Given the degeneracy of the genetic code, multiple 3-letter combinations of nucleotides can code for the same amino acid. Such synonymous codons are nevertheless not uniformly distributed along the genomes and can significantly deviate between organisms 1 . Evolutionary forces that explain the existence of the so-called codon bias include (1) a mutation pressure for a certain GC base composition depending on the species and chromosomal location, and (2) the translational selection for codons corresponding to highly expressed tRNA isoacceptors [2] [3] [4] .
Viruses strongly depend on the translational machinery of the host for the expression of their own proteins and, ultimately, their replication. For instance, given the small size of most viral genomes, no or very few tRNA genes are generally autonomously encoded 5 . In terms of codon usage, it has indeed been shown that bacteriophages are specifically adapted to their microbial hosts 6, 7 . This information has been applied in the prediction of viral hosts from metagenomics data 8, 9 . The codon usage of human-infecting viruses is similarly adapted to the host 10, 11 , and actually the concept of codon deoptimization has been applied in the design of attenuated vaccines 12 .
Although translational selection has long been under debate in human 13 , recent studies indicate that different tissues and conditions showcase distinct tRNA expression profiles, leading to changes in their respective translational efficiency 2, 14 . In agreement with this observation, the codon usage of papillomavirus capsid proteins is adapted to the tRNAs of differentiated keratinocytes, where their translation becomes specifically efficient 15, 16 . In addition, upon HIV-1 infection, the host tRNA pool is reprogrammed to favor translation of late viral genes 17 , a phenomenon that is indeed exploited by host antiviral mechanisms 18 . Furthermore, some viruses with a specific tissue tropism resemble the codon bias of highly expressed proteins of their respective infecting tissues 19 . Nevertheless, despite the few aforementioned studies, a high-throughput analysis of the translational selection of viral genomes to their tissue tropism has been heretofore hindered by the absence of tissue-wide tRNA expression data.
Here, we systematically analyze the relative codon usage landscape of over 500 humaninfecting viruses together with the recently published tRNA expression profiles of human tissues 2 . Among other viral annotated features, including phylogeny and Baltimore classification, their tissue tropism explains more variance in codon usage than the other tested features. In consequence, tropism corresponds with codon optimization patterns that can be associated with tissue-specific profiles of tRNA-based translation efficiencies.
The SARS-CoV-2 coronavirus constitutes the etiologic agent of the biggest pandemic of the 21 st century, causing the COVID-19 pneumonia-like disease. Originated in Wuhan (China), the virus has already caused 40,598 deaths as of 1 st April 2020 20 . In this context, recent scientific efforts suggest that the novel coronavirus strain has evolved to preferentially infect nasal goblet and ciliated cells as well as the type-2 alveolar epithelial cells 21, 22 . However, some non-respiratory-related symptoms such as loss of smell and taste hint as well to other tropism potential outside the respiratory tract 23, 24 . In terms of translational efficiency, we here discover that the coronavirus proteome is especially adapted not only to upper respiratory airways and alveoli, but also to the gastrointestinal tract and brain.
Further, by studying the tissue-adaptation among the viral proteome, we also determine that early replication-related proteins are more translationally-adapted than the late structural counterparts. Overall, we observe a tropism-specific adaptation of the viral proteome to the tRNA profiles of their target tissues, which opens the door to the development of tissue-specific codon-deoptimized vaccines and targeted antiviral therapies.
Publicly available genomic data comprised a total of 564 human-infecting viruses, distributed among 33 families and covering all seven Baltimore categories (Sup. Table 1 ). Across this diversity, six main viral tropisms were defined for 182 viruses based on the ViralZone curated database 25 : neurons, immune cells, respiratory tract, hepatocytes, intestine, and epithelial cells (Fig. 1A) . Their corresponding coding sequences constituted a total of 4935 viral proteins (Sup. Table 1 ), for which we determined the Relative Codon Usage (RCU, i.e. the contribution of each synonymous codon to the amino acid it encodes, see Methods).
In order to understand the main factors driving differences between viral RCU, we used three internal clustering indexes that assess how similar each virus is to a certain group compared to other groups. Taking the average RCU over each of the 564 viral proteomes, we applied this framework to assess the grouping performance of six different viral features: tropism, type of genetic material, family, genus, Baltimore category, and a sequence-based classification by Aiewsakun and Simmonds (2018) . In such analysis, the tropism leads the best classification of viral RCUs, followed by the viral genetic type (Fig. 1B) . On the other hand, classical and sequence-based phylogenetic classifications show poor clustering performances.
Given the impact of viral tropism on the RCU, we sought to determine the main codon differences between the six defined target tissues. By using a linear discriminant analysis (see Methods), the 182 tropism-defined viruses were classified in six clear clusters, regardless of other factors such as the phylogenetic lineage (Fig. 1C) . In consequence, we observe that specific codon usage profiles are associated with the tissue tropism of human-infecting viruses.
Based on the RCU differences between viruses with distinct tropism, we hypothesize that distinct tissues impose selection towards a certain set of translationally-efficient codons. However, a validation for this hypothesis requires the accurate quantification of tissue-specific tRNA profiles, which has been hitherto missing. With the advent of such high-throughput expression data 27, 28 , here we retrieved the previously-published Supply-to-Demand Adaptation (SDA) estimate for translational efficiency 2 , which computes the balance between the supply (i.e., the anticodon tRNA abundances) and demand (i.e., the codons expressed in mRNAs) of each codon (see Methods).
Using a total of 620 healthy samples from The Cancer Genome Atlas (TCGA) dataset 2 , we first computed the SDA of all viral-protein-coding sequences based on the SDA weights of their constituent codons. Therefore, taking the average of all healthy samples across each of the 23 TCGA cancer types (Sup. Table 2 ), we determined the estimated translational efficiencies of viral proteins in different human tissues (Sup. Table 3 ).
Next, from the perspective of the translational selection hypothesis, we would expect that viral proteins are translationally adapted to their target tissues. In consequence, we tried to test our hypothesis using a completely blind and unbiased random forest classifier, which applies machine learning in order to predict the tropism of each viral protein based on the SDA to different tissues (see Methods). The resulting performance of the models, based on the Area Under the Curve (AUC) of their Receiver Operating Characteristic (ROC) curves, ranges between 0.74-0.91 ( Fig. 2A) , clearly higher than the no-skill model of 0.5. Similar results are also obtained from complementary prediction performance metrics such as Precision-Recall curves ( Fig. 2A) . These results indicate that our machine learning model is able to predict the tropism of a viral protein based on its SDA to tissues with high accuracy. In concordance, a linear discriminant analysis of the average SDA of each virus across tissues can similarly separate different clusters of viral tropism based on their translational efficiencies (Extended Data Fig. 1 ).
In an attempt to understand which tissues are the most predictive in identifying the viral tropism of proteins, we analyzed the relative feature importance within each random forest classifier, which measures the contribution of each tissue SDA in the decision trees (Fig. 2B) . The main observation is that no single tissue alone is able to discriminate against the specific tropism, since all feature importances lie below 0.10. However, it is also clear that translational adaptation to bile duct (CHOL, for healthy samples of cholangiocarcinoma) is a recurrent discriminant feature, while other tissues are specifically important for just one or few tropisms, such as rectum (READ, for healthy samples of rectum adenocarcinoma) in predicting intestinal viruses. In any case, the directionality of these features cannot be established.
Overall, as tropism of proteins can be predicted from their translational adaptation to tissues, these results indicate that viral proteomes are specifically adapted to certain tRNA-based translational efficiencies. In consequence, and complementary to the observations of mutational pressure driving viral codon bias 11, 29, 30 , we describe the basis for a potential tissuespecific translational selection of the viral codon usage.
As our systematic analysis suggests that the codon usage of viruses tend to be adapted to the tissue they infect, we selected the novel coronavirus SARS-CoV-2 and other related respiratory viruses to further explore their translational adaptation profile over tissues.
We initially reconstructed tRNA expression profiles along the respiratory tract making use of the spatial information associated with healthy TCGA samples from head and neck squamous cell carcinoma (HNSC), lung squamous cell carcinoma (LUSC) and lung adenocarcinoma (LUAD) (Sup. Table 4 ). We then computed the SDA of viral proteins from the three pandemic coronaviruses of the last two decades SARS-CoV 31 , MERS-CoV 32 , and SARS-CoV-2 33 , as well as the common flu-causing influenza A virus (H1N1) along the respiratory tract. We find that the new coronavirus strain SARS-CoV-2 is the most translationally adapted virus across all tissues, especially in comparison with MERS-CoV and influenza A virus (Fig. 3A) . Further, the novel coronavirus is highly adapted throughout the whole respiratory tract, and specifically the upper respiratory airways and the alveolar region in the lung periphery are the most efficient tissues.
Moreover, given that the tropisms not only depend on the translational adaptation to the host, but also on the expression of the required entry receptors, we measure the respective receptors of each virus. Influenza A viruses bind to α(2,6)-linked and α(2,3)-linked sialic acids, which are synthesized by the enzymes ST6GAL1 and ST3GAL4, respectively 34 . While their expression is relatively uniform along the airways, the MERS-CoV uses the parenchymaspecific receptor DPP4 (Fig. 3B) 35 . In both cases, the expression of their entry receptor coincides with an optimal translational adaptation in the lower respiratory tract. On the other hand, both the SARS-CoV and SARS-CoV-2 strains bind to the ACE2 protein and require the proteolytic priming of the viral spike protein by TMPRSS2 36 , although the receptor BSG/CD147 has also been proposed 37 . In all cases, their expression throughout the airways (Fig. 3B ) alongside their efficient translational adaptation defines the upper respiratory tract and the alveoli as their particular optimal tropism (Fig. 3C ). This is in agreement with recent single-cell transcriptomic studies reporting the expression of ACE2 in the nasal goblet and ciliated cells as well as the type-2 alveolar epithelial cells 21, 22 .
Apart from the clear viral tropism of SARS-CoV-2 to the respiratory tract, recent studies propose that their tropism can expand to other tissues such as the digestive system or the brain 23, 24 . For this reason, we also extended our translational analysis to all the 23 tissues of the TCGA dataset (Extended Data Fig. 2A) , together with the expression of the corresponding receptors (Sup. Table 4 ). In agreement with the clinical findings 23, 24 , the gastrointestinal tract emerges as the most translationally adapted tissue, followed by the other epithelial-like tissues and the brain. Therefore, in terms of translational efficiency, the novel SARS-CoV-2 is widely adapted across tissues.
In an attempt to elucidate the translational selection that could have benefitted the evolution of the new coronavirus, we also compared the SDA adaptation of SARS-CoV-2 to those of close phylogenetic strains: the human-infecting SARS-CoV and the bat coronavirus RatG13, with 79.6% and 96.2% of sequence identity, respectively 33 . We observe that all four main proteins of the new strain have optimized their codon usage with regard to the previous 2003pandemic SARS-CoV (Extended Data Fig. 2B ). On the other hand, even though the ratio of synonymous versus nonsynonymous mutation rate of the bat coronavirus compared to SARS-CoV-2 is exceptionally high 38 , their proteins are very similarly adapted to most human tissues.
In short, we report that the new pandemic SARS-CoV-2 coronavirus has evolved to significantly increase its translational adaptation to the human tissues. While it might not be sufficient on its own, the coincidence of both translational efficiency and entry receptor expression supports an optimal tropism of the alveoli but also the upper respiratory tract (Fig.  3C ), which has recently been related to the higher transmissibility of the novel strain 21 .
Given the tropism-specific adaptation of viral RCU towards the translational machinery of tissues, we wondered whether certain genomic subsets were specifically adapted to the tissue of infection. In particular, we speculated that early replication-related proteins would further benefit from such adaptation than late structural proteins, since once the virus takes control of the cell it could change its tRNA expression program 17 .
To estimate the adaptation of each protein to the tRNA-based codon efficiencies of each tissue, we computed their SDA 2 (Sup. Table 5 ). For that purpose, we matched each virus to the tRNAs of their tissues of infection (Sup. Table 5 ). In concordance with our hypothesis, based on current viral annotations (VOGdb, vogdb.org), we observed a small but highly significant shift in SDA between structural and replication proteins in all but hepatocyteinfecting viruses (Fig. 4A) . However, the proteomes of this latter group are poorly annotated at the viral orthology database VOGdb, and separation between classes is often hindered by the polyprotein structure of their genomes 39 . Similarly, we performed a Gene Set Enrichment Analysis to identify which Virus Orthologous Groups (VOGs) were enriched in high-SDA or low-SDA proteins (Fig. 4B) . As determined by current annotations 39 , top-VOGs mostly contained replication-related early proteins, whereas bottom-VOGs had structural late functions.
Previous studies on the translational adaptation of the human immunodeficiency virus 1 suggested that the host tRNA pool is reprogramed upon viral infection in order to favor the expression of late genes 17 . In this direction, we wanted to test whether this tRNA reprogramming is a general adaptive mechanism among viral species. Using three previously published small RNA-sequencing datasets of human cell lines upon viral infection [40] [41] [42] , we quantified the tRNA of mock and productive infections at different time points (Sup. Table 6 ). Therefore, for all datasets, we detected a general decrease in translational efficiency upon viral infection, with late viral proteins being more favored than early ones specifically at longer infection times (Fig. 4C) .
Overall, we determine that the tropism-specific adaptation of viruses is specifically pronounced among early proteins. However, later in infection, the tRNA pool of the host cells is reprogrammed to favor the expression of late viral genes, reproducing previous findings of HIV1 infection and extending the mechanism to dsDNA-based viruses such as human cytomegaloviruses and human simplex viruses.
Tropism is determined by an ensemble of different factors, including the mechanism of viral entry to the host, the immune responses to the infection, or the viral hijacking of the cellular machinery in the interest of replication and propagation. In this article, we study the latter by focusing on the translational adaptation of viral genomes to the host.
While previous studies on the base composition and codon usage of both DNA and RNA viruses 11, 30 have attributed most of the codon usage variability to the mutational pressure of viral genomes, our analysis proposes tropism as an important driving force. By systematically interrogating all human-infecting viruses, we uncover that tissue tropism explains changes in their codon usage more than other viral properties such as type or family. Therefore, as mutational pressure would act more similarly within phylogenetically closer species, such tropism-related differences in codon usage suggests that tissue-specific tRNA expression could be driving a translational selection on viral genomes.
Although high-throughput sequencing of tRNAs has been only recently developed, cases of natural selection of codon usage towards the host have been previously proposed. For instance, codon usage of Parvovirus has been progressively adapted from dogs to cats after the host jump 43 . Influenzaviruses show a similar adaptation over time of viral isolation, deviating from the codon usage of avian hosts 44, 45 . However, whether these progressive changes in codon usage over time are directly driven by translational selection has remained elusive. With the advent of tissue-wide datasets of tRNAs and their translational efficiencies 2 , we can now compute the Supply-to-Demand Adaptation (SDA) of all viral proteomes in different tissues. From there, we then created a random forest model that predicts with high accuracy the viral tropism of proteins based on their profile of adaptation to human tissues. In consequence, the tRNA-based adaptation profile of a protein is descriptive of their viral tropism, indicating that translational selection could indeed drive tropism differences of codon usage. It is important to remark that viruses could still have a good SDA to non-target tissues with similar tRNA expression patterns that are not infected because they are not exposed to the virus.
Specifically, we therefore focus our analysis on the new SARS-CoV-2 coronavirus, causative of the current COVID-19 pandemic. As has been recently suggested based on the expression of their entry receptor 21, 22 , SARS-CoV-2 is also translationally adapted to the upper respiratory airways, as well as the lung parenchyma. This upper tract tropism has indeed been linked to the higher transmissibility of the strain 21 . The novel coronavirus showcases the highest SDA among all studied coronaviruses, including lung but also other tissues such as the digestive system and brain. Interestingly, recent reports have shown that COVID-19 patients very frequently have problems in the digestive tract 24,46,47 as well as in neural tissue 23, 48, 49 .
One major open question in the field still persists: how much natural selection before or after zoonosis shapes the current coronavirus 50 . On the one hand, viruses tend to gradually change their codon usage after a zoonotic jump 43, 45 . On the other hand, given the similarity of SARS-CoV-2 SDA with the phylogenetically closest bat coronavirus, it seems that a translational selection to increase SDA would have acted before the putative zoonosis from bats or other intermediate hosts. Furthermore, in agreement with the highest translational potential of SARS-CoV-2 in their target tissues, a recent model of viral tropism suggested that a tradeoff exists between the efficiency of viral translation and the translational load on the host, indicating that an improved codon usage can make the difference between symptomatic and natural hosts 51 .
On the other hand, it is known that host tRNA pools undergo reprogramming upon viral infection of HIV-1, vaccinia virus, and influenza A virus 17, 52 . In this context, differences in codon usage between early and late viral genes have been previously reported, but the directionality of such changes remained unclear 10, 53 . Based on our concept of tissue-specific adaptation, we therefore validate that early replication-related proteins are better adapted to the tissue of infection. Upon infection, we then unveil that changes in tRNA abundance switch the adaptation towards the expression of late structural proteins, confirming a general trend that had previously only been observed in HIV-1 17 .
Overall, this is the first systematic analysis establishing a link between the codon usage of human viruses and the translational efficiency of their tissue of infection. This correspondence is particularly observed in early viral proteins, while late counterparts benefit from the tRNA reprogramming that underlies the process of infection. We therefore envision the development of ad hoc gene therapies specifically targeting the tissue of interest. Table 2 for full TCGA cancer type names. See also Extended Data Fig. 1 . 36, 37 , influenza A virus binds to sialic acids, which are synthesised by the enzymes ST3GAL4 and ST6GAL1 34 . RSEM receptor expression is re-scaled between 0 and 1 among all tissues, being 1 the tissue with highest expression. Refer to Sup. Table 3 Table 6 ). Center values within the violin plot represent the median. Using a two-tailed Wilcoxon rank-sum test, only significant differences are shown: * (p <= 0.05), ** (p <= 0.01), *** (p <= 0.001), **** (p <= 0.0001).
We included in the analysis all human-infecting viruses from the NCBI Viral Genome Browser, downloaded as of August 29, 2019. Additionally, for its interest, we added a posteriori the new SARS-CoV-2 virus. Viral metadata including family, genus, genetic material type and Baltimore category were retrieved either from the ICTV 2018b Master Species List 54 or the ICTV Virus Metadata Resource (talk.ictvonline.org/taxonomy/vmr/). The sequence-based phylogenetic information was obtained from Aiewsakun and Simmonds (2018). Tissue and cell type tropism was determined based on the curated database ViralZone 25 , and allocated to each of the six main classes based on the main annotation. Sup. Table 1 contains all human-infecting viruses and their associated metadata.
The coding sequences of human-infecting viruses from RefSeq were downloaded from the Codon/Codon Pair Usage Tables (CoCoPUTs) project release as of August 29, 2019 55,56 (Sup. Table 1 ). The SARS-CoV-2 and RaTG13 sequences were directly downloaded from GenBank (Sup . Table 4 ).
Virus Orthologous Groups and their functional annotations (virus structure and replication) were downloaded from VOGdb (vogdb.org, release number vog94). The protein sets of each VOG were formatted to a Gene Matrix Transposed (GMT) file for custom GSEA analyses. Trimmed FASTQ files were then mapped using a specific pipeline for tRNAs 2, 58 . Summarizing, an artificial genome is first generated by masking all annotated tRNA genes and adding pre-tRNAs (i.e. tRNA genes with 3' and 5' genomic flanking regions) as extra chromosomes. Upon mapping to this artificial genome with Segemehl [v0.3.1] 59 , reads that map to the tRNAmasked chromosomes or to the tRNA flanking regions are filtered out in order to remove non-tRNA reads and unmature-tRNA reads respectively.
After this first mapping step, a second library is generated by adding 3' CCA tails and removing introns from tRNA genes. All 100% identical sequences of this so-called mature tRNAs are clustered to avoid redundancy. Next, the subset of filtered reads from the first mapping is aligned against the clustered mature tRNAs using Segemehl [v0. 3.1] 59 . Mapped reads are then realigned with GATK IndelRealigner [v3.8] 60 to reduce the number of mismatching bases across all reads.
For quantification, isoacceptors were quantified as reads per million (RPM). In order to increase the coverage for anticodon-level quantification, we consider all reads that map unambiguously to a certain isoacceptor, even though they ambiguously map to different isodecoders (i.e. tRNA genes that differ in their sequence but share the same anticodon). Ambiguous reads mapping to genes of different isoacceptors were discarded.
The RCU is defined as the contribution of a certain codon to the amino acid it belongs to. The RCU of all synonymous codons therefore sum up to 1.
where refers to the abundance of the codon , and is the set of all synonymous codons.
Relative tRNA Adaptation Index (RtAI)
As described by dos Reis et al. (2003, 2004) , the tAI weights every codon based on the wobble-base codon-anticodon interaction rules. Let be a codon, then the decoding weight is a weighted sum of the square-root-normalized tRNA abundances for all tRNA isoacceptors that bind with affinity The SDA aims to consider not only tRNA abundances, but also the codon usage demand. In doing so, it constitutes a global measure of translation control, since the efficiency of a certain codon depends both on its complementary anticodon abundance as well as the demand for such anticodon by other transcripts. This global control has been indeed established to play an important role in defining optimal translation programs 63 .
The definition of the SDA is based on similar previously published metrics 2,64,65 , which consists of a ratio between the anticodon supply and demand. On the one hand, the anticodon supply is defined as the relative tAI weights (see previous section). On the other, the anticodon demand is estimated from the codon usage at the transcriptome level. It is computed as the frequency of each codon in a transcript weighted by the corresponding transcript expression, and finally summing up over all transcripts. Let be a codon, then the codon usage is a weighted sum of the counts of codon in gene weighted by the mRNA-seq abundance for all genes in the genome :
Similarly to the supply, the anticodon demand is then normalized within each amino acid family:
Finally, the SDA weights (SDAw) are defined as the ratio between the codon supply and demand :
= And therefore the SDA of a certain protein is the product of weights of each codon at the triplet position throughout the full gene length , and normalized by the length.
Three indexes were used to determine the clustering performance of the RCUs based on different viral features. These are "internal" metrics, since they evaluate the quality of a certain grouping using measures of the dataset itself (homogeneity of clusters, distances within and between clusters, etc.).
• WB index is a ratio of the sum-of-squares (SS) within clusters and the SS between clusters, normalized by the number of clusters 66 . Therefore, low values of the WB index are indicative of good clustering. • Dunn index considers the inter-cluster distance and diameter of the cluster hypersphere 67 . A higher Dunn index indicates better clustering. • Silhouette Coefficient ranges from -1 to +1, and measures how similar an object is to its own cluster (intra-cluster distance) compared to other clusters (nearest-cluster distance) 68 . A high value indicates a correct clustering.
We applied a Linear Discriminant Analysis (LDA) to the viral RCUs, taking for each virus the average RCU of its proteins. We assigned each virus to its corresponding tropism (Sup. Table  1 ) in order to find the linear combination of codon features that maximized differences between viral target tissues. Given the collinear nature of RCUs by definition, the estimated coefficients are impossible to interpret, although it does not hamper the classification performance.
To evaluate the adaptation of the viral proteins to the SDAw of human tissues, we computed their average SDA to each of the 23 TCGA tissues (Sup. Table 3 ). Using the set of 182 tropism-defined viruses, we had a total of 2891 viral proteins. Taking the 23 tissue-specific SDAs as features, we applied a Random Forest (RF) Classifier, populated with 100 decision trees, using the scikit-learn package 69 . Therefore, for each of the six viral tropisms, we developed a model for predicting the tropism-positive versus tropism-negative proteins based on the translational adaptation across tissues. Given that the size of the tropism-positive and tropism-negative groups were often unbalanced, we iteratively sampled equal-sized groups, for n=100 iterations. Furthermore, we validated the results with a stratified 5-fold crossvalidation.
In order to evaluate the performance of the RF models, we computed the Area Under the Curve (AUC) of Receiver Operating Characteristic (ROC) and Precision-Recall (PR) plots ( Fig.  2A) . We took the average and standard deviation across all iterations. Similarly, we computed the relative feature weights corresponding to each of the 23 TCGA tissues (Fig. 2B) .
Similar to the RF classifier, we also computed the average proteome SDA per virus in each of the 23 tissues. We then applied a Linear Discriminant Analysis (LDA) to these averaged SDAs. We assigned each virus to its corresponding tropism (Sup. Table 1 ) in order to find the linear combination of tissue adaptation features that maximized differences between viral target tissues (Extended Data Fig. 1 , Sup. Table 3 ).
We analyzed the enrichment of gene sets of the Virus Orthologous Groups using the GSEA algorithm 70 . The score used to generate the ranked list input is specified in the text.
All details of the statistical analyses can be found in the figure legends. For hypothesis testing, a Wilcoxon rank-sum test was performed. We used a significance value of 0.05.
The code used in this study is available at GitHub (https://github.com/hexavier/tRNA_viruses, https://github.com/hexavier/tRNA_mapping). The published article includes all datasets generated or analyzed during this study.