Biochemical analysis of a virulent and an avirulent strain of foot-and-mouth disease virus

The pseudoknot region and poly-(C) tract comprise an essential RNA packaging signal for assembly of foot-and-mouth disease virus

Virus assembly is a crucial step for the completion of the viral replication cycle. In addition to ensuring efficient incorporation of viral genomes into nascent virions, high specificity is required to prevent incorporation of host nucleic acids. For picornaviruses, including FMDV, the mechanisms required to fulfil these requirements are not well understood. However, recent evidence has suggested that specific RNA sequences dispersed throughout picornavirus genomes are involved in packaging. Here, we have shown that such sequences are essential for FMDV RNA packaging and have demonstrated roles for both the pseudoknot (PK) region and the poly-(C) tract in this process, where the length of the poly-(C) tract was found to influence the efficiency of RNA encapsidation. Sub-genomic replicons containing longer poly-(C) tracts were packaged with greater efficiency in trans, and viruses recovered from transcripts containing short poly-(C) tracts were found to have greatly extended poly-(C) tracts after only a single passage in cells, suggesting that maintaining a long poly-(C) tract provides a selective advantage. We also demonstrated a critical role for a packaging signal (PS) located in the pseudoknot (PK) region, adjacent to the poly-(C) tract, as well as several other non-essential but beneficial PSs elsewhere in the genome. Collectively, these PSs greatly enhanced encapsidation efficiency, with the poly-(C) tract possibly facilitating nearby PSs to adopt the correct conformation. Using these data, we have proposed a model where interactions with capsid precursors control a transition between two RNA conformations, directing the fate of nascent genomes to either be packaged or alternatively to act as templates for replication and/or for protein translation.

Virulence and Immune Evasion Strategies of FMDV: Implications for Vaccine Design

Foot-and-mouth disease (FMD) is globally recognized as a highly economically devastating and prioritized viral disease affecting livestock. Vaccination remains a crucial preventive measure against FMD. The improvement of current vaccine platforms could help control outbreaks, leading to the potential eradication of the disease. In this review, we describe the variances in virulence and immune responses among FMD-susceptible host species, specifically bovines and pigs, highlighting the details of host-pathogen interactions and their impact on the severity of the disease. This knowledge serves as an important foundation for translating our insights into the rational design of vaccines and countermeasure strategies, including the use of interferon as a biotherapeutic agent. Ultimately, in this review, we aim to bridge the gap between our understanding of FMDV biology and the practical approaches to control and potentially eradicate FMD.

The long-lasting enigma of polycytidine (polyC) tract

Long polycytidine (polyC) tracts varying in length from 50 to 400 nucleotides were first described in the 5'-noncoding region (NCR) of genomes of picornaviruses belonging to the Cardio- and Aphthovirus genera over 50 years ago, but the molecular basis of their function is still unknown. Truncation or complete deletion of the polyC tracts in picornaviruses compromises virulence and pathogenicity but do not affect replicative fitness in vitro, suggesting a role as "viral security" RNA element. The evidence available suggests that the presence of a long polyC tract is required for replication in immune cells, which impacts viral distribution and targeting, and, consequently, pathogenic progression. Viral attenuation achieved by reduction of the polyC tract length has been successfully used for vaccine strategies. Further elucidation of the role of the polyC tract in viral replication cycle and its connection with replication in immune cells has the potential to expand the arsenal of tools in the fight against cancer in oncolytic virotherapy (OV). Here, we review the published data on the biological significance and mechanisms of action of the polyC tract in viral pathogenesis in Cardio- and Aphthoviruses.

Construction and characterization of an infectious cDNA clone of encephalomyocarditis virus from pigs in China

Encephalomyocarditis virus (EMCV) infects animals of various species and causes a variety of clinical symptoms. In this study, an infectious full-length cDNA clone was constructed, and the characteristics of the rescued virus were investigated in vitro and in vivo. Our data demonstrated that the growth kinetics in vitro and plaque morphology of the rescued EMCV rNJ08 strain were similar to those of the parental strain. Although rNJ08 infected BALB/c mice, none of the mice died during the observation period of 14 days post-inoculation. The availability of the infectious cDNA clone provides a genetic platform for studying gene function and for the rational design of vaccines.

A Brief Review on Diagnosis of Foot-and-Mouth Disease of Livestock: Conventional to Molecular Tools

Foot-and-mouth disease (FMD) is one of the highly contagious diseases of domestic animals. Effective control of this disease needs sensitive, specific, and quick diagnostic tools at each tier of control strategy. In this paper we have outlined various diagnostic approaches from old to new generation in a nutshell. Presently FMD diagnosis is being carried out using techniques such as Virus Isolation (VI), Sandwich-ELISA (S-ELISA), Liquid-Phase Blocking ELISA (LPBE), Multiplex-PCR (m-PCR), and indirect ELISA (DIVA), and real time-PCR can be used for detection of antibody against nonstructural proteins. Nucleotide sequencing for serotyping, microarray as well as recombinant antigen-based detection, biosensor, phage display, and nucleic-acid-based diagnostic are on the way for rapid and specific detection of FMDV. Various pen side tests, namely, lateral flow, RT-LAMP, Immunostrip tests, and so forth. are also developed for detection of the virus in field condition.

Recovery of infectious foot-and-mouth disease virus from full-length genomic cDNA clones using an RNA polymerase I system

The prototypic foot-and-mouth disease virus (FMDV) was shown more than a century ago to be the first filterable agent capable of causing FMD, and it has served as an important model for studying basic principles of Aphthovirus molecular biology. However, the complex structure and antigenic diversity of FMDV have posed a major obstacle to the attempts at manipulating the infectious virus by reverse genetic techniques. Here, we report the recovery of infectious FMDV from cDNAs based on an efficient in vivo RNA polymerase I (polI) transcription system. Intracellular transcription of the full-length viral genome from polI-based vectors resulted in efficient formation of infectious virus displaying a genetic marker. Compared with wild-type virus, an abundance of genomic mRNA and elevated expression levels of viral antigens were indicative of the hyperfunction throughout the life-cycle of this cDNA-derived virus at transcription, replication, and translation levels. The technology described here could be an extremely valuable molecular biology tool for studying FMDV complex infectious characteristics. It is an operating platform for studying FMDV functional genomics, molecular mechanism of pathogenicity and variation, and lays a solid foundation for the development of viral chimeras toward the prospect of a genetically engineered vaccine.

Construction of an infectious cDNA clone of foot-and-mouth disease virus type O 1 BFS 1860 and its use in the preparation of candidate vaccine

Foot-and-mouth disease virus (FMDV) serotype O is the most predominant among the endemic serotypes in India. A stable,full-length cDNA clone of FMDV type O 1 BFS 1860 preceded by a bacteriophage T7 polymerase promoter was assembled in a plasmid vector pGEM R- - 7Zf(-). An 8.2 kb PCR product was amplified from the cDNA clone and a full-length RNA was generated from it by in vitro transcription.Transfection of BHK-21 cells with the in vitro transcripts resulted in the production of infectious recombinant FMDV particles as evidenced by cytopathic effects (CPE). Further, characterization of the recombinant virus by immunofluorescence, microneutralization test (MNT), antigen ELISA,RT-PCR, plaque assay and electron microscopy revealed similarity to the parental strain. The immunogenicity of an oil-adjuvant vaccine prepared using the inactivated recombinant virus was tested in guinea pigs and cattle. Neutralizing antibodies were produced in both vaccinated guinea pigs and cattle. Vaccinated animals were protected on challenge. The results demonstrated that the recombinant virus was as stable and effective as the parental strain for the preparation of inactivated vaccine, suggesting the potential application of this strategy to make genetically engineered FMDV vaccines.

Translation and replication of FMDV RNA

Foot-and-mouth disease virus (FMDV) RNA is infectious. After delivery of the RNA (about 8.3 kb) into the cytoplasm of a cell, the RNA must initially be translated to produce the viral proteins required for RNA replication and for the packaging of the RNA into new virions. Subsequently there has to be a switch in the function of the RNA; translation has to be stopped to permit RNA replication. The signals required for the control of the different roles of viral RNA must be included within the viral RNA sequence. Many cellular proteins interact with the viral RNA and probably also with the virus-encoded proteins. The functions of different RNA elements within the viral RNA and the various virus-encoded proteins in determining the efficiency of virus replication are discussed. Unique aspects of FMDV RNA translation and replication are emphasised.

Generation of an infectious cDNA clone of an FMDV strain isolated from swine

A full-length cDNA clone of a foot-and-mouth disease virus (FMDV) isolated from swine was assembled in, the plasmid vector pBluescript II SK+ downstream of a T7 promoter. RNA synthesized in vitro using T7 polymerase lead to the production of infectious particles upon transfection of BHK-21 cells, as shown by cytopathic effects. The rescued virus was also found to be highly pathogenic for mice by intradermal injection producing a fatal disease indistinguishable from that of wild-type virus. The availability of this cDNA clone will allow examination of the molecular mechanisms behind FMDV virulence and attenuation, which might in turn allow the production of second-generation, genetically engineered FMDV vaccines.

Foot-and-mouth disease

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals. The disease was initially described in the 16th century and was the first animal pathogen identified as a virus. Recent FMD outbreaks in developed countries and their significant economic impact have increased the concern of governments worldwide. This review describes the reemergence of FMD in developed countries that had been disease free for many years and the effect that this has had on disease control strategies. The etiologic agent, FMD virus (FMDV), a member of the Picornaviridae family, is examined in detail at the genetic, structural, and biochemical levels and in terms of its antigenic diversity. The virus replication cycle, including virus-receptor interactions as well as unique aspects of virus translation and shutoff of host macromolecular synthesis, is discussed. This information has been the basis for the development of improved protocols to rapidly identify disease outbreaks, to differentiate vaccinated from infected animals, and to begin to identify and test novel vaccine candidates. Furthermore, this knowledge, coupled with the ability to manipulate FMDV genomes at the molecular level, has provided the framework for examination of disease pathogenesis and the development of a more complete understanding of the virus and host factors involved.

Molecular basis of pathogenesis of FMDV

Current understanding of the molecular basis of pathogenesis of foot-and-mouth disease (FMD) has been achieved through over 100 years of study into the biology of the etiologic agent, FMDV. Over the last 40 years, classical biochemical and physical analyses of FMDV grown in cell culture have helped to reveal the structure and function of the viral proteins, while knowledge gained by the study of the virus' genetic diversity has helped define structures that are essential for replication and production of disease. More recently, the availability of genetic engineering methodology has permitted the direct testing of hypotheses formulated concerning the role of individual RNA structures, coding regions and polypeptides in viral replication and disease. All of these approaches have been aided by the simultaneous study of other picornavirus pathogens of animals and man, most notably poliovirus. Although many questions of how FMDV causes its devastating disease remain, the following review provides a summary of the current state of knowledge into the molecular basis of the virus' interaction with its host that produces one of the most contagious and frightening diseases of animals or man.

Multiple virulence determinants of foot-and-mouth disease virus in cell culture

Hypervirulent variants of foot-and-mouth disease virus (FMDV) of serotype C arise upon serial cytolytic or persistent infections in cell culture. A specific mutation in the internal ribosome entry site of persistent FMDV was previously associated with enhanced translation initiation activity that could contribute to the hypervirulent phenotype for BHK-21 cells. Here we report that several hypervirulent FMDV variants arising upon serial cytolytic passage show an invariant internal ribosome entry site but have a number of mutations affecting structural and nonstructural viral proteins. The construction of chimeric type O-type C infectious transcripts has allowed the mapping of a major determinant of hypervirulence to the viral capsid. Tissue culture-adapted FMDV displayed enhanced affinity for heparin, but binding to cell surface heparan sulfate moieties was not required for expression of the hypervirulent phenotype in Chinese hamster ovary (CHO) cells. Virulence was identical or even higher for glycosaminoglycan-deficient CHO cells than for wild-type CHO cells. FMDV variants with decreased affinity for heparin were selected from a high-binding parental population and analyzed. Substitutions associated with decreased heparin binding were located at positions 173 of capsid protein VP3 and 144 of capsid protein VP1. These substitutions had a moderate effect on virulence for BHK-21 cells but completely abrogated infection of CHO cells. The comparative results with several FMDV isolates show that (i) increased affinity for heparin and alterations in cell tropism may be mediated by a number of independent sites on the viral capsid and (ii) the same capsid modifications may have different effects on different cell types.

Evolution of a persistent aphthovirus in cytolytic infections: partial reversion of phenotypic traits accompanied by genetic diversification

Foot-and-mouth disease virus (FMDV) shows a dual potential to be cytolytic or to establish persistent infections in cell culture. FMDV R100, a virus rescued after 100 passages of carrier BHK-21 cells persistently infected with FMDV clone C-S8c1, showed multiple genetic and phenotypic alterations relative to the parental clone C-S8c1. Several FMDV R100 populations have been subjected to 100 serial cytolytic infections in BHK-21 cells, and the reversion of phenotypic and genetic alterations has been analyzed. An extreme temperature sensitivity of R100 reverted totally or partially in some passage series but not in others. The small-plaque morphology reverted to normal size in all cases. The hypervirulence for BHK-21 cells did not revert, and even showed an increase, upon cytolytic passage. Most of the mutations that had been fixed in the R100 genome during persistence did not revert in the course of cytolytic passages, but the extended polyribocytidylate tract of R100 (about 460 residues, versus 290 in C-S8c1) decreased dramatically in length, to the range of 220 to 260 residues in all passage series examined. In passages involving very large viral populations, a variant with two amino acid substitutions (L-144-->V and A-145-->P) next to the highly conserved Arg-Gly-Asp (RGD motif; positions 141 to 143) within the G-H loop of capsid protein VP1 became dominant. A clonal analysis allowed isolation of a mutant with the single replacement A-145-->P. Viral production and growth competition experiments showed the two variants to have a fitness very close to that of the parental virus. The results provide evidence that the repertoire of variants that could potentially become dominant in viral quasispecies may be influenced by the population size of the evolving virus. The net results of a series of persistent-infection passages followed by a series of cytolytic passages was progressive genomic diversification despite reversion or stasis of phenotypic traits. Implications for the evolution of RNA viruses are discussed.

Genetically engineered foot-and-mouth disease viruses with poly(C) tracts of two nucleotides are virulent in mice

To determine the role of the poly(C) tract found at the 5' end of the genome of foot-and-mouth disease virus, synthetic RNAs (in vitro transcripts) with poly(C) tracts of different lengths have been produced and evaluated. RNAs with poly(C) tracts of 35, 25, 16, 6, or 2 residues displayed similar specific infectivities in baby hamster kidney (BHK) cells. Viruses recovered from cells transfected with in vitro transcripts containing 6 to 35 Cs had properties similar to those of the wild-type virus in cell culture, and poly(C) tracts present in the synthetic RNA-derived viruses ranged from 75 to 140 bases in length. Viruses recovered from transcripts containing only two Cs showed very different properties. Specifically, viruses grew to much lower levels in cell culture and maintained a poly(C) tract of only two residues. The pool of viruses harvested from cells transfected with the synthetic C2 RNA also contained a small amount of a virus with a 42-base deletion in the region of the poly(C) tract, which appeared to have arisen by recombination. Taken together, these data suggest that recombination provides the mechanism of poly(C) elongation and that viruses with poly(C) tracts over 75 bases in length have a selective advantage in cell culture. Interestingly, all of the in vitro transcript-derived viruses [including viruses with poly(C) tracts of only two residues] were equally virulent in mice, indicating that poly(C) tract length has no effect on virulence in this animal model.

Modifications of the 5' untranslated region of foot-and-mouth disease virus after prolonged persistence in cell culture

The nucleotide sequence of the 5'-untranslated region (5'UTR) of the genome of foot-and-mouth disease virus (FMDV) R100, rescued after 100 passages of persistently infected BHK-21 cells, has been compared with that of the parental FMDV C-S8c1. The nucleotide sequence divergence between the two viruses in heteropolymeric regions is 1%. The few mutations located at the 5'-most terminal region (S fragment) and at the internal ribosome entry site (IRES) do not appear to affect significantly the tight secondary structure predicted for these RNA segments. Comparison of the 5'UTR of C-S8c1 or R100 RNA with that of other FMDV serotypes and subtypes indicates the presence of block deletions (or insertions) which do not correlate with the serological classification of FMDV. Remarkably, FMDV R100, a virus highly attenuated for mice and cattle, contains a polyribocytidylate (poly C) tract of about 420 nucleotides, 145 residues longer than its parental, virulent FMDV C-S8c1. This long poly C of R100 RNA includes a few uridine residues interspersed at fairly regular intervals. This is the longest highly homopolymeric tract described in a viral genome and, to our knowledge, in any informational biomolecule.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

Infectious foot-and-mouth disease virus derived from a cloned full-length cDNA

A full-length cDNA plasmid of foot-and-mouth disease virus has been constructed. RNA synthesized in vitro by means of a bacteriophage SP6 promoter inserted in front of the cDNA led to the production of infectious particles upon transfection of BHK-21 cells. These particles were also found to be highly infectious for primary bovine kidney cells as well as for baby mice. The difficulty in cloning the foot-and-mouth disease virus cytidyl tract in Escherichia coli was circumvented by joining two separate cloned parts, representing the S and L fragments of the genome, and, in a second step, inserting a dC-dG homopolymer. Homopolymeric sequences of up to 25 cytidyl residues did not lead to the production of virus. Replicons containing poly(C) tracts long enough to permit virus replication were first established in yeast cells. One of these constructs could also be maintained in E. coli and was used to produce infectious RNA in vitro. The length of the poly(C) sequence in this cDNA plasmid was 32 nucleotides. However, the poly(C) tracts of two recombinant viruses found in transfected BHK-21 cells were 60 and 80 nucleotides long, respectively. Possible mechanisms leading to the enlargement of the poly(C) tract during virus replication are discussed.

Attenuation of Mengo virus through genetic engineering of the 5' noncoding poly(C) tract

The murine cardioviruses, such as the Mengo and encephalomyocarditis viruses, and the bovine aphthoviruses, such as foot-and-mouth disease virus, are distinguished among positive-strand RNA viruses by the presence of long homopolymeric poly(C) tracts within their 5' noncoding sequences. Although the specific lengths (60-350 bases) and sequence discontinuities (for example, uridine residues) that sometimes disrupt the homopolymer have served to characterize natural viral isolates, the biological function of the poly(C) region has never been clear. We now report that complementary DNA-mediated truncation of the Mengo virus poly(C) tract dramatically attenuates the pathogenicity of the virus in mice. Animals injected with viruses with short tracts not only survived inoculation of up to 50 micrograms live virus (10(11) plaque-forming units) but consistently produced high titres of neutralizing antibodies, which conferred long-term immunogenic protection from (normally) lethal virus challenge. We propose that analogous synthetic strains of foot and mouth disease virus could serve as the basis for new attenuated vaccines.

Genomic differences between the diabetogenic and nondiabetogenic variants of encephalomyocarditis virus

Plaque purification of the M variant of encephalomyocarditis (EMC-M) virus resulted in the isolation of two stable variants. One is a highly diabetogenic D variant (EMC-D) and the other is a nondiabetogenic B variant (EMC-B). The cDNA of EMC-D and EMC-B genomes were cloned and seven overlapping cDNA clones were selected to cover the entire genome except the 5'-end 310 bases which were determined by RNA-dependent DNA sequencing and enzymatic RNA sequencing. Each clone was restriction-mapped, subcloned, and sequenced. The genomes of EMC-D and EMC-B are composed of 7829 and 7825 bases, respectively. Both genomes contain a long open reading frame of 6876 nucleotides starting at position 830 on the consensus sequence, which encodes a polyprotein of 2292 amino acids. The sequences of EMC-D and EMC-B differ by two deletions, one insertion, and eight point mutations. The first deletion of 3 nucleotides is located in the 5' poly(C) tract where EMC-B has 127 nucleotides compared with 130 nucleotides in EMC-D. The second deletion in EMC-B involves 2 nucleotides at the 3'-end polyadenylation site. A single base insertion of U occurs at the 5' noncoding region of EMC-B. The eight point mutations are located in the polyprotein coding region. Two are silent and are each located in the structural gene 1B and in the nonstructural gene 2B. The remaining six mutations, one on the L gene and the other five on the 1D gene, introduce respective amino acid changes. It is concluded that the diabetogenic EMC-D viral genome (7829 bases) differs from the nondiabetogenic EMC-B viral genome (7825 bases) by 14 nucleotides out of 7829.

The genomic RNA of diabetogenic encephalomyocarditis virus: characterization and molecular cloning

The RNA of a diabetogenic variant of encephalomyocarditis (EMC) virus (D variant, ifp- phenotype) and a nondiabetogenic variant of EMC virus (B variant, ifp+ phenotype) which were derived from the same virus stock (M strain) have been compared. The size of both genomes is estimated at 7.7 kb. The poly(C) tract of EMC-D is estimated at 144 bases, whereas that of EMC-B is 141 bases in length. The untranslated 5' terminal 103 nucleotides are identical for B and D with preservation of a stable terminal hairpin structure. The entire open reading frame of both variants has been cloned and the restriction maps of 12 different enzymes are identical. These maps were compared to a computer-generated restriction map of another strain of EMC virus which has been cloned and sequenced by Palmenberg et al. (1984, Nucleic Acids Res. 12, 2969-2985). Approximately 50% of the restriction sites of the B and D variants had similar locations in this strain of EMC. We conclude that significant genetic variation exists between the M strain (B and D variants) and the Palmenberg strain of EMC virus. However, the B and D variants are very similar at the molecular level and comparison of their nucleotide sequences will be necessary to reveal the basis for their different biological properties.

Biochemical characterization of an aphthovirus type C3 strain Resende attenuated for cattle by serial passages in chicken embryos

We have compared several aspects of an aphthovirus strain attenuated for cattle (C3R-O/E) with the original strain (C3Res) from which it was derived after serial passages in chicken embryos. Biochemical differences detected by protein analysis in regular polyacrylamide gels (SDS-PAGE) and on electrofocusing gels (NEPHGE) suggest the presence of mutations throughout the genome. Changes were located in coat proteins VP1 and VP3 and in the polymerase precursor P100 (P3/ABCD). No other differences were found at the protein level by means of the techniques used. Polypeptide P100 of the attenuated strain showed a faster electrophoretic mobility in SDS-PAGE with respect to that of the wild-type strain, and the change seems to be located on its amino terminus half. Several functional differences were also found between the two viruses. Both strains grew equally well in BHK cells reaching roughly similar titers in plaque assays. However, the wild-type strain maintained its titer in cells of bovine origin (BK), whereas the titer of C3R-O/E strain decreased approximately one log in this cell system; moreover, plaques elicited by the attenuated strain were much smaller than the ones produced by C3Res. A diminution in the rate of RNA synthesis induced by C3R-O/E in BK cells compared with that of the wild-type strain was also detected; this trait was not observed in BHK cells. A delay in the kinetics of RNA synthesis was also detected in this strain. The virus yield of attenuated strain in BK cells was four times lower than in BHK cells.

Isolation and characterization of recombinants between attenuated and virulent aphthovirus strains

A guanidine-resistant mutant of the attenuated strain of aphthovirus type 01 strain Campos and the original wild-type strain were crossed to generate recombinant viruses. Two independently derived recombinant viruses were isolated. One isolate (RI) contained the P1 (structural proteins) gene region of attenuated strain and P3 (polymerase precursor) gene region of the wild-type strain. The other isolate (RII) had a genomic structure complementary to that of RI, this is, P1 of the wild-type strain and P3 of the attenuated virus. Recombinant RII inherited some in vitro phenotypic markers that were characteristic of the attenuated strain, whereas the RI recombinant had in vitro behavior that was similar to that of the wild-type strain. The data obtained suggest that the polymerase precursor (P3) of the attenuated strain (01 Campos) could be involved in the determination of the attenuated phenotype for fetal bovine kidney cells and, eventually, for cattle.

Biochemical characterization of a foot-and-mouth disease virus strain attenuated for cattle. Brief report

Wild-type, virulent (A-24 Cruzeiro subtype) foot-and-mouth disease virus (FMDV), a related attenuated strain and revertants of the attenuated strain were examined by titration on primary bovine kidney (PBK) and baby hamster kidney (BHK-21) cells, as well as, by infection of unweaned mice. Wild type virus grew equally well in all three systems, whereas the attenuated strain had a titer 2-3 log lower in PBK cells than in the other 2 assays. Within 9 successive passages in BHK-21 cells the attenuated strain gave rise to revertants that had regained the growth properties of wild-type virus in PBK cells. After cloning of the attenuated strain by plaque isolations, the same revertant phenotype was obtained within 9 successive passages. Oligonucleotide mapping indicated that the attenuated strain differed from the wild-type and revertants by at least one additional oligonucleotide. Differences in poly(C) length were not found among any of the three strains of FMDV. These results correlate attenuation and virulence with point mutation(s) and not with deletions. Possible reversions in nature with this attenuated strain may be anticipated.

Biochemical characterization of an aphthovirus type 0(1) strain campos attenuated for cattle by serial passages in chicken embryos

The biochemical properties of a virulent and an attenuated strain of foot-and-mouth disease virus (FMDV) Type 0(1) Campos (0(1)C) were compared in order to establish differences that could account for their altered biological functions. The avirulent strain (0(1)C-O/E) was derived from the virulent strain 0(1)C by serial passages in chicken embryos. Analysis of the RNase T1-generated oligonucleotides of the viral RNA through one- and two-dimensional (2D) gel electrophoresis (fingerprints) revealed a few changes in the genome structure of the 0(1)C-O/E strain compared to the wild type strain. In addition there was a significant decrease in the length of the poly(C) rich tract of the 0(1)C-O/E RNA. All virion structural proteins, except VP4, their precursors, and the viral RNA polymerase (p56a) show charge differences. In addition a significant decrease in the apparent molecular weight of polypeptide p100 (primary translational product from the 3' end region of the genome) of the attenuated strain was observed.

Applications of Oligonucleotide Fingerprinting to the Identification of Viruses

This chapter focuses on applications of oligonucleotide fingerprinting to the identification of viruses. Fingerprinting is a technique by which oligonucleotides, produced by cleavage of RNA molecules with specific ribonucleases, are separated in two dimensions. It is a definitive method of identifying RNA viruses according to their genotypes. It is not subject to the problems of antigenic drift or antigenic convergence that complicate serological identification. Furthermore, it provides a semiquantitative means of following the evolution of viral genomes in nature. Because all regions of the genome are represented by the large diagnostic oligonucleotides, a survey of the total genomic changes can be monitored. Fingerprinting has two limitations as a diagnostic tool. First, although highly definitive, fingerprinting is not as rapid or inexpensive as serological techniques and cannot be as easily scaled up for routine identification of a large number of samples. Second, the evolutionary range of fingerprinting is short and relationships may not be evident for isolates of rapidly evolving viruses obtained over long intervals. However, these limitations are not large, compared to the full benefits offered to the virologist by the fingerprinting method.

Application of RNase T1 one- and two-dimensional analyses to the rapid identification of foot-and-mouth disease viruses

The analysis of several isolates of foot-and-mouth disease virus by RNase T1 fingerprinting of the 32P-labeled RNA is described. It has been shown that use of the 35S induced RNA instead of the virus particle RNA has two advantages. (i) About 40 times more radioactivity is incorporated into the induced RNA. (ii) The RNA can be prepared much more rapidly, thus increasing the value of the technique in rapid diagnosis. One-dimensional maps, in which the RNase T1 oligonucleotides are separated according to size, have been shown to provide a valuable screening method for distinguishing between viruses. Those viruses giving similar one-dimensional maps also gave similar two-dimensional maps. The value of using the length of the polycytidylic acid tract of foot-and-mouth disease virus as a diagnostic tool is also discussed.

Picornaviral structure and assembly

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Location of the initiation site for protein synthesis on foot-and-mouth disease virus RNA by in vitro translation of defined fragments of the RNA

An mRNA-dependent reticulocyte lysate has been used to translate foot-and-mouth disease virus RNA in vitro. Polypeptides P16, P20a, and P88, which have been shown to be derived from the 5' end of the RNA by pactamycin mapping experiments with infected cells, were preferentially synthesized in vitro. Removal of VPg, the small protein covalently linked to the 5' end of the genome RNA, had no effect on the translation of the RNA. The two RNA fragments (L and S) produced by specific digestion of the polycytidylic acid [poly(C)] tract with RNase H were also translated in vitro. The L fragment, consisting of RNA to the 3' side of the poly(C) tract and including the polyadenylic acid [poly(A)] tract, directed the synthesis of the same products as those made by full-length RNA. However, no small defined products were produced when the S fragment, which contains the 5' end of the RNA, was translated. These results show that the major initiation site for protein synthesis on foot-and-mouth disease virus RNA is to the 3' side of the poly(C) tract. Furthermore, the use of N-formyl [35S]methionine tRNAfMet as a label for the initiation peptides showed that the major polypeptide labeled in lysates primed with both full-length RNA and the L fragment was P16, i.e., the protein nearest the initiation site for translation as deduced from pactamycin mapping experiments. Fragments of RNA were also translated in vitro. Those containing the poly(C) tract gave products similar to those produced when full-length RNA was translated. The polypeptides synthesized when fragments containing the poly(A) tract were used, however, did not resemble those made from full-length RNA.

Enzyme-labelled immunosorbent assay techniques in foot-and-mouth disease virus research

The indirect ELISA technique has been developed successfully to measure antibodies to foot-and-mouth disease virus (FMDV) in cattle sera. Preliminary studies using a standard serum assay show that reproducible results are obtained. The method should prove useful for the examination of antibody titres in sera from large numbers of cattle or other animals.

Ribonuclease activities associated with purified foot and mouth disease virus

Ribonuclease activities internally and externally associated with purified foot-and mouth disease virus were detected. The outer activity was easily removed by cesium chloride or by detergent (Sarkosyl). The inner activity is not removable by any procedure used and could be the enzyme responsible for the heterogeneity normally observed in the extracted FMDV-RNA. It is not known at present if both activities are related to the same or to different enzymes.