Analysis of cauliflower mosaic virus RNAs in Brassica species showing a range of susceptibility to infection.

Cauliflower mosaic virus (CaMV) is a plant pararetrovirus i.e., a DNA virus that replicates through reverse transcription of its terminally redundant genomic RNA (the 35 S RNA). In this study, the absolute levels and relative ratios of CaMV-encoded RNA species were analyzed in Brassica host plants with different susceptibilities to infection. As reported previously, only very low levels of CaMV RNAs were detected in plants of low susceptibility such as cauliflower. Early in infection, a large proportion of these RNAs were the "short-stop" RNA: a 180-nucleotide RNA generated by mRNA 3' end processing at the first encounter of the polyadenylation [poly(A)] signal rather than at the second encounter by which the 35 S RNA is generated. In contrast, in highly susceptible plants such as turnip, high levels of CaMV RNAs were detected, and the short-stop RNA represented only a small fraction of the RNA. In leaf protoplasts, bypass of the poly(A) signal was similar in all Brassica species. Finally, the ratio of the 19 S RNA, a subgenomic RNA encoding a post-transcriptional trans-activator, to the 35 S RNA was lower in cauliflower than in turnip. These results are discussed in light of the CaMV life cycle.