Transcription fidelity and its roles in the cell

•The Trigger Loop is one of the major determinants of transcription fidelity.•Intrinsic proofreading occurs via transcript-assisted cleavage.•Factor-assisted proofreading takes place via exchange of RNAP active centres.•Misincorporation is a major source of transcription pausing.•Another role of fidelity is the prevention of conflicts with other cellular processes. Accuracy of transcription is essential for productive gene expression, and the past decade has brought new understanding of the mechanisms ensuring transcription fidelity. The discovery of a new catalytic domain, the Trigger Loop, revealed that RNA polymerase can actively choose the correct substrates. Also, the intrinsic proofreading activity was found to proceed via a ribozyme-like mechanism, whereby the erroneous nucleoside triphosphate (NTP) helps its own excision. Factor-assisted proofreading was shown to proceed through an exchange of active centres, a unique phenomenon among proteinaceous enzymes. Furthermore, most recent in vivo studies have revised the roles of transcription accuracy and proofreading factors, as not only required for production of errorless RNAs, but also for prevention of frequent misincorporation-induced pausing that may cause conflicts with fellow RNA polymerases and the replication machinery.