Abstract
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 mRNAs1,2. 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.
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Acknowledgements
This project was supported by the Israel Science Foundation (543165), the European Research Council under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 714023), by the Abisch-Frenkel-Stiftung, by research grants from The Abramson Family Center for Young Scientists, the David and Fela Shapell Family Foundation INCPM Fund for Preclinical Studies, the Estate of David Turner, and the Berlin Family Foundation New Scientist Fund.
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Contributions
M.S., A.S.-C., R.N., D.B., M.E., W.R., N.S.-G., and S.S. designed the experiments. M.S. performed the m1A-seq optimizations and experiments, the luciferase assays, and the genetic perturbations. M.S. and A.S.-C. performed the targeted sequencing experiments and the mitochondrial RNA half-life experiments. M.S. and R.W. performed the polysomal fractionations. R.N. performed the oligoarray experiment. A.N. assisted in the GTEx analysis. S.S. and M.S. performed the computational analysis. S.S. wrote the manuscript with input from all authors.
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Extended data figures and tables
Extended Data Figure 1 Analytical pipeline for identification of m1A sites.
a, Overview of the analytical pipeline, which was applied to each of the four data sets. The pipeline utilizes two key statistical tests to identify ‘A’ containing sites that either harbour misincorporations at higher levels following immunoprecipitation with an anti-m1A antibody compared with input samples or compared with Dimroth-treated samples. In addition, the pipeline requires that minimum levels of at least two distinct types of misincorporation be observed for a given site. b–d, Truncation and misincorporation rates are reproducible between replicates, both with SuperScript and with TGIRT enzymes. Shown are scatterplots based on 205 putative m1A sites, indicating the correlation between replicates of truncation rates using SuperScript (b) misincorporation rates using SuperScript (c), and misincorporation rates using TGIRT (d). e, Comparison of truncation rates using TGIRT (y axis) and SuperScript (x axis), on the basis of the 205 detected sites. Sites are colour-coded on the basis of the indicated classes of RNA. f, Comparison of misincorporation rates using TGIRT and SuperScript, plotted as in e. g, Comparison of misincorporation rates in IP samples compared with IP + Dimroth, in both cases using TGIRT. h, Comparison of misincorporation rates in IP samples compared with input, using TGIRT. i, Serial dilutions of RNA extracted from cells overexpressing TRMT6/TRMT61A within RNA extracted from cells depleted of TRMT6/TRMT61A. The x axis denotes the percentage of the RNA originating from cells overexpressing TRMT6/TRMT61A, and the y axis captures the misincorporation rate, as measured via targeted sequencing of the three presented sites.
Extended Data Figure 2 Misincorporation levels at putative sites on the basis of GTEx data set.
Shown are misincorporation levels across eight lncRNA + mRNA and two rRNA sites on the basis of a random sampling of 4,114 GTEx RNA-seq samples. Boxplots represent median, interquartile range, distribution, and outliers, as indicated in Fig. 2h.
Extended Data Figure 3 m1A detection as a function of gene expression, on the basis of analysis of sites identified following TRMT6/TRMT61A overexpression.
a, All genes were divided into five bins, on the basis of the indicated levels of expression. The number of m1A harbouring sites in each bin is plotted. b, As in a, but normalized by number of genes within each bin. Error bars, binomial confidence intervals.
Extended Data Figure 4 Peak detection following m1A-seq reveals a strong enrichment towards the 5′ terminus.
a, Representative meta-gene profiles of read coverage in m1A-IP (top) or input (bottom). Shown are the first 500 nt immediately following the transcription start site (left) and immediately preceding the annotated 3′ terminus of the gene (right). Genes are divided into five bins, on the basis of expression levels. b, Distribution of expression levels (on the basis of input samples) of all genes harbouring high-confidence peaks. On this distribution are overlaid, in red, the genes in which putative m1A sites were detected. c, Consistently identified peaks are highly enriched towards the 5′ terminus of the gene. Each peak was classified into one of five segments, as in ref. 41, in the following order: transcription start site (TSS) if the peak is present in the first 200 nt of the gene; 5′ UTR if in the 5′ UTR region but outside the transcription start site region; stop codon region, comprising 200 nt on both sides of the stop window; CDS region for peaks within the CDS; and 3′ UTR for remaining 3′ UTR peaks. Each peak is scored on the basis of the number of experiments in which it was detected, whereby more robustly identified peaks should be considered as ones of higher confidence. The stacked bar plots summarize the relative proportion of peaks in each segment, and the right-most bar plots the relative amount of ‘gene architecture’ taken up by each of these segments. d, Analysis of transcription start site peaks. The number of reads beginning (rather than overlapping) at each of the first 50 annotated transcribed bases was calculated across IP and input samples (Methods), and the log(fold change) between the two was derived. All fold changes were binned into six bins (as plotted) and the fraction of positions harbouring an ‘A’ are plotted as a function of this binned fold change, revealing that positions at the transcription start site that are enriched in IP samples over input samples are biased towards beginning with an ‘A’. Error bars, binomial error.
Extended Data Figure 5 Characterization of sequence and structure at m1A sites, and requirement of the TRMT6/TRMT61A machinery for its catalysis in the cytosol.
a, Predicted secondary structures of sequence environment surrounding putative m1A-containing sites in cytoplasmic mRNA and lncRNAs (see also Fig. 2a). b, Predicted secondary structure at the vicinity of mitochondrial m1A sites. A common secondary structure surrounding mitochondrial sites is depicted, featuring a ‘UAAA’ motif in the loop, stabilized by a stem structure. c, Misincorporation levels at the indicated sites, measured by targeted sequencing, in cells depleted of TRMT6/TRMT61A via siRNAs, compared with mock-treated controls. Error bars, binomial confidence intervals. d, e, Distributions of loop length (d) and stem length (e) among the 384 putative m1A sites in mRNA + lncRNAs, compared with randomly shuffled controls. These values are derived from RNAfold.
Extended Data Figure 6 Validation of putative m1A sites, via m6A-seq of Dimroth-converted RNA extracted from TRMT6/TRMT61A mRNA.
a, Coverage plot along the cytosolic 28S rRNA across the four indicated samples. The known m1A and m6A sites are indicated. b, Coverage plots as in a for the mitochondrial 16S rRNA. The normalized coverage levels are indicated to the left of the track (note the orders-of-magnitude higher coverage at the m1A sites upon m6A-IP in Dimroth-treated samples, compared with the controls). c, Quantification of percentage of coverage in a 100 nt centred region around putative m1A sites out of overall coverage of the gene in two biological replicates of mRNA extracted from cells overexpressing TRMT6/TRMT61A; this mRNA was subjected to Dimroth treatment followed by m6A-seq. Quantifications were obtained for 40 sites exhibiting misincorporation levels greater than 10% upon TRMT6/TRMT61A overexpression in addition to the 2 sites on the 16S and 28S rRNA molecules. d, Comparison of ‘% Coverage in peak’ (as in c) between RNA subjected to Dimroth treatment and m6A-IP compared with RNA only subjected to m6A-IP. e, Misincorporation levels across the indicated conditions across 8 sites (of the 42 tested) in which a significant P value (P < 0.05) was obtained when comparing Dimroth + m6A-IP samples with their corresponding input (significance indicated by ‘*’), and/or when comparing Dimroth + m6A-IP to no-Dimroth + m6A-IP (significance indicated by ‘#’). Error bars, binomial error after pooling of two biological replicates.
Extended Data Figure 7 ND5 methylation levels are genetically determined, are in part controlled by a SNP two bases upstream, and are increased in stable transcripts.
a, Snapshot of randomly sampled reads aligned to the ND5 locus. Reads originating from the heavy strand are depicted in red, reads from the light strand in purple. Top: IP sample; bottom: input sample. Misincorporations are apparent only in reads originating from the heavy strand. b, Misincorporation levels at the ND5 locus in DNA and RNA samples of five individuals. For RNA, the distribution of misincorporation reads are shown across all tissues; for DNA, the measurement consists of a single measurement available in GTEx. c, Misincorporation levels at ND5:1374 measured at the indicated time-points following ethidium-bromide-mediated transcriptional arrest (n = 3); Points, mean; error bars, s.e.m. d, Correlations between misincorporation levels at ND5:1374 in skin versus brain samples, from the same individuals. e, Histogram of all pairwise correlation coefficients between tissues (but from the same individuals); note that values are centred around 0.5, rather than around 0 if they were independent of each other. f, Misincorporation rates at ND5:1374 obtained via strand-specific targeted sequencing of the ND5 locus across six lymphoblastoid cell lines, two harbouring a G13708A SNP and four wild-type samples.
Extended Data Figure 8 m1A represses translation.
a, Representative sucrose gradient, indicating the division into fractions on the basis of the number of polysomes associated with them. b, Scheme of experimental design. The wild-type m1A-containing stretch from the PRUNE gene was cloned either in-frame and upstream of firefly luciferase (CDS construct) or as a 3′ UTR element (3′ UTR construct). Control or TRMT6/TRMT61A-overexpressing cells were co-transfected with each of these plasmids along with a plasmid expressing Renilla luciferase. c, d, Renilla-normalized firefly luciferase levels in TRMT6/TRMT61A-overexpressing cells, standardized by this value in the non-overexpressing (control) cells (n = 3). Note that for the CDS construct the presented data are identical to those in Fig. 4 and are re-plotted for convenience.
Extended Data Figure 9 Cytosolic m1A does not affect steady-state levels of mRNA or mRNA stability.
a, Comparison of expression levels of all genes acquiring robust levels of m1A (defined as misincorporation levels greater than 10%) upon overexpression of TRMT6/TRMT61A, compared with wild-type (non-overexpressing) counterparts. b, Misincorporation levels in the four indicated genes, in cells overexpressing TRMT6/TRMT61A, in a 6 h time course following cycloheximide treatment (n = 3). Error bars, s.e.m.
Supplementary information
Supplementary Information
This file contains Supplementary Notes 1-10 and Supplementary References.
Supplementary Table 1
Alignment statistics for all four analysed datasets, including total number of reads and the percentage of aligned reads.
Supplementary Table 2
Dataset of 277 putative m1A sites, obtained after intersection of the four analysed datasets – see Supplementary Information document for full description.
Supplementary Table 3
Dataset of 690 consistently identified peaks, used in the context of analyses presented in Extended Data Fig. 4 revealing strong bias towards 5’ termini of genes. The dataset is in bed format.
Supplementary Table 4
Dataset of 495 putative m1A sites, obtained following overexpression of TRMT6/TRMT61A.
Supplementary Table 5
A list of primer sequences used in this study.
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Safra, M., Sas-Chen, A., Nir, R. et al. The m1A landscape on cytosolic and mitochondrial mRNA at single-base resolution. Nature 551, 251–255 (2017). https://doi.org/10.1038/nature24456
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DOI: https://doi.org/10.1038/nature24456
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