The poly(A) tail inhibits the assembly of a 3'-to-5' exonuclease in an in vitro RNA stability system.

We have developed an in vitro system which faithfully reproduces several aspects of general mRNA stability. Poly(A)- RNAs were rapidly and efficiently degraded in this system with no detectable intermediates by a highly processive 3'-to-5' exonuclease activity. The addition of a poly(A) tail of at least 30 bases, or a 3' histone stem-loop element, specifically stabilized these transcripts. Stabilization by poly(A) required the interaction of proteins with the poly(A) tail but did not apparently require a 3' OH or interaction with the 5' cap structure. Finally, movement of the poly(A) tract internal to the 3' end caused a loss of its ability to stabilize transcripts incubated in the system but did not affect its ability to interact with poly(A) binding proteins. The requirement for the poly(A) tail to be proximal to the 3' end indicates that it mediates RNA stability by blocking the assembly, but not the action, of an exonuclease involved in RNA degradation in vitro.