In a paper in the 15 November 2007 issue of Nature, Lehmann et al. explore in detail the RNA-dependent activity of RNA polymerase II from Saccharomyces cerevisiae. RNA polymerase normally transcribes RNA from a DNA template, but the ability of RNA polymerase to use an RNA template suggests that RNA polymerase could have evolved from an enzyme that replicated viral RNA genomes. It is hypothesized that, during the transition from the RNA world to our DNA-based world, this ancient replication enzyme evolved to use DNA as a template.
There is evidence that RNA polymerase II replicates hepatitis delta virus, which has an RNA genome, but this replication is slow in vitro. The authors speculate that unidentified factors present in the cell increase the processivity of RNA polymerase II when it uses an RNA template.