Restoration of the 3' end of potyvirus RNA derived from Poly(A)-deficient infectious cDNA clones

Poly(A)-deficient full-length cDNA clones of clover yellow vein virus (ClYVV), a member of the genus Potyvirus, were found to be infectious when expressed from the CaMV 35S promoter. The poly(A) tail was replaced with different short sequences and the infectivities of the cDNA constructs were examined. Although the infectivity of the plasmid varied depending on the sequences introduced, all the constructs were infectious. In all cases, progeny viral RNAs from the cDNA clones had an authentic viral sequence at their 3' regions with poly(A) tails and the downstream nonviral sequences were completely lost. However, two minor mutations, a two-nucleotide deletion at the 3' end and a single-nucleotide addition at the second nucleotide position downstream of the poly(A) site, were also observed. The clones of the viral (-) strand RNAs had poly(U) tracts at their 5' ends, suggesting that their synthesis is primed by the poly(U) sequence. It furthermore suggests that the mutations were introduced during or after primary transcription from the cDNA and were maintained during authentic viral replication. Although the mechanism involved is not known, recovery of the poly(A) tail is an essential step for maintaining the infectivity of the viral cDNAs.