The varicella-zoster virus ORFS/L (ORF0) gene is required for efficient viral replication and contains an element involved in DNA cleavage

Creating the "Dew Drop on a Rose Petal": the Molecular Pathogenesis of Varicella-Zoster Virus Skin Lesions

Varicella-zoster virus (VZV) is a human alphaherpesvirus that causes varicella (chicken pox) as the primary infection in a susceptible host. Varicella is very contagious through its transmission by direct contact with vesicular skin lesions that contain high titers of infectious virus and respiratory droplets. While the clinical manifestations of primary VZV infection are well recognized, defining the molecular mechanisms that drive VZV pathogenesis in the naive host before adaptive antiviral immunity is induced has been a challenge due to species specificity. This review focuses on advances made in identifying the differentiated human host cells targeted by VZV to cause varicella, the processes involved in viral takeover of these heterogenous cell types, and the host cell countermeasures that typically culminate in a benign illness. This work has revealed many unexpected and multifaceted mechanisms used by VZV to achieve its high prevalence and persistence in the human population.

Comparative Analysis of the Simian Varicella Virus and Varicella Zoster Virus Genomes

Varicella zoster virus (VZV) and simian varicella virus (SVV) cause varicella (chickenpox) in children and nonhuman primates, respectively. After resolution of acute disease, the viruses establish latent infection in neural ganglia, after which they may reactivate to cause a secondary disease, such as herpes zoster. SVV infection of nonhuman primates provides a model to investigate VZV pathogenesis and antiviral strategies. The VZV and SVV genomes are similar in size and structure and share 70–75% DNA homology. SVV and VZV DNAs are co-linear in gene arrangement with the exception of the left end of the viral genomes. Viral gene expression is regulated into immediate early, early, and late transcription during in vitro and in vivo infection. During viral latency, VZV and SVV gene expression is limited to transcription of a viral latency-associated transcript (VLT). VZV and SVV are closely related alphaherpesviruses that likely arose from an ancestral varicella virus that evolved through cospeciation into species-specific viruses.

The architecture of the simian varicella virus transcriptome

Primary infection with varicella-zoster virus (VZV) causes varicella and the establishment of lifelong latency in sensory ganglion neurons. In one-third of infected individuals VZV reactivates from latency to cause herpes zoster, often complicated by difficult-to-treat chronic pain. Experimental infection of non-human primates with simian varicella virus (SVV) recapitulates most features of human VZV disease, thereby providing the opportunity to study the pathogenesis of varicella and herpes zoster in vivo. However, compared to VZV, the transcriptome and the full coding potential of SVV remains incompletely understood. Here, we performed nanopore direct RNA sequencing to annotate the SVV transcriptome in lytically SVV-infected African green monkey (AGM) and rhesus macaque (RM) kidney epithelial cells. We refined structures of canonical SVV transcripts and uncovered numerous RNA isoforms, splicing events, fusion transcripts and non-coding RNAs, mostly unique to SVV. We verified the expression of canonical and newly identified SVV transcripts in vivo, using lung samples from acutely SVV-infected cynomolgus macaques. Expression of selected transcript isoforms, including those located in the unique left-end of the SVV genome, was confirmed by reverse transcription PCR. Finally, we performed detailed characterization of the SVV homologue of the VZV latency-associated transcript (VLT), located antisense to ORF61. Analogous to VZV VLT, SVV VLT is multiply spliced and numerous isoforms are generated using alternative transcription start sites and extensive splicing. Conversely, low level expression of a single spliced SVV VLT isoform defines in vivo latency. Notably, the genomic location of VLT core exons is highly conserved between SVV and VZV. This work thus highlights the complexity of lytic SVV gene expression and provides new insights into the molecular biology underlying lytic and latent SVV infection. The identification of the SVV VLT homolog further underlines the value of the SVV non-human primate model to develop new strategies for prevention of herpes zoster.

Varicella-zoster virus (VZV)–a ubiquitous human pathogen–infects most individuals during childhood, leading to chickenpox, after which the virus persists in the host for decades. Later in life, VZV reactivates to cause shingles, frequently associated with difficult-to-treat chronic pain. Our limited understanding of the viral life-cycle hampers the development of more effective treatment options. Simian varicella virus (SVV) is the non-human primate homologue of VZV and causes a natural disease in Old World monkeys with clinical, pathological, and immunological features resembling human VZV infection. However, it is unclear how similar both viruses are at the molecular level. Here, we have revisited the genome-wide transcriptional activity of SVV during lytic infection of kidney epithelial cells derived from two non-human primate species and validated expression of newly identified viral transcripts in lung tissue from SVV-infected animals. Together, this has led to the identification of numerous alternative RNA isoforms, mostly unique to SVV, and some of which may have functional implications for the virus. Notably, we defined the SVV latency-associated transcript, which is highly similar to its VZV counterpart. In conclusion, our study shows the value of understanding the molecular biology of a given animal model and identifies potentially conserved mechanism of latency.

The Structures and Functions of VZV Glycoproteins

The virions of all enveloped viruses, including those of the Herpesviridae, must bind to the cell surface then undergo a process of membrane fusion between the cell plasma membrane and the virus particle envelope. As for all herpesviruses, glycoproteins in the virion envelope are the modus operandi of these events.

Varicella-Zoster Virus-Genetics, Molecular Evolution and Recombination

This chapter first details the structure, organization and coding content of the VZV genome to provide a foundation on which the molecular evolution of the virus can be projected. We subsequently describe the evolution of molecular profiling approaches from restriction fragment length polymorphisms to single nucleotide polymorphism profiling to modern day high-throughput sequencing approaches. We describe how the application of these methodologies led to our current model of VZV phylogeograpy including the number and structure of geographic clades and the role of recombination in reshaping these.

Comparison of the Whole-Genome Sequence of an Oka Varicella Vaccine from China with Other Oka Vaccine Strains Reveals Sites Putatively Critical for Vaccine Efficacy

Varicella-zoster virus (VZV) infection results in varicella mostly in children. Reactivation of the virus causes herpes zoster (HZ), mostly in adults. A live attenuated vaccine (vOka-Biken) was originally derived from the parental strain pOka. Several live attenuated vaccines based on the Oka strain are currently available worldwide. In China, varicella vaccines have been licensed by four manufacturers. In this study, we analyze the whole-genome sequence (WGS) of vOka-BK produced by Changchun BCHT Biotechnology also known as Baike. vOka-BK WGS was compared against the genomic sequences of four other Oka strains: pOka, vOka-Biken, vOka-Varilrix from GlaxoSmithKline, and vOka-Varivax from Merck & Co. A previous study identified 137 single nucleotide polymorphisms (SNPs) shared by all vOkas. The current analysis used these data as a reference to compare with vOka-BK WGS and focused on 54 SNPs located in the unique regions of the genome. Twenty-eight nonsynonymous substitutions were identified, ORF62 and ORF55 featuring the most amino acid changes with 9 and 3, respectively. Among the 54 SNPs, 10 had a different mutation profile in vOka-BK compared to the other three vaccines. A comparison with the clade 3 strain Ellen, known to be attenuated, identified three shared amino acid changes: *130R in ORF0 and R958G and S628G in ORF62. This analysis provides the first comparison of a Chinese varicella vaccine to the other vaccines available worldwide and identifies sites potentially critical for VZV vaccine efficacy.IMPORTANCE Varicella, also known as chickenpox, is a highly contagious disease, caused by varicella-zoster virus (VZV). Varicella is a common childhood disease that can be prevented by a live attenuated vaccine. The first available vaccine was derived from the parental Oka strain in Japan in 1974. Several live attenuated vaccines based on the Oka strain are currently available worldwide. Among the four vaccines produced in China, the vaccine manufactured by Changchun BCHT Biotechnology, also known as Baike, has been reported to be very efficacious. Comparative genomic analysis of the Baike vaccine with other Oka vaccine strains identified sites that might be involved in vaccine efficacy, as well as important for the biology of the virus.

Molecular Aspects of Varicella-Zoster Virus Latency

Primary varicella-zoster virus (VZV) infection causes varicella (chickenpox) and the establishment of a lifelong latent infection in ganglionic neurons. VZV reactivates in about one-third of infected individuals to cause herpes zoster, often accompanied by neurological complications. The restricted host range of VZV and, until recently, a lack of suitable in vitro models have seriously hampered molecular studies of VZV latency. Nevertheless, recent technological advances facilitated a series of exciting studies that resulted in the discovery of a VZV latency-associated transcript (VLT) and provide novel insights into our understanding of VZV latency and factors that may initiate reactivation. Deducing the function(s) of VLT and the molecular mechanisms involved should now be considered a priority to improve our understanding of factors that govern VZV latency and reactivation. In this review, we summarize the implications of recent discoveries in the VZV latency field from both a virus and host perspective and provide a roadmap for future studies.

Vaccine-type mutations identified in Varicella zoster virus passaged in cell culture

Varicella-zoster virus (VZV) is a causative agent for chickenpox and shingles. Comparative genomic sequence analysis of clinical and vaccine strains suggested potential sites responsible for attenuation. In this study, low and high passages of two VZV clinical strains cultured in human fibroblast cells were compared for genomic DNA sequences and growth characteristics. Mutations were detected at 187 and 162 sites in the strain YC01 and YC02, respectively. More than 86% of mutations were found in open reading frames, and ORF62 exhibited highest frequency of mutations. T to C and A to G transitions accounted for more 90% of all possible substitutions. Forty mutations were common to two strains, including 27 in ORF62. Mutations found in attenuated vaccine strains were also detected at 7 positions. Both high and low passage strains were infectious and grew similarly in human fibroblast cells. In guinea pig cells, however, high passage strain remained infectious while low passage strain lost infectivity. This study may provide new insight into the attenuating mutations associated with in vitro passaging of VZV.

Targeted Genome Sequencing Reveals Varicella-Zoster Virus Open Reading Frame 12 Deletion

The neurotropic herpesvirus varicella-zoster virus (VZV) establishes a lifelong latent infection in humans following primary infection. The low abundance of VZV nucleic acids in human neurons has hindered an understanding of the mechanisms that regulate viral gene transcription during latency. To overcome this critical barrier, we optimized a targeted capture protocol to enrich VZV DNA and cDNA prior to whole-genome/transcriptome sequence analysis. Since the VZV genome is remarkably stable, it was surprising to detect that VZV32, a VZV laboratory strain with no discernible growth defect in tissue culture, contained a 2,158-bp deletion in open reading frame (ORF) 12. Consequently, ORF 12 and 13 protein expression was abolished and Akt phosphorylation was inhibited. The discovery of the ORF 12 deletion, revealed through targeted genome sequencing analysis, points to the need to authenticate the VZV genome when the virus is propagated in tissue culture.IMPORTANCE Viruses isolated from clinical samples often undergo genetic modifications when cultured in the laboratory. Historically, VZV is among the most genetically stable herpesviruses, a notion supported by more than 60 complete genome sequences from multiple isolates and following multiple in vitro passages. However, application of enrichment protocols to targeted genome sequencing revealed the unexpected deletion of a significant portion of VZV ORF 12 following propagation in cultured human fibroblast cells. While the enrichment protocol did not introduce bias in either the virus genome or transcriptome, the findings indicate the need for authentication of VZV by sequencing when the virus is propagated in tissue culture.

Deep Sequencing of Distinct Preparations of the Live Attenuated Varicella-Zoster Virus Vaccine Reveals a Conserved Core of Attenuating Single-Nucleotide Polymorphisms

The continued success of the live attenuated varicella-zoster virus vaccine in preventing varicella-zoster and herpes zoster is well documented, as are many of the mutations that contribute to the attenuation of the vOka virus for replication in skin. At least three different preparations of vOka are marketed. Here, we show using deep sequencing of seven batches of vOka vaccine (including ZostaVax, VariVax, VarilRix, and the Oka/Biken working seed) from three different manufacturers (VariVax, GSK, and Biken) that 137 single-nucleotide polymorphism (SNP) mutations are present in all vaccine batches. This includes six sites at which the vaccine allele is fixed or near fixation, which we speculate are likely to be important for attenuation. We also show that despite differences in the vaccine populations between preparations, batch-to-batch variation is minimal, as is the number and frequency of mutations unique to individual batches. This suggests that the vaccine manufacturing processes are not introducing new mutations and that, notwithstanding the mixture of variants present, VZV live vaccines are extremely stable.

IMPORTANCE

The continued success of vaccinations to prevent chickenpox and shingles, combined with the extremely low incidence of adverse reactions, indicates the quality of these vaccines. The vaccine itself is comprised of a heterogeneous live attenuated virus population and thus requires deep-sequencing technologies to explore the differences and similarities in the virus populations between different preparations and batches of the vaccines. Our data demonstrate minimal variation between batches, an important safety feature, and provide new insights into the extent of the mutations present in this attenuated virus.

A comparison of herpes simplex virus type 1 and varicella-zoster virus latency and reactivation

Herpes simplex virus type 1 (HSV-1; human herpesvirus 1) and varicella-zoster virus (VZV; human herpesvirus 3) are human neurotropic alphaherpesviruses that cause lifelong infections in ganglia. Following primary infection and establishment of latency, HSV-1 reactivation typically results in herpes labialis (cold sores), but can occur frequently elsewhere on the body at the site of primary infection (e.g. whitlow), particularly at the genitals. Rarely, HSV-1 reactivation can cause encephalitis; however, a third of the cases of HSV-1 encephalitis are associated with HSV-1 primary infection. Primary VZV infection causes varicella (chickenpox) following which latent virus may reactivate decades later to produce herpes zoster (shingles), as well as an increasingly recognized number of subacute, acute and chronic neurological conditions. Following primary infection, both viruses establish a latent infection in neuronal cells in human peripheral ganglia. However, the detailed mechanisms of viral latency and reactivation have yet to be unravelled. In both cases latent viral DNA exists in an 'end-less' state where the ends of the virus genome are joined to form structures consistent with unit length episomes and concatemers, from which viral gene transcription is restricted. In latently infected ganglia, the most abundantly detected HSV-1 RNAs are the spliced products originating from the primary latency associated transcript (LAT). This primary LAT is an 8.3 kb unstable transcript from which two stable (1.5 and 2.0 kb) introns are spliced. Transcripts mapping to 12 VZV genes have been detected in human ganglia removed at autopsy; however, it is difficult to ascribe these as transcripts present during latent infection as early-stage virus reactivation may have transpired in the post-mortem time period in the ganglia. Nonetheless, low-level transcription of VZV ORF63 has been repeatedly detected in multiple ganglia removed as close to death as possible. There is increasing evidence that HSV-1 and VZV latency is epigenetically regulated. In vitro models that permit pathway analysis and identification of both epigenetic modulations and global transcriptional mechanisms of HSV-1 and VZV latency hold much promise for our future understanding in this complex area. This review summarizes the molecular biology of HSV-1 and VZV latency and reactivation, and also presents future directions for study.

Three-dimensional normal human neural progenitor tissue-like assemblies: a model of persistent varicella-zoster virus infection

Varicella-zoster virus (VZV) is a neurotropic human alphaherpesvirus that causes varicella upon primary infection, establishes latency in multiple ganglionic neurons, and can reactivate to cause zoster. Live attenuated VZV vaccines are available; however, they can also establish latent infections and reactivate. Studies of VZV latency have been limited to the analyses of human ganglia removed at autopsy, as the virus is strictly a human pathogen. Recently, terminally differentiated human neurons have received much attention as a means to study the interaction between VZV and human neurons; however, the short life-span of these cells in culture has limited their application. Herein, we describe the construction of a model of normal human neural progenitor cells (NHNP) in tissue-like assemblies (TLAs), which can be successfully maintained for at least 180 days in three-dimensional (3D) culture, and exhibit an expression profile similar to that of human trigeminal ganglia. Infection of NHNP TLAs with cell-free VZV resulted in a persistent infection that was maintained for three months, during which the virus genome remained stable. Immediate-early, early and late VZV genes were transcribed, and low-levels of infectious VZV were recurrently detected in the culture supernatant. Our data suggest that NHNP TLAs are an effective system to investigate long-term interactions of VZV with complex assemblies of human neuronal cells.

Varicella-zoster virus (VZV), the alphaherpesvirus that typically causes childhood chickenpox and shingles in adults, becomes latent in neurons, thus remaining in the body for a lifetime. Unfortunately, few models are available to study the establishment of VZV latency since the virus infects only humans and establishes persistent infections and latency only in neurons, a slowly proliferating, short-lived cell in culture. We have successfully maintained normal human neural progenitor cells (NHNP) in tissue-like assemblies (TLAs) in 3-dimensional (3D) cultures for up to 6 months. The 3D NHNP TLAs show some characteristics as those found in the human trigeminal ganglia, the site of VZV latency. NHNP TLAs infected with VZV remain viable for 3 months during which time VZV DNA replicates and remains genetically stable, virus genes are transcribed, and infectious VZV is sporadically released. The ability to maintain VZV infected NHNP cells in culture for extended times provides the unique opportunity to study the molecular interactions between this important human pathogen and neuronal tissue to an extent previously unattainable.

Comprehensive analysis of varicella-zoster virus proteins using a new monoclonal antibody collection

Varicella-zoster virus (VZV) is the etiological agent of chickenpox and shingles. Due to the virus's restricted host and cell type tropism and the lack of tools for VZV proteomics, it is one of the least-characterized human herpesviruses. We generated 251 monoclonal antibodies (MAbs) against 59 of the 71 (83%) currently known unique VZV proteins to characterize VZV protein expression in vitro and in situ. Using this new set of MAbs, 44 viral proteins were detected by Western blotting (WB) and indirect immunofluorescence (IF); 13 were detected by WB only, and 2 were detected by IF only. A large proportion of viral proteins was analyzed for the first time in the context of virus infection. Our study revealed the subcellular localization of 46 proteins, 14 of which were analyzed in detail by confocal microscopy. Seven viral proteins were analyzed in time course experiments and showed a cascade-like temporal gene expression pattern similar to those of other herpesviruses. Furthermore, selected MAbs tested positive on human skin lesions by using immunohistochemistry, demonstrating the wide applicability of the MAb collection. Finally, a significant portion of the VZV-specific antibodies reacted with orthologs of simian varicella virus (SVV), thus enabling the systematic analysis of varicella in a nonhuman primate model system. In summary, this study provides insight into the potential function of numerous VZV proteins and novel tools to systematically study VZV and SVV pathogenesis.

Marek's disease virus (MDV) ubiquitin-specific protease (USP) performs critical functions beyond its enzymatic activity during virus replication

Marek's disease virus (MDV) encodes an ubiquitin-specific protease (USP) within its UL36 gene. USP is highly conserved among herpesviruses and was shown to be important for MDV replication and pathogenesis in MDV's natural host, the chicken. To further investigate the role of MDV USP, several recombinant (r) MDVs were generated and their in vitro phenotypes were evaluated using plaque size and growth kinetics assays. We discovered that the N-terminus of pUL36 is essential for MDV replication and could not be complemented by ectopic expression of MDV USP. In addition, we demonstrated that the region located between the conserved glutamine (Q85) and leucine (L106) residues comprising the active site cysteine (C98) is also essential for MDV replication. Based on the analyses of the rMDVs generated here, we concluded that MDV USP likely contributes to the structure and/or stability of pUL36 and affects replication and oncogenesis of MDV beyond its enzymatic activity.

The attenuated genotype of varicella-zoster virus includes an ORF0 transitional stop codon mutation

Varicella-zoster virus (VZV) is the first of the human herpesviruses to be attenuated and subsequently approved as a live vaccine to prevent varicella and herpes zoster. Both the attenuated VZV vaccine, called vaccine Oka or vOka, and the parental strain pOka have been completely sequenced. Yet the specific determinants of attenuation are uncertain. The open reading frame (ORF) with the most single nucleotide polymorphisms (SNPs), ORF62, encodes the regulatory protein IE62, but IE62 studies have failed to define a specific SNP associated with attenuation. We have completed next-generation sequencing of the VZV Ellen genome, a strain known to be highly attenuated by its very limited replication in human skin xenografts in the SCID mouse model of VZV pathogenesis. A comparative analysis of the Ellen sequence with all other complete VZV sequences was extremely informative. In particular, an unexpected finding was a stop codon mutation in Ellen ORF0 (herpes simplex virus UL56 homolog) identical to one found in vOka, combined with the absence of polymorphisms in most Ellen ORFs that were known to be mutated in vOka. The mutated ORF0 protein was also imaged in both two dimensions and three dimensions by confocal microscopy. The probability of two VZV strains not connected by a recent common ancestor having an identical ORF0 SNP by chance would be 1 × 10(-8), in other words, extremely unlikely. Taken together, these bioinformatics analyses strongly suggest that the stop codon ORF0 SNP is one of the determinants of the attenuation genotype of live VZV vaccines.

Herpes simplex virus and varicella zoster virus, the house guests who never leave

Human alphaherpesviruses including herpes simplex viruses (HSV-1, HSV-2) and varicella zoster virus (VZV) establish persistent latent infection in sensory neurons for the life of the host. All three viruses have the potential to reactivate causing recurrent disease. Regardless of the homology between the different virus strains, the three viruses are characterized by varying pathologies. This review will highlight the differences in infection pattern, immune response, and pathogenesis associated with HSV-1 and VZV.

Simian varicella virus gene expression during acute and latent infection of rhesus macaques

Varicella zoster virus (VZV) is a neurotropic α-herpesvirus that causes chickenpox during primary infection and establishes latency in sensory ganglia. Reactivation of VZV results in herpes zoster and other neurological complications. Our understanding of the VZV transcriptome during acute and latent infection in immune competent individuals remains incomplete. Infection of rhesus macaques with the homologous simian varicella virus (SVV) recapitulates the hallmarks of VZV infection. We therefore characterized the SVV transcriptome by quantitative real-time reverse transcriptase PCR during acute infection in bronchial alveolar lavage (BAL) cells and peripheral blood mononuclear cells, and during latency in sensory ganglia obtained from the same rhesus macaques. During acute infection, all known SVV open reading frames (ORFs) were detected, and the most abundantly expressed ORFs are involved in virus replication and assembly such as the transcriptional activator ORF 63 and the structural proteins ORF 41 and ORF 49. In contrast, latent SVV gene expression is highly restricted. ORF 61, a viral transactivator and latency-associated transcript, is the most prevalent transcript detected in sensory ganglia. We also detected ORFs A, B, 4, 10, 63, 64, 65, 66, and 68 though significantly less frequently than ORF 61. This comprehensive analysis has revealed genes that potentially play a role in the establishment and/or maintenance of SVV latency.

Herpesvirus telomeric repeats facilitate genomic integration into host telomeres and mobilization of viral DNA during reactivation

Herpesvirus telomeric repeats facilitate virus integration into host telomeres, a process which is required for the establishment of virus latency.

Some herpesviruses, particularly lymphotropic viruses such as Marek’s disease virus (MDV) and human herpesvirus 6 (HHV-6), integrate their DNA into host chromosomes. MDV and HHV-6, among other herpesviruses, harbor telomeric repeats (TMRs) identical to host telomeres at either end of their linear genomes. Using MDV as a natural virus-host model, we show that herpesvirus TMRs facilitate viral genome integration into host telomeres and that integration is important for establishment of latency and lymphoma formation. Integration into host telomeres also aids in reactivation from the quiescent state of infection. Our results and the presence of TMRs in many herpesviruses suggest that integration mediated by viral TMRs is a conserved mechanism, which ensures faithful virus genome maintenance in host cells during cell division and allows efficient mobilization of dormant viral genomes. This finding is of particular importance as reactivation is critical for virus spread between susceptible individuals and is necessary for continued herpesvirus evolution and survival.

A 128-base-pair sequence containing the pac1 and a presumed cryptic pac2 sequence includes cis elements sufficient to mediate efficient genome maturation of human cytomegalovirus

Herpesvirus DNA replication proceeds via concatemeric replicative intermediates that are comprised of head-to-tail linked genomes. Genome maturation is carried out by the terminase, an enzyme complex that mediates both the insertion of concatemer DNA into capsids and its subsequent cleavage to release genomes within these capsids. This cleavage is sequence specific, but the governing cis-acting DNA sequences are only partially characterized. Two highly conserved motifs, the pac1 and pac2 motifs, lie near the ends of herpesvirus genomes and are known to be critical for genome maturation. In murine cytomegalovirus, poorly conserved sequences distal to the pac2 motif up to 150 bp from the point of cleavage are also important for cleavage. Here, we sought to identify the cleavage/packaging signals of human cytomegalovirus. Our results show that a previously proposed pac2-like poly(A) tract is dispensable for cleavage/packaging function and suggest that human cytomegalovirus may utilize a cryptic pac2 motif that lacks a poly(A) tract characteristic of pac2 motifs in other herpesviruses. Additional distal sequences 47 to 100 bp from the point of cleavage were found to enhance cleavage efficiency. These results should facilitate the identification of trans-acting factors that bind to these cis elements and elucidation of their functions. Such information will be critical for understanding the molecular basis of this complex process.