Endonucleolytic cleavage separates the pre-mRNA into an upstream fragment destined to become the mature mRNA and a downstream fragment that is rapidly degraded. Polyadenylation and cleavage occur concurrently, with complexes co-assembling, cleavage depends on two signals in the RNA, a highly conserved hexanucleotide, AAUAAA, 10 to 30 nucleotides upstream of the cleavage site, and a poorly conserved GU- or U-rich downstream element. Additional sequences, often upstream of AAUAAA, can enhance the efficiency of the reaction. Cleavage occurs most often after a CA dinucleotide. A single gene can have more than one 3' processing site.
Cleavage is preceded by the assembly of a large processing complex, the composition of which is poorly defined. ATP, but not its hydrolysis, is required for assembly. Cleavage depends on a number of protein factor. CPSF, a heterotetramer, binds specifically to the AAUAAA sequence. The heterotrimer CstF binds the GU- or U-rich downstream element.