Human rhinovirus serotype 2: in vitro synthesis of an infectious RNA

A novel mechanism of antibody-mediated enhancement of flavivirus infection

Antibody-dependent enhancement of viral infection is a well-described phenomenon that is based on the cellular uptake of infectious virus-antibody complexes following their interaction with Fcγ receptors expressed on myeloid cells. Here we describe a novel mechanism of antibody-mediated enhancement of infection by a flavivirus (tick-borne encephalitis virus) in transformed and primary human cells, which is independent of the presence of Fcγ receptors. Using chemical cross-linking and immunoassays, we demonstrate that the monoclonal antibody (mab) A5, recognizing an epitope at the interface of the dimeric envelope protein E, causes dimer dissociation and leads to the exposure of the fusion loop (FL). Under normal conditions of infection, this process is triggered only after virus uptake by the acidic pH in endosomes, resulting in the initiation of membrane fusion through the interaction of the FL with the endosomal membrane. Analysis of virus binding and cellular infection, together with inhibition by the FL-specific mab 4G2, indicated that the FL, exposed after mab A5- induced dimer-dissociation, mediated attachment of the virus to the plasma membrane also at neutral pH, thereby increasing viral infectivity. Since antibody-induced enhancement of binding was not only observed with cells but also with liposomes, it is likely that increased infection was due to FL-lipid interactions and not to interactions with cellular plasma membrane proteins. The novel mechanism of antibody-induced infection enhancement adds a new facet to the complexity of antibody interactions with flaviviruses and may have implications for yet unresolved effects of polyclonal antibody responses on biological properties of these viruses.

Application of FCS in studies of rhinovirus receptor binding and uncoating

Fluorescence correlation spectroscopy (FCS) allows determining diffusion and relaxation properties of fluorescent molecules. It requires only extremely small amounts of sample, down to picomolar concentrations, in an effective analysis volume of a few femtoliters. In essence, FCS determines the autocorrelation of fluorescence fluctuations caused by single labeled molecules passing through the confocal volume of a microscope equipped with a suitable detector; it permits investigating interactions of (macro)molecules, even in single cells. We present an FCS protocol for exploring, under in vitro conditions, the dynamic processes that take place during the early steps of virus infection. We cover two important issues of rhinovirus research, the kinetics of directional RNA release, and virus-receptor interactions exemplified by using human rhinovirus type A2 (HRV-A2) as a model.

Specificity of human rhinovirus 2A(pro) is determined by combined spatial properties of four cleavage site residues

The 2A proteinase (2A(pro)) of human rhinoviruses cleaves the virally encoded polyprotein between the C terminus of VP1 and its own N terminus. Poor understanding of the 2A(pro) substrate specificity of this enzyme has hampered progress in developing inhibitors that may serve as antiviral agents. We show here that the 2A(pro) of human rhinovirus (HRV) 1A and 2 (rhinoviruses from genetic group A) cannot self-process at the HRV14 (a genetic group B rhinovirus) cleavage site. When the amino acids in the cleavage site of HRV2 2A(pro) (Ile-Ile-Thr-Thr-Ala*Gly-Pro-Ser-Asp) were singly or doubly replaced with the corresponding HRV14 residues (Asp-Ile-Lys-Ser-Tyr*Gly-Leu-Gly-Pro) at positions from P3 to P2', HRV1A and HRV2 2A(pro) cleavage took place at WT levels. However, when three or more positions of the HRV1A or 2 2A(pro) were substituted (e.g. at P2, P1 and P2'), cleavage in vitro was essentially eliminated. Introduction of the full HRV14 cleavage site into a full-length clone of the HRV1A and transfection of HeLa cells with a transcribed RNA did not give rise to viable virus. In contrast, revertant viruses bearing cysteine at the P1 position or proline at P2' were obtained when an RNA bearing the three inhibitory amino acids was transfected. Reversions in the enzyme affecting substrate specificity were not found in any of the in vivo experiments. Modelling of oligopeptide substrates onto the structure of HRV2 2A(pro) revealed no appreciable differences in residues of HRV2 and HRV14 in the respective substrate binding sites, suggesting that the overall shape of the substrate is important in determining binding efficiency.

Fragmentation of the Golgi apparatus provides replication membranes for human rhinovirus 1A

All viruses with a positive-stranded RNA genome replicate their genomic RNA in association with membranes from the host cell. Here we demonstrate a novel organelle source of replication membranes for human rhinovirus 1A (HRV-1A). HRV-1A infection induces fragmentation of the Golgi apparatus, and Golgi membranes are rearranged into vesicles of approximately 250–500 nm diameter. The newly distributed Golgi membranes co-localize with viral RNA replication templates, strongly suggesting that the observed vesicles are the sites of viral RNA replication. Expression of the HRV-1A 3A protein induces alterations in the Golgi staining pattern similar to those seen during viral infection, and expressed 3A localizes to the Golgi-derived membranes. Taken together, these data show that in HRV-1A infection, the 3A protein plays a role in fragmenting the Golgi complex and generating vesicles that are used as the site of viral RNA replication.

Engineering infectious foot-and-mouth disease virus in vivo from a full-length genomic cDNA clone of the A/AKT/58 strain

Two full-length genomic cDNA clones, pTA/FMDV and pCA/FMDV, were constructed that contained three point-mutants [A174G and A308G (not present in pTA/FMDV); T1029G] in the genome compared with the wild type A/AKT/58 strain of foot-and-mouth disease virus. These two viruses were rescued by co-transfection of pCA/FMDV with pCT7RNAP, which can express T7 RNA polymerase in BHK-21 cell-lines, or by transfection of the in vitro transcribed RNA. Their biological properties were analyzed for their antigenicity, virulence in suckling-mice (LD50) and growth kinetics in BHK-21 cells. The in vivo rescued viruses showed high pathogenicity for 3-day-old unweaned mice (LD50=10(-7.5)). However, the in vitro transcribed RNA derived from pTA/FMDV had lower pathogenicity for suckling-mice (LD50=10(-6)), and the in vivo transcribed RNA recovered from pCA/FMDV co-transfected with pCT7RNAP showed no significant differences from the wild type virus. These data showed that recovery of the infectious foot-and-mouth disease virus directly from the use of in vivo techniques was better than from in vitro methods. Furthermore, the reverse genetic procedure technique was simplified to a faster one-step procedure based on co-transfection with pCT7RNAP. These results suggest that in vivo RNA transcripts may be more valuable for engineering recombinant foot-and-mouth disease virus than in vitro RNA transcripts, and may contribute to further understanding of the biological properties, such as replication, maturation and quasispecies, of the foot-and-mouth disease virus.

Site of human rhinovirus RNA uncoating revealed by fluorescent in situ hybridization

By using fluorescent in situ hybridization (FISH), we visualized viral RNA of human rhinovirus type 2 (HRV2) during its entry into HeLa cells. RNA uncoating of HRV2 is entirely dependent on low endosomal pH (< or =5.6). When internalized into cells treated with bafilomycin, which results in neutralization of the endosomal pH, no FISH signal was recorded, whereas in the absence of the drug, fluorescent dots were seen. Therefore, FISH detects the genomic viral RNA only upon its release from the capsid. Free viral RNA was first seen at 10 min postinfection (p.i.) in the perinuclear area of the cell, which is indicative of RNA release in/from late endosomal compartments. Pulse-chase experiments and observation of HRV2 RNA and capsid proteins via microscopy, Western blotting, and reverse transcription-PCR revealed that the RNA signal persisted whereas the protein signal disappeared. This demonstrates transport of capsids to lysosomes and degradation. In contrast, viral RNA that had already been transferred into the cytoplasm escaped lysosomal breakdown as indicated by a persistent FISH signal. Taken together, our results demonstrate by direct means RNA arrival in the cytosol within 10 min p.i. Based on persistence of the FISH signal and productive infection in the presence of the microtubule-depolymerizing drug nocodazole, we localized this process to endosomal carrier vesicles/late endosomes.

Dynamic force microscopy imaging of plasmid DNA and viral RNA

Plasmid DNA and viral RNA were imaged in a liquid environment by dynamic force microscopy (DFM) and fine structures of DNA with heights of 1.82+/-0.66 nm were obtained in topographical images. In simultaneously acquired phase images, DNA could be imaged with better contrast at lower imaging forces. By splitting the cantilever oscillation signal into lower and upper parts, the contribution of the adhesion between tip and sample to the topographical images was eliminated, resulting in better signal-to-noise ratio. DFM of the single stranded RNA genome of a human rhinovirus showed loops protruding from a condensed RNA core, 20-50 nm in height. The mechanical rigidity of the RNA was determined by single molecule pulling experiments. From fitting RNA stretching curves to the Worm-Like-Chain (WLC) model a persistence length of 1.0+/-0.17 nm was obtained.

Effects of picornavirus 3A Proteins on Protein Transport and GBF1-dependent COP-I recruitment

The 3A protein of the coxsackievirus B3 (CVB3), an enterovirus that belongs to the family of the picornaviruses, inhibits endoplasmic reticulum-to-Golgi transport. Recently, we elucidated the underlying mechanism by showing that CVB3 3A interferes with ADP-ribosylation factor 1 (Arf1)-dependent COP-I recruitment to membranes by binding and inhibiting the function of GBF1, a guanine nucleotide exchange factor that is required for the activation of Arf1 (E. Wessels et al., Dev. Cell 11:191-201, 2006). Here, we show that the 3A protein of poliovirus, another enterovirus, is also able to interfere with COP-I recruitment through the same mechanism. No interference with protein transport or COP-I recruitment was observed for the 3A proteins of any of the other picornaviruses tested here (human rhinovirus [HRV], encephalomyocarditis virus, foot-and-mouth disease virus, and hepatitis A virus). We show that the 3A proteins of HRV, which are the most closely related to the enteroviruses, are unable to inhibit COP-I recruitment, due to a reduced ability to bind GBF1. When the N-terminal residues of the HRV 3A proteins are replaced by those of CVB3 3A, chimeric proteins are produced that have gained the ability to bind GBF1 and, by consequence, to inhibit protein transport. These results show that the N terminus of the CVB3 3A protein is important for binding of GBF1 and its transport-inhibiting function. Taken together, our data demonstrate that the activity of the enterovirus 3A protein to inhibit GBF1-dependent COP-I recruitment is unique among the picornaviruses.

The Microbial Rosetta Stone Database: a compilation of global and emerging infectious microorganisms and bioterrorist threat agents

BACKGROUND

Thousands of different microorganisms affect the health, safety, and economic stability of populations. Many different medical and governmental organizations have created lists of the pathogenic microorganisms relevant to their missions; however, the nomenclature for biological agents on these lists and pathogens described in the literature is inexact. This ambiguity can be a significant block to effective communication among the diverse communities that must deal with epidemics or bioterrorist attacks.

RESULTS

We have developed a database known as the Microbial Rosetta Stone. The database relates microorganism names, taxonomic classifications, diseases, specific detection and treatment protocols, and relevant literature. The database structure facilitates linkage to public genomic databases. This paper focuses on the information in the database for pathogens that impact global public health, emerging infectious organisms, and bioterrorist threat agents.

CONCLUSION

The Microbial Rosetta Stone is available at http://www.microbialrosettastone.com/. The database provides public access to up-to-date taxonomic classifications of organisms that cause human diseases, improves the consistency of nomenclature in disease reporting, and provides useful links between different public genomic and public health databases.

Binding of fluorescent dye to genomic RNA inside intact human rhinovirus after viral capsid penetration investigated by capillary electrophoresis

RiboGreen is used for concentration measurements of RNA. Upon binding to the RNA, an approximately 1000-fold increase in sensitivity in comparison with the UV absorbance of the free polynucleotide is observed. In the present work, we demonstrate that this dye can penetrate in a time- and temperature-dependent manner the intact viral capsids of human rhinovirus serotypes 2 and 14, where it forms a fluorescent complex with the viral RNA. Capillary electrophoresis with laser-induced fluorescence detection of virus incubated with RiboGreen shows that the electrophoretic mobility of the viruses remained unchanged upon dye-binding. As shown for human rhinovirus serotype 2, its native conformation was conserved, since it still bound a recombinant soluble receptor fragment derived from the very low density lipoprotein receptor. The labeled RNA was released by heat-induced uncoating of the virus, and the RNA-dye complex could be directly detected if degradation was prevented with an RNase inhibitor. This in vitro labeling of viral RNA encased within a protein shell demonstrates the virion's dynamic nature that temporarily allows access of a low-molecular-mass compound to the otherwise protected RNA. It might be of great value for experiments requiring fluorescent viral particles with an unmodified surface, such as investigations of endocytosis and viral uncoating on the single molecule level.

Biochemical and genetic studies of the initiation of human rhinovirus 2 RNA replication: purification and enzymatic analysis of the RNA-dependent RNA polymerase 3D(pol)

The replication of human rhinovirus 2 (HRV2), a positive-stranded RNA virus belonging to the Picornaviridae, requires a virus-encoded RNA polymerase. We have expressed in Escherichia coli and purified both a glutathione S-transferase fusion polypeptide and an untagged form of the HRV2 RNA polymerase 3D(pol). Using in vitro assay systems previously described for poliovirus RNA polymerase 3D(pol) (J. B. Flanegan and D. Baltimore, Proc. Natl. Acad. Sci. USA 74:3677-3680, 1977; A. V. Paul, J. H. van Boom, D. Filippov, and E. Wimmer, Nature 393:280-284, 1998), we have analyzed the biochemical properties of the two different enzyme preparations. HRV2 3D(pol) is both template and primer dependent, and it catalyzes two types of synthetic reactions in the presence of UTP, Mn(2+), and a poly(A) template. The first consists of an elongation reaction of an oligo(dT)(15) primer into poly(U). The second is a protein-priming reaction in which the enzyme covalently links UMP to the hydroxyl group of tyrosine in the terminal protein VPg, yielding VPgpU. This precursor is elongated first into VPgpUpU and then into VPg-linked poly(U), which is identical to the 5' end of picornavirus minus strands. The two forms of the enzyme are about equally active both in the oligonucleotide elongation and in the VPg-primed reaction. Various synthetic mutant VPgs were tested as substrates in the VPg uridylylation reaction.

Biochemical and genetic studies of the initiation of human rhinovirus 2 RNA replication: identification of a cis-replicating element in the coding sequence of 2A(pro)

We have previously shown that the RNA polymerase 3D(pol) of human rhinovirus 2 (HRV2) catalyzes the covalent linkage of UMP to the terminal protein (VPg) using poly(A) as a template (K. Gerber, E. Wimmer, and A. V. Paul, J. Virol. 75:10969-10978, 2001). The products of this in vitro reaction are VPgpU, VPgpUpU, and VPg-poly(U), the 5' end of minus-strand RNA. In the present study we used an assay system developed for poliovirus 3D(pol) (A. V. Paul, E. Rieder, D. W. Kim, J. H. van Boom, and E. Wimmer, J. Virol. 74: 10359-10370, 2000) to search for a viral sequence or structure in HRV2 RNA that would provide specificity to this reaction. We now show that a small hairpin in HRV2 RNA [cre(2A)], located in the coding sequence of 2A(pro), serves as the primary template for HRV2 3D(pol) in the uridylylation of HRV2 VPg, yielding VPgpU and VPgpUpU. The in vitro reaction is strongly stimulated by the addition of purified HRV2 3CD(pro). Our analyses suggest that HRV2 3D(pol) uses a "slide-back" mechanism during synthesis of the VPg-linked precursors. The corresponding cis- replicating RNA elements in the 2C(ATPase) coding region of poliovirus type 1 Mahoney (I. Goodfellow, Y. Chaudhry, A. Richardson, J. Meredith, J. W. Almond, W. Barclay, and D. J. Evans, J. Virol. 74:4590-4600, 2000) and VP1 of HRV14 (K. L. McKnight and S. M. Lemon, RNA 4:1569-1584, 1998) can be functionally exchanged in the assay with cre(2A) of HRV2. Mutations of either the first or the second A in the conserved A(1)A(2)A(3)CA sequence in the loop of HRV2 cre(2A) abolished both viral growth and the RNA's ability to serve as a template in the in vitro VPg uridylylation reaction.

Nucleotide sequence of classical swine fever virus strain Alfort/187 and transcription of infectious RNA from stably cloned full-length cDNA

The complete nucleotide sequence of the genome of classical swine fever virus (CSFV) strain Alfort/187 was determined from three cDNA libraries constructed by cloning of DNA fragments obtained from independent sets of reverse transcription and PCR. The cDNA fragments were then assembled and inserted downstream of a T7 promoter in a P15A-derived plasmid vector to obtain the full-length cDNA clone pA187-1. The first nucleotide of the CSFV genome was positioned at the transcription start site of the T7 promoter. Cleavage at an SrfI restriction site introduced at the exact 3' end of the cloned viral cDNA allowed the in vitro synthesis of full-length viral RNA by runoff transcription. This RNA proved to be infectious after transfection into porcine kidney cells. Infectivity was not increased after capping of the synthetic RNA. Virus recovered from transfected cells was titrated in porcine kidney cells by endpoint dilution using indirect immunofluorescence and a CSFV-specific monoclonal antibody. RNA transcripts generated from plasmid DNA isolated from bacteria which had been cultured and cloned 10 times remained infectious, indicating that the full-length clone is stable in bacterial cells. A silent point mutation introduced at position 11842 of the genome was retained in the recombinant virus recovered from transfected cells. An infectious chimeric construct was obtained by replacing a 696-bp fragment in pA187-1 with the corresponding cDNA fragment from the CSFV strain CAP. The stably cloned full-length CSFV cDNA allows site-specific mutagenesis of the viral genome and thus will be useful for detailed molecular characterization of the virus as well as for studies of viral pathogenesis.

Capsid coding sequence is required for efficient replication of human rhinovirus 14 RNA

Mechanisms by which the plus-sense RNA genomes of picornaviruses are replicated remain poorly defined, but existing models do not suggest a role for sequences encoding the capsid proteins. However, candidate RNA replicons (delta P1 beta gal and delta P1Luc), representing the sequence of human rhinovirus 14 virus (HRV-14) with reporter protein sequences (beta-galactosidase or luciferase, respectively) replacing most of the P1 capsid-coding region, failed to replicate in transfected H1-HeLa cells despite efficient primary cleavage of the polyprotein. To determine which P1 sequences might be required for RNA replication, HRV-14 mutants in which segments of the P1 region were removed to frame from the genome were constructed. Mutants with deletions involving the 5'proximal 1,489 nucleotides of the P1 region replicated efficiently, while those with deletions involving the 3' 1,079 nucleotides did not. Reintroduction of the 3' P1 sequence into the nonreplicating delta P1Luc construct resulted in a new candidate replicon, delta P1Luc/VP3, which replicated well and expressed luciferase efficiently. Capsid proteins provided in trans by helper virus failed to rescue the nonreplicating delta P1Luc genome but were able to package the larger-than-genome-length delta P1Luc/VP3 replicon. Thus, a 3'-distal P1 capsid-coding sequence has a previously unrecognized cis-active function related to replication of HRV-14 RNA.

Development of a recombinant RNA technique for the construction of chimeric RNA with a long poly(C) tract

The murine cardioviruses and bovine aphthoviruses are distinguished from other (+) strand RNA viruses by their long poly(C) tract in the 5'-noncoding region. The presence of this poly(C) tract has long hampered the construction of full-length cDNA with the complete poly(C) tract, because long poly(dC-dG) homopolymer-containing plasmids are difficult to amplify in bacterial systems. To overcome this problem, we constructed a chimeric RNA by joining the poly(C) region of the viral RNA to the 5'-truncated RNA transcript of the encephalomyocarditis (EMC) virus cDNA. The non-chimeric, recombinant EMC virus with a short poly(C) tract produces recombinant progeny virus, but this is not pathogenic in vivo. On the other hand, the EMC viral RNA chimera with the complete poly(C) tract produces recombinant progeny virus that is pathogenic in vivo. This method of viral RNA construction will be invaluable for functional studies of other cardioviruses and aphthoviruses, as well as for recombinant RNA manipulations.

Role of maturation cleavage in infectivity of picornaviruses: activation of an infectosome

Maturation of picornaviruses involves assembly of a "provirion," which undergoes an autocatalytic cleavage of VP0 to VP2 plus VP4. RNA transcripts from a cDNA clone of human rhinovirus 14 mutated at asparagine 68, one of the residues in the maturation cleavage site, generated normal yields of 150S particles which were noninfectious in the plaque assay because they were unable to initiate a second cycle of infection. These cleavage-defective provirions were otherwise indistinguishable from mature virions in sedimentation coefficient, binding affinity to monoclonal antibodies against neutralization sites IA, II, and III, attachment to HeLa cell receptors, and rate of cell-mediated conformational changes to form 125S A-particles and 80S empty capsids. These results suggest that maturation cleavage is required for the function of a previously undescribed intermediate which transfers packaged RNA across the membrane and into the cytosol. For this hypothetical intermediate, we propose the name infectosome. Since the native virus has a particle/PFU ratio of about 800, such an intermediate will be difficult to find. Mutations at serine 10 in VP2 reduced maturation cleavage to a rate sufficiently slow to show that the infectivity of virus particles increased with the degree of cleavage of VP0 to VP4 and VP2. This article describes the first characterization of a pure form of a picornaviral provirion, and hence the first direct evidence that provirions of picornaviruses lack infectivity.

Initiation of translation of human rhinovirus RNA: mapping the internal ribosome entry site

In order to map the 3' boundary of the segments needed for translation initiation at the correct site on human rhinovirus 2, deletions were made from the 3' end of the viral 5'-untranslated region. These truncated viral segments were placed immediately upstream of a reporter gene, a derivative of the influenza virus NS cDNA, either as monocistronic constructs or as dicistronic constructs in which the upstream cistron was the Xenopus laevis cyclin B2 cDNA. In vitro transcripts of these clones were translated in the rabbit reticulocyte lysate system, with or without supplementation with crude HeLa cell initiation factors, or in a HeLa cell-free system. When the full-length viral 5'-untranslated region was present, the HeLa cell factors strongly stimulated the synthesis of the NS-related polypeptides, especially in the case of the dicistronic mRNAs. Deletions from the 3' end extending up to nt 562 had little effect on translation efficiency or the response to HeLa cell factors, but more extensive deletions resulted in the complete loss of response to these factors, an almost total inhibition of NS synthesis from dicistronic mRNAs, and a partial inhibition in the case of the monocistronic mRNAs. In the case of a deletion extending to nt 554, insertion of a 15 nucleotide residue linker failed to restore efficient translation initiation. We conclude that the essential sequences for internal initiation extend to a point located between nt 554 and 562, and that the ribosome entry site, defined as the most 5'-proximal point where the ribosome can bind in an initiation-competent manner, must lie within 6 residues on either side of nt 562, and certainly not further downstream than nt 568.

Genetic analysis of picornaviruses

During the past year, genetic studies of picornaviruses, vastly facilitated by the application of infectious picornaviral cDNAs and RNAs, have contributed to our understanding of the function of individual picornavirus polypeptides and to the genetic processes that operate in these small RNA viruses. Especially notable were the demonstrations that the RNA-dependent RNA polymerase may have a function in RNA synthesis as an uncleaved precursor polypeptide, and that a mutation in the polymerase can be complemented in trans, in contrast to data obtained from previously studied polymerase mutants. A new in vitro system, in which positive-strand synthesis, negative-strand synthesis and RNA packaging were all observed, will facilitate further studies into the mechanism of these processes.

Human rhinovirus mutants resistant to low pH

Mutants of human rhinovirus serotype 14 (HRV14) with increased resistance to treatment at low pH were obtained by repeated cycles of exposure to pH 4.5 and propagation in HeLa cells. Whereas wild-type virus lost more than 5 logs of infectivity upon incubation at pH 4.3, the three isolates examined were essentially unaffected. Conformational change of the viral capsid upon exposure to low pH was assessed as an increase of hydrophobicity by partition between an aqueous phase and a Triton X-114 phase; the mutants required exposure to a much lower pH to accumulate in the Triton phase than wild-type HRV14. The sequence of the capsid region was determined for three isolates; two isolates were found to have the changes Thr17 to lie in VP2 and Asn 100 to lie in VP1. The third isolate also had the change Thr17 to Ile in VP2; however, in VP1, Asp101 was replaced by Glu. Separate introduction of the mutations into full length cDNA clones of the wild-type sequence of HRV14 showed that only the changes in VP1 were necessary for the increased stability at pH 4.5. The implications of the mutations for the three-dimensional structure of the viral capsid are discussed.

Conditional poliovirus mutants made by random deletion mutagenesis of infectious cDNA

Small deletions were introduced into DNA plasmids bearing cDNA copies of Mahoney type 1 poliovirus RNA. The procedure used was similar to that of P. Hearing and T. Shenk (J. Mol. Biol. 167:809-822, 1983), with modifications designed to introduce only one lesion randomly into each DNA molecule. Methods to map small deletions in either large DNA or RNA molecules were employed. Two poliovirus mutants, VP1-101 and VP1-102, were selected from mutagenized populations on the basis of their host range phenotype, showing a large reduction in the relative numbers of plaques on CV1 and HeLa cells compared with wild-type virus. The deletions borne by the mutant genomes were mapped to the region encoding the amino terminus of VP1. That these lesions were responsible for the mutant phenotypes was substantiated by reintroduction of the sequenced lesions into a wild-type poliovirus cDNA by deoxyoligonucleotide-directed mutagenesis. The deletion of nucleotides encoding amino acids 8 and 9 of VP1 was responsible for the VP1-101 phenotype; the VP1-102 defect was caused by the deletion of the sequences encoding the first four amino acids of VP1. The peptide sequence at the VP1-VP3 proteolytic cleavage site was altered from glutamine-glycine to glutamine-methionine in VP1-102; this apparently did not alter the proteolytic cleavage pattern. The biochemical defects resulting from these mutations are discussed in the accompanying report.