Primer-independent initiation of RNA synthesis by SeMV recombinant RNA-dependent RNA polymerase

Sesbania mosaic virus (SeMV), a single-strand positive-sense RNA plant virus, belongs to the genus Sobemoviruses. Mechanism of replication in Sobemoviruses is poorly understood. In the present study, SeMV RNA-dependent RNA polymerase (RdRp) was overexpressed and purified as a thioredoxin-tagged protein. The recombinant SeMV RdRp could synthesize RNA from genomic or subgenomic RNA templates, even in the absence of the protein primer, VPg. Analysis of the product indicated that it was double-stranded and that the mode of initiation was de novo. Mutational analysis of the 3′ UTR of subgenomic RNA revealed that a stem–loop structure at the 3′ end was important. Further, analysis of this stem–loop showed that the SeMV RdRp was capable of recognizing stem–loop structures of various lengths and forms. These results demonstrate that the SeMV RdRp is capable of primer-independent RNA synthesis in vitro.