Activity and roles of Arabidopsis thaliana XRN family exoribonucleases in noncoding RNA pathways

RNA metabolism is mediated by several sophisticated exo- or endo- ribonucleases. XRN family proteins are the conserved 5'-3' exoribonucleases in eukaryotes. A. thaliana genome encodes three XRN homologs (AtXRN2, AtXRN3 and AtXRN4) and their independent or redundant roles, which are possibly plant-specific in some cases, have been reported. AtXRN2 acts in maturation of ribosomal RNAs partially with AtXRN3. AtXRN3 is also involved in elimination of 3' remnants of microRNA precursors and in termination of mRNA transcription events. AtXRN4 degrades not only a small fraction of mRNAs in stress response but also 3' cleavage products of miRNA-mediated cleavage of target mRNAs. Moreover, all AtXRNs are important factors to suppress unexpected RNA silencing occurrence. Thus, this review summarizes and discusses multiple roles of AtXRN exoribonucleases and their relationship with noncoding RNA pathways including RNA silencing pathways.