Poly(A) signal sequence is
Polyadenylation signal in nascent messenger RNA consists of highly conserved hexamer AAUAAA located ten to thirty nucleotides upstream of CA dinucleotide cleavage site. Recognized by WDR33 and CPSF30 subunits of cleavage and polyadenylation specificity factor CPSF through base stacking in zinc finger pockets. Mutation of AAUAAA to AAGAAA drastically reduces cleavage efficiency and causes read-through transcription. Auxiliary upstream sequence elements USE enriched in uridine and downstream GU-rich downstream sequence element DSE bound by CstF enhance recognition. Identical DNA motif is AATAAA.
Ref: Berg et al., Biochemistry, 9th ed., Chapter 28: Poly(A) signal AAUAAA; Lodish 9th ed., CPSF recognition of AAUAAA hexamer