Changes

===Virus Structure===

===Virus Structure===

Donis, 1995 Dubovi, 1990

Donis, 1995 Dubovi, 1990

The BVDV genome comprises a single strand of positive sense RNA which is around 12.3 kilobases in length³. The genome is read in one 3898-codon open reading frame that contains no non-coding sequences. BVDV polyprotein is translated directly from the ORF and is cleaved by viral and cellular proteinases to form mature viral proteins^{3, 4}.  

The BVDV genome comprises a single strand of positive sense RNA which is around 12.3 kilobases in length³. The genome is read in one 3898-codon open reading frame that contains no non-coding sequences. BVDV polyprotein is translated directly from the ORF and is cleaved by viral and cellular proteinases to form mature viral proteins^{3, 4}. At either end of the ORF, 5’ and 3’ untranslated regions exist. These regions are long, allowing them to accomodate fuctions conferred in eukaryotic DNA by the 5’ cap and the 3' poly-A tail, such as controlling the initiation of translation, facilitating the entry of replicases, and contributing to RNA stability.  

 

The 5’ and 3’ untranslated regions (UTRs) are located at either end of the ORF. Unlike eukaryotic mRNA, BVDV genomic RNA has neither the 5’ methylated cap nor the 3’ poly-A tail at these positions. Instead, the UTRs are significantly long, at approximately 385 and 226 nucleotides for the 5’ and 3’ UTRs respectively (Donis, 1995). This allows them to accommodate a variety of functions normally conferred by the 5’ cap and the poly-A region, including the control of translation initiation and RNA stability. Entry of replicases required to produce viral progeny from the RNA template is also guided by the UTRs.

 

Figure 1.1 also shows the proteins encoded by the BVDV genome. The polyprotein produced is 3898 amino acids long, and has a molecular weight of 438kD (Donis, 1995). The genomic RNA codes both structural and non-structural proteins.  

The first structural protein coded is Npro, which has a protease action that generates the N-terminus of the next protein, C. C packages genomic RNA and provides interactions necessary for formation of the enveloped virion. Erns, E1 and E2 are all glycoproteins. Erns has an RNase activity that is probably involved in viral replication and pathogenesis (van Gennip, 2005). E1 is membrane-anchored and initiates the translocation of E2 to the virion envelope where it is highly antigenic, causing the production of neutralising antibodies following infection or vaccination (Donis, 1995).  

The first structural protein coded is Npro, which has a protease action that generates the N-terminus of the next protein, C. C packages genomic RNA and provides interactions necessary for formation of the enveloped virion. Erns, E1 and E2 are all glycoproteins. Erns has an RNase activity that is probably involved in viral replication and pathogenesis (van Gennip, 2005). E1 is membrane-anchored and initiates the translocation of E2 to the virion envelope where it is highly antigenic, causing the production of neutralising antibodies following infection or vaccination (Donis, 1995).