Previously, I summarized the importance of the poly(A) tail of mRNAs. As I stated in that essay (and as one can tell by perusing the background stuff and some of the links here), what I am interested in is how the poly(A) tail is actually added to a mRNA. The point of this essay is to provide an overview of what we know, so that future essays on the subject may have a better context. I’ll structure this essay as a series of bullets (ideally, they’d be concise, but knowing me that’s not too likely).
- The poly(A) tail is not encoded in the DNA
The first and most important point is that poly(A) tails are not “parts” of the genes that encode mRNAs. In other words, if you look at the DNA that yields a particular polyadenylated mRNA, you will not find a complementary stretch of T’s (hundreds of bases long) on the DNA strand that is copied by RNA polymerase.
- The addition of the poly(A) tail is an RNA processing event
If poly(A) tails are not coded by DNA, and are not produced by transcribing RNA polymerase, then where do they come from? In a nutshell, they are added to the ends of precursor mRNAs by different polymerases, called (naturally enough) poly(A) polymerases. Poly(A) polymerase adds tracts of poly(A) to the 3’ ends of RNA made by RNA polymerase. Moreover, these 3’ ends are generated, NOT by the cessation of transcription by RNA polymerase, but by the cutting, or processing, of the precursor mRNA. This processing generates two RNA pieces; the “upstream” or 5’ piece contains the capped 5’ end of the mRNA and a 3’ end that is a suitable substrate for poly(A) polymerase. The “downstream” piece possesses an uncapped 5’ end, something that, as we learned in the earlier essay, is an excellent substrate for the Xrn class of exonucleases. It is thus not surprising that the downstream pieces are not very stable.
- Polyadenylation is guided by specific parts of the precursor RNA
As one might expect, if poly(A) tail addition is preceded by an RNA processing event, it follows that there must be a way for the responsible enzymes to identify the site of processing. This is in fact the case. It turns out that each precursor mRNA possesses an array of RNA sequence elements that direct the processing and polyadenylation complex. These elements are collectively known as the polyadenylation signal. (Readers who have taken genetics or molecular biology classes may recall the motif AAUAAA as the canonical polyadenylation signal in mammalian mRNAs. Other eukaryotes have “fuzzier” signals, and in fact a large proportion of mammalian signals are not as clear-cut as this, either. This will be discussed in a future essay.)
- Poly(A) tail addition involves a complex of factors
While the process of polyadenylation seems simple enough, it actually involves a rather large number of proteins (more than 20, if one counts the more peripheral members of the complex). These subunits include a number of RNA binding proteins, at least one endonuclease, and some proteins that act as scaffolds and bridges for the apparatus.
- Poly(A) tail addition is physically linked to other processes
While it may be easy to view polyadenylation as something that occurs once the processes RNA has been released from the processing complex, in fact the polyadenylation complex has several physical and functional connections with other steps of the transcription process. These include links with transcription initiation factors, RNA polymerase, transcription elongation and termination factors, RNA splicing enzymes, the machinery that transports mRNAs from the nucleus to the cytoplasm, cytoplasmic RNA metabolic enzymes, and complexes involved in RNA degradation. All of these links have important functions; as time goes on, I hope to elaborate on various and sundry of these. The take-home message, however, is that mRNA biogenesis occurs within the context of a large system, the individual functionalities of which are physically and conceptually linked.
- A good review (importantly, freely available)
Proudfoot N, O’Sullivan J. 2002. Polyadenylation: A tail of two complexes. Current Biology, Vol 12, R855-R857 link
- A brief pictorial overview