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Stasevich, T. J.

Publications and source records attributed to Stasevich, T. J..

At least 19 recordsLinked to original sources

Live-cell, single-molecule imaging resolves the dual nature of mRNA silencing by SARS-CoV-2 Nsp1

SARS-CoV-2 nonstructural protein 1 (Nsp1) is a conserved viral factor that potently silences host transcripts by binding ribosomes. Yet how it acts at the level of individual mRNAs remains unclear, with competing models arguing that Nsp1 blocks translation initiation, stalls ribosome elongation, cleaves mRNA, or does all three. Here, we use Nascent Chain Tracking to quantify in living cells how Nsp1 reshapes mRNA abundance and translation dynamics with single-molecule resolution. Using reporter transcripts that vary in UTR architecture, initiation mode, and open reading frame length, we find that Nsp1 mainly silences our reporter transcripts by reducing their numbers rather than inhibiting initiation. Two routes to Nsp1-mediated mRNA loss are identified, both requiring cap-dependent initiation: an early route acting during or shortly after initiation and a later route associated with downstream translation that depends in part on ZNF598. Together, these two mechanisms make Nsp1 an efficient and robust suppressor of host gene expression. We further find that reporters containing the SARS-CoV-2 UTRs remain susceptible to Nsp1-mediated mRNA loss, but the 5' and 3' UTRs cooperate to increase translation output. Neither viral UTR alone increased translation beyond the host reporter, whereas combining both UTRs substantially enhanced translation in the absence of Nsp1. Together, our results reveal that Nsp1 uses dual, cap-dependent routes to promote mRNA loss across the translation cycle, while the SARS-CoV-2 UTRs provide a cooperative translational advantage rather than robust protection from Nsp1.

molecular biology↗

UTag, a cysteine-free thermostable tagging system for tracking single mRNA translation live

Spatiotemporal regulation of mRNA translation is central to gene expression. Over the past decade, translation has become directly observable in live cells at single-mRNA resolution by tagging nascent chains with tandem arrays of short epitope tags recognized by genetically encodable fluorescent intracellular antibodies (intrabodies). While this technology has revolutionized our understanding of translation regulation, the current toolbox of tagging systems remains limited. Here, we developed a novel and tight-binding intrabody against a short (11-amino acid) HIV protease epitope (named UTag). To ensure robust intracellular folding of the anti-UTag intrabody, we further engineered a cysteine-free variant that folds and functions independently of disulfide-bond formation, as validated by X-ray crystallography. The cysteine-free anti-UTag intrabody retains high binding affinity comparable to the parental intrabody while exhibiting significantly improved thermostability ([~]80 {degrees}C). Importantly, the cysteine-free UTag system enables real-time tracking of single-mRNA translation in live cells with performance on par with the parental UTag system as well as the established SunTag and ALFA-tag. Collectively, these results demonstrate that the newly developed UTag system expands the toolbox for live-cell translation tracking and provides complementary tools for multiplexed applications.

biophysics↗

Mechanism of ATXN8OS CTA/CTG repeat-associated non-AUG translation revealed by approaches ranging from cell-free translation to live-cell imaging.

Microsatellite repeat expansions contribute to the pathogenesis of many neurodegenerative disorders. In spinocerebellar ataxia type 8 (SCA8), abnormal expansion of CTA/CTG repeats in the 3' untranslated region of the ATXN8OS (ATXN8 Opposite Strand) gene has been implicated in disease pathology. Although the occurrence of repeat-associated non-AUG (RAN) translation from the ATXN8 transcript has been reported, whether and how RAN translation occurs from the ATXN8OS transcript has remained unexplored. Here, using a cell-free translation system and cultured cells, we showed that ATXN8OS undergoes robust AUG-independent translation in a repeat length-dependent manner. Mechanistic analyses revealed that translation of the poly L (0) frame initiates at a non-AUG codon located upstream of the repeats. Moreover, using live-cell imaging at a single mRNA level, we directly visualized ribosomal -1 frameshifting from the poly L (0) frame to the poly T-poly A (+2) frame during translation elongation. We further showed that ATXN8OS translation was enhanced upon activation of the integrated stress response. Together, these findings establish both the occurrence and the molecular mechanisms of ATXN8OS translation from expanded CTA/CTG repeats and provide insights into the pathogenic processes underlying SCA8. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=176 SRC="FIGDIR/small/700793v1_ufig1.gif" ALT="Figure 1"> View larger version (42K): org.highwire.dtl.DTLVardef@1f836fborg.highwire.dtl.DTLVardef@e1b77forg.highwire.dtl.DTLVardef@d605c3org.highwire.dtl.DTLVardef@11475d9_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗

Real-time single-molecule imaging in zebrafish embryos uncovers non-canonical translation

Precise spatiotemporal control of protein synthesis is essential during embryogenesis, yet directly measuring translation kinetics in vivo remains challenging in vertebrates. In particular, it remains unclear how translation efficiency is determined for key developmental regulators, and which kinetic steps limit their production. Here, we used ALFA array-based nascent chain labeling combined with lattice light-sheet microscopy to visualize bmp2b translation in real time and at single-molecule resolution in early zebrafish embryos. When combined with MS2/MCP labeling to visualize all bmp2b mRNAs, we found that only some of them are being actively translated, suggesting that limited mRNA translation competence contributes to overall translation efficiency. We found that bmp2b translation operates below a maximal initiation regime, as replacement of its UTRs with viral UTRs increases ribosome loading. Furthermore, the bmp2b, but not actb2, 5'UTR supports cap-independent translation with ribosome loading comparable to cap-dependent initiation. Altogether, this approach provides a quantitative in vivo framework to dissect translation kinetics during early vertebrate development.

developmental biology↗

Fragile nucleosomes are essential for RNA Polymerase II to transcribe in eukaryotes

Nucleosomes are barriers to RNA Polymerase II (Pol II) transcribing along gene bodies in eukaryotes. We found that a fragile "tailless nucleosome" could be generated to resolve the Pol II pausing caused by the +1 nucleosome in metazoans. We hypothesized that "pan-acetylated nucleosomes" along gene bodies are fragile and are generated to resolve the resistance of nucleosomes to the elongation of Pol II. To confirm this hypothesis, we systematically analyzed pan-acetylation and pan-phosphorylation of the entire human genome in HEK293T cells. Herein, we show that both pan-acetylation and pan-phosphorylation of histones in nucleosomes along gene bodies are coupled with the activities of Pol II. Our results show that there are four major types of fragile nucleosomes required for Pol II to transcribe along gene bodies: nucleosomes without tails, nucleosomes with dominant acetylation, nucleosomes with dominant phosphorylation, and nucleosomes with both acetylation and phosphorylation.

genetics↗

AI-assisted protein design to rapidly convert antibody sequences to intrabodies targeting diverse peptides and histone modifications

Intrabodies are engineered antibodies that function inside living cells, enabling therapeutic, diagnostic, and imaging applications. While powerful, their development has been hindered by challenges associated with their folding, solubility, and stability in the reduced intracellular environment. Here, we present an AI-driven pipeline integrating AlphaFold2, ProteinMPNN, and live-cell screening to optimize antibody framework regions while preserving epitope-binding complementarity-determining regions. Using this approach, we successfully converted 19 out of 26 antibody sequences into functional single-chain variable fragment (scFv) intrabodies, including a panel targeting diverse histone modifications for real-time imaging of chromatin dynamics and gene regulation. Notably, 18 of these 19 sequences had failed to convert using the standard approach, demonstrating the unique effectiveness of our method. As antibody sequence databases expand, our method will accelerate intrabody design, making their development easier, more cost-effective, and broadly accessible for biological research.

cell biology↗

PAbFold: Linear Antibody Epitope Prediction using AlphaFold2

Defining the binding epitopes of antibodies is essential for understanding how they bind to their antigens and perform their molecular functions. However, while determining linear epitopes of monoclonal antibodies can be accomplished utilizing well-established empirical procedures, these approaches are generally labor- and time-intensive and costly. To take advantage of the recent advances in protein structure prediction algorithms available to the scientific community, we developed a calculation pipeline based on the localColabFold implementation of AlphaFold2 that can predict linear antibody epitopes by predicting the structure of the complex between antibody heavy and light chains and target peptide sequences derived from antigens. We found that this AlphaFold2 pipeline, which we call PAbFold, was able to accurately flag known epitope sequences for several well-known antibody targets (HA / Myc) when the target sequence was broken into small overlapping linear peptides and antibody complementarity determining regions (CDRs) were grafted onto several different antibody framework regions in the single-chain antibody fragment (scFv) format. To determine if this pipeline was able to identify the epitope of a novel antibody with no structural information publicly available, we determined the epitope of a novel anti-SARS-CoV-2 nucleocapsid targeted antibody using our method and then experimentally validated our computational results using peptide competition ELISA assays. These results indicate that the AlphaFold2-based PAbFold pipeline we developed is capable of accurately identifying linear antibody epitopes in a short time using just antibody and target protein sequences. This emergent capability of the method is sensitive to methodological details such as peptide length, AlphaFold2 neural network versions, and multiple-sequence alignment database. PAbFold is available at https://github.com/jbderoo/PAbFold.

molecular biology↗

Single molecule imaging of the central dogma reveals myosin-2A gene expression is regulated by contextual translational buffering

While protein homeostasis is a hallmark of gene regulation, unraveling the hidden regulatory mechanisms that maintain homeostasis is difficult using traditional methods. To confront this problem, we CRISPR engineered a human cell line with multiple tags in the endogenous MYH9 gene, which encodes the essential and ubiquitous myosin-2A cytoskeletal motor. Using these cells, we imaged MYH9 transcription, translation, and mature mRNA and protein in distinct colors, enabling a full dissection of the central dogma. Our data show that MYH9 transcription is upregulated in an SRF-dependent manner in response to cytoskeletal cues and that MYH9 translation can either buffer or match the transcriptional response depending on context. Upon knockdown of actin-depolymerizing proteins like cofilin, translation efficiency drops by a factor of two to buffer strong transcriptional upregulation, likely to help prevent excessive myosin activity. In contrast, following serum stimulation, translation matches the transcriptional response to readily reestablish steady state. Our results identify contextual translational buffering as an important regulatory mechanism driving stable MYH9 expression. They also demonstrate the power and broad applicability of our cell line, which can now be used to accurately quantify central dogma dynamics in response to diverse forms of cellular perturbations.

cell biology↗

Genetically encoded affinity reagents (GEARs): A toolkit for visualizing and manipulating endogenous protein function in vivo.

Probing endogenous protein localization and function in vivo remains challenging due to laborious gene targeting and monofunctional alleles. Here, we develop a multifunctional, universal, and adaptable toolkit based on genetically encoded affinity reagents (GEARs). GEARs use nanobodies and single chain variable fragments (scFv), which recognize small epitopes, enabling fluorescent visualization and selective degradation of protein targets in vivo. Furthermore, we delineate a CRISPR/Cas9-based epitope tagging pipeline to demonstrate its utility for producing knock-in alleles that have broad multifunctionality. We use GEARs to examine the native behaviour of the pioneer transcription factor Nanog and the planar cell polarity protein Vangl2 during early zebrafish development. Together, this toolkit provides a versatile system for probing and perturbing endogenous protein function while circumventing challenges associated with conventional gene targeting and is broadly available to the model organism community.

developmental biology↗

Live-cell imaging uncovers the relationship between histone acetylation, transcription initiation, and nucleosome mobility

Post-translational protein modifications play an important role in the regulation of gene dynamics. Certain modifications, such as histone acetylation and RNA polymerase II phosphorylation, are associated with transcriptionally active chromatin. However, the spatial and temporal relationship between chromatin and post-translational protein modifications, and how these dynamics facilitate selective gene expression, remain poorly understood. In this study, we address this problem by developing a general methodology for quantifying in live cells the dynamics of chromatin across multiple time and length scales in the context of residue-specific protein modifications. By combining Fab-based labeling of endogenous protein modifications with single-molecule imaging, we track the dynamics of chromatin enriched with histone H3 Lysine-27 acetylation (H3K27ac) and RNA polymerase II Serine-5 phosphorylation (RNAP2-Ser5ph). Our analysis reveals chromatin enriched with H3K27ac is separated from chromatin enriched with RNAP2-Ser5ph. Furthermore, in these separated sites, we show the presence of the two modifications are inversely correlated with one another on the minutes timescale. We then track single nucleosomes in both types of sites on the sub-second timescale and again find evidence for distinct and opposing changes in their diffusive behavior. While nucleosomes diffuse [~]15% faster in chromatin enriched with H3K27ac, they diffuse [~]15% slower in chromatin enriched with RNAP2-Ser5ph. Taken together, these results argue that high levels of H3K27ac and RNAP2-Ser5ph are not often present together at the same place and time, but rather each modification marks distinct sites that are transcriptionally poised or active, respectively.

molecular biology↗

Generation and diversification of recombinant monoclonal antibodies for studying mitosis

Antibodies are indispensable tools used for a large number of applications in both foundational and translational bioscience research; however, there are drawbacks to using traditional antibodies generated in animals. These include a lack of standardization leading to problems with reproducibility, high costs of antibodies purchased from commercial sources, and ethical concerns regarding the large number of animals used to generate antibodies. To address these issues, we have developed practical methodologies and tools for generating low-cost, high-yield preparations of recombinant monoclonal antibodies and antibody fragments directed to protein epitopes from primary sequences. We describe these methods here, as well as approaches to diversify monoclonal antibodies, including customization of antibody species specificity, generation of genetically encoded small antibody fragments, and conversion of single chain antibody fragments (e.g. scFv) into full-length, bivalent antibodies. This study focuses on antibodies directed to epitopes important for mitotic cell division; however, the methods and reagents described here are applicable to antibodies and antibody fragments for use in any field.

cell biology↗

Visualizing looping of two endogenous genomic loci using synthetic zinc-finger proteins with anti-FLAG and anti-HA frankenbodies in living cells

In eukaryotic nuclei, chromatin loops mediated through cohesin are critical structures that regulate gene expression and DNA replication. Here we demonstrate a new method to visualize endogenous genomic loci using synthetic zinc-finger proteins harboring repeat epitope tags (ZF probes) for signal amplification via binding of tag-specific intracellular antibodies, or frankenbodies, fused with fluorescent proteins. We achieve this in two steps. First, we develop an anti-FLAG frankenbody that can bind FLAG-tagged proteins in diverse live-cell environments. The anti-FLAG frankenbody complements the anti-HA frankenbody, enabling two-color signal amplification from FLAG and HA-tagged proteins. Second, we develop a pair of cell-permeable ZF probes that specifically bind two endogenous chromatin loci predicted to be involved in chromatin looping. By coupling our anti-FLAG and anti-HA frankenbodies with FLAG- and HA-tagged ZF probes, we simultaneously visualize the dynamics of the two loci in single living cells. This reveals close association between the two loci in the majority of cells, but the loci markedly separate upon the triggered degradation of the cohesin subunit RAD21. Our ability to image two endogenous genomic loci simultaneously in single living cells provides a proof-of-principle that ZF probes coupled with frankenbodies are useful new tools for exploring genome dynamics in multiple colors.

cell biology↗

Protein manipulation using single copies of short peptide tags in cultured cells and in Drosophila melanogaster

Cellular development and specialized cellular functions are regulated processes which rely on highly dynamic molecular interactions among proteins, distributed in all cell compartments. Analysis of these interactions and their mechanisms of action has been one of the main topics in cellular and developmental research over the last fifty years. Studying and understanding the functions of proteins of interest (POIs) has been mostly achieved by their alteration at the genetic level and the analysis of the phenotypic changes generated by these alterations. Although genetic and reverse genetic technologies contributed to the vast majority of information and knowledge we have gathered so far, targeting specific interactions of POIs in a time- and space-controlled manner or analyzing the role of POIs in dynamic cellular processes such as cell migration or cell division would require more direct approaches. The recent development of specific protein binders, which can be expressed and function intracellularly, together with several improvements in synthetic biology techniques, have contributed to the creation of a new toolbox for direct protein manipulations. We selected a number of short tag epitopes for which protein binders from different scaffolds have been developed and tested whether these tags can be bound by the corresponding protein binders in living cells when they are inserted in a single copy in a POI. We indeed find that in all cases, a single copy of a short tag allows protein binding and manipulation. Using Drosophila, we also find that single short tags can be recognized and allow degradation and relocalization of POIs in vivo.

cell biology↗

Live-cell imaging reveals the spatiotemporal organization of endogenous RNA polymerase II phosphorylation at a single gene

The carboxyl-terminal domain of RNA polymerase II is dynamically phosphorylated during transcription in eukaryotic cells. While residue-specific phosphorylation has been mapped with exquisite spatial resolution along the 1D genome in a population of fixed cells using immunoprecipitation-based assays, the timing, kinetics, and spatial organization of phosphorylation along a single-copy gene have not yet been measured in living cells. Here, we achieve this by combining multi-color, single-molecule microscopy with fluorescent antibody-based probes that specifically bind to unphosphorylated and phosphorylated forms of endogenous RNAP2 in living cells. Applying this methodology to a single-copy HIV-1 reporter gene provides live-cell evidence for heterogeneity in the distribution of RNAP2 along the length of the gene as well as clusters of Serine 5 phosphorylated RNAP2 that form around active genes and are separated in both space and time from nascent mRNA synthesis. Computational models fit to our data determine that 5 to 40 RNAP2 cluster around the promoter of a gene during typical transcriptional bursts. Rapid imaging demonstrates nearly all RNAP2 in the cluster acquire Serine 5 phosphorylation within 3-6 seconds of arrival. Transcription from the cluster appears to be highly efficient, with nearly half of the clustered RNAP2 ultimately escaping the promoter in 1.5 minutes on average to elongate a full-length mRNA in approximately five minutes. The highly dynamic and spatially organized concentrations of RNAP2 we observe support the notion of highly efficient transcription clusters that form around promoters and contain high concentrations of RNAP2 phosphorylated at Serine 5.

molecular biology↗

Quantifying the spatiotemporal dynamics of IRES versus Cap translation with single-molecule resolution in living cells

Viruses use IRES sequences within their RNA to hijack translation machinery and thereby rapidly replicate in host cells. While this process has been extensively studied in bulk assays, the dynamics of hijacking at the single-molecule level remain unexplored in living cells. To achieve this, we developed a bicistronic biosensor encoding complementary repeat epitopes in two ORFs, one translated in a Cap-dependent manner and the other translated in an IRES-mediated manner. Using a pair of complementary probes that bind the epitopes co-translationally, our biosensor lights up in different colors depending on which ORF is being translated. In combination with single-molecule tracking and computational modeling, we measured the relative kinetics of Cap versus IRES translation and show: (1) Two non-overlapping ORFs can be simultaneously translated within a single mRNA; (2) EMCV IRES-mediated translation sites recruit ribosomes less efficiently than Cap-dependent translation sites but are otherwise nearly indistinguishable, having similar mobilities, sizes, spatial distributions, and ribosomal initiation and elongation rates; (3) Both Cap-dependent and IRES-mediated ribosomes tend to stretch out translation sites; (4) Although the IRES recruits two to three times fewer ribosomes than the Cap in normal conditions, the balance shifts dramatically in favor of the IRES during oxidative and ER stresses that mimic viral infection; and (5) Translation of the IRES is enhanced by translation of the Cap, demonstrating upstream translation can positively impact the downstream translation of a non-overlapping ORF. With the ability to simultaneously quantify two distinct translation mechanisms in physiologically relevant live-cell environments, we anticipate bicistronic biosensors like the one we developed here will become powerful new tools to dissect both canonical and non-canonical translation dynamics with single-molecule precision. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=177 HEIGHT=200 SRC="FIGDIR/small/900829v1_ufig1.gif" ALT="Figure 1"> View larger version (37K): org.highwire.dtl.DTLVardef@a44259org.highwire.dtl.DTLVardef@651org.highwire.dtl.DTLVardef@19ebc26org.highwire.dtl.DTLVardef@527822_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗

Coupling of translation quality control and mRNA targeting to stress granules

Stress granules (SGs) are dynamic assemblies of non-translating RNAs and proteins that form with translation inhibition1. Stress granules are similar to neuronal and germ cell granules, play a role in survival during stress, and aberrant, cytotoxic SGs are implicated in neurodegeneration2-4. Perturbations in the ubiquitin-proteasome (UPS) system also cause neurodegeneration5-10, and alter the dynamicity and kinetics of SGs11-14. Using single mRNA imaging in live cells15, 16, we took an unbiased approach to determine if defects in the UPS perturb mRNA translation and partitioning into SGs during acute stress. We observe ribosomes stall on mRNAs during arsenite stress, and the release of transcripts from stalled ribosomes for their partitioning into SGs requires the activities of valosin-containing protein (VCP) and the proteasome, which is in contrast to previous work showing VCP primarily affected SG disassembly 11, 13, 14, 17. Moreover, members of a specialized complex in the UPS that targets aberrant nascent proteins for decay upon ribosome stalling, referred to as ribosome-associated quality control complex (RQC)18, are also required for mRNA release from ribosomes and partitioning into SGs. VCP alleles that increase segregase activity and cause neurodegeneration and inclusion body myopathies5, 6, 19, 20 increase mRNA recruitment to SGs, suggesting aberrant mRNA localization to SGs in disease contexts. This work identifies a new type of stress-activated RQC (saRQC) distinct from canonical RQC pathways in mRNA substrates, cellular context and mRNA fate.

molecular biology↗

Detection and quantification of single mRNA dynamics with the Riboglow fluorescent RNA tag

Labeling and tracking biomolecules with fluorescent probes on the single molecule level enables quantitative insights into their dynamics in living cells. We previously developed Riboglow, a platform to label RNAs in live mammalian cells, consisting of a short RNA tag and a small organic probe that increases fluorescence upon binding RNA. Here, we demonstrate that Riboglow is capable of detecting and tracking single RNA molecules. We benchmark RNA tracking by comparing results with the established MS2 RNA tagging system. To demonstrate versatility of Riboglow, we assay translation on the single molecule level, where the translated mRNA is tagged with Riboglow and the nascent polypeptide is labeled with a fluorescent antibody. The growing effort to investigate RNA biology on the single molecule level requires sophisticated and diverse fluorescent probes for multiplexed, multi-color labeling of biomolecules of interest, and we present Riboglow as a new member in this toolbox.

cell biology↗

Computational design and interpretation of live-cell, single-RNA translation experiments

Advances in fluorescence microscopy have introduced new assays to quantify live-cell translation dynamics at single-RNA resolution. We introduce a detailed, yet efficient sequence-based stochastic model that generates realistic synthetic data for several such assays, including Fluorescence Correlation Spectroscopy (FCS), ribosome Run-Off Assays (ROA) after Harringtonine application, and Fluorescence Recovery After Photobleaching (FRAP). We simulate these experiments under multiple imaging conditions and for thousands of human genes, and we evaluate through simulations which experiments are most likely to provide accurate estimates of elongation kinetics. Finding that FCS analyses are optimal for both short and long length genes, we integrate our model with experimental FCS data to capture the nascent protein statistics and temporal dynamics for three human genes: KDM5B, {beta}-actin, and H2B. Finally, we introduce a new open-source software package, RNA Sequence to NAscent Protein Simulator (O_SCPCAPRC_SCPCAPSNAPO_SCPCAPSIMC_SCPCAP), to easily simulate the single-molecule translation dynamics of any gene sequence for any of these assays and for different assumptions regarding synonymous codon usage, tRNA level modifications, or ribosome pauses. O_SCPCAPRC_SCPCAPSNAPO_SCPCAPSIMC_SCPCAP is implemented in Python and is available at: https://github.com/MunskyGroup/rSNAPsim.git.\n\nAuthor summaryTranslation is an essential step in which ribosomes decipher mRNA sequences to manufacture proteins. Recent advances in time-lapse fluorescence microscopy allow live-cell quantification of translation dynamics at the resolution of single mRNA molecules. Here, we develop a flexible computational framework to reproduce and interpret such experiments. We use this framework to explore how well different single-mRNA translation experiment designs would perform to estimate key translation parameters. We then integrate experimental data from the most flexible design with our stochastic model framework to reproduce the statistics and temporal dynamics of nascent protein elongation for three different human genes. Our validated computational method is packaged with a simple graphical user interface that (1) starts with mRNA sequences, (2) generates discrete, codon-dependent translation models, (3) provides visualization of ribosome movement as trajectories or kymographs, and (4) allows the user to estimate how optical single-mRNA translation experiments would be affected by different genetic alterations (e.g., codon substitutions) or environmental perturbations (e.g., tRNA titrations or drug treatments).

systems biology↗