Polyadenylation of RNAs associated with a nucleus-localized phosphorolytic nuclease

The exosome is a protein complex consisting of a variety of 3′→5′ exoribonucleases that functions both in the processing of rRNA precursors and in the degradation of mRNA. A prokaryotic counterpart of the exosome known as the degradosome exists in bacteria and chloroplasts. Interestingly, RNA polyadenylation has been implicated in degradosome functioning, giving rise to the possibility of a similar role in exosome function. Using phosphorolytic breakdown of RNA as an assay, we have purified an exosome-like activity from pea nuclear extracts. This activity copurifies with at least one Arabidopsis exosome subunit homologue. Recombinant Arabidopsis poly(A) polymerase and purified chloroplast poly(A) polymerase can polyadenylate RNAs that copurify with the exosome-like activity, even though the quantity of this co-purifying RNA is well below the affinity of the PAPs for free RNA. These results suggest a role for polyadenylation in exosome function, perhaps analogous to the role that polyadenylation plays in facilitating RNA breakdown by the bacterial degradosome.