Reverse genetics system for the avian coronavirus infectious bronchitis virus

Construction of a severe acute respiratory syndrome coronavirus infectious cDNA clone and a replicon to study coronavirus RNA synthesis

The engineering of a full-length infectious cDNA clone and a functional replicon of the severe acute respiratory syndrome coronavirus (SARS-CoV) Urbani strain as bacterial artificial chromosomes (BACs) is described in this study. In this system, the viral RNA was expressed in the cell nucleus under the control of the cytomegalovirus promoter and further amplified in the cytoplasm by the viral replicase. Both the infectious clone and the replicon were fully stable in Escherichia coli. Using the SARS-CoV replicon, we have shown that the recently described RNA-processing enzymes exoribonuclease, endoribonuclease, and 2'-O-ribose methyltransferase were essential for efficient coronavirus RNA synthesis. The SARS reverse genetic system developed as a BAC constitutes a useful tool for the study of fundamental viral processes and also for developing genetically defined vaccines.

Nidovirus transcription: how to make sense...?

Many positive-stranded RNA viruses use subgenomic mRNAs to express part of their genetic information. To produce structural and accessory proteins, members of the order Nidovirales (corona-, toro-, arteri- and roniviruses) generate a 3' co-terminal nested set of at least three and often seven to nine mRNAs. Coronavirus and arterivirus subgenomic transcripts are not only 3' co-terminal but also contain a common 5' leader sequence, which is derived from the genomic 5' end. Their synthesis involves a process of discontinuous RNA synthesis that resembles similarity-assisted RNA recombination. Most models proposed over the past 25 years assume co-transcriptional fusion of subgenomic RNA leader and body sequences, but there has been controversy over the question of whether this occurs during plus- or minus-strand synthesis. In the latter model, which has now gained considerable support, subgenomic mRNA synthesis takes place from a complementary set of subgenome-size minus-strand RNAs, produced by discontinuous minus-strand synthesis. Sense-antisense base-pairing interactions between short conserved sequences play a key regulatory role in this process. In view of the presumed common ancestry of nidoviruses, the recent finding that ronivirus and torovirus mRNAs do not contain a common 5' leader sequence is surprising. Apparently, major mechanistic differences must exist between nidoviruses, which raises questions about the functions of the common leader sequence and nidovirus transcriptase proteins and the evolution of nidovirus transcription. In this review, nidovirus transcription mechanisms are compared, the experimental systems used are critically assessed and, in particular, the impact of recently developed reverse genetic systems is discussed.

Safety and efficacy of an infectious bronchitis virus used for chicken embryo vaccination

Commercial vaccines for in ovo vaccination have not yet been developed for infectious bronchitis virus (IBV), the major coronavirus in the poultry industry. Recombinant IBVs based on the Beaudette strain expressing the Beaudette spike protein (Beau-R) or that from the virulent M41 strain (BeauR-M41(S)) were assessed for their potential as prototype vaccines for application to 18-day-old embryos. Pathogenicity was assessed by observing the effect on hatchability, and/or the production of nasal discharge and/or the effects on ciliary activity in the trachea at various time points post hatch. In contrast to commercial IBV vaccines given in ovo, the Beau-R and BeauR-M41(S) strains did not reduce hatchability or cause nasal discharge, and caused minimal damage to the ciliated epithelium of the trachea. The presence of the spike protein from a virulent virus did not increase the pathogenicity of the virus according to the criteria used. Assessment of the BeauR-M41(S) strain for efficacy showed that it protected up to 90% of chicks against challenge with virulent IB virus (M41) in a dose dependent manner. Further egg passage of the BeauR-M41(S) strain (BeauR-M41(S) EP10) did not increase its pathogenicity though it did improve its efficacy, based on serology and protection against a virulent challenge. BeauR-M41(S) EP10 was more efficacious than BeauR-M41(S) protecting more birds against virulent challenge and providing a better serological antibody response. BeauR-M41(S) EP10 induced a serological response similar to that of a commercial vaccine given at day-old though the commercial vaccine provided slightly higher efficacy. These results are promising for the development of embryo safe efficacious IBV vaccines for in ovo application.

Identification of functionally important negatively charged residues in the carboxy end of mouse hepatitis coronavirus A59 nucleocapsid protein

The coronavirus nucleocapsid (N) protein is a multifunctional viral gene product that encapsidates the RNA genome and also plays some as yet not fully defined role in viral RNA replication and/or transcription. A number of conserved negatively charged amino acids are located within domain III in the carboxy end of all coronavirus N proteins. Previous studies suggested that the negatively charged residues are involved in virus assembly by mediating interaction between the membrane (M) protein carboxy tail and nucleocapsids. To determine the importance of these negatively charged residues, a series of alanine and other charged-residue substitutions were introduced in place of those in the N gene within a mouse hepatitis coronavirus A59 infectious clone. Aspartic acid residues 440 and 441 were identified as functionally important. Viruses could not be isolated when both residues were replaced by positively charged amino acids. When either amino acid was replaced by a positively charged residue or both were changed to alanine, viruses were recovered that contained second-site changes within N, but not in the M or envelope protein. The compensatory role of the new changes was confirmed by the construction of new viruses. A few viruses were recovered that retained the D441-to-arginine change and no compensatory changes. These viruses exhibited a small-plaque phenotype and produced significantly less virus. Overall, results from our analysis of a large panel of plaque-purified recovered viruses indicate that the negatively charged residues at positions 440 and 441 are key residues that appear to be involved in virus assembly.

Analysis of rubella virus capsid protein-mediated enhancement of replicon replication and mutant rescue

The rubella virus capsid protein (C) has been shown to complement a lethal deletion (termed deltaNotI) in P150 replicase protein. To investigate this phenomenon, we generated two lines of Vero cells that stably expressed either C (C-Vero cells) or C lacking the eight N-terminal residues (Cdelta8-Vero cells), a construct previously shown to be unable to complement DeltaNotI. In C-Vero cells but not Vero or Cdelta8-Vero cells, replication of a wild-type (wt) replicon expressing the green fluorescent protein (GFP) reporter gene (RUBrep/GFP) was enhanced, and replication of a replicon with deltaNotI (RUBrep/GFP-deltaNotI) was rescued. Surprisingly, replicons with deleterious mutations in the 5' and 3' cis-acting elements were also rescued in C-Vero cells. Interestingly, the Cdelta8 construct localized to the nucleus while the C construct localized in the cytoplasm, explaining the lack of enhancement and rescue in Cdelta8-Vero cells since rubella virus replication occurs in the cytoplasm. Enhancement and rescue in C-Vero cells were at a basic step in the replication cycle, resulting in a substantial increase in the accumulation of replicon-specific RNAs. There was no difference in translation of the nonstructural proteins in C-Vero and Vero cells transfected with the wt and mutant replicons, demonstrating that enhancement and rescue were not due to an increase in the efficiency of translation of the transfected replicon transcripts. In replicon-transfected C-Vero cells, C and the P150 replicase protein associated by coimmunoprecipitation, suggesting that C might play a role in RNA replication, which could explain the enhancement and rescue phenomena. A unifying model that accounts for enhancement of wt replicon replication and rescue of diverse mutations by the rubella virus C protein is proposed.

Recovery of a neurovirulent human coronavirus OC43 from an infectious cDNA clone

This study describes the assembly of a full-length cDNA clone of human coronavirus (HCoV)-OC43 in a bacterial artificial chromosome (BAC). The BAC containing the full-length infectious cDNA (pBAC-OC43(FL)) was assembled using a two-part strategy. The first step consisted in the introduction of each end of the viral genome into the BAC with accessory sequences allowing proper transcription. The second step consisted in the insertion of the whole HCoV-OC43 cDNA genome into the BAC. To produce recombinant viral particles, pBAC-OC43(FL) was transfected into BHK-21 cells. Recombinant virus displayed the same phenotypic properties as the wild-type virus, including infectious virus titers produced in cell culture and neurovirulence in mice.

Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

Coronaviruses are a family of enveloped, single-stranded, positive-strand RNA viruses classified within the Nidovirales order. This coronavirus family consists of pathogens of many animal species and of humans, including the recently isolated severe acute respiratory syndrome coronavirus (SARS-CoV). This review is divided into two main parts; the first concerns the animal coronaviruses and their pathogenesis, with an emphasis on the functions of individual viral genes, and the second discusses the newly described human emerging pathogen, SARS-CoV. The coronavirus part covers (i) a description of a group of coronaviruses and the diseases they cause, including the prototype coronavirus, murine hepatitis virus, which is one of the recognized animal models for multiple sclerosis, as well as viruses of veterinary importance that infect the pig, chicken, and cat and a summary of the human viruses; (ii) a short summary of the replication cycle of coronaviruses in cell culture; (iii) the development and application of reverse genetics systems; and (iv) the roles of individual coronavirus proteins in replication and pathogenesis. The SARS-CoV part covers the pathogenesis of SARS, the developing animal models for infection, and the progress in vaccine development and antiviral therapies. The data gathered on the animal coronaviruses continue to be helpful in understanding SARS-CoV.

Coronavirus pathogenesis and the emerging pathogen severe acute respiratory syndrome coronavirus

Coronaviruses are a family of enveloped, single-stranded, positive-strand RNA viruses classified within the Nidovirales order. This coronavirus family consists of pathogens of many animal species and of humans, including the recently isolated severe acute respiratory syndrome coronavirus (SARS-CoV). This review is divided into two main parts; the first concerns the animal coronaviruses and their pathogenesis, with an emphasis on the functions of individual viral genes, and the second discusses the newly described human emerging pathogen, SARS-CoV. The coronavirus part covers (i) a description of a group of coronaviruses and the diseases they cause, including the prototype coronavirus, murine hepatitis virus, which is one of the recognized animal models for multiple sclerosis, as well as viruses of veterinary importance that infect the pig, chicken, and cat and a summary of the human viruses; (ii) a short summary of the replication cycle of coronaviruses in cell culture; (iii) the development and application of reverse genetics systems; and (iv) the roles of individual coronavirus proteins in replication and pathogenesis. The SARS-CoV part covers the pathogenesis of SARS, the developing animal models for infection, and the progress in vaccine development and antiviral therapies. The data gathered on the animal coronaviruses continue to be helpful in understanding SARS-CoV.

Gene 5 of the avian coronavirus infectious bronchitis virus is not essential for replication

The avian coronavirus Infectious bronchitis virus (IBV), like other coronaviruses, expresses several small nonstructural (ns) proteins in addition to those from gene 1 (replicase) and the structural proteins. These coronavirus ns genes differ both in number and in amino acid similarity between the coronavirus groups but show some concordance within a group or subgroup. The functions and requirements of the small ns gene products remain to be elucidated. With the advent of reverse genetics for coronaviruses, the first steps in elucidating their role can be investigated. We have used our reverse genetics system for IBV (R. Casais, V. Thiel, S. G. Siddell, D. Cavanagh, and P. Britton, J. Virol. 75:12359-12369, 2001) to investigate the requirement of IBV gene 5 for replication in vivo, in ovo, and ex vivo. We produced a series of recombinant viruses, with an isogenic background, in which complete expression of gene 5 products was prevented by the inactivation of gene 5 following scrambling of the transcription-associated sequence, thereby preventing the expression of IBV subgenomic mRNA 5, or scrambling either separately or together of the translation initiation codons for the two gene 5 products. As all of the recombinant viruses replicated very similarly to the wild-type virus, Beau-R, we conclude that the IBV gene 5 products are not essential for IBV replication per se and that they are accessory proteins.

Selective replication of coronavirus genomes that express nucleocapsid protein

The coronavirus nucleocapsid (N) protein is a structural protein that forms a ribonucleoprotein complex with genomic RNA. In addition to its structural role, it has been described as an RNA-binding protein that might be involved in coronavirus RNA synthesis. Here, we report a reverse genetic approach to elucidate the role of N in coronavirus replication and transcription. We found that human coronavirus 229E (HCoV-229E) vector RNAs that lack the N gene were greatly impaired in their ability to replicate, whereas the transcription of subgenomic mRNA from these vectors was easily detectable. In contrast, vector RNAs encoding a functional N protein were able to carry out both replication and transcription. Furthermore, modification of the transcription signal required for the synthesis of N protein mRNAs in the HCoV-229E genome resulted in the selective replication of genomes that are able to express the N protein. This genetic evidence leads us to conclude that at least one coronavirus structural protein, the N protein, is involved in coronavirus replication.

Development of a transgenic mouse model susceptible to human coronavirus 229E

Human coronavirus (HCoV) 229E is a group 1 coronavirus and is specific to humans. So far, no animal model is available to study the pathogenesis of infection by HCoV-229E. We show here that the expression of aminopeptidase N (APN, also termed CD13), the receptor for HCoV-229E, is required but not sufficient to confer susceptibility in vivo. HCoV-229E infection was facilitated by crossing APN transgenic mice into signal transducers and activators of transcription (Stat) 1 null mice and by adaptation of HCoV-229E to grow in primary APN transgenic, Stat1 null fibroblasts. Double transgenic mice allow the study of human coronavirus group 1 infections in an animal model, in particular, viral tropism, replication, recombination, and spread in an immunocompromised situation. Furthermore, these mice provide an important tool for the evaluation of biosafety and efficacy of coronavirus-based vectors.

In vitro assembled, recombinant infectious bronchitis viruses demonstrate that the 5a open reading frame is not essential for replication

Molecular clones of infectious bronchitis virus (IBV), derived from the Vero cell adapted Beaudette strain, were constructed, using an in vitro assembly method. In vitro transcribed RNA from a cDNA template that had been constructed from seven cDNA fragments, encompassing the entire genome of IBV, was electroporated into BHK-21 cells. The cells were overlaid onto the susceptible Vero cells and viable virus was recovered from the molecular clone. The molecularly cloned IBV (MIBV) demonstrated growth kinetics, and plaque size and morphology that resembled the parental Beaudette strain IBV. The recombinant virus was further manipulated to express enhanced green fluorescent protein (EGFP) by replacing an open reading frame (ORF) of the group-specific gene, ORF 5a, with the EGFP ORF. The rescued recombinant virus, expressing EGFP (GIBV), replicated to lower viral titers and formed smaller plaques compared to the parental virus and the MIBV. After six passages of GIBV, a minority of plaques were observed that had reverted to the larger plaque size and virus from these plaques no longer expressed EGFP. Direct sequencing of RT-PCR products derived from cells infected with the plaque-purified virus, which had lost expression of EGFP, confirmed loss of the EGFP ORF. The loss of EGFP expression (Delta5a IBV) was also accompanied by reversion to growth kinetics resembling the standard virus and intact recombinant virus. This study demonstrates that the 5a ORF is not essential for viral multiplication in Vero cells.

Development and use of the H strain of avian infectious bronchitis virus from the Netherlands as a vaccine: a review

The H strain of infectious bronchitis (IB) was one of the earliest live attenuated IB vaccines to be developed and has continued to be use in most parts of the world for almost 50 years. It was developed for used at both the 52nd (H52) and 120th (H120) vaccine levels and, because of it ability to provide heterologous cross-protection against a number of IB viruses of different serotypes, has proved to be one of the most enduring live attenuated IB vaccines. In fact, the H120 vaccine is possibly the most widely used live attenuated IB vaccine globally to this day. The use of H52 has, however, declined with the introduction of safe and highly efficacious inactivated IB vaccines. This review documents the original studies to isolate and attenuate the H strain by serial embryo passage, and describes the early studies to demonstrate its efficacy in laboratory studies and under field conditions. The efficacy of the H vaccine in providing cross-protection against some of the many IB variants now reported worldwide is also discussed, and possible future vaccination strategies for IB considered.

Recombinant mouse hepatitis virus strain A59 from cloned, full-length cDNA replicates to high titers in vitro and is fully pathogenic in vivo

Mouse hepatitis virus (MHV) is the prototype of group II coronaviruses and one of the most extensively studied coronaviruses. Here, we describe a reverse genetic system for MHV (strain A59) based upon the cloning of a full-length genomic cDNA in vaccinia virus. We show that the recombinant virus generated from cloned cDNA replicates to the same titers as the parental virus in cell culture ( approximately 10(9) PFU/ml), has the same plaque morphology, and produces the same amounts and proportions of genomic and subgenomic mRNAs in virus-infected cells. In a mouse model of neurological infection, the recombinant and parental viruses are equally virulent, they replicate to the same titers in brain and liver, and they induce similar patterns of acute hepatitis, acute meningoencephalitis, and chronic demyelination. We also describe improvements in the use of the coronavirus reverse genetic system based on vaccinia virus cloning vectors. These modifications facilitate (i) the mutagenesis of cloned cDNA by using vaccinia virus-mediated homologous recombination and (ii) the rescue of recombinant coronaviruses by using a stable nucleocapsid protein-expressing cell line for the electroporation of infectious full-length genomes. Thus, our system represents a versatile and universal tool to study all aspects of MHV molecular biology and pathogenesis. We expect this system to provide valuable insights into the replication of group II coronaviruses that may lead to the development of novel strategies against coronavirus infections, including the related severe acute respiratory syndrome coronavirus.

Coronavirus genome structure and replication

In addition to the SARS coronavirus (treated separately elsewhere in this volume), the complete genome sequences of six species in the coronavirus genus of the coronavirus family [avian infectious bronchitis virus-Beaudette strain (IBV-Beaudette), bovine coronavirus-ENT strain (BCoV-ENT), human coronavirus-229E strain (HCoV-229E), murine hepatitis virus-A59 strain (MHV-A59), porcine transmissible gastroenteritis-Purdue 115 strain (TGEV-Purdue 115), and porcine epidemic diarrhea virus-CV777 strain (PEDV-CV777)] have now been reported. Their lengths range from 27,317 nt for HCoV-229E to 31,357 nt for the murine hepatitis virus-A59, establishing the coronavirus genome as the largest known among RNA viruses. The basic organization of the coronavirus genome is shared with other members of the Nidovirus order (the torovirus genus, also in the family Coronaviridae, and members of the family Arteriviridae) in that the nonstructural proteins involved in proteolytic processing, genome replication, and subgenomic mRNA synthesis (transcription) (an estimated 14–16 end products for coronaviruses) are encoded within the 5′-proximal two-thirds of the genome on gene 1 and the (mostly) structural proteins are encoded within the 3′-proximal one-third of the genome (8–9 genes for coronaviruses). Genes for the major structural proteins in all coronaviruses occur in the 5′ to 3′ order as S, E, M, and N. The precise strategy used by coronaviruses for genome replication is not yet known, but many features have been established. This chapter focuses on some of the known features and presents some current questions regarding genome replication strategy, the cis-acting elements necessary for genome replication [as inferred from defective interfering (DI) RNA molecules], the minimum sequence requirements for autonomous replication of an RNA replicon, and the importance of gene order in genome replication.

Development of mouse hepatitis virus and SARS-CoV infectious cDNA constructs

The genomes of transmissible gastroenteritis virus (TGEV) and mouse hepatitis virus (MHV) have been generated with a novel construction strategy that allows for the assembly of very large RNA and DNA genomes from a panel of contiguous cDNA subclones. Recombinant viruses generated from these methods contained the appropriate marker mutations and replicated as efficiently as wild-type virus. The MHV cloning strategy can also be used to generate recombinant viruses that contain foreign genes or mutations at virtually any given nucleotide. MHV molecular viruses were engineered to express green fluorescent protein (GFP), demonstrating the feasibility of the systematic assembly approach to create recombinant viruses expressing foreign genes. The systematic assembly approach was used to develop an infectious clone of the newly identified human coronavirus, the serve acute respiratory syndrome virus (SARS-CoV). Our cloning and assembly strategy generated an infectious clone within 2 months of identification of the causative agent of SARS, providing a critical tool to study coronavirus pathogenesis and replication. The availability of coronavirus infectious cDNAs heralds a new era in coronavirus genetics and genomic applications, especially within the replicase proteins whose functions in replication and pathogenesis are virtually unknown.

The coronavirus replicase

Coronavirus genome replication and transcription take place at cytoplasmic membranes and involve coordinated processes of both continuous and discontinuous RNA synthesis that are mediated by the viral replicase, a huge protein complex encoded by the 20-kb replicase gene. The replicase complex is believed to be comprised of up to 16 viral subunits and a number of cellular proteins. Besides RNA-dependent RNA polymerase, RNA helicase, and protease activities, which are common to RNA viruses, the coronavirus replicase was recently predicted to employ a variety of RNA processing enzymes that are not (or extremely rarely) found in other RNA viruses and include putative sequence-specific endoribonuclease, 3′-to-5′ exoribonuclease, 2′-O-ribose methyltransferase, ADP ribose 1′-phosphatase and, in a subset of group 2 coronaviruses, cyclic phosphodiesterase activities. This chapter reviews (1) the organization of the coronavirus replicase gene, (2) the proteolytic processing of the replicase by viral proteases, (3) the available functional and structural information on individual subunits of the replicase, such as proteases, RNA helicase, and the RNA-dependent RNA polymerase, and (4) the subcellular localization of coronavirus proteins involved in RNA synthesis. Although many molecular details of the coronavirus life cycle remain to be investigated, the available information suggests that these viruses and their distant nidovirus relatives employ a unique collection of enzymatic activities and other protein functions to synthesize a set of 5′-leader-containing subgenomic mRNAs and to replicate the largest RNA virus genomes currently known.

Reverse genetics of coronaviruses using vaccinia virus vectors

In this article, we describe the reverse genetic system that is based on the use of vaccinia virus cloning vectors. This system represents a generic approach to coronavirus reverse genetics and was first described for the generation of recombinant human coronavirus 229E representing a group I coronavirus. Subsequently, the same approach has been used to generate recombinant avian infectious bronchitis coronavirus and, recently, recombinant mouse hepatitis virus, representing group III and group II coronaviruses, respectively. We describe how vaccinia virus-mediated homologous recombination can be used to introduce specific mutations into the coronavirus genomic cDNA during its propagation in vaccinia virus and how recombinant coronaviruses can be isolated. Finally, we describe how the coronavirus reverse genetic system has now been extended to the generation of coronavirus replicon RNAs.

Generation of a recombinant avian coronavirus infectious bronchitis virus using transient dominant selection

A reverse genetics system for the avian coronavirus infectious bronchitis virus (IBV) has been described in which a full-length cDNA, corresponding to the IBV (Beaudette-CK) genome, was inserted into the vaccinia virus genome following in vitro assembly of three contiguous cDNAs [Casais, R., Thiel, V., Siddell, S.G., Cavanagh, D., Britton, P., 2001. Reverse genetics system for the avian coronavirus infectious bronchitis virus. J. Virol. 75, 12359-12369]. The method has subsequently been used to generate a recombinant IBV expressing a chimaeric S gene [Casais, R., Dove, B., Cavanagh, D., Britton, P., 2003. Recombinant avian infectious bronchitis virus expressing a heterologous spike gene demonstrates that the spike protein is a determinant of cell tropism. J. Virol. 77, 9084-9089]. Use of vaccinia virus as a vector for the full-length cDNA of the IBV genome has the advantage that modifications can be made to the IBV cDNA using homologous recombination, a method frequently used to insert and delete sequences from the vaccinia virus genome. We describe the use of homologous recombination as a method for modifying the Beaudette full-length cDNA, within the vaccinia virus genome, without the requirement for in vitro assembly of the IBV cDNA. To demonstrate the feasibility of the method we exchanged the ectodomain of the Beaudette spike gene for the corresponding region from IBV M41 and generated two recombinant infectious bronchitis viruses (rIBVs) expressing the chimaeric S protein, validating the method as an alternative way for generating rIBVs.

Coronavirus reverse genetics and development of vectors for gene expression

Knowledge of coronavirus replication, transcription, and virus-host interaction has been recently improved by engineering of coronavirus infectious cDNAs. With the transmissible gastroenteritis virus (TGEV) genome the efficient (>40 microg per 106 cells) and stable (>20 passages) expression of the foreign genes has been shown. Knowledge of the transcription mechanism in coronaviruses has been significantly increased, making possible the fine regulation of foreign gene expression. A new family of vectors based on single coronavirus genomes, in which essential genes have been deleted, has emerged including replication-competent, propagation-deficient vectors. Vector biosafety is being increased by relocating the RNA packaging signal to the position previously occupied by deleted essential genes, to prevent the rescue of fully competent viruses that might arise from recombination events with wild-type field coronaviruses. The large cloning capacity of coronaviruses (>5 kb) and the possibility of engineering the tissue and species tropism to target expression to different organs and animal species, including humans, has increased the potential of coronaviruses as vectors for vaccine development and, possibly, gene therapy.

Coronavirus reverse genetics by targeted RNA recombination

Targeted RNA recombination was the first reverse genetics system devised for coronaviruses at a time when it was not clear whether the construction of full-length infectious cDNA clones would become possible. In its current state targeted RNA recombination offers a versatile and powerful method for the site-directed mutagenesis of the downstream third of the coronavirus genome, which encodes all the viral structural proteins. The development of this system is described, with an emphasis on recent improvements, and multiple applications of this technique to the study of coronavirus molecular biology and pathogenesis are reviewed. Additionally, the relative strengths and limitations of targeted RNA recombination and infectious cDNA systems are contrasted.

Diversity of Coronavirus Spikes: Relationship to Pathogen Entry and Dissemination

Coronaviruses are widespread in the environment, infecting humans, domesticated and wild mammals, and birds. Infections cause a variety of diseases including bronchitis, gastroenteritis, hepatitis, and encephalitis, with symptoms ranging from being nearly undetectable to rapidly fatal. A combination of interacting variables determine the pattern and severity of coronavirus-induced disease, including the infecting virus strain, its transmission strategy, and the age and immune status of the infected host. Coronavirus pathogenesis is best understood by discerning how each of these variables dictates clinical outcomes. This chapter focuses on variabilities amongst the spike (S) proteins of infecting virus strains. Diversity of coronavirus surface proteins likely contributes to epidemic disease, an important and timely topic given the recent emergence of the human SARS coronavirus.

Mass spectroscopic characterization of the coronavirus infectious bronchitis virus nucleoprotein and elucidation of the role of phosphorylation in RNA binding by using surface plasmon resonance

Phosphorylation of the coronavirus nucleoprotein (N protein) has been predicted to play a role in RNA binding. To investigate this hypothesis, we examined the kinetics of RNA binding between nonphosphorylated and phosphorylated infectious bronchitis virus N protein with nonviral and viral RNA by surface plasmon resonance (Biacore). Mass spectroscopic analysis of N protein identified phosphorylation sites that were proximal to RNA binding domains. Kinetic analysis, by surface plasmon resonance, indicated that nonphosphorylated N protein bound with the same affinity to viral RNA as phosphorylated N protein. However, phosphorylated N protein bound to viral RNA with a higher binding affinity than nonviral RNA, suggesting that phosphorylation of N protein determined the recognition of virus RNA. The data also indicated that a known N protein binding site (involved in transcriptional regulation) consisting of a conserved core sequence present near the 5' end of the genome (in the leader sequence) functioned by promoting high association rates of N protein binding. Further analysis of the leader sequence indicated that the core element was not the only binding site for N protein and that other regions functioned to promote high-affinity binding.

Recombinant infectious bronchitis coronavirus Beaudette with the spike protein gene of the pathogenic M41 strain remains attenuated but induces protective immunity

We have replaced the ectodomain of the spike (S) protein of the Beaudette strain (Beau-R; apathogenic for Gallus domesticus chickens) of avian infectious bronchitis coronavirus (IBV) with that from the pathogenic M41 strain to produce recombinant IBV BeauR-M41(S). We have previously shown that this changed the tropism of the virus in vitro (R. Casais, B. Dove, D. Cavanagh, and P. Britton, J. Virol. 77:9084-9089, 2003). Herein we have assessed the pathogenicity and immunogenicity of BeauR-M41(S). There were no consistent differences in pathogenicity between the recombinant BeauR-M41(S) and its apathogenic parent Beau-R (based on snicking, nasal discharge, wheezing, watery eyes, rales, and ciliostasis in trachea), and both replicated poorly in trachea and nose compared to M41; the S protein from the pathogenic M41 had not altered the apathogenic nature of Beau-R. Both Beau-R and BeauR-M41(S) induced protection against challenge with M41 as assessed by absence of recovery of challenge virus and nasal exudate. With regard to snicking and ciliostasis, BeauR-M41(S) induced greater protection (seven out of nine chicks [77%]; assessed by ciliostasis) than Beau-R (one out of nine; 11%) but less than M41 (100%). The greater protection induced by BeauR-M41(S) against M41 may be related to the ectodomain of the spike protein of Beau-R differing from that of M41 by 4.1%; a small number of epitopes on the S protein may play a disproportionate role in the induction of immunity. The results are promising for the prospects of S-gene exchange for IBV vaccine development.

The nucleoprotein is required for efficient coronavirus genome replication

The construction of a set of transmissible gastroenteritis coronavirus (TGEV)-derived replicons as bacterial artificial chromosomes is reported. These replicons were generated by sequential deletion of nonessential genes for virus replication, using a modified TGEV full-length cDNA clone containing unique restriction sites between each pair of consecutive genes. Efficient activity of TGEV replicons was associated with the presence of the nucleoprotein provided either in cis or in trans. TGEV replicons were functional in several cell lines, including the human cell line 293T, in which no or very low cytopathic effect was observed, and expressed high amounts of heterologous protein.

Identification and characterization of severe acute respiratory syndrome coronavirus replicase proteins

The severe acute respiratory syndrome coronavirus (SARS-CoV) encodes proteins required for RNA transcription and genome replication as large polyproteins that are proteolytically processed by virus-encoded proteinases to produce mature replicase proteins. In this report, we generated antibodies against SARS-CoV predicted replicase protein and used the antibodies to identify and characterize 12 of the 16 predicted mature replicase proteins (nsp1, nsp2, nsp3, nsp4, nsp5, nsp8, nsp9, nsp12, nsp13, nsp14, nsp15, and nsp16) in SARS-CoV-infected Vero cells. Immunoblot analysis of infected-cell lysates identified proteins of the predicted sizes. Immunofluorescence microscopy detected similar patterns of punctate perinuclear and distributed cytoplasmic foci with all replicase antibodies and as early as 6 h postinfection. Dual-labeling studies demonstrated colocalization of replicase protein nsp8 with nsp2 and nsp3 in cytoplasmic complexes and also with LC3, a protein marker for autophagic vacuoles. Antibodies directed against mouse hepatitis virus (MHV) virions and against the putative RNA-dependent RNA polymerase (Pol) detected SARS-CoV nucleocapsid and nsp12 (Pol), respectively, in SARS-CoV-infected Vero cells. These results confirm the predicted protein processing pattern for mature SARS-CoV replicase proteins, demonstrate localization of replicase proteins to cytoplasmic complexes containing markers for autophagosome membranes, and suggest conservation of protein epitopes in the replicase and nucleocapsid of SARS-CoV and the group II coronavirus, MHV. Further, the results demonstrate the ability of replicase antibodies to detect SARS-CoV-infected cells as early as 6 h postinfection and thus represent important tools for studies of SARS-CoV replication, inhibition, and diagnosis.

Human respiratory coronavirus OC43: genetic stability and neuroinvasion

The complete genome sequences of the human coronavirus OC43 (HCoV-OC43) laboratory strain from the American Type Culture Collection (ATCC), and a HCoV-OC43 clinical isolate, designated Paris, were obtained. Both genomes are 30,713 nucleotides long, excluding the poly(A) tail, and only differ by 6 nucleotides. These six mutations are scattered throughout the genome and give rise to only two amino acid substitutions: one in the spike protein gene (I958F) and the other in the nucleocapsid protein gene (V81A). Furthermore, the two variants were shown to reach the central nervous system (CNS) after intranasal inoculation in BALB/c mice, demonstrating neuroinvasive properties. Even though the ATCC strain could penetrate the CNS more effectively than the Paris 2001 isolate, these results suggest that intrinsic neuroinvasive properties already existed for the HCoV-OC43 ATCC human respiratory isolate from the 1960s before it was propagated in newborn mouse brains. It also demonstrates that the molecular structure of HCoV-OC43 is very stable in the environment (the two variants were isolated ca. 40 years apart) despite virus shedding and chances of persistence in the host. The genomes of the two HCoV-OC43 variants display 71, 53.1, and 51.2% identity with those of mouse hepatitis virus A59, severe acute respiratory syndrome human coronavirus Tor2 strain (SARS-HCoV Tor2), and human coronavirus 229E (HCoV-229E), respectively. HCoV-OC43 also possesses well-conserved motifs with regard to the genome sequence of the SARS-HCoV Tor2, especially in open reading frame 1b. These results suggest that HCoV-OC43 and SARS-HCoV may share several important functional properties and that HCoV-OC43 may be used as a model to study the biology of SARS-HCoV without the need for level three biological facilities.

The 3' cis-acting genomic replication element of the severe acute respiratory syndrome coronavirus can function in the murine coronavirus genome

The 3' untranslated region (3' UTR) of the genome of the severe acute respiratory syndrome coronavirus can functionally replace its counterpart in the prototype group 2 coronavirus mouse hepatitis virus (MHV). By contrast, the 3' UTRs of representative group 1 or group 3 coronaviruses cannot operate as substitutes for the MHV 3' UTR.

Severe acute respiratory syndrome vaccine development: experiences of vaccination against avian infectious bronchitis coronavirus

Vaccines against infectious bronchitis of chickens (Gallus gallus domesticus) have arguably been the most successful, and certainly the most widely used, of vaccines for diseases caused by coronaviruses, the others being against bovine, canine, feline and porcine coronaviruses. Infectious bronchitis virus (IBV), together with the genetically related coronaviruses of turkey (Meleagris gallopovo) and ring-necked pheasant (Phasianus colchicus), is a group 3 coronavirus, severe acute respiratory syndrome (SARS) coronavirus being tentatively in group 4, the other known mammalian coronaviruses being in groups 1 and 2. IBV replicates not only in respiratory tissues (including the nose, trachea, lungs and airsacs, causing respiratory disease), but also in the kidney (associated with minor or major nephritis), oviduct, and in many parts of the alimentary tract--the oesophagus, proventriculus, duodenum, jejunum, bursa of Fabricius, caecal tonsils (near the distal end of the tract), rectum and cloaca (the common opening for release of eggs and faeces), usually without clinical effects. The virus can persist, being re-excreted at the onset of egg laying (4 to 5 months of age), believed to be a consequence of the stress of coming into lay. Genetic lines of chickens differ in the extent to which IBV causes mortality in chicks, and in respect of clearance of the virus after the acute phase. Live attenuated (by passage in chicken embryonated eggs) IBV strains were introduced as vaccines in the 1950s, followed a couple of decades later by inactivated vaccines for boosting protection in egg-laying birds. Live vaccines are usually applied to meat-type chickens at 1 day of age. In experimental situations this can result in sterile immunity when challenged by virulent homologous virus. Although 100% of chickens may be protected (against clinical signs and loss of ciliary activity in trachea), sometimes 10% of vaccinated chicks do not respond with a protective immune response. Protection is short lived, the start of the decline being apparent 9 weeks after vaccination with vaccines based on highly attenuated strains. IBV exists as scores of serotypes (defined by the neutralization test), cross-protection often being poor. Consequently, chickens may be re-vaccinated, with the same or another serotype, two or three weeks later. Single applications of inactivated virus has generally led to protection of <50% of chickens. Two applications have led to 90 to 100% protection in some reports, but remaining below 50% in others. In practice in the field, inactivated vaccines are used in laying birds that have previously been primed with two or three live attenuated virus vaccinations. This increases protection of the laying birds against egg production losses and induces a sustained level of serum antibody, which is passed to progeny. The large spike glycoprotein (S) comprises a carboxy-terminal S2 subunit (approximately 625 amino acid residues), which anchors S in the virus envelope, and an amino-terminal S1 subunit (approximately 520 residues), believed to largely form the distal bulbous part of S. The S1 subunit (purified from IBV virus, expressed using baculovirus or expressed in birds from a fowlpoxvirus vector) induced virus neutralizing antibody. Although protective immune responses were induced, multiple inoculations were required and the percentage of protected chickens was too low (<50%) for commercial application. Remarkably, expression of S1 in birds using a non-pathogenic fowl adenovirus vector induced protection in 90% and 100% of chickens in two experiments. Differences of as little as 5% between the S1 sequences can result in poor cross-protection. Differences in S1 of 2 to 3% (10 to 15 amino acids) can change serotype, suggesting that a small number of epitopes are immunodominant with respect to neutralizing antibody. Initial studies of the role of the IBV nucleocapsid protein (N) in immunity suggested that immunization with bacterially expressed N, while not inducing protection directly, improved the induction of protection by a subsequent inoculation with inactivated IBV. In another study, two intramuscular immunizations of a plasmid expressing N induced protective immunity. The basis of immunity to IBV is not well understood. Serum antibody levels do not correlate with protection, although local antibody is believed to play a role. Adoptive transfer of IBV-infection-induced alphabeta T cells bearing CD8 antigen protected chicks from challenge infection. In conclusion, live attenuated IBV vaccines induce good, although short-lived, protection against homologous challenge, although a minority of individuals may respond poorly. Inactivated IBV vaccines are insufficiently efficacious when applied only once and in the absence of priming by live vaccine. Two applications of inactivated IBV are much more efficacious, although this is not a commercially viable proposition in the poultry industry. However, the cost and logistics of multiple application of a SARS inactivated vaccine would be more acceptable for the protection of human populations, especially if limited to targeted groups (e.g. health care workers and high-risk contacts). Application of a SARS vaccine is perhaps best limited to a minimal number of targeted individuals who can be monitored, as some vaccinated persons might, if infected by SARS coronavirus, become asymptomatic excretors of virus, thereby posing a risk to non-vaccinated people. Looking further into the future, the high efficacy of the fowl adenovirus vector expressing the IBV S1 subunit provides optimism for a live SARS vaccine, if that were deemed to be necessary, with the possibility of including the N protein gene.

Rapid identification of coronavirus replicase inhibitors using a selectable replicon RNA

A previously unknown coronavirus (CoV) is the aetiological agent causing severe acute respiratory syndrome (SARS), for which an effective antiviral treatment is urgently needed. To enable the rapid and biosafe identification of coronavirus replicase inhibitors, we have generated a non-cytopathic, selectable replicon RNA (based on human CoV 229E) that can be stably maintained in eukaryotic cells. Most importantly, the replicon RNA mediates reporter gene expression as a marker for coronavirus replication. We have used a replicon RNA-containing cell line to test the inhibitory effect of several compounds that are currently being assessed for SARS treatment. Amongst those, interferon-alpha displayed the strongest inhibitory activity. Our results demonstrate that coronavirus replicon cell lines provide a versatile and safe assay for the identification of coronavirus replicase inhibitors. Once this technology is adapted to SARS-CoV replicon RNAs, it will allow high throughput screening for SARS-CoV replicase inhibitors without the need to grow infectious SARS-CoV.

Multiple enzymatic activities associated with severe acute respiratory syndrome coronavirus helicase

Severe acute respiratory syndrome coronavirus (SARS-CoV), a newly identified group 2 coronavirus, is the causative agent of severe acute respiratory syndrome, a life-threatening form of pneumonia in humans. Coronavirus replication and transcription are highly specialized processes of cytoplasmic RNA synthesis that localize to virus-induced membrane structures and were recently proposed to involve a complex enzymatic machinery that, besides RNA-dependent RNA polymerase, helicase, and protease activities, also involves a series of RNA-processing enzymes that are not found in most other RNA virus families. Here, we characterized the enzymatic activities of a recombinant form of the SARS-CoV helicase (nonstructural protein [nsp] 13), a superfamily 1 helicase with an N-terminal zinc-binding domain. We report that nsp13 has both RNA and DNA duplex-unwinding activities. SARS-CoV nsp13 unwinds its substrates in a 5'-to-3' direction and features a remarkable processivity, allowing efficient strand separation of extended regions of double-stranded RNA and DNA. Characterization of the nsp13-associated (deoxy)nucleoside triphosphatase ([dNTPase) activities revealed that all natural nucleotides and deoxynucleotides are substrates of nsp13, with ATP, dATP, and GTP being hydrolyzed slightly more efficiently than other nucleotides. Furthermore, we established an RNA 5'-triphosphatase activity for the SARS-CoV nsp13 helicase which may be involved in the formation of the 5' cap structure of viral RNAs. The data suggest that the (d)NTPase and RNA 5'-triphosphatase activities of nsp13 have a common active site. Finally, we established that, in SARS-CoV-infected Vero E6 cells, nsp13 localizes to membranes that appear to be derived from the endoplasmic reticulum and are the likely site of SARS-CoV RNA synthesis.

Presence of an encephalomyocarditis virus internal ribosome entry site sequence in avian infectious bronchitis virus defective RNAs abolishes rescue by helper virus

Avian infectious bronchitis virus (IBV) defective RNAs (D-RNAs) have been used for the expression of heterologous genes in a helper-virus-dependent expression system. The heterologous genes were expressed under the control of an IBV transcription-associated sequence (TAS) derived from gene 5 of IBV Beaudette. However, coronavirus D-RNA expression vectors display an inherent instability following serial passage with helper virus, resulting in the eventual loss of the heterologous genes. The use of the picornavirus encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) sequence to initiate gene translation was investigated as an alternative method to the coronavirus-mediated TAS-controlled heterologous gene expression system. IBV D-RNAs containing the chloramphenicol acetyltransferase (CAT) reporter gene, under EMCV IRES control, were assessed for IRES-mediated CAT protein translation. CAT protein was detected from T7-derived IBV D-RNA transcripts in a cell-free protein synthesis system and in situ in avian chick kidney (CK) cells following T7-derived D-RNA synthesis from a recombinant fowlpox virus expressing the bacteriophage T7 DNA-dependent RNA polymerase. However, CAT protein was not detected in CK cells from IRES-containing IBV D-RNAs, in which the IRES-CAT construct was inserted at two different positions within the D-RNA, in the presence of helper IBV. Northern blot analysis demonstrated that the IRES-containing D-RNAs were not rescued on serial passage with helper virus, indicating that the EMCV IRES sequence had a detrimental effect on IBV D-RNA rescue.

SARS virus: the beginning of the unraveling of a new coronavirus

Severe acute respiratory syndrome (SARS) virus caused a severe outbreak in several regions of the world in 2003. The virus is a novel coronavirus, which may have an origin in wild animals such as civet cats in southern China. Its genome structure, gene expression pattern and protein profiles are similar to those of other coronaviruses. However, distinct patterns of several open reading frames in the SARS virus genome may contribute to its severe virulence. The potential mutability of the coronavirus genome may pose problems in the control of future SARS outbreaks. The mechanism of SARS pathogenesis may involve both direct viral cytocidal effects on the target cells and immune-mediated mechanisms. The life cycle of the SARS virus is largely unknown; however, based on the analogy with other coronaviruses, several potential targets for antiviral development are identified. Vaccines offer an important preventive measure for possible future recurrences of SARS, but the prospect for their development is still unknown because of the uncertainty regarding the role of immune responses in SARS virus pathogenesis. The comparative studies of other coronaviruses offer insights into the understanding of SARS virus.

SARS virus: the beginning of the unraveling of a new coronavirus

Severe acute respiratory syndrome (SARS) virus caused a severe outbreak in several regions of the world in 2003. The virus is a novel coronavirus, which may have an origin in wild animals such as civet cats in southern China. Its genome structure, gene expression pattern and protein profiles are similar to those of other coronaviruses. However, distinct patterns of several open reading frames in the SARS virus genome may contribute to its severe virulence. The potential mutability of the coronavirus genome may pose problems in the control of future SARS outbreaks. The mechanism of SARS pathogenesis may involve both direct viral cytocidal effects on the target cells and immune-mediated mechanisms. The life cycle of the SARS virus is largely unknown; however, based on the analogy with other coronaviruses, several potential targets for antiviral development are identified. Vaccines offer an important preventive measure for possible future recurrences of SARS, but the prospect for their development is still unknown because of the uncertainty regarding the role of immune responses in SARS virus pathogenesis. The comparative studies of other coronaviruses offer insights into the understanding of SARS virus.

Coronaviruses as vectors: position dependence of foreign gene expression

Coronaviruses are the enveloped, positive-stranded RNA viruses with the largest RNA genomes known. Several features make these viruses attractive as vaccine and therapeutic vectors: (i) deletion of their nonessential genes is strongly attenuating; (ii) the genetic space thus created allows insertion of foreign information; and (iii) their tropism can be modified by manipulation of the viral spike. We studied here their ability to serve as expression vectors by inserting two different foreign genes and evaluating systematically the genomic position dependence of their expression, using a murine coronavirus as a model. Renilla and firefly luciferase expression cassettes, each provided with viral transcription regulatory sequences (TRSs), were inserted at several genomic positions, both independently in different viruses and combined within one viral genome. Recombinant viruses were generated by using a convenient method based on targeted recombination and host cell switching. In all cases high expression levels of the foreign genes were observed without severe effects on viral replication in vitro. The expression of the inserted gene appeared to be dependent on its genomic position, as well as on the identity of the gene. Expression levels increased when the luciferase gene was inserted closer to the 3' end of the genome. The foreign gene insertions generally reduced the expression of upstream viral genes. The results are consistent with coronavirus transcription models in which the transcription from upstream TRSs is attenuated by downstream TRSs. Altogether, our observations clearly demonstrate the potential of coronaviruses as (multivalent) expression vectors.

Reverse genetics with a full-length infectious cDNA of severe acute respiratory syndrome coronavirus

A previously undescribed coronavirus (CoV) is the etiologic agent responsible for severe acute respiratory syndrome (SARS). Using a panel of contiguous cDNAs that span the entire genome, we have assembled a full-length cDNA of the SARS-CoV Urbani strain, and have rescued molecularly cloned SARS viruses (infectious clone SARS-CoV) that contained the expected marker mutations inserted into the component clones. Recombinant viruses replicated as efficiently as WT virus and both were inhibited by treatment with the cysteine proteinase inhibitor (2S,3S)-transepoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester. In addition, subgenomic transcripts were initiated from the consensus sequence ACGAAC in both the WT and infectious clone SARS-CoV. Availability of a SARS-CoV full-length cDNA provides a template for manipulation of the viral genome, allowing for the rapid and rational development and testing of candidate vaccines and therapeutics against this important human pathogen.

Enhanced virulence mediated by the murine coronavirus, mouse hepatitis virus strain JHM, is associated with a glycine at residue 310 of the spike glycoprotein

The coronavirus, mouse hepatitis virus strain JHM, causes acute and chronic neurological diseases in rodents. Here we demonstrate that two closely related virus variants, both of which cause acute encephalitis in susceptible strains of mice, cause markedly different diseases if mice are protected with a suboptimal amount of an anti-JHM neutralizing antibody. One strain, JHM.SD, caused acute encephalitis, while infection with JHM.IA resulted in no acute disease. Using recombinant virus technology, we found that the differences between the two viruses mapped to the spike (S) glycoprotein and that the two S proteins differed at four amino acids. By engineering viruses that differed by only one amino acid, we identified a serine-to-glycine change at position 310 of the S protein (S310G) that recapitulated the more neurovirulent phenotype. The increased neurovirulence mediated by the virus encoding glycine at position S310 was not associated with a different tropism within the central nervous system (CNS) but was associated with increased lateral spread in the CNS, leading to significantly higher brain viral titers. In vitro studies revealed that S310G was associated with decreased S1-S2 stability and with enhanced ability to mediate infection of cells lacking the primary receptor for JHM ("receptor-independent spread"). These enhanced fusogenic properties of viruses encoding a glycine at position 310 of the S protein may contribute to spread within the CNS, a tissue in which expression of conventional JHM receptors is low.

Multigene RNA vector based on coronavirus transcription

Coronavirus genomes are the largest known autonomously replicating RNAs with a size of ca. 30 kb. They are of positive polarity and are translated to produce the viral proteins needed for the assembly of an active replicase-transcriptase complex. In addition to replicating the genomic RNA, a key feature of this complex is a unique transcription process that results in the synthesis of a nested set of six to eight subgenomic mRNAs. These subgenomic mRNAs are produced in constant but nonequimolar amounts and, in general, each is translated to produce a single protein. To take advantage of these features, we have developed a multigene expression vector based on human coronavirus 229E. We have constructed a prototype RNA vector containing the 5' and 3' ends of the human coronavirus genome, the entire human coronavirus replicase gene, and three reporter genes (i.e., the chloramphenicol acetyltransferase [CAT] gene, the firefly luciferase [LUC] gene, and the green fluorescent protein [GFP] gene). Each reporter gene is located downstream of a human coronavirus transcription-associated sequence, which is required for the synthesis of individual subgenomic mRNAs. The transfection of vector RNA and human coronavirus nucleocapsid protein mRNA into BHK-21 cells resulted in the expression of the CAT, LUC, and GFP reporter proteins. Sequence analysis confirmed the synthesis of coronavirus-specific mRNAs encoding CAT, LUC, and GFP. In addition, we have shown that human coronavirus-based vector RNA can be packaged into virus-like particles that, in turn, can be used to transduce immature and mature human dendritic cells. In summary, we describe a new class of eukaryotic, multigene expression vectors that are based on the human coronavirus 229E and have the ability to transduce human dendritic cells.

Frameshift mutations in infectious cDNA clones of Citrus tristeza virus: a strategy to minimize the toxicity of viral sequences to Escherichia coli

The advent of reverse genetics revolutionized the study of positive-stranded RNA viruses that were amenable for cloning as cDNAs into high-copy-number plasmids of Escherichia coli. However, some viruses are inherently refractory to cloning in high-copy-number plasmids due to toxicity of viral sequences to E. coli. We report a strategy that is a compromise between infectivity of the RNA transcripts and toxicity to E. coli effected by introducing frameshift mutations into "slippery sequences" near the viral "toxicity sequences" in the viral cDNA. Citrus tristeza virus (CTV) has cDNA sequences that are toxic to E. coli. The original full-length infectious cDNA of CTV and a derivative replicon, CTV-DeltaCla, cloned into pUC119, resulted in unusually limited E. coli growth. However, upon sequencing of these cDNAs, an additional uridinylate (U) was found in a stretch of U's between nts 3726 and 3731 that resulted in a change to a reading frame with a stop codon at nt 3734. Yet, in vitro produced RNA transcripts from these clones infected protoplasts, and the resulting progeny virus was repaired. Correction of the frameshift mutation in the CTV cDNA constructs resulted in increased infectivity of in vitro produced RNA transcripts, but also caused a substantial increase of toxicity to E. coli, now requiring 3 days to develop visible colonies. Frameshift mutations created in sequences not suspected to facilitate reading frame shifting and silent mutations introduced into oligo(U) regions resulted in complete loss of infectivity, suggesting that the oligo(U) region facilitated the repair of the frameshift mutation. Additional frameshift mutations introduced into other oligo(U) regions also resulted in transcripts with reduced infectivity similarly to the original clones with the +1 insertion. However, only the frameshift mutations introduced into oligo(U) regions that were near and before the toxicity region improved growth and stability in E. coli. These data demonstrate that, when hosts are sufficiently susceptible for infection by transcripts of reduced specific infectivity, introduction of frameshift mutations at "slippery sequences" near toxic regions of viral cDNAs can be used as an additional strategy to clone recalcitrant viral sequences in high-copy-number plasmids for reverse genetics.

Recombinant avian infectious bronchitis virus expressing a heterologous spike gene demonstrates that the spike protein is a determinant of cell tropism

A recombinant infectious bronchitis virus (IBV), BeauR-M41(S), was generated using our reverse genetics system (R. Casais, V. Thiel, S. G. Siddell, D. Cavanagh, and P. Britton, J. Virol. 75:12359-12369, 2001), in which the ectodomain region of the spike gene from IBV M41-CK replaced the corresponding region of the IBV Beaudette genome. BeauR-M41(S) acquired the same cell tropism phenotype as IBV M41-CK in four different cell types, demonstrating that the IBV spike glycoprotein is a determinant of cell tropism.

Transmissible gastroenteritis coronavirus packaging signal is located at the 5' end of the virus genome

To locate the transmissible gastroenteritis coronavirus (TGEV) packaging signal, the incorporation of TGEV subgenomic mRNAs (sgmRNAs) into virions was first addressed. TGEV virions were purified by three different techniques, including an immunopurification using an M protein-specific monoclonal antibody. Detection of sgmRNAs in virions by specific reverse transcription-PCRs (RT-PCRs) was related to the purity of virus preparations. Interestingly, virus mRNAs were detected in partially purified virus but not in virus immunopurified using stringent conditions. Analyses by quantitative RT-PCR confirmed that virus mRNAs were not present in highly purified preparations. Lack of sgmRNA encapsidation was probably due to the absence of a packaging signal (Psi) within these mRNAs. This information plus that from the encapsidation of a collection of TGEV-derived minigenomes suggested that Psi is located at the 5' end of the genome. To confirm that this was the case, a set of minigenomes was expressed that included an expression cassette for an mRNA including the beta-glucuronidase gene (GUS) plus variable sequence fragments from the 5' end of the virus genome potentially including Psi. Insertion of the first 649 nucleotides (nt) of the TGEV genome led to the specific encapsidation of the mRNA, indicating that a Psi was located within this region which was absent from all of the other virus mRNAs. The presence of this packaging signal was further confirmed by showing the expression and rescue of the mRNA including the first 649 nt of the TGEV genome under control of the cytomegalovirus promoter in TGEV-infected cells. This mRNA was successfully amplified and encapsidated, indicating that the first 649 nt of TGEV genome also contained the 5' cis-acting replication signals. The encapsidation efficiency of this mRNA was about 30-fold higher than the genome encapsidation efficiency, as estimated by quantitative RT-PCR. In contrast, viral mRNAs presented significantly lower encapsidation efficiencies (about 100-fold) than those of the virus genome, strongly suggesting that TGEV mRNAs in fact lacked an alternative TGEV Psi.

Identification of the murine coronavirus MP1 cleavage site recognized by papain-like proteinase 2

The replicase polyprotein of murine coronavirus is extensively processed by three proteinases, two papain-like proteinases (PLPs), termed PLP1 and PLP2, and a picornavirus 3C-like proteinase (3CLpro). Previously, we established a trans-cleavage assay and showed that PLP2 cleaves the replicase polyprotein between p210 and membrane protein 1 (MP1) (A. Kanjanahaluethai and S. C. Baker, J. Virol. 74:7911-7921, 2000). Here, we report the results of our studies identifying and characterizing this cleavage site. To determine the approximate position of the cleavage site, we expressed constructs that extended various distances upstream from the previously defined C-terminal end of MP1. We found that the construct extending from the putative PLP2 cleavage site at glycine 2840-alanine 2841 was most similar in size to the processed MP1 replicase product generated in a trans-cleavage assay. To determine which amino acids are critical for PLP2 recognition and processing, we generated 14 constructs with amino acid substitutions upstream and downstream of the putative cleavage site and assessed the effects of the mutations in the PLP2 trans-cleavage assay. We found that substitutions at phenylalanine 2835, glycine 2839, or glycine 2840 resulted in a reduction in cleavage of MP1. Finally, to unequivocally identify this cleavage site, we isolated radiolabeled MP1 protein and determined the position of [(35)S]methionine residues released by Edman degradation reaction. We found that the amino-terminal residue of MP1 corresponds to alanine 2841. Therefore, murine coronavirus PLP2 cleaves the replicase polyprotein between glycine 2840 and alanine 2841, and the critical determinants for PLP2 recognition and processing occupy the P6, P2, and P1 positions of the cleavage site. This study is the first report of the identification and characterization of a cleavage site recognized by murine coronavirus PLP2 activity.

Development and application of a reverse genetics system for Japanese encephalitis virus

Japanese encephalitis virus (JEV) is a common agent of viral encephalitis that causes high mortality and morbidity among children. Molecular genetic studies of JEV are hampered by the lack of a genetically stable full-length infectious JEV cDNA clone. We describe here the development of such a clone. A JEV isolate was fully sequenced, and then its full-length cDNA was cloned into a bacterial artificial chromosome. This was then further engineered so that transcription of the cDNA in vitro would generate synthetic RNAs with authentic 5' and 3' ends. The synthetic RNAs thus produced were highly infectious in susceptible cells (>10(6) PFU/ micro g), and these cells rapidly generated a high titer of synthetic viruses (>5 x 10(6) PFU/ml). The recovered viruses were indistinguishable from the parental virus in terms of plaque morphology, growth kinetics, RNA accumulation, protein expression, and cytopathogenicity. Significantly, the structural and functional integrity of the cDNA was maintained even after 180 generations of growth in Escherichia coli. A single point mutation acting as a genetic marker was introduced into the cDNA and was found in the genome of the recovered virus, indicating that the cDNA can be manipulated. Furthermore, we showed that JEV is an attractive vector for the expression of heterologous genes in a wide variety of cell types. This novel reverse genetics system for JEV will greatly facilitate research into JEV biology. It will also be useful as a heterologous gene expression vector and will aid the development of a vaccine against JEV.

Switching species tropism: an effective way to manipulate the feline coronavirus genome

Feline infectious peritonitis virus (FIPV), a coronavirus, is the causative agent of an invariably lethal infection in cats. Like other coronaviruses, FIPV contains an extremely large positive-strand RNA genome of ca. 30 kb. We describe here the development and use of a reverse genetics strategy for FIPV based on targeted RNA recombination that is analogous to what has been described for the mouse hepatitis virus (MHV) (L. Kuo et al., J. Virol. 74:1393-1406, 2000). In this two-step process, we first constructed by targeted recombination a mutant of FIPV, designated mFIPV, in which the ectodomain of the spike glycoprotein was replaced by that of MHV. This switch allowed for the selection of the recombinant virus in murine cells: mFIPV grows to high titers in these cells but has lost the ability to grow in feline cells. In a second, reverse process, mFIPV was used as the recipient, and the reintroduction of the FIPV spike now allowed for selection of candidate recombinants by their regained ability to grow in feline cells. In this fashion, we reconstructed a wild-type recombinant virus (r-wtFIPV) and generated a directed mutant FIPV in which the initiation codon of the nonstructural gene 7b had been disrupted (FIPV Delta 7b). The r-wtFIPV was indistinguishable from its parental virus FIPV 79-1146 not only for its growth characteristics in tissue culture but also in cats, exhibiting a highly lethal phenotype. FIPV Delta 7b had lost the expression of its 7b gene but grew unimpaired in cell culture, confirming that the 7b glycoprotein is not required in vitro. We establish the second targeted RNA recombination system for coronaviruses and provide a powerful tool for the genetic engineering of the FIPV genome.

Systematic assembly of a full-length infectious cDNA of mouse hepatitis virus strain A59

A novel method was developed to assemble a full-length infectious cDNA of the group II coronavirus mouse hepatitis virus strain A59 (MHV-A59). Seven contiguous cDNA clones that spanned the 31.5-kb MHV genome were isolated. The ends of the cDNAs were engineered with unique junctions and assembled with only the adjacent cDNA subclones, resulting in an intact MHV-A59 cDNA construct of approximately 31.5 kb in length. The interconnecting restriction site junctions that are located at the ends of each cDNA are systematically removed during the assembly of the complete full-length cDNA product, allowing reassembly without the introduction of nucleotide changes. RNA transcripts derived from the full-length MHV-A59 construct were infectious, although transfection frequencies were enhanced 10- to 15-fold in the presence of transcripts encoding the nucleocapsid protein N. Plaque-purified virus derived from the infectious construct replicated efficiently and displayed similar growth kinetics, plaque morphology, and cytopathology in murine cells as did wild-type MHV-A59. Molecularly cloned viruses recognized the MHV receptor (MHVR) for docking and entry, and pretreatment of cells with monoclonal antibodies against MHVR blocked virus entry and replication. Cells infected with molecularly cloned MHV-A59 virus expressed replicase (gene 1) proteins identical to those of laboratory MHV-A59. Importantly, the molecularly cloned viruses contained three marker mutations that had been derived from the engineered component clones. Full-length infectious constructs of MHV-A59 will permit genetic modifications of the entire coronavirus genome, particularly in the replicase gene. The method has the potential to be used to construct viral, microbial, or eukaryotic genomes approaching several million base pairs in length and used to insert restriction sites at any given nucleotide in a microbial genome.

Stabilization of a full-length infectious cDNA clone of transmissible gastroenteritis coronavirus by insertion of an intron

The stable propagation of a full-length transmissible gastroenteritis coronavirus (TGEV) cDNA in Escherichia coli cells as a bacterial artificial chromosome has been considerably improved by the insertion of an intron to disrupt a toxic region identified in the viral genome. The viral RNA was expressed in the cell nucleus under the control of the cytomegalovirus promoter and the intron was efficiently removed during translocation of this RNA to the cytoplasm. The insertion in two different positions allowed stable plasmid amplification for at least 200 generations. Infectious TGEV was efficiently recovered from cells transfected with the modified cDNAs.

Genetic evidence for a structural interaction between the carboxy termini of the membrane and nucleocapsid proteins of mouse hepatitis virus

The coronavirus membrane (M) protein is the most abundant virion protein and the key component in viral assembly and morphogenesis. The M protein of mouse hepatitis virus (MHV) is an integral membrane protein with a short ectodomain, three transmembrane segments, and a large carboxy-terminal endodomain facing the interior of the viral envelope. The carboxy terminus of MHV M has previously been shown to be extremely sensitive to mutation, both in a virus-like particle expression system and in the intact virion. We have constructed a mutant, M(Delta)2, containing a two-amino-acid truncation of the M protein that was previously thought to be lethal. This mutant was isolated by means of targeted RNA recombination with a powerful host range-based selection allowed by the interspecies chimeric virus fMHV (MHV containing the ectodomain of the feline infectious peritonitis virus S protein). Analysis of multiple second-site revertants of the M(Delta)2 mutant has revealed changes in regions of both the M protein and the nucleocapsid (N) protein that can compensate for the loss of the last two residues of the M protein. Our data thus provide the first genetic evidence for a structural interaction between the carboxy termini of the M and N proteins of MHV. In addition, this work demonstrates the efficacy of targeted recombination with fMHV for the systematic genetic analysis of coronavirus structural protein interactions.