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PubMed1990

Characterization of reverse transcriptase from feline immunodeficiency virus.

Reverse transcriptase has been purified from feline immunodeficiency virus (FIV) by DEAE-cellulose and phosphocellulose chromatography. The purified enzyme consists of a single protein with a Mr of 67,000. When proteolysis is not minimized during purification, a fragment of Mr 54,000 is also observed. This is similar to the reverse transcriptase from human immunodeficiency virus type 1 (HIV), which consists of a polypeptide of Mr 66,000; when proteolysis is not minimized during purification, a fragment of Mr 51,000 is also observed. In direct comparisons, the FIV reverse transcriptase is very similar to the HIV reverse transcriptase in template specificity and requirements for Mg2+. In contrast to these similarities, the FIV and HIV reverse transcriptases are substantially different in primary sequence, as determined by peptide mapping.

Animals↗

PubMed1995

Enhancement of HIV-1 proteinase activity by HIV-1 reverse transcriptase.

HIV-1 reverse transcriptase (RT) was found to increase the activity of HIV-1 proteinase in vitro and in eukaryotic cells. The effect of RT on proteinase activity was dose-dependent and independent of pH or salt concentration. The cleavage of sequences corresponding to all the naturally occurring cleavage sites that could be tested in vitro was enhanced. The effect of RT on cleavage was greatest at the cleavage site between RT and integrase. The enhancement of viral proteinase activity by the virus RT may contribute to regulation of the order and/or efficiency of cleavage at different sites during virus replication and maturation.

Base Sequence↗

PubMed1992

Resolution of microheterogeneity associated with recombinant HIV-1 heterodimeric reverse transcriptase.

HIV-1 reverse transcriptase (RT) has been successfully expressed as a biologically active recombinant protein in Escherichia coli and purified to homogeneity. After partial purification, RT was obtained primarily in a heterodimeric form represented by two subunits of 66 and 51 kDa, but the preparation also included several forms distinguishable in size and charge by chromatography on ionic-exchange and gel-filtration columns. We have developed a purification method that yields a single heterodimeric form of RT. Our strategy involves the selection of RT molecules exhibiting uniformity in elution from QAE Sepharose anion-exchange columns and Superose 12 gel-filtration columns. In the former, RT is resolved into multiple peaks on the basis of enzymatic activity, one of which represents highly active and pure p66:p51 heterodimeric RT. This highly active RT fraction, after gel-filtration chromatography, yields a compositionally pure protein product free of observable microheterogeneity by 1D and 2D polyacrylamide gel electrophoresis under a variety of conditions. Furthermore, the RNAse H enzymatic activity associated with HIV-1 RT has been demonstrated to coelute with the purified polymerase activity during gel filtration at a size (120 kDa) consistent with its location on the heterodimeric protein molecule.

Amino Acid Sequence↗

PubMed1989

Fidelity of human immunodeficiency virus type I reverse transcriptase in copying natural DNA.

Reverse transcriptase from the human immunodeficiency virus type I (HIV-1) was expressed in E. coli and purified to near homogeneity. The enzyme was shown to contain reverse transcriptase, DNA polymerase and ribonuclease H activities. The DNA polymerase activity converted singly-primed phi X174 (+) DNA into the double-stranded form. Two third of the replication product is ligatable to covalently closed circular DNA (RFIV-form DNA) indicating that DNA synthesis by HIV reverse transcriptase can proceed until the enzyme matches the 5'-end of a pre-existing primer molecule. The in vitro accuracy of HIV reverse transcriptase was measured with the phi X174am16 reversion assay to be 1/7,400. Reversion rates for the individual mispairs were determined from pool bias studies to be 1/8,000 for the dGMP:T template mismatch, 1/35,000 for the dGMP:A template mismatch, 1/45,000 for the dAMP:G template mismatch, 1/73,000 for the dCMP:T template mispair, 1/140,000 for the dCMP:A template mispair, and 1/180,000 for the dGMP:G template mismatch. The dTMP:T template mispair was below the detection limit of the assay indicating a reversion rate of less than 1/300,000 for this particular mispair.

Cloning, Molecular↗

PubMed1991

The p15 carboxyl-terminal proteolysis product of the human immunodeficiency virus type 1 reverse transcriptase p66 has DNA polymerase activity.

The reverse transcriptase of human immunodeficiency virus type 1 is a heterodimeric protein consisting of two polypeptides with masses of 66 and 51 kDa and has, as a second enzymatic activity, RNase H activity. The 66-kDa polypeptide can be cleaved by the virus-encoded protease to yield polypeptides of 51 and 15 kDa. The latter has been characterized as possessing RNase H activity [Hansen, J., Schultze, T., Mellert, W. & Moelling, K. (1988) EMBO J. 7, 239-243]. We have purified simultaneously the heterodimeric reverse transcriptase/RNase H containing the 66/51-kDa polypeptides and the 15-kDa RNase H from Escherichia coli containing the expression vector pJS 3.7 by a procedure including chromatography on DEAE-cellulose, phosphocellulose, and heparin-Sepharose. Two RNase H and reverse transcriptase peaks were separated on phosphocellulose, one coinciding with the heterodimeric protein and the other with the 15-kDa protein. On the basis of the following findings it appears that the 15-kDa polypeptide has both RNase H and reverse transcriptase activities: (i) it copurified with both activities; (ii) it functioned as a reverse transcriptase in an in situ assay after SDS/polyacrylamide gel electrophoresis; (iii) polyclonal antibodies raised against the 66-kDa polypeptide reacted in immunoblots exclusively with a 15-kDa polypeptide, reacted in immunoblots exclusively with a 15-kDa polypeptide, while no immunoreactive bands in the range of 51-66 kDa were seen in the 15-kDa polypeptide preparation; (iv) the p15 and the p66/51 reverse transcriptase could be quantitatively pelleted in an enzymatically active form only when antibodies specific for the p66 carboxyl terminus were used; and (v) the p15 protein had bona fide properties of a reverse transcriptase and could enzymatically synthesize a high molecular weight, alkali-resistant product. The two reverse transcriptases appear to have different behaviors on various template/primer systems tested. Conceivably different forms of human immunodeficiency virus type 1 reverse transcriptases might be used in individual steps of (+)- and (-)-strand replication.

Blotting, Western↗

PubMedDate not supplied

Evaluation of natural products as inhibitors of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase.

Inhibition of human immunodeficiency virus reverse transcriptase is currently considered a useful approach in the prophylaxis and intervention of acquired immunodeficiency syndrome (AIDS), and natural products have not been extensively explored as inhibitors of this enzyme. We currently report that the reverse transcriptase assay developed for the detection of the enzyme in virions involving polyadenylic acid.oligodeoxythymidylic acid (poly rA.oligo dT) and radiolabeled thymidine 5'-triphosphate (TTP), can be applied as a simple method for screening the human immunodeficiency virus type 1 reverse transcriptase (HIV-1 RT) inhibitory potential of natural products. As reported herein, 156 pure natural products have been examined in this system. Benzophenanthridine alkaloids such as faragaronine chloride [1] and nitidine chloride, which are known inhibitors of avian myeloblastosis virus reverse transcriptase, demonstrated potent activity in the HIV-1 RT system, and 1 (IC50 10 micrograms/ml) was adopted as a positive-control substance. Additional inhibitors found were columbamine iodide [2] and other protoberberine alkaloids, the isoquinoline alkaloid O-methylpsychotrine sulfate [3], and the iridoid fulvoplumierin [4]. A number of indolizidine, pyrrolizidine, quinolizidine, indole, and other alkaloids, as well as compounds of many other structural classes, were tested and found to be inactive. A total of 100 plant extracts have also been evaluated, and 15 of these extracts showed significant inhibitory activity. Because tannins and other polyphenolic compounds are potent reverse transcriptase inhibitors, methods were evaluated for the removal of these from plant extracts prior to testing. Polyphenolic compounds were found to be responsible for the activity demonstrated by the majority of plant extracts. After appropriate tannin removal procedures were established, the bioassay system was shown to be generally applicable to both pure natural products and plant extracts. The method also proved useful in directing an isolation procedure with Plumeria rubra to yield fulvoplumierin [4] as an active compound (IC50 45 micrograms/ml).

Antiviral Agents↗

PubMed1988

Isolation of cloned Moloney murine leukemia virus reverse transcriptase lacking ribonuclease H activity.

Retroviral reverse transcriptase possesses DNA polymerase and ribonuclease H (RNase H) activity within a single polypeptide. Chemical or proteolytic treatment of reverse transcriptase has been used in the past to produce enzyme that is missing DNA polymerase activity and retains RNase H activity. It has not been possible to obtain reverse transcriptase that lacks RNase H but retains DNA polymerase activity. We have constructed a novel deletion derivative of the cloned Moloney murine leukemia virus (M-MLV) reverse transcriptase gene, expressed the gene in E. coli, and purified the protein to near homogeneity. The purified enzyme has a fully active DNA polymerase, but has no detectable RNase H activity. These results are consistent with, but do not prove, the conclusion that the DNA polymerase and RNase H activities of M-MLV reverse transcriptase reside within separate structural domains.

Chromosome Deletion↗

PubMed1991

Reverse-transcriptase-associated RNaseH activity mediates template switching during reverse transcription in vitro.

During the first steps of reverse transcription of the retroviral genome, sequences present at the extremities of the RNA are used to reconstitute a host cell PolII promoter. The assembly of the promoter occurs by template switching, which takes advantage of a direct repeat at the ends of the RNA molecule. These steps are catalysed by the viral reverse transcriptase, which carries an intrinsic RNaseH activity that is probably also involved therein. To study the role of the RNaseH activity in this first template-switching event, an in vitro system has been developed based on primer extensions of synthetic RNAs. When an RNA was reverse transcribed with wild-type reverse transcriptase in the presence of a second RNA the 3' part of which was repeated at the 5' end of the first one, extension products could be observed corresponding to a chimeric cDNA comprising both RNA species. This template switching could not be detected when a mutant reverse transcriptase lacking the RNaseH activity was used. The results show that the RNaseH activity is needed to remove the 5' RNA sequences from the cDNA:RNA hybrid thereby enabling its translocation to another RNA containing an appropriate complementary target sequence.

Endoribonucleases↗

PubMed1993

Expression and mutational analysis of the reverse transcriptase of the lentivirus equine infectious anemia virus.

The reverse transcriptase of equine infectious anemia virus (EIAV) shows sequence similarity with the reverse transcriptases of other lentiviruses, particularly with those of human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2). We have constructed a plasmid that when introduced into E. coli induces the synthesis of substantial quantities of the nearly authentic EIAV reverse transcriptase. The viral and bacterially expressed reverse transcriptases are similar in their molecular weights. The bacterial expression clone was used to generate deletion mutants of the protein. Mutations in both amino and carboxyl terminal regions of the polypeptide strongly affect the DNA polymerase activity of the enzyme. Thus, EIAV reverse transcriptase resembles the reverse transcriptases of HIV-1 and HIV-2 and can serve as a suitable enzyme for studying the structure-function relationship in lentiviral reverse transcriptase.

Cloning, Molecular↗

PubMed1990

Immunologic and proteolytic analysis of HIV-1 reverse transcriptase structure.

HIV-1 virions contain two reverse transcriptase polypeptides that have apparent molecular weights of 66 and 51 kDa. The 51-kDa form lacks the carboxy-terminal sequences found in the 66-kDa form, and is believed to be a proteolytic digestion product. We have treated purified 66-kDa reverse transcriptase with viral and nonviral proteases. The digestion products were characterized by their ability to react with monoclonal antibodies known to recognize particular segments of the HIV-1 reverse transcriptase. The approximate location of the segments recognized by the monoclonal antibodies was determined by testing the ability of the antibodies to recognize a series of amino- and carboxy-terminal-deleted forms of HIV-1 reverse transcriptase. The segments recognized are not uniformly distributed along the primary amino acid sequence of HIV-1 reverse transcriptase. We suggest that these segments are probably on the surface of the properly folded form of reverse transcriptase. Of the tested proteases, only the viral protease was able to cleave the 66-kDa form to the 51-kDa form without producing additional cleavage products, suggesting that the viral protease cleaves the 66-kDa protein to the 51-kDa form in virions.

Animals↗

PubMed1992

Structure-activity analyses of HIV-1 reverse transcriptase.

HIV-1 reverse transcriptase is a dimeric enzyme which can exist in both homodimeric (p66/p66) and heterodimeric (p66/p51) forms. The monomeric subunits are catalytically inert. However, during DNA synthesis by the dimeric enzyme, only one subunit (p66) appears to carry out the catalysis, while the second subunit serves only a supportive role. In the case of the p66/p66 homodimers, we find that both the subunits are catalytically competent as judged by the observation that a) primer binding occurs to both subunits and b) catalytically inert dimers can be partially activated by replacement of one of the two inactive p66 subunits.

Acquired Immunodeficiency Syndrome↗

PubMed1985

Expression of enzymatically active reverse transcriptase in Escherichia coli.

Reverse transcriptase of murine retroviruses is a monomeric protein of approximately 80,000 daltons, which is encoded by the central portion of the viral pol gene. To prepare large quantities of the enzyme, we have constructed gene fusions between the trpE gene and portions of the pol gene of Moloney murine leukemia virus. The inserted pol gene sequences include the entire coding region for the mature enzyme and various amounts of additional coding sequences. Many of these constructs express high levels of reverse transcriptase activity even though the NH2 and COOH termini of the protein product only approximate the correct termini of the authentic protein.

DNA, Recombinant↗

PubMed1993

Kinetics of interaction of HIV reverse transcriptase with primer/template.

Intrinsic protein fluorescence of reverse transcriptases from HIV-1 and HIV-2 provides a sensitive signal for monitoring the interaction of the enzymes with primer/template duplex molecules. Kd values for 18/36-mer DNA/DNA duplexes were found to be in the range of a few nanomolar (about 3 times higher for the enzyme from HIV-2 than for that from HIV-1). The quenching of protein fluorescence induced on binding primer/template, together with an increase in extrinsic fluorescence on interaction with primer/template containing a fluorescent nucleotide at the 3'-end of the primer, was used to investigate the kinetics of interaction with reverse transcriptase from HIV-1. The results can be explained in terms of a two-step binding model, with a rapid diffusion-limited initial association (k(ass) = ca. 5 x 10(8) M-1 s-1) followed by a slow isomerization step (k = ca. 0.5 s-1). These (forward) rate constants are increased in the presence of a non-nucleoside inhibitor (S-TIBO) of HIV-1 reverse transcriptase, while the reverse rate constant for the second step is decreased, leading to an increase in affinity between the enzyme and primer/template by a factor of at least 10 when S-TIBO is bound. The results are discussed in terms of present knowledge of the structure of reverse transcriptase.

Base Sequence↗

PubMed1988

Kinetics and inhibition of reverse transcriptase from human and simian immunodeficiency viruses.

Reverse transcriptase from the simian immunodeficiency virus (SIV) was found to have kinetic behavior similar to that of enzyme from the human immunodeficiency virus (HIV). Michaelis constants for the substrates TTP and dGTP and inhibition constants for the inhibitors 3'-azido-3'-deoxythymidine 5'-triphosphate, 2',3'-dideoxythymidine 5'-triphosphate, and 2'-3'-dideoxyguanosine 5'-triphosphate were obtained for SIV reverse transcriptase and were found to be similar to the corresponding values for HIV reverse transcriptase. Thus, the interaction of SIV reverse transcriptase with nucleotide analogs appears to be indistinguishable from that of the HIV enzyme, suggesting that SIV/simian acquired immunodeficiency syndrome (SAIDS) is a potentially good model of AIDS.

Deoxyguanine Nucleotides↗

PubMed2025

Kinetic Investigation of Resistance to Islatravir Conferred by Mutations in HIV-1 Reverse Transcriptase.

Islatravir (EFdA) is a novel nucleoside reverse transcriptase translocation inhibitor (NRTTI) that potently blocks HIV-1 replication in vivo. Its unique structural features in contrast to nucleoside reverse transcriptase inhibitors (NRTIs), particularly the 4'-ethynyl and 3'-hydroxy groups, contribute to its high clinical potency. Once intracellularly activated to EFdA 5'-triphosphate (EFdA-TP), it competes with dATP for incorporation by HIV-1 reverse transcriptase (RT) during HIV-1 genomic replication. The 4'-ethynyl group of incorporated EFdA-MP interacts with a hydrophobic pocket of HIV-1 RT, hindering DNA translocation and terminating DNA synthesis. The M184V mutation, commonly associated with resistance to NRTIs such as lamivudine and emtricitabine, and the M184V/A114S mutations, both located within the hydrophobic pocket, were shown to reduce Islatravir susceptibility in cell-based viral resistance selection assays. To elucidate the mechanisms by which these mutations affect Islatravir inhibition, we employed pre-steady-state kinetics to investigate their impact on EFdA-TP incorporation by HIV-1 RT using both DNA and RNA templates. We found that M184V had a modest effect on EFdA-TP incorporation efficiency, increasing it 2-fold with the DNA template and decreasing it 3-fold with the RNA template. In contrast, M184V/A114S significantly inhibited EFdA-TP incorporation, reducing its incorporation efficiency 5.4-fold with the DNA template and 181-fold with the RNA template. These reductions were primarily attributable to corresponding decreases in EFdA-TP incorporation rate constants of 18-fold and 105-fold, respectively. These results suggest that, unlike FDA-approved NRTIs, the clinical efficacy of Islatravir, may not be substantially compromised by the M184V mutation alone but will be significantly reduced by the M184V/A114S mutations.

HIV Reverse Transcriptase↗

PubMed1994

The genetic and functional basis of HIV-1 resistance to nonnucleoside reverse transcriptase inhibitors.

The nonnucleoside reverse transcriptase (RT) inhibitors are structurally diverse compounds that are specific inhibitors of the human immunodeficiency virus type 1 RT enzyme. The compounds are largely functionally identical and bind to a common site in the enzyme. HIV-1 variants that exhibit reduced susceptibility to these inhibitors have been derived in cell culture and, more recently, from HIV-1-infected patients undergoing experimental therapy. The variants express amino acid substitutions at RT positions that apparently interact directly with the inhibitors. Effects of specific substitutions at these positions vary among the compounds, suggesting subtle differences in how the compounds physically interact with the enzyme.

Antiviral Agents↗

PubMed1995

Action of uracil analogs on human immunodeficiency virus type 1 and its reverse transcriptase.

Three structural analogs of 5-ethyl-1-benzyloxymethyl-6-(phenylthio)uracil (E-BPU) inhibited human immunodeficiency virus type 1 (HIV-1) replication without cytotoxicity in vitro and were more potent than azidothymidine and were as potent as E-BPU. The target of these compounds is HIV-1 reverse transcriptase. Reverse transcriptases resistant to nevirapine (tyrosine at position 181 to cysteine) and TIBO R82150 (leucine at position 100 to isoleucine) are cross resistant to E-BPU analogs. Nevirapine- or TIBO R82150-resistant HIV-1 were cross resistant to E-BPU analogs but were inhibited at concentrations 11- to 135-fold lower than the cytotoxic doses.

Antiviral Agents↗

PubMed1989

Comparative studies of the inhibitory properties of antibiotics on human immunodeficiency virus and avian myeloblastosis virus reverse transcriptases and cellular DNA polymerases.

The inhibition of human immunodeficiency virus (HIV) reverse transcriptase by certain antibiotics and related compounds was studied in comparison with that of avian myeloblastosis virus (AMV) reverse transcriptase and cellular DNA polymerases alpha and beta. In general, compounds that inhibited HIV reverse transcriptase also inhibited AMV reverse transcriptase. For example, 10 micrograms/ml of the isoquinoline quinones used in this study inhibited approximately 80% of the activity of reverse transcriptases of HIV and AMV, but did not inhibit the activity of DNA polymerases alpha and beta even at 50 micrograms/ml. AMV enzyme was more sensitive than HIV enzyme to colistin, enduracidins A and B, janiemycin, glysperin A, and thielavins A and B. The streptonigrin alkyl esters, however, inhibited HIV reverse transcriptase only. Sakyomicin A, luzopeptins, ellagic acid and suramine inhibited the activities of reverse transcriptases and cellular DNA polymerases.

Anti-Bacterial Agents↗