Plant mitochondrial polyadenylated mRNAs are degraded by a 3 '- to 5 '-exoribonuclease activity, which proceeds unimpeded by stable secondary structures

Recently, we and others have reported that mRNAs may be polyadenylated in p lant mitochondria, and that polyadenylation accelerates the degradation rat e of mRNAs. To further characterize the molecular mechanisms involved in pl ant mitochondrial mRNA degradation, we have analyzed the polyadenylation an d degradation processes of potato atp9 mRNAs. The overall majority of polya denylation sites of potato atp9 mRNAs is located at or in the vicinity of t heir mature 3'-extremities. We show that a 3'- to 5'-exoribonuclease activi ty is responsible for the preferential degradation of polyadenylated mRNAs as compared with non-polyadenylated mRNAs, and that 20-30 adenosine residue s constitute the optimal poly(A) tail size for inducing degradation of RNA substrates in vitro. The addition of as few as seven non-adenosine nucleoti des 3' to the poly(A) tail is sufficient to aim st completely inhibit the i n vitro degradation of the RNA substrate. Interestingly, the exoribonucleas e activity proceeds unimpeded by stable secondary structures present in RNA substrates. From these results, we propose that in plant mitochondria, pol y(A) tails added at the 3' ends of mRNAs promote an efficient 3'- to 5'-deg radation process.