Abstract
Efficient termination is required for robust gene transcription. Eukaryotic organisms use a conserved exoribonuclease-mediated mechanism to terminate the mRNA transcription by RNA polymerase II (PolII)1-5. Here we report two cryogenic electron microscopy structures of Saccharomyces cerevisiae PolII pre-termination transcription complexes bound to the 5'-to-3' exoribonuclease Rat1 and its partner Rai1. Our structures show that Rat1 displaces the elongation factor Spt5 to dock at the PolII stalk domain. Rat1 shields the RNA exit channel of PolII, guides the nascent RNA towards its active centre and stacks three nucleotides at the 5' terminus of the nascent RNA. The structures further show that Rat1 rotates towards PolII as it shortens RNA. Our results provide the structural mechanism for the Rat1-mediated termination of mRNA transcription by PolII in yeast and the exoribonuclease-mediated termination of mRNA transcription in other eukaryotes.
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