Mechanism of DI RNA formation in tombusviruses: dissecting the requirement for primer extension by the tombusvirus RNA dependent RNA polymerase in vitro.

Tombusviruses, which are positive-strand RNA viruses of plants, frequently generate defective interfering (DI) RNAs that consist of three to four noncontiguous segments of the parental RNA. Replicase jumping was postulated to cause multiple deletions leading to the de novo formation of DI RNAs in planta. This model was tested using a partially purified RNA-dependent RNA polymerase (RdRp) preparation from tombusvirus-infected plants in vitro. The tombusvirus RdRp was capable of primer extension without the need for sequence complementarity between the primer and the acceptor template in vitro, although the most efficient primer extension was obtained with primers forming a 5-bp duplex with the acceptor region. Primers forming 14- to 20-bp duplexes with the acceptor region were used less efficiently by the tombusvirus RdRp in vitro. In addition, primers with 3' noncomplementary nucleotides were also extended by the tombusvirus RdRp, albeit with a reduced efficiency. The preference of the tombusvirus RdRp for short base-paired primers in vitro is consistent with the lack of extended sequence similarities at the junction sites in the de novo generated tombusvirus-associated DI RNAs. The in vitro experiments also revealed that the acceptor region plays a significant role in primer extension. Comparison of tombusvirus-derived, heterologous and artificial acceptor regions revealed that the conserved regions present in DI RNAs are the best acceptor regions when they are available in the minus-strand orientation. These data suggest that recombination/deletion events may be more frequent at some regions, rather than occurring randomly throughout the parental genome. In addition, these findings support a model that predicts a higher frequency of replicase jumping, i.e., recombination/deletion events, during plus-strand synthesis than during minus-strand synthesis.