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The Infectious bronchitis virus D-RNA is an RNA element known as defective RNA or D-RNA. This element is thought to be essential for viral replication and efficient packaging of avian infectious bronchitis virus particles.
Coronavirus D-RNA like that of IBV, are produced during high multiplicity of infection and contain cis-acting sequences which are required for viral replication. While it is unclear exactly how IBV D-RNA is made, it is thought to be synthesized in a similar manner as subgenomic mRNA , with most of the genomic sequence left out of the product. Additionally, sg mRNA can also be synthesized from the IBV D-RNA, although the mechanism of that process is still largely unknown.
IBV D-RNA is often used in the reverse genetics approach to experimentally induce heterologous gene expression and site-specific mutagenesis of the coronavirus genome. However, a translation associated sequence , which is normally used to transcribe sg mRNA and is derived from gene 5 of the Beaudette strain of IBV, is needed as a promoter to regulate heterologous gene expression. It is also thought that TAS may program some IBV D-RNA to synthesize sg mRNA, which are necessary for homologous gene protein synthesis. In particular, IBV D-RNA CD-61 is used to experimentally produce recombinant IBV vaccines. D-RNA CD-61 was created from the naturally occurring IBV D-RNA CD-91, which is produced by multiple passage of high concentration IBV in chick kidney cells. The IBV D-RNA CD-61 resulted from deletion mutagenesis of CD-91 and lacks much of the genome but retains the sequences necessary for replication and packaging of viral particles in the presence of a helper virus.
One particularly promising method of IBV D-RNA-mediated heterologous gene expression uses the helper virus dependent system to promote IBV immunity. The helper virus identifies and responds to signals within the IBV D-RNA that are responsible for replication and packaging of IBV particles. Those sequences are thought to be contained within the 5’ and 3’ UTRs of the D-RNA. Analysis of the packaged IBV particles revealed that leader sequence switching occurs between the D-RNA and the IBV helper viruses, which was similarly observed in bovine coronavirus. In addition, it was found that the TAS of the IBV D-RNA contained a consensus sequence that can accept the switched leader sequence and can also be involved in the expression of mRNA from D-RNA.