Sequences at the ends of RNA-2 of I6, a recombinant tobravirus.

Tobravirus isolate I6 is an anomalous strain of tobacco rattle virus (TRV), which has biological properties typical of TRV, but which is serologically related to pea early-browning virus (PEBV). Its RNA-2 species is a natural recombinant molecule which contains internal sequences, including the particle protein gene, resembling those of PEBV, but with some TRV-like sequences at each end. Sequencing of RNA-2 of isolate I6 showed that, at the 3' end, it is almost identical to RNA-2 of PEBV strain SP5, and TRV-like sequences are limited to the 25 residues that are common to both viruses. However, unlike PEBV RNA-2, I6 RNA-2 can be replicated by TRV RNA-1-coded enzymes. Thus, specific recognition of RNA-2 by TRV replicase seems not to involve sequences at the 3' end. About 275 residues at the 5' end of I6 RNA-2 are similar to those in RNA-2 of TRV strain TCM, and are jointed to the PEBV-SP5-like subgenomic promoter and coding sequences by about 100 residues of uncertain origin. These results support the idea that I6 RNA-2 originated by recombination between TRV and PEBV RNA-2 molecules, although the exact position of the junction and therefore the mechanism involved are unclear.