CPSF protein

In vitro experiments show that correct modification of the 3 -end requires at least three protein factors the CPSF protein, the poly-A polymerase and the poly-A binding protein. The CPSF protein (CPSF cleavage and polyadenylation specificity factor) binds to the AAUAA signal and brings the poly-A polymerase to the polyadenylation site. The poly-A polymerase is supported by the poly-A binding protein. The latter binds to the poly-A sequence and is required for the transition from the phase of synthesis of short poly-A sequences to the formation of mature poly-A sequences (ca. 200 A-residues). 

Much less is known about termination of transcription in eukaryotes than in bacteria. Once transcribed, specific termination sequences of mRNA are bound by protein complexes (such as CPSF cleavage and poly(A)denylation specificity factor, and CstF cleavage stimulation factor) that process cleavage of the message and subsequent poly(A)denylation. RNA polymerase continues transcription for up to several hundred nucleotides before dissociating from the DNA. 

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