October 2008 Molecule of the Month by David Goodsell
Keywords: polynucleotide adenylyltransferase activity, RNA polyadenylation, mRNA processing, poly(A) RNA binding
Most of the RNA found in our cells is built using our DNA genome as a template. In special cases, however, our cells also build RNA strands without a template. For instance, the end of (almost) every messenger RNA strand is composed of a long string of repeated adenosine nucleotides. These long poly(A) tails are not encoded in the genome. Instead, they are added after RNA polymerase finishes its normal process of transcription. After RNA polymerase releases the RNA strand, other enzymes add the finishing touches, editing out introns, adding a cap to the front end, and building the long poly(A) tail at the other end.
The Tail End
A complex of over a dozen enzymes oversees the creation of a poly(A) tail on messenger RNA molecules. Several special sequences at the end of the RNA recruit this complex to the proper place. Then the RNA strand is cleaved, and about 250 adenosine nucleotides are added to the new end. The enzyme poly(A) polymerase, in PDB entries 1f5a (cow) and 1fa0 (yeast, shown here), is responsible for the creation of the poly(A) tail. With the help of two magnesium ions, it binds to the messenger RNA and adds adenosine nucleotides one at a time to the end of the strand.
Heads and Tails
The poly(A) tail plays several important roles in the function of messenger RNA molecules. With the help of poly(A)-binding protein (shown on the next page), it protects the end of the RNA strand, shielding it from RNA-cutting nucleases. It also assists with the transport of the messenger RNA out of the nucleus through nuclear pores. Surprisingly, the poly(A) tail, which is at the end the messenger RNA, also stimulates the start of protein synthesis by helping to recruit translation initiation factors at the front end of the RNA. Some researchers actually think that poly(A)-binding protein links the RNA strand into a big circle. This could have a very useful consequence: since the beginning of the messenger RNA is so close to the end, ribosomes that have just finished making a protein could jump immediately to the beginning and start again.