Ty3 reverse transcriptase complexed with an RNA-DNA hybrid shows structural and functional asymmetry

Structural and biochemical characterization of cauliflower mosaic virus reverse transcriptase

Reverse transcriptases (RTs) are enzymes with DNA polymerase and RNase H activities. They convert ssRNA into dsDNA and are key enzymes for the replication of retroviruses and retroelements. Caulimoviridae is a major family of plant-infecting viruses. Caulimoviruses have a circular dsDNA genome that is replicated by reverse transcription, but in contrast to retroviruses, they lack integrase. Caulimoviruses are related to Ty3 retroelements. Ty3 RT has been extensively studied structurally and biochemically, but corresponding information for caulimoviral RTs is unavailable. In the present study, we report the first crystal structure of cauliflower mosaic virus (CaMV) RT in complex with a duplex made of RNA and DNA strands (RNA/DNA hybrid). CaMV RT forms a monomeric complex with the hybrid, unlike Ty3 RT, which does so as a dimer. Results of the RNA-dependent DNA polymerase and DNA-dependent DNA polymerase activity assays showed that individual CaMV RT molecules are able to perform full polymerase functions. However, our analyses showed that an additional CaMV RT molecule needs to transiently associate with a polymerase-competent RT molecule to execute RNase H cuts of the RNA strand. Collectively, our results provide details into the structure and function of CaMV RT and describe how the enzyme compares to other related RTs.

Origin and Evolution of Plant Long Terminal Repeat Retrotransposons with Additional Ribonuclease H

Retroviruses originated from long terminal repeat retrotransposons (LTR-RTs) through several structural adaptations. One such modification was the arrangement of an additional ribonuclease H (aRH) domain next to native RH, followed by degradation and subfunctionalization of the latter. We previously showed that this retrovirus-like structure independently evolved in Tat LTR-RTs in flowering plants, proposing its origin from sequential rearrangements of ancestral Tat structures identified in lycophytes and conifers. However, most nonflowering plant genome assemblies were not available at that time, therefore masking the history of aRH acquisition by Tat and challenging our hypothesis. Here, we revisited Tat's evolution scenario upon the aRH acquisition by covering most of the extant plant phyla. We show that Tat evolved and obtained aRH in an ancestor of land plants. Importantly, we found the retrovirus-like structure in clubmosses, hornworts, ferns, and gymnosperms, suggesting its ancient origin, broad propagation, and yet-to-be-understood benefit for the LTR-RTs' adaptation.

Orchestrating nucleic acid-protein interactions at chromosome ends: telomerase mechanisms come into focus

Telomerase is a special reverse transcriptase ribonucleoprotein dedicated to the synthesis of telomere repeats that protect chromosome ends. Among reverse transcriptases, telomerase is unique in using a stably associated RNA with an embedded template to synthesize a specified sequence. Moreover, it is capable of iteratively copying the same template region (repeat addition processivity) through multiple rounds of RNA-DNA unpairing and reannealing, that is, the translocation reaction. Biochemical analyses of telomerase over the past 3 decades in protozoa, fungi and mammals have identified structural elements that underpin telomerase mechanisms and have led to models that account for the special attributes of telomerase. Notably, these findings and models can now be interpreted and adjudicated through recent cryo-EM structures of Tetrahymena and human telomerase holoenzyme complexes in association with substrates and regulatory proteins. Collectively, these structures reveal the intricate protein-nucleic acid interactions that potentiate telomerase's unique translocation reaction and clarify how this enzyme reconfigures the basic reverse transcriptase scaffold to craft a polymerase dedicated to the synthesis of telomere DNA. Among the many new insights is the resolution of the telomerase 'anchor site' proposed more than 3 decades ago. The structures also highlight the nearly universal conservation of a protein-protein interface between an oligonucleotide/oligosaccharide-binding (OB)-fold regulatory protein and the telomerase catalytic subunit, which enables spatial and temporal regulation of telomerase function in vivo. In this Review, we discuss key features of the structures in combination with relevant functional analyses. We also examine conserved and divergent aspects of telomerase mechanisms as gleaned from studies in different model organisms.

Human endogenous retrovirus-K (HERV-K) reverse transcriptase (RT) structure and biochemistry reveals remarkable similarities to HIV-1 RT and opportunities for HERV-K-specific inhibition

A large percentage of the human genome is composed of repetitive elements that are relics of past viral infections. Expression of these human endogenous retroviruses (HERVs) is associated with a variety of diseases, including cancer; however, causality remains to be established. A subset of these HERVs express proteins with reverse transcriptase (RT) activity. This has inspired several clinical studies of antiviral RT inhibitors for indications in which HERV expression is associated with disease. We have determined the X-ray structure of an HERV reverse transcriptase. This structure clarifies the reasons for poor inhibition by 3TC (lamivudine) and lack of inhibition by nonnucleoside inhibitors nevirapine and efavirenz. This structure will enable the design of selective HERV-K RT tools for drug target validation.

Human endogenous retroviruses (HERVs) comprise nearly 8% of the human genome and are derived from ancient integrations of retroviruses into the germline. The biology of HERVs is poorly defined, but there is accumulating evidence supporting pathological roles in diverse diseases, such as cancer, autoimmune, and neurodegenerative diseases. Functional proteins are produced by HERV-encoded genes, including reverse transcriptases (RTs), which could be a contributor to the pathology attributed to aberrant HERV-K expression. To facilitate the discovery and development of HERV-K RT potent and selective inhibitors, we expressed active HERV-K RT and determined the crystal structure of a ternary complex of this enzyme with a double-stranded DNA substrate. We demonstrate a range of RT inhibition with antiretroviral nucleotide analogs, while classic nonnucleoside analogs do not inhibit HERV-K RT. Detailed comparisons of HERV-K RT with other known RTs demonstrate similarities to diverse RT families and a striking similarity to the HIV-1 RT asymmetric heterodimer. Our analysis further reveals opportunities for selective HERV-K RT inhibition.

Retroviral RNase H: Structure, mechanism, and inhibition

All retroviruses encode the enzyme, reverse transcriptase (RT), which is involved in the conversion of the single-stranded viral RNA genome into double-stranded DNA. RT is a multifunctional enzyme and exhibits DNA polymerase and ribonuclease H (RNH) activities, both of which are essential to the reverse-transcription process. Despite the successful development of polymerase-targeting antiviral drugs over the last three decades, no bona fide inhibitor against the RNH activity of HIV-1 RT has progressed to clinical evaluation. In this review article, we describe the retroviral RNH function and inhibition, with primary consideration of the structural aspects of inhibition.

Structural basis of Ty3 retrotransposon integration at RNA Polymerase III-transcribed genes

Retrotransposons are endogenous elements that have the ability to mobilise their DNA between different locations in the host genome. The Ty3 retrotransposon integrates with an exquisite specificity in a narrow window upstream of RNA Polymerase (Pol) III-transcribed genes, representing a paradigm for harmless targeted integration. Here we present the cryo-EM reconstruction at 4.0 Å of an active Ty3 strand transfer complex bound to TFIIIB transcription factor and a tRNA gene. The structure unravels the molecular mechanisms underlying Ty3 targeting specificity at Pol III-transcribed genes and sheds light into the architecture of retrotransposon machinery during integration. Ty3 intasome contacts a region of TBP, a subunit of TFIIIB, which is blocked by NC2 transcription regulator in RNA Pol II-transcribed genes. A newly-identified chromodomain on Ty3 integrase interacts with TFIIIB and the tRNA gene, defining with extreme precision the integration site position.

Structures of Substrate Complexes of Foamy Viral Protease-Reverse Transcriptase

Reverse transcriptases (RTs) use their DNA polymerase and RNase H activities to catalyze the conversion of single-stranded RNA to double-stranded DNA (dsDNA), a crucial process for the replication of retroviruses. Foamy viruses (FVs) possess a unique RT, which is a fusion with the protease (PR) domain. The mechanism of substrate binding by this enzyme has been unknown. Here, we report a crystal structure of monomeric full-length marmoset FV (MFV) PR-RT in complex with an RNA/DNA hybrid substrate. We also describe a structure of MFV PR-RT with an RNase H deletion in complex with a dsDNA substrate in which the enzyme forms an asymmetric homodimer. Cryo-electron microscopy reconstruction of the full-length MFV PR-RT–dsDNA complex confirmed the dimeric architecture. These findings represent the first structural description of nucleic acid binding by a foamy viral RT and demonstrate its ability to change its oligomeric state depending on the type of bound nucleic acid.

IMPORTANCE Reverse transcriptases (RTs) are intriguing enzymes converting single-stranded RNA to dsDNA. Their activity is essential for retroviruses, which are divided into two subfamilies differing significantly in their life cycles: Orthoretrovirinae and Spumaretrovirinae. The latter family is much more ancient and comprises five genera. A unique feature of foamy viral RTs is that they contain N-terminal protease (PR) domains, which are not present in orthoretroviral enzymes. So far, no structural information for full-length foamy viral PR-RT interacting with nucleic substrates has been reported. Here, we present crystal and cryo-electron microscopy structures of marmoset foamy virus (MFV) PR-RT. These structures revealed the mode of binding of RNA/DNA and dsDNA substrates. Moreover, unexpectedly, the structures and biochemical data showed that foamy viral PR-RT can adopt both a monomeric configuration, which is observed in our structures in the presence of an RNA/DNA hybrid, and an asymmetric dimer arrangement, which we observed in the presence of dsDNA.

Crystal Structure of a Retroviral Polyprotein: Prototype Foamy Virus Protease-Reverse Transcriptase (PR-RT)

In most cases, proteolytic processing of the retroviral Pol portion of the Gag-Pol polyprotein precursor produces protease (PR), reverse transcriptase (RT), and integrase (IN). However, foamy viruses (FVs) express Pol separately from Gag and, when Pol is processed, only the IN domain is released. Here, we report a 2.9 Å resolution crystal structure of the mature PR-RT from prototype FV (PFV) that can carry out both proteolytic processing and reverse transcription but is in a configuration not competent for proteolytic or polymerase activity. PFV PR-RT is monomeric and the architecture of PFV PR is similar to one of the subunits of HIV-1 PR, which is a dimer. There is a C-terminal extension of PFV PR (101-145) that consists of two helices which are adjacent to the base of the RT palm subdomain, and anchors PR to RT. The polymerase domain of PFV RT consists of fingers, palm, thumb, and connection subdomains whose spatial arrangements are similar to the p51 subunit of HIV-1 RT. The RNase H and polymerase domains of PFV RT are connected by flexible linkers. Significant spatial and conformational (sub)domain rearrangements are therefore required for nucleic acid binding. The structure of PFV PR-RT provides insights into the conformational maturation of retroviral Pol polyproteins.

Structural basis for template switching by a group II intron-encoded non-LTR-retroelement reverse transcriptase

Reverse transcriptases (RTs) can switch template strands during complementary DNA synthesis, enabling them to join discontinuous nucleic acid sequences. Template switching (TS) plays crucial roles in retroviral replication and recombination, is used for adapter addition in RNA-Seq, and may contribute to retroelement fitness by increasing evolutionary diversity and enabling continuous complementary DNA synthesis on damaged templates. Here, we determined an X-ray crystal structure of a TS complex of a group II intron RT bound simultaneously to an acceptor RNA and donor RNA template–DNA primer heteroduplex with a 1-nt 3′-DNA overhang. The structure showed that the 3′ end of the acceptor RNA binds in a pocket formed by an N-terminal extension present in non–long terminal repeat–retroelement RTs and the RT fingertips loop, with the 3′ nucleotide of the acceptor base paired to the 1-nt 3′-DNA overhang and its penultimate nucleotide base paired to the incoming dNTP at the RT active site. Analysis of structure-guided mutations identified amino acids that contribute to acceptor RNA binding and a phenylalanine residue near the RT active site that mediates nontemplated nucleotide addition. Mutation of the latter residue decreased multiple sequential template switches in RNA-Seq. Our results provide new insights into the mechanisms of TS and nontemplated nucleotide addition by RTs, suggest how these reactions could be improved for RNA-Seq, and reveal common structural features for TS by non–long terminal repeat–retroelement RTs and viral RNA–dependent RNA polymerases.

Convergent evolution of integration site selection upstream of tRNA genes by yeast and amoeba retrotransposons

Transposable elements amplify in genomes as selfish DNA elements and challenge host fitness because their intrinsic integration steps during mobilization can compromise genome integrity. In gene-dense genomes, transposable elements are notably under selection to avoid insertional mutagenesis of host protein-coding genes. We describe an example of convergent evolution in the distantly related amoebozoan Dictyostelium discoideum and the yeast Saccharomyces cerevisiae, in which the D. discoideum retrotransposon DGLT-A and the yeast Ty3 element developed different mechanisms to facilitate position-specific integration at similar sites upstream of tRNA genes. Transcription of tRNA genes by RNA polymerase III requires the transcription factor complexes TFIIIB and TFIIIC. Whereas Ty3 recognizes tRNA genes mainly through interactions of its integrase with TFIIIB subunits, the DGLT-A-encoded ribonuclease H contacts TFIIIC subunit Tfc4 at an interface that covers tetratricopeptide repeats (TPRs) 7 and 8. A major function of this interface is to connect TFIIIC subcomplexes τA and τB and to facilitate TFIIIB assembly. During the initiation of tRNA gene transcription τB is displaced from τA, which transiently exposes the TPR 7/8 surface of Tfc4 on τA. We propose that the DGLT-A intasome uses this binding site to obtain access to genomic DNA for integration during tRNA gene transcription.

RNases H: Structure and mechanism

RNases H are a family of endonucleases that hydrolyze RNA residues in various nucleic acids. These enzymes are present in all branches of life, and their counterpart domains are also found in reverse transcriptases (RTs) from retroviruses and retroelements. RNases H are divided into two main classes (RNases H1 and H2 or type 1 and type 2 enzymes) with common structural features of the catalytic domain but different range of substrates for enzymatic cleavage. Additionally, a third class is found in some Archaea and bacteria. Besides distinct cellular functions specific for each type of RNases H, this family of proteins is generally involved in the maintenance of genome stability with overlapping and cooperative role in removal of R-loops thus preventing their accumulation. Extensive biochemical and structural studies of RNases H provided not only a comprehensive and complete picture of their mechanism but also revealed key basic principles of nucleic acid recognition and processing. RNase H1 is present in prokaryotes and eukaryotes and cleaves RNA in RNA/DNA hybrids. Its main function is hybrid removal, notably in the context of R-loops. RNase H2, which is also present in all branches of life, can play a similar role but it also has a specialized function in the cleavage of single ribonucleotides embedded in the DNA. RNase H3 is present in Archaea and bacteria and is closely related to RNase H2 in sequence and structure but has RNase H1-like biochemical properties. This review summarizes the mechanisms of substrate recognition and enzymatic cleavage by different classes of RNases H with particular insights into structural features of nucleic acid binding, specificity towards RNA and/or DNA strands and catalysis.

Structural and Functional Aspects of Foamy Virus Protease-Reverse Transcriptase

Reverse transcription describes the process of the transformation of single-stranded RNA into double-stranded DNA via an RNA/DNA duplex intermediate, and is catalyzed by the viral enzyme reverse transcriptase (RT). This event is a pivotal step in the life cycle of all retroviruses. In contrast to orthoretroviruses, the domain structure of the mature RT of foamy viruses is different, i.e., it harbors the protease (PR) domain at its N-terminus, thus being a PR-RT. This structural feature has consequences on PR activation, since the enzyme is monomeric in solution and retroviral PRs are only active as dimers. This review focuses on the structural and functional aspects of simian and prototype foamy virus reverse transcription and reverse transcriptase, as well as special features of reverse transcription that deviate from orthoretroviral processes, e.g., PR activation.

Multiple Partial Regularized Nonnegative Matrix Factorization for Predicting Ontological Functions of lncRNAs

Long non-coding RNAs (LncRNA) are critical regulators for biological processes, which are highly related to complex diseases. Even though the next generation sequence technology facilitates the discovery of a great number of lncRNAs, the knowledge about the functions of lncRNAs is limited. Thus, it is promising to predict the functions of lncRNAs, which shed light on revealing the mechanisms of complex diseases. The current algorithms predict the functions of lncRNA by using the features of protein-coding genes. Generally speaking, these algorithms fuse heterogeneous genomic data to construct lncRNA-gene associations via a linear combination, which cannot fully characterize the function-lncRNA relations. To overcome this issue, we present an nonnegative matrix factorization algorithm with multiple partial regularization (aka MPrNMF) to predict the functions of lncRNAs without fusing the heterogeneous genomic data. In details, for each type of genomic data, we construct the lncRNA-gene associations, resulting in multiple associations. The proposed method integrates separately them via regularization strategy, rather than fuse them into a single type of associations. The results demonstrate that the proposed algorithm outperforms state-of-the-art methods based network-analysis. The model and algorithm provide an effective way to explore the functions of lncRNAs.

Specific reverse transcriptase slippage at the HIV ribosomal frameshift sequence: potential implications for modulation of GagPol synthesis

Synthesis of HIV GagPol involves a proportion of ribosomes translating a U6A shift site at the distal end of the gag gene performing a programmed -1 ribosomal frameshift event to enter the overlapping pol gene. In vitro studies here show that at the same shift motif HIV reverse transcriptase generates -1 and +1 indels with their ratio being sensitive to the relative concentration ratio of dNTPs specified by the RNA template slippage-prone sequence and its 5' adjacent base. The GGG sequence 3' adjacent to the U6A shift/slippage site, which is important for ribosomal frameshifting, is shown here to limit reverse transcriptase base substitution and indel 'errors' in the run of A's in the product. The indels characterized here have either 1 more or less A, than the corresponding number of template U's. cDNA with 5 A's may yield novel Gag product(s), while cDNA with an extra base, 7 A's, may only be a minor contributor to GagPol polyprotein. Synthesis of a proportion of non-ribosomal frameshift derived GagPol would be relevant in efforts to identify therapeutically useful compounds that perturb the ratio of GagPol to Gag, and pertinent to the extent in which specific polymerase slippage is utilized in gene expression.

Arm-specific cleavage and mutation during reverse transcription of 2΄,5΄-branched RNA by Moloney murine leukemia virus reverse transcriptase

Branchpoint nucleotides of intron lariats induce pausing of DNA synthesis by reverse transcriptases (RTs), but it is not known yet how they direct RT RNase H activity on branched RNA (bRNA). Here, we report the effects of the two arms of bRNA on branchpoint-directed RNA cleavage and mutation produced by Moloney murine leukemia virus (M-MLV) RT during DNA polymerization. We constructed a long-chained bRNA template by splinted-ligation. The bRNA oligonucleotide is chimeric and contains DNA to identify RNA cleavage products by probe hybridization. Unique sequences surrounding the branchpoint facilitate monitoring of bRNA purification by terminal-restriction fragment length polymorphism analysis. We evaluate the M-MLV RT-generated cleavage and mutational patterns. We find that cleavage of bRNA and misprocessing of the branched nucleotide proceed arm-specifically. Bypass of the branchpoint from the 2΄-arm causes single-mismatch errors, whereas bypass from the 3΄-arm leads to deletion mutations. The non-template arm is cleaved when reverse transcription is primed from the 3΄-arm but not from the 2΄-arm. This suggests that RTs flip ∼180° at branchpoints and RNases H cleave the non-template arm depending on its accessibility. Our observed interplay between M-MLV RT and bRNA would be compatible with a bRNA-mediated control of retroviral and related retrotransposon replication.

Characterization of Echinostoma cinetorchis endoribonuclease, RNase H

Echinostoma cinetorchis is an oriental intestinal fluke causing significant pathological damage to the small intestine. The aim of this study was to determine a full-length cDNA sequence of E. cinetorchis endoribonuclease (RNase H; EcRNH) and to elucidate its molecular biological characters. EcRNH consisted of 308 amino acids and showed low similarity to endoribonucleases of other parasites (<40%). EcRNH had an active site centered on a putative DDEED motif instead of DEDD conserved in other species. A recombinant EcRNH produced as a soluble form in Escherichia coli showed enzymatic activity to cleave the 3'-O-P bond of RNA in a DNA-RNA duplex, producing 3'-hydroxyl and 5'-phosphate. These findings may contribute to develop antisense oligonucleotides which could damage echinostomes and other flukes.

Reverse Transcription in the Saccharomyces cerevisiae Long-Terminal Repeat Retrotransposon Ty3

Converting the single-stranded retroviral RNA into integration-competent double-stranded DNA is achieved through a multi-step process mediated by the virus-coded reverse transcriptase (RT). With the exception that it is restricted to an intracellular life cycle, replication of the Saccharomyces cerevisiae long terminal repeat (LTR)-retrotransposon Ty3 genome is guided by equivalent events that, while generally similar, show many unique and subtle differences relative to the retroviral counterparts. Until only recently, our knowledge of RT structure and function was guided by a vast body of literature on the human immunodeficiency virus (HIV) enzyme. Although the recently-solved structure of Ty3 RT in the presence of an RNA/DNA hybrid adds little in terms of novelty to the mechanistic basis underlying DNA polymerase and ribonuclease H activity, it highlights quite remarkable topological differences between retroviral and LTR-retrotransposon RTs. The theme of overall similarity but distinct differences extends to the priming mechanisms used by Ty3 RT to initiate (−) and (+) strand DNA synthesis. The unique structural organization of the retrotransposon enzyme and interaction with its nucleic acid substrates, with emphasis on polypurine tract (PPT)-primed initiation of (+) strand synthesis, is the subject of this review.

Convergence of retrotransposons in oomycetes and plants

Background

Retrotransposons comprise a ubiquitous and abundant class of eukaryotic transposable elements. All members of this class rely on reverse transcriptase activity to produce a DNA copy of the element from the RNA template. However, other activities of the retrotransposon-encoded polyprotein may differ between diverse retrotransposons. The polyprotein domains corresponding to each of these activities may have their own evolutionary history independent from that of the reverse transcriptase, thus underlying the modular view on the evolution of retrotransposons. Furthermore, some transposable elements can independently evolve similar domain architectures by acquiring functionally similar but phylogenetically distinct modules. This convergent evolution of retrotransposons may ultimately suggest similar regulatory pathways underlying the lifecycle of the elements.

Results

Here, we provide new examples of the convergent evolution of retrotransposons of species from two unrelated taxa: green plants and parasitic protozoan oomycetes. In the present study we first analyzed the available genomic sequences of oomycete species and characterized two groups of Ty3/Gypsy long terminal repeat retrotransposons, namely Chronos and Archon, and a subgroup of L1 non-long terminal repeat retrotransposons. The results demonstrated that the retroelements from these three groups each have independently acquired plant-related ribonuclease H domains. This process closely resembles the evolution of retrotransposons in the genomes of green plants. In addition, we showed that Chronos elements captured a chromodomain, mimicking the process of chromodomain acquisition by Chromoviruses, another group of Ty3/Gypsy retrotransposons of plants, fungi, and vertebrates.

Conclusions

Repeated and strikingly similar acquisitions of ribonuclease H domains and chromodomains by different retrotransposon groups from unrelated taxa indicate similar selection pressure acting on these elements. Thus, there are some major trends in the evolution of the structural composition of retrotransposons, and characterizing these trends may enhance the current understanding of the retrotransposon life cycle.

Electronic supplementary material

The online version of this article (doi:10.1186/s13100-017-0087-y) contains supplementary material, which is available to authorized users.

Viral reverse transcriptases

Reverse transcriptases (RTs) play a major role in the replication of Retroviridae, Metaviridae, Pseudoviridae, Hepadnaviridae and Caulimoviridae. RTs are enzymes that are able to synthesize DNA using RNA or DNA as templates (DNA polymerase activity), and degrade RNA when forming RNA/DNA hybrids (ribonuclease H activity). In retroviruses and LTR retrotransposons (Metaviridae and Pseudoviridae), the coordinated action of both enzymatic activities converts single-stranded RNA into a double-stranded DNA that is flanked by identical sequences known as long terminal repeats (LTRs). RTs of retroviruses and LTR retrotransposons are active as monomers (e.g. murine leukemia virus RT), homodimers (e.g. Ty3 RT) or heterodimers (e.g. human immunodeficiency virus type 1 (HIV-1) RT). RTs lack proofreading activity and display high intrinsic error rates. Besides, high recombination rates observed in retroviruses are promoted by poor processivity that causes template switching, a hallmark of reverse transcription. HIV-1 RT inhibitors acting on its polymerase activity constitute the backbone of current antiretroviral therapies, although novel drugs, including ribonuclease H inhibitors, are still necessary to fight HIV infections. In Hepadnaviridae and Caulimoviridae, reverse transcription leads to the formation of nicked circular DNAs that will be converted into episomal DNA in the host cell nucleus. Structural and biochemical information on their polymerases is limited, although several drugs inhibiting HIV-1 RT are known to be effective against the human hepatitis B virus polymerase. In this review, we summarize current knowledge on reverse transcription in the five virus families and discuss available biochemical and structural information on RTs, including their biosynthesis, enzymatic activities, and potential inhibition.

Reverse Transcription of Retroviruses and LTR Retrotransposons

The enzyme reverse transcriptase (RT) was discovered in retroviruses almost 50 years ago. The demonstration that other types of viruses, and what are now called retrotransposons, also replicated using an enzyme that could copy RNA into DNA came a few years later. The intensity of the research in both the process of reverse transcription and the enzyme RT was greatly stimulated by the recognition, in the mid-1980s, that human immunodeficiency virus (HIV) was a retrovirus and by the fact that the first successful anti-HIV drug, azidothymidine (AZT), is a substrate for RT. Although AZT monotherapy is a thing of the past, the most commonly prescribed, and most successful, combination therapies still involve one or both of the two major classes of anti-RT drugs. Although the basic mechanics of reverse transcription were worked out many years ago, and the first high-resolution structures of HIV RT are now more than 20 years old, we still have much to learn, particularly about the roles played by the host and viral factors that make the process of reverse transcription much more efficient in the cell than in the test tube. Moreover, we are only now beginning to understand how various host factors that are part of the innate immunity system interact with the process of reverse transcription to protect the host-cell genome, the host cell, and the whole host, from retroviral infection, and from unwanted retrotransposition.

Ty3, a Position-specific Retrotransposon in Budding Yeast

Long terminal repeat (LTR) retrotransposons constitute significant fractions of many eukaryotic genomes. Two ancient families are Ty1/Copia (Pseudoviridae) and Ty3/Gypsy (Metaviridae). The Ty3/Gypsy family probably gave rise to retroviruses based on the domain order, similarity of sequences, and the envelopes encoded by some members. The Ty3 element of Saccharomyces cerevisiae is one of the most completely characterized elements at the molecular level. Ty3 is induced in mating cells by pheromone stimulation of the mitogen-activated protein kinase pathway as cells accumulate in G1. The two Ty3 open reading frames are translated into Gag3 and Gag3-Pol3 polyprotein precursors. In haploid mating cells Gag3 and Gag3-Pol3 are assembled together with Ty3 genomic RNA into immature virus-like particles in cellular foci containing RNA processing body proteins. Virus-like particle Gag3 is then processed by Ty3 protease into capsid, spacer, and nucleocapsid, and Gag3-Pol3 into those proteins and additionally, protease, reverse transcriptase, and integrase. After haploid cells mate and become diploid, genomic RNA is reverse transcribed into cDNA. Ty3 integration complexes interact with components of the RNA polymerase III transcription complex resulting in Ty3 integration precisely at the transcription start site. Ty3 activation during mating enables proliferation of Ty3 between genomes and has intriguing parallels with metazoan retrotransposon activation in germ cell lineages. Identification of nuclear pore, DNA replication, transcription, and repair host factors that affect retrotransposition has provided insights into how hosts and retrotransposons interact to balance genome stability and plasticity.

Detection of treatment-resistant infectious HIV after genome-directed antiviral endonuclease therapy

Incurable chronic viral infections are a major cause of morbidity and mortality worldwide. One potential approach to cure persistent viral infections is via the use of targeted endonucleases. Nevertheless, a potential concern for endonuclease-based antiviral therapies is the emergence of treatment resistance. Here we detect for the first time an endonuclease-resistant infectious virus that is found with high frequency after antiviral endonuclease therapy. While testing the activity of HIV pol-specific zinc finger nucleases (ZFNs) alone or in combination with three prime repair exonuclease 2 (Trex2), we identified a treatment-resistant and infectious mutant virus that was derived from a ZFN-mediated disruption of reverse transcriptase (RT). Although gene disruption of HIV protease, RT and integrase could inhibit viral replication, a chance single amino acid insertion within the thumb domain of RT produced a virus that could actively replicate. The endonuclease-resistant virus could replicate in primary CD4(+) T cells, but remained susceptible to treatment with antiretroviral RT inhibitors. When secondary ZFN-derived mutations were introduced into the mutant virus's RT or integrase domains, replication could be abolished. Our observations suggest that caution should be exercised during endonuclease-based antiviral therapies; however, combination endonuclease therapies may prevent the emergence of resistance.

Transcriptionally active LTR retrotransposons in Eucalyptus genus are differentially expressed and insertionally polymorphic

BACKGROUND

In Eucalyptus genus, studies on genome composition and transposable elements (TEs) are particularly scarce. Nearly half of the recently released Eucalyptus grandis genome is composed by retrotransposons and this data provides an important opportunity to understand TE dynamics in Eucalyptus genome and transcriptome.

RESULTS

We characterized nine families of transcriptionally active LTR retrotransposons from Copia and Gypsy superfamilies in Eucalyptus grandis genome and we depicted genomic distribution and copy number in two Eucalyptus species. We also evaluated genomic polymorphism and transcriptional profile in three organs of five Eucalyptus species. We observed contrasting genomic and transcriptional behavior in the same family among different species. RLC_egMax_1 was the most prevalent family and RLC_egAngela_1 was the family with the lowest copy number. Most families of both superfamilies have their insertions occurring <3 million years, except one Copia family, RLC_egBianca_1. Protein theoretical models suggest different properties between Copia and Gypsy domains. IRAP and REMAP markers suggested genomic polymorphisms among Eucalyptus species. Using EST analysis and qRT-PCRs, we observed transcriptional activity in several tissues and in all evaluated species. In some families, osmotic stress increases transcript values.

CONCLUSION

Our strategy was successful in isolating transcriptionally active retrotransposons in Eucalyptus, and each family has a particular genomic and transcriptional pattern. Overall, our results show that retrotransposon activity have differentially affected genome and transcriptome among Eucalyptus species.

Convergent evolution of ribonuclease h in LTR retrotransposons and retroviruses

Ty3/Gypsy long terminals repeat (LTR) retrotransposons are structurally and phylogenetically close to retroviruses. Two notable structural differences between these groups of genetic elements are 1) the presence in retroviruses of an additional envelope gene, env, which mediates infection, and 2) a specific dual ribonuclease H (RNH) domain encoded by the retroviral pol gene. However, similar to retroviruses, many Ty3/Gypsy LTR retrotransposons harbor additional env-like genes, promoting concepts of the infective mode of these retrotransposons. Here, we provide a further line of evidence of similarity between retroviruses and some Ty3/Gypsy LTR retrotransposons. We identify that, together with their additional genes, plant Ty3/Gypsy LTR retrotransposons of the Tat group have a second RNH, as do retroviruses. Most importantly, we show that the resulting dual RNHs of Tat LTR retrotransposons and retroviruses emerged independently, providing strong evidence for their convergent evolution. The convergent resemblance of Tat LTR retrotransposons and retroviruses may indicate similar selection pressures acting on these diverse groups of elements and reveal potential evolutionary constraints on their structure. We speculate that dual RNH is required to accelerate retrotransposon evolution through increased rates of strand transfer events and subsequent recombination events.

Retroviral DNA Transposition: Themes and Variations

Retroviruses and LTR retrotransposons are transposable elements that encapsidate the RNAs that are intermediates in the transposition of DNA copies of their genomes (proviruses), from one cell (or one locus) to another. Mechanistic similarities in DNA transposase enzymes and retroviral/retrotransposon integrases underscore the close evolutionary relationship among these elements. The retroviruses are very ancient infectious agents, presumed to have evolved from Ty3/Gypsy LTR retrotransposons (1), and DNA copies of their sequences can be found embedded in the genomes of most, if not all, members of the tree of life. All retroviruses share a specific gene arrangement and similar replication strategies. However, given their ancestries and occupation of diverse evolutionary niches, it should not be surprising that unique sequences have been acquired in some retroviral genomes and that the details of the mechanism by which their transposition is accomplished can vary. While every step in the retrovirus lifecycle is, in some sense, relevant to transposition, this Chapter focuses mainly on the early phase of retroviral replication, during which viral DNA is synthesized and integrated into its host genome. Some of the initial studies that set the stage for current understanding are highlighted, as well as more recent findings obtained through use of an ever-expanding technological toolbox including genomics, proteomics, and siRNA screening. Persistence in the area of structural biology has provided new insight into conserved mechanisms as well as variations in detail among retroviruses, which can also be instructive.