The relation of poly(A) length to specific infectivity of viral RNA: a comparison of different types of foot-and-mouth disease virus

A cDNA-based reverse genetics system for feline calicivirus identifies the 3' untranslated region as an essential element for viral replication

Virulent systemic feline calicivirus (VS-FCV) is a newly emerging FCV variant that is associated with a severe acute multisystem disease in cats that is characterized by jaundice, oedema, and high mortality (approximately 70%). VS-FCV has spread throughout the world, but there are no effective vaccines or therapeutic options to combat infection. VS-FCV may therefore pose a serious threat to the health of felines. The genomic characteristics and functions of VS-FCV are still poorly understood, and the reason for its increased pathogenicity is unknown. Reverse genetics systems are powerful tools for studying the molecular biology of RNA viruses, but a reverse genetics system for VS-FCV has not yet been reported. In this study, we developed a plasmid-based reverse genetics system for VS-FCV in which infectious progeny virus is produced in plasmid-transfected CRFK cells. Using this system, we found that the 3' untranslated region (UTR) and poly(A) tail are important for maintaining the infection and replication capacity of VS-FCV and that shortening of the poly(A) tail to less than 28 bases eliminated the ability to rescue infectious progeny virus. Whether these observations are unique to VS-FCV or represent more-general features of FCV remains to be determined. In conclusion, we successfully established a rapid and efficient VS-FCV reverse genetics system, which provides a good platform for future research on the gene functions and pathogenesis of VS-FCV. The effects of the deletion of 3' UTR and poly(A) tail on VS-FCV infectivity and replication also provided new information about the pathogenesis of VS-FCV.

Genomic comparison of foot-and-mouth disease virus R strain and its chick-passaged attenuated strain

The present study examined the genomic differences between foot-and-mouth disease virus (FMDV) R strain and its attenuated, chick-passaged (R(304)) strain. Eleven pairs of primers were used to amplify the complete genome of FMDV R and R(304) by RT-PCR. Each fragment was cloned into pMD18-T vector and sequenced. Nucleotide analyses showed that the genome encoding regions of R and R(304) strains open reading frame (ORF) were both 6966 nucleotides (nt) in length, encoding 2322 amino acids. One hundred and ten nucleotides or 32 amino acids were found to be mutated most frequently were in the 3A gene. The next highest rates of mutation were observed in the LP and 1D genes. No mutations were found in either the 2A or 2C genes. The length of 5'IRES region and 3'UTR were 450 nt and 94 nt, respectively. The 5'IRES region and 3'UTR had only 4 nt and 3 nt mutation, respectively after attenuation. The R(304) poly(A) tail length of 18 nt, while that of the R strain was 30 nt. This result demonstrated the primary genomic changes of a FMDV and its attenuated strain, which has important implications in understanding the molecular epidemiology and functional genomics of FMDV.

Rabbit hemorrhagic disease virus poly(A) tail is not essential for the infectivity of the virus and can be restored in vivo

The precise role of the poly(A) tail at the 3'-end of the calicivirus RNA genome is unknown. To study the relationship between the presence of the poly(A) tail and the infectivity and replication of rabbit hemorrhagic disease virus (RHDV), mutants of an infectious cDNA clone of RHDV were constructed, and RK13 cells were transfected with transcripts from these mutants. Transcripts with and without a poly(A) had a fairly similar ability to infect and replicate, suggesting that a long 3'-terminal poly(A) is not essential for infectivity and replication. RT-PCR with specific primers, using viral RNA recovered from RK13 cells transfected with poly(A)-deficient RNA transcripts, showed that the poly(A) tail was restored in vivo.

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.

Nucleotide sequence and construction of an infectious cDNA clone of an EMCV strain isolated from aborted swine fetus

A full-length cDNA clone of an Encephalomyocarditis virus (EMCV) strain (2887A) isolated from aborted swine fetus was constructed and sequenced. Sequence comparison showed more than 99% nucleotide and amino acid sequence identity with two other EMCV strains, EMCV-PV21 and -R. However, the 2887A genomic sequence showed only about 84% nucleotide identity and 96% amino acid identity with EMCV-B, -D and -PV2 variants. RNA synthesized by in vitro transcription of this cDNA clone was infectious upon transfection of BHK21 cells, as shown by cytopathic effects and identification by neutralization test, and by propagation of the virus released into the culture media. The transcript RNA led to the production of infectious particles despite the presence of two nongenomic nucleotide residues at the 5' end, the short poly(C) tract (C(10)TCTC(3)TC(10)), the short poly(A) tail (7A), and the presence of six nongenomic nucleotides at the 3' end. The rescued virus was also found to be highly pathogenic for mice by intra-peritoneal inoculation producing a fatal disease indistinguishable from that of wild-type virus. An important finding concerning the molecular basis of infectivity was that the in vitro synthesized EMCV RNA transcript is infectious, although it contains a very short poly(A). The availability of the infectious cDNA clone of the reproductive failure strain of EMCV should prove to be useful for studying the molecular basis of the pathogenicity of EMCV in pig.

Picornaviral structure and assembly

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