Polyadenylation of mRNA

Manley J. L. (1995) A complex protein assembly catalyzes polyadenylation of mRNA precursors. Current Opin. Genet. Develop., 5, 222-228. 

Polyadenylation of mRNAs preceded by specific endonuclease cleavage has been demonstrated in Ch. reinhardtii chloroplasts. The plastid gene named me for its similarity to the E. coli RNase E is involved in this process. The chloroplast me genes were discovered in the same algal genomes that contain the mpB gene. In land plants, the me genes are located in the nucleus. 

Further proteolytic cleavages occur carboxy-terminal to Arg 334 and Arg 343 and give rise to beta-factor Xlla. During the formation of beta-factor Xlla, residues 1-334 and 345-353 of factor XII are removed. The codon for the carboxy-terminal residue of the heavy chain of beta-factor Xlla (Ser-596) is followed by a T6A stop codon, a 3 untranslated region of 150 nucleotides and a poly(A) tail. Nucleotides 1937 to 1942 contain the sequence AATAAA that is involved in the polyadenylation of mRNA (134,181). This sequence occurs 17 nucleotides upstream of the poly(A) tail. From the predicted amino acid sequence, the molecular weight of plasma factor XII is 66,915 in the absence of carbohydrate and 80,427 with the addition of 16.8% carbohydrate (207). 

Manley JL (1983) Accurate and specific polyadenylation of mRNA precursors in a soluble whole-cell lysate. Cell 33(2) 595-605... 

Ara-A is phosphorylated in mammalian cells to ara-AMP by adenosine kinase and deoxycytidine kinase. Further phosphorylation to the di- and triphosphates, ara-ADP and ara-ATP, also occurs. In HSV-1 infected cells, ara-A also is converted to ara-ATP. Levels of ara-ATP correlate directly with HSV rephcation. It has recently been suggested that ara-A also may exhibit an antiviral effect against adenovims by inhibiting polyadenylation of viral messenger RNA (mRNA), which may then inhibit the proper transport of the viral mRNA from the cell nucleus. 

Figure 37-13. Mechanisms of alternative processing of mRNA precursors. This form of RNA processing involves the selective inclusion or exclusion of exons, the use of alternative 5 donor or 3 acceptor sites, and the use of different polyadenylation sites.

The polyadenylation of c-mos mRNA in maturing oocytes is similar to that of other maternal RNAs that are accumulated during oocyte... 

Vassalli, J. D., Huarte, J., Belin, D., Gubler, P., Vassalli, A., O Connell, M. L., Parton, L. A., Rickies, R. J., and Strickland, S. (1989). Regulated polyadenylation controls mRNA translation during meiotic maturation of mouse oocytes. Genes Dev. 3 2163-2171. 

Salles, F. J., and Strickland, S. (1995). Rapid and sensitive analysis of mRNA polyadenylation states by PCR. PCR Methods and Applications 4, 317-321. 

Answer A. Polyadenylation of pre-mRNA occurs in the nucleoplasm. Generally associ-... 

A poly(A) "tail" consisting of -250 residues of adenylic acid is added next by poly(A) polymerase, a component of an enzyme complex that also cleaves the RNA chains.545 57111 Most eukaryotic mRNA is polyadenylated with the exception of that encoding histones. The function of the poly(A) is unclear. It is needed for transport of mRNA out of the nucleus, but it does confer a greatly increased stability to the mRNA in the cytoplasm where the adenylate irnits are gradually removed.307 308 In contrast, in chloroplasts and plant mitochondria polyadenylation is required for rapid degradation of mRNA.571c d Polyadenylation may also increase the efficiency of translation.572 Polyadenylation occurs rapidly within -1 min after transcription is completed. 

The processing pathways that lead to the two forms of mRNA are illustrated in Figure 8. To produce mRNA encoding ps, a primary transcript is cleaved and polyadenylated at a proximal poly(A) site located 3 to the exon encoding C 4 and the terminal segment of the secreted chain. To produce mRNA encoding pm, the transcript is cleaved and polyadenylated at a distal poly(A) site 3 to the membrane exons. The primary transcript for pm is spliced from the border between... 

Cp4 and the secreted segment to the 5 border of the first membrane exon the donor splice site lies within a GGU glycine codon that is retained in ps mRNA. Which form of mRNA is produced depends on competition between splicing and polyadenylation at the p.s and pm poly(A) sites (Peterson and Perry, 1986 Tsurushita et al., 1987 Galli et al., 1988 Peterson and Perry, 1989). The balance between the two mRNA products determines the proportion of membrane and secreted IgM. Activation of the B lymphocyte appears to tip the balance in favor of secretion, but the regulatory signals are not known. 

In the cytoplasm the codons of mRNA are translated into amino acids via the concerted action of the ribosomes, transfer RNA (tRNA), and a large protein complex. In addition to the mature RNA sequence (capped and polyadenylated) other structural elements located in the 5 and 3 untranslated regions (UTRs) of the mRNA may influence its translation. 

Eukaryotic mRNAs often have long 3 untranslated sequences— sequences that follow the stop codon for the protein they encode. These mRNAs generally conclude with a sequence of up to 200 adenosines, the polyadenylic acid (polyA) sequence at the 3 end. This sequence isn t coded by the DNA template for the gene it is added post-transcriptionally. Not all mRNAs are polyadenylated. For example, histone mRNAs lack polyA tails. Polyadenylation seems to play a role in regulating the stability of mRNAs. An early event in the breakdown of some mRNAs is the removal of their polyA tails. 

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