Dissecting the basis for differential substrate specificity of ADAR1 and ADAR2.

Millions of adenosines are deaminated throughout the transcriptome by ADAR1 and/or ADAR2 at varying levels, raising the question of what are the determinants guiding substrate specificity and how these differ between the two enzymes. We monitor how secondary structure modulates ADAR2 vs ADAR1 substrate selectivity, on the basis of systematic probing of thousands of synthetic sequences transfected into cell lines expressing exclusively ADAR1 or ADAR2. Both enzymes induce symmetric, strand-specific editing, yet with distinct offsets with respect to structural disruptions: -26 nt for ADAR2 and -35 nt for ADAR1. We unravel the basis for these differences in offsets through mutants, domain-swaps, and ADAR homologs, and find it to be encoded by the differential RNA binding domain (RBD) architecture. Finally, we demonstrate that this offset-enhanced editing can allow an improved design of ADAR2-recruiting therapeutics, with proof-of-concept experiments demonstrating increased on-target and potentially decreased off-target editing.

Exclusion of m6A from splice-site proximal regions by the exon junction complex dictates m6A topologies and mRNA stability.

N6-methyladenosine (m6A), a widespread destabilizing mark on mRNA, is non-uniformly distributed across the transcriptome, yet the basis for its selective deposition is unknown. Here, we propose that m6A deposition is not selective. Instead, it is exclusion based: m6A consensus motifs are methylated by default, unless they are within a window of ∼100 nt from a splice junction. A simple model which we extensively validate, relying exclusively on presence of m6A motifs and exon-intron architecture, allows in silico recapitulation of experimentally measured m6A profiles. We provide evidence that exclusion from splice junctions is mediated by the exon junction complex (EJC), potentially via physical occlusion, and that previously observed associations between exon-intron architecture and mRNA decay are mechanistically mediated via m6A. Our findings establish a mechanism coupling nuclear mRNA splicing and packaging with the covalent installation of m6A, in turn controlling cytoplasmic decay.

A systematic dissection of determinants and consequences of snoRNA-guided pseudouridylation of human mRNA.

RNA can be extensively modified post-transcriptionally with >170 covalent modifications, expanding its functional and structural repertoire. Pseudouridine (Ψ), the most abundant modified nucleoside in rRNA and tRNA, has recently been found within mRNA molecules. It remains unclear whether pseudouridylation of mRNA can be snoRNA-guided, bearing important implications for understanding the physiological target spectrum of snoRNAs and for their potential therapeutic exploitation in genetic diseases. Here, using a massively parallel reporter based strategy we simultaneously interrogate Ψ levels across hundreds of synthetic constructs with predesigned complementarity against endogenous snoRNAs. Our results demonstrate that snoRNA-mediated pseudouridylation can occur on mRNA targets. However, this is typically achieved at relatively low efficiencies, and is constrained by mRNA localization, snoRNA expression levels and the length of the snoRNA:mRNA complementarity stretches. We exploited these insights for the design of snoRNAs targeting pseudouridylation at premature termination codons, which was previously shown to suppress translational termination. However, in this and follow-up experiments in human cells we observe no evidence for significant levels of readthrough of pseudouridylated stop codons. Our study enhances our understanding of the scope, 'design rules', constraints and consequences of snoRNA-mediated pseudouridylation.

Cloning of DNA oligo pools for in vitro expression.

Oligo library pools are powerful tools for systematic investigation of genetic and transcriptomic machinery such as promoter function and gene regulation, non-coding RNAs, or RNA modifications. Here, we provide a detailed protocol for cloning DNA oligo pools made up of tens of thousands of different constructs, aiming to preserve the complexity of the pools. This system would be suitable for expression in cell lines and can be followed up by next-generation sequencing analysis. For complete details on the use and execution of this profile, please refer to Uzonyi et al. (2021).

Deciphering the principles of the RNA editing code via large-scale systematic probing.

Adenosine-to-inosine editing is catalyzed by ADAR1 at thousands of sites transcriptome-wide. Despite intense interest in ADAR1 from physiological, bioengineering, and therapeutic perspectives, the rules of ADAR1 substrate selection are poorly understood. Here, we used large-scale systematic probing of ∼2,000 synthetic constructs to explore the structure and sequence context determining editability. We uncover two structural layers determining the formation and propagation of A-to-I editing, independent of sequence. First, editing is robustly induced at fixed intervals of 35 bp upstream and 30 bp downstream of structural disruptions. Second, editing is symmetrically introduced on opposite sites on a double-stranded structure. Our findings suggest a recursive model for RNA editing, whereby the structural alteration induced by the editing at one site iteratively gives rise to the formation of an additional editing site at a fixed periodicity, serving as a basis for the propagation of editing along and across both strands of double-stranded RNA structures.

mito-Ψ-Seq: A High-Throughput Method for Systematic Mapping of Pseudouridine Within Mitochondrial RNA.

RNA modifications are present in most cellular RNAs and are formed posttranscriptionally by enzymatic machineries that involve hundreds of enzymes and cofactors. RNA modifications impact the life cycle of the RNA, its stability, folding, cellular localization, as well as interactions with RNA and protein partners. RNA modifications are important for mitochondrial function and are required for proper processing and function of mitochondrial (mt) tRNA and rRNA. Underscoring their importance, several mitochondrial diseases are caused by defects in mt-RNA modifications, stemming from mutations in mtDNA at or near mt-RNA modification sites or in nuclear-encoded mt-RNA modifying enzymes. A highly abundant RNA modification, involved in mitochondrial physiology and pathology is pseudouridylation (Ψ), which is catalyzed by enzymes of the Pseudouridine Synthase (PUS) family. Although some Ψ sites in mt-rRNA and mt-tRNA have been identified, little is known about the functional role of these modifications. Furthermore, it is unknown which enzyme facilitates the modification of each site and it is likely that many yet undiscovered mt-RNA modifications exist, as is evidenced by recent work showing some Ψ sites on mRNA. Here, we present mito-Ψ-Seq, a high-throughput method for semiquantitative mapping of Ψ in mt-RNA.

Dynamic RNA acetylation revealed by quantitative cross-evolutionary mapping.

N4-acetylcytidine (ac4C) is an ancient and highly conserved RNA modification that is present on tRNA and rRNA and has recently been investigated in eukaryotic mRNA1-3. However, the distribution, dynamics and functions of cytidine acetylation have yet to be fully elucidated. Here we report ac4C-seq, a chemical genomic method for the transcriptome-wide quantitative mapping of ac4C at single-nucleotide resolution. In human and yeast mRNAs, ac4C sites are not detected but can be induced-at a conserved sequence motif-via the ectopic overexpression of eukaryotic acetyltransferase complexes. By contrast, cross-evolutionary profiling revealed unprecedented levels of ac4C across hundreds of residues in rRNA, tRNA, non-coding RNA and mRNA from hyperthermophilic archaea. Ac4C is markedly induced in response to increases in temperature, and acetyltransferase-deficient archaeal strains exhibit temperature-dependent growth defects. Visualization of wild-type and acetyltransferase-deficient archaeal ribosomes by cryo-electron microscopy provided structural insights into the temperature-dependent distribution of ac4C and its potential thermoadaptive role. Our studies quantitatively define the ac4C landscape, providing a technical and conceptual foundation for elucidating the role of this modification in biology and disease4-6.

Deciphering the "m6A Code" via Antibody-Independent Quantitative Profiling.

N6-methyladenosine (m6A) is the most abundant modification on mRNA and is implicated in critical roles in development, physiology, and disease. A major limitation has been the inability to quantify m6A stoichiometry and the lack of antibody-independent methodologies for interrogating m6A. Here, we develop MAZTER-seq for systematic quantitative profiling of m6A at single-nucleotide resolution at 16%-25% of expressed sites, building on differential cleavage by an RNase. MAZTER-seq permits validation and de novo discovery of m6A sites, calibration of the performance of antibody-based approaches, and quantitative tracking of m6A dynamics in yeast gametogenesis and mammalian differentiation. We discover that m6A stoichiometry is "hard coded" in cis via a simple and predictable code, accounting for 33%-46% of the variability in methylation levels and allowing accurate prediction of m6A loss and acquisition events across evolution. MAZTER-seq allows quantitative investigation of m6A regulation in subcellular fractions, diverse cell types, and disease states.

Systematic interrogation of human promoters.

Despite much research, our understanding of the architecture and cis-regulatory elements of human promoters is still lacking. Here, we devised a high-throughput assay to quantify the activity of approximately 15,000 fully designed sequences that we integrated and expressed from a fixed location within the human genome. We used this method to investigate thousands of native promoters and preinitiation complex (PIC) binding regions followed by in-depth characterization of the sequence motifs underlying promoter activity, including core promoter elements and TF binding sites. We find that core promoters drive transcription mostly unidirectionally and that sequences originating from promoters exhibit stronger activity than those originating from enhancers. By testing multiple synthetic configurations of core promoter elements, we dissect the motifs that positively and negatively regulate transcription as well as the effect of their combinations and distances, including a 10-bp periodicity in the optimal distance between the TATA and the initiator. By comprehensively screening 133 TF binding sites, we find that in contrast to core promoters, TF binding sites maintain similar activity levels in both orientations, supporting a model by which divergent transcription is driven by two distinct unidirectional core promoters sharing bidirectional TF binding sites. Finally, we find a striking agreement between the effect of binding site multiplicity of individual TFs in our assay and their tendency to appear in homotypic clusters throughout the genome. Overall, our study systematically assays the elements that drive expression in core and proximal promoter regions and sheds light on organization principles of regulatory regions in the human genome.

Unraveling the determinants of microRNA mediated regulation using a massively parallel reporter assay.

Despite extensive research, the sequence features affecting microRNA-mediated regulation are not well understood, limiting our ability to predict gene expression levels in both native and synthetic sequences. Here we employed a massively parallel reporter assay to investigate the effect of over 14,000 rationally designed 3' UTR sequences on reporter construct repression. We found that multiple factors, including microRNA identity, hybridization energy, target accessibility, and target multiplicity, can be manipulated to achieve a predictable, up to 57-fold, change in protein repression. Moreover, we predict protein repression and RNA levels with high accuracy (R = 0.84 and R = 0.80, respectively) using only 3' UTR sequence, as well as the effect of mutation in native 3' UTRs on protein repression (R = 0.63). Taken together, our results elucidate the effect of different sequence features on miRNA-mediated regulation and demonstrate the predictability of their effect on gene expression with applications in regulatory genomics and synthetic biology.

The m1A landscape on cytosolic and mitochondrial mRNA at single-base resolution.

Modifications on mRNA offer the potential of regulating mRNA fate post-transcriptionally. Recent studies suggested the widespread presence of N1-methyladenosine (m1A), which disrupts Watson-Crick base pairing, at internal sites of mRNAs. These studies lacked the resolution of identifying individual modified bases, and did not identify specific sequence motifs undergoing the modification or an enzymatic machinery catalysing them, rendering it challenging to validate and functionally characterize putative sites. Here we develop an approach that allows the transcriptome-wide mapping of m1A at single-nucleotide resolution. Within the cytosol, m1A is present in a low number of mRNAs, typically at low stoichiometries, and almost invariably in tRNA T-loop-like structures, where it is introduced by the TRMT6/TRMT61A complex. We identify a single m1A site in the mitochondrial ND5 mRNA, catalysed by TRMT10C, with methylation levels that are highly tissue specific and tightly developmentally controlled. m1A leads to translational repression, probably through a mechanism involving ribosomal scanning or translation. Our findings suggest that m1A on mRNA, probably because of its disruptive impact on base pairing, leads to translational repression, and is generally avoided by cells, while revealing one case in mitochondria where tight spatiotemporal control over m1A levels was adopted as a potential means of post-transcriptional regulation.

Sequence features of viral and human Internal Ribosome Entry Sites predictive of their activity.

Translation of mRNAs through Internal Ribosome Entry Sites (IRESs) has emerged as a prominent mechanism of cellular and viral initiation. It supports cap-independent translation of select cellular genes under normal conditions, and in conditions when cap-dependent translation is inhibited. IRES structure and sequence are believed to be involved in this process. However due to the small number of IRESs known, there have been no systematic investigations of the determinants of IRES activity. With the recent discovery of thousands of novel IRESs in human and viruses, the next challenge is to decipher the sequence determinants of IRES activity. We present the first in-depth computational analysis of a large body of IRESs, exploring RNA sequence features predictive of IRES activity. We identified predictive k-mer features resembling IRES trans-acting factor (ITAF) binding motifs across human and viral IRESs, and found that their effect on expression depends on their sequence, number and position. Our results also suggest that the architecture of retroviral IRESs differs from that of other viruses, presumably due to their exposure to the nuclear environment. Finally, we measured IRES activity of synthetically designed sequences to confirm our prediction of increasing activity as a function of the number of short IRES elements.

TRUB1 is the predominant pseudouridine synthase acting on mammalian mRNA via a predictable and conserved code.

Following synthesis, RNA can be modified with over 100 chemically distinct modifications, which can potentially regulate RNA expression post-transcriptionally. Pseudouridine (Ψ) was recently established to be widespread and dynamically regulated on yeast mRNA, but less is known about Ψ presence, regulation, and biogenesis in mammalian mRNA. Here, we sought to characterize the Ψ landscape on mammalian mRNA, to identify the main Ψ-synthases (PUSs) catalyzing Ψ formation, and to understand the factors governing their specificity toward selected targets. We first developed a framework allowing analysis, evaluation, and integration of Ψ mappings, which we applied to >2.5 billion reads from 30 human samples. These maps, complemented with genetic perturbations, allowed us to uncover TRUB1 and PUS7 as the two key PUSs acting on mammalian mRNA and to computationally model the sequence and structural elements governing the specificity of TRUB1, achieving near-perfect prediction of its substrates (AUC = 0.974). We then validated and extended these maps and the inferred specificity of TRUB1 using massively parallel reporter assays in which we monitored Ψ levels at thousands of synthetically designed sequence variants comprising either the sequences surrounding pseudouridylation targets or systematically designed mutants perturbing RNA sequence and structure. Our findings provide an extensive and high-quality characterization of the transcriptome-wide distribution of pseudouridine in human and the factors governing it and provide an important resource for the community, paving the path toward functional and mechanistic dissection of this emerging layer of post-transcriptional regulation.

Comparative genetics. Systematic discovery of cap-independent translation sequences in human and viral genomes.

To investigate gene specificity at the level of translation in both the human genome and viruses, we devised a high-throughput bicistronic assay to quantify cap-independent translation. We uncovered thousands of novel cap-independent translation sequences, and we provide insights on the landscape of translational regulation in both humans and viruses. We find extensive translational elements in the 3' untranslated region of human transcripts and the polyprotein region of uncapped RNA viruses. Through the characterization of regulatory elements underlying cap-independent translation activity, we identify potential mechanisms of secondary structure, short sequence motif, and base pairing with the 18S ribosomal RNA (rRNA). Furthermore, we systematically map the 18S rRNA regions for which reverse complementarity enhances translation. Thus, we make available insights into the mechanisms of translational control in humans and viruses.

Phosphorylation of the Drosophila melanogaster RNA-binding protein HOW by MAPK/ERK enhances its dimerization and activity.

Drosophila melanogaster Held Out Wings (HOW) is a conserved RNA-binding protein (RBP) belonging to the STAR family, whose closest mammalian ortholog Quaking (QKI) has been implicated in embryonic development and nervous system myelination. The HOW RBP modulates a variety of developmental processes by controlling mRNA levels and the splicing profile of multiple key regulatory genes; however, mechanisms regulating its activity in tissues have yet to be elucidated. Here, we link receptor tyrosine kinase (RTK) signaling to the regulation of QKI subfamily of STAR proteins, by showing that HOW undergoes phosphorylation by MAPK/ERK. Importantly, we show that this modification facilitates HOW dimerization and potentiates its ability to bind RNA and regulate its levels. Employing an antibody that specifically recognizes phosphorylated HOW, we show that HOW is phosphorylated in embryonic muscles and heart cardioblasts in vivo, thus documenting for the first time Serine/Threonine (Ser/Thr) phosphorylation of a STAR protein in the context of an intact organism. We also identify the sallimus/D-titin (sls) gene as a novel muscle target of HOW-mediated negative regulation and further show that this regulation is phosphorylation-dependent, underscoring the physiological relevance of this modification. Importantly, we demonstrate that HOW Thr phosphorylation is reduced following muscle-specific knock down of Drosophila MAPK rolled and that, correspondingly, Sls is elevated in these muscles, similarly to the HOW RNAi effect. Taken together, our results provide a coherent mechanism of differential HOW activation; MAPK/ERK-dependent phosphorylation of HOW promotes the formation of HOW dimers and thus enhances its activity in controlling mRNA levels of key muscle-specific genes. Hence, our findings bridge between MAPK/ERK signaling and RNA regulation in developing muscles.

Tissue development and RNA control: "HOW" is it coordinated?

The regulation of developmental processes at the RNA level enables selective and rapid modulation of gene expression. Studies in model organisms revealed the essential contribution of the signal transduction and activation of RNA (STAR) family of RNA binding proteins to developmental processes. STAR proteins coordinate the proper timing of developmental events by delaying expression or altering the mRNA or protein levels of essential genes. Recent functional analysis of the Drosophila melanogaster STAR protein, Held Out Wing (HOW), in the context of embryonic development, provided insight into its mode of activity. Here, we describe HOW's activity in the temporal repression or elevation of gene expression that is essential for coordinating the correct timing of instructive signals.

Post-transcriptional repression of the Drosophila midkine and pleiotrophin homolog miple by HOW is essential for correct mesoderm spreading.

The even spreading of mesoderm cells in the Drosophila embryo is essential for its proper patterning by ectodermally derived signals. In how germline clone embryos, defects in mesoderm spreading lead to a partial loss of dorsal mesoderm derivatives. HOW is an RNA-binding protein that is thought to regulate diverse mRNA targets. To identify direct HOW targets, we implemented a series of selection methods on mRNAs whose levels were elevated in how germline clone embryos during the stage of mesoderm spreading. Four mRNAs were found to be specifically elevated in the mesoderm of how germline clone embryos, and to exhibit specific binding to HOW via their 3' UTRs. Importantly, overexpression of three of these genes phenocopied the mesoderm-spreading phenotype of how germline clone embryos. Further analysis showed that overexpressing one of these genes, miple (a Drosophila midkine and pleiotrophin heparin-binding growth factor), in the mesoderm led to abnormal scattered MAPK activation, a phenotype that might explain the abnormal mesoderm spreading. In addition, the number of EVE-positive cells, which are responsive to receptor tyrosine kinase (RTK) signaling, was increased following Miple overexpression in the mesoderm and appeared to be dependent on Heartless function. In summary, our analysis suggests that HOW downregulates the levels of a number of mRNA species in the mesoderm in order to enable proper mesoderm spreading during early embryogenesis.

Dissection of the target specificity of the RNA-binding protein HOW reveals dpp mRNA as a novel HOW target.

Regulation of RNA metabolism plays a major role in controlling gene expression during developmental processes. The Drosophila RNA-binding protein Held out wing (HOW), regulates an array of developmental processes in embryonic and adult growth. We have characterized the primary sequence and secondary structural requirements for the HOW response element (HRE), and show that this site is necessary and sufficient for HOW binding. Based on this analysis, we have identified the Drosophila TGFbeta homolog, dpp, as a novel direct target for HOW negative regulation in the wing imaginal disc. The binding of the repressor isoform HOW(L) to the dpp 3' untranslated region (UTR) leads to a reduction of GFP-dpp3'UTR reporter levels in S-2 cells, in an HRE site-dependent manner. Moreover, co-expression of HOW(L) in the wing imaginal disc with a dpp-GFP fusion construct led to a reduction in DPP-GFP levels in a dpp-3'UTR-dependent manner. Conversely, a reduction of the endogenous levels of HOW by targeted expression of HOW-specific double-stranded RNA led to a corresponding elevation in dpp mRNA level in the wing imaginal disc. Thus, by characterizing the RNA sequences that bind HOW, we demonstrate a novel aspect of regulation, at the mRNA level, of Drosophila DPP.

DCX's phosphorylation by not just another kinase (JNK).

The mammalian cortex is generally subdivided into six organized layers, which are formed during development in an organized fashion. This organized cortical layering is disrupted in case of mutations in the doublecortin (DCX) gene. DCX is a Microtubule Associated Protein (MAP). However, besides stabilization of microtubules, it may be involved in additional functions. The participation of this molecule in signal transduction is beginning to emerge via discovery of interacting molecules and its regulation by phosphorylation using several different kinases. We raise the hypothesis, that the combinatorial phosphorylation of DCX by different kinases and at different sites may be a molecular regulatory switch in the transition of a migrating neuron through multiple phases of migration. Our recent research has suggested the involvement of DCX in the JNK (Jun-N-terminal Kinase) pathway. The JNK pathway is linked to the reelin pathway, known to regulate cortical layering. Positioning of DCX in this signaling pathway opens up additional possibilities of understanding how migrating neurons are controlled.