Molecular Genetics

3' ends of mRNAs are generated by specific cleavage and polyadenylation.

Facts

• Recall that transcription termination often occurs at sites considerably downstream of the sites that, after polyadenylation, are the 3' ends of most eukaryotic mRNAs. Polyadenylation is the non-templated addition of a 50 to 200 nt chain of polyadenylic acid (polyA). Cleavage must precede polyadenylation.
• A consensus AAUAAA was found 10-30 nucleotides 5' of the polyA site and a GU rich and/or U rich element 3' of the site. When AATAAA is deleted from the DNA template, no mature mRNA is made. When the signal is changed to AAGAAA (in adenovirus DNA) transcripts are longer than usual. Deletion of other sequences in the vicinity, including the actual 3' cleavage site, results in normal polyadenylation at the original distance from the AAUAAA signal.
• Transplacement of parts of the sequence surrounding the polyA site from SV40 DNA into the adenovirus E1A gene revealed a requirement for both AAUAAA and GU or U-rich signals for efficient mature 3' end formation.
• A polyA polymerase has been isolated. In vitro transcripts whose 3' ends are at the 3' cleavage site can be polyadenylated by the enzyme. It will not add polyA tails to in vitro RNA transcripts unless those transcripts contain the region surrounding the residue polyadenylated in in vivo transcripts. When addition does occur, it is not to the 3' end of the RNA. Rather, cleavage occurs, making the normal polyadenylation site available to the enzyme. Both cleavage and polymerase activities appear to be catalyzed by the same enzyme.

Interpretations

• The AAUAAA signal is sufficient for polyadenylation if it is located at the appropriate distance from the end of a molecule.
• For cleavage of the RNA, both the AAUAAA and the 3' signal are required.

Further information

• One recognition site can have several adenylation sites. In the adenovirus E3 mRNA polyadenylated sites are 17, 26 or 29 nucleotides beyond AATAAA. Where several alternative polyadenylation sites are found, preference is governed by a third sequence element upstream of AAUAAA.
• PolyA tails are thought to function in mRNA stability and in initiation of translation.
• The formation of proper 3' ends that are polyadenylated is important. For some individuals with hemoglobin deficiencies, the defects were traced to mutations in the AATAAA sequence.
• Cleavage/polyadenylation signals vary in their efficiency. Functioning of low efficiency signals depends on the concentrations of the protein factors. In cells with low concentrations, the low efficiency site is not used. Instead, a downstream higher efficiency site is used. In cells with high concentrations, the high efficiency site is used. This difference is used in the switching of immune system cells from the synthesis of membrane-bound IgM to the synthesis of secreted IgM. That switch results from the presence of two membrane-specific exons between the two signals (ref).
• In flower development of plants a protein complex may direct the polyadenylation/cleavage machinery to a low efficiency signal, thus regulating the timing of flowering (ref)
• Histone mRNAs are not polyadenylated. Their 3' cleavage requires, 5' of the cleavage site, a hairpin following an ACCA sequence and a CAAGAAAGA sequence in the 3' flanking region. The U7 snRNP is required for this maturation.
• SnRNAs U1 to U5 and U7 are transcribed by RNA polymerase II. Their 3' end formation, which may be coincident with RNA polymerase termination, requires a 3' flanking GTTTAAANNNAGA signal. The processing of yeast pre mRNA differs from that in other organisms.
• Cleavage is required also in the processing of non mRNAs. 3'end formation for tRNAs proceeds by a different pathway.
• A nucleotide, or nucleotides, is added to the 5' end of mRNAs. In addition to adding nucleotides to the ends of RNA, nuceltoides are sometimes added to the middle of RNAs or the bases of the RNAs are modified.

This is page 234 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999, 2003