Polyadenylation at the 3! end

Eukaryotic termination signals have been less
well recognized than the regulators of gene activity
at the 5! end. Eukaryotic primary transcripts
are split by a specific endonuclease
shortly after the sequence AAAUAA. Subsequently,
about 100 –250 adenine nucleotides
are attached to the 3! end of the transcript by
means of a poly(A)-polymerase (polyadenylation).
The poly(A) end binds to a protein. All
mRNAs, except those that code for histone proteins,
possess a poly(A) terminus.