The genome of the Cauliflower mosaic virus, a plant pararetrovirus, is highly methylated in the nucleus

Abstract

Cytosine methylation is an important defense against invasive DNAs. Here, cytosine methylation profiles of a plant pararetrovirus, Cauliflower mosaic virus (CaMV), were investigated. Nuclear CaMV DNA is highly methylated throughout the genome including at transcription regulatory regions, but the virion DNA is unmethylated. In vitro CG methylation of the viral 35S promoter reduces transcription from the downstream gene. Although nuclear CaMV DNA is highly methylated, its transcripts are accumulated in the nucleus. The data suggest that a small population of unmethylated viral genomes produced through reverse transcription are constantly delivered back to the nucleus. Small RNA profiles suggest that methylation of the CaMV DNA may be due to de novo methylation through 21-, 22-, and 24-nt small RNAs with adenines at their 5' terminus.