DNA-[Adenine] Methylation in Lower Eukaryotes

The fungicide pyraclostrobin affects gene expression by altering the DNA methylation pattern in Magnaporthe oryzae

Introduction

Rice blast disease caused by Magnaporthe oryzae has long been the main cause of rice (Oryza sativa L.) yield reduction worldwide. The quinone external inhibitor pyraclostrobin is widely used as a fungicide to effectively control the spread of pathogenic fungi, including M. oryzae. However, M. oryzae can develop resistance through multiple levels of mutation, such as target protein cytb mutation G143A/S, leading to a decrease in the effectiveness of the biocide after a period of application. Therefore, uncovering the possible mutational mechanisms from multiple perspectives will further provide feasible targets for drug development.

Methods

In this work, we determined the gene expression changes in M. oryzae in response to pyraclostrobin stress and their relationship with DNA methylation by transcriptome and methylome.

Results

The results showed that under pyraclostrobin treatment, endoplasmic reticulum (ER)-associated and ubiquitin-mediated proteolysis were enhanced, suggesting that more aberrant proteins may be generated that need to be cleared. DNA replication and repair processes were inhibited. Glutathione metabolism was enhanced, while lipid metabolism was impaired. The number of alternative splicing events increased. These changes may be related to the elevated methylation levels of cytosine and adenine in gene bodies. Both hypermethylation and hypomethylation of differentially methylated genes (DMGs) mainly occurred in exons and promoters. Some DMGs and differentially expressed genes (DEGs) were annotated to the same pathways by GO and KEGG, including protein processing in the ER, ubiquitin-mediated proteolysis, RNA transport and glutathione metabolism, suggesting that pyraclostrobin may affect gene expression by altering the methylation patterns of cytosine and adenine.

Discussion

Our results revealed that 5mC and 6mA in the gene body are associated with gene expression and contribute to adversity adaptation in M. oryzae. This enriched the understanding for potential mechanism of quinone inhibitor resistance, which will facilitate the development of feasible strategies for maintaining the high efficacy of this kind of fungicide.

Shaping eukaryotic epigenetic systems by horizontal gene transfer

DNA methylation constitutes one of the pillars of epigenetics, relying on covalent bonds for addition and/or removal of chemically distinct marks within the major groove of the double helix. DNA methyltransferases, enzymes which introduce methyl marks, initially evolved in prokaryotes as components of restriction-modification systems protecting host genomes from bacteriophages and other invading foreign DNA. In early eukaryotic evolution, DNA methyltransferases were horizontally transferred from bacteria into eukaryotes several times and independently co-opted into epigenetic regulatory systems, primarily via establishing connections with the chromatin environment. While C5-methylcytosine is the cornerstone of plant and animal epigenetics and has been investigated in much detail, the epigenetic role of other methylated bases is less clear. The recent addition of N4-methylcytosine of bacterial origin as a metazoan DNA modification highlights the prerequisites for foreign gene co-option into the host regulatory networks, and challenges the existing paradigms concerning the origin and evolution of eukaryotic regulatory systems.

ZayyuNet - A Unified Deep Learning Model for the Identification of Epigenetic Modifications Using Raw Genomic Sequences

Epigenetic modifications have a vital role in gene expression and are linked to cellular processes such as differentiation, development, and tumorigenesis. Thus, the availability of reliable and accurate methods for identifying and defining these changes facilitates greater insights into the regulatory mechanisms that rely on epigenetic modifications. The current experimental methods provide a genome-wide identification of epigenetic modifications; however, they are expensive and time-consuming. To date, several machine learning methods have been proposed for identifying modifications such as DNA N6-Methyladenine (6mA), RNA N6-Methyladenosine (m6A), DNA N4-methylcytosine (4mC), and RNA pseudouridine ( Ψ). However, these methods are task-specific computational tools and require different encoding representations of DNA/RNA sequences. In this study, we propose a unified deep learning model, called ZayyuNet, for the identification of various epigenetic modifications. The proposed model is based on an architecture called, SpinalNet, inspired by the human somatosensory system that can efficiently receive large inputs and achieve better performance. The proposed model has been evaluated on various epigenetic modifications such as 6mA, m6A, 4mC, and Ψ and the results achieved outperform current state-of-the-art models. A user-friendly web server has been built and made freely available at http://nsclbio.jbnu.ac.kr/tools/ZayyuNet/.

Research progress in predicting DNA methylation modifications and the relation with human diseases

DNA methylation is an important mode of regulation in epigenetic mechanisms, and it is one of the research foci in the field of epigenetics. DNA methylation modification affects a series of biological processes, such as eukaryotic cell growth, differentiation and transformation mechanisms, by regulating gene expression. In this review, we systematically summarized the DNA methylation databases, prediction tools for DNA methylation modification, machine learning algorithms for predicting DNA methylation modification, and the relationship between DNA methylation modification and diseases such as hypertension, Alzheimer's disease, diabetic nephropathy, and cancer. An in-depth understanding of DNA methylation mechanisms can promote accurate prediction of DNA methylation modifications and the treatment and diagnosis of related diseases.

Biodiversity-based development and evolution: the emerging research systems in model and non-model organisms

Evolutionary developmental biology, or Evo-Devo for short, has become an established field that, broadly speaking, seeks to understand how changes in development drive major transitions and innovation in organismal evolution. It does so via integrating the principles and methods of many subdisciplines of biology. Although we have gained unprecedented knowledge from the studies on model organisms in the past decades, many fundamental and crucially essential processes remain a mystery. Considering the tremendous biodiversity of our planet, the current model organisms seem insufficient for us to understand the evolutionary and physiological processes of life and its adaptation to exterior environments. The currently increasing genomic data and the recently available gene-editing tools make it possible to extend our studies to non-model organisms. In this review, we review the recent work on the regulatory signaling of developmental and regeneration processes, environmental adaptation, and evolutionary mechanisms using both the existing model animals such as zebrafish and Drosophila, and the emerging nonstandard model organisms including amphioxus, ascidian, ciliates, single-celled phytoplankton, and marine nematode. In addition, the challenging questions and new directions in these systems are outlined as well.

Toward incorporating epigenetics into regulation of gene expression in the parasite Trichomonas vaginalis

Trichomonas vaginalis is an extracellular parasite that colonizes the human urogenital tract, causing a highly prevalent sexually transmitted infection. The parasite must change its transcriptional profile in order to establish and maintain infection. However, few core regulatory elements and transcription factors have been identified to date and little is known about other mechanisms that may control these rapid changes in gene expression during parasite infection. In the last years, epigenetic mechanisms involved in the regulation of gene expression have been gaining major attention in this parasite. In this review, we summarize and discuss the major advances of the last few years with regard to epigenetics (DNA methylation, post-translational histone modifications, and histone variants) in the parasite T. vaginalis. These studies can shed light into our current understanding of this parasite's biology with far-reaching implications for the prognosis and treatment of trichomoniasis.

DeepTorrent: a deep learning-based approach for predicting DNA N4-methylcytosine sites

DNA N4-methylcytosine (4mC) is an important epigenetic modification that plays a vital role in regulating DNA replication and expression. However, it is challenging to detect 4mC sites through experimental methods, which are time-consuming and costly. Thus, computational tools that can identify 4mC sites would be very useful for understanding the mechanism of this important type of DNA modification. Several machine learning-based 4mC predictors have been proposed in the past 3 years, although their performance is unsatisfactory. Deep learning is a promising technique for the development of more accurate 4mC site predictions. In this work, we propose a deep learning-based approach, called DeepTorrent, for improved prediction of 4mC sites from DNA sequences. It combines four different feature encoding schemes to encode raw DNA sequences and employs multi-layer convolutional neural networks with an inception module integrated with bidirectional long short-term memory to effectively learn the higher-order feature representations. Dimension reduction and concatenated feature maps from the filters of different sizes are then applied to the inception module. In addition, an attention mechanism and transfer learning techniques are also employed to train the robust predictor. Extensive benchmarking experiments demonstrate that DeepTorrent significantly improves the performance of 4mC site prediction compared with several state-of-the-art methods.

The clever strategies used by intracellular parasites to hijack host gene expression

Intracellular pathogens need to develop sophisticated mechanisms to survive and thrive in the hostile environment within host cells. Unicellular, eukaryotic parasites from the Apicomplexa phylum have become masters of manipulating their host cells, exploiting signaling, and metabolic pathways to hijack host gene expression to their own advantage. These intracellular parasites have developed a wide range of strategies that affect transcriptional machineries and epigenetic events in the host cell nucleus. In recent years, many laboratories have risen to the challenge of studying the epigenetics of host-pathogen interactions with the hope that unraveling the complexity of the mechanisms involved will provide important insights into parasitism and provide clues to fight infection. In this review, we survey some of these many strategies that Apicomplexan parasites employ to hijack their hosts, including inducing epigenetic enzymes, secreting epigenators into host cells, sequestering host signaling proteins, and co-opting non-coding RNAs to change gene and protein expression. We cite selected examples from the literature on Apicomplexa parasites (including Toxoplasma, Theileria, and Cryptosporidium) to highlight the success of these parasitic processes. We marvel at the effectiveness of the strategies that these pathogens have evolved and wonder what mysteries lie ahead in exploring the epigenetics of host-parasite interactions.

The DNA modification N6-methyl-2'-deoxyadenosine (m6dA) drives activity-induced gene expression and is required for fear extinction

DNA modification is known to regulate experience-dependent gene expression. However, beyond cytosine methylation and its oxidated derivatives, very little is known about the functional importance of chemical modifications on other nucleobases in the brain. Here we report that in adult mice trained in fear extinction the DNA modification N6-methyl-2’-deoxyadenosine (m6dA) accumulates along promoters and coding sequences in activated prefrontal cortical neurons. The deposition of m6dA is associated with increased genome-wide occupancy of the mammalian m6dA methyltransferase, N6amt1, and this correlates with extinction-induced gene expression. The accumulation of m6dA is associated with transcriptional activation at the brain-derived neurotrophic factor (Bdnf) P4 promoter, which is required for Bdnf exon IV mRNA expression and for the extinction of conditioned fear. These results expand the scope of DNA modifications in the adult brain and highlight changes in m6dA as an epigenetic mechanism associated with activity-induced gene expression and the formation of fear extinction memory.

N6-Methyladenosine in RNA and DNA: An Epitranscriptomic and Epigenetic Player Implicated in Determination of Stem Cell Fate

Vast emerging evidences are linking the base modifications and determination of stem cell fate such as proliferation and differentiation. Among the base modification markers extensively studied, 5-methylcytosine (5-mC) and its oxidative derivatives (5-hydroxymethylcytosine (5-hmC), 5-formylcytosine (5-fC), and 5-carboxylcytosine (5-caC)) dynamically occur in DNA and RNA and have been acknowledged as important epigenetic markers involved in regulation of cellular biological processes. N6-Methyladenosine modification in DNA (m6dA), mRNA (m6A), tRNA, and other noncoding RNAs has been defined as another important epigenetic and epitranscriptomic marker in eukaryotes in recent years. The mRNA m6A modification has been characterized biochemically, molecularly, and phenotypically, including elucidation of its methyltransferase complexes (m6A writer), demethylases (m6A eraser), and direct interaction proteins (readers), while limited information on the DNA m6dA is available. The levels and the landscapes of m6A in the epitranscriptomes and epigenomes are precisely and dynamically regulated by the fine-tuned coordination of the writers and erasers in accordance with stages of the growth, development, and reproduction as naturally programmed during the lifespan. Additionally, progress has been made in appreciation of the link between aberrant m6A modification in stem cells and diseases, like cancers and neurodegenerative disorders. These achievements are inspiring scientists to further uncover the epigenetic mechanisms for stem cell development and to dissect pathogenesis of the multiple diseases conferred by development aberration of the stem cells. This review article will highlight the research advances in the role of m6A methylation modifications of DNA and RNA in the regulation of stem cell and genesis of the closely related disorders. Additionally, this article will also address the research directions in the future.

DNA N6-methyladenine in metazoans: functional epigenetic mark or bystander?

The DNA-adenine modification N⁶-methyladenine (6mA), initially thought to be mainly restricted to prokaryotes and certain unicellular eukaryotes, has recently been found in metazoans. Proposed functions vary from gene activation to transposon suppression. However, since most metazoan genomes possess 5-methylcytosine (5mC) as a dominant epigenetic mark, it raises the question of why 6mA is required. This Perspective summarizes the latest discoveries and suggests potential functional roles for 6mA in metazoan genomes.

Trait changes induced by species interactions in two phenotypically distinct strains of a marine dinoflagellate

Populations of the toxigenic marine dinoflagellate Alexandrium are composed of multiple genotypes that display phenotypic variation for traits known to influence top-down processes, such as the ability to lyse co-occurring competitors and prospective grazers. We performed a detailed molecular analysis of species interactions to determine how different genotypes perceive and respond to other species. In a controlled laboratory culture study, we exposed two A. fundyense strains that differ in their capacity to produce lytic compounds to the dinoflagellate grazer Polykrikos kofoidii, and analyzed transcriptomic changes during this interaction. Approximately 5% of all analyzed genes were differentially expressed between the two Alexandrium strains under control conditions (without grazer presence) with fold-change differences that were proportionally higher than those observed in grazer treatments. Species interactions led to the genotype-specific expression of genes involved in endocytotic processes, cell cycle control and outer membrane properties, and signal transduction and gene expression regulatory processes followed similar patterns for both genotypes. The genotype-specific trait changes observed in this study exemplify the complex responses to chemically mediated species interactions within the plankton and their regulation at the gene level.

Characterization of eukaryotic DNA N(6)-methyladenine by a highly sensitive restriction enzyme-assisted sequencing

Although extensively studied in prokaryotes, the prevalence and significance of DNA N(6)-methyladenine (6mA or m(6)dA) in eukaryotes had been underappreciated until recent studies, which have demonstrated that 6mA regulates gene expression as a potential heritable mark. To interrogate 6mA sites at single-base resolution, we report DA-6mA-seq (DpnI-Assisted N(6)-methylAdenine sequencing), an approach that uses DpnI to cleave methylated adenine sites in duplex DNA. We find that DpnI cuts other sequence motifs besides the canonical GATC restriction sites, thereby expanding the utility of this method. DA-6mA-seq achieves higher sensitivity with nanograms of input DNA and lower sequencing depth than conventional approaches. We study 6mA at base resolution in the Chlamydomonas genome and apply the new method to two other eukaryotic organisms, Plasmodium and Penicillium. Combined with conventional approaches, our method further shows that most 6mA sites are fully methylated on both strands of DNA at various sequence contexts.

Programmed Genome Rearrangements in the Ciliate Oxytricha

The ciliate Oxytricha is a microbial eukaryote with two genomes, one of which experiences extensive genome remodeling during development. Each round of conjugation initiates a cascade of events that construct a transcriptionally active somatic genome from a scrambled germline genome, with considerable help from both long and small noncoding RNAs. This process of genome remodeling entails massive DNA deletion and reshuffling of remaining DNA segments to form functional genes from their interrupted and scrambled germline precursors. The use of Oxytricha as a model system provides an opportunity to study an exaggerated form of programmed genome rearrangement. Furthermore, studying the mechanisms that maintain nuclear dimorphism and mediate genome rearrangement has demonstrated a surprising plasticity and diversity of noncoding RNA pathways, with new roles that go beyond conventional gene silencing. Another aspect of ciliate genetics is their unorthodox patterns of RNA-mediated, epigenetic inheritance that rival Mendelian inheritance. This review takes the reader through the key experiments in a model eukaryote that led to fundamental discoveries in RNA biology and pushes the biological limits of DNA processing.

DNA Base Flipping: A General Mechanism for Writing, Reading, and Erasing DNA Modifications

The modification of DNA bases is a classic hallmark of epigenetics. Four forms of modified cytosine-5-methylcytosine, 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine-have been discovered in eukaryotic DNA. In addition to cytosine carbon-5 modifications, cytosine and adenine methylated in the exocyclic amine-N4-methylcytosine and N6-methyladenine-are other modified DNA bases discovered even earlier. Each modified base can be considered a distinct epigenetic signal with broader biological implications beyond simple chemical changes. Since 1994, crystal structures of proteins and enzymes involved in writing, reading, and erasing modified bases have become available. Here, we present a structural synopsis of writers, readers, and erasers of the modified bases from prokaryotes and eukaryotes. Despite significant differences in structures and functions, they are remarkably similar regarding their engagement in flipping a target base/nucleotide within DNA for specific recognitions and/or reactions. We thus highlight base flipping as a common structural framework broadly applied by distinct classes of proteins and enzymes across phyla for epigenetic regulations of DNA.

DNA N(6)-methyladenine: a new epigenetic mark in eukaryotes?

DNA N(6)-adenine methylation (N(6)-methyladenine; 6mA) in prokaryotes functions primarily in the host defence system. The prevalence and significance of this modification in eukaryotes had been unclear until recently. Here, we discuss recent publications documenting the presence of 6mA in Chlamydomonas reinhardtii, Drosophila melanogaster and Caenorhabditis elegans; consider possible roles for this DNA modification in regulating transcription, the activity of transposable elements and transgenerational epigenetic inheritance; and propose 6mA as a new epigenetic mark in eukaryotes.

NanoCluster Beacons Enable Detection of a Single N⁶-Methyladenine

While N(6)-methyladenine (m(6)A) is a common modification in prokaryotic and lower eukaryotic genomes and has many biological functions, there is no simple and cost-effective way to identify a single N(6)-methyladenine in a nucleic acid target. Here we introduce a robust, simple, enzyme-free and hybridization-based method using a new silver cluster probe, termed methyladenine-specific NanoCluster Beacon (maNCB), which can detect single m(6)A in DNA targets based on the fluorescence emission spectra of silver clusters. Not only can maNCB identify m(6)A at the single-base level but it also can quantify the extent of adenine methylation in heterogeneous samples. Our method is superior to high-resolution melting analysis as we can pinpoint the location of m(6)A in the target.

Epigenetic modifications underlying symbiont-host interactions

The development, existence, and functioning of numerous animals and plants depend on their symbiotic interactions with other organisms, mainly microorganisms. In return, the symbionts benefit from safe habitats and nutrient-rich environments provided by their hosts. In these interactions, genetic changes in either of the partners may provide fitness advantages and become subjects to natural selection. Recent findings suggest that epigenetic changes, heritable or within the organism's life time, in either of the partners play significant roles in the establishment of symbiotic relationships. In this review, a variety of epigenetic effects underlying the most common host-symbiont interactions will be examined to determine to what extent these effects are shared in various interactions and how the epigenetic pathways could possibly be manipulated to benefit the interacting symbionts.

Epi-fingerprinting and epi-interventions for improved crop production and food quality

Increasing crop production at a time of rapid climate change represents the greatest challenge facing contemporary agricultural research. Our understanding of the genetic control of yield derives from controlled field experiments designed to minimize environmental variance. In spite of these efforts there is substantial residual variability among plants attributable to Genotype × Environment interactions. Recent advances in the field of epigenetics have revealed a plethora of gene control mechanisms that could account for much of this unassigned variation. These systems act as a regulatory interface between the perception of the environment and associated alterations in gene expression. Direct intervention of epigenetic control systems hold the enticing promise of creating new sources of variability that could enhance crop performance. Equally, understanding the relationship between various epigenetic states and responses of the crop to specific aspects of the growing environment (epigenetic fingerprinting) could allow for a more tailored approach to plant agronomy. In this review, we explore the many ways in which epigenetic interventions and epigenetic fingerprinting can be deployed for the improvement of crop production and quality.

Cancer Stem Cells: Dynamic Entities in an Ever-Evolving Paradigm

The cancer stem cell (CSC) hypothesis postulates that there is a hierarchy of cellular differentiation within cancers and that the bulk population of tumor cells is derived from a relatively small population of multi-potent neoplastic stem-like cells (CSCs). This tumor-initiating cell population plays an important role in maintaining tumor growth through their unlimited self-renewal, therapeutic resistance, and capacity to propagate tumors through asymmetric cell division. Recent findings from multiple laboratories show that cancer progenitor cells have the capacity to de-differentiate and acquire a stem-like phenotype in response to either genetic manipulation or environmental cues. These findings suggest that CSCs and relatively differentiated progenitors coexist in dynamic equilibrium and are subject to bidirectional conversion. In this review, we discuss emerging concepts regarding the stem-like phenotype, its acquisition by cancer progenitor cells, and the molecular mechanisms involved. Understanding the dynamic equilibrium between CSCs and cancer progenitor cells is critical for the development of novel therapeutic strategies that focus on depleting tumors of their tumor-propagating cell population.

Unexplored potentials of epigenetic mechanisms of plants and animals-theoretical considerations

Morphological and functional changes of cells are important for adapting to environmental changes and associated with continuous regulation of gene expressions. Genes are regulated–in part–by epigenetic mechanisms resulting in alternating patterns of gene expressions throughout life. Epigenetic changes responding to the environmental and intercellular signals can turn on/off specific genes, but do not modify the DNA sequence. Most epigenetic mechanisms are evolutionary conserved in eukaryotic organisms, and several homologs of epigenetic factors are present in plants and animals. Moreover, in vitro studies suggest that the plant cytoplasm is able to induce a nuclear reassembly of the animal cell, whereas others suggest that the ooplasm is able to induce condensation of plant chromatin. Here, we provide an overview of the main epigenetic mechanisms regulating gene expression and discuss fundamental epigenetic mechanisms and factors functioning in both plants and animals. Finally, we hypothesize that animal genome can be reprogrammed by epigenetic factors from the plant protoplast.

Nuclear repositioning precedes promoter accessibility and is linked to the switching frequency of a Plasmodium falciparum invasion gene

Variation of surface adhesins, such as the Plasmodium falciparum erythrocyte invasion ligand PfRh4, is critical for virulence and immune evasion in many microbes. While phenotypic switching is linked to transcriptional changes and chromatin function, the determinants of switching frequency remain poorly defined. By expressing a prokaryotic DNA methylase in P. falciparum, we directly assayed accessibility of transcriptionally active and silent chromatin at the PfRh4 locus. Parasites selected for in vivo PfRh4 activation show a reversible increase in promoter accessibility and exhibit perinuclear repositioning of the locus from a silent to a conserved activation domain. Forced activation of a proximal gene results in a similar repositioning of the PfRh4 locus, with a concomitant increase in PfRh4 activation in a subpopulation of parasites and promoter accessibility correlating with actively transcribed loci. Thus, nuclear repositioning is associated with increased P. falciparum switching frequency, while promoter accessibility is tightly linked to clonally active PfRh4 promoters.

The incorporation of epigenetics in artificial gene regulatory networks

Artificial gene regulatory networks are computational models that draw inspiration from biological networks of gene regulation. Since their inception they have been used to infer knowledge about gene regulation and as methods of computation. These computational models have been shown to possess properties typically found in the biological world, such as robustness and self organisation. Recently, it has become apparent that epigenetic mechanisms play an important role in gene regulation. This paper describes a new model, the Artificial Epigenetic Regulatory Network (AERN) which builds upon existing models by adding an epigenetic control layer. Our results demonstrate that AERNs are more adept at controlling multiple opposing trajectories when applied to a chaos control task within a conservative dynamical system, suggesting that AERNs are an interesting area for further investigation.

Differential effect of three base modifications on DNA thermostability revealed by high resolution melting

High resolution melting (HRM) can detect and quantify the presence of 5-methylcytosine (5mC) in DNA samples, but the ability of HRM to diagnose other DNA modifications remains unexplored. The DNA bases N6-methyladenine and 5-hydroxymethylcytosine occur across almost all phyla. While their function remains controversial, their presence perturbs DNA structure. Such modifications could affect gene regulation, chromatin condensation and DNA packaging. Here, we reveal that DNA containing N6-methyladenine or 5-hydroxymethylcytosine exhibits reduced thermal stability compared to cytosine-methylated DNA. These thermostability changes are sufficiently divergent to allow detection and quantification by HRM analysis. Thus, we report that HRM distinguishes between sequence-identical DNA differing only in the modification type of one base. This approach is also able to distinguish between two DNA fragments carrying both N6-methyladenine and 5-methylcytosine but differing only in the distance separating the modified bases. This finding provides scope for the development of new methods to characterize DNA chemically and to allow for low cost screening of mutant populations of genes involved in base modification. More fundamentally, contrast between the thermostabilizing effects of 5mC on dsDNA compared with the destabilizing effects of N6-methyladenine (m6A) and 5-hydroxymethylcytosine (5hmC) raises the intriguing possibility of an antagonistic relationship between modification types with functional significance.

Epigenetic regulation and measurement of epigenetic changes

Epigenetic mechanisms provide an adaptive layer of control in the regulation of gene expression that enables an organism to adjust to a changing environment. Epigenetic regulation increases the functional complexity of deoxyribonucleic acid (DNA) by altering chromatin structure, nuclear organization, and transcript stability. These changes may additively or synergistically influence gene expression and result in long-term molecular and functional consequences independent of the DNA sequence that may ultimately define an individual's phenotype. This article (1) describes histone modification, DNA methylation, and expression of small noncoding RNA species; (2) reviews the most common methods used to measure these epigenetic changes; and (3) presents factors that need to be considered when choosing a specific tissue to evaluate for epigenetic changes.

Environmentally induced phenotypes and DNA methylation: how to deal with unpredictable conditions until the next generation and after

Organisms often respond to environmental changes by producing alternative phenotypes. Epigenetic processes such as DNA methylation may contribute to environmentally induced phenotypic variation by modifying gene expression. Changes in DNA methylation, unlike DNA mutations, can be influenced by the environment; they are stable at the time scale of an individual and present different levels of heritability. These characteristics make DNA methylation a potentially important molecular process to respond to environmental change. The aim of this review is to present the implications of DNA methylation on phenotypic variations driven by environmental changes. More specifically, we explore epigenetic concepts concerning phenotypic change in response to the environment and heritability of DNA methylation, namely the Baldwin effect and genetic accommodation. Before addressing this point, we report major differences in DNA methylation across taxa and the role of this modification in producing and maintaining environmentally induced phenotypic variation. We also present the different methods allowing the detection of methylation polymorphism. We believe this review will be helpful to molecular ecologists, in that it highlights the importance of epigenetic processes in ecological and evolutionary studies.

Dimeric/oligomeric DNA methyltransferases: an unfinished story

DNA methyltransferases (MTases) are enzymes that carry out post-replicative sequence-specific modifications. The initial experimental data on the structure and kinetic characteristics of the EcoRI MTase led to the paradigm that type II systems comprise dimeric endonucleases and monomeric MTases. In retrospect, this was logical because, while the biological substrate of the restriction endonuclease is two-fold symmetrical, the in vivo substrate for the MTase is generally hemi-methylated and, hence, inherently asymmetric. Thus, the paradigm was extended to include all DNA MTases except the more complex bifunctional type I and type III enzymes. Nevertheless, a gradual enlightenment grew over the last decade that has changed the accepted view on the structure of DNA MTases. These results necessitate a more complex view of the structure and function of these important enzymes.

What do unicellular organisms teach us about DNA methylation?

DNA methylation is an epigenetic hallmark that has been studied intensively in mammals and plants. However, knowledge of this phenomenon in unicellular organisms is scanty. Examining epigenetic regulation, and more specifically DNA methylation, in these organisms represents a unique opportunity to better understand their biology. The determination of their methylation status is often complicated by the presence of several differentiation stages in their life cycle. This article focuses on some recent advances that have revealed the unexpected nature of the epigenetic determinants present in protozoa. The role of the enigmatic DNA methyltransferase Dnmt2 in unicellular organisms is discussed.

RNAi-mediated knock-down of gene mN6A1 reduces cell proliferation and decreases protein translation

Methyltransferases play essential roles in modulating important cellular and metabolic processes. A mouse putative N6-DNA methyltransferase gene (GenBank No AY456393) is a novel gene named mN6amt1(mN6A1). To investigate its function in cell fate and protein translation, RNA interference (RNAi)-mediated knock-down method was established. Cell cycle analysis suggests that the cell proliferation decreases after RNAi with mN6A1. The expression plasmid of luciferase was used to detect protein translation, and the results showed that luciferase expression decreased after RNAi with mN6A1, whereas increased after over-expression of mN6A1 or/and eRF1. The binding between mN6A1 and eRF1 was identified by co-immunoprecipitation and pull-down experiments. It might be suggested that mN6A1 participates in protein translation through interaction with eRF1.

Molecular cloning and characterization of the DNA adenine methyltransferase gene in Feldmannia sp. virus

The genome of Feldmannia sp. virus (FsV), a marine brown alga virus, contains a putative DNA adenine methyltransferase (dam) gene of 1,245 bp that encodes a polypeptide of 45.8 kDa. A BLAST search with the FsV dam gene showed high amino acid identity to two putative methyltransferase genes, ORF B29 of Feldmannia irregularis virus (FirrV, 54%) and ORF129 of Ectocarpus siliculosus virus (EsV, 36%); and a PSI BLAST search revealed similarity to the N(6)-adenine methyltransferases (MTases) of other species. Most conserved motifs of beta-class MTases were observed in the FsV dam gene. However, neither of the highly conserved sequences in motifs I (FxGxG) or IV [(S/N/D)PP(Y/F/W)] perfectly matched those in the FsV dam gene. The highly conserved DPPY consensus sequence in motif IV was NTPW in the FsV dam gene, perfectly matching the sequences in ORF B29 of FirrV and ORF129 of EsV. Therefore, the dam genes in brown algae viruses may belong to a yet undiscovered group. The FsV Dam protein expressed from the cloned FsV dam gene methylated E. coli chromosomal DNA. This is the first report showing that a virus infecting marine filamentous brown algae encodes a functional Dam protein.

Purification, crystallization and preliminary X-ray analysis of the BseCI DNA methyltransferase from Bacillus stearothermophilus in complex with its cognate DNA

The DNA methyltransferase M.BseCI from Bacillus stearothermophilus (EC 2.1.1.72), a 579-amino-acid enzyme, methylates the N6 atom of the 3' adenine in the sequence 5'-ATCGAT-3'. M.BseCI was crystallized in complex with its cognate DNA. The crystals were found to belong to the hexagonal space group P6, with unit-cell parameters a = b = 87.0, c = 156.1 A, beta = 120.0 degrees and one molecule in the asymmetric unit. Two complete data sets were collected at wavelengths of 1.1 and 2.0 A to 2.5 and 2.8 A resolution, respectively, using synchrotron radiation at 100 K.

Mutations within the catalytic motif of DNA adenine methyltransferase (Dam) of Aeromonas hydrophila cause the virulence of the Dam-overproducing strain to revert to that of the wild-type phenotype

In this study, we demonstrated that the methyltransferase activity associated with Dam was essential for attenuation of Aeromonas hydrophila virulence. We mutated aspartic acid and tyrosine residues to alanine within the conserved DPPY catalytic motif of Dam and transformed the pBAD/damD/A, pBAD/damY/A, and pBAD/damAhSSU (with the native dam gene) recombinant plasmids into the Escherichia coli GM33 (dam-deficient) strain. Genomic DNA (gDNA) isolated from either of the E. coli GM33 strains harboring the pBAD vector with the mutated dam gene was resistant to DpnI digestion and sensitive to DpnII restriction endonuclease cutting. These findings were contrary to those with the gDNA of E. coli GM33 strain containing the pBAD/damAhSSU plasmid, indicating nonmethylation of E. coli gDNA with mutated Dam. Overproduction of mutated Dam in A. hydrophila resulted in bacterial motility, hemolytic and cytotoxic activities associated with the cytotoxic enterotoxin (Act), and protease activity similar to that of the wild-type (WT) bacterium, which harbored the pBAD vector and served as a control strain. On the contrary, overproduction of native Dam resulted in decreased bacterial motility, increased Act-associated biological effects, and increased protease activity. Lactone production, an indicator of quorum sensing, was increased when the native dam gene was overexpressed, with its levels returning to that of the control strain when the dam gene was mutated. These effects of Dam appeared to be mediated through a regulatory glucose-inhibited division A protein. Infection of mice with the mutated Dam-overproducing strains resulted in mortality rates similar to those for the control strain, with 100% of the animals dying within 2 to 3 days with two 50% lethal doses (LD50s) of the WT bacterium. Importantly, immunization of mice with a native-Dam-overproducing strain at the same LD50 did not result in any lethality and provided protection to animals after subsequent challenge with a lethal dose of the control strain.

Undetectable levels of N6-methyl adenine in mouse DNA: Cloning and analysis of PRED28, a gene coding for a putative mammalian DNA adenine methyltransferase

Three methylated bases, 5-methylcytosine, N4-methylcytosine and N6-methyladenine (m6A), can be found in DNA. However, to date, only 5-methylcytosine has been detected in mammalian genomes. To reinvestigate the presence of m6A in mammalian DNA, we used a highly sensitive method capable of detecting one N6-methyldeoxyadenosine per million nucleosides. Our results suggest that the total mouse genome contains, if any, less than 10(3) m6A. Experiments were next performed on PRED28, a putative mammalian N6-DNA methyltransferase. The murine PRED28 encodes two alternatively spliced RNA. However, although recombinant PRED28 proteins are found in the nucleus, no evidence for an adenine-methyltransferase activity was detected.

N6-methyladenine: the other methylated base of DNA

Contrary to mammalian DNA, which is thought to contain only 5-methylcytosine (m5C), bacterial DNA contains two additional methylated bases, namely N6-methyladenine (m6A), and N4-methylcytosine (m4C). However, if the main function of m5C and m4C in bacteria is protection against restriction enzymes, the roles of m6A are multiple and include, for example, the regulation of virulence and the control of many bacterial DNA functions such as the replication, repair, expression and transposition of DNA. Interestingly, even if adenine methylation is usually considered a bacterial DNA feature, the presence of m6A has been found in protist and plant DNAs. Furthermore, indirect evidence suggests the presence of m6A in mammal DNA, raising the possibility that this base has remained undetected due to the low sensitivity of the analytical methods used. This highlights the importance of considering m6A as the sixth element of DNA.

N6-methyl-adenine: an epigenetic signal for DNA-protein interactions

N(6)-methyl-adenine is found in the genomes of bacteria, archaea, protists and fungi. Most bacterial DNA adenine methyltransferases are part of restriction-modification systems. Certain groups of Proteobacteria also harbour solitary DNA adenine methyltransferases that provide signals for DNA-protein interactions. In gamma-proteobacteria, Dam methylation regulates chromosome replication, nucleoid segregation, DNA repair, transposition of insertion elements and transcription of specific genes. In Salmonella, Haemophilus, Yersinia and Vibrio species and in pathogenic Escherichia coli, Dam methylation is required for virulence. In alpha-proteobacteria, CcrM methylation regulates the cell cycle in Caulobacter, Rhizobium and Agrobacterium, and has a role in Brucella abortus infection.