The fate of RNAs in the nucleus is deter-mined by the polymerase that made them. For example, the same primary transcript that will give rise to a messenger RNA if made by RNA polymerase Ⅱ (pol Ⅱ), will never mature if is made by pol Ⅰ, pol Ⅲ or a bacteriophage RNA polymerase. How can we explain this link between the machines that make and process mRNAs? On page 93 of this issue, Hirose and Manley bring us one step closer to an answer. They report that the carboxy-terminal domain (CTD) — a protein domain unique to pol Ⅱ — acts as part of the protein complex that fashions the 3' end of the mature mRNA by cleavage and polyadenylation.
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