polyadenatering

Polyadenatering, commonly called polyadenylation, is the cellular process that adds a chain of adenine nucleotides to the 3' end of a eukaryotic pre-mRNA. This maturation step is tightly coordinated with transcription termination and pre-mRNA cleavage. A conserved polyadenylation signal, usually AAUAAA, lies upstream of the cleavage site, and downstream elements rich in GU and U nucleotides help recruit the processing complex.

The cleavage and polyadenylation specificity factor (CPSF) and the cleavage stimulation factor (CstF) recognize these signals and mediate endonucleolytic cleavage. Poly(A) polymerase (PAP) then extends the newly generated 3' end with adenosines to form the poly(A) tail. In the nucleus, the tail is bound by poly(A) binding proteins (PABPN1 and others) that regulate tail length. The mature mRNA is then exported to the cytoplasm, where PABP family proteins continue to modulate stability and translation.

Tail length varies by organism and transcript; in mammals, tails are commonly around 200–250 nucleotides, though much shorter tails occur in many transcripts. Longer tails generally correlate with higher stability and greater translational efficiency. Alternative polyadenylation can generate mRNA isoforms with different 3'UTRs, altering regulatory element content and protein expression.

Histone mRNAs are an important exception: they are not polyadenylated, ending instead in a conserved stem-loop structure. Polyadenylation is also a key regulatory point in gene expression and can be affected in disease and development; defects in processing factors can alter mRNA stability and translation.

Overall, polyadenatering is essential for mRNA maturation and proper gene expression in eukaryotes.