Host involvement in the replication of potato spindle tuber viroid and the evolutionary relationship between plant viroids and the hepatitis delta virus
The present study examines the interaction between host RNA polymerase II (RNAP II) and potato spindle tuber viroid (PSTVd), with the goal of locating and characterizing a putative RNAP II promoter within the viroid's RNA genome. By using a co-immunoprecipitation approach coupled with deletion and mutational analysis, RNAP II was shown to specifically bind the left terminal hairpin loop of PSTVd(+) RNA. The interaction with RNAP II appears to be dependent on PSTVd secondary structure features,
... tructure features, rather than a particular sequence. These findings provide direct evidence of association between RNAP II and PSTVd RNA, and render a unique example of a possible RNA promoter for RNAP II. The second part of the study examines the evolutionary relationship between viroids and the hepatitis delta virus (HDV), as these pathogens share key structural and functional characteristics. We conclude, based on infection experiments, that HDV and viroids share common strategies and host factors to fulfill their respective life-cycles. We found that both HDV and an HDV mutant lacking the HDAg protein-coding region (miniHDV) can replicate in a plant host. However, miniHDV and PSTVd can replicate in human cells only in the presence of the small delta antigen (HDAg-S). Together, these results provide support for the hypothesis that HDV evolved from a viroid-like element through the capture of a cellular transcript necessary for its adaptation to a human host.