Identification of sites of Infectious Bronchitis Virus RNA synthesis

Infectious bronchitis virus (IBV), an avian gammacoronavirus, is an important pathogen causing significant animal welfare problems and economic losses to the global poultry industry. Positive-strand RNA viruses, including coronaviruses, induce cellular membrane rearrangements during replication forming replication organelles, which are thought to support efficient viral RNA synthesis. IBV replication has been shown to induce the formation of double membrane vesicles (DMVs), zippered ER and tethered vesicles, known as spherules. Although these are proposed to be the site of viral RNA synthesis, this is as yet unconfirmed and is therefore the focus of these studies. Historically, dsRNA has been used as a marker for sites of coronavirus RNA synthesis, however IBV-associated dsRNA and nsp12 (the viral RNA-dependent RNA polymerase) do not colocalise in infected cells. We have determined the cellular location of the viral genome using Fluorescence In Situ Hybridisation (FISH). By comparing the immunofluorescence labelling of cells permeabilised with different detergents, we have demonstrated that dsRNA can be found within membrane-protected compartments, while nsp12 is not, indicating that the virus could be isolating the dsRNA in DMVs or spherules, affording protection from the host immune response. By incorporating uridine analogues over the course of infection with IBV, we have visualised sites of nascent viral RNA synthesis using super-resolution microscopy. This has shown that dsRNA appears to colocalise more strongly with nascent RNA than nsp12. Using these methods as well as looking at the ultrastructural level we are able to begin to discover the location of sites of IBV RNA synthesis.