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Use of Degenerate Primers for Partial Sequencing and RT-PCR-Based Assays of Grapevine Leafroll-Associated Viruses 4 and 5

Double-stranded RNA (dsRNA) was purified from grapevines infected with grapevine leafroll-associated viruses 4 (GLRaV-4) and 5 (GLRaV-5), two putative closteroviruses. Reverse-transcriptase polymerase chain reaction (RT-PCR) was performed on this dsRNA using degenerate oligonucleotides designed to amplify an approximately 550- to 650-nucleotide fragment from the heat shock protein 70 homolog (HSP70) of the known closteroviruses. RT-PCR products of the appropriate molecular weight were gel-isolated and cloned into the plasmid vector pGEM-T. Clones of RT-PCR products generated by using these primers on dsRNA isolated from a plant infected with GLRaV-4 were sequenced. This sequence was used to develop an immunocapture RT-PCR (IC-RT-PCR) detection protocol capable of detecting GLRaV-4. Similar clones were made from dsRNA isolated from a plant infected with GLRaV-5. These clones were also sequenced. The two sequences were compared, and RT-PCR primers were developed that were able to amplify cDNA from both. These experiments demonstrate that degenerate primers that amplify closterovirus HSP70 sequences can be used to successfully generate sequences useful for IC-RT-PCR detection of these viruses. These data also suggest that it is feasible to use HSP70 sequences to design PCR primers capable of more general PCR detection of multiple GLRaV serotypes. Lastly, the presence of closterovirus-like HSP70 sequences in these putative closteroviruses implies that they are indeed members of this taxonomic group.