What makes a strong polyadenylation signal a strong signal? I was wondering why is it that "AAUAAA" is a strong Polyadenylation signal whereas the rest of the polyadenylation signals show reduced cleavage and polyadenylation efficiency?

_Just expanding on aandreev's comment._

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Polyadenylation is initiated by binding of CPSF (Cleavage and Polyadenylation specificity factor) after its binding to the `AAUAAA` motif.
From the introduction of this article:

> CPSF (cleavage and polyadenylation specificityfactor) binds to the AAUAAA hexamer via its 160-kDa subunit and possibly its 30-kDa subunit (Keller et al., 1991; Murthy & Manley,1992; Jenny et al.,1994; Barabino et al.,1997). CstF (cleavage stimulation factor) binds to the U- or G/U-rich element downstream of the cleavage site, and stabilizes the binding of CPSF to the AAUAAA sequence of the pre-mRNA (reviewed in Colgan & Manley,1997; Wahle & Kuhn,1997).

Any alteration in the CPSF binding site will lead to reduced polyadenylation efficiency. Polyadenylation efficiency also depends on other factors such as secondary structure and presence of proximal CstF sites.