PolyA tail

PolyA Signal This is the polyadenylation signal for attachment of the polyA tail to generate mature mRNA it is important for transcription termination. 

In eukaryotic cells, the number of initiation factors is larger and initiation is therefore more complex than in prokaryotes. The cap at the 5 end of mRNA and the polyA tail (see p. 246) play important parts in initiation. However, the elongation and termination processes are similar in all organisms. The individual steps of bacterial translation can be inhibited by antibiotics (see p. 254). 

Most eukaryotic mRNAs end approximately 20 nucleotides downstream of the sequence, AAUAA, which permits addition of a polyA tail that protects the message from cleavage by S to S exonucleases. 

The first technical ditference between the DASL assay and traditional microarray assays is that a random priming technique is utilized, and as such an intact polyA tail is not required for efficient labeling. Other differences include a mechanism by which two oligonucleotide probes must recognize the RNA of interest in order to be extended, ligated, and later amplified via PCR. This technique leads to both high specificity and high sensitivity (due to the PCR reaction). 

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. 

Both capping and poly A formation precede intron removal. Thus, the first steps of processing result in a pre-mRNA that has a 5 cap and a 3 polyA tail, but with all its introns present. 

A long polyA tail increases the half life of the m-RNA. 

After the primary transcript is complete, a polyA tail (-AAA Aoh) is added to the 3 terminus. 

Processing the primary transcript (this includes capping, splicing out introns, addition of the polyA tail, and RNA editing in which specific nucleotides are changed),... 

Fig. 8.1. Diagram of a typical eukaryote gene that is transcribed by RNA polymerase II. Control regions upstream of the promoter are involved in the regulation of transcription. The primary transcript is processed to remove the introns. The 5 cap and 3 polyA tail are added before the mature mRNA is transported from the nucleus.

Figure 3. One orientation cloning and its application - Steps for directional cDNA cloning in Agt22. The experimental details of the procedure are presented in the text. At indicates the polyA tail of mRNA, C, G, T indicate the oligonucleotide primer-adapter which has a dC, dG, and dT homopolymer tail, respectively. SP6 and T7 indicate promoters for SP6 and T7 polymerase.

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