The enzymes for genome size increase and maintenance of large (+)RNA viruses

With sizes <50 kb, viral RNA genomes are at the crossroads of genetic, biophysical, and biochemical stability in their host cell. Here, we analyze the enzymatic assets accompanying large RNA genome viruses, mostly based on recent scientific advances in Coronaviridae. We argue that, in addition to the presence of an RNA exonuclease (ExoN), two markers for the large size of viral RNA genomes are (i) the presence of one or more RNA methyltransferases (MTases) and (ii) a specific architecture of the RNA-dependent RNA polymerase active site. We propose that RNA genome expansion and maintenance are driven by an evolutionary ménage-à-trois made of fast and processive RNA polymerases, RNA repair ExoNs, and RNA MTases that relates to the transition between RNA- to DNA-based life.