Coronavirus proofreading system

The replication and maintenance of the coronavirus genome, the largest known viral RNA, is a hallmark of nidoviruses. These viruses encode a unique set of RNA-modifying activities that are not present in other viral RNA genomes. One of them is the exonuclese activity of nsp14 (ExoN), related to the DEDD superfamily of exonucleases. In addition to the N-terminal ExoN domain, nsp14 also contains a C-terminal N7-methyltransferase (N7-MTase) domain, which is involved in viral mRNA capping.