RNA2 of TRV SYM breaks the rules for tobravirus genome structure

Currently, all of the RNA2 molecules described for all of the more than thirty sequenced isolates of the three tobraviruses, Tobacco rattle virus (TRV), Pea early-browning virus (PEBV) and Pepper ringspot virus (PepRSV), have the virus coat protein (CP) gene located in the 5' proximal position. However, sequencing of the RNA2 of the SYM isolate of TRV revealed that this isolate has a unique genome structure in which the virus CP gene is located in the central region of RNA2 downstream of three completely novel open reading frames (ORFN1, ORFN2 and ORFN3). An infectious clone of SYM RNA2 was constructed and mutations were introduced separately into each of the novel genes to interrupt their translation. However, none of the mutations resulted in any noticeable change in the ability of TRV RNA1 or RNA2 to replicate and move systemically in the leaves or roots of infected plants. In addition, individual expression of the novel ORFs either from a Potato virus X (PVX) vector or from a binary plasmid in Agrobacterium tumefaciens did not reveal any potential function.