Interactions of the RNA polymerase with the viral genome at the 5′- and 3′-ends contribute to 20S RNA narnavirus persistence in yeast

摘要

20S RNA narnavirus is a positive strand RNA virus found in the yeast Saccharomyces cerevisiae. The viral genome (2514 nucleotides) only encodes a single protein (p91), the RNA-dependent RNA polymerase and does not have capsid proteins to form intracellular virions. The genomic RNA has no 3′ poly(A) tail and perhaps no cap structure at the 5′-end; thus resembling an intermediate of mRNA degradation. The virus, however, escapes the host surveillance and replicates in the yeast cytoplasm persistently. The viral genome is not naked but exists in the form of a ribonucleoprotein complex with p91 in a 1:1 stoichiometry. Here we investigated interactions between p91 and the viral genome. Our results indicate that p91 directly or indirectly interacts with the RNA at the 5′- and 3′-end regions and to a lesser extent at a central part. The 3′-end site is identical to or overlaps with the 3′ cis signal for replication identified previously. The 5′-site is at the second stem loop structure from the 5′-end (nucleotides 72–104), and this structure also contains a cis signal for replication. Analysis of mutants in the structure revealed a tight correlation between replication and formation of complexes. These results highlight the importance of ribonucleoprotein complexes for the viral life cycle. We will discuss implications of these findings especially on how the virus escapes from mRNA degradation pathways and resides in the cytoplasm persistently despite the lack of a protective capsid.