3polyadenylaatio

3' polyadenylation, usually referred to as polyadenylation, is a RNA processing step in eukaryotes that adds a string of adenine nucleotides to the 3' end of a precursor mRNA. The poly(A) tail enhances mRNA stability, promotes export from the nucleus, and supports efficient translation.

During maturation of most mRNAs, transcription and processing are coupled. The pre-mRNA contains a polyadenylation signal, typically AAUAAA, and a downstream GU- or U-rich element. The cleavage and polyadenylation specificity factor (CPSF) and the cleavage stimulatory factor (CstF) bind these elements and recruit the endonuclease CPSF-73 to cleave the transcript. Poly(A) polymerase (PAP) then adds a tail of adenosines to the new 3' end. In the nucleus, the tail length is regulated by the poly(A)-binding protein PABPN1, which increases the processivity of PAP and promotes sustained tail extension.

Typical mature mRNAs acquire tails of about 100–250 adenines, with organisms differing in average length. After assembly, the tail interacts with poly(A)-binding proteins (PABPC in the cytoplasm to stabilize and enhance translation). The tail also influences mRNA turnover and translation efficiency.

Alternative polyadenylation (APA) uses different polyadenylation sites within the same gene, producing mRNA isoforms with different 3' UTRs. APA can affect mRNA stability, localization, and translation and is regulated by multiple factors, including CPSF, CstF, and CFIm. Cytoplasmic polyadenylation, controlled by CPEB and CPSF, can activate translation of dormant mRNAs in oocytes and neurons.

Not all transcripts are polyadenylated. Replication-dependent histone mRNAs, for example, terminate in a conserved stem-loop structure instead of a poly(A) tail. Dysregulation of polyadenylation is linked to development and disease, including altered gene expression in cancer and neurological disorders.