Lysine acetylation of the Mycobacterium tuberculosis HU protein modulates its DNA binding and genome organization

Nucleoid-associated proteins of mycobacteria come with a distinctive flavor

In every bacterium, nucleoid-associated proteins (NAPs) play crucial roles in chromosome organization, replication, repair, gene expression, and other DNA transactions. Their central role in controlling the chromatin dynamics and transcription has been well-appreciated in several well-studied organisms. Here, we review the diversity, distribution, structure, and function of NAPs from the genus Mycobacterium. We highlight the progress made in our understanding of the effects of these proteins on various processes and in responding to environmental stimuli and stress of mycobacteria in their free-living as well as during distinctive intracellular lifestyles. We project them as potential drug targets and discuss future studies to bridge the information gap with NAPs from well-studied systems.

Epigenetic maneuvering: an emerging strategy for mycobacterial intracellular survival

Mycobacterium tuberculosis (Mtb) has elaborated numerous mechanisms for its pathogenesis. Mtb manipulates host signaling pathways to interfere with the immune response and cell death pathways. By employing virulence factors - of which secretory proteins are emerging as significant components - it ensures successful survival in the host. In this review, we discuss advances made on the largely unexplored secretory modifiers of Mtb that alter the host epigenome to impact host pathways for the pathogen's advantage. We highlight the findings on the Mtb-encoded modification enzymes and their role in maneuvering the host machinery. We also provide pointers to the gaps that still exist in this area and approaches to address these questions for a better appreciation of the uncanny success of Mtb as an intracellular pathogen.

Future Directions of the Prokaryotic Chromosome Field

In September 2023, the Biology and Physics of Prokaryotic Chromosomes meeting ran at the Lorentz Center in Leiden, The Netherlands. As part of the workshop, those in attendance developed a series of discussion points centered around current challenges for the field, how these might be addressed, and how the field is likely to develop over the next 10 years. The Lorentz Center staff facilitated these discussions via tools aimed at optimizing productive interactions. This Perspective article is a summary of these discussions and reflects the state-of-the-art of the field. It is expected to be of help to colleagues in advancing their own research related to prokaryotic chromosomes and inspiring novel interdisciplinary collaborations. This forward-looking perspective highlights the open questions driving current research and builds on the impressive recent progress in these areas as represented by the accompanying reviews, perspectives, and research articles in this issue. These articles underline the multi-disciplinary nature of the field, the multiple length scales at which chromatin is studied in vitro and in and highlight the differences and similarities of bacterial and archaeal chromatin and chromatin-associated processes.

Acetylomics reveals an extensive acetylation diversity within Pseudomonas aeruginosa

Bacteria employ a myriad of regulatory mechanisms to adapt to the continuously changing environments that they face. They can, for example, use post-translational modifications, such as Nε-lysine acetylation, to alter enzyme activity. Although a lot of progress has been made, the extent and role of lysine acetylation in many bacterial strains remains uncharted. Here, we applied stable isotope labeling by amino acids in cell culture (SILAC) in combination with the immunoprecipitation of acetylated peptides and LC-MS/MS to measure the first Pseudomonas aeruginosa PAO1 acetylome, revealing 1076 unique acetylation sites in 508 proteins. Next, we assessed interstrain acetylome differences within P. aeruginosa by comparing our PAO1 acetylome with two publicly available PA14 acetylomes, and postulate that the overall acetylation patterns are not driven by strain-specific factors. In addition, the comparison of the P. aeruginosa acetylome to 30 other bacterial acetylomes revealed that a high percentage of transcription related proteins are acetylated in the majority of bacterial species. This conservation could help prioritize the characterization of functional consequences of individual acetylation sites.

Biomolecular condensates as stress sensors and modulators of bacterial signaling

Microbes exhibit remarkable adaptability to environmental fluctuations. Signaling mechanisms, such as two-component systems and secondary messengers, have long been recognized as critical for sensing and responding to environmental cues. However, recent research has illuminated the potential of a physical adaptation mechanism in signaling-phase separation, which may represent a ubiquitous mechanism for compartmentalizing biochemistry within the cytoplasm in the context of bacteria that frequently lack membrane-bound organelles. This review considers the broader prospect that phase separation may play critical roles as rapid stress sensing and response mechanisms within pathogens. It is well established that weak multivalent interactions between disordered regions, coiled-coils, and other structured domains can form condensates via phase separation and be regulated by specific environmental parameters in some cases. The process of phase separation itself acts as a responsive sensor, influenced by changes in protein concentration, posttranslational modifications, temperature, salts, pH, and oxidative stresses. This environmentally triggered phase separation can, in turn, regulate the functions of recruited biomolecules, providing a rapid response to stressful conditions. As examples, we describe biochemical pathways organized by condensates that are essential for cell physiology and exhibit signaling features. These include proteins that organize and modify the chromosome (Dps, Hu, SSB), regulate the decay, and modification of RNA (RNase E, Hfq, Rho, RNA polymerase), those involved in signal transduction (PopZ, PodJ, and SpmX) and stress response (aggresomes and polyphosphate granules). We also summarize the potential of proteins within pathogens to function as condensates and the potential and challenges in targeting biomolecular condensates for next-generation antimicrobial therapeutics. Together, this review illuminates the emerging significance of biomolecular condensates in microbial signaling, stress responses, and regulation of cell physiology and provides a framework for microbiologists to consider the function of biomolecular condensates in microbial adaptation and response to diverse environmental conditions.

N - acetyl-transferases required for iron uptake and aminoglycoside resistance promote virulence lipid production in M. marinum

Phagosomal lysis is a key aspect of mycobacterial infection of host macrophages. Acetylation is a protein modification mediated enzymatically by N-acetyltransferases (NATs) that impacts bacterial pathogenesis and physiology. To identify NATs required for lytic activity, we leveraged Mycobacterium marinum, a nontubercular pathogen and an established model for M. tuberculosis. M. marinum hemolysis is a proxy for phagolytic activity. We generated M. marinum strains with deletions in conserved NAT genes and screened for hemolytic activity. Several conserved lysine acetyltransferases (KATs) contributed to hemolysis. Hemolysis is mediated by the ESX-1 secretion system and by phthiocerol dimycocerosate (PDIM), a virulence lipid. For several strains, the hemolytic activity was restored by the addition of second copy of the ESX-1 locus. Using thin-layer chromatography (TLC), we found a single NAT required for PDIM and phenolic glycolipid (PGL) production. MbtK is a conserved KAT required for mycobactin siderophore synthesis and virulence. Mycobactin J exogenously complemented PDIM/PGL production in the Δ mbtK strain. The Δ mbtK M. marinum strain was attenuated in macrophage and Galleria mellonella infection models. Constitutive expression of either eis or papA5, which encode a KAT required for aminoglycoside resistance and a PDIM/PGL biosynthetic enzyme, rescued PDIM/PGL production and virulence of the Δ mbtK strain. Eis N-terminally acetylated PapA5 in vitro , supporting a mechanism for restored lipid production. Overall, our study establishes connections between the MbtK and Eis NATs, and between iron uptake and PDIM and PGL synthesis in M. marinum . Our findings underscore the multifunctional nature of mycobacterial NATs and their connection to key virulence pathways.

Significance Statement

Acetylation is a modification of protein N-termini, lysine residues, antibiotics and lipids. Many of the enzymes that promote acetylation belong to the GNAT family of proteins. M. marinum is a well-established as a model to understand how M. tuberculosis causes tuberculosis. In this study we sought to identify conserved GNAT proteins required for early stages of mycobacterial infection. Using M. marinum, we determined that several GNAT proteins are required for the lytic activity of M. marinum. We uncovered previously unknown connections between acetyl-transferases required for iron uptake and antimicrobial resistance, and the production of the unique mycobacterial lipids, PDIM and PGLOur data support that acetyl-transferases from the GNAT family are interconnected, and have activities beyond those previously reported.

The activity of CobB1 protein deacetylase contributes to nucleoid compaction in Streptomyces venezuelae spores by increasing HupS affinity for DNA

Streptomyces are soil bacteria with complex life cycle. During sporulation Streptomyces linear chromosomes become highly compacted so that the genetic material fits within limited spore volume. The key players in this process are nucleoid-associated proteins (NAPs). Among them, HU (heat unstable) proteins are the most abundant NAPs in the cell and the most conserved in bacteria. HupS, one of the two HU homologues encoded by the Streptomyces genome, is the best-studied spore-associated NAP. In contrast to other HU homologues, HupS contains a long, C-terminal domain that is extremely rich in lysine repeats (LR domain) similar to eukaryotic histone H2B and mycobacterial HupB protein. Here, we have investigated, whether lysine residues in HupS are posttranslationally modified by reversible lysine acetylation. We have confirmed that Streptomyces venezuelae HupS is acetylated in vivo. We showed that HupS binding to DNA in vitro is controlled by the acetylation. Moreover, we identified that CobB1, one of two Sir2 homologues in Streptomyces, controls HupS acetylation levels in vivo. We demonstrate that the elimination of CobB1 increases HupS mobility, reduces chromosome compaction in spores, and affects spores maturation. Thus, our studies indicate that HupS acetylation affects its function by diminishing DNA binding and disturbing chromosome organization.

Bacterial protein acetylation: mechanisms, functions, and methods for study

Lysine acetylation is an evolutionarily conserved protein modification that changes protein functions and plays an essential role in many cellular processes, such as central metabolism, transcriptional regulation, chemotaxis, and pathogen virulence. It can alter DNA binding, enzymatic activity, protein-protein interactions, protein stability, or protein localization. In prokaryotes, lysine acetylation occurs non-enzymatically and by the action of lysine acetyltransferases (KAT). In enzymatic acetylation, KAT transfers the acetyl group from acetyl-CoA (AcCoA) to the lysine side chain. In contrast, acetyl phosphate (AcP) is the acetyl donor of chemical acetylation. Regardless of the acetylation type, the removal of acetyl groups from acetyl lysines occurs only enzymatically by lysine deacetylases (KDAC). KATs are grouped into three main superfamilies based on their catalytic domain sequences and biochemical characteristics of catalysis. Specifically, members of the GNAT are found in eukaryotes and prokaryotes and have a core structural domain architecture. These enzymes can acetylate small molecules, metabolites, peptides, and proteins. This review presents current knowledge of acetylation mechanisms and functional implications in bacterial metabolism, pathogenicity, stress response, translation, and the emerging topic of protein acetylation in the gut microbiome. Additionally, the methods used to elucidate the biological significance of acetylation in bacteria, such as relative quantification and stoichiometry quantification, and the genetic code expansion tool (CGE), are reviewed.

Stress-induced nucleoid remodeling in Deinococcus radiodurans is associated with major changes in Heat Unstable (HU) protein dynamics

Bacteria have developed a wide range of strategies to respond to stress, one of which is the rapid large-scale reorganization of their nucleoid. Nucleoid associated proteins (NAPs) are believed to be major actors in nucleoid remodeling, but the details of this process remain poorly understood. Here, using the radiation resistant bacterium D. radiodurans as a model, and advanced fluorescence microscopy, we examined the changes in nucleoid morphology and volume induced by either entry into stationary phase or exposure to UV-C light, and characterized the associated changes in mobility of the major NAP in D. radiodurans, the heat-unstable (HU) protein. While both types of stress induced nucleoid compaction, HU diffusion was reduced in stationary phase cells, but was instead increased following exposure to UV-C, suggesting distinct underlying mechanisms. Furthermore, we show that UV-C-induced nucleoid remodeling involves a rapid nucleoid condensation step associated with increased HU diffusion, followed by a slower decompaction phase to restore normal nucleoid morphology and HU dynamics, before cell division can resume. These findings shed light on the diversity of nucleoid remodeling processes in bacteria and underline the key role of HU in regulating this process through changes in its mode of assembly on DNA.

Antimicrobial resistance in the wild: Insights from epigenetics

Spreading of bacterial and fungal strains that are resistant to antimicrobials poses a serious threat to the well-being of humans, animals, and plants. Antimicrobial resistance has been mainly investigated in clinical settings. However, throughout their evolutionary history microorganisms in the wild have encountered antimicrobial substances, forcing them to evolve strategies to combat antimicrobial action. It is well known that many of these strategies are based on genetic mechanisms, but these do not fully explain important aspects of the antimicrobial response such as the rapid development of resistance, reversible phenotypes, and hetero-resistance. Consequently, attention has turned toward epigenetic pathways that may offer additional insights into antimicrobial mechanisms. The aim of this review is to explore the epigenetic mechanisms that confer antimicrobial resistance, focusing on those that might be relevant for resistance in the wild. First, we examine the presence of antimicrobials in natural settings. Then we describe the documented epigenetic mechanisms in bacteria and fungi associated with antimicrobial resistance and discuss innovative epigenetic editing techniques to establish causality in this context. Finally, we discuss the relevance of these epigenetic mechanisms on the evolutionary dynamics of antimicrobial resistance in the wild, emphasizing the critical role of priming in the adaptation process. We underscore the necessity of incorporating non-genetic mechanisms into our understanding of antimicrobial resistance evolution. These mechanisms offer invaluable insights into the dynamics of antimicrobial adaptation within natural ecosystems.

Nε-lysine acetylation of the histone-like protein HBsu influences antibiotic survival and persistence in Bacillus subtilis

Nε-lysine acetylation is recognized as a prevalent post-translational modification (PTM) that regulates proteins across all three domains of life. In Bacillus subtilis, the histone-like protein HBsu is acetylated at seven sites, which regulates DNA compaction and the process of sporulation. In Mycobacteria, DNA compaction is a survival strategy in response antibiotic exposure. Acetylation of the HBsu ortholog HupB decondenses the chromosome to escape this drug-induced, non-growing state, and in addition, regulates the formation of drug-tolerant subpopulations by altering gene expression. We hypothesized that the acetylation of HBsu plays similar regulatory roles. First, we measured nucleoid area by fluorescence microscopy and in agreement, we found that wild-type cells compacted their nucleoids upon kanamycin exposure, but not exposure to tetracycline. We analyzed a collection of HBsu mutants that contain lysine substitutions that mimic the acetylated (glutamine) or unacetylated (arginine) forms of the protein. Our findings indicate that some level of acetylation is required at K3 for a proper response and K75 must be deacetylated. Next, we performed time-kill assays of wild-type and mutant strains in the presence of different antibiotics and found that interfering with HBsu acetylation led to faster killing rates. Finally, we examined the persistent subpopulation and found that altering the acetylation status of HBsu led to an increase in persister cell formation. In addition, we found that most of the deacetylation-mimic mutants, which have compacted nucleoids, were delayed in resuming growth following removal of the antibiotic, suggesting that acetylation is required to escape the persistent state. Together, this data adds an additional regulatory role for HBsu acetylation and further supports the existence of a histone-like code in bacteria.

The Mycobacterium tuberculosis methyltransferase Rv2067c manipulates host epigenetic programming to promote its own survival

Mycobacterium tuberculosis has evolved several mechanisms to counter host defense arsenals for its proliferation. Here we report that M. tuberculosis employs a multi-pronged approach to modify host epigenetic machinery for its survival. It secretes methyltransferase (MTase) Rv2067c into macrophages, trimethylating histone H3K79 in a non-nucleosomal context. Rv2067c downregulates host MTase DOT1L, decreasing DOT1L-mediated nucleosomally added H3K79me3 mark on pro-inflammatory response genes. Consequent inhibition of caspase-8-dependent apoptosis and enhancement of RIPK3-mediated necrosis results in increased pathogenesis. In parallel, Rv2067c enhances the expression of SESTRIN3, NLRC3, and TMTC1, enabling the pathogen to overcome host inflammatory and oxidative responses. We provide the structural basis for differential methylation of H3K79 by Rv2067c and DOT1L. The structures of Rv2067c and DOT1L explain how their action on H3K79 is spatially and temporally separated, enabling Rv2067c to effectively intercept the host epigenetic circuit and downstream signaling.

Limited proteolysis of mycobacterial DNA-binding protein 1 with an extended, lysine-rich, intrinsically disordered region to unveil posttranslational modifications

The basic, intrinsically disordered regions of eukaryotic histones and their bacterial counterparts are presumed to act as signaling hubs to regulate the compaction of chromosomes or nucleoids and various DNA processes such as gene expression, recombination, and DNA replication. Posttranslational modifications (PTMs) on these regions are pivotal in regulating chromosomal or nucleoid compaction and DNA processes. However, the low sequence complexity and the presence of short lysine-rich repeats in the regions have hindered the accurate determination of types and locations of PTMs using conventional proteomic procedures. We described a limited proteolysis protocol using trypsin to analyze PTMs on mycobacterial DNA-binding protein 1 (MDP1), a nucleoid-associated protein in mycobacterial species that possesses an extended, lysine-rich, intrinsically disordered region in its C-terminal domain. This limited proteolysis approach successfully revealed significant methylation on many lysine residues in the C-terminal domain of MDP1 purified from Mycobacterium tuberculosis, which was lacking in the corresponding region of recombinant MDP1 expressed in Escherichia coli.

The roles of nucleoid-associated proteins and topoisomerases in chromosome structure, strand segregation, and the generation of phenotypic heterogeneity in bacteria

How to adapt to a changing environment is a fundamental, recurrent problem confronting cells. One solution is for cells to organize their constituents into a limited number of spatially extended, functionally relevant, macromolecular assemblies or hyperstructures, and then to segregate these hyperstructures asymmetrically into daughter cells. This asymmetric segregation becomes a particularly powerful way of generating a coherent phenotypic diversity when the segregation of certain hyperstructures is with only one of the parental DNA strands and when this pattern of segregation continues over successive generations. Candidate hyperstructures for such asymmetric segregation in prokaryotes include those containing the nucleoid-associated proteins (NAPs) and the topoisomerases. Another solution to the problem of creating a coherent phenotypic diversity is by creating a growth-environment-dependent gradient of supercoiling generated along the replication origin-to-terminus axis of the bacterial chromosome. This gradient is modulated by transcription, NAPs, and topoisomerases. Here, we focus primarily on two topoisomerases, TopoIV and DNA gyrase in Escherichia coli, on three of its NAPs (H-NS, HU, and IHF), and on the single-stranded binding protein, SSB. We propose that the combination of supercoiling-gradient-dependent and strand-segregation-dependent topoisomerase activities result in significant differences in the supercoiling of daughter chromosomes, and hence in the phenotypes of daughter cells.

Mycobacterial DNA-binding protein 1 is critical for BCG survival in stressful environments and simultaneously regulates gene expression

Survival of the live attenuated Bacillus Calmette-Guérin (BCG) vaccine amidst harsh host environments is key for BCG effectiveness as it allows continuous immune response induction and protection against tuberculosis. Mycobacterial DNA binding protein 1 (MDP1), a nucleoid associated protein, is essential in BCG. However, there is limited knowledge on the extent of MDP1 gene regulation and how this influences BCG survival. Here, we demonstrate that MDP1 conditional knockdown (cKD) BCG grows slower than vector control in vitro, and dies faster upon exposure to antibiotics (bedaquiline) and oxidative stress (H2O2 and menadione). MDP1-cKD BCG also exhibited low infectivity and survival in THP-1 macrophages and mice indicating possible susceptibility to host mediated stress. Consequently, low in vivo survival resulted in reduced cytokine (IFN-gamma and TNF-alpha) production by splenocytes. Temporal transcriptome profiling showed more upregulated (81-240) than downregulated (5-175) genes in response to MDP1 suppression. Pathway analysis showed suppression of biosynthetic pathways that coincide with low in vitro growth. Notable was the deferential expression of genes involved in stress response (sigI), maintenance of DNA integrity (mutT1), REDOX balance (WhiB3), and host interactions (PE/PE_PGRS). Thus, this study shows MDP1's importance in BCG survival and highlights MDP1-dependent gene regulation suggesting its role in growth and stress adaptation.

The role of Mycobacterium tuberculosis acetyltransferase and protein acetylation modifications in tuberculosis

Tuberculosis (TB) is a widespread infectious disease caused by Mycobacterium tuberculosis (M. tb), which has been a significant burden for a long time. Post-translational modifications (PTMs) are essential for protein function in both eukaryotic and prokaryotic cells. This review focuses on the contribution of protein acetylation to the function of M. tb and its infected macrophages. The acetylation of M. tb proteins plays a critical role in virulence, drug resistance, regulation of metabolism, and host anti-TB immune response. Similarly, the PTMs of host proteins induced by M. tb are crucial for the development, treatment, and prevention of diseases. Host protein acetylation induced by M. tb is significant in regulating host immunity against TB, which substantially affects the disease's development. The review summarizes the functions and mechanisms of M. tb acetyltransferase in virulence and drug resistance. It also discusses the role and mechanism of M. tb in regulating host protein acetylation and immune response regulation. Furthermore, the current scenario of isoniazid usage in M. tb therapy treatment is examined. Overall, this review provides valuable information that can serve as a preliminary basis for studying pathogenic research, developing new drugs, exploring in-depth drug resistance mechanisms, and providing precise treatment for TB.

Antimicrobial resistance and mechanisms of epigenetic regulation

The rampant use of antibiotics in animal husbandry, farming and clinical disease treatment has led to a significant issue with pathogen resistance worldwide over the past decades. The classical mechanisms of resistance typically investigate antimicrobial resistance resulting from natural resistance, mutation, gene transfer and other processes. However, the emergence and development of bacterial resistance cannot be fully explained from a genetic and biochemical standpoint. Evolution necessitates phenotypic variation, selection, and inheritance. There are indications that epigenetic modifications also play a role in antimicrobial resistance. This review will specifically focus on the effects of DNA modification, histone modification, rRNA methylation and the regulation of non-coding RNAs expression on antimicrobial resistance. In particular, we highlight critical work that how DNA methyltransferases and non-coding RNAs act as transcriptional regulators that allow bacteria to rapidly adapt to environmental changes and control their gene expressions to resist antibiotic stress. Additionally, it will delve into how Nucleolar-associated proteins in bacteria perform histone functions akin to eukaryotes. Epigenetics, a non-classical regulatory mechanism of bacterial resistance, may offer new avenues for antibiotic target selection and the development of novel antibiotics.

TgKDAC4: A Unique Deacetylase of Toxoplasma's Apicoplast

Toxoplasma gondii is an obligate intracellular parasite of the phylum Apicomplexa and causes toxoplasmosis infections, a disease that affects a quarter of the world's population and has no effective cure. Epigenetic regulation is one of the mechanisms controlling gene expression and plays an essential role in all organisms. Lysine deacetylases (KDACs) act as epigenetic regulators affecting gene silencing in many eukaryotes. Here, we focus on TgKDAC4, an enzyme unique to apicomplexan parasites, and a class IV KDAC, the least-studied class of deacetylases so far. This enzyme shares only a portion of the specific KDAC domain with other organisms. Phylogenetic analysis from the TgKDAC4 domain shows a putative prokaryotic origin. Surprisingly, TgKDAC4 is located in the apicoplast, making it the only KDAC found in this organelle to date. Transmission electron microscopy assays confirmed the presence of TgKDAC4 in the periphery of the apicoplast. We identified possible targets or/and partners of TgKDAC4 by immunoprecipitation assays followed by mass spectrometry analysis, including TgCPN60 and TgGAPDH2, both located at the apicoplast and containing acetylation sites. Understanding how the protein works could provide new insights into the metabolism of the apicoplast, an essential organelle for parasite survival.

Protein acylation links metabolism and the control of signal transduction, transcription regulation, growth, and pathogenicity in Actinobacteria

Actinobacteria have a complex life cycle, including morphological and physiological differentiation which are often associated with the biosynthesis of secondary metabolites. Recently, increased interest in post-translational modifications (PTMs) in these Gram-positive bacteria has highlighted the importance of PTMs as signals that provide functional diversity and regulation by modifying proteins to respond to diverse stimuli. Here, we review the developments in research on acylation, a typical PTM that uses acyl-CoA or related metabolites as donors, as well as the understanding of the direct link provided by acylation between cell metabolism and signal transduction, transcriptional regulation, cell growth, and pathogenicity in Actinobacteria.

Mycobacterial IHF is a highly dynamic nucleoid-associated protein that assists HupB in organizing chromatin

Nucleoid-associated proteins (NAPs) crucially contribute to organizing bacterial chromatin and regulating gene expression. Among the most highly expressed NAPs are the HU and integration host factor (IHF) proteins, whose functional homologues, HupB and mycobacterial integration host factor (mIHF), are found in mycobacteria. Despite their importance for the pathogenicity and/or survival of tubercle bacilli, the role of these proteins in mycobacterial chromosome organization remains unknown. Here, we used various approaches, including super-resolution microscopy, to perform a comprehensive analysis of the roles of HupB and mIHF in chromosome organization. We report that HupB is a structural agent that maintains chromosome integrity on a local scale, and that the lack of this protein alters chromosome morphology. In contrast, mIHF is a highly dynamic protein that binds DNA only transiently, exhibits susceptibility to the chromosomal DNA topology changes and whose depletion leads to the growth arrest of tubercle bacilli. Additionally, we have shown that depletion of Mycobacterium smegmatis integration host factor (msIHF) leads to chromosome shrinkage and replication inhibition.

Systematic analysis of lysine malonylation in Streptococcus mutans

Protein lysine malonylation (Kmal) is a novel post-translational modification (PTM) that regulates various biological pathways such as energy metabolism and translation. Malonylation in prokaryotes, however, is still poorly understood. In this study, we performed a global Kmal analysis of the cariogenic organism Streptococcus mutans by combining antibody-based affinity enrichment and high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis. Altogether, 392 malonyllysine sites in 159 proteins were identified. Subsequent bioinformatic analysis revealed that Kmal occurs in proteins involved in various metabolic pathways including translation machinery, energy metabolism, RNA degradation, and biosynthesis of various secondary metabolites. Quantitative analysis demonstrated that Kmal substrates were globally altered in the biofilm growth state compared to the planktonic growth state. Furthermore, a comparative analysis of the lysine malonylome of our study with previously determined lysine acetylome in S. mutans revealed that a small proportion of Kmal sites overlapped with acetylated sites, whereby suggesting that these two acylations have distinct functional implications. These results expand our knowledge of Kmal in prokaryotes, providing a resource for researching metabolic regulation of bacterial virulence and physiological functions by PTM.

Phosphorylation Regulation of a Histone-like HU Protein from Deinococcus radiodurans

BACKGROUND

Histone-like proteins are small molecular weight DNA-binding proteins that are widely distributed in prokaryotes. These proteins have multiple functions in cellular structures and processes, including the morphological stability of the nucleoid, DNA compactness, DNA replication, and DNA repair. Deinococcus radiodurans, an extremophilic microorganism, has extraordinary DNA repair capability and encodes an essential histone-like protein, DrHU.

OBJECTIVE

We aim to investigate the phosphorylation regulation role of a histone-like HU protein from Deinococcus radiodurans.

METHODS

LC-MS/MS analysis was used to determine the phosphorylation site of endogenous DrHU. The predicted structure of DrHU-DNA was obtained from homology modeling (Swissmodel) using Staphylococcus aureus HU-DNA structure (PDB ID: 4QJU) as the starting model. Two types of mutant proteins T37E and T37A were generated to explore their DNA binding affinity. Complemented-knockout strategy was used to generate the ΔDrHU/pk-T37A and ΔDrHU/pk-T37E strains for growth curves and phenotypical analyses.

RESULTS AND DISCUSSION

The phosphorylation site Thr37, which is present in most bacterial HU proteins, is located at the putative protein-DNA interaction interface of DrHU. Compared to the wild-type protein, one in which this threonine is replaced by glutamate to mimic a permanent state of phosphorylation (T37E) showed enhanced double-stranded DNA binding but a weakened protective effect against hydroxyl radical cleavage. Complementation of T37E in a DrHU-knockout strain caused growth defects and sensitized the cells to UV radiation and oxidative stress.

CONCLUSIONS

Phosphorylation modulates the DNA-binding capabilities of the histone-like HU protein from D. radiodurans, which contributes to the environmental adaptation of this organism.

HupB, a nucleoid-associated protein, is critical for survival of Mycobacterium tuberculosis under host-mediated stresses and for enhanced tolerance to key first-line antibiotics

To survive and establish its niche, Mycobacterium tuberculosis (Mtb) engages in a steady battle against an array of host defenses and a barrage of antibiotics. Here, we demonstrate that Mtb employs HupB, a nucleoid-associated protein (NAP) as its key player to simultaneously battle and survive in these two stress-inducing fronts. Typically, NAPs are key to bacterial survival under a wide array of environmental or host-mediated stresses. Here, we report that for Mtb to survive under different macrophage-induced assaults including acidic pH, nutrient depletion, oxidative and nitrosative stresses, HupB presence is critical. As expected, the hupB knockout mutant is highly sensitive to these host-mediated stresses. Furthermore, Mtb aptly modulates HupB protein levels to overcome these stresses. We also report that HupB aids Mtb to gain tolerance to high levels of rifampicin (RIF) and isoniazid (INH) exposure. Loss of hupB makes Mtb highly susceptible to even short exposures to reduced amounts of RIF and INH. Overexpressing hupB in Mtb or complementing hupB in the hupB knockout mutant triggers enhanced survival of Mtb under these stresses. We also find that upon loss of hupB, Mtb significantly enhances the permeability of its cell wall by modulating the levels of several surface lipids including phthiocerol dimycocerosates (PDIMs), thus possibly influencing overall susceptibility to host-mediated stresses. Loss of hupB also downregulates efflux pump expression possibly influencing increased susceptibility to INH and RIF. Finally, we find that therapeutic targeting of HupB with SD1, a known small molecule inhibitor, significantly enhances Mtb susceptibility to INH and THP-1 macrophages and significantly reduces MIC to INH. Thus, our data strongly indicate that HupB is a highly promising therapeutic target especially for potential combinatorial shortened therapy with reduced INH and RIF doses.

Small Prokaryotic DNA-Binding Proteins Protect Genome Integrity throughout the Life Cycle

Genomes of all organisms are persistently threatened by endogenous and exogenous assaults. Bacterial mechanisms of genome maintenance must provide protection throughout the physiologically distinct phases of the life cycle. Spore-forming bacteria must also maintain genome integrity within the dormant endospore. The nucleoid-associated proteins (NAPs) influence nucleoid organization and may alter DNA topology to protect DNA or to alter gene expression patterns. NAPs are characteristically multifunctional; nevertheless, Dps, HU and CbpA are most strongly associated with DNA protection. Archaea display great variety in genome organization and many inhabit extreme environments. As of yet, only MC1, an archaeal NAP, has been shown to protect DNA against thermal denaturation and radiolysis. ssDNA are intermediates in vital cellular processes, such as DNA replication and recombination. Single-stranded binding proteins (SSBs) prevent the formation of secondary structures but also protect the hypersensitive ssDNA against chemical and nuclease degradation. Ionizing radiation upregulates SSBs in the extremophile Deinococcus radiodurans.

Types and functions of heterogeneity in mycobacteria

The remarkable ability of Mycobacterium tuberculosis to survive attacks from the host immune response and drug treatment is due to the resilience of a few bacilli rather than a result of survival of the entire population. Maintenance of mycobacterial subpopulations with distinct phenotypic characteristics is key for survival in the face of dynamic and variable stressors encountered during infection. Mycobacterial populations develop a wide range of phenotypes through an innate asymmetric growth pattern and adaptation to fluctuating microenvironments during infection that point to heterogeneity being a vital survival strategy. In this Review, we describe different types of mycobacterial heterogeneity and discuss how heterogeneity is generated and regulated in response to environmental cues. We discuss how this heterogeneity may have a key role in recording memory of their environment at both the single-cell level and the population level to give mycobacterial populations plasticity to withstand complex stressors.

Phosphorylation and acetylation of mitochondrial transcription factor A promote transcription processivity without compromising initiation or DNA compaction

Mitochondrial transcription factor A (TFAM) plays important roles in mitochondrial DNA compaction, transcription initiation, and in the regulation of processes like transcription and replication processivity. It is possible that TFAM is locally regulated within the mitochondrial matrix via such mechanisms as phosphorylation by protein kinase A and nonenzymatic acetylation by acetyl-CoA. Here, we demonstrate that DNA-bound TFAM is less susceptible to these modifications. We confirmed using EMSAs that phosphorylated or acetylated TFAM compacted circular double-stranded DNA just as well as unmodified TFAM and provide an in-depth analysis of acetylated sites on TFAM. We show that both modifications of TFAM increase the processivity of mitochondrial RNA polymerase during transcription through TFAM-imposed barriers on DNA, but that TFAM bearing either modification retains its full activity in transcription initiation. We conclude that TFAM phosphorylation by protein kinase A and nonenzymatic acetylation by acetyl-CoA are unlikely to occur at the mitochondrial DNA and that modified free TFAM retains its vital functionalities like compaction and transcription initiation while enhancing transcription processivity.

Acetylation at Lysine 86 of Escherichia coli HUβ Modulates the DNA-Binding Capability of the Protein

DNA-binding protein HU is highly conserved in bacteria and has been implicated in a range of cellular processes and phenotypes. Like eukaryotic histones, HU is subjected to post-translational modifications. Specifically, acetylation of several lysine residues have been reported in both homologs of Escherichia coli HU. Here, we investigated the effect of acetylation at Lys67 and Lys86, located in the DNA binding-loop and interface of E. coli HUβ, respectively. Using the technique of genetic code expansion, homogeneous HUβ(K67ac) and HUβ(K86ac) protein units were obtained. Acetylation at Lys86 seemed to have negligible effects on protein secondary structure and thermal stability. Nevertheless, we found that this site-specific acetylation can regulate DNA binding by the HU homodimer but not the heterodimer. Intriguingly, while Lys86 acetylation reduced the interaction of the HU homodimer with short double-stranded DNA containing a 2-nucleotide gap or nick, it enhanced the interaction with longer DNA fragments and had minimal effect on a short, fully complementary DNA fragment. These results demonstrate the complexity of post-translational modifications in functional regulation, as well as indicating the role of lysine acetylation in tuning bacterial gene transcription and epigenetic regulation.

Structure-based design of haloperidol analogues as inhibitors of acetyltransferase Eis from Mycobacterium tuberculosis to overcome kanamycin resistance

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), is a deadly bacterial disease. Drug-resistant strains of Mtb make eradication of TB a daunting task. Overexpression of the enhanced intracellular survival (Eis) protein by Mtb confers resistance to the second-line antibiotic kanamycin (KAN). Eis is an acetyltransferase that acetylates KAN, inactivating its antimicrobial function. Development of Eis inhibitors as KAN adjuvant therapeutics is an attractive path to forestall and overcome KAN resistance. We discovered that an antipsychotic drug, haloperidol (HPD, 1), was a potent Eis inhibitor with IC₅₀ = 0.39 ± 0.08 μM. We determined the crystal structure of the Eis-haloperidol (1) complex, which guided synthesis of 34 analogues. The structure-activity relationship study showed that in addition to haloperidol (1), eight analogues, some of which were smaller than 1, potently inhibited Eis (IC₅₀ ≤ 1 μM). Crystal structures of Eis in complexes with three potent analogues and droperidol (DPD), an antiemetic and antipsychotic, were determined. Three compounds partially restored KAN sensitivity of a KAN-resistant Mtb strain K204 overexpressing Eis. The Eis inhibitors generally did not exhibit cytotoxicity against mammalian cells. All tested compounds were modestly metabolically stable in human liver microsomes, exhibiting 30-60% metabolism over the course of the assay. While direct repurposing of haloperidol as an anti-TB agent is unlikely due to its neurotoxicity, this study reveals potential approaches to modifying this chemical scaffold to minimize toxicity and improve metabolic stability, while preserving potent Eis inhibition.

Post-translational Lysine Ac(et)ylation in Bacteria: A Biochemical, Structural, and Synthetic Biological Perspective

Ac(et)ylation is a post-translational modification present in all domains of life. First identified in mammals in histones to regulate RNA synthesis, today it is known that is regulates fundamental cellular processes also in bacteria: transcription, translation, metabolism, cell motility. Ac(et)ylation can occur at the ε-amino group of lysine side chains or at the α-amino group of a protein. Furthermore small molecules such as polyamines and antibiotics can be acetylated and deacetylated enzymatically at amino groups. While much research focused on N-(ε)-ac(et)ylation of lysine side chains, much less is known about the occurrence, the regulation and the physiological roles on N-(α)-ac(et)ylation of protein amino termini in bacteria. Lysine ac(et)ylation was shown to affect protein function by various mechanisms ranging from quenching of the positive charge, increasing the lysine side chains’ size affecting the protein surface complementarity, increasing the hydrophobicity and by interfering with other post-translational modifications. While N-(ε)-lysine ac(et)ylation was shown to be reversible, dynamically regulated by lysine acetyltransferases and lysine deacetylases, for N-(α)-ac(et)ylation only N-terminal acetyltransferases were identified and so far no deacetylases were discovered neither in bacteria nor in mammals. To this end, N-terminal ac(et)ylation is regarded as being irreversible. Besides enzymatic ac(et)ylation, recent data showed that ac(et)ylation of lysine side chains and of the proteins N-termini can also occur non-enzymatically by the high-energy molecules acetyl-coenzyme A and acetyl-phosphate. Acetyl-phosphate is supposed to be the key molecule that drives non-enzymatic ac(et)ylation in bacteria. Non-enzymatic ac(et)ylation can occur site-specifically with both, the protein primary sequence and the three dimensional structure affecting its efficiency. Ac(et)ylation is tightly controlled by the cellular metabolic state as acetyltransferases use ac(et)yl-CoA as donor molecule for the ac(et)ylation and sirtuin deacetylases use NAD⁺ as co-substrate for the deac(et)ylation. Moreover, the accumulation of ac(et)yl-CoA and acetyl-phosphate is dependent on the cellular metabolic state. This constitutes a feedback control mechanism as activities of many metabolic enzymes were shown to be regulated by lysine ac(et)ylation. Our knowledge on lysine ac(et)ylation significantly increased in the last decade predominantly due to the huge methodological advances that were made in fields such as mass-spectrometry, structural biology and synthetic biology. This also includes the identification of additional acylations occurring on lysine side chains with supposedly different regulatory potential. This review highlights recent advances in the research field. Our knowledge on enzymatic regulation of lysine ac(et)ylation will be summarized with a special focus on structural and mechanistic characterization of the enzymes, the mechanisms underlying non-enzymatic/chemical ac(et)ylation are explained, recent technological progress in the field are presented and selected examples highlighting the important physiological roles of lysine ac(et)ylation are summarized.

Bacterial protein acetylation and its role in cellular physiology and metabolic regulation

Protein acetylation is an evolutionarily conserved posttranslational modification. It affects enzyme activity, metabolic flux distribution, and other critical physiological and biochemical processes by altering protein size and charge. Protein acetylation may thus be a promising tool for metabolic regulation to improve target production and conversion efficiency in fermentation. Here we review the role of protein acetylation in bacterial physiology and metabolism and describe applications of protein acetylation in fermentation engineering and strategies for regulating acetylation status. Although protein acetylation has become a hot topic, the regulatory mechanisms have not been fully characterized. We propose future research directions in protein acetylation.

Characterization of the Radiation Desiccation Response Regulon of the Radioresistant Bacterium Deinococcus radiodurans by Integrative Genomic Analyses

Numerous genes are overexpressed in the radioresistant bacterium Deinococcus radiodurans after exposure to radiation or prolonged desiccation. It was shown that the DdrO and IrrE proteins play a major role in regulating the expression of approximately twenty genes. The transcriptional repressor DdrO blocks the expression of these genes under normal growth conditions. After exposure to genotoxic agents, the IrrE metalloprotease cleaves DdrO and relieves gene repression. At present, many questions remain, such as the number of genes regulated by DdrO. Here, we present the first ChIP-seq analysis performed at the genome level in Deinococcus species coupled with RNA-seq, which was achieved in the presence or not of DdrO. We also resequenced our laboratory stock strain of D. radiodurans R1 ATCC 13939 to obtain an accurate reference for read alignments and gene expression quantifications. We highlighted genes that are directly under the control of this transcriptional repressor and showed that the DdrO regulon in D. radiodurans includes numerous other genes than those previously described, including DNA and RNA metabolism proteins. These results thus pave the way to better understand the radioresistance pathways encoded by this bacterium and to compare the stress-induced responses mediated by this pair of proteins in diverse bacteria.

Contribution of Nε-lysine Acetylation towards Regulation of Bacterial Pathogenesis

N^ε-lysine acetylation is an important, dynamic regulatory posttranslational modification (PTM) that is common in bacteria. Protein acetylomes have been characterized for more than 30 different species, and it is known that acetylation plays important regulatory roles in many essential biological processes. The levels of acetylation are enzymatically controlled by the opposing actions of lysine acetyltransferases and deacetylases. In bacteria, a second mechanism of acetylation exists and occurs via an enzyme-independent manner using the secondary metabolite acetyl-phosphate. Nonenzymatic acetylation accounts for global low levels of acetylation. Recently, studies concerning the role of protein acetylation in bacterial virulence have begun. Acetylated virulence factors have been identified and further characterized. The roles of the enzymes that acetylate and deacetylate proteins in the establishment of infection and biofilm formation have also been investigated. In this review, we discuss the acetylomes of human bacterial pathogens. We highlight examples of known acetylated virulence proteins and examine how they affect survival in the host. Finally, we discuss how acetylation might influence host-pathogen interactions and look at the contribution of acetylation to antimicrobial resistance.

Epigenetic code during mycobacterial infections: therapeutic implications for tuberculosis

Epigenetics involves changing the gene function without any change in the sequence of the genes. In the case of tuberculosis (TB) infections, the bacilli, Mycobacterium tuberculosis (M.tb), uses epigenetics as a tool to protect itself from the host immune system. TB is a deadly disease-causing maximum death per year due to a single infectious agent. In the case of TB, there is an urgent need for novel host-directed therapies which can effectively target the survival and long-term persistence of the bacteria without developing drug resistance in the bacterial strains while also reducing the duration and toxicity associated with the mainstream anti-TB drugs. Recent studies have suggested that TB infection has a significant effect on the host epigenome thereby manipulating the host immune response in the favor of the pathogen. M.tb alters the activation status of key genes involved in the immune response against TB to promote its survival and subvert the antibacterial strategies of the host. These changes are reversible and can be exploited to design very efficient host-directed therapies to fight against TB. This review has been written with the purpose of discussing the role of epigenetic changes in TB pathogenesis and the therapeutic approaches involving epigenetics, which can be utilized for targeting the pathogen.

Protein acetyltransferases mediate bacterial adaptation to a diverse environment

Protein lysine acetylation is a conserved post-translational modification that modulates several cellular processes. Protein acetylation and its physiological implications are well understood in eukaryotes; however, its role is emerging in bacteria. Lysine acetylation in bacteria is fine-tuned by the concerted action of lysine acetyltransferases (KATs), protein deacetylases (KDACs), metabolic intermediates- acetyl-coenzyme A (Ac-CoA) and acetyl phosphate (AcP). AcP mediated nonenzymatic acetylation is predominant in bacteria due to its high acetyl transfer potential whereas, enzymatic acetylation by bacterial KATs (bKAT) are considered less abundant. SePat, the first bKAT discovered in Salmonella enterica, regulates the activity of the central metabolic enzyme- acetyl-CoA synthetase, through its acetylation. Recent studies have highlighted the role of bKATs in stress responses like pH tolerance, nutrient stress, persister cell formation, antibiotic resistance and pathogenesis. Bacterial genomes encode many putative bKATs of unknown biological function and significance. Detailed characterization of putative and partially characterized bKATs is important to decipher the acetylation mediated regulation in bacteria. Proper synthesis of information about the diverse roles of bKATs is missing to date, which can lead to the discovery of new antimicrobial targets in future. In this review, we provide an overview of the diverse physiological roles of known bKATs, and their mode of regulation in different bacteria. We also highlight existing gaps in the literature and present questions that may help understand the regulatory mechanisms mediated by bKATs in adaptation to a diverse habitat.

Rv0802c is an acyltransferase that succinylates and acetylates Mycobacterium tuberculosis nucleoid-associated protein HU

Among the nucleoid-associated proteins (NAPs), HU is the most conserved in eubacteria, engaged in overall chromosome organization and regulation of gene expression. Unlike other bacteria, HU from Mycobacterium tuberculosis (MtHU), has a long carboxyl terminal domain enriched in basic amino acids, resembling eukaryotic histone N-terminal tails. As with histones, MtHU undergoes post-translational modifications and we have previously identified interacting kinases, methyltransferases, an acetyltransferase and a deacetylase. Here we show that Rv0802c interacts and succinylates MtHU. Although categorized as a succinyltransferase, we show that this GNAT superfamily member can catalyse both succinylation and acetylation of MtHU with comparable kinetic parameters. Like acetylation of MtHU, succinylation of MtHU caused reduced interaction of the NAP with DNA, determined by electrophoretic mobility shift assay and surface plasmon resonance. However, in vivo expression of Rv0802c did not significantly alter the nucleoid architecture. Although such succinylation of NAPs is rare, these modifications of the archetypal NAP may provide avenues to the organism to compensate for the underrepresentation of NAPs in its genome to control the dynamics of nucleoid architecture and cellular functions.

Immunomodulation by epigenome alterations in Mycobacterium tuberculosis infection

Mycobacterium tuberculosis (MTB) has co-evolved with humans for decades and developed several mechanisms to evade host immunity. It can efficiently alter the host epigenome, thus playing a major role in immunomodulation by either activating or suppressing genes responsible for mounting an immune response against the pathogen. Epigenetic modifications such as DNA methylation and chromatin remodelling regulate gene expression and influence several cellular processes. The involvement of epigenetic factors in disease onset and development had been overlooked upon in comparison to genetic mutations. It is now believed that assessment of epigenetic changes hold great potential in diagnosis, prevention and treatment strategies for a wide range of diseases. In this review, we unravel the principles of epigenetics and the numerous ways by which MTB re-shapes the host epigenetic landscape as a strategy to overpower the host immune system for its survival and persistence.

HDAC3 inhibitor RGFP966 controls bacterial growth and modulates macrophage signaling during Mycobacterium tuberculosis infection

RATIONALE

Host-directed therapeutics for Mycobacterium tuberculosis (Mtb) offer potential strategies for combatting antibiotic resistance and for killing non-replicating bacilli. Phenylbutyrate, a partially selective histone-deacetylase (HDAC) inhibitor, was previously shown to control Mtb growth and alter macrophage inflammatory pathways at 2-4 mM concentrations.

OBJECTIVE

To identify a more potent and selective HDAC inhibitor that modulates macrophage responses to mycobacteria and has direct antibacterial effects against Mtb.

METHODS

We used cellular approaches to characterize the role of pharmacologic inhibition of HDAC3 on Mtb growth and Mtb-induced peripheral and alveolar macrophage immune functions.

MEASUREMENTS AND MAIN RESULTS

RGFP966, an HDAC3 inhibitor, controlled Mtb, BCG and M. avium growth directly in broth culture and in human peripheral blood monocyte-derived and alveolar macrophages with an MIC50 of approximately 5-10 μM. In contrast, RGFP966 did not inhibit growth of several other intracellular and extracellular bacteria. We also found that RGFP966 modulated macrophage pro-inflammatory cytokine secretion in response to Mtb infection with decreased IL6 and TNF secretion.

CONCLUSIONS

We identified a potent and selective small molecule inhibitor of HDAC3 with direct antimicrobial activity against Mtb and modulation of macrophage signaling pathways.

Structural analysis of the N-acetyltransferase Eis1 from Mycobacterium abscessus reveals the molecular determinants of its incapacity to modify aminoglycosides

Enhanced intracellular survival (Eis) proteins belonging to the superfamily of the GCN5-related N-acetyltransferases play important functions in mycobacterial pathogenesis. In Mycobacterium tuberculosis, Eis enhances the intracellular survival of the bacilli in macrophages by modulating the host immune response and is capable to chemically modify and inactivate aminoglycosides. In nontuberculous mycobacteria (NTM), Eis shares similar functions. However, Mycobacterium abscessus, a multidrug resistant NTM, possesses two functionally distinct Eis homologues, Eis1_Mab and Eis2_Mab . While Eis2_Mab participates in virulence and aminoglycosides resistance, this is not the case for Eis1_Mab, whose exact biological function remains to be determined. Herein, we show that overexpression of Eis1_Mab in M. abscessus fails to induce resistance to aminoglycosides. To clarify why Eis1_Mab is unable to modify this class of antibiotics, we solved its crystal structure bound to its cofactor, acetyl-CoA. The structure revealed that Eis1_Mab has a typical homohexameric Eis-like organization. The structural analysis supported by biochemical approaches demonstrated that while Eis1_Mab can acetylate small substrates, its active site is too narrow to accommodate aminoglycosides. Comparison with other Eis structures showed that an extended loop between strands 9 and 10 is blocking the access of large substrates to the active site and movement of helices 4 and 5 reduces the volume of the substrate-binding pocket to these compounds in Eis1_Mab . Overall, this study underscores the molecular determinants explaining functional differences between Eis1_Mab and Eis2_Mab, especially those inherent to their capacity to modify aminoglycosides.

Methylation of two-component response regulator MtrA in mycobacteria negatively modulates its DNA binding and transcriptional activation

Post-translational modifications such as phosphorylation, nitrosylation, and pupylation modulate multiple cellular processes in Mycobacterium tuberculosis. While protein methylation at lysine and arginine residues is widespread in eukaryotes, to date only two methylated proteins in Mtb have been identified. Here, we report the identification of methylation at lysine and/or arginine residues in nine mycobacterial proteins. Among the proteins identified, we chose MtrA, an essential response regulator of a two-component signaling system, which gets methylated on multiple lysine and arginine residues to examine the functional consequences of methylation. While methylation of K207 confers a marginal decrease in the DNA-binding ability of MtrA, methylation of R122 or K204 significantly reduces the interaction with the DNA. Overexpression of S-adenosyl homocysteine hydrolase (SahH), an enzyme that modulates the levels of S-adenosyl methionine in mycobacteria decreases the extent of MtrA methylation. Most importantly, we show that decreased MtrA methylation results in transcriptional activation of mtrA and sahH promoters. Collectively, we identify novel methylated proteins, expand the list of modifications in mycobacteria by adding arginine methylation, and show that methylation regulates MtrA activity. We propose that protein methylation could be a more prevalent modification in mycobacterial proteins.

Regulation of gene expression by protein lysine acetylation in Salmonella

Protein lysine acetylation influences many physiological functions, such as gene regulation, metabolism, and disease in eukaryotes. Although little is known about the role of lysine acetylation in bacteria, several reports have proposed its importance in various cellular processes. Here, we discussed the function of the protein lysine acetylation and the post-translational modifications (PTMs) of histone-like proteins in bacteria focusing on Salmonella pathogenicity. The protein lysine residue in Salmonella is acetylated by the Pat-mediated enzymatic pathway or by the acetyl phosphate-mediated non-enzymatic pathway. In Salmonella, the acetylation of lysine 102 and lysine 201 on PhoP inhibits its protein activity and DNA-binding, respectively. Lysine acetylation of the transcriptional regulator, HilD, also inhibits pathogenic gene expression. Moreover, it has been reported that the protein acetylation patterns significantly differ in the drug-resistant and -sensitive Salmonella strains. In addition, nucleoid-associated proteins such as histone-like nucleoid structuring protein (H-NS) are critical for the gene silencing in bacteria, and PTMs in H-NS also affect the gene expression. In this review, we suggest that protein lysine acetylation and the post-translational modifications of H-NS are important factors in understanding the regulation of gene expression responsible for pathogenicity in Salmonella.

Lysine-specific acetylated proteome from the archaeon Thermococcus gammatolerans reveals the presence of acetylated histones

Thermococcus gammatolerans EJ3 is an extremophile archaeon which was revealed as one of the most radioresistant organisms known on Earth, withstanding up to 30 kGy gamma-ray radiations. While its theoretical proteome is rather small, T. gammatolerans may enhance its toolbox by post-translational modification of its proteins. Here, we explored its extent of Nε-acetylation of lysines. For this, we immunopurified with two acetylated-lysine antibodies the acetylated peptides resulting from a proteolysis of soluble proteins with trypsin. The comparison of acetylated proteomes of two archaea highlights some common acetylation patterns but only 4 out of 26 orthologous proteins found to be acetylated in both species, are acetylated on the same lysine site. We evidenced that histone B is acetylated in T. gammatolerans at least at two different sites (K27 and K36), and a peptide common at the C-terminus of histones A and B is also acetylated. We verified that acetylation of histones is a common trait among Thermococcales after recording data on Thermococcus kodakaraensis histones and identifying three acetylated sites. This discovery reinforces the strong evolutionary link between Archaea and Eukaryotes and should be an incentive for further investigation on the extent and role of acetylation of histones in Archaea. SIGNIFICANCE: Acetylation is an important post-translational modification of proteins that has been extensively described in Eukaryotes, and more recently in Bacteria. Here, we report for the first time ever that histones in Archaea are also modified by acetylation after a systematic survey of acetylated peptides in Thermococcus gammatolerans. Structural models of histones A and B indicates that acetylation of the identified modified residues may play an important role in histone assembly and/or interaction with DNA. The in-depth protein acetylome landscape in T. gammatolerans includes at least 181 unique protein sequences, some of them being modified on numerous residues. Proteins involved in metabolic processes, information storage and processing mechanisms are over-represented categories in this dataset, highlighting the ancient role of this protein post-translational modification in primitive cells.

Role of post-translational modifications in the acquisition of drug resistance in Mycobacterium tuberculosis

Tuberculosis (TB) is one of the primary causes of deaths due to infectious diseases. The current TB regimen is long and complex, failing of which leads to relapse and/or the emergence of drug resistance. There is a critical need to understand the mechanisms of resistance development. With increasing drug pressure, Mycobacterium tuberculosis (Mtb) activates various pathways to counter drug-related toxicity. Signaling modules steer the evolution of Mtb to a variant that can survive, persist, adapt, and emerge as a form that is resistant to one or more drugs. Recent studies reveal that about 1/3rd of the annotated Mtb proteome is modified post-translationally, with a large number of these proteins being essential for mycobacterial survival. Post-translational modifications (PTMs) such as phosphorylation, acetylation, and pupylation play a salient role in mycobacterial virulence, pathogenesis, and metabolism. The role of many other PTMs is still emerging. Understanding the signaling pathways and PTMs may assist clinical strategies and drug development for Mtb. In this review, we explore the contribution of PTMs to mycobacterial physiology, describe the related cellular processes, and discuss how these processes are linked to drug resistance. A significant number of drug targets, InhA, RpoB, EmbR, and KatG, are modified at multiple residues via PTMs. A better understanding of drug-resistance regulons and associated PTMs will aid in developing effective drugs against TB.

Hypothesis: nucleoid-associated proteins segregate with a parental DNA strand to generate coherent phenotypic diversity

The generation of a phenotypic diversity that is coherent across a bacterial population is a fundamental problem. We propose here that the DNA strand-specific segregation of certain nucleoid-associated proteins or NAPs results in these proteins being asymmetrically distributed to the daughter cells. We invoke a variety of mechanisms as responsible for this asymmetrical segregation including those based on differences between the leading and lagging strands, post-translational modifications, oligomerisation and association with membrane domains.

Addressing the Possibility of a Histone-Like Code in Bacteria

Acetylation was initially discovered as a post-translational modification (PTM) on the unstructured, highly basic N-terminal tails of eukaryotic histones in the 1960s. Histone acetylation constitutes part of the "histone code", which regulates chromosome compaction and various DNA processes such as gene expression, recombination, and DNA replication. In bacteria, nucleoid-associated proteins (NAPs) are responsible these functions in that they organize and compact the chromosome and regulate some DNA processes. The highly conserved DNABII family of proteins are considered functional homologues of eukaryotic histones despite having no sequence or structural conservation. Within the past decade, a growing interest in N^ε-lysine acetylation led to the discovery that hundreds of bacterial proteins are acetylated with diverse cellular functions, in direct contrast to the original thought that this was a rare phenomenon. Similarly, other previously undiscovered bacterial PTMs, like serine, threonine, and tyrosine phosphorylation, have also been characterized. In this review, the various PTMs that were discovered among DNABII family proteins, specifically histone-like protein (HU) orthologues, from large-scale proteomic studies are discussed. The functional significance of these modifications and the enzymes involved are also addressed. The discovery of novel PTMs on these proteins begs this question: is there a histone-like code in bacteria?

Bordetella pertussis Acetylome is Shaped by Lysine Deacetylase Bkd1

Post-translational modifications of proteins enable swift physiological adaptation of cells to altered growth conditions and stress. Aside from protein phosphorylation, acetylation on ε-amino groups of lysine residues (N-ε-lysine acetylation) represents another important post-translational modification of proteins. For many bacterial pathogens, including the whooping cough agent Bordetella pertussis, the role and extent of protein acetylation remain to be defined. We expressed in Escherichia coli the BP0960 and BP3063 genes encoding two putative deacetylases of B. pertussis and show that BP0960 encodes a lysine deacetylase enzyme, named Bkd1, that regulates acetylation of a range of B. pertussis proteins. Comparison of the proteome and acetylome of a Δbkd1 mutant with the proteome and acetylome of wild-type B. pertussis (PRIDE ID. PXD016384) revealed that acetylation on lysine residues may modulate activities or stabilities of proteins involved in bacterial metabolism and histone-like proteins. However, increased acetylation of the BvgA response regulator protein of the B. pertussis master virulence-regulating BvgAS two-component system affected neither the total levels of produced BvgA nor its phosphorylation status. Indeed, the Δbkd1 mutant was not impaired in the production of key virulence factors and its survival within human macrophages in vitro was not affected. The Δbkd1 mutant exhibited an increased growth rate under carbon source-limiting conditions and its virulence in the in vivo mouse lung infection model was somewhat affected. These results indicate that the lysine deacetylase Bkd1 and N-ε-lysine acetylation primarily modulate the general metabolism rather than the virulence of B. pertussis.

Chromosome Segregation Proteins as Coordinators of Cell Cycle in Response to Environmental Conditions

Chromosome segregation is a crucial stage of the cell cycle. In general, proteins involved in this process are DNA-binding proteins, and in most bacteria, ParA and ParB are the main players; however, some bacteria manage this process by employing other proteins, such as condensins. The dynamic interaction between ParA and ParB drives movement and exerts positioning of the chromosomal origin of replication (oriC) within the cell. In addition, both ParA and ParB were shown to interact with the other proteins, including those involved in cell division or cell elongation. The significance of these interactions for the progression of the cell cycle is currently under investigation. Remarkably, DNA binding by ParA and ParB as well as their interactions with protein partners conceivably may be modulated by intra- and extracellular conditions. This notion provokes the question of whether chromosome segregation can be regarded as a regulatory stage of the cell cycle. To address this question, we discuss how environmental conditions affect chromosome segregation and how segregation proteins influence other cell cycle processes.

Nucleoid Associated Proteins: The Small Organizers That Help to Cope With Stress

The bacterial chromosome must be efficiently compacted to fit inside the small and crowded cell while remaining accessible for the protein complexes involved in replication, transcription, and DNA repair. The dynamic organization of the nucleoid is a consequence of both intracellular factors (i.e., simultaneously occurring cell processes) and extracellular factors (e.g., environmental conditions, stress agents). Recent studies have revealed that the bacterial chromosome undergoes profound topological changes under stress. Among the many DNA-binding proteins that shape the bacterial chromosome structure in response to various signals, NAPs (nucleoid associated proteins) are the most abundant. These small, basic proteins bind DNA with low specificity and can influence chromosome organization under changing environmental conditions (i.e., by coating the chromosome in response to stress) or regulate the transcription of specific genes (e.g., those involved in virulence).

The inter-talk between Mycobacterium tuberculosis and the epigenetic mechanisms

Epigenetics regulate gene function without any alteration in the DNA sequence. The epigenetics represent one of the most important regulators in different cellular processes and have initially been developed in microorganisms as a protective strategy. The evaluation of the epigenetic mechanisms is also important in achieving an efficient control strategy in tuberculosis (TB). TB is one of the most significant epidemiological concerns in human history. Despite several in vivo and in vitro studies that have evaluated different epigenetic modifications in TB, many aspects of the association between epigenetics and TB are not fully understood. The current paper is aimed at reviewing our knowledge on histone modifications and DNA methylation modifications, as well as miRNAs regulation in TB.