Evidence suggestive of Cauliflower mosaic virus transcribing complementary DNA from Cucumber mosaic virus RNA in mixed infection

The aim of this study was to investigate if Cauliflower mosaic virus (CaMV) reverse transcriptase (RT) can make complementary DNA (cDNA) from Cucumber mosaic virus (CMV) genomic RNAs in the mixed infection. First, symptoms of single infection with CaMV or CMV, and dual-infection with CaMV + CMV were investigated in cauliflower ( Brassica oleracea ) plants. CMV symptoms disappeared in the mixed-infection after about 45 days post infection (dpi). The efficiency of the CaMV RT in converting CMV RNA genome to cDNA was investigated by extracting total DNA from the dually-infected plants and subjecting to polymerase chain reaction (PCR) by the use of CMV coat protein (CP)-specific primers which gave the anticipated ~ 675 bp DNA. To further verify the identity of the PCR products, the amplified CMV CP cDNA or CaMV RT DNA were sequenced and the resultant nucleotide data were compared with the counterpart genomic region of other isolates of the viruses available in the Genbank. PCR assays on total DNA from plants with single CMV-infection gave no amplification which ruled out the nonspecific activity of plant endogenous RTs in converting the RNA to cDNA. Also, possible amplification of CMV CP cDNA from traces amount of CMV RNA from the infected plants by a weak reverse transcription activity of Taq DNA polymerase was ruled out by performing PCR on the virus RNA which did not yield any DNA. The results showed CMV-associated symptoms attenuation in the presence of CaMV. This study opens up new insights into the interaction between the viruses. This is the first report, to our knowledge, of cDNA synthesis from an RNA virus by CaMV RT in the mixed-infection.