Abstract

Polyadenylation is best known for its role in stabilizing polII transcripts and facilitating both mRNA export from the nucleus and translation initiation. Paradoxically, the ancestral role of polyadenylation is to tag RNA for degradation by facilitating the recognition of transcripts by 3’ to 5’ exoribonucleases. In fact, the connection between polyadenylation and RNA degradation is conserved in the three domains of life: Eucaryotes, Bacteria and some Archaea. In plants, polyadenylation-mediated RNA degradation operates in the three genetic compartments: the mitochondrion, the chloroplast and the nucleus. In plant mitochondria and chloroplasts, polyadenylated RNAs are degraded by polynucleotide phosphorylases (mtPNPase and cpPNPase, respectively). Both cpPNPase and mtPNPase are involved in mRNA and rRNA degradation/maturation. In addition, the mtPNPase degrades cryptic transcripts that are abundantly produced in plant mitochondria as a consequence of complex genome organisation and expression. Polyadenylation-mediated RNA degradation by mtPNPase thus plays an essential role in establishing the final transcriptome in plant mitochondria. In the nucleus, polyadenylated RNAs destined for degradation are eliminated by the exosome and one of its co-factors, Rrp6, a 3’ to 5’ exoribonuclease. In Arabidopsis, rice and poplar, three genes encode RRP6 -like proteins, while in other eukaryotes RRP6 is encoded by a single gene. The three Arabidopsis RRP6 -like proteins are likely to be functionally specialized as they have distinctive localisations. Initial functional characterization implicates RRP6L2 in nuclear polyadenylation-mediated RNA degradation. The roles of poly(A)-mediated RNA degradation pathways are just beginning to be unraveled in plants. Non-canonical poly(A) polymerases are encoded by small multigene gene families in plants and likely play additional roles in genome expression besides RNA recognition in degradative pathways.

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