The Coronavirus Replication–Transcription Complex

) include major human and animal pathogens. They have exceptionally large RNA genomes and use complex strategies to replicate and express these genomes. Intensive research activities in recent years have significantly advanced our knowledge of the molecular mechanisms involved in coronavirus RNA synthesis. Here, we briefly review these mechanisms and focus in particular on the structures and functions of the core replication-transcription complex (RTC) and other enzyme functions that can be recruited to this complex to fulfil additional functions, for example, in the context of 5' capping of viral mRNAs or in the context of mechanisms that control the processivity, replication fidelity, and backtracking of RTCs. Some of these recent studies provided fundamentally new insight into specific roles of previously identified genetic markers of coronaviruses and other nidoviruses, including specific functions in an unconventional RNA capping mechanism and potential roles in proofreading and discontinuous negative-strand RNA synthesis.