XVI International Botanical Congess

Poly(A)is attached to the plastid psbA mRNA at specific endonucleolytic positions. Unlike eukaryotic and bacterial mRNAs, the sequence moiety does not consist exclusively of poly(A) but is rather composed mostly of purines, 70% adenosines and 25% guanosines. The addition of poly(A)-rich sequences to endonucleolytic cleavage products of chloroplast mRNA is required to target these RNAs for degradation. We found that the chloroplast exoribonuclease 100RNP/PNPase preferably degrades polyadenylated RNA because it bound the poly(A) sequence with much higher affinity than other RNA molecules, thereby defining the poly(A)-rich RNA as a preferential substrate for this enzyme. Our results suggest a mechanism for the degradation of chloroplast mRNA in which endonucleolytic cleavages are followed by the addition of poly(A)-rich sequences to the upstream cleavage products, which target these RNAs for rapid decay.