Summary auto-generated
Ovine adenovirus (OAV287) is a phylogenetically distinct adenovirus prototype that appears to lack virus-associated (VA) RNA genes. While all known adenoviruses typically contain one or two VA RNA genes important for subverting host cell defenses during replication, OAV's genome arrangement—where the terminal protein and 52/55K protein genes overlap—provided no space for VA genes in their typical location. Researchers used multiple approaches to determine whether VA genes exist elsewhere in the OAV genome. In vitro transcription of OAV genome fragments with nuclear extracts containing RNA polymerase III activity produced ~120 bp products from two widely separated genomic regions. However, when these regions were used as probes to analyze RNA from infected cells via Northern blotting and RNase protection assays, no corresponding VA-like RNA was detected. Direct radiolabeling and analysis of small RNA species in infected versus uninfected cells also failed to identify any novel VA RNA. Additionally, protein synthesis studies showed that OAV infection did not result in preferential translation of viral proteins relative to host proteins—unlike human adenoviruses with functional VA genes. Comparison with the phylogenetically related egg drop syndrome virus, which contains a VA gene in a region absent from OAV, further supports this conclusion. This represents the first known adenovirus lacking a VA RNA gene.
Key findings
- OAV is the first known adenovirus that lacks a virus-associated (VA) RNA gene, which are otherwise universally present in all other characterized adenovirus genomes
- Multiple experimental approaches (in vitro transcription, Northern analysis, RNase protection assays, and direct RNA labeling) consistently failed to detect VA RNA or VA-like RNA species in OAV-infected cells
- The absence of VA RNA correlates with inefficient preferential translation of viral proteins during OAV infection compared to human adenoviruses, which typically produce abundant VA RNA to enhance viral protein synthesis
- OAV's genome structure, with overlapping terminal protein and 52/55K protein genes, leaves no space for VA genes in their typical genomic location
- Comparative genome analysis with the related egg drop syndrome virus confirms that the VA gene present in EDS is located in a genomic region entirely absent from OAV
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Abstract
Ovine adenovirus OAV287 (OAV) is the prototype of a virus group which is phylogenetically distinct from the mastadenoviruses and aviadenoviruses. The genome arrangement of OAV showed that virus- associated (VA) RNA genes were not located between the reading frames for p52/55K and terminal protein as these overlapped. To determine whether VA genes were located elsewhere, several approaches were used. Nuclear extracts containing RNA polymerase III activity were used to transcribe OAV genome fragments in vitro. A product of approximately 120 bp was produced from two widely separated coding regions of the genome. However, when these were subcloned and used as radiolabelled probes to analyse RNA from OAV-infected cells, no VA-like RNA was detected, although late mRNAs that were transcribed from the regions were identified. In addition, 5' radiolabelling of small RNA species in control- and OAV-infected cells followed by gel analysis did not identify candidate VA RNAs. Radiolabelling of proteins in control- and OAV-infected cells showed that there was little preferential translation of viral proteins compared with host polypeptides, in contrast to the situation for adenovirus 5 (Ad5). In addition, the kinetics of OAV infection were slower than observed for human adenoviruses. Collectively, the data suggest that OAV is unique in that it does not produce VA RNA during infection. This conclusion is supported by a comparison of the genomes of the phylogenetically related OAV and egg drop syndrome viruses which shows that the VA gene identified in the latter is located in a region absent from OAV.