Further Evidence that Viroplasms Are the Site of Cauliflower Mosaic Virus Genome Replication by Reverse Transcription during Viral Infection

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Abstract
Cauliflower mosaic virus (CaMV) is a DNA plant virus that replicates its genome through an RNA intermediate. The cytoplasmic step of CaMV DNA replication was studied using a fraction consisting of purified viroplasms, which are virus-specific inclusion bodies accumulating in the infected plant cells. The isolated viroplasms retain a DNA polymerase activity able to synthesize CaMV DNA from endogenous templates. A further characterization of the viral DNA sequences produced in the isolated inclusion bodies indicates that newly synthesized DNA, mostly of polarity opposite to that of viral RNA, is single-stranded and partly associated with RNA by base-pairing. In addition to an RNA-dependent DNA polymerase activity, RNA molecules, which presumably originate from the viral RNA template used for reverse transcription, are found to accumulate in the purified inclusion bodies. Furthermore, a small DNA molecule strongly labelled in the purified fraction has been characterized and corresponds to the CaMV reverse transcription intermediate sa-DNA. These results provide further evidence that the reverse transcription of CaMV RNA occurs in the viroplasms. Additional data are presented which suggest that CaMV replication could occur in virion-related particles.

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