Polyadenylation of mRNA precursors by poly(A) polymerase depends on two 
specificity factors and their recognition sequences. These are cleavage and 
polyadenylation specificity factor (CPSF), recognizing the polyadenylation 
signal AAUAAA, and poly(A) binding protein II (PAB II), interacting with the 
growing poly(A) tail. Their effects are independent of ATP and an RNA 5'-cap. 
Analysis of RNA-protein interactions by non-denaturing gel electrophoresis shows 
that CPSF, PAB II and poly(A) polymerase form a quaternary complex with the 
substrate RNA that transiently stabilizes the binding of poly(A) polymerase to 
the RNA 3'-end. Only the complex formed from all three proteins is competent for 
the processive synthesis of a full-length poly(A) tail.