Cleavage and polyadenylation of mRNAs in eukaryotic cells (see also Figure 28.29 on page 1096). Cleavage-and-polyadenylation specificity factor (CPSF) binds to an upstream AU-rich polyadenylation (polyA) signal. Cleavage stimulatory factor (CStF) interacts with a downstream GU- or U-rich sequence, stabilizing the complex that also includes cleavage factors CFI and CFII. Binding of polyA polymerase (PAP) then stimulates cleavage at the polyA site, which is usually 10-35 nucleotides 3' of the upstream polyA signal. The cleavage factors are released, as is the downstream RNA cleavage product, which is rapidly degraded. Bound PAP adds about 12 A residues at a slow rate to the 3'-hydroxyl group generated by the cleavage reaction. Binding of polyA-binding protein II (PABII) to the initial short polyA tail accelerates the rate of addition by PAP. After 200-250 A residues have been added, PABII signals PAP to stop polymerization.