RNA polymerase II is an essential mRNA polyadenylation factor

Production of messenger RNA in eukaryotic cells is a complex, multistep process. mRNA polyadenylation, or 3′ processing, requires several protein factors, including cleavage/polyadenylation-specificity factor (CPSF), cleavage-stimulation factor, two cleavage factors and poly(A) polymerase (reviewed in refs 1, 2). These proteins seem to be unnecessary for other steps in mRNA synthesis such as transcription and splicing, and factors required for these processes were not considered to be essential for polyadenylation. Nonetheless, these reactions may be linked so that they are effectively coordinated in vivo3,4,5,6,7,8,9. For example, the CTD carboxy-terminal domain of the largest subunit of RNA polymerase II (RNAP II) is required for efficient splicing and polyadenylation in vivo8, and CPSF is brought to a promoter by the transcription factor TFIID and transferred to RNAP II at the time of transcription initiation9. These findings suggest that polyadenylation factors can be recruited to an RNA 3′-processing signal by RNAP II, where they dissociate from the polymerase and initiate polyadenylation. Here we present results that extend this model by showing that RNAP II is actually required, in the absence of transcription, for 3′ processing in vitro.