polyadenylaatioon

Polyadenylation is the process by which a poly(A) tail is added to the 3' end of most eukaryotic messenger RNAs during transcription and processing. The tail, typically several hundred adenosine residues long in vertebrates, is important for mRNA stability, export from the nucleus, and efficient translation. The addition of the tail occurs after cleavage of the pre-mRNA at a defined polyadenylation site, usually marked by the polyadenylation signal AAUAAA.

The 3' end is defined by a multisubunit complex that recognizes AAUAAA and downstream sequence elements. CPSF binds the signal; CstF binds a GU-rich downstream region; together they recruit cleavage factors and the endonuclease CPSF-73 to perform site-specific cleavage. Poly(A) polymerase then extends the 3' end with a stretch of adenosines; a poly(A)-binding protein (PABP) coats the tail and interacts with translation factors to promote initiation. In mammals the mature tail length is commonly about 200–250 nucleotides, though length varies by species and transcript.

Polyadenylation influences RNA fate by affecting stability, nuclear export, and translational efficiency. The tail is gradually shortened by deadenylases in the cytoplasm, which can lead to mRNA decay or translational repression. The tail length can also regulate microRNA access and localization.

Alternative polyadenylation (APA) uses different polyadenylation sites to generate mRNA isoforms with distinct 3' untranslated regions. APA is developmentally regulated and tissue-specific; misregulation has been linked to cancer and neurological disorders.

While polyadenylation is characteristic of most nuclear-encoded eukaryotic mRNAs, some RNAs are not polyadenylated, and in bacteria polyadenylation can target RNAs for degradation.