RNA polymerase I and III: similar yet unique

•Cryo-EM structures of unbound and elongating Pol III complete the gallery of eukaryotic RNA polymerases.•Transition from dimeric Pol I to transcribing Pol I involves closing of the DNA-binding cleft and folding of the bridge helix.•During Pol I transcription initiation TFIIB-like factor Rrn7 binds more upstream to promoter DNA compared to TFIIB.•Brf2-TBP-DNA complex structure reveals a redox-sensor in Brf2 and suggests a mechanism for the oncogenic activity of Brf2.

The majority of non-protein-coding RNAs present in eukaryotic cells comprises rRNAs, tRNAs and U6 snRNA that are involved in protein biosynthesis and are synthesized by DNA-dependent-RNA polymerase I and III. The transcription cycle (initiation, elongation and termination) has similar principles in all three nuclear RNA polymerases with specific features that are reflected back in their structures. Recently, owing to the 'resolution revolution' in electron cryo-microscopy, there has been a significant advancement in the understanding of these molecular machines. Here, we highlight the structure-function adaptation in specificity and activity of these molecular machines and present parallels and distinctions between their transcription mechanisms.

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