5' and 3' untranslated regions of pestivirus genome: primary and secondary structure analyses

A common structural core in the internal ribosome entry sites of picornavirus, hepatitis C virus, and pestivirus

Cap-independent translations of viral RNAs of enteroviruses and rhinoviruses, cardioviruses and aphthoviruses, hepatitis A and C viruses (HAV and HCV), and pestivirus are initiated by the direct binding of 40S ribosomal subunits to a cis-acting genetic element termed the internal ribosome entry site (IRES) or ribosome landing pad (RLP) in the 5' noncoding region (5'NCR). RNA higher ordered structure models for these IRES elements were derived by a combined approach using thermodynamic RNA folding, Monte Carlo simulation, and phylogenetic comparative analysis. The structural differences among the three groups of picornaviruses arise not only from point mutations, but also from the addition or deletion of structural domains. However, a common core can be identified in the proposed structural models of these IRES elements from enteroviruses and rhinoviruses, cardioviruses and aphthoviruses, and HAV. The common structural core identified within the picornavirus IRES is also conserved in the 5'NCR of the divergent viruses, HCV, and pestiviruses. Furthermore, the proposed structural motif shares a structural feature similar to that observed in the catalytic core of the group 1 intron. The conserved structural motif from these divergent sequences that looks like the common core region of group 1 introns is probably a crucial element involved in the IRES-dependent translation.

Demonstration and genotyping of pestivirus RNA from mammalian cell lines

We examined 20 cell lines of various animal origins for the presence of pestivirus contamination by the reverse transcription-polymerase chain reaction (RT-PCR), and found 15 (75%) cell lines were positive. The RT-PCR products of the 5' untranslated region (UTR) of pestivirus genome were sequenced and subjected to genotyping. Stem-loop structures at three variable regions in the 5' UTR render genotyping of the contaminated pestiviruses. Bovine cell lines tested were all contaminated with genotypes I, II, or III of bovine diarrhea virus (BVDV). Cell lines of canine, feline, and primate origin were contaminated with genotype II of BVDV. Cell line Ch1Es of caprine origin was contaminated with border disease virus (BDV).

Molecular biology of bovine viral diarrhea virus and its interactions with the host

The contributions of pestivirus molecular biology research to our understanding of Bovine Viral Diarrhea Virus (BVDV) biology and disease have been remarkable. Completion of nucleotide sequence information for genomes of NCP and CP-BVDV isolates was an important milestone. Subsequent work on the protein map of BVDV and polyprotein processing pathways paved the way for the interpretation of many other virologic and immunologic studies. Discovery of a correlation between genotype II and virulence (hemorrhagic syndrome) will help to clarify previously controversial data and to improve disease control. Description of multiple pathways of p80 expression in CP-BVDV offered insight into the pathogenesis of mucosal disease. Identification of gp53/ E2 as the target of neutralizing antibodies and source of antigenic hypervariability helped us to understand immunity to BVDV. Collectively, the advances described contribute to the implementation of improved diagnostic and control strategies to reduce losses inflicted by the bovine pestivirus.

Characterization of the hog cholera virus 5' terminus

Hog cholera virus (HoCV) 5' terminus of the ALD and GPE(-) strains were analyzed by using rapid amplification of cDNA end method (5'RACE). An additional nine nucleotides were found at the 5' termini of genomic RNA in the ALD and GPE(-) strains of HoCV. These nine nucleotides were also conserved in BVDV and were suggested to form a hairpin structure at the 5' terminus by computer-assisted analysis. It seems possible that the secondary structure and/or the 5' terminus sequence has a significant role in the HoCV virus genome.

Unusual folding regions and ribosome landing pad within hepatitis C virus and pestivirus RNAs

A statistically significant folding region is identified in the 5' untranslated region (5'-UTR) of hepatitis C virus (HCV), bovine viral diarrhea virus and hog cholera virus. This unusual folding region (UFR) detected in HCV encompasses 199 nucleotides (nt) and coincides with the reported internal ribosome entry site or ribosome landing pad (RLP), as determined by the 5' and 3' deletions [Tsukiyama-Kohara et al., J. Virol. 66 (1992) 1476-1483]. The RNA structure predicted in the UFR of HCV consists of a large stem-loop and a pseudoknot. The proposed structural model is consistent with RNase sensitivity studies [Brown et al., Nucleic Acids Res. 20 (1992) 5041-5045]. Moreover, the structure is highly conserved among these divergent HCV and pestivirus RNAs. The covariation of paired bases in the helical regions offers support for the proposed structural models. The pseudoknot predicted in these UFR shares a similar structural feature to those proposed in the RLP of cardioviruses, aphthoviruses and hepatitis A virus. Based on the common structural motif, a putative base-pairing model between HCV RNA and 18S rRNA, as well as pestiviral RNAs and 18S rRNA are suggested. Intriguingly, the proposed base-pairing models in this study are comparable to those proposed in picornaviruses in terms of their folded shape and location of the predicted complementary sequences between viral RNAs and 18S rRNA. Taken together, we suggest that the common base-pairing model between the UFR detected in the 5'-UTR of pestivirus and HCV and 18S rRNA have a general function in the internal initiation of cap-independent translation.

Pestivirus translation initiation occurs by internal ribosome entry

The role of the 385 nucleotide 5' noncoding region (NCR) in the translation of the pestivirus genome was investigated. In vitro translation of an RNA transcript containing the 5' NCR of the bovine viral diarrhea virus (BVDV) genome followed by the coding sequence of the first gene product (p20) of the BVDV large open reading frame resulted in the synthesis of a 20-kDa polypeptide. Results from hybrid-arrest translation studies identified a region involving a predicted RNA stem-loop structure spanning nucleotides 154-261 within the 5' NCR that was important for p20 synthesis. An additional inhibitory oligonucleotide was complementary to the sequence at the base of this stem-loop and encompassed the initiating AUG at nucleotide 386. Antisense oligonucleotides both upstream and downstream of those that were inhibitory had no effect on p20 translation. RNA from a dicistronic expression vector in which the BVDV 5' NCR was inserted between two reporter genes, CAT and LUC, showed strong expression of the second (LUC) cistron upon in vitro translation. This expression was dramatically reduced in an analogous construct in which nucleotides 173-236 of the 5' NCR were deleted. Similar results were obtained when RNA from these same vectors was evaluated for expression after transfection into BHK cells. These results suggest that the BVDV 5' NCR contains an internal ribosome entry site for translation initiation. This translational mechanism is similar to that shown for hepatitis C virus, further demonstrating the close relationship between viruses of these two genera within the family Flaviviridae.

A conserved helical element is essential for internal initiation of translation of hepatitis C virus RNA

Translation of hepatitis C virus (HCV) RNA is initiated by cap-independent internal ribosome binding to the 5' noncoding region (NCR). To identify the sequences and structural elements within the 5' NCR of HCV RNA that contribute to the initiation of translation, a series of point mutations was introduced within this sequence. Since the pyrimidine-rich tract is considered a characteristic feature of picornavirus internal ribosome entry site (IRES) elements, our mutational analysis focused on two putative pyrimidine tracts (Py-I and Py-II) within the HCV 5' NCR. Translational efficiency of these mutant RNAs was examined by in vitro translation and after RNA transfection into liver-derived cells. Mutational analysis of Py-I (nucleotides 120 to 130), supported by compensatory mutants, demonstrates that the primary sequence of this motif is not important but that a helical structural element associated with this region is critical for HCV IRES function. Mutations in Py-II (nucleotides 191 to 199) show that this motif is dispensable for IRES function as well. Thus, the pyrimidine-rich tract motif, which is considered as an essential element of the picornavirus IRES elements, does not appear to be a functional component of the HCV IRES. Further, the insertional mutagenesis study suggests a requirement for proper spacing between the initiator AUG and the upstream structures of the HCV IRES element for internal initiation of translation.

Rapid characterization of new pestivirus strains by direct sequencing of PCR-amplified cDNA from the 5' noncoding region

Reverse transcription coupled with the polymerase chain reaction (RT-PCR) was used for the rapid laboratory diagnosis of pestivirus infections. A direct DNA sequencing method was developed for the analysis of the amplified cDNA from the 5' noncoding region of the viral genome. 70 pestivirus strains were compared in this study. Sequence analysis allowed the characterization of each isolate as either classical swine fever virus (CSFV), bovine viral diarrhea virus, or border disease virus, respectively. The 48 CSFV strains could be further classified into several subgroups, which correlated either with the geographical origin or the date of the first isolation of the respective isolate.