polyadenylation signal - What is it? (Mar/20/2006 )
I am trying to understand some of the characteristics of plasmids:
- What is it a polyadenylation signal? Eg. BGH polyA or SV40 polyA?
- Can you give me eg. of procaryotic and eucaryotic promoters? High and Low copy number?
basically, we should return to the basics of a gene.
A gene is a promoter, a coding seq and a termination sequence.
Regarding pol III promoters, transcription termination occurs after TTTT
Regarding pol II-U1 promoter, transcription occurs in the 3' box, a well known sequence.
Regarding general pol II-"mRNA-giving" genes, transcription termination is little more confuse. you have at the end of the DNA sequence a "polyadenylation" sequence (consensus one is AATAAA on DNA, AAUAAA on mRNA) and the termination of transcription is 20 - 50 bp after it. But does it play a role in transcription termination? nothing is very clear at time
This sequence is also found in the mature mRNA transcript, and recruits the Poly A polymerase, which add several A to the end of mRNA. Then this sequence (the poly A tail) is bound by poly A binding protein, which recruits others proteins, and the complex goes to connect by non covalent bonds the 7Me-guanosine cap. Toghether, these partenaries ensure the stability of the mRNA transcript.
Regarding stability, a mRNA lacking poly A tail does not survive more than 5-10' (if my memory doesn't fail ) but the destruction process may be shorter.
So to sum up, the polyadenylation signal is added to the end of an insert, to ensure the polyadenylation of the mRNA, and a to participate to correct termination of transcription, in same instance. As SV40polyA signal is very well known and as the SV40 PA signal play a role in terminationof transcription, it's the one routinely used.
So i hope ths reply gives you the needed infos.