Crystal structures of a natural DNA polymerase that functions as an XNA reverse transcriptase.

Replicative DNA polymerases are highly efficient enzymes that maintain stringent geometric control over shape and orientation of the template and incoming nucleoside triphosphate. In a surprising twist to this paradigm, a naturally occurring bacterial DNA polymerase I member isolated from Geobacillus stearothermophilus (Bst) exhibits an innate ability to reverse transcribe RNA and other synthetic congeners (XNAs) into DNA. This observation raises the interesting question of how a replicative DNA polymerase is able to recognize templates of diverse chemical composition. Here, we present crystal structures of natural Bst DNA polymerase that capture the post-translocated product of DNA synthesis on templates composed entirely of 2'-deoxy-2'-fluoro-ß-d-arabino nucleic acid (FANA) and α-l-threofuranosyl nucleic acid (TNA). Analysis of the enzyme active site reveals the importance of structural plasticity as a possible mechanism for XNA-dependent DNA synthesis and provides insights into the construction of variants with improved activity.

Mimivirus encodes a multifunctional primase with DNA/RNA polymerase, terminal transferase and translesion synthesis activities.

Acanthamoeba polyphaga mimivirus is an amoeba-infecting giant virus with over 1000 genes including several involved in DNA replication and repair. Here, we report the biochemical characterization of gene product 577 (gp577), a hypothetical protein (product of L537 gene) encoded by mimivirus. Sequence analysis and phylogeny suggested gp577 to be a primase-polymerase (PrimPol)-the first PrimPol to be identified in a nucleocytoplasmic large DNA virus (NCLDV). Recombinant gp577 protein purified as a homodimer and exhibited de novo RNA as well as DNA synthesis on circular and linear single-stranded DNA templates. Further, gp577 extends a DNA/RNA primer annealed to a DNA or RNA template using deoxyribonucleoties (dNTPs) or ribonucleotides (NTPs) demonstrating its DNA/RNA polymerase and reverse transcriptase activity. We also show that gp577 possesses terminal transferase activity and is capable of extending ssDNA and dsDNA with NTPs and dNTPs. Mutation of the conserved primase motif residues of gp577 resulted in the loss of primase, polymerase, reverse transcriptase and terminal transferase activities. Additionally, we show that gp577 possesses translesion synthesis (TLS) activity. Mimiviral gp577 represents the first protein from an NCLDV endowed with primase, polymerase, reverse transcriptase, terminal transferase and TLS activities.

Identification of Thermus aquaticus DNA polymerase variants with increased mismatch discrimination and reverse transcriptase activity from a smart enzyme mutant library.

DNA polymerases the key enzymes for several biotechnological applications. Obviously, nature has not evolved these enzymes to be compatible with applications in biotechnology. Thus, engineering of a natural scaffold of DNA polymerases may lead to enzymes improved for several applications. Here, we investigated a two-step approach for the design and construction of a combinatorial library of mutants of KlenTaq DNA polymerase. First, we selected amino acid sites for saturation mutagenesis that interact with the primer/template strands or are evolutionarily conserved. From this library, we identified mutations that little interfere with DNA polymerase activity. Next, these functionally active mutants were combined randomly to construct a second library with enriched sequence diversity. We reasoned that the combination of mutants that have minuscule effect on enzyme activity and thermostability, will result in entities that have an increased mutation load but still retain activity. Besides activity and thermostability, we screened the library for entities with two distinct properties. Indeed, we identified two different KlenTaq DNA polymerase variants that either exhibit increased mismatch extension discrimination or increased reverse transcription PCR activity, respectively.

Preparation and characterization of the RNase H domain of Moloney murine leukemia virus reverse transcriptase.

Moloney murine leukemia virus reverse transcriptase (MMLV RT) contains fingers, palm, thumb, and connection subdomains as well as an RNase H domain. The DNA polymerase active site resides in the palm subdomain, and the RNase H active site is located in the RNase H domain. The RNase H domain contains a positively charged α-helix called the C helix (H(594)GEIYRRR(601)), that is thought to be involved in substrate recognition. In this study, we expressed three versions of the RNase H domain in Escherichia coli, the wild-type domain (WT) (residues Ile498-Leu671) and two variants that lack the regions containing the C helix (Ile593-Leu603 and Gly595-Thr605, which we called ΔC1 and ΔC2, respectively) with a strep-tag at the N-terminus and a deca-histidine tag at the C-terminus. These peptides were purified from the cells by anion-exchange, Ni(2+) affinity, and Strep-Tactin affinity column chromatography, and then the tags were removed by proteolysis. In an RNase H assay using a 25-bp RNA-DNA heteroduplex, WT, ΔC1, and ΔC2 produced RNA fragments ranging from 7 to 16 nucleotides (nt) whereas the full-length MMLV RT (Thr24-Leu671) produced 14-20-nt RNA fragments, suggesting that elimination of the fingers, palm, thumb, and connection subdomains affects the binding of the RNase H domain to the RNA-DNA heteroduplex. The activity levels of WT, ΔC1, and ΔC2 were estimated to be 1%, 0.01%, and 0.01% of full-length MMLV RT activity, indicating that the C helix is important, but not critical, for the activity of the isolated RNase H domain.

Adventitious agents in viral vaccines: lessons learned from 4 case studies.

Since the earliest days of biological product manufacture, there have been a number of instances where laboratory studies provided evidence for the presence of adventitious agents in a marketed product. Lessons learned from such events can be used to strengthen regulatory preparedness for the future. We have therefore selected four instances where an adventitious agent, or a signal suggesting the presence of an agent, was found in a viral vaccine, and have developed a case study for each. The four cases are: a) SV40 in polio vaccines; b) bacteriophage in measles and polio vaccines; c) reverse transcriptase in measles and mumps vaccines; and d) porcine circovirus and porcine circovirus DNA sequences in rotavirus vaccines. The lessons learned from each event are discussed. Based in part on those experiences, certain scientific principles have been identified by WHO that should be considered in regulatory risk evaluation if an adventitious agent is found in a marketed vaccine in the future.

In vitro epsilon RNA-dependent protein priming activity of human hepatitis B virus polymerase.

Hepatitis B virus (HBV) replicates its DNA genome through reverse transcription of a pregenomic RNA (pgRNA) by using a multifunctional polymerase (HP). A critical function of HP is its specific recognition of a viral RNA signal termed ε (Hε) located on pgRNA, which is required for specific packaging of pgRNA into viral nucleocapsids and initiation of viral reverse transcription. HP initiates reverse transcription by using itself as a protein primer (protein priming) and Hε as the obligatory template. We have purified HP from human cells that retained Hε binding activity in vitro. Furthermore, HP purified as a complex with Hε, but not HP alone, displayed in vitro protein priming activity. While the HP-Hε interaction in vitro and in vivo required the Hε internal bulge, but not its apical loop, and was not significantly affected by the cap-Hε distance, protein priming required both the Hε apical loop and internal bulge, as well as a short distance between the cap and Hε, mirroring the requirements for RNA packaging. These studies have thus established new HBV protein priming and RNA binding assays that should greatly facilitate the dissection of the requirements and molecular mechanisms of HP-Hε interactions, RNA packaging, and protein priming.

Purification of M-MLVH- RT on a 9-aminoethyladenine-(1,6-diamine-hexane)-triazine selected from a combinatorial library of dNTP-mimetic ligands.

Reverse transcriptase (RT) catalyzes the formation of dsDNA from single-stranded retroviral RNA genome. This enzyme is unique among DNA polymerases in its ability to use either RNA or DNA as a template. Moloney Murine Leukemia virus reverse transcriptase lacking RNase H activity (M-MLVH- RT) especially holds particular interest because of its ability to eliminate the deleterious effect of RNase H, which results in more efficient synthesis of full-length cDNA from mRNA. Therefore, the development of a simple purification method attracts the attention of retroviral drug and enzyme researchers and manufacturers. The present work is the first purification example of a non-tagged (native) RT by affinity chromatography using synthetic affinity ligands. In this study, the ligand was selected from a structure-biased combinatorial library of dNTP-mimetic ligands, and it was evaluated for its ability to bind and purify M-MLVH- RT from inclusion bodies of recombinant E. coli. The selected ligand (AEAd), bearing 9-aminoethyladenine and 1,6-diamine-hexane both linked on the same triazine scaffold, displayed the highest enzyme purifying ability after applying mild desorption conditions (6 mM MnCl(2) in 20 mM Tris-HCl buffer, pH 7.5). The binding capacity of immobilized AEAd with M-MLVH- RT was determined to be equal to approximately 1 mg enzyme/g moist weight gel. Adsorption studies with immobilized AEAd and soluble M-MLVH- RT demonstrated that the formation of the respective complex was perturbed by ATP. Quality control tests of the purified M-MLVH- RT essentially showed a single band (sodium dodecyl sulfate polyacrylamide gel electrophoresis) and absence of nucleic acids and contaminating nuclease activities.

A novel and simple method for high-level production of reverse transcriptase from Moloney murine leukemia virus (MMLV-RT) in Escherichia coli.

A novel protocol for producing recombinant Moloney murine leukemia virus (MMLV-RT) in Escherichia coli is reported. The optimized coding sequence for mature MMLV-RT was cloned into pET28a and over-expressed as an N-terminal His6-tagged fusion protein. An enterokinase (EK) recognition site was introduced between the His6-tag and MMLV-RT to release tag-free enzyme. Optimal expression of soluble His6-MMLV-RT was achieved by chaperone co-expression and lower temperature fermentation. The His6-tagged enzyme was first purified by Ni(2+) affinity chromatography. The bound enzyme was then eluted by EK digestion and the eluate was purified on an anion-exchange Q column to remove DNA and EK. Twenty-one milligram MMLV-RT was obtained from 1 l of bacterial culture.

Expression and characterization of a novel reverse transcriptase of the LTR retrotransposon Tf1.

The LTR retrotransposon of Schizosacharomyces pombe, Tf1, has several distinctive properties that can be related to the unique properties of its reverse transcriptase (RT). Consequently, we expressed, purified and studied the recombinant Tf1 RT. This monomeric protein possesses all activities typical to RTs: DNA and RNA-dependent DNA polymerase as well as an inherent ribonuclease H. The DNA polymerase activity shows preference to Mn(+)(2) or Mg(+)(2), depending on the substrate used, whereas the ribonuclease H strongly prefers Mn(+)(2). The most outstanding feature of Tf1 RT is its capacity to add non-templated nucleotides to the 3'-ends of the nascent DNA. This is mainly apparent in the presence of Mn(+)(2), as is the noticeable low fidelity of DNA synthesis. In all, Tf1 RT has a marked infidelity in synthesizing DNA at template ends, a phenomenon that can explain, as discussed herein, some of the features of Tf1 replication in the host cells.

Reverse transcriptases: intron-encoded proteins found in thermophilic bacteria.

A number of thermophilic bacteria have been surveyed for possessing reverse transcriptase genes using a degenerate primer approach derived from an alignment of known group II intron encoded reverse transcriptases (RT) from mesophilic prokaryotes and eukaryotes. Six out of 34 thermophilic isolates gave a PCR product that was indicative of an RT internal fragment on sequencing. A putative RT from Bacillus caldolyticus strain EA1 was isolated by genomic walking and cloned into an Escherichia coli expression vector. The recombinant protein proved to be insoluble and was unable to be recovered from the insoluble fraction of lysates of E. coli. The RT was successfully expressed in a baculovirus vector although yields remained low. We followed RT activity during purification using the poly(rC)*p(dG)(12-18), which specifically detects only RNA-dependent DNA polymerase activity. We could not detect incorporation of dTTP into poly(rC) )*p(dG)(12-18) when using uninfected Sf21 lysates and conclude that the substrate is not a template for DNA-dependent DNA polymerase. Although a high level of RT activity was detected in the total cell protein, when compared to the activity detected in the soluble fraction, only about 10% of the activity was soluble. Sequence comparisons showed significant differences between the EA1-IEP and a Geobacillus RT expressed by others. We conclude that it may be necessary to isolate the IEP RT as a ribonucleoprotein to obtain sufficient material for further analysis.

Reverse transcriptase-like sequences related to retrotransposon in a red alga, Porphyra yezoensis.

Four DNA fragments encoding a reverse transcriptase (RT)-like gene related to that of long terminal repeat (LTR) retrotransposons were isolated from the red alga Porphyra yezoensis by genomic PCR. Southern blot analysis suggested that one clone exists as a single copy per genome. Its full-length cDNA (PyRE2A) contained RT/RNase H-like sequences, which are most closely related to those of the Volvox LTR retrotransposon, although two stop codons were present within the RT region. We did not find any sequence related to LTR retrotransposons other than RT/RNase H in RyRE2A. These results indicate that PyRE2A is a single RT/RNase H-like gene and a defective progenitor of LTR retrotransposons.

Polyadenylation-dependent screening assay for respiratory syncytial virus RNA transcriptase activity and identification of an inhibitor.

RNA-dependent RNA polymerase from respiratory syncytial virus (RSV) is a multi-subunit ribonucleoprotein (RNP) complex that, in addition to synthesizing the full 15 222 nt viral genomic RNA, is able to synthesize all 10 viral mRNAs. We have prepared crude RNP from RSV-infected HEp-2 cells, based on a method previously used for Newcastle disease virus, and established a novel polyadenylation-dependent capture [poly(A) capture] assay to screen for potential inhibitors of RSV transcriptase activity. In this homogeneous assay, radiolabeled full-length polyadenylated mRNAs produced by the viral RNP are detected through capture on immobilized biotinylated oligo(dT) in a 96-well streptavidin-coated FlashPlate. Possible inhibitors identified with this assay could interfere at any step required for the production of complete RSV mRNAs, including transcription, polyadenylation and, potentially, co-transcriptional guanylylation. A specific inhibitor of RSV transcriptase with antiviral activity was identified through screening of this assay.

Assembly of an active group II intron-maturase complex by protein dimerization.

Group II intron-encoded proteins promote both splicing and mobility of the intron RNA through formation of a specific RNA-protein complex. The Lactococcus lactis L1.LtrB intron encodes a maturase, LtrA, with reverse transcriptase homology and specific binding affinity for two domains of the intron RNA. The catalytically active ribonucleoprotein (RNP) has splicing, endonuclease, and reverse transcriptase activity, enabling efficient insertion of the intron sequence by a retro-homing mechanism. To determine the composition and assembly mechanism of the RNP complex, purified LtrA protein was analyzed for its ability to recognize a series of intron-derived RNAs. Equilibrium dissociation measurements show that LtrA recognizes two intronic domains, DI and DIV. However, distinct electrostatic requirements for binding imply different modes of molecular recognition in each case. Stoichiometric binding experiments show that the functional RNP complex consists of a dimeric protein species bound to a single intron RNA. Significant differences between the measured equilibrium dissociation constants and kinetically derived values suggest that conformational changes accompany assembly of the intron-maturase complex, and results of limited proteolysis and fluorescence spectroscopy experiments suggest that significant RNA-dependent structural changes within the maturase occur upon association with the intron. These results support a mutually induced fit model in which RNA-dependent conformational changes within LtrA enable stable association of the protein dimer with two independent intron domains to form a functional RNP.

Inhibition of HIV-1 reverse transcriptase and protease by phlorotannins from the brown alga Ecklonia cava.

The bioassay-directed isolation of a marine brown alga, Ecklonia cava, afforded four phlorotannin derivatives, eckol (1), 8,8'-bieckol (2), 8,4"'-dieckol (3), and phlorofucofuroeckol A (4). Among these compounds, 2 and 3 exhibited an inhibitory effect on human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) and protease. Specifically, they inhibited the RT more potently than the protease. The inhibitory activity of compound 2 (IC(50), 0.51 microM) against HIV-1 RT was comparable to that of nevirapine (IC(50), 0.28 microM), a reference compound. An enzyme kinetic assay showed that this compound inhibited the RNA-dependent DNA synthesis activity of HIV-1 RT noncompetitively against dUTP/dTTP with a K(i) value of 0.78 microM. With respect to the homopolymeric template/primer, (rA)n(dT)15, 8,8'-bieckol (2) displayed an uncompetitive type of inhibition (K(i), 0.23 microM).

Functional reverse transcriptase encoded by the human LINE-1 from baculovirus-infected insect cells.

The human LINE-1 ORF2, which encodes reverse transcriptase, was inserted into a baculovirus shuttle vector and expressed in Sf 21 cells. An immunoreactive polypeptide (149kDa) synthesized by infected cells had reverse transcriptase activity. A procedure for purification of functional ORF2 protein from insect cells was developed. The enzyme was purified with good recovery to near homogeneity and retained stable DNA polymerase activity. The optimum reaction conditions of the enzyme were determined with respect to salts, pH, and temperature. Substrate specificities and divalent cation requirements were investigated. The recombinant enzyme had a 3-fold preference for Mg2+ over Mn2+ for reverse transcriptase activity on poly(rA).oligo(dT)(12). As for DNA synthesis, the recombinant ORF2 protein was found to possess both RNA-dependent and DNA-dependent DNA polymerase activities.

Cloning, expression, purification, and crystallisation of HIV-2 reverse transcriptase.

A purification procedure is described for the isolation of recombinant HIV-2 reverse transcriptase expressed in Escherichia coli. The p68 subunit is expressed, in the absence of induction, and use of a heparin-Sepharose column produces substantially pure protein. Concentration of the homodimeric p68 reverse transcriptase pool, followed by incubation at room temperature for several days, results in full conversion by E. coli proteases to the heterodimer (p68/p55). This extended incubation simplifies the purification process and improves the yield of heterodimeric reverse transcriptase, which shows a truncation of the smaller subunit to 427 residues. The protein is then purified further by hydroxyapatite and gel-filtration chromatography to homogeneity. The HIV-2 RT is active and has been used to produce crystals that diffract to beyond 3.0 A.

Lysine 152 of MuLV reverse transcriptase is required for the integrity of the active site.

Comparison of the three-dimensional structure of the active sites of MuLV and HIV-1 reverse transcriptases shows the presence of a lysine residue (K152) in the substrate-binding region in MuLV RT, while its equivalent position in HIV-1 RT is occupied by a glycine (G112). To investigate the role of K152 in the mechanism of the polymerase reaction catalyzed by MuLV RT, four mutant RTs, namely, K152A, K152R, K152E, and K152G, were generated and biochemically characterized. All muteins exhibited reduced polymerase activity on both RNA and DNA template-primers with K152E being the most defective. The template-primer binding affinity and the processivity of DNA synthesis, however, remained unchanged. The steady-state kinetic characterization showed little change in K(m.dNTP) (except for that of K152E) and an approximately 3-10-fold decrease in k(cat) depending upon the template-primer and mutational substitutions. The ddNTP resistance patterns were unchanged for all muteins, suggesting no participation of K152 in ddNTP recognition. The ability of individual muteins to add dNTP on the covalently cross-linked enzyme-template-primer complex was significantly decreased. These results together with the analysis of the ion pairs in the catalytic apparatus of MuLV RT suggest that K152 participates in maintaining the integrity of the active site of MuLV RT. Examination of the prepolymerase ternary complex formation showed that neither the wild type nor any of the K152 muteins of MuLV RT are capable of forming stable ternary complexes. This property is in contrast to that of HIV-1 RT, which readily forms stable ternary complexes under similar conditions. These results further indicate that the catalytic mechanism of MuLV RT is significantly different from that of HIV-1 RT, despite the presence of a number of conserved motifs and amino acid residues.

Characterization of the interaction between the nuclease and reverse transcriptase activity of the yeast telomerase complex.

Telomerase is a ribonucleoprotein that mediates extension of the dG-rich strand of telomeres in most eukaryotes. Like telomerase derived from ciliated protozoa, yeast telomerase is found to possess a tightly associated endonuclease activity that copurifies with the polymerization activity over different affinity-chromatographic steps. As is the case for ciliate telomerase, primers containing sequences that are not complementary to the RNA template can be efficiently cleaved by the yeast enzyme. More interestingly, we found that for the yeast enzyme, cleavage site selection is not stringent, since blocking cleavage at one site by the introduction of a nonhydrolyzable linkage can lead to the utilization of other sites. In addition, the reverse transcriptase activity of yeast telomerase can extend either the 5'- or 3'-end fragment following cleavage. Two general models that are consistent with the biochemical properties of the enzyme are presented: one model postulates two distinct active sites for the nuclease and reverse transcriptase, and the other invokes a multimeric enzyme with each protomer containing a single active site capable of mediating both cleavage and extension.

The majority of duck hepatitis B virus reverse transcriptase in cells is nonencapsidated and is bound to a cytoplasmic structure.

The hepadnavirus reverse transcriptase binds cotranslationally to the viral pregenomic RNA. This ribonucleoprotein complex is then encapsidated into nascent viral core particles, where the reverse transcriptase copies the viral RNA into DNA. Here we report that 75% of the duck hepatitis B virus reverse transcriptase present in transfected LMH cells does not follow this well-known pathway but rather exists in the cell separate from the core protein or nucleocapsids. The nonencapsidated reverse transcriptase is also abundant in infected duck liver. The nonencapsidated reverse transcriptase exists as a complex set of isoforms that are most likely produced by posttranslational modification. Interestingly, only the smallest of these isoforms is encapsidated into viral core particles. The nonencapsidated reverse transcriptase is bound to a large cellular cytoplasmic structure(s) in a detergent-sensitive complex. The cellular distribution of the reverse transcriptase only partially overlaps that of the core protein, and this distribution is unaffected by blocking encapsidation. These observations raise the possibilities that the metabolic fate of the reverse transcriptase may be posttranscriptionally regulated and that the reverse transcriptase may have roles in the viral replication cycle beyond its well-known function in copying the viral genome.