RNA-POLYMERASE-II IS AN ESSENTIAL MESSENGER-RNA POLYADENYLATION FACTOR

Production of messenger RNA in eukaryotic cells is a complex, multiste p process. mRNA polyadenylation, or 3' processing, requires several pr otein factors, including cleavage/polyadenylation-specificity factor ( CPSF), cleavage-stimulation factor, two cleavage factors and poly(A) p olymerase (reviewed in refs 1, 2). These proteins seem to be unnecessa ry for other steps in mRNA synthesis such as transcription and splicin g, and factors required for these processes were not considered to be essential for polyadenylation. Nonetheless, these reactions may be lin ked so that they are effectively coordinated in vivo(3-9). For example , the CTD carboxy-terminal domain of the largest subunit of RNA polyme rase II (RNAP II) is required for efficient splicing and polyadenylati on in vivo(8), and CPSF is brought to a promoter by the transcription factor TFIID and transferred to RNAP II at the time of transcription i nitiation(9). These findings suggest that polyadenylation factors can be recruited to an RNA 3'-processing signal by RNAP II, where they dis sociate from the polymerase and initiate polyadenylation. Here we pres ent results that extend this model by showing that RNAP II is actually required, in the absence of transcription, for 3' processing in vitro .