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PubMed1990

Differential inhibitory effects of some catechin derivatives on the activities of human immunodeficiency virus reverse transcriptase and cellular deoxyribonucleic and ribonucleic acid polymerases.

The two components of Camellia sinensis (tea plant) [i.e., (-)-epicatechin gallate and (-)-epigallocatechin gallate] were found to differentially inhibit the activities of reverse transcriptase and cellular DNA and RNA polymerases. Under the assay conditions optimized for each enzyme species, the strongest inhibition by these compounds was observed with reverse transcriptase. The concentrations of (-)-epicatechin gallate and (-)-epigallocatechin gallate required for 50% inhibition of the activity of human immunodeficiency virus (HIV) reverse transcriptase were in the range of 0.01-0.02 microgram/mL. On the other hand, neither (-)-epicatechin, (-)-epigallocatechin, nor gallic acid, the constituents of (-)-epicatechin gallate and (-)-epigallocatechin gallate, was inhibitory to the activity of HIV reverse transcriptase at concentrations up to 1 microgram/mL. The mode of inhibition of reverse transcriptase and other DNA polymerases by these compounds was competitive with respect to the template-primer, whereas the mode of inhibition of RNA polymerase was competitive with respect to the nucleotide substrate. The Ki values of HIV reverse transcriptase for (-)-epicatechin gallate and (-)-epigallocatechin gallate were determined to be 7.2 and 2.8 nM, respectively, which are smaller by 1-2 orders of magnitude than the Ki's of other DNA and RNA polymerases for these compounds.

Animals↗
PubMed1989

Structural characterization of HIV reverse transcriptase: a target for the design of specific virus inhibitors.

The reverse transcriptase (RT) of HIV is an important target for chemotherapy as demonstrated by the effective treatment of AIDS patients with zidovudine, a potent inhibitor of RT. Structural studies of HIV RT were therefore undertaken with a view to designing more effective inhibitors. To obtain sufficient quantities of enzyme for these studies the reverse transcriptase gene of HIV was cloned into a high level expression plasmid yielding reverse transcriptase at a level of 10% of the total Escherichia coli proteins. Monoclonal antibodies to RT were raised in mice and have been used to purify the enzyme by immunoaffinity chromatography. Crystallization of the enzyme has been achieved and studies are underway to determine its three-dimensional structure. In addition, carboxy-terminal truncated mutants were prepared by inserting stop codons into the gene at appropriate sites. The proteins expressed were analysed for RT and RNase H activity and used for mapping RT epitopes. This, together with previous data on site-directed mutagenesis of conserved regions of HIV RT has helped to map some of the structural and functional regions of the enzyme.

Animals↗
PubMed1978

Discrimination of DNA polymerase and RNase H activities in reverse transcriptase of avian myeloblastosis virus.

The active sites in reverse transcriptase of avian myeloblastosis virus have been selectively modified by various chemical reagents. The DNA polymerase activity is very sensitive to hydrophobic sulfhydryl reagents such as 5,5'-dithiobis(2-nitrobenzoic acid) and p-hydroxymercuribenzoate but resistant to sulfhydryl reagents with hydrophilic properties. The RNase H activity, on the other hand, is resistant to both hydrophobic and hydrophilic sulfhydryl reagents, indicating the absence of cysteinyl residues essential for RNase H activity. N-Ethylmaleimide (NEM), an amino and sulfhydryl group specific reagent, inactivates both DNA polymerase and RNase H, the later activity being fourfold more stable. Polynucleotides, but not nucleotide triphosphates, protect the two enzymatic activites of reverse transcriptase against NEM. Since pretreatment of the enzyme with 5,5' -dithiobis(2-nitrobenzoic acid) does not prevent N-ethylmaleimide from reacting with a residue necessary for DNA polymerase activity, two different reactive groups are probably involved with this enzymatic activity. The pH profile of reverse transcriptase inhibition by N-ethylmaleimide also suggests the involvement of two reactive groups essential for the DNA polymerase activity with apparent pKas of 5.5 and 6.5. Only one reactive group with a pKa of 7.5 is found associated with the RNase H activity.

Avian Leukosis Virus↗
PubMed1986

Murine leukemia virus pol gene products: analysis with antisera generated against reverse transcriptase and endonuclease fusion proteins expressed in Escherichia coli.

The organization of the murine leukemia virus (MuLV) pol gene was investigated by expressing molecular clones containing AKR MuLV reverse transcriptase or endonuclease or both gene segments in Escherichia coli and generating specific antisera against the expressed bacterial proteins. Reaction of these antisera with detergent-disrupted virus precipitated an 80-kilodalton (kDa) protein, the MuLV reverse transcriptase, and a 46-kDa protein which we believe is the viral endonuclease. A third (50-kDa) protein, related to reverse transcriptase, was also precipitated. Bacterial extracts of clones expressing reverse transcriptase and endonuclease sequences competed with the viral 80- and 46-kDa proteins, respectively. These results demonstrate that the antisera are specific for viral reverse transcriptase and endonuclease. Immunoprecipitation of AKR MuLV with antisera prepared against a bacterial protein containing only endonuclease sequences led to the observation that reverse transcriptase and endonuclease can be associated as a complex involving a disulfide bond(s).

AKR murine leukemia virus↗
PubMed1977

Purification, biochemical characterization and serological analysis of cellular deoxyribonucleic acid polymerases and a reverse transcriptase from spleen of a patient with myelofibrotic syndrome.

The present study describes the separation and purification of a reverse transcriptase and cellular DNA polymerases from the human spleen of a patient with myelofibrotic syndrome. The specific requirements with respect to bivalent cations and template-primers for DNA polymerase-alpha, DNA polymerase-beta and DNA polymerase-gamma, as well as for the reverse transcriptase, are reported. Sedimentation-velocity measurements of the purified enzymes gave values of 150000, 40000, 100000 and 70000 daltons for DNA polymerase-alpha DNA polymerase-beta, DNA polymerase-gamma and the reverse transcriptase respectively. Serological studies have shown that the reverse transcriptase from human spleen is not antigenically related to cellular DNA polymerase-alpha, -beta or -gamma, but is antigenically related to reverse transcriptase from simian sarcoma virus and gibbon-ape leukaemia virus.

Child, Preschool↗
PubMed1971

Distinguishing reverse transcriptase of an RNA tumor virus from other known DNA polymerases.

Assays are described that permit one to distinguish the reverse transcriptase of RNA tumor viruses from known normal cellular DNA-instructed DNA polymerases. Template responses of purified reverse transcriptase were compared with those of similar preparations of the DNA polymerase I of Escherichia coli and of calf-thymus polymerase. All three enzymes responded well to the synthetic duplexes poly(dT).poly(A), poly(U).poly(A), and poly(dT).poly(dA). Hence, these duplexes can detect, but cannot distinguish reverse, transcriptase from the known normal DNA polymerases. However, certain oligomer-homopolymer complexes serve as excellent distinguishing agents. The reverse transcriptase responds very well to (dT)(10).poly(A) and very poorly to (dT)(10).poly(dA), whereas both cellular DNA polymerases do not exhibit this behavior.Purified single-stranded RNA also serves as a diagnostic device, since only reverse transcriptase gives a detectable response. To be definitive, a positive response to RNA must be accompanied by a demonstration via molecular hybridization that the DNA product is complementary to the RNA and not to some minor DNA contaminant.

Adenine Nucleotides↗
PubMed1990

[Application of sensitive reverse transcriptase assay to human immunodeficiency virus (HIV) type 2].

Sensitive reverse transcriptase assay was applied to human immunodeficiency virus type 2. The kinetics of this assay, stability of the enzyme and the effect of BSA to this assay indicated that the condition of this assay should be 37 degrees C in reaction temperature. The sensitivity of this assay increased by adding more than 10 micrograms/ml of BSA. The sensitivity of this assay is at least four times more than that of CPE assay using Molt-4 cell. Other HIV-2 isolate, LAV-2 in culture medium was also detectable in this condition. Moreover reverse transcriptase inhibiting antibody that specifically inhibits HIV-2 reverse transcriptase was found by this assay.

HIV↗
PubMed1989

Effects of small insertions on the RNA-dependent DNA polymerase activity of HIV-1 reverse transcriptase.

We have described a strain of Escherichia coli that expresses high levels of enzymatically active, soluble, HIV-1 reverse transcriptase (A. Hizi, C. McGill, and S. H. Hughes, Proc. Natl. Acad. Sci. USA, 85, 1218-1222, 1988). The clone can be used as a source of the enzyme and to generate and characterize mutations in the reverse transcriptase. We have made a series of small in-frame insertions in the region that encodes the reverse transcriptase. When the mutant plasmids are reintroduced into E. coli, they induce the synthesis of mutant forms of the enzyme. With one interesting exception, the reduction in RNA-dependent DNA polymerizing activity seen in the mutants correlates well with the degree of sequence conservation among the various reverse transcriptases. Insertions into regions that are evolutionarily conserved have a more profound effect on RNA-dependent DNA polymerase activity than do insertions into regions that are less conserved. The exception to this simple correlation is that a small insertion into the region encoding RNase H gives rise to a protein with essentially no RNA-dependent DNA polymerase activity. We suggest that this mutation may affect the ability of the reverse transcriptase to fold properly, which might explain our previous observation that small carboxyl terminal deletions profoundly affect RNA-dependent NAD polymerase activity.

Binding Sites↗
PubMed1972

Immunological relationships of reverse transcriptases from ribonucleic acid tumor viruses.

Antiserum to partially purified reverse transcriptase from the Schmidt-Ruppin strain of Rous sarcoma virus has been prepared and characterized. Antibody to the avian polymerase inhibited the reverse transcriptase activity of avian C-type viruses but had no effect on the polymerase activity from C-type viruses of other classes. The known mammalian C-type viral polymerases were significantly inhibited only by the antiserum to murine C-type viral polymerases; reverse transcriptases from four other mammalian viruses were immunologically distinct from both avian and mammalian C-type viral polymerases. Partially purified murine leukemia viral DNA polymerase activity was comparably reduced by specific antibody regardless of the template used for enzyme detection.

Animals↗
PubMed1988

Reverse transcriptases from human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus (SIVMAC) are susceptible to inhibition by foscarnet and 3'-azido-3'-deoxythymidine triphosphate.

Reverse transcriptases from human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus (SIV) were investigated with respect to susceptibilities to the reverse transcriptase inhibitors foscarnet and 3'-azido-3'-deoxythymidine triphosphate (AZTTP). The different reverse transcriptases had the same sensitivity to foscarnet (50% inhibition at 0.10 to 0.16 microM). The Ki values for AZTTP were 0.01 to 0.02 microM for HIV-1 reverse transcriptase and 0.02 to 0.03 microM for HIV-2 and SIVMAC reverse transcriptases.

Animals↗
PubMed1988

Fidelity of HIV-1 reverse transcriptase.

The human immunodeficiency virus type 1 (HIV-1) shows extensive genetic variation and undergoes rapid evolution. The fidelity of purified HIV-1 reverse transcriptase was measured during DNA polymerization in vitro by means of three different assays. Reverse transcriptase from HIV-1 introduced base-substitution errors in DNA from the bacteriophage phi X174 amber3 at estimated frequencies of 1/2000 to 1/4000. Analyses of misincorporation rates opposite a single template adenine residue showed that HIV-1 reverse transcriptase catalyzed nucleotide mismatches with a specificity of A:C much greater than A:G greater than A:A. The high error rate of HIV-1 reverse transcriptase in vitro translates to approximately five to ten errors per HIV-1 genome per round of replication in vivo. This high error rate suggests that misincorporation by HIV-1 reverse transcriptase is, at least in part, responsible for the hypermutability of the AIDS virus. The specificity of misincorporation may provide a basis for the systematic construction of antiviral nucleosides.

Avian Myeloblastosis Virus↗
PubMed1992

Regulation of the reverse transcriptase of human immunodeficiency virus type 1 by dNTPs.

Reverse transcriptase (RNA-directed DNA polymerase, EC 2.7.7.49) of human immunodeficiency virus type 1 has been examined with respect to the steady-state kinetics of polymerization of dNTPs into product DNA. With dNTPs as variable substrate, the kinetics of polymerization deviated from standard Michaelis-Menten kinetics. Substrate inhibition was observed at high substrate concentrations and negative cooperativity was seen at lower substrate concentrations. Examination of incorporation of substrate dNMPs in the presence of nucleotides not complementing the template demonstrated that dNTPs may act as noncompetitive inhibitors, as well as substrate. The Ki of the enzyme for dNTPs was 104 microM. A working model is presented that accounts for the substrate inhibition. In this model, the reverse transcriptase is a multisubunit holoenzyme, where noncompetitive inhibition is mediated by one subunit binding nucleotide and down-regulating the enzymatically active 64-kDa subunit. With additional assumptions, this model can accommodate the negative cooperativity observed.

Allosteric Regulation↗
PubMed1989

Ribonuclease H activities associated with viral reverse transcriptases are endonucleases.

A series of test substrates have been synthesized to establish the effect of termini on the putative exoribonuclease H activity of reverse transcriptase. Recombinant reverse transcriptase from human immunodeficiency virus, natural enzyme from avian myeloblastosis virus, and a known endonuclease, Escherichia coli ribonuclease H, cleaved relaxed, circular, covalently closed plasmids in which 770 consecutive residues of one strand were ribonucleotides. The avian enzyme also deadenylated capped globin mRNA with a covalently attached oligo(dT) tail at the 3' end. These results resolve a long-standing controversy--that the viral enzymes are obligatory exonucleases in vitro, based on their failure to cleave certain substrates for E. coli ribonuclease H, including circular poly(A).linear poly(T) and ribonucleotide-substituted supercoiled plasmids, but resemble endonucleases in vivo, based on their ability to degrade RNA in complex DNA.RNA hybrids. The data strongly suggest that the viral enzymes are endonucleases with exquisite sensitivity to the conformation of heteroduplexes. Inhibition of viral, but not cellular, ribonuclease H with ribonucleoside-vanadyl complexes further distinguishes these enzymes.

Avian Myeloblastosis Virus↗
PubMed1988

The alpha and beta chains of avian retrovirus reverse transcriptase independently expressed in Escherichia coli: characterization of enzymatic activities.

Reverse transcriptase of the avian sarcoma and leukosis retroviruses is a heterodimer composed of a 63-kDa alpha and a 95-kDa beta polypeptide chain, both of which are encoded in the pol gene and are produced by proteolytic processing of a larger precursor. We previously constructed a bacterial expression clone of the entire pol coding region that produces a protein 4 kDa larger than the mature viral beta subunit. By use of this clone and synthetic oligonucleotides to introduce stop codons, two derivatives have been constructed: one that directs synthesis of a protein equivalent to the mature beta subunit and the other that directs synthesis of a protein equivalent to alpha subunit. Predicted amino acid sequences of these proteins differ from their viral counterparts only by an initiator methionine that was added to the N termini for expression in Escherichia coli. Both bacterially expressed proteins exhibit reverse transcriptase activity and appear to function as homodimers. The properties of these proteins resemble those of the viral reverse transcriptase heterodimer; however, the bacterially produced alpha dimer protein could be distinguished from the other proteins by its increased sensitivity to heat inactivation, which also has been reported for the corresponding viral product. These results show that correct folding and expression of enzymatic function does not require formation of a precursor. The alpha and beta clones provide a convenient source of individual pol gene products for further evaluation of their roles in the synthesis and integration of retroviral DNA.

Amino Acid Sequence↗
PubMed1977

Interactions of murine leukemia virus core components: characterization of reverse transcriptase packaged in the absence of 70S genomic RNA.

Virions produced by cells in the presence of actinomycin D (Act D virions) contain reverse transcriptase but are deficient in 70S genomic RNA. To assess the role of genomic RNA in encapsidation of a functional reverse transcriptase and to study the interaction of the enzyme and its template in the cores of intact virions, the reverse transcriptase enzymes of normal and Act D virions were compared. The enzymes were indistinguishable by column chromatography, sedimentation velocity, or template/primer preferences. In addition, these enzymes showed equal sensitivity to inactivation by antibodies directed against Rauscher murine leukemia virus DNA polymerase. The enzymes from Act D and normal virions had similar thermal decay rates and were both protected against heat denaturation by natural and synthetic template/primers. By these criteria, the DNA polymerase molecules synthesized and assembled into virions in the absence of genomic RNA are identical to those packaged under normal conditions. Additional studies designed to measure protection of reverse transcriptase by genomic RNA were carried out by comparing the thermal lability of the enzyme in intact Act D and normal virions. The thermal decay rate of reverse transcriptase in Act D virions was identical to that in control virions. In contrast to the lability of the virion-associated enzyme, however, genomic RNA in control virions was stable to heat treatment.

AKR murine leukemia virus↗
PubMed1984

Increase of cellular murine leukemia virus reverse transcriptase in interferon-treated cells.

The addition of mouse interferon (IFN) to AKR murine leukemia virus (MuLV)-producing NIH3T3 cells inhibited the extracellular appearance of AKR MuLV when assayed for reverse transcriptase activity or infectious virus production. IFN treatment had no detectable effect on proviral DNA formation during infection nor on the level of viral RNA in virus-producing cells. However, addition of IFN did alter the level of cellular viral reverse transcriptase activity. Chromatography of extracts from virus-producing cells on poly(G)-Sepharose columns revealed two peaks of reverse transcriptase activity. Peaks I and II eluted at 0.45 M and 0.65 M NaCl, respectively, while the cellular DNA polymerase beta eluted earlier at 0.3 M NaCl. IFN treatment of these chronic virus producer cells resulted in a 5-fold increase in peak I whereas peak II and polymerase beta remained essentially unchanged. When reverse transcriptase from purified virions was similarly chromatographed on poly(G)-Sepharose, all of the enzymatic activity eluted as peak I. Thus, the reverse transcriptase in peak I from cell extracts appears to be the form which is present in mature virions. Contrary to the results with chronic virus-producing cells, IFN treatment prior to exogenous infection with MuLV did not alter levels of reverse transcriptase peaks I and II or polymerase beta. These results provide further evidence that the major effect of IFN occurs at the level of MuLV maturation and assembly.

AKR murine leukemia virus↗
PubMed1990

Site-directed mutagenesis of the conserved Asp-443 and Asp-498 carboxy-terminal residues of HIV-1 reverse transcriptase.

Substitution of the conserved Asp-443 residue of HIV-1 reverse transcriptase by asparagine specifically suppressed the ribonuclease H activity of the enzyme without affecting the reverse transcriptase activity, suggesting involvement of this ionizable residue at the ribonuclease H active site. An analogous asparagine substitution of the Asp-498 residue yielded an unstable enzyme that was difficult to enzymatically characterize. However, the instability caused by the Asn-498 mutation was relieved by the introduction of a second distal Asn-443 substitution, yielding an enzyme with wild type reverse transcriptase activity, but lacking ribonuclease H activity.

Amino Acid Sequence↗
PubMed1990

HIV-1 reverse transcriptase: structure predictions for the polymerase domain.

Reverse transcriptase (RT) plays an essential role in the life cycle of the human immunodeficiency viruses (HIV). A better understanding of this enzyme, and its two catalytic functions, the DNA polymerase and the RNase H, could lead to the development of new drugs that would specifically block HIV replication. The available genetic, sequence, biochemical, and immunological data on the reverse transcriptase of HIV-1 constrain the possible structure of the DNA polymerase domain. The purpose of this review is to correlate the data and to discuss, in light of that data, a model for the structure of the polymerase domain. In this model, the polymerase domain is approximately 50 to 60 A in diameter with a 20 A opening to accommodate the nucleic acid duplex. The most evolutionarily conserved region of RT (amino acids 20-190 of HIV-1 RT) is proposed to form the inner surface of the 20 A opening to which the nucleic acid hemiduplex is bound.

Amino Acid Sequence↗