BLOCKING POLYADENYLATION OF MESSENGER-RNA IN THE CHLOROPLAST INHIBITSITS DEGRADATION

The addition of poly(A)-rich sequences to endonuclease cleavage produc ts of chloroplast mRNA has recently been suggested to target the polya denylated RNA for rapid exonucleolytic degradation. This study analyze d whether the addition of a poly(A)-rich tail to RNA molecules is requ ired for degradation by chloroplast exonuclease(s). In lyzed chloropla sts from spinach, addition of the polyadenylation inhibitor, cordycepi n triphosphate (3'-dATP), inhibited the degradation of psbA and rbcL m RNAs. Furthermore, degradation intermediates generated by endonucleoly tic cleavages accumulated. Similar results were obtained when yeast tR NA was added to the mRNA degradation system as a non-specific exoribon uclease inhibitor. Nevertheless, the stabilization mechanisms differ: while tRNA directly affects the exonuclease activity, 3' dATP has an i ndirect effect by inhibiting polyadenylation. The results indicate tha t the addition of poly(A)-rich sequences to endonucleolytic cleavage p roducts of chloroplast mRNA is required to target these RNAs for rapid exonucleolytic degradation. Together with previous work, the data rep orted here support a model for mRNA degradation in the chloroplast in which endonucleolytic cleavages are followed by the addition of poly(A )-rich sequences to the proximal cleavage products, targeting these RN As for rapid exonucleolytic decay.