NMR detection of intermolecular interaction sites in the dimeric 5'-leader of the HIV-1 genome

Role of RNA structural plasticity in modulating HIV-1 genome packaging and translation

HIV-1 transcript function is controlled in part by twinned transcriptional start site usage, where 5' capped RNAs beginning with a single guanosine (1G) are preferentially packaged into progeny virions as genomic RNA (gRNA) whereas those beginning with three sequential guanosines (3G) are retained in cells as mRNAs. In 3G transcripts, one of the additional guanosines base pairs with a cytosine located within a conserved 5' polyA element, resulting in formation of an extended 5' polyA structure as opposed to the hairpin structure formed in 1G RNAs. To understand how this remodeling influences overall transcript function, we applied in vitro biophysical studies with in-cell genome packaging and competitive translation assays to native and 5' polyA mutant transcripts generated with promoters that differentially produce 1G or 3G RNAs. We identified mutations that stabilize the 5' polyA hairpin structure in 3G RNAs, which promote RNA dimerization and Gag binding without sequestering the 5' cap. None of these 3G transcripts were competitively packaged, confirming that cap exposure is a dominant negative determinant of viral genome packaging. For all RNAs examined, conformations that favored 5' cap exposure were both poorly packaged and more efficiently translated than those that favored 5' cap sequestration. We propose that structural plasticity of 5' polyA and other conserved RNA elements place the 5' leader on a thermodynamic tipping point for low-energetic (~3 kcal/mol) control of global transcript structure and function.

Retroviral PBS-segment sequence and structure: Orchestrating early and late replication events

An essential regulatory hub for retroviral replication events, the 5' untranslated region (UTR) encodes an ensemble of cis-acting replication elements that overlap in a logical manner to carry out divergent RNA activities in cells and in virions. The primer binding site (PBS) and primer activation sequence initiate the reverse transcription process in virions, yet overlap with structural elements that regulate expression of the complex viral proteome. PBS-segment also encompasses the attachment site for Integrase to cut and paste the 3' long terminal repeat into the host chromosome to form the provirus and purine residues necessary to execute the precise stoichiometry of genome-length transcripts and spliced viral RNAs. Recent genetic mapping, cofactor affinity experiments, NMR and SAXS have elucidated that the HIV-1 PBS-segment folds into a three-way junction structure. The three-way junction structure is recognized by the host's nuclear RNA helicase A/DHX9 (RHA). RHA tethers host trimethyl guanosine synthase 1 to the Rev/Rev responsive element (RRE)-containing RNAs for m7-guanosine Cap hyper methylation that bolsters virion infectivity significantly. The HIV-1 trimethylated (TMG) Cap licenses specialized translation of virion proteins under conditions that repress translation of the regulatory proteins. Clearly host-adaption and RNA shapeshifting comprise the fundamental basis for PBS-segment orchestrating both reverse transcription of virion RNA and the nuclear modification of m7G-Cap for biphasic translation of the complex viral proteome. These recent observations, which have exposed even greater complexity of retroviral RNA biology than previously established, are the impetus for this article. Basic research to fully comprehend the marriage of PBS-segment structures and host RNA binding proteins that carry out retroviral early and late replication events is likely to expose an immutable virus-specific therapeutic target to attenuate retrovirus proliferation.

HIV-1 RNA genome packaging: it's G-rated

A member of the Retroviridae, human immunodeficiency virus type 1 (HIV-1), uses the RNA genome packaged into nascent virions to transfer genetic information to its progeny. The genome packaging step is a highly regulated and extremely efficient process as a vast majority of virus particles contain two copies of full-length unspliced HIV-1 RNA that form a dimer. Thus, during virus assembly HIV-1 can identify and selectively encapsidate HIV-1 unspliced RNA from an abundant pool of cellular RNAs and various spliced HIV-1 RNAs. Several "G" features facilitate the packaging of a dimeric RNA genome. The viral polyprotein Gag orchestrates virus assembly and mediates RNA genome packaging. During this process, Gag preferentially binds unpaired guanosines within the highly structured 5' untranslated region (UTR) of HIV-1 RNA. In addition, the HIV-1 unspliced RNA provides a scaffold that promotes Gag:Gag interactions and virus assembly, thereby ensuring its packaging. Intriguingly, recent studies have shown that the use of different guanosines at the junction of U3 and R as transcription start sites results in HIV-1 unspliced RNA species with 99.9% identical sequences but dramatically distinct 5' UTR conformations. Consequently, one species of unspliced RNA is preferentially packaged over other nearly identical RNAs. These studies reveal how conformations affect the functions of HIV-1 RNA elements and the complex regulation of HIV-1 replication. In this review, we summarize cis- and trans-acting elements critical for HIV-1 RNA packaging, locations of Gag:RNA interactions that mediate genome encapsidation, and the effects of transcription start sites on the structure and packaging of HIV-1 RNA.

Structural and computational studies of HIV-1 RNA

Viruses remain a global threat to animals, plants, and humans. The type 1 human immunodeficiency virus (HIV-1) is a member of the retrovirus family and carries an RNA genome, which is reverse transcribed into viral DNA and further integrated into the host-cell DNA for viral replication and proliferation. The RNA structures from the HIV-1 genome provide valuable insights into the mechanisms underlying the viral replication cycle. Moreover, these structures serve as models for designing novel therapeutic approaches. Here, we review structural data on RNA from the HIV-1 genome as well as computational studies based on these structural data. The review is organized according to the type of structured RNA element which contributes to different steps in the viral replication cycle. This is followed by an overview of the HIV-1 transactivation response element (TAR) RNA as a model system for understanding dynamics and interactions in the viral RNA systems. The review concludes with a description of computational studies, highlighting the impact of biomolecular simulations in elucidating the mechanistic details of various steps in the HIV-1's replication cycle.

Single-cell multiomic understanding of HIV-1 reservoir at epigenetic, transcriptional, and protein levels

PURPOSE OF REVIEW

The success of HIV-1 eradication strategies relies on in-depth understanding of HIV-1-infected cells. However, HIV-1-infected cells are extremely heterogeneous and rare. Single-cell multiomic approaches resolve the heterogeneity and rarity of HIV-1-infected cells.

RECENT FINDINGS

Advancement in single-cell multiomic approaches enabled HIV-1 reservoir profiling across the epigenetic (ATAC-seq), transcriptional (RNA-seq), and protein levels (CITE-seq). Using HIV-1 RNA as a surrogate, ECCITE-seq identified enrichment of HIV-1-infected cells in clonally expanded cytotoxic CD4+ T cells. Using HIV-1 DNA PCR-activated microfluidic sorting, FIND-seq captured the bulk transcriptome of HIV-1 DNA+ cells. Using targeted HIV-1 DNA amplification, PheP-seq identified surface protein expression of intact versus defective HIV-1-infected cells. Using ATAC-seq to identify HIV-1 DNA, ASAP-seq captured transcription factor activity and surface protein expression of HIV-1 DNA+ cells. Combining HIV-1 mapping by ATAC-seq and HIV-1 RNA mapping by RNA-seq, DOGMA-seq captured the epigenetic, transcriptional, and surface protein expression of latent and transcriptionally active HIV-1-infected cells. To identify reproducible biological insights and authentic HIV-1-infected cells and avoid false-positive discovery of artifacts, we reviewed current practices of single-cell multiomic experimental design and bioinformatic analysis.

SUMMARY

Single-cell multiomic approaches may identify innovative mechanisms of HIV-1 persistence, nominate therapeutic strategies, and accelerate discoveries.

Human Retrovirus Genomic RNA Packaging

Two non-covalently linked copies of the retrovirus genome are specifically recruited to the site of virus particle assembly and packaged into released particles. Retroviral RNA packaging requires RNA export of the unspliced genomic RNA from the nucleus, translocation of the genome to virus assembly sites, and specific interaction with Gag, the main viral structural protein. While some aspects of the RNA packaging process are understood, many others remain poorly understood. In this review, we provide an update on recent advancements in understanding the mechanism of RNA packaging for retroviruses that cause disease in humans, i.e., HIV-1, HIV-2, and HTLV-1, as well as advances in the understanding of the details of genomic RNA nuclear export, genome translocation to virus assembly sites, and genomic RNA dimerization.

RNA Structural Requirements for Nucleocapsid Protein-Mediated Extended Dimer Formation

Retroviruses package two copies of their genomic RNA (gRNA) as non-covalently linked dimers. Many studies suggest that the retroviral nucleocapsid protein (NC) plays an important role in gRNA dimerization. The upper part of the L3 RNA stem-loop in the 5' leader of the avian leukosis virus (ALV) is converted to the extended dimer by ALV NC. The L3 hairpin contains three stems and two internal loops. To investigate the roles of internal loops and stems in the NC-mediated extended dimer formation, we performed site-directed mutagenesis, gel electrophoresis, and analysis of thermostability of dimeric RNAs. We showed that the internal loops are necessary for efficient extended dimer formation. Destabilization of the lower stem of L3 is necessary for RNA dimerization, although it is not involved in the linkage structure of the extended dimer. We found that NCs from ALV, human immunodeficiency virus type 1 (HIV-1), and Moloney murine leukemia virus (M-MuLV) cannot promote the formation of the extended dimer when the apical stem contains ten consecutive base pairs. Five base pairs correspond to the maximum length for efficient L3 dimerization induced by the three NCs. L3 dimerization was less efficient with M-MuLV NC than with ALV NC and HIV-1 NC.

Epitranscriptomic regulation of HIV-1 full-length RNA packaging

During retroviral replication, the full-length RNA serves both as mRNA and genomic RNA. However, the mechanisms by which the HIV-1 Gag protein selects the two RNA molecules that will be packaged into nascent virions remain poorly understood. Here, we demonstrate that deposition of N6-methyladenosine (m6A) regulates full-length RNA packaging. While m6A deposition by METTL3/METTL14 onto the full-length RNA was associated with increased Gag synthesis and reduced packaging, FTO-mediated demethylation promoted the incorporation of the full-length RNA into viral particles. Interestingly, HIV-1 Gag associates with the RNA demethylase FTO in the nucleus and contributes to full-length RNA demethylation. We further identified two highly conserved adenosines within the 5'-UTR that have a crucial functional role in m6A methylation and packaging of the full-length RNA. Together, our data propose a novel epitranscriptomic mechanism allowing the selection of the HIV-1 full-length RNA molecules that will be used as viral genomes.

Overview of Methods for Large-Scale RNA Synthesis

In recent years, it has become clear that RNA molecules are involved in almost all vital cellular processes and pathogenesis of human disorders. The functional diversity of RNA comes from its structural richness. Although composed of only four nucleotides, RNA molecules present a plethora of secondary and tertiary structures critical for intra and intermolecular contacts with other RNAs and ligands (proteins, small metabolites, etc.). In order to fully understand RNA function it is necessary to define its spatial structure. Crystallography, nuclear magnetic resonance and cryogenic electron microscopy have demonstrated considerable success in determining the structures of biologically important RNA molecules. However, these powerful methods require large amounts of sample. Despite their limitations, chemical synthesis and in vitro transcription are usually employed to obtain milligram quantities of RNA for structural studies, delivering simple and effective methods for large-scale production of homogenous samples. The aim of this paper is to provide an overview of methods for large-scale RNA synthesis with emphasis on chemical synthesis and in vitro transcription. We also present our own results of testing the efficiency of these approaches in order to adapt the material acquisition strategy depending on the desired RNA construct.

HIV-1 Packaging Visualised by In-Gel SHAPE

HIV-1 packages two copies of its gRNA into virions via an interaction with the viral structural protein Gag. Both copies and their native RNA structure are essential for virion infectivity. The precise stepwise nature of the packaging process has not been resolved. This is largely due to a prior lack of structural techniques that follow RNA structural changes within an RNA-protein complex. Here, we apply the in-gel SHAPE (selective 2'OH acylation analysed by primer extension) technique to study the initiation of HIV-1 packaging, examining the interaction between the packaging signal RNA and the Gag polyprotein, and compare it with that of the NC domain of Gag alone. Our results imply interactions between Gag and monomeric packaging signal RNA in switching the RNA conformation into a dimerisation-competent structure, and show that the Gag-dimer complex then continues to stabilise. These data provide a novel insight into how HIV-1 regulates the translation and packaging of its genome.

A Stretch of Unpaired Purines in the Leader Region of Simian Immunodeficiency Virus (SIV) Genomic RNA is Critical for its Packaging into Virions

Simian immunodeficiency virus (SIV) is an important lentivirus used as a non-human primate model to study HIV replication, and pathogenesis of human AIDS, as well as a potential vector for human gene therapy. This study investigated the role of single-stranded purines (ssPurines) as potential genomic RNA (gRNA) packaging determinants in SIV replication. Similar ssPurines have been implicated as important motifs for gRNA packaging in many retroviruses like, HIV-1, MPMV, and MMTV by serving as Gag binding sites during virion assembly. In examining the secondary structure of the SIV 5' leader region, as recently deduced using SHAPE methodology, we identified four specific stretches of ssPurines (I-IV) in the region that harbors major packaging determinants of SIV. The significance of these ssPurine motifs were investigated by mutational analysis coupled with a biologically relevant single round of replication assay. These analyses revealed that while ssPurine II was essential, the others (ssPurines I, III, & IV) did not significantly contribute to SIV gRNA packaging. Any mutation in the ssPurine II, such as its deletion or substitution, or other mutations that caused base pairing of ssPurine II loop resulted in near abrogation of RNA packaging, further substantiating the crucial role of ssPurine II and its looped conformation in SIV gRNA packaging. Structure prediction analysis of these mutants further corroborated the biological results and further revealed that the unpaired nature of ssPurine II is critical for its function during SIV RNA packaging perhaps by enabling it to function as a specific binding site for SIV Gag.

Stability and conformation of the dimeric HIV-1 genomic RNA 5'UTR

During HIV-1 assembly, the viral Gag polyprotein specifically selects the dimeric RNA genome for packaging into new virions. The 5' untranslated region (5'UTR) of the dimeric genome may adopt a conformation that is optimal for recognition by Gag. Further conformational rearrangement of the 5'UTR, promoted by the nucleocapsid (NC) domain of Gag, is predicted during virus maturation. Two 5'UTR dimer conformations, the kissing dimer (KD) and the extended dimer (ED), have been identified in vitro, which differ in the extent of intermolecular basepairing. Whether 5'UTRs from different HIV-1 strains with distinct sequences have access to the same dimer conformations has not been determined. Here, we applied fluorescence cross-correlation spectroscopy and single-molecule Förster resonance energy transfer imaging to demonstrate that 5'UTRs from two different HIV-1 subtypes form (KDs) with divergent stabilities. We further show that both 5'UTRs convert to a stable dimer in the presence of the viral NC protein, adopting a conformation consistent with extensive intermolecular contacts. These results support a unified model in which the genomes of diverse HIV-1 strains adopt an ED conformation.

Evaluating RNA Structural Flexibility: Viruses Lead the Way

Our understanding of RNA structure has lagged behind that of proteins and most other biological polymers, largely because of its ability to adopt multiple, and often very different, functional conformations within a single molecule. Flexibility and multifunctionality appear to be its hallmarks. Conventional biochemical and biophysical techniques all have limitations in solving RNA structure and to address this in recent years we have seen the emergence of a wide diversity of techniques applied to RNA structural analysis and an accompanying appreciation of its ubiquity and versatility. Viral RNA is a particularly productive area to study in that this economy of function within a single molecule admirably suits the minimalist lifestyle of viruses. Here, we review the major techniques that are being used to elucidate RNA conformational flexibility and exemplify how the structure and function are, as in all biology, tightly linked.

RNA Structures and Their Role in Selective Genome Packaging

To generate infectious viral particles, viruses must specifically select their genomic RNA from milieu that contains a complex mixture of cellular or non-genomic viral RNAs. In this review, we focus on the role of viral encoded RNA structures in genome packaging. We first discuss how packaging signals are constructed from local and long-range base pairings within viral genomes, as well as inter-molecular interactions between viral and host RNAs. Then, how genome packaging is regulated by the biophysical properties of RNA. Finally, we examine the impact of RNA packaging signals on viral evolution.

A Tale of Two Transitions: The Unfolding Mechanism of the prfA RNA Thermosensor

RNA thermosensors (RNATs), found in the 5' untranslated region (UTR) of some bacterial messenger RNAs (mRNAs), control the translation of the downstream gene in a temperature-dependent manner. In Listeria monocytogenes, the expression of a key transcription factor, PrfA, is mediated by an RNAT in its 5' UTR. PrfA functions as a master regulator of virulence in L. monocytogenes, controlling the expression of many virulence factors. The temperature-regulated expression of PrfA by its RNAT element serves as a signal of successful host invasion for the bacteria. Structurally, the prfA RNAT bears little resemblance to known families of RNATs, and prior studies demonstrated that the prfA RNAT is highly responsive over a narrow temperature range. Herein, we have undertaken a comprehensive mutational and thermodynamic analysis to ascertain the molecular determinants of temperature sensitivity. We provide evidence to support the idea that the prfA RNAT unfolding is different from that of cssA, a well-characterized RNAT, suggesting that these RNATs function via distinct mechanisms. Our data show that the unfolding of the prfA RNAT occurs in two distinct events and that the internal loops play an important role in mediating the cooperativity of RNAT unfolding. We further demonstrated that regions distal to the ribosome binding site (RBS) not only contribute to RNAT structural stability but also impact translation of the downstream message. Our collective results provide insight connecting the thermal stability of the prfA RNAT structure, unfolding energetics, and translational control.

Advances in understanding the initiation of HIV-1 reverse transcription

Many viruses, including Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) and Human Immunodeficiency Virus (HIV), use RNA as their genetic material. How viruses harness RNA structure and RNA-protein interactions to control their replication remains obscure. Recent advances in the characterization of HIV-1 reverse transcriptase, the enzyme that converts its single-stranded RNA genome into a double-stranded DNA copy, reveal how the reverse transcription complex evolves during initiation. Here we highlight these advances in HIV-1 structural biology and discuss how they are furthering our understanding of HIV and related ribonucleoprotein complexes implicated in viral disease.

Specific Guanosines in the HIV-2 Leader RNA are Essential for Efficient Viral Genome Packaging

HIV-2, a human pathogen that causes acquired immunodeficiency syndrome, is distinct from the more prevalent HIV-1 in several features including its evolutionary history and certain aspects of viral replication. Like other retroviruses, HIV-2 packages two copies of full-length viral RNA during virus assembly and efficient genome encapsidation is mediated by the viral protein Gag. We sought to define cis-acting elements in the HIV-2 genome that are important for the encapsidation of full-length RNA into viral particles. Based on previous studies of murine leukemia virus and HIV-1, we hypothesized that unpaired guanosines in the 5' untranslated region (UTR) play an important role in Gag:RNA interactions leading to genome packaging. To test our hypothesis, we targeted 18 guanosines located in 9 sites within the HIV-2 5' UTR and performed substitution analyses. We found that mutating as few as three guanosines significantly reduce RNA packaging efficiency. However, not all guanosines examined have the same effect; instead, a hierarchical order exists wherein a primary site, a secondary site, and three tertiary sites are identified. Additionally, there are functional overlaps in these sites and mutations of more than one site can act synergistically to cause genome packaging defects. These studies demonstrate the importance of specific guanosines in HIV-2 5'UTR in mediating genome packaging. Our results also demonstrate an interchangeable and hierarchical nature of guanosine-containing sites, which was not previously established, thereby revealing key insights into the replication mechanisms of HIV-2.

NMR Studies of Retroviral Genome Packaging

Nearly all retroviruses selectively package two copies of their unspliced RNA genomes from a cellular milieu that contains a substantial excess of non-viral and spliced viral RNAs. Over the past four decades, combinations of genetic experiments, phylogenetic analyses, nucleotide accessibility mapping, in silico RNA structure predictions, and biophysical experiments were employed to understand how retroviral genomes are selected for packaging. Genetic studies provided early clues regarding the protein and RNA elements required for packaging, and nucleotide accessibility mapping experiments provided insights into the secondary structures of functionally important elements in the genome. Three-dimensional structural determinants of packaging were primarily derived by nuclear magnetic resonance (NMR) spectroscopy. A key advantage of NMR, relative to other methods for determining biomolecular structure (such as X-ray crystallography), is that it is well suited for studies of conformationally dynamic and heterogeneous systems—a hallmark of the retrovirus packaging machinery. Here, we review advances in understanding of the structures, dynamics, and interactions of the proteins and RNA elements involved in retroviral genome selection and packaging that are facilitated by NMR.

Identification of the initial nucleocapsid recognition element in the HIV-1 RNA packaging signal

Understanding the molecular determinants of retroviral genome packaging is important for drug discovery and development of vectors for gene delivery. We show that the HIV-1 leader, which contains the RNA elements necessary for genome packaging, binds approximately two dozen copies of the cognate NC protein with affinities ranging from ∼40 nM to 1.4 µM. Binding to the four highest-affinity “initial” binding sites occurs with endothermic energetics attributed to NC-induced localized RNA melting. Mutations that stabilize these sites inhibit NC binding in vitro and RNA packaging in transfected cells. A small-molecule inhibitor of RNA packaging binds specifically to the initial NC binding sites and stabilizes the RNA structure. Our findings identify a potential RNA Achilles’ heel for HIV therapeutic development.

Selective packaging of the HIV-1 genome during virus assembly is mediated by interactions between the dimeric 5ʹ-leader of the unspliced viral RNA and the nucleocapsid (NC) domains of a small number of assembling viral Gag polyproteins. Here, we show that the dimeric 5′-leader contains more than two dozen NC binding sites with affinities ranging from 40 nM to 1.4 μM, and that all high-affinity sites (K_d ≲ 400 nM) reside within a ∼150-nt region of the leader sufficient to promote RNA packaging (core encapsidation signal, Ψ^CES). The four initial binding sites with highest affinity reside near two symmetrically equivalent three-way junction structures. Unlike the other high-affinity sites, which bind NC with exothermic energetics, binding to these sites occurs endothermically due to concomitant unwinding of a weakly base-paired [UUUU]:[GGAG] helical element. Mutations that stabilize base pairing within this element eliminate NC binding to this site and severely impair RNA packaging into virus-like particles. NMR studies reveal that a recently discovered small-molecule inhibitor of HIV-1 RNA packaging that appears to function by stabilizing the structure of the leader binds directly to the [UUUU]:[GGAG] helix. Our findings suggest a sequential NC binding mechanism for Gag-genome assembly and identify a potential RNA Achilles’ heel to which HIV therapeutics may be targeted.

Structural Insights into RNA Dimerization: Motifs, Interfaces and Functions

In comparison with the pervasive use of protein dimers and multimers in all domains of life, functional RNA oligomers have so far rarely been observed in nature. Their diminished occurrence contrasts starkly with the robust intrinsic potential of RNA to multimerize through long-range base-pairing ("kissing") interactions, self-annealing of palindromic or complementary sequences, and stable tertiary contact motifs, such as the GNRA tetraloop-receptors. To explore the general mechanics of RNA dimerization, we performed a meta-analysis of a collection of exemplary RNA homodimer structures consisting of viral genomic elements, ribozymes, riboswitches, etc., encompassing both functional and fortuitous dimers. Globally, we found that domain-swapped dimers and antiparallel, head-to-tail arrangements are predominant architectural themes. Locally, we observed that the same structural motifs, interfaces and forces that enable tertiary RNA folding also drive their higher-order assemblies. These feature prominently long-range kissing loops, pseudoknots, reciprocal base intercalations and A-minor interactions. We postulate that the scarcity of functional RNA multimers and limited diversity in multimerization motifs may reflect evolutionary constraints imposed by host antiviral immune surveillance and stress sensing. A deepening mechanistic understanding of RNA multimerization is expected to facilitate investigations into RNA and RNP assemblies, condensates, and granules and enable their potential therapeutical targeting.

HIV-1 spliced RNAs display transcription start site bias

Human immunodeficiency virus type 1 (HIV-1) transcripts have three fates: to serve as genomic RNAs, unspliced mRNAs, or spliced subgenomic mRNAs. Recent structural studies have shown that sequences near the 5' end of HIV-1 RNA can adopt at least two alternate three-dimensional conformations, and that these structures dictate genome versus unspliced mRNA fates. HIV-1's use of alternate transcription start sites (TSS) can influence which RNA conformer is generated, and this choice, in turn, dictates the fate of the unspliced RNA. The structural context of HIV-1's major 5' splice site differs in these two RNA conformers, suggesting that the conformers may differ in their ability to support HIV-1 splicing events. Here, we tested the hypothesis that TSS that shift the RNA monomer/dimer structural equilibrium away from the splice site sequestering dimer-competent fold would favor splicing. Consistent with this hypothesis, the results showed that the 5' ends of spliced HIV-1 RNAs were enriched in 3G^Cap structures and depleted of 1G^Cap RNAs relative to the total intracellular RNA population. These findings expand the functional significance of HIV-1 RNA structural dynamics by demonstrating roles for RNA structure in defining all three classes of HIV-1 RNAs, and suggest that HIV-1 TSS choice initiates a cascade of molecular events that dictate the fates of nascent HIV-1 RNAs.

The roles of structural dynamics in the cellular functions of RNAs

RNAs fold into 3D structures that range from simple helical elements to complex tertiary structures and quaternary ribonucleoprotein assemblies. The functions of many regulatory RNAs depend on how their 3D structure changes in response to a diverse array of cellular conditions. In this Review, we examine how the structural characterization of RNA as dynamic ensembles of conformations, which form with different probabilities and at different timescales, is improving our understanding of RNA function in cells. We discuss the mechanisms of gene regulation by microRNAs, riboswitches, ribozymes, post-transcriptional RNA modifications and RNA-binding proteins, and how the cellular environment and processes such as liquid-liquid phase separation may affect RNA folding and activity. The emerging RNA-ensemble-function paradigm is changing our perspective and understanding of RNA regulation, from in vitro to in vivo and from descriptive to predictive.

Benefits of stable isotope labeling in RNA analysis

RNAs are key players in life as they connect the genetic code (DNA) with all cellular processes dominated by proteins. They contain a variety of chemical modifications and many RNAs fold into complex structures. Here, we review recent progress in the analysis of RNA modification and structure on the basis of stable isotope labeling techniques. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy are the key tools and many breakthrough developments were made possible by the analysis of stable isotope labeled RNA. Therefore, we discuss current stable isotope labeling techniques such as metabolic labeling, enzymatic labeling and chemical synthesis. RNA structure analysis by NMR is challenging due to two major problems that become even more salient when the size of the RNA increases, namely chemical shift overlaps and line broadening leading to complete signal loss. Several isotope labeling strategies have been developed to provide solutions to these major issues, such as deuteration, segmental isotope labeling or site-specific labeling. Quantification of modified nucleosides in RNA by MS is only possible through the application of stable isotope labeled internal standards. With nucleic acid isotope labeling coupled mass spectrometry (NAIL-MS), it is now possible to analyze the dynamic processes of post-transcriptional RNA modification and demodification. The trend, in both NMR and MS RNA analytics, is without doubt shifting from the analysis of snapshot moments towards the development and application of tools capable of analyzing the dynamics of RNA structure and modification profiles.

Biochemical Reconstitution of HIV-1 Assembly and Maturation

The assembly of an orthoretrovirus such as HIV-1 requires the coordinated functioning of multiple biochemical activities of the viral Gag protein. These activities include membrane targeting, lattice formation, packaging of the RNA genome, and recruitment of cellular cofactors that modulate assembly. In most previous studies, these Gag activities have been investigated individually, which provided somewhat limited insight into how they functionally integrate during the assembly process. Here, we report the development of a biochemical reconstitution system that allowed us to investigate how Gag lattice formation, RNA binding, and the assembly cofactor inositol hexakisphosphate (IP6) synergize to generate immature virus particles in vitro The results identify an important rate-limiting step in assembly and reveal new insights into how RNA and IP6 promote immature Gag lattice formation. The immature virus-like particles can be converted into mature capsid-like particles by the simple addition of viral protease, suggesting that it is possible in principle to fully biochemically reconstitute the sequential processes of HIV-1 assembly and maturation from purified components.IMPORTANCE Assembly and maturation are essential steps in the replication of orthoretroviruses such as HIV-1 and are proven therapeutic targets. These processes require the coordinated functioning of the viral Gag protein's multiple biochemical activities. We describe here the development of an experimental system that allows an integrative analysis of how Gag's multiple functionalities cooperate to generate a retrovirus particle. Our current studies help to illuminate how Gag synergizes the formation of the virus compartment with RNA binding and how these activities are modulated by the small molecule IP6. Further development and use of this system should lead to a more comprehensive understanding of the molecular mechanisms of HIV-1 assembly and maturation and may provide new insights for the development of antiretroviral drugs.

Coronavirus genomic RNA packaging

RNA viruses carry out selective packaging of their genomes in a variety of ways, many involving a genomic packaging signal. The first coronavirus packaging signal was discovered nearly thirty years ago, but how it functions remains incompletely understood. This review addresses the current state of knowledge of coronavirus genome packaging, which has mainly been studied in two prototype species, mouse hepatitis virus and transmissible gastroenteritis virus. Despite the progress that has been made in the mapping and characterization of some packaging signals, there is conflicting evidence as to whether the viral nucleocapsid protein or the membrane protein plays the primary role in packaging signal recognition. The different models for the mechanism of genomic RNA packaging that have been prompted by these competing views are described. Also discussed is the recent exciting discovery that selective coronavirus genome packaging is critical for in vivo evasion of the host innate immune response.

•

Selective incorporation of the coronavirus genome into virions is mediated by a cis-acting RNA packaging signal.

•

Packaging signals vary across different coronavirus genera and lineages.

•

Different lines of evidence attribute packaging signal recognition to either the nucleocapsid or the membrane protein.

•

Selective coronavirus genome packaging plays a role in evasion of host innate immunity.

RNA Packaging in HIV

Successful replication of the AIDS retrovirus, HIV, requires that its genomic RNA be packaged in assembling virus particles with high fidelity. However, cellular mRNAs can also be packaged under some conditions. Viral RNA (vRNA) contains a 'packaging signal' (ψ) and is packaged as a dimer, with two vRNA monomers joined by a limited number of base pairs. It has two conformers, only one of which is capable of dimerization and packaging. Recent years have seen important progress on the 3D structure of dimeric ψ. Gag, the protein that assembles into the virus particle, interacts specifically with ψ, but this is obscured under physiological conditions by its high nonspecific affinity for any RNA. New results suggest that vRNA is selected for packaging because ψ nucleates assembly more efficiently than other RNAs.

Intrinsic conformational dynamics of the HIV-1 genomic RNA 5'UTR

Here we have directly visualized conformational changes in the 5′UTR of the HIV-1 genome using single-molecule fluorescence techniques. We find that the monomeric 5′UTR can spontaneously transition between two conformations, which have distinct intramolecular base pairing. One of the observed conformations is competent for dimerization with a second 5′UTR molecule. Our results are consistent with a model in which dimerization initiates by way of localized intermolecular kissing-loop base pairing, which is promoted by tRNA primer annealing. The intermolecular interface then extends, giving rise to the putative extended dimer, which is stabilized by HIV-1 NC. Thus, the 5′UTR is intrinsically dynamic, and both viral and host factors play a role in modulating the RNA conformation and dynamics.

The highly conserved 5′ untranslated region (5′UTR) of the HIV-1 RNA genome is central to the regulation of virus replication. NMR and biochemical experiments support a model in which the 5′UTR can transition between at least two conformational states. In one state the genome remains a monomer, as the palindromic dimerization initiation site (DIS) is sequestered via base pairing to upstream sequences. In the second state, the DIS is exposed, and the genome is competent for kissing loop dimerization and packaging into assembling virions where an extended dimer is formed. According to this model the conformation of the 5′UTR determines the fate of the genome. In this work, the dynamics of this proposed conformational switch and the factors that regulate it were probed using multiple single-molecule and in-gel ensemble FRET assays. Our results show that the HIV-1 5′UTR intrinsically samples conformations that are stabilized by both viral and host factor binding. Annealing of tRNA^Lys3, the primer for initiation of reverse transcription, can promote the kissing dimer but not the extended dimer. In contrast, HIV-1 nucleocapsid (NC) promotes formation of the extended dimer in both the absence and presence of tRNA^Lys3. Our data are consistent with an ordered series of events that involves primer annealing, genome dimerization, and virion assembly.

The search for a PKR code-differential regulation of protein kinase R activity by diverse RNA and protein regulators

The interferon-inducible protein kinase R (PKR) is a key component of host innate immunity that restricts viral replication and propagation. As one of the four eIF2α kinases that sense diverse stresses and direct the integrated stress response (ISR) crucial for cell survival and proliferation, PKR's versatile roles extend well beyond antiviral defense. Targeted by numerous host and viral regulators made of RNA and proteins, PKR is subject to multiple layers of endogenous control and external manipulation, driving its rapid evolution. These versatile regulators include not only the canonical double-stranded RNA (dsRNA) that activates the kinase activity of PKR, but also highly structured viral, host, and artificial RNAs that exert a full spectrum of effects. In this review, we discuss our deepening understanding of the allosteric mechanism that connects the regulatory and effector domains of PKR, with an emphasis on diverse structured RNA regulators in comparison to their protein counterparts. Through this analysis, we conclude that much of the mechanistic details that underlie this RNA-regulated kinase await structural and functional elucidation, upon which we can then describe a "PKR code," a set of structural and chemical features of RNA that are both descriptive and predictive for their effects on PKR.

Advances that facilitate the study of large RNA structure and dynamics by nuclear magnetic resonance spectroscopy

The characterization of functional yet nonprotein coding (nc) RNAs has expanded the role of RNA in the cell from a passive player in the central dogma of molecular biology to an active regulator of gene expression. The misregulation of ncRNA function has been linked with a variety of diseases and disorders ranging from cancers to neurodegeneration. However, a detailed molecular understanding of how ncRNAs function has been limited; due, in part, to the difficulties associated with obtaining high-resolution structures of large RNAs. Tertiary structure determination of RNA as a whole is hampered by various technical challenges, all of which are exacerbated as the size of the RNA increases. Namely, RNAs tend to be highly flexible and dynamic molecules, which are difficult to crystallize. Biomolecular nuclear magnetic resonance (NMR) spectroscopy offers a viable alternative to determining the structure of large RNA molecules that do not readily crystallize, but is itself hindered by some technical limitations. Recently, a series of advancements have allowed the biomolecular NMR field to overcome, at least in part, some of these limitations. These advances include improvements in sample preparation strategies as well as methodological improvements. Together, these innovations pave the way for the study of ever larger RNA molecules that have important biological function. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems.

Challenges and approaches to predicting RNA with multiple functional structures

The revolution in sequencing technology demands new tools to interpret the genetic code. As in vivo transcriptome-wide chemical probing techniques advance, new challenges emerge in the RNA folding problem. The emphasis on one sequence folding into a single minimum free energy structure is fading as a new focus develops on generating RNA structural ensembles and identifying functional structural features in ensembles. This review describes an efficient combinatorially complete method and three free energy minimization approaches to predicting RNA structures with more than one functional fold, as well as two methods for analysis of a thermodynamics-based Boltzmann ensemble of structures. The review then highlights two examples of viral RNA 3'-UTR regions that fold into more than one conformation and have been characterized by single molecule fluorescence energy resonance transfer or NMR spectroscopy. These examples highlight the different approaches and challenges in predicting structure and function from sequence for RNA with multiple biological roles and folds. More well-defined examples and new metrics for measuring differences in RNA structures will guide future improvements in prediction of RNA structure and function from sequence.

Thermodynamic investigation of kissing-loop interactions

Kissing loop interactions (KLIs) are a common motif that is critical in retroviral dimerization, viroid replication, mRNA, and riboswitches. In addition, KLIs are currently used in a variety of biotechnology applications, such as in aptamer sensors, RNA scaffolds and to stabilize vaccines for therapeutics. Here we describe the thermodynamics of a basic intramolecular DNA capable of engaging in a KLI, consisting of two hairpins connected by a flexible linker. Each hairpin loop has a five-nucleotide complementary sequence theoretically capable of engaging in a KLI. On either side of each loop is two thymines which will not engage in kissing but are present to provide more flexibility and optimal KLI positioning. Our results suggest that the KLI occurs even at physiological salt levels, and that the KLI does not alter the thermodynamics and stability of the two stem structures. The KLI does not involve all five nucleotides, or at least each base-pair stack is not making full contact. Adding a second strand complementary to the bottom of the kissing complex removes flexibility and causes destabilization of the stems. The KLI of this less flexible complex is maintained but the T_M is reduced, indicating an entopic penalty to its formation.

RNA Structure-A Neglected Puppet Master for the Evolution of Virus and Host Immunity

The central dogma of molecular biology describes the flow of genetic information from DNA to protein via an RNA intermediate. For many years, RNA has been considered simply as a messenger relaying information between DNA and proteins. Recent advances in next generation sequencing technology, bioinformatics, and non-coding RNA biology have highlighted the many important roles of RNA in virtually every biological process. Our understanding of RNA biology has been further enriched by a number of significant advances in probing RNA structures. It is now appreciated that many cellular and viral biological processes are highly dependent on specific RNA structures and/or sequences, and such reliance will undoubtedly impact on the evolution of both hosts and viruses. As a contribution to this special issue on host immunity and virus evolution, it is timely to consider how RNA sequences and structures could directly influence the co-evolution between hosts and viruses. In this manuscript, we begin by stating some of the basic principles of RNA structures, followed by describing some of the critical RNA structures in both viruses and hosts. More importantly, we highlight a number of available new tools to predict and to evaluate novel RNA structures, pointing out some of the limitations readers should be aware of in their own analyses.

Multiple, Switchable Protein:RNA Interactions Regulate Human Immunodeficiency Virus Type 1 Assembly

Human immunodeficiency virus type 1 (HIV-1) particle assembly requires several protein:RNA interactions that vary widely in their character, from specific recognition of highly conserved and structured viral RNA elements to less specific interactions with variable RNA sequences. Genetic, biochemical, biophysical, and structural studies have illuminated how virion morphogenesis is accompanied by dramatic changes in the interactions among the protein and RNA virion components. The 5' leader RNA element drives RNA recognition by Gag upon initiation of HIV-1 assembly and can assume variable conformations that influence translation, dimerization, and Gag recognition. As Gag multimerizes on the plasma membrane, forming immature particles, its RNA binding specificity transiently changes, enabling recognition of the A-rich composition of the viral genome. Initiation of assembly may also be regulated by occlusion of the membrane binding surface of Gag by tRNA. Finally, recent work has suggested that RNA interactions with viral enzymes may activate and ensure the accuracy of virion maturation.

Influence of gag and RRE Sequences on HIV-1 RNA Packaging Signal Structure and Function

The packaging signal (Ψ) and Rev-responsive element (RRE) enable unspliced HIV-1 RNAs' export from the nucleus and packaging into virions. For some retroviruses, engrafting Ψ onto a heterologous RNA is sufficient to direct encapsidation. In contrast, HIV-1 RNA packaging requires 5' leader Ψ elements plus poorly defined additional features. We previously defined minimal 5' leader sequences competitive with intact Ψ for HIV-1 packaging, and here examined the potential roles of additional downstream elements. The findings confirmed that together, HIV-1 5' leader Ψ sequences plus a nuclear export element are sufficient to specify packaging. However, RNAs trafficked using a heterologous export element did not compete well with RNAs using HIV-1's RRE. Furthermore, some RNA additions to well-packaged minimal vectors rendered them packaging-defective. These defects were rescued by extending gag sequences in their native context. To understand these packaging defects' causes, in vitro dimerization properties of RNAs containing minimal packaging elements were compared to RNAs with sequence extensions that were or were not compatible with packaging. In vitro dimerization was found to correlate with packaging phenotypes, suggesting that HIV-1 evolved to prevent 5' leader residues' base pairing with downstream residues and misfolding of the packaging signal. Our findings explain why gag sequences have been implicated in packaging and show that RRE's packaging contributions appear more specific than nuclear export alone. Paired with recent work showing that sequences upstream of Ψ can dictate RNA folds, the current work explains how genetic context of minimal packaging elements contributes to HIV-1 RNA fate determination.

HIV-1 Matrix Protein Interactions with tRNA: Implications for Membrane Targeting

The N-terminally myristoylated matrix (MA) domain of the HIV-1 Gag polyprotein promotes virus assembly by targeting Gag to the inner leaflet of the plasma membrane. Recent studies indicate that, prior to membrane binding, MA associates with cytoplasmic tRNAs (including tRNA^Lys3), and in vitro studies of tRNA-dependent MA interactions with model membranes have led to proposals that competitive tRNA interactions contribute to membrane discrimination. We have characterized interactions between native, mutant, and unmyristylated (myr-) MA proteins and recombinant tRNA^Lys3 by NMR spectroscopy and isothermal titration calorimetry. NMR experiments confirm that tRNA^Lys3 interacts with a patch of basic residues that are also important for binding to the plasma membrane marker, phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂]. Unexpectedly, the affinity of MA for tRNA^Lys3 (K_d = 0.63 ± 0.03 μM) is approximately 1 order of magnitude greater than its affinity for PI(4,5)P₂-enriched liposomes (K_d(apparent) = 10.2 ± 2.1 μM), and NMR studies indicate that tRNA^Lys3 binding blocks MA association with liposomes, including those enriched with PI(4,5)P₂, phosphatidylserine, and cholesterol. However, the affinity of MA for tRNA^Lys3 is diminished by mutations or sample conditions that promote myristate exposure. Since Gag-Gag interactions are known to promote myristate exposure, our findings support virus assembly models in which membrane targeting and genome binding are mechanistically coupled.

CLIP-related methodologies and their application to retrovirology

Virtually every step of HIV-1 replication and numerous cellular antiviral defense mechanisms are regulated by the binding of a viral or cellular RNA-binding protein (RBP) to distinct sequence or structural elements on HIV-1 RNAs. Until recently, these protein-RNA interactions were studied largely by in vitro binding assays complemented with genetics approaches. However, these methods are highly limited in the identification of the relevant targets of RBPs in physiologically relevant settings. Development of crosslinking-immunoprecipitation sequencing (CLIP) methodology has revolutionized the analysis of protein-nucleic acid complexes. CLIP combines immunoprecipitation of covalently crosslinked protein-RNA complexes with high-throughput sequencing, providing a global account of RNA sequences bound by a RBP of interest in cells (or virions) at near-nucleotide resolution. Numerous variants of the CLIP protocol have recently been developed, some with major improvements over the original. Herein, we briefly review these methodologies and give examples of how CLIP has been successfully applied to retrovirology research.

A guide to large-scale RNA sample preparation

RNA is becoming more important as an increasing number of functions, both regulatory and enzymatic, are being discovered on a daily basis. As the RNA boom has just begun, most techniques are still in development and changes occur frequently. To understand RNA functions, revealing the structure of RNA is of utmost importance, which requires sample preparation. We review the latest methods to produce and purify a variation of RNA molecules for different purposes with the main focus on structural biology and biophysics. We present a guide aimed at identifying the most suitable method for your RNA and your biological question and highlighting the advantages of different methods. Graphical abstract In this review we present different methods for large-scale production and purification of RNAs for structural and biophysical studies.

Retroviral RNA Dimerization: From Structure to Functions

The genome of the retroviruses is a dimer composed by two homologous copies of genomic RNA (gRNA) molecules of positive polarity. The dimerization process allows two gRNA molecules to be non-covalently linked together through intermolecular base-pairing. This step is critical for the viral life cycle and is highly conserved among retroviruses with the exception of spumaretroviruses. Furthermore, packaging of two gRNA copies into viral particles presents an important evolutionary advantage for immune system evasion and drug resistance. Recent studies reported RNA switches models regulating not only gRNA dimerization, but also translation and packaging, and a spatio-temporal characterization of viral gRNA dimerization within cells are now at hand. This review summarizes our current understanding on the structural features of the dimerization signals for a variety of retroviruses (HIVs, MLV, RSV, BLV, MMTV, MPMV…), the mechanisms of RNA dimer formation and functional implications in the retroviral cycle.

An RNA-binding compound that stabilizes the HIV-1 gRNA packaging signal structure and specifically blocks HIV-1 RNA encapsidation

Background

NSC260594, a quinolinium derivative from the NCI diversity set II compound library, was previously identified in a target-based assay as an inhibitor of the interaction between the HIV-1 (ψ) stem-loop 3 (SL3) RNA and Gag. This compound was shown to exhibit potent antiviral activity. Here, the effects of this compound on individual stages of the viral lifecycle were examined by qRT-PCR, ELISA and Western blot, to see if its actions were specific to the viral packaging stage. The structural effects of NSC260594 binding to the HIV-1 gRNA were also examined by SHAPE and dimerization assays.

Results

Treatment of cells with NSC260594 did not reduce the number of integration events of incoming virus, and treatment of virus producing cells did not affect the level of intracellular Gag protein or viral particle release as determined by immunoblot. However, NSC260594 reduced the incorporation of gRNA into virions by up to 82%, without affecting levels of gRNA inside the cell. This reduction in packaging correlated closely with the reduction in infectivity of the released viral particles. To establish the structural effects of NSC260594 on the HIV-1 gRNA, we performed SHAPE analyses to pinpoint RNA structural changes. NSC260594 had a stabilizing effect on the wild type RNA that was not confined to SL3, but that was propagated across the structure. A packaging mutant lacking SL3 did not show this effect.

Conclusions

NSC260594 acts as a specific inhibitor of HIV-1 RNA packaging. No other viral functions are affected. Its action involves preventing the interaction of Gag with SL3 by stabilizing this small RNA stem-loop which then leads to stabilization of the global packaging signal region (psi or ψ). This confirms data, previously only shown in analyses of isolated SL3 oligonucleotides, that SL3 is structurally labile in the presence of Gag and that this is critical for the complete psi region to be able to adopt different conformations. Since replication is otherwise unaffected by NSC260594 the flexibility of SL3 appears to be a unique requirement for genome encapsidation and identifies this process as a highly specific drug target. This study is proof of principle that development of a new class of antiretroviral drugs that specifically target viral packaging by binding to the viral genomic RNA is achievable.

Electronic supplementary material

The online version of this article (10.1186/s12977-018-0407-4) contains supplementary material, which is available to authorized users.

Structure of the 30 kDa HIV-1 RNA Dimerization Signal by a Hybrid Cryo-EM, NMR, and Molecular Dynamics Approach

Cryoelectron microscopy (cryo-EM) and nuclear magnetic resonance (NMR) spectroscopy are routinely used to determine structures of macromolecules with molecular weights over 65 and under 25 kDa, respectively. We combined these techniques to study a 30 kDa HIV-1 dimer initiation site RNA ([DIS]2; 47 nt/strand). A 9 Å cryo-EM map clearly shows major groove features of the double helix and a right-handed superhelical twist. Simulated cryo-EM maps generated from time-averaged molecular dynamics trajectories (10 ns) exhibited levels of detail similar to those in the experimental maps, suggesting internal structural flexibility limits the cryo-EM resolution. Simultaneous inclusion of the cryo-EM map and 2H-edited NMR-derived distance restraints during structure refinement generates a structure consistent with both datasets and supporting a flipped-out base within a conserved purine-rich bulge. Our findings demonstrate the power of combining global and local structural information from these techniques for structure determination of modest-sized RNAs.

New windows into retroviral RNA structures

Background

The multiple roles of both viral and cellular RNAs have become increasingly apparent in recent years, and techniques to model them have become significantly more powerful, enabling faster and more accurate visualization of RNA structures.

Main body

Techniques such as SHAPE (selective 2’OH acylation analysed by primer extension) have revolutionized the field, and have been used to examine RNAs belonging to many and diverse retroviruses. Secondary structure probing reagents such as these have been aided by the development of faster methods of analysis either via capillary or next-generation sequencing, allowing the analysis of entire genomes, and of retroviral RNA structures within virions. Techniques to model the three-dimensional structures of these large RNAs have also recently developed.

Conclusions

The flexibility of retroviral RNAs, both structural and functional, is clear from the results of these new experimental techniques. Retroviral RNA structures and structural changes control many stages of the lifecycle, and both the RNA structures themselves and their interactions with ligands are potential new drug targets. In addition, our growing understanding of retroviral RNA structures is aiding our knowledge of cellular RNA form and function.

Applications of NMR to structure determination of RNAs large and small

Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool to investigate the structure and dynamics of RNA, because many biologically important RNAs have conformationally flexible structures, which makes them difficult to crystallize. Functional, independently folded RNA domains, range in size between simple stem-loops of as few as 10-20 nucleotides, to 50-70 nucleotides, the size of tRNA and many small ribozymes, to a few hundred nucleotides, the size of more complex RNA enzymes and of the functional domains of non-coding transcripts. In this review, we discuss new methods for sample preparation, assignment strategies and structure determination for independently folded RNA domains of up to 100 kDa in molecular weight.

NMR Studies of the Structure and Function of the HIV-1 5'-Leader

The 5′-leader of the human immunodeficiency virus type 1 (HIV-1) genome plays several critical roles during viral replication, including differentially establishing mRNA versus genomic RNA (gRNA) fates. As observed for proteins, the function of the RNA is tightly regulated by its structure, and a common paradigm has been that genome function is temporally modulated by structural changes in the 5′-leader. Over the past 30 years, combinations of nucleotide reactivity mapping experiments with biochemistry, mutagenesis, and phylogenetic studies have provided clues regarding the secondary structures of stretches of residues within the leader that adopt functionally discrete domains. More recently, nuclear magnetic resonance (NMR) spectroscopy approaches have been developed that enable direct detection of intra- and inter-molecular interactions within the intact leader, providing detailed insights into the structural determinants and mechanisms that regulate HIV-1 genome packaging and function.

Transcriptional start site heterogeneity modulates the structure and function of the HIV-1 genome

The promoter in HIV type 1 (HIV-1) proviral DNA contains three sequential guanosines at the U3-R boundary that have been proposed to function as sites for transcription initiation. Here we show that all three sites are used in cells infected with HIV-1 and that viral RNAs containing a single 5' capped guanosine (^Cap1G) are specifically selected for packaging in virions, consistent with a recent report [Masuda et al. (2015) Sci Rep 5:17680]. In addition, we now show that transcripts that begin with two or three capped guanosines (^Cap2G or ^Cap3G) are enriched on polysomes, indicating that RNAs synthesized from different transcription start sites have different functions in viral replication. Because genomes are selected for packaging as dimers, we examined the in vitro monomer-dimer equilibrium properties of ^Cap1G, ^Cap2G, and ^Cap3G 5'-leader RNAs in the NL4-3 strain of HIV-1. Strikingly, under physiological-like ionic conditions in which the ^Cap1G 5'-leader RNA adopts a dimeric structure, the ^Cap2G and ^Cap3G 5'-leader RNAs exist predominantly as monomers. Mutagenesis studies designed to probe for base-pairing interactions suggest that the additional guanosines of the 2G and 3G RNAs remodel the base of the PolyA hairpin, resulting in enhanced sequestration of dimer-promoting residues and stabilization of the monomer. Our studies suggest a mechanism through which the structure, function, and fate of the viral genome can be modulated by the transcriptionally controlled presence or absence of a single 5' guanosine.