The Addition of the PolyA Tail

Most mature eukaryotic mRNAs have from 50 to 250 adenine nucleotides at the 3' end (a poly(A) tail). These nucleotides are not encoded in the DNA but are added after transcription (IFigure 14.7) in a process termed poly-adenylation. Many eukaryotic genes transcribed by RNA polymerase II are transcribed well beyond the end of the coding sequence (see Chapter 13); the extra material at the 3' end is then cleaved and the poly(A) tail is added. For some pre-mRNA molecules, more than 1000 nucleotides may be cleaved from the 3' end.

Processing of the 3' end of pre-mRNA requires sequences both upstream and downstream of the cleavage site (I Figure 14.8a). The consensus sequence AAUAAA is usually from 11 to 30 nucleotides upstream of the cleavage site (see Figure 14.7) and determines the point at which cleavage will take place. A sequence rich in Us (or Gs and Us) is typically downstream of the cleavage site.

In mammals, 3' cleavage and the addition of the poly(A) tail requires a complex consisting of several proteins: cleavage and polyadenylation specificity factor (CPSF); cleavage stimulation factor (CstF); at least two cleavage factors (CFI and CFII); and polyadenylate polymerase (PAP). CPSF binds to the upstream AAUAAA consensus sequence, whereas CstF binds to the downstream sequence (IFigure 14.8b).

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