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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.
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, PDB entry 1fa0 from 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.
More from the PDB here.
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