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Figure 1.
RNA processing in the Arabidopsis nucleus
In addition to its role in transcribing and processing rRNA into ribosomal subunits for export to the cytoplasm, the Arabidopsis nucleolus has also been implicated in other pathways, including nonsense-mediated decay (NMD). Both exon-joining complex (EJC) members and NMD effector proteins were identified by proteomic analysis and shown to localize to this subnuclear organelle, and aberrant mRNAs were shown to be more abundant in nucleolar fractions than in nucleoplasmic fractions. The mechanism of this nucleolar targeting of aberrant mRNA is unclear, and may take place in the nucleus, cytoplasm, or both. Equally unclear is whether NMD takes place in the nucleolus and/or nucleus, or whether transcripts are prepared here for export to the cytoplasm, where UPF1 is recruited to activated degradation. Both pathways may exist, with the latter primarily handling premature termination codon (PTC)+ mRNAs. A similar combination of nuclear and cytoplasmic pathways has been proposed for small interfering RNA (siRNA) and microRNA (miRNA) processing in Arabidopsis, which take place in nucleolar-associated bodies (Cajal bodies and D-bodies). AGO, argonaute; DCL, dicer-like; eIF4A-III, eukaryotic initiation factor 4AIII; HYL1, hyponastic leaves 1; P-body, processing body; Pol IVa, RNA polymerase IVa; RDR2, RNA-dependent RNA polymerase 2; RISC, RNA-induced silencing complex; RNPS1, RNA binding protein S1, serine-rich domain; SE, serrate; UPF, up-frameshift. |
