poly-A tail

**Overview**: Part of eukaryotic mRNA processing (along with 5' capping and splicing), occurring in the nucleus. Not templated by DNA; added post-transcriptionally.

**Mechanism**: - RNA polymerase II transcribes past the end of the gene. - The polyadenylation signal AAUAAA (located ~10–30 nucleotides upstream of the cleavage site) is recognized by CPSF (cleavage and polyadenylation specificity factor). A downstream GU-rich element is recognized by CstF. - Endonuclease cleaves the transcript ~10–30 nt downstream of AAUAAA. - Poly-A polymerase adds ~200 adenines to the 3' end (no template required). Poly-A binding protein (PABP) then coats the tail. - Transcription termination follows (torpedo model: Xrn2 degrades the remaining RNA and displaces Pol II).

**Functions**: - Stabilizes mRNA by protecting against 3'→5' exonuclease degradation (tail length shortens over time; deadenylation precedes decay). - Promotes nuclear export. - Enhances translation initiation (PABP interacts with the 5' cap-binding complex eIF4F/eIF4G, circularizing the mRNA).

**Key high-yield points**: - Present on most eukaryotic mRNAs; notable exception is replication-dependent histone mRNAs (which have a stem-loop instead). - Prokaryotic mRNA generally lacks poly-A tails (polyadenylation there marks RNA for degradation). - Mature mRNA = 5' cap (7-methylguanosine) + exons + poly-A tail. - Clinical/lab: Oligo-dT primers/columns bind poly-A tails to isolate mRNA and for cDNA synthesis (RT-PCR). Many viruses (e.g., positive-sense ssRNA viruses such as Picornaviruses, Coronaviruses, Hepatitis C is an exception lacking poly-A) have poly-A tails that allow their genomes to act as mRNA. Poxviruses and others encode their own poly-A polymerase. - Mutations in the AAUAAA signal can cause disease: e.g., α-thalassemia (AATAAA→AATAAG in the α-globin gene) and IPEX syndrome (FOXP3 polyadenylation signal mutation). - Alpha-amanitin (Amanita phalloides) inhibits RNA polymerase II, blocking mRNA synthesis and thus poly-A tail addition. - Pre-mRNA processing occurs in the nucleus; poly-A tail and 5' cap help distinguish mRNA from other RNAs. Splicing (snRNPs, spliceosome) is also in the nucleus. Anti-Sm antibodies (SLE) target snRNPs. - Cordycepin (3'-deoxyadenosine) terminates poly-A chain elongation (experimental).