How a small DNA virus uses dsRNA but not RNAi to regulate its life cycle

Gene Regulation and Quality Control in Murine Polyomavirus Infection

Murine polyomavirus (MPyV) infects mouse cells and is highly oncogenic in immunocompromised hosts and in other rodents. Its genome is a small, circular DNA molecule of just over 5000 base pairs and it encodes only seven polypeptides. While seemingly simply organized, this virus has adopted an unusual genome structure and some unusual uses of cellular quality control pathways that, together, allow an amazingly complex and varied pattern of gene regulation. In this review we discuss how MPyV leverages these various pathways to control its life cycle.

The Cre/loxP recombination system for production of infectious mouse polyomavirus

Murine polyomavirus mutants are frequently produced for experimental as well as therapy purposes. Commonly used methods for preparation of mutant viral genomes from recombinant vectors are laborious and give variable yields and quality. We describe an efficient and reproducible Cre/loxP-mediated recombination system that generates polyomavirus genomes from recombinant plasmid in vivo. We designed and constructed two variants of recombinant vectors containing the wild-type polyomavirus genome flanked by loxP homologous sites. The loxP sites were introduced either into the intronic region of early genes or between the two poly(A) signal sites of convergent transcriptional units. After cotransfection of the recombinant plasmids with the Cre-expressing vector into mouse 3T6 cells, we obtained infectious virus from the genome variant containing loxP site in the intronic region, but we failed to isolate any infectious virus from the viral genome containing loxP site between poly(A) signals. We show that the Cre/loxP-based method of polyomavirus production is simple, expedient, and reproducible and works with satisfactory efficiency.

Evidence for ADAR-induced hypermutation of the Drosophila sigma virus (Rhabdoviridae)

Background

ADARs are RNA editing enzymes that target double stranded RNA and convert adenosine to inosine, which is read by translation machinery as if it were guanosine. Aside from their role in generating protein diversity in the central nervous system, ADARs have been implicated in the hypermutation of some RNA viruses, although why this hypermutation occurs is not well understood.

Results

Here we describe the hypermutation of adenosines to guanosines in the genome of the sigma virus--a negative sense RNA virus that infects Drosophila melanogaster. The clustering of these mutations and the context in which they occur indicates that they have been caused by ADARs. However, ADAR-editing of viral RNA is either rare or edited viral RNA are rapidly degraded, as we only detected evidence for editing in two of the 104 viral isolates we studied.

Conclusion

This is the first evidence for ADARs targeting viruses outside of mammals, and it raises the possibility that ADARs could play a role in the antiviral defences of insects.

RNA processing in the polyoma virus life cycle

Not only is gene regulation in polyoma interesting, but it has also proven to be highly informative and illustrative of a number of novel concepts in gene regulation. Of special interest and importance are the mechanisms by which this virus switches from the expression of early gene products to late gene products after the onset of viral DNA replication. This switch is mediated at least in part by changes in transcription elongation and polyadenylation in the late region, and by the formation and editing of dsRNA in the nucleus. In this review we will summarize the regulation of RNA synthesis and processing during polyoma infection, and will point out in particular those aspects that have been most novel.

Gene regulation by sense-antisense overlap of polyadenylation signals

We show here that expression of genes from convergent transcription units can be regulated by the formation of double-stranded RNA (dsRNA) in the region of overlapping polyadenylation signals. The model system employed is the mouse polyomavirus. The early and late genes of polyomavirus are transcribed from opposite strands of the circular viral genome. At early times after infection, the early genes are expressed predominantly. Late gene expression increases dramatically upon the onset of DNA replication, when a major defect in polyadenylation of the late primary transcripts generates multigenomic RNAs that are precursors to the mature late mRNAs. Embedded in these late pre-mRNAs are sequences complementary to the early RNAs that act to down-regulate early gene expression via A-to-I editing of dsRNAs. In this system, the defective polyadenylation, and consequently the production of multigenomic late RNAs, depends on the context, and perhaps also, on the A-to-I editing of the poly(A) signal that overlaps the 3'-end of early transcripts.