Two Anomalous Tobravirus Isolates: Evidence For RNA Recombination in Nature

Abstract

SUMMARY16 and N5 are naturally occurring tobravirus isolates that produce symptoms in herbaceous plants similar to those induced by strains of tobacco rattle virus (TRV). In immunosorbent electron microscopy tests, however, they reacted with antisera to particles of pea early-browning virus (PEBV), not TRV. Furthermore, these tests indicated that 16 was related to the British serotype of PEBV and N5 to the Dutch. Pseudo-recombinant isolates were produced by reassortment of the genome parts of 16 or N5 with those of TRV, in any combination, but not in most combinations with those of PEBV. However, I6 RNA-2 was replicated in plants inoculated also with RNA-1 from an isolate of the British serotype of PEBV, but the PEBV RNA-1 was imperfectly packaged by 16 coat protein, and the virus particles seemed to have only limited stability. Nucleic acid hybridization experiments showed that the RNA-1 sequences of both I6 and N5 were similar to those of TRV strains. I6 RNA-2 contained sequences resembling those of the British serotype of PEBV, but with some TRV-like sequences at the 3' and 5' ends, whereas N5 RNA-2 contained more extensive TRV-like 3' and 5' ends flanking sequences that were related, but perhaps not closely, to those of the Dutch serotype of PEBV. Thus, the RNA-2 species of 16 and N5 were recombinant molecules that contained sequences typical of both TRV and PEBV, and which probably had separate but similar evolutionary origins. As a result of their hybrid nature, I6 and N5 were part of the gene pool and had the pathogenicity of TRV, while possessing the serological properties of PEBV.

The Complete Nucleotide Sequence of Tobacco Rattle Virus RNA-1

“…Hybridization studies show that both have TRV-like RNA-1 species but their coat proteins are antigenically related to PEBV. The 3' terminal homology between SYM RNA-I and RNA-2 is less than 100 nucleotides (Robinson et al,, 1987). It is possible, therefore, that the essential regions for minus strand replication are contained in this short 3' sequence.…”

“…However PRV RNA-2 had a 3' non-coding region of 554 bases of which only the 3' terminal 45 nucleotides showed any homology (78 ~) to the SYM, PSG and TCM RNA sequences. Robinson et al (1987) compared the RNA species of various tobravirus strains by hybridization methods and detected significant homology between the 5' terminal regions of RNA-1 and RNA-2. Similarly, the sequence of SYM RNA-1 was found to be homologous with the 5' terminus of RNA-2 from strains PSG and TCM (Angenent et al, 1986).…”

“…The mechanism of such a process might involve RNA recombination in which the terminal homologies ( Fig. 5 and Robinson et al, 1987) help align the two molecules. RNA recombination has been shown in an experimental situation with BMV (Bujarski et al, 1986) and is the probable explanation for the evolution of tobraviral RNAs, such as N5, which are hybrid between PEBV and TRV.…”

A model for the generation of tobacco rattle virus (TRV) anomalous isolates: pea early browning virus RNA-2 acquires TRV sequences from both RNA-1 and RNA-2

“…TRV isolates I6 and N5 are considered anomalous because they appear to be exceptions to this rule . Despite having RNA-1 molecules homologous to those of other TRV isolates and resembling TRV strains in other ways, they produce virus particles serologically closely related to those of PEBV and appear to have PEBV-derived RNA-2 molecules (Harrison et al, 1983;Robinson et al, 1987;Van Hoof et al, 1966). Thus they apparently represent TRV RNA-1/PEBV RNA-2 pseudorecombinants.…”

Transgenic resistance genes from nepoviruses: Efficacy and other properties

“…There are strong reasons to accept that recombination between ssRNA genomes of viruses has facilitated virus evolution (e.g., Bujarski & Kaesberg 1986;Robinson et al 1987;Chang et al 1988;van der Kuyl et al 1991;Gal et al 1992). The fact that numerous viruses (including CLRV; Scott et al 1992) have short base sequences in common with plants may also imply that recombination has occurred between virion and host sequences.…”