Gene I, a potential cell-to-cell movement locus of cauliflower mosaic virus, encodes an RNA-binding protein

Cauliflower mosaic virus (CaMV) is a double-stranded DNA (dsDNA) pararetrovirus capable of cell-to-cell movement presumably through intercellular connections, the plasmodesmata, of the infected plant. This movement is likely mediated by a specific viral protein encoded by the gene I locus. Here we report that the purified gene I protein binds RNA and single-stranded DNA (ssDNA) but not dsDNA regardless of nucleotide sequence specificity. The binding is highly cooperative, and the affinity of the gene I protein for RNA is 10-fold higher than for ssDNA. CaMV replicates by reverse transcription of a 35S RNA that is homologous to the entire genome. We propose that the 35S RNA may be involved in cell-to-cell movement of CaMV as an intermediate that is transported through plasmodesmata as an RNA-gene I protein complex.