Project description:Upon the environmental changes, cells flexibly and rapidly alter the gene expression by the translation control. In plants, the translation of NIP5;1, a boric acid diffusion facilitator, is upregulated through the upstream open reading frame (uORF), which comprises only AUG and stop codons, in response to the shortage of boric acid. However, the molecular details of how the minimum uORF controls the translation of downstream main ORF depending on the boric acid concentration remained unclear. Here we showed that the 80S ribosome (80S) assembled at AUG-stop migrates into the subsequent RNA segment for downstream translation initiation and that the presence of boric acid impedes the process by stable confinement of eukaryotic release factor 1 (eRF1) on 80S on AUG-stop. Ribosome profiling along the in vitro-translated reporter mRNA revealed that the number of the ribosomes on AUG-stop and that along the downstream are anti-correlated in response to boric acid, reflecting the 80S scanning leaving AUG-stop for translation initiation from downstream ORF. Moreover, cryo-electron microscopy (cryo-EM) analysis revealed that, in the presence of boric acid, the initiator methionyl-tRNA (Met-tRNAi) in the P site stably contacts eRF1 in the A site on AUG-stop. In contrast, the absence of boric acid led to weaker eRF1 density, suggesting that eRF1 cannot be stably positioned without boric acid. Consistent with the structural features, the hydrolysis of Met-tRNAi on AUG-stop was accelerated by boric acid, which is likely to be the checkpoint for the subsequent 80S scanning of the downstream. Our results provide a molecular insight into the translation regulation by a minimum and environment-responsive uORF.
Project description:In response to environmental changes, cells flexibly and rapidly alter gene expression, through translational controls. In plants, the translation of NIP5;1, a boric acid diffusion facilitator, is upregulated through upstream open reading frames (uORFs) that comprise only AUG and stop codons, in response to a shortage of boric acid in the environment. However, the molecular details of how the minimum uORF controls the translation of the downstream main ORF, in a boric acid concentration-dependent manner, have remained unclear. Here, combining ribosome profiling, TCP-Seq, structural analysis with cryo-electron microscopy, and biochemical assays, we showed that the 80S ribosome (80S) assembled at AUG-stop migrates into the subsequent RNA segment, followed by downstream translation initiation, and that boric acid impedes this process by the stable confinement of eukaryotic release factor 1 (eRF1) on the 80S on AUG-stop. Our results provide molecular insight into translation regulation by a minimum and environment-responsive uORF.
Project description:Translation of mRNA into a polypeptide is terminated when the release factor eRF1 recognizes a UAA, UAG, or UGA stop codon in the ribosomal A site and stimulates nascent peptide release. However, stop codon readthrough can occur when a near-cognate tRNA outcompetes eRF1 in decoding the stop codon, resulting in the continuation of the elongation phase of protein synthesis. At the end of a conventional mRNA coding region readthrough allows translation into the mRNA 3’-UTR. Previous studies with reporter systems have shown that the efficiency of termination or readthrough is modulated by cis-acting elements other than stop codon identity, including two nucleotides 5’ of the stop codon, six nucleotides 3’ of the stop codon in the ribosomal mRNA channel, and stem-loop structures in the mRNA 3’-UTR. It is unknown whether these elements are important at a genome-wide level and whether other mRNA features proximal to the stop codon significantly affect termination and readthrough efficiencies in vivo. Accordingly, we carried out ribosome profiling analyses of yeast cells expressing wild-type or temperature-sensitive eRF1 and developed bioinformatics strategies to calculate readthrough efficiency, and to identify mRNA and peptide features which influence that efficiency. We found that the stop codon (nt +1 to +3), the nucleotide after it (nt +4), the codon in the P site (nt -3 to -1), and 3’-UTR length are the most influential features in the control of readthrough efficiency, while nts +5 to +9 and mRNA secondary structure in the 3’-UTR had milder effects. Additionally, we found low readthrough genes to have shorter 3’-UTRs compared to high readthrough genes in cells with thermally inactivated eRF1, while this trend was reversed in wild-type cells. Together, our results demonstrated the general roles of known regulatory elements in genome-wide regulation and identified several new mRNA or peptide features important for translation termination and readthrough.
Project description:The ribosome-associated quality control (RQC) is a surveillance system for aberrant translation, sensing ribosome collisions. Although the molecular mechanism has been extensively studied, the endogenous targets of RQC in human cells were poorly understood. Here, starting from the study of codon specificity of eukaryotic termination factor eRF1, we unexpectedly find that misrecognition of UUA sense codon by eRF1 leads to ribosome collisions and provides the source of RQC substrates in humans. eRF1-selective Monosome-Seq and Disome-Seq reveal that eRF1 recruitment to ribosome was not restricted to the stop codons but also the sub-cognate sense codons, including the UUA codon. The UUA misrecognition by eRF1 causes ribosome collision without termination reaction. Remarkably, Disome-Seq with the depletion of ASCC3 and 4EHP, key factors in RQC, showed that ribosome stalled at UUA codons are the predominant sub-populations rescued by RQC. Failure to resolve ribosome collisions by RQC triggers p38 phosphorylation and upregulation of stress response transcription factor ATF3. This study presents the impact of sense codon misrecognition by the termination factor on translation homeostasis in human cells.
Project description:Translational regulation at the stage of initiation impacts the number of ribosomes translating each mRNA molecule. For example, multiple ribosomes can engage on a single mRNA forming a polysome, resulting in highly efficient protein synthesis. However, the translational activity of single 80S ribosomes on mRNA (monosomes) is less well understood, even though these 80S monosomes represent the dominant ribosomal complexes in many tissues. Here, we used cryo-EM to determine the translational activity of 80S monosomes across different tissues in Drosophila melanogaster. We discovered that while head and embryo 80S monosomes are highly translationally active, testis and ovary 80S monosomes are translationally inactive. RNA-Seq analysis of head monosome- and polysome-translated mRNAs, revealed that head 80S monosomes preferentially translate mRNAs with TOP motifs, short 5’-UTRs, short ORFs and are enriched for uORFs. Overall, these findings highlight that regulation of translation initiation, and therefore the number of ribosomes bound per mRNA, varies substantially across tissues.
Project description:We performed ribosome profiles in the eRF1 depleted cells under the wild-type backgound and GCN2 knockout background in mouse embryonic stem cells
Project description:We present RNA-Seq data for mice with mutated TERP (TERP-AUG and TERP-del7) and for mice from control groups (TERP-wt-AUG and TERP-del7-wt). TERP-AUG mutation leads to increased translation of TERP, and TERP-del7 mutation leads to the formation of nonfunctional TERP.
Project description:Translation termination is an essential cellular process that is also of therapeutic interest for diseases that manifest from premature stop codons. In eukaryotes, translation termination requires eRF1, which recognizes stop codons, catalyzes the release of nascent proteins fom ribosomes, and facilitates ribosome recycling. The small molecule SRI-41315 triggers eRF1 degradation and enhances translational readthrough of premature stop codons. SRI-41315 promotes retention of eRF1 on ribosomes and leads to a higher frequency of translation termination at near-cognate stop codons. In this study, the systematic effect of SRI-41315 on translation termination at cognate and near-cognate stop codons is evaluated using a rabbit reticulocyte lysate-based in vitro translation system with endogenous transcript as well as reporter transcripts containing defined near-cognate stop codon.
Project description:The recycling of ribosomal subunits after translation termination is critical for efficient gene expression. Tma64 (eIF2D), Tma20 (MCT-1), and Tma22 (DENR) function as 40S recycling factors in vitro, but it is unknown whether they perform this function in vivo or serve as alternative initiation factors. Ribosome profiling of strains missing these factors revealed 80S ribosomes queued behind the stop codon, consistent with a block in 40S recycling. We found that unrecycled ribosomes could reinitiate translation at AUG codons in the 3’UTR, as evidenced by peaks in the footprint data and 3’UTR reporter analysis. In vitro translation experiments using reporter mRNAs containing upstream ORFs (uORFs) further established that reinitiation increased in the absence of these proteins. In some cases, 40S ribosomes appeared to rejoin with 60S subunits and undergo an alternative 80S reinitiation process in 3’UTRs. These results support a crucial role for Tma64, Tma20, and Tma22 in the recycling of 40S ribosomal subunits at stop codons and translation reinitiation.