Three Rat1/Xrn2 homologues exist in Arabidopsis thaliana: nuclear AtXRN2 and AtXRN3, and cytoplasmic AtXRN4. The latter has a role in degrading 3′ products of miRNA-mediated mRNA cleavage, whereas all three proteins act as endogenous post-transcriptional gene silencing suppressors. Here we show that, similar to yeast nuclear Rat1, AtXRN2 has a role in ribosomal RNA processing. The lack of AtXRN2, however, does not result in defective formation of rRNA 5′-ends but inhibits endonucleolytic cleavage at the primary site P in the pre-rRNA resulting in the accumulation of the 35S* precursor. This does not lead to a decrease in mature rRNAs, as additional cleavages occur downstream of site P. Supplementing a P-site cleavage-deficient xrn2 plant extract with the recombinant protein restores processing activity, indicating direct participation of AtXRN2 in this process. Our data suggest that the 5′ external transcribed spacer is shortened by AtXRN2 prior to cleavage at site P and that this initial exonucleolytic trimming is required to expose site P for subsequent endonucleolytic processing by the U3 snoRNP complex. We also show that some rRNA precursors and excised spacer fragments that accumulate in the absence of AtXRN2 and AtXRN3 are polyadenylated, indicating that these nucleases contribute to polyadenylation-dependent nuclear RNA surveillance.

Edited by Michael IbbaProper regulation of ribosome biosynthesis is mandatory for cellular adaptation, growth and proliferation. Ribosome biogenesis is the most energetically demanding cellular process, which requires tight control. Abnormalities in ribosome production have severe consequences, including developmental defects in plants and genetic diseases (ribosomopathies) in humans. One of the processes occurring during eukaryotic ribosome biogenesis is processing of the ribosomal RNA precursor molecule (pre-rRNA), synthesized by RNA polymerase I, into mature rRNAs. It must not only be accurate but must also be precisely coordinated with other phenomena leading to the synthesis of functional ribosomes: RNA modification, RNA folding, assembly with ribosomal proteins and nucleocytoplasmic RNP export. A multitude of ribosome biogenesis factors ensure that these events take place in a correct temporal order. Among them are endo-and exoribonucleases involved in pre-rRNA processing. Here, we thoroughly present a wide spectrum of ribonucleases participating in rRNA maturation, focusing on their biochemical properties, regulatory mechanisms and substrate specificity. We also discuss cooperation between various ribonucleolytic activities in particular stages of pre-rRNA processing, delineating major similarities and differences between three representative groups of eukaryotes: yeast, plants and humans.Keywords: endoribonuclease; exoribonuclease; pre-rRNA processing; ribosome biogenesis; RNA quality control; RNA trimming Ribosome biosynthesis is a multistep process that includes several tightly controlled events -ribosomal DNA (rDNA) transcription, processing of rRNA precursors into mature molecules, accompanied by binding of ribosome biogenesis factors (RBFs) and ribosomal proteins (RPs), and the final assembly of all Abbreviations CD, catalytic domain; dsRBD, double-stranded RNA-binding domain; ETS, external transcribed spacer; ITS, internal transcribed spacer; LSU, large ribosome subunit (60S); miRNA, microRNA; mRNA, messenger RNA; MRP RNA, noncoding RNA component of the RNase MRP; mTOR, mammalian target of rapamycin; nt(s), nucleotide(s); PIN domain, PilT N-terminal domain; Pol I/II/III, RNA polymerase I/II/III; prerRNA, ribosomal RNA precursor; RBF, ribosome biogenesis factor; RMRP, RNase MRP (mitochondrial RNA processing ribonuclease); RNase, ribonuclease; RNB domain, RNase II domain; RNP, ribonucleoprotein; RP, ribosomal protein; rRNA, ribosomal RNA; siRNA, short interfering RNA; snoRNA, small nucleolar RNA; snoRNP, small nucleolar ribonucleoprotein; snRNA, small nuclear RNA; SSU, small ribosome subunit (40S); TRAMP4 or TRAMP5, Trf4/Air/Mtr4 or Trf5/Air/Mtr4 polyadenylation complex; tRNA, transfer RNA.

Arabidopsis thaliana contains two nuclear XRN2/3 5'-3' exonucleases that are homologs of yeast and human Rat1/Xrn2 proteins involved in the processing and degradation of several classes of nuclear RNAs and in transcription termination of RNA polymerase II. Using strand-specific short read sequencing we show that knockdown of XRN3 leads to an altered expression of hundreds of genes and the accumulation of uncapped and polyadenylated read-through transcripts generated by inefficiently terminated Pol II. Our data support the notion that XRN3-mediated changes in the expression of a subset of genes are caused by upstream read-through transcription and these effects are enhanced by RNA-mRNA chimeras generated in xrn3 plants. In turn, read-through transcripts that are antisense to downstream genes may trigger production of siRNA. Our results highlight the importance of XRN3 exoribonuclease in Pol II transcription termination in plants and show that disturbance in this process may significantly alter gene expression.