Polyadenylation

Polyadenylation

1. Polyadenylation _________________________________________________AAAAAAAAAAAAAAAAAAAAAA Dr. Emasushan Minj Assistant Professor Department of Botany

2. What is Polyadenylation ? Polyadenylation is the addition of a poly tail to a messenger RNA. The poly tail consists of multiple adenosine monophosphates ; in other words, it is a stretch of RNA that has only adenine bases.

3. All eukaryotic mRNA have a series of up to 250 adenosines at their 3’ ends called poly(A) tail. It has an important role in mRNA stability, nucleocytoplasmic export and translation. The poly-A tail is not specified by the DNA and is added to the transcript by a template-independent RNA polymerase called poly(A) polymerase after endonucleolytic cleavage of the nascent RNA near the 3’ terminus. There two coupled reactions (cleavage and formation of poly (A) tail), are collectively referred to as cleavage and polyadenylation or simply, polyadenylation .

4. Cleavage and polyadenylation is controlled by cis elements located upstream and downstream of the polyadenylation site. The upstream element includes the hexameric poly(A) signal and downstream GU-rich or U-rich elements. Both the poly(A) signal and GU-rich downstream elements are binding sites for multi- subnit protein complexes, which are, respectively, the cleavage and polyadenylation specificity factor (CPSF) and the cleavage stimulation FACTOR ( CstF ). Endonuclease (CFI and CFII) cleave pre-mRNA at polyadenylation site. Finally , the poly(A) polymerase (PAP) catalyzes the polyadenylation reaction in template independent manner.

5. Polyadenylation also occurs in some mRNA in bacteria. In E.coli it results the formation of poly(A) tail of 10-40 nucleotides. It is catalyzed by poly(A) polymerase associated with ribosomes . Here poly(A) tail acts as a binding site for the nucleases ( PNPase and RNaseE ) responsible for degradation of mRNA.

6. Alternative polyadenylation : In mRNAs, protein-coding genes have more than one polyadenylation site, so a gene can code for several mRNAs that differ in the position of poly(A) tail. This process is termed as alternative polyadenylation . This causes the formation of more than one transcript from a single gene. One common example is occurring of poly(A) tail during the development of B-lymphocytes.

7. Cytoplasmic polyadenylation : The length of the poly(A) tail can be regulated in the cytoplasm. In some species, for example the egg cell store mRNA in the cytoplasm for later use after fertilization. The stores mRNA has a short poly(A) tail. Activation of the mRNA for translation includes lengthening of the poly(A) tail . It has also been degraded. Cytoplasmic polyadenylation targets mRNAs that already contain a short poly(A) tail, usually 20-30 nucleotides long and catalyzed by a cytoplamic poly(A) polymerase.

8. Thank you