- Volume 75, Issue 8, 1994

RNA 3 of rice hoja blanca tenuivirus (RHBV) has 2299 nucleotides and resembles RNA 3 of other tenuiviruses such as maize stripe (MStV) and rice stripe (RStV) viruses in potentially coding with an ambisense strategy for two proteins. Both the viral-sense protein of 23K and the complementary-sense protein of 35K have about 46% amino acid identity with the analogous proteins encoded by RNA 3 of MStV and RStV. As the proteins of MStV and RStV have about 65 % identity between themselves, RHBV cannot be a South and Central American strain of the Asian RStV. The intergenic region resembles those of other tenuiviruses, being rich in A and U residues, but its predicted folding pattern is unlike those of other tenuiviruses. Instead, the predicted folding of the intergenic region was indistinguishable from that of the coding regions and there was no evidence for a distinct hairpin-loop structure. The significance to the evolution of tenuiviruses of the similarities that the two proteins have with their analogues in other tenuiviruses is discussed.

The 3′ non-coding regions (NCR) of RNA1 and RNA2 of blueberry leaf mottle nepovirus (BBLMV) are nearly identical with differences occurring at only four positions. The presence of this 1·4 kb duplication indicates that recombination has occurred at least once in the evolutionary history of BBLMV. Since high mutation rates are common in RNA viruses, strong selection pressure and/or high frequency of recombination must be operating in order to maintain identity in this duplicated region. The possible involvement of high frequency RNA recombination in maintaining identity was investigated. The four conserved differences between the 3′ NCR of RNA1 and RNA2 were used as markers to detect recombinants in a viral population. Nucleotide sequences of BBLMV cDNA clones were compared to the 3′ consensus sequence and deviations were examined to determine whether they were due to single base mutations or recombinational events. No evidence of recombination was found in any of the cDNA clones sequenced and all differences were attributed to mutations. If recombination occurred in the 3′ NCR of BBLMV, the frequency was below 1·1% between markers. The data indicate that identity in the 3′ NCR of RNA1 and RNA2 of BBLMV was maintained without high levels of recombination. The high number of mutations observed in a BBLMV population and lack of observable recombination indicate that other mechanisms, such as selection, play an important role in the conservation of identity in the 3′ NCR.