Modern mRNA proofreading and repair: Clues that the Last Universal Common Ancestor possessed an RNA genome?

RNA repair has now been demonstrated to be a genuine biological process and appears to be present in all three domains of life. In this article, we consider what this might mean for the transition from an early RNA-dominated world to modem cells possessing genetically encoded proteins and DNA. There are significant gaps in our understanding of how the modem protein-DNA world could have evolved from a simpler system, and it is currently uncertain whether DNA genomes evolved once or twice. Against this backdrop, the discovery of RNA repair in modem cells is timely food for thought and brings us conceptually one step closer to understanding how RNA genomes were replaced by DNA genomes. We have examined the available literature on multisubunit RNA polymerase structure and function and conclude that a strong case can be made that the Last Universal Common Ancestor (LUCA) possessed a repair-competent RNA polymerase, which would have been capable of acting on an RNA genome. However, while this]ends credibility to the proposal that the LUCA had an RNA genome, the alternative, that LUCA had a DNA genome, cannot be completely ruled out.