The Nonstop decay and the RNA silencing systems operate cooperatively in plants
Ontology highlight
ABSTRACT: Translation-dependent mRNA quality control systems protect the protein homeostasis of eukaryotic cells by eliminating aberrant transcripts and stimulating the decay of their protein products. Although these systems are intensively studied in animals, little is known about the translation-dependent quality control systems in plants. Here we characterize the mechanism of nonstop decay (NSD) system in Nicotiana benthamiana model plant. We show that plant NSD efficiently degrades nonstop mRNAs, which can be generated by premature polyadenylation, and stop codon-less transcripts, which can be produced by endonucleolytic cleavage. We demonstrate that in plants, like in animals, Pelota, Hbs1 and SKI2 proteins are required for NSD, supporting that NSD is an ancient and conserved eukaryotic quality control system. Relevantly, we found that NSD and RNA silencing systems cooperate in plants. Plant silencing predominantly represses target mRNAs through endonucleolytic cleavage in the coding region. The cleavage products are degraded by general decay systems or subjected to silencing amplification. The balance between decay and silencing amplification might be finely regulated by unknown mechanisms. Here we show that NSD is required for the elimination of 5’cleavage product when mi- or siRNA-guided silencing complex cleaves in the coding region. Thus NSD might regulate the frequency of silencing amplification.
Project description:Translation of poly(A) tails leads to mRNA cleavage but the mechanism and global pervasiveness of this “nonstop/no-go” decay process is not understood. Here we performed ribosome profiling of short 15-18 nt mRNA footprints to identify ribosomes stalled at 3’ ends of mRNA decay intermediates. We found mRNA cleavage extending hundreds of nucleotides upstream of ribosome stalling in poly(A) and predominantly in one reading frame. These observations suggest that cleavage is closely associated with the ribosome. Surprisingly, ribosomes appeared to stall when as few as 3 consecutive ORF-internal lysine codons were positioned in the A, P, and E sites though significant mRNA degradation was not observed. Endonucleolytic cleavage was widespread, however, at sites of premature polyadenylation and rescue of the ribosomes stalled at these sites was dependent on Dom34. These results suggest this process may be critical when changes in polyadenylation occur during development, tumorigenesis, or when translation termination/recycling is impaired.
Project description:XRN 5′-3′ exoribonucleases play crucial roles in the control of RNA processing, quality, and quantity in eukaryotes. Although genome-wide profiling of RNA decay fragments is now feasible, how XRNs shape the plant mRNA degradome remains elusive. Here, we profiled and analyzed the RNA degradomes of the Arabidopsis wild type and mutants with defects in XRN activity. Deficiency of nuclear XRN3 or cytoplasmic XRN4 but not nuclear XRN2 activity largely altered Arabidopsis mRNA decay profiles. In addition to the primary XRN4 substrates derived from decapping and microRNA-directed slicing, terminating ribosome- and exon junction complex-protected fragments produced from XRN4-mediated cytoplasmic decay also represent the most abundant decay intermediates of Arabidopsis mRNAs. Short excised linear introns and cleaved pre-mRNA fragments downstream of polyadenylation sites were polyadenylated and stabilized in the xrn3 mutant, demonstrating the function of XRN3 in the removal of cleavage remnants from pre-mRNA processing. Further analysis of stabilized XRN3 substrates confirmed that polyadenylation cleavage frequently occurs after an adenosine. An increase in decay intermediates with 5′ ends upstream of a consensus motif in the xrn4 mutant suggests an endonucleolytic cleavage mechanism targeting the 3′ untranslated region of many Arabidopsis mRNAs. However, analysis of decay fragments stabilized in the xrn4 mutant indicated that, except for microRNA-directed slicing, endonucleolytic cleavage events in the coding sequence might rarely result in major decay intermediates. Together, the results of this study reveal major substrates and products of nuclear and cytoplasmic XRNs along Arabidopsis transcripts and provide a basis for precise interpretation of RNA degradome data.
Project description:Ribosome stalling at problematic sequences in mRNAs leads to collisions that trigger a collection of quality control events including ribosome rescue, targeting the nascent polypeptide for decay (Ribosome-mediated Quality Control or RQC), and targeting of the mRNA for decay (No Go Decay or NGD). Using a reverse genetic screen in yeast, we identify Cue2 as the endonuclease that is recruited to stalled ribosomes to promote NGD. Following Cue2-mediated cleavage, ribosomes upstream of the cleavage site translate to the end of the truncated mRNA and are rescued by the Dom34:Hbs1 complex. We also show that the putative helicase Slh1 (part of the RQC Trigger or RQT complex) removes collided ribosomes behind the lead stalled ribosome and thereby reduces endonucleolytic cleavage by Cue2. The synergistic activities of Cue2 and Slh1 define two parallel pathways that allow cells to recognize and respond to ribosomes trapped on problematic mRNAs.
Project description:We aimed to study the cleavage sites of nonsense-mediated mRNA decay (NMD) substrates. Therefore, we depleted the major exoribonuclease XRN1 in human cell culture, which degrades the 3' fragments generated by SMG6-mediated endonucleolytic cleavage. Different reporter mRNAs or endogenous NMD targets were investigated and the 3' fragments were cloned, amplified and subsequently subjected to high throughput sequencing.
Project description:Messenger RNAs are regulated by a variety of degradation mechanisms in mammalian cells. In the canonical animal microRNA pathway, microRNAs in complex with Argonaute proteins bind to many mRNA targets with imperfect complementarity, leading to degradation of the mRNA through the regular decay machinery. The ancestral “slicer” endonuclease activity of Argonaute2 itself, which requires more extensive complementarity with the target RNA, is not used in this pathway, and has only been observed in two microRNA-guided cases. Nevertheless, the cleavage capacity of mammalian Ago2 is conserved and essential for viability. Here, we assess the endonucleolytic function of Ago2 and other nucleases by identifying cleavage products retaining 5`-phosphate groups in mouse ES cells on a transcriptome-wide scale. We detect a significant signature of Ago2-dependent cleavage events and validate several targets. Unexpectedly, a broader class of Ago2-independent cleavage sites is also observed, indicating participation of additional nucleases in this mode of mRNA regulation. Within this class, we identify a cohort of Drosha-dependent mRNA cleavage events, including one in the Dgcr8 mRNA, that functionally regulate mRNA levels in mES cells. Together, these results highlight the underappreciated role of endonucleolytic cleavage in controlling mRNA fates in mammals. Global 5`-phosphate-dependent RACE in WT, Ago2-KO and Drosha-excised mouse ES cells and human 293S cells
Project description:Messenger RNAs are regulated by a variety of degradation mechanisms in mammalian cells. In the canonical animal microRNA pathway, microRNAs in complex with Argonaute proteins bind to many mRNA targets with imperfect complementarity, leading to degradation of the mRNA through the regular decay machinery. The ancestral “slicer” endonuclease activity of Argonaute2 itself, which requires more extensive complementarity with the target RNA, is not used in this pathway, and has only been observed in two microRNA-guided cases. Nevertheless, the cleavage capacity of mammalian Ago2 is conserved and essential for viability. Here, we assess the endonucleolytic function of Ago2 and other nucleases by identifying cleavage products retaining 5`-phosphate groups in mouse ES cells on a transcriptome-wide scale. We detect a significant signature of Ago2-dependent cleavage events and validate several targets. Unexpectedly, a broader class of Ago2-independent cleavage sites is also observed, indicating participation of additional nucleases in this mode of mRNA regulation. Within this class, we identify a cohort of Drosha-dependent mRNA cleavage events, including one in the Dgcr8 mRNA, that functionally regulate mRNA levels in mES cells. Together, these results highlight the underappreciated role of endonucleolytic cleavage in controlling mRNA fates in mammals.
Project description:MicroRNAs (miRNAs) and small-interfering RNAs (siRNAs) negatively regulate their targets by 1) repressing translation, 2) endonucleolytic RNA cleavage, or 3) DNA methylation resulting in transcriptional silencing. P-body/decapping components are likely required for translational repression, but are not known to function in other posttranscriptional regulatory pathways or to affect smRNA levels. Here, we show that the P-body/decapping protein DCP5 is required for miRNA-mediated translational repression but not cleavage, and to regulate the transcription of specific miRNAs. We find that this protein also affects the abundance of tRNA-derived smRNAs. Significantly, DCP5 is required for the transcriptional silencing and DNA methylation of numerous transposable/repetitive elements and imprinted genes, indicating that it is a novel component of the RNA-directed DNA methylation pathway. Our results demonstrate that DCP5 and likely the P-body itself are required for multiple smRNA-mediated silencing pathways and provide the first evidence for the spatial separation of translational inhibition and cleavage by miRNAs. small RNA (smRNA) expression comparison between wildtype (Col-0) and dcp5 mutant plants in Arabidopsis
Project description:MicroRNAs (miRNAs) and small-interfering RNAs (siRNAs) negatively regulate their targets by 1) repressing translation, 2) endonucleolytic RNA cleavage, or 3) DNA methylation resulting in transcriptional silencing. P-body/decapping components are likely required for translational repression, but are not known to function in other posttranscriptional regulatory pathways or to affect smRNA levels. Here, we show that the P-body/decapping protein DCP5 is required for miRNA-mediated translational repression but not cleavage, and to regulate the transcription of specific miRNAs. We find that this protein also affects the abundance of tRNA-derived smRNAs. Significantly, DCP5 is required for the transcriptional silencing and DNA methylation of numerous transposable/repetitive elements and imprinted genes, indicating that it is a novel component of the RNA-directed DNA methylation pathway. Our results demonstrate that DCP5 and likely the P-body itself are required for multiple smRNA-mediated silencing pathways and provide the first evidence for the spatial separation of translational inhibition and cleavage by miRNAs. total RNA expression comparison with between wildtype (Col-0) and dcp5 mutant plants in Arabidopsis