High-throughput 5'P sequencing enables the study of degradation-associated ribosome stalls

The ABCF ATPase New1 resolves translation termination defects associated with specific tRNAArg and tRNALys isoacceptors in the P site

The efficiency of translation termination is determined by the nature of the stop codon as well as its context. In eukaryotes, recognition of the A-site stop codon and release of the polypeptide are mediated by release factors eRF1 and eRF3, respectively. Translation termination is modulated by other factors which either directly interact with release factors or bind to the E-site and modulate the activity of the peptidyl transferase center. Previous studies suggested that the Saccharomyces cerevisiae ABCF ATPase New1 is involved in translation termination and/or ribosome recycling, however, the exact function remained unclear. Here, we have applied 5PSeq, single-particle cryo-EM and readthrough reporter assays to provide insight into the biological function of New1. We show that the lack of New1 results in ribosomal stalling at stop codons preceded by a lysine or arginine codon and that the stalling is not defined by the nature of the C-terminal amino acid but rather by the identity of the tRNA isoacceptor in the P-site. Collectively, our results suggest that translation termination is inefficient when ribosomes have specific tRNA isoacceptors in the P-site and that the recruitment of New1 rescues ribosomes at these problematic termination contexts.

Genome-wide analysis of mRNA decay in Arabidopsis shoot and root reveals the importance of co-translational mRNA decay in the general mRNA turnover

Until recently, the general 5'-3' mRNA decay was placed in the cytosol after the mRNA was released from ribosomes. However, the discovery of an additional 5' to 3' pathway, the Co-Translational mRNA Decay (CTRD), changed this paradigm. Up to date, defining the real contribution of CTRD in the general mRNA turnover has been hardly possible as the enzyme involved in this pathway is also involved in cytosolic decay. Here we overcame this obstacle and created an Arabidopsis line specifically impaired for CTRD called XRN4ΔCTRD. Through a genome-wide analysis of mRNA decay rate in shoot and root, we tested the importance of CTRD in mRNA turnover. First, we observed that mRNAs tend to be more stable in root than in shoot. Next, using XRN4ΔCTRD line, we demonstrated that CTRD is a major determinant in mRNA turnover. In shoot, the absence of CTRD leads to the stabilization of thousands of transcripts while in root its absence is highly compensated resulting in faster decay rates. We demonstrated that this faster decay rate is partially due to the XRN4-dependent cytosolic decay. Finally, we correlated this organ-specific effect with XRN4ΔCTRD line phenotypes revealing a crucial role of CTRD in mRNA homeostasis and proper organ development.

The ABCF ATPase New1 resolves translation termination defects associated with specific tRNAArg and tRNALys isoacceptors in the P site

The efficiency of translation termination is determined by the nature of the stop codon as well as its context. In eukaryotes, recognition of the A-site stop codon and release of the polypeptide are mediated by release factors eRF1 and eRF3, respectively. Translation termination is modulated by other factors which either directly interact with release factors or bind to the E-site and modulate the activity of the peptidyl transferase center. Previous studies suggested that the Saccharomyces cerevisiae ABCF ATPase New1 is involved in translation termination and/or ribosome recycling, however, the exact function remained unclear. Here, we have applied 5PSeq, single-particle cryo-EM and readthrough reporter assays to provide insight into the biological function of New1. We show that the lack of New1 results in ribosomal stalling at stop codons preceded by a lysine or arginine codon and that the stalling is not defined by the nature of the C-terminal amino acid but rather by the identity of the tRNA isoacceptor in the P-site. Collectively, our results suggest that translation termination is inefficient when ribosomes have specific tRNA isoacceptors in the P-site and that the recruitment of New1 rescues ribosomes at these problematic termination contexts.

Pervasive translation of Xrn1-sensitive unstable long noncoding RNAs in yeast

Despite being predicted to lack coding potential, cytoplasmic long noncoding (lnc)RNAs can associate with ribosomes. However, the landscape and biological relevance of lncRNA translation remain poorly studied. In yeast, cytoplasmic Xrn1-sensitive unstable transcripts (XUTs) are targeted by nonsense-mediated mRNA decay (NMD), suggesting a translation-dependent degradation process. Here, we report that XUTs are pervasively translated, which impacts their decay. We show that XUTs globally accumulate upon translation elongation inhibition, but not when initial ribosome loading is impaired. Ribo-seq confirmed ribosomes binding to XUTs and identified ribosome-associated 5'-proximal small ORFs. Mechanistically, the NMD-sensitivity of XUTs mainly depends on the 3'-untranslated region length. Finally, we show that the peptide resulting from the translation of an NMD-sensitive XUT reporter exists in NMD-competent cells. Our work highlights the role of translation in the posttranscriptional metabolism of XUTs. We propose that XUT-derived peptides could be exposed to natural selection, while NMD restricts XUT levels.

Cellular energy regulates mRNA degradation in a codon-specific manner

Codon optimality is a major determinant of mRNA translation and degradation rates. However, whether and through which mechanisms its effects are regulated remains poorly understood. Here we show that codon optimality associates with up to 2-fold change in mRNA stability variations between human tissues, and that its effect is attenuated in tissues with high energy metabolism and amplifies with age. Mathematical modeling and perturbation data through oxygen deprivation and ATP synthesis inhibition reveal that cellular energy variations non-uniformly alter the effect of codon usage. This new mode of codon effect regulation, independent of tRNA regulation, provides a fundamental mechanistic link between cellular energy metabolism and eukaryotic gene expression.

Atlas of mRNA translation and decay for bacteria

Regulation of messenger RNA stability is pivotal for programmed gene expression in bacteria and is achieved by a myriad of molecular mechanisms. By bulk sequencing of 5' monophosphorylated mRNA decay intermediates (5'P), we show that cotranslational mRNA degradation is conserved among both Gram-positive and -negative bacteria. We demonstrate that, in species with 5'-3' exonucleases, the exoribonuclease RNase J tracks the trailing ribosome to produce an in vivo single-nucleotide toeprint of the 5' position of the ribosome. In other species lacking 5'-3' exonucleases, ribosome positioning alters endonucleolytic cleavage sites. Using our metadegradome (5'P degradome) sequencing approach, we characterize 5'P mRNA decay intermediates in 96 species including Bacillus subtilis, Escherichia coli, Synechocystis spp. and Prevotella copri and identify codon- and gene-level ribosome stalling responses to stress and drug treatment. We also apply 5'P sequencing to complex clinical and environmental microbiomes and demonstrate that metadegradome sequencing provides fast, species-specific posttranscriptional characterization of responses to drug or environmental perturbations. Finally we produce a degradome atlas for 96 species to enable analysis of mechanisms of RNA degradation in bacteria. Our work paves the way for the application of metadegradome sequencing to investigation of posttranscriptional regulation in unculturable species and complex microbial communities.

Not1 and Not4 inversely determine mRNA solubility that sets the dynamics of co-translational events

BACKGROUND

The Ccr4-Not complex is mostly known as the major eukaryotic deadenylase. However, several studies have uncovered roles of the complex, in particular of the Not subunits, unrelated to deadenylation and relevant for translation. In particular, the existence of Not condensates that regulate translation elongation dynamics has been reported. Typical studies that evaluate translation efficiency rely on soluble extracts obtained after the disruption of cells and ribosome profiling. Yet cellular mRNAs in condensates can be actively translated and may not be present in such extracts.

RESULTS

In this work, by analyzing soluble and insoluble mRNA decay intermediates in yeast, we determine that insoluble mRNAs are enriched for ribosomes dwelling at non-optimal codons compared to soluble mRNAs. mRNA decay is higher for soluble RNAs, but the proportion of co-translational degradation relative to the overall mRNA decay is higher for insoluble mRNAs. We show that depletion of Not1 and Not4 inversely impacts mRNA solubilities and, for soluble mRNAs, ribosome dwelling according to codon optimality. Depletion of Not4 solubilizes mRNAs with lower non-optimal codon content and higher expression that are rendered insoluble by Not1 depletion. By contrast, depletion of Not1 solubilizes mitochondrial mRNAs, which are rendered insoluble upon Not4 depletion.

CONCLUSIONS

Our results reveal that mRNA solubility defines the dynamics of co-translation events and is oppositely regulated by Not1 and Not4, a mechanism that we additionally determine may already be set by Not1 promoter association in the nucleus.

Transcriptomic diversity in human medullary thymic epithelial cells

The induction of central T cell tolerance in the thymus depends on the presentation of peripheral self-epitopes by medullary thymic epithelial cells (mTECs). This promiscuous gene expression (pGE) drives mTEC transcriptomic diversity, with non-canonical transcript initiation, alternative splicing, and expression of endogenous retroelements (EREs) representing important but incompletely understood contributors. Here we map the expression of genome-wide transcripts in immature and mature human mTECs using high-throughput 5' cap and RNA sequencing. Both mTEC populations show high splicing entropy, potentially driven by the expression of peripheral splicing factors. During mTEC maturation, rates of global transcript mis-initiation increase and EREs enriched in long terminal repeat retrotransposons are up-regulated, the latter often found in proximity to differentially expressed genes. As a resource, we provide an interactive public interface for exploring mTEC transcriptomic diversity. Our findings therefore help construct a map of transcriptomic diversity in the healthy human thymus and may ultimately facilitate the identification of those epitopes which contribute to autoimmunity and immune recognition of tumor antigens.

Structural basis for PoxtA-mediated resistance to phenicol and oxazolidinone antibiotics

PoxtA and OptrA are ATP binding cassette (ABC) proteins of the F subtype (ABCF). They confer resistance to oxazolidinone and phenicol antibiotics, such as linezolid and chloramphenicol, which stall translating ribosomes when certain amino acids are present at a defined position in the nascent polypeptide chain. These proteins are often encoded on mobile genetic elements, facilitating their rapid spread amongst Gram-positive bacteria, and are thought to confer resistance by binding to the ribosome and dislodging the bound antibiotic. However, the mechanistic basis of this resistance remains unclear. Here we refine the PoxtA spectrum of action, demonstrate alleviation of linezolid-induced context-dependent translational stalling, and present cryo-electron microscopy structures of PoxtA in complex with the Enterococcus faecalis 70S ribosome. PoxtA perturbs the CCA-end of the P-site tRNA, causing it to shift by ∼4 Å out of the ribosome, corresponding to a register shift of approximately one amino acid for an attached nascent polypeptide chain. We postulate that the perturbation of the P-site tRNA by PoxtA thereby alters the conformation of the attached nascent chain to disrupt the drug binding site.

Not4 and Not5 modulate translation elongation by Rps7A ubiquitination, Rli1 moonlighting, and condensates that exclude eIF5A

In this work, we show that Not4 and Not5 from the Ccr4-Not complex modulate translation elongation dynamics and change ribosome A-site dwelling occupancy in a codon-dependent fashion. These codon-specific changes in not5Δ cells are very robust and independent of codon position within the mRNA, the overall mRNA codon composition, or changes of mRNA expression levels. They inversely correlate with codon-specific changes in cells depleted for eIF5A and positively correlate with those in cells depleted for ribosome-recycling factor Rli1. Not5 resides in punctate loci, co-purifies with ribosomes and Rli1, but not with eIF5A, and limits mRNA solubility. Overexpression of wild-type or non-complementing Rli1 and loss of Rps7A ubiquitination enable Not4 E3 ligase-dependent translation of polyarginine stretches. We propose that Not4 and Not5 modulate translation elongation dynamics to produce a soluble proteome by Rps7A ubiquitination, dynamic condensates that limit mRNA solubility and exclude eIF5A, and a moonlighting function of Rli1.