3′ Polyadenylation

Core Concept

Nearly all eukaryotic mRNAs carry a poly(A) tail — a string of 50–250 adenine nucleotides added to their 3′ end. The tail is not encoded in the DNA: it is synthesized post-transcriptionally by poly(A) polymerase, which adds adenosines one at a time without a template. The process has two coupled steps: endonucleolytic cleavage of the pre-mRNA at a specific site, followed by untemplated addition of the poly(A) tail. The tail protects the mRNA from 3′ exonucleases and works with the 5′ cap to promote export and efficient translation.

Key Components

• AAUAAA (polyadenylation signal) — conserved hexamer ~10–30 nt upstream of the cleavage site.
• Cleavage and polyadenylation specificity factor (CPSF) — binds AAUAAA and defines the cleavage site.
• Cleavage stimulation factor (CstF) — binds a GU/U-rich downstream element and stimulates cleavage.
• Cleavage factors (CF I/II) — execute the endonucleolytic cut.
• Poly(A) polymerase (PAP) — adds adenosines to the new 3′-OH, untemplated.
• Poly(A)-binding protein (PABP) — coats the tail and links it to translation and stability.

Mechanism

The polyadenylation complex recognizes the AAUAAA signal and a downstream GU-rich element. CPSF binds AAUAAA, CstF binds the downstream element, and together they position the cleavage factors. The pre-mRNA is cut ~10–30 nt downstream of AAUAAA, generating a free 3′-OH. Poly(A) polymerase then adds adenosines in rapid succession using ATP. Early addition is slow and requires CPSF; once ~10 adenines are added, PABP binds and converts PAP to a processive, CPSF-independent mode, extending the tail to ~200–250 adenines.

How It Works

1. The Pol II CTD recruits CPSF and CstF as transcription passes the polyadenylation signals.
2. CPSF binds the AAUAAA hexamer; CstF binds the GU/U-rich downstream sequence.
3. Cleavage factors cut the nascent RNA ~10–30 nt after AAUAAA.
4. The upstream fragment (now with a free 3′-OH) is retained as the mRNA; the downstream fragment is degraded.
5. Poly(A) polymerase adds ~10 adenines slowly (CPSF-dependent initiation).
6. PABP binds the short tail and makes PAP processive, extending it to ~250 adenines.
7. The polyadenylated mRNA is exported; in the cytoplasm the tail shortens over time (deadenylation), regulating mRNA lifetime.

Energy and Directionality

Polyadenylation consumes ATP: each added adenine comes from ATP, with pyrophosphate released on phosphodiester-bond formation. The reaction is untemplated and directional — adenines are added only to the 3′-OH end (5′→3′ addition onto the mRNA's 3′ terminus), so the tail is a pure homopolymer of A that lengthens the mRNA's 3′ end. Cleavage separates the productive (capped, polyadenylated) transcript from the discarded downstream RNA, coupling polyadenylation to Pol II transcription termination.

Experimental Evidence

• AAUAAA mutagenesis — mutating the hexamer abolishes cleavage and polyadenylation, identifying it as the essential signal.
• In vitro processing — nuclear extracts cleave and polyadenylate synthetic pre-mRNA, and omission of ATP or PAP blocks tail addition, proving PAP's untemplated activity.
• α-amanitin/CTD studies — the Pol II CTD is required for efficient processing, showing coupling to transcription.
• Deadenylation assays — progressive 3′ shortening by deadenylases correlates with mRNA decay, demonstrating the tail's protective role.

Technique

Methods include 3′ RACE (map polyadenylation sites), RNA-seq with poly(A) selection (isolate mature mRNAs via oligo(dT)), oligo(dT) chromatography, PAT (poly(A) test) assays to measure tail length, and in vitro cleavage/polyadenylation using radiolabeled substrates.

How it works

1. The Pol II CTD recruits CPSF and CstF as transcription passes the polyadenylation signals.
2. CPSF binds the AAUAAA hexamer; CstF binds the GU/U-rich downstream sequence.
3. Cleavage factors cut the nascent RNA ~10–30 nt after AAUAAA.
4. The upstream fragment (now with a free 3′-OH) is retained as the mRNA; the downstream fragment is degraded.
5. Poly(A) polymerase adds ~10 adenines slowly (CPSF-dependent initiation).
6. PABP binds the short tail and makes PAP processive, extending it to ~250 adenines.
7. The polyadenylated mRNA is exported; in the cytoplasm the tail shortens over time (deadenylation), regulating mRNA lifetime.

Common confusions

• "The poly(A) tail is coded by a run of T's in the DNA" — it is not; it is added post-transcriptionally by poly(A) polymerase without a template (the DNA only has the signal AAUAAA).
• "The tail is added to the 5′ end" — it is added to the 3′ end (opposite the cap).
• "A longer tail always means more protein" — tail length tunes stability/translation, but other factors (cap, miRNA) also regulate expression.
• "Polyadenylation and transcription termination are unrelated" — polyadenylation site cleavage triggers Pol II termination.
• "All RNAs get poly(A) tails" — mainly mRNA (and some lncRNAs); tRNA, rRNA, and histone mRNAs generally do not.

Quick review

• Poly(A) tail: 50–250 A's, untemplated, added by poly(A) polymerase (ATP).
• CPSF binds AAUAAA; CstF binds downstream GU/U; cleavage factors cut the RNA.
• PABP confers processivity and links tail to translation/stability.
• Functions: 3′ protection, export, translation, deadenylation-regulated decay.
• Coupled to Pol II termination; key to mRNA-based assays (oligo(dT), 3′ RACE).