in early cleaving embryos

Early studies suggested that exogenous retroviral genes are not expressed by mouse embryos infected at early cleavage stages. The general view has therefore been that, since human eggs are non-dividing cells, they could not be infected with HIV before fertilization, nor would HIV be expressed if infection occurred during early cleavage stages following fertilization. In contrast with this view, more recent work has shown that genes coupled to the long terminal repeat (LTR) promotor elements in Rous Sarcoma Virus are readily expressed in early cleaving mouse embryos; in addition, pHIV-LTR, a plasmid construct with HIV-LTR coupled to the reporter gene, lacZ (which encodes beta-galactosidase), is expressed when transfected into a human embryonic carcinoma cell line. To determine if HIV LTR coupled genes would be expressed in early cleaving embryos, one nucleus of mouse zygotes were microinjected with approximately 5000 copies of the pHIV-LTR plasmid. Following microinjection, zygotes were cultured for 48 h to the four cell stage and stained for expression of beta-galactosidase. A plasmid construct containing lacZ without HIV LTR was microinjected as control. A total of 25 of the 111 mouse zygotes microinjected with pHIV-LTR stained positively for beta-galactosidase activity. Blastomeres within individual embryos were not uniformly stained, suggesting a mosaic pattern of distribution of the microinjected plasmids among the cleaving blastomeres. None of the 22 mouse zygotes microinjected with control plasmid stained positively. A total of six polypronuclear human eggs were obtained as discarded by-products of a human IVF program. All four human eggs microinjected with approximately 6000 copies of HIV-LTR reporter gene construct and cultured for 48 h stained highly positive for beta-galactasidase. Staining intensity varied among blastomeres, suggesting a mosaic pattern similar to the mouse embryos. The two human eggs microinjected with control plasmid were negative for beta-galactosidase activity. These results suggest that HIV-LTR is active in both early cleaving mouse embryos and in early cleaving polypronuclear human eggs. These findings indicate that human eggs infected at the time of fertilization with HIV could express viral proteins during early embryonic development.

Presence of retrovirus reverse transcriptase-related gene sequences in avian cells lacking endogenous avian leukosis viruses

Using a molecularly cloned viral DNA probe representing the entire avian sarcoma virus (ASV) reverse transcriptase (pol) gene, we have detected related sequences in DNA preparations from two avain species, ev- chickens and Japanese quail, previously demonstrated to lack all endogenous avain leukosis viruses. Nucleotide sequence homology was detected only when hybridization conditions, which allowed the formation of stable duplexes with as much as 30% base mismatch, were used. No sequence homology could be detected when stringent hybridization conditions were used. Nucleotide sequence analysis of a clone representing the major pol-specific EcoRI restriction fragment from ev- chicken embryo fibroblasts revealed DNA homology as high as 72% and implied amino acid homology as high as 82% when compared to the sequence of the ASV strain Prague C pol gene. These data reveal the presence of retroviral pol gene sequences in avian cell lines that lack endogenous retrovirus sequences, suggesting that a reverse transcriptase-related gene exists in these cells as either part of a more distantly evolved retrovirus or a cellular gene.

Possible role of RNA-dependent DNA-polymerase in early stages of evolution

The tDNA cistrons have permuted sequences of triplets corresponding to anti-codons in tRNA at specific regions in their sequences. We invoke reverse transcription for the generation of such sequences in the genome during early stages of evolution. Making the assumption that a single tDNA cistron, in a genome might have come into existence by an 'accident', after transcription, tRNA is expected to fold into a three-dimensional shape analogous to the contemporary tRNA, where the anti-codon triplet bases are sticking out well-exposed for chemical mutagens. The mutated tRNAs would have been reverse-transcribed into the genome by crude analogs of now-known reverse-transcriptases. The back and forth process of transcription and reverse transcription would give rise to all the tDNA cistrons with the required anti-codons. This process may act as an important feedback mechanism for the efficient progress of evolution.

RNase-sensitive DNA polymerase activity in cell fractions and mutants of Neurospora crassa

RNase-sensitive DNA polymerase activity (RSDP) was tested in different cell fractions of Neurospora crassa cell types and its morphological mutants. This RSDP was found localized in the microsomal pellet fraction and absent in the purified nuclear pellets isolated from different N. crassa cell types: conidia, germinated conidia, and mycelia. This enzyme is capable of synthesizing a DNA product only in the presence of all four deoxyribonucleoside-5' - triphosphates and Mg2+. Removal of RNA from the pellet fraction by RNase strongly inhibited the DNA synthesis. The endogenous synthesis of DNA in the microsomal pellet fraction was associated with the formation of an RNA:DNA hybrid as analyzed by Cs2SO4 equilibrium density gradient centrifugation. The DNA product after alkali hydrolysis hybridizes with the RNA isolated from the same pellet fraction, as analyzed by elution from hydroxylapatite column at 60C. This DNa product did not hybridize with poly (A). A few mutants tested showed this RNase-sensitive DNA polymerase activity.

Formation of Rous associated virus-60: origin of the polymerase gene

The DNA of normal chicken embryos contains sequences related to the avian leukosis-sarcoma viruses. RNA-dependent DNA polymerase of these viruses is encoded by a genetic element known as the pol gene. The nature of the endogenous virus pol gene in chicken cells was investigated by testing its ability to participate in genetic recombination. Rous-associated virus-60-type recombinant viruses isolated after infection of chicken cells with strains tsLA337PR-B or tsNY21SR-A, both of which produce a temperature-sensitive DNA polymerase, also possessed the temperature-sensitive lesion. These results are consistent with the hypothesis that the endogenous viral information used for the generation of Rous-associated virus-60 is deficient in at least part of the pol gene and that the defect includes that portion represented by the lesions in NY21 and LA337. The frequency of polymerase-negative BH-Rous sarcoma virus alpha formation was not affected by the levels of endogenous viral expression, which suggests that the alpha defect is not derived from the endogenous pol gene.

RNA-directed DNA polymerase from particles released by normal goose cells

Cells from a goose embryo were shown to release particle-associated RNA-directed DNA polymerase and RNase H activities that required the presence of Nonidet P-40 for detection. The particles were not infectious and did not have endogenous DNA synthesis. The goose particle DNA polymerase was related to the DNA polymerase of spleen necrosis virus with respect to size and was inhibited by immunoglobulin G to spleen necrosis virus DNA polymerase. However, goose cells producing DNA polymerase-containing particles did not contain reticuloendotheliosis virus-related nucleotide sequences in their DNA.

Isolation and characterization of a virus-specific ribonucleoprotein complex from reticuloendotheliosis virus-transformed chicken bone marrow cells

Chicken bone marrow cells transformed by reticuloendotheliosis virus (REV) produce in the cytoplasm a ribonucleoprotein (RNP) complex which has a sedimentation value of approximately 80 to 100S and a density of 1.23 g/cm3. This RNP complex is not derived from the mature virion. An endogenous RNA-directed DNA polymerase activity is associated with the RNP complex. The enzyme activity was completely neutralized by anti-REV DNA polymerase antibody but not by anti-avian myeloblastosis virus DNA polymerase antibody. The DNA product from the endogenous RNA-directed DNA polymerase reaction of the RNP complex hybridized to REV RNA but not to avian leukosis virus RNA. The RNA extracted from the RNP hybridized only to REV-specific complementary DNA synthesized from an endogenous DNA polymerase reaction of purified REV. The size of the RNA in the RNP is 30 to 35S, which represents the subunit size of the genomic RNA. No 60S mature genomic RNA was found within the RNP complex. The significance of finding the endogenous DNA polymerase activity in the viral RNP in infected cells and the maturation process of 60S virion RNA of REV are discussed.

Marker rescue of endogenous cellular genetic information related to the avian leukosis virus gene encoding RNA-directed DNA polymerase

Endogenous cellular genetic information related to the avian leukosis virus gene encoding RNA-directed DNA polymerase was studied, using a marker rescue assay to detect biological activity of subgenomic fragments of virus-related DNAs of uninfected avian cells. Recipient cultures of chicken embryo fibroblasts were treated with sonicated DNA fragments and were infected with a temperature-sensitive mutant of Rous sarcoma virus that encoded a thermolabile DNA polymerase. Wild-type progeny viruses were isolated by marker rescue with fragments of DNA of uninfected chicken, pheasant, quail, and turkey cells. The DNAs of these uninfected avian cells, therefore, appeared to contain endogenous genetic information related to the avian leukosis virus DNA polymerase gene.

Chick-embryo DNA polymerase gamma. Identity of gamma-polymerases purified from nuclei and mitochondria

The level of DNA polymerase gamma as compared to DNA polymerases alpha and beta has been determined in chick embryo by means of specific tests: the amount of gamma-polymerase in the 12-day-old chick embryo reaches about 15% of the total polymerase activity. This enzyme is mainly localized in nuclei and mitochondria, where it represents the prevailing if not the unique DNA polymerase activity. The mitochondrial DNA polymerase gamma is likely to be associated with the internal membrane or the matrix of this organelle since it is not removed by digitonin treatment. The gamma-polymerases have been purified from chick embryo nuclei and mitochondria 500-700 times by means of DEAE-cellulose, phosphocellulose and hydroxyapatite chromatographies. The purified mitochondrial DNA polymerase gamma is closely related to the homologous enzyme purified from the nuclei of the same cells. So far, they cannot be distinguished on the basis of their sedimentation, catalytical properties and response to inhibitors or denaturating agents. The purified gamma enzymes are distinct from the chick embryo DNA polymerases alpha and beta and are not inhibited by antibodies prepared against the latter enzymes. The nuclear and mitochondrial gamma-polymerases do not respond to the oncogenic RNA virus DNA polymerase assay with natural mRNAs.

Purification and further characterization of an RNA-dependent DNA polymerase from the allantoic fluid of leukosis-virus-free chicken eggs

The purification of an RNA-dependent DNA polymerase from the allantoic fluid of uninfected, embryonated chicken eggs is described in detail. Comparison to the polymerase of avian myeloblastosis virus shows that the two enzymes are different with respect to ion concentrations for optimal reaction, response to increasing concentrations of substrate, thermal stability and protection from thermal inactivation by viral RNA. It is concluded that the enzymes compared to each other are different proteins, which must have been coded by different genes. The RNA-dependent DNA polymerase in the allantoic fluid, therefore, does not derive from the partial or complete expression of the endogenous virus genome of the normal chicken cell or from infection by exogenous viruses.

Endogenous DNA-directed DNA synthesising system in a microsomal fraction of embryonic chick brain

A DNA polymerising complex directed by endogenous DNA has been partially purified from 11-day-old embryonic chick brain microsomes by DEAE-cellulose and phosphocellulose column chromatography. The active fractions are eluted together with an exogenous DNA-directed DNA polymerase; after Sephadex gel filtration, the endogenous activity remains associated with a high molecular weight DNA-directed DNA polymerase. The endogenous activity of the complex has been shown to be RNase-resistant and actinomycin-sensitive. It requires potassium, an ATP-regenerating system and all four deoxyribonucleoside triphosphates for full activity. The significance of this activity with regard to the protovirus hypothesis is discussed.

An RNA-dependent DNA polymerase, different from the known viral reverse transcriptases, in the chicken system

The properties of an RNA-dependent DNA polymerase (an RNA-dependent DNA nucleotidyltransferase), which occurs ubiquitously in the allantoic fluid of uninfected, leukosis-virus-free eggs, are described. It is shown that the enzyme can synthesize faithful transcripts from natural RNA (globin mRNA). By biochemical and immunological methods, the enzyme can be clearly distinguished from the reverse transcriptases of the known chicken RNA tumor viruses and therefore seems to be a member of a so far unknown class of chicken polymerases.

Screen for type-C ribonucleic acid viruses in vaccines using the ribonucleic acid-dependent deoxyribonucleic acid polymerase assay

The ribonucleic acid-dependent deoxyribonucleic acid polymerase assay was used to detect type-C viruses in live virus vaccines. Conditions were first established to maximize the sensitivity of the assay. Vaccines tested included live poliomyelitis, rubella, measles, mumps, and yellow fever. Only yellow fever and measles vaccines known to have been produced in avian leukosis-contaminated cells showed evidence of type-C viruses using the assay. The result of the survey show that the assay has direct practical application to the problem of detecting latent agents in biological products intended for human use.

Introduction to virus-caused cancers

Members of four different groups of animal viruses are known to cause cancer in animals. (Only two of them, the leukoviruses and herpesviruses, cause cancer in nonlaboratory situations.) All the members of these groups of viruses form integrated viral DNA in infected cells. However, the efficiencies with which they cause cancer vary by over a dozen orders of magnitude. These differences in efficiency are a result of differences in efficiency of formation and expression of the genes for neoplastic transformation. Four models of mechanisms for formation of the genes for neoplastic transformation are presented. Two involve the formation of new DNA sequences. No efficient human cancer‐causing viruses are known. Therefore, it is proposed that human cancer is a result of formation of the genes for neoplastic transformation by misevolution of a normal cellular information transferring process. This misevolution is caused by chemicals, physical agents, or viruses.

Introduction to virus-caused cancers

Members of four different groups of animal viruses are known to cause cancer in animals. (Only two of them, the leukoviruses and herpesviruses, cause cancer in nonlaboratory situations.) All the members of these groups of viruses form integrated viral DNA in infected cells. However, the efficiencies with which they cause cancer vary by over a dozen orders of magnitude. These differences in efficiency are a result of differences in efficiency of formation and expression of the genes for neoplastic transformation. Four models of mechanisms for formation of the genes for neoplastic transformation are presented. Two involve the formation of new DNA sequences. No efficient human cancer‐causing viruses are known. Therefore, it is proposed that human cancer is a result of formation of the genes for neoplastic transformation by misevolution of a normal cellular information transferring process. This misevolution is caused by chemicals, physical agents, or viruses.

Lack of sequence homology among RNAs of avian leukosis-sarcoma viruses, reticuloendotheliosis viruses, and chicken endogenous RNA-directed DNA polymerase activity

The relatedness of the RNAs of the three avian systems, including six avian leukosis-sarcoma viruses, four reticuloendotheliosis viruses, and the microsome fraction of normal uninfected chicken embryo cells, containing RNA and a DNA polymerase have been studied by nucleic acid hybridization. All six avian leukosis-sarcoma viruses have closely related nucleotide sequences; and all four reticuloendotheliosis viruses have closely related nucleotide sequences. But, almost no similarities were detected between the RNAs of avian leukosis-sarcoma viruses and reticuloendotheliosis viruses. The RNA template of the endogenous RNA-directed DNA polymerase activity of normal uninfected chicken cells had no detectable relationship to RNAs of avian leukosis-sarcoma and reticuloendotheliosis viruses.

Lack of serological relationship among DNA polymerases of avian leukosis-sarcoma viruses, reticuloendotheliosis viruses, and chicken cells

Antibodies against a large and a small DNA polymerase isolated from chicken embryos and against avian myeloblastosis virus DNA polymerase were used to study the serological relationships of the DNA polymerase activities of three avian systems with RNA and a DNA polymerase-avian leukosis-sarcoma viruses, reticuloendotheliosis viruses, and a fraction from uninfected chicken cells. The DNA polymerase activity of disrupted virions of all avian leukosis-sarcoma viruses tested was neutralized to the same extent by antibody against avian myeloblastosis virus DNA polymerase and was not neutralized by the antibodies against chicken cellular DNA polymerases. The viruses tested included induced leukosis viruses and Rous-associated virus-O. The DNA polymerase activity of disrupted virions of all of the reticuloendotheliosis viruses was not neutralized by any of the antibodies. The chicken endogenous RNA-directed DNA polymerase activity was neutralized partially or completely, in different experiments, by antibody against the small DNA polymerase isolated from chicken embryos, but was not neutralized by the other two antibodies.

Relationship of reticuloendotheliosis virus to the avian tumor viruses: nucleic acid and polypeptide composition

The RNA content and polypeptide composition of reticuloendotheliosis virus (REV) was compared to that of C-type RNA tumor viruses. Two RNA species with approximate sedimentation values of 64S and 4S were observed after sucrose gradient centrifugation of RNA extracted from purified REV. The high-molecular-weight RNA species of REV sedimented slightly faster than that of the Bryan strain of Rous sarcoma virus (RSV). Although these characteristics were consistent with those of other C-type RNA tumor viruses, significant differences were observed when the polypeptide composition of REV was compared with that of RSV possessing envelope determinants of Rous-associated virus RAV-2 and RAV-3. Five polypeptides of which two were glycosylated were resolved by polyacrylamide gel electrophoresis. The major nonglycosylated polypeptide of REV did not comigrate with that of RSV (RAV-2)-RSV(RAV-3). The majority of the group-specific antigen reactivity resides in this major nonglycosylated polypeptide of avian tumor viruses and comigrates when proteins of several avian tumor viruses are subjected to coelectrophoresis. This difference in the migration of the major polypeptide of REV and RSV(RAV-2)-RSV(RAV-3) may explain the absence of avian tumor virus group-specific antigen in REV.

RNA-directed DNA polymerase and virus-induced leukemia in mice

Rifamycin antibiotics, which inhibit RNA-directed DNA polymerase of Rauscher leuckemia virus, prevent the leukemogenic activity of the virus, and the effect of leukemogenesis correlates with the magnitude of inhibition of the purified enzyme. This inhibition of enzyme activity by rifamycin SV derivatives is due to a relatively tight binding between the enzymes and the inhibitors, yet the binding can be reversed by nonionic detergent. The results of this study suggest that RNA-directed DNA polymerase is essential for induction of leukemia by exogenous virus and correlate with the previous observation that the same derivatives block viral transformation in vitro.

Primer requirement and template specificity of a DNA polymerase of chick embryo

A DNA polymerase purified from extracts of chick embryo catalyzes the synthesis of poly(dT) in the presence of poly(rA) or poly(dA) as template and poly(dT) or oligo(dT) as primer. Oligo- or polyribonucleotides are relatively ineffective initiators of polydeoxynucleotide polymerization. Using activated DNA as template, the enzyme catalyzes a repair-like reaction that resembles that catalyzed by RNA-directed DNA polymerase of RNA tumor viruses and Escherichia coli DNA polymerase II. With natural templates containing poly(rA) sequences, such as avian myeloblastosis virus RNA or rabbit globin mRNA, and oligo(dT) as initiator, principally poly(dT) is synthesized. Consequently, the enzyme differs from the RNA-directed DNA polymerase associated with RNA tumor viruses by its incapacity to transcribe heteropolymeric regions of RNA templates. The possible role of the enzyme in repair of DNA during transcription is discussed.