Effects of methylation on the stability of nucleic acid conformations. Studies at the polymer level

Methyl-dependent auto-regulation of the DNA N6-adenine methyltransferase AMT1 in the unicellular eukaryote Tetrahymena thermophila

DNA N6-methyladenine (6mA) is a potential epigenetic mark involved in gene transcription in eukaryotes, yet the regulatory mechanism governing its methyltransferase (MTase) activity remains obscure. Here, we exploited the 6mA MTase AMT1 to elucidate its auto-regulation in the unicellular eukaryote Tetrahymena thermophila. The detailed endogenous localization of AMT1 in vegetative and sexual stages revealed a correlation between the 6mA reestablishment in the new MAC and the occurrence of zygotically expressed AMT1. Catalytically inactive AMT1 reduced 6mA level on the AMT1 gene and its expression, suggesting that AMT1 modulated its own transcription via 6mA. Furthermore, AMT1-dependent 6mA regulated the transcription of its target genes, thereby affecting cell fitness. Our findings unveil a positive feedback loop of transcriptional activation on the AMT1 gene and highlight the crucial role of AMT1-dependent 6mA in gene transcription.

Epigenetic control of tetrapyrrole biosynthesis by m4C DNA methylation in a cyanobacterium

Epigenetic DNA modifications are pivotal in eukaryotic gene expression, but their regulatory significance in bacteria is less understood. In Synechocystis 6803, the DNA methyltransferase M.Ssp6803II modifies the first cytosine in the GGCC motif, forming N4-methylcytosine (GGm4CC). Deletion of the sll0729 gene encoding M.Ssp6803II (∆sll0729) caused a bluish phenotype due to reduced chlorophyll levels, which was reversed by suppressor mutations. Re-sequencing of 7 suppressor clones revealed a common GGCC to GGTC mutation in the slr1790 promoter's discriminator sequence, encoding protoporphyrinogen IX oxidase, HemJ, crucial for tetrapyrrole biosynthesis. Transcriptomic and qPCR analyses indicated aberrant slr1790 expression in ∆sll0729 mutants. This aberration led to the accumulation of coproporphyrin III and protoporphyrin IX, indicative of impaired HemJ activity. To confirm the importance of DNA methylation in hemJ expression, hemJ promoter variants with varying discriminator sequences were introduced into the wild type, followed by sll0729 deletion. The sll0729 deletion segregated in strains with the GGTC discriminator motif, resulting in wild-type-like pigmentation, whereas freshly prepared ∆sll0729 mutants with the native hemJ promoter exhibited the bluish phenotype. These findings demonstrate that hemJ is tightly regulated in Synechocystis and that N4-methylcytosine is essential for proper hemJ expression. Thus, cytosine N4-methylation is a relevant epigenetic marker in Synechocystis and likely other cyanobacteria.

Sequencing of N6-methyl-deoxyadenosine at single-base resolution across the mammalian genome

Although DNA N6-methyl-deoxyadenosine (6mA) is abundant in bacteria and protists, its presence and function in mammalian genomes have been less clear. We present Direct-Read 6mA sequencing (DR-6mA-seq), an antibody-independent method, to measure 6mA at base resolution. DR-6mA-seq employs a unique mutation-based strategy to reveal 6mA sites as misincorporation signatures without any chemical or enzymatic modulation of 6mA. We validated DR-6mA-seq through the successful mapping of the well-characterized G(6mA)TC motif in the E. coli DNA. As expected, when applying DR-6mA-seq to mammalian systems, we found that genomic DNA (gDNA) 6mA abundance is generally low in most mammalian tissues and cells; however, we did observe distinct gDNA 6mA sites in mouse testis and glioblastoma cells. DR-6mA-seq provides an enabling tool to detect 6mA at single-base resolution for a comprehensive understanding of DNA 6mA in eukaryotes.

Studies of DNA 'Breathing' by Polarization-Sweep Single-Molecule Fluorescence Microscopy of Exciton-Coupled (iCy3)2 Dimer-Labeled DNA Fork Constructs

Local fluctuations of the sugar-phosphate backbones and bases of DNA (often called DNA 'breathing') play a variety of critical roles in controlling the functional interactions of the DNA genome with the protein complexes that regulate it. Here, we present a single-molecule fluorescence method that we have used to measure and characterize such conformational fluctuations at and near biologically important positions in model DNA replication fork constructs labeled with exciton-coupled cyanine [(iCy3)2] dimer probes. Previous work has shown that the constructs that we tested here exhibit a broad range of spectral properties at the ensemble level, and these differences can be structurally and dynamically interpreted using our present methodology at the single-molecule level. The (iCy3)2 dimer has one symmetric (+) and one antisymmetric (-) exciton, with the respective transition dipole moments oriented perpendicular to one another. We excite single-molecule samples using a continuous-wave linearly polarized laser, with the polarization direction continuously rotated at the frequency of 1 MHz. The ensuing fluorescence signal is modulated as the laser polarization alternately excites the symmetric and antisymmetric excitons of the (iCy3)2 dimer probe. Phase-sensitive detection of the modulated signal provides information about the distribution of local conformations and the conformational interconversion dynamics of the (iCy3)2 probe. We find that at most construct positions that we examined, the (iCy3)2 dimer-labeled DNA fork constructs can adopt four topologically distinct conformational macrostates. These results suggest that in addition to observing DNA breathing at and near ss-dsDNA junctions, our new methodology should be useful to determine which of these pre-existing macrostates are recognized by, bind to, and are stabilized by various genome-regulatory proteins.

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.

Navigating the pitfalls of mapping DNA and RNA modifications

Chemical modifications to nucleic acids occur across the kingdoms of life and carry important regulatory information. Reliable high-resolution mapping of these modifications is the foundation of functional and mechanistic studies, and recent methodological advances based on next-generation sequencing and long-read sequencing platforms are critical to achieving this aim. However, mapping technologies may have limitations that sometimes lead to inconsistent results. Some of these limitations are technical in nature and specific to certain types of technology. Here, however, we focus on common (yet not always widely recognized) pitfalls that are shared among frequently used mapping technologies and discuss strategies to help technology developers and users mitigate their effects. Although the emphasis is primarily on DNA modifications, RNA modifications are also discussed.

Interaction preferences between protein side chains and key epigenetic modifications 5-methylcytosine, 5-hydroxymethycytosine and N6-methyladenine

Covalent modifications of standard DNA/RNA nucleobases affect epigenetic regulation of gene expression by modulating interactions between nucleic acids and protein readers. We derive here the absolute binding free energies and analyze the binding modalities between key modified nucleobases 5-methylcytosine (5mC), 5-hydroxymethylcytosine (5hmC) and N⁶-methyladenine (m⁶A) and all non-prolyl/non-glycyl protein side chains using molecular dynamics simulations and umbrella sampling in both water and methanol, the latter mimicking the low dielectric environment at the dehydrated nucleic-acid/protein interfaces. We verify the derived affinities by comparing against a comprehensive set of high-resolution structures of nucleic-protein complexes involving 5mC. Our analysis identifies protein side chains that are highly tuned for detecting cytosine methylation as a function of the environment and can thus serve as microscopic readers of epigenetic marks. Conversely, we show that the relative ordering of sidechain affinities for 5hmC and m⁶A does not differ significantly from those for their precursor bases, cytosine and adenine, respectively, especially in the low dielectric environment. For those two modified bases, the effect is more nuanced and manifests itself primarily at the level of absolute changes in the binding free energy. Our results contribute towards establishing a quantitative foundation for understanding, predicting and modulating the interactions between modified nucleic acids and proteins at the atomistic level.

Same modification, different location: the mythical role of N6-adenine methylation in plant genomes

The present review summarizes recent advances in the understanding of 6mA in DNA as an emergent epigenetic mark with distinctive characteristics, discusses its importance in plant genomes, and highlights its chemical nature and functions. Adenine methylation is an epigenetic modification present in DNA (6mA) and RNA (m6A) that has a regulatory function in many cellular processes. This modification occurs through a reversible reaction that covalently binds a methyl group, usually at the N⁶ position of the purine ring. This modification carries biophysical properties that affect the stability of nucleic acids as well as their binding affinity with other molecules. DNA 6mA has been related to genome stability, gene expression, DNA replication, and repair mechanisms. Recent advances have shown that 6mA in plant genomes is related to development and stress response. In this review, we present recent advances in the understanding of 6mA in DNA as an emergent epigenetic mark with distinctive characteristics. We discuss the key elements of this modification, focusing mainly on its importance in plant genomes. Furthermore, we highlight its chemical nature and the regulatory effects that it exerts on gene expression and plant development. Finally, we emphasize the functions of 6mA in photosynthesis, stress, and flowering.

Isotope Labels Combined with Solution NMR Spectroscopy Make Visible the Invisible Conformations of Small-to-Large RNAs

RNA is central to the proper function of cellular processes important for life on earth and implicated in various medical dysfunctions. Yet, RNA structural biology lags significantly behind that of proteins, limiting mechanistic understanding of RNA chemical biology. Fortunately, solution NMR spectroscopy can probe the structural dynamics of RNA in solution at atomic resolution, opening the door to their functional understanding. However, NMR analysis of RNA, with only four unique ribonucleotide building blocks, suffers from spectral crowding and broad linewidths, especially as RNAs grow in size. One effective strategy to overcome these challenges is to introduce NMR-active stable isotopes into RNA. However, traditional uniform labeling methods introduce scalar and dipolar couplings that complicate the implementation and analysis of NMR measurements. This challenge can be circumvented with selective isotope labeling. In this review, we outline the development of labeling technologies and their application to study biologically relevant RNAs and their complexes ranging in size from 5 to 300 kDa by NMR spectroscopy.

Destabilization of DNA and RNA G-quadruplex structures formed by GGA repeat due to N6-methyladenine modification

N6-methyladenine (m6A) is the most abundant RNA modification in eukaryotic RNA. Further, m6A has been identified in the genomic DNA of both eukaryotes and prokaryotes. The G-quadruplex (G4) structure is a non-canonical nucleic acid structure formed by the stacking of G:G:G:G tetrads. In this study, we evaluated the effect of m6A modifications on G4 structures formed by GGA repeat oligonucleotides, d(GGA)8, d(GGA)4, and r(GGA)4. The d(GGA)8 forms an intramolecular tetrad:heptad:heptad:tetrad G4 structure, while d(GGA)4 forms a dimerized intermolecular tetrad:heptad:heptad:tetrad G4 structure. r(GGA)4 forms a dimerized intermolecular tetrad:hexad:hexad:tetrad G4 structure. Circular dichroism melting analysis demonstrated that (1) m6A modifications destabilized the G4 structure formed by d(GGA)8, (2) m6A modification at A3 disrupted the G4 structure formed by d(GGA)4, and (3) m6A modification at A3 destabilized the G4 structure formed by r(GGA)4. m6A modifications may be involved in controlling G4 structure formation to regulate biological functions.

Means, mechanisms and consequences of adenine methylation in DNA

N⁶-methyl-2'-deoxyadenosine (6mA or m⁶dA) has been reported in the DNA of prokaryotes and eukaryotes ranging from unicellular protozoa and algae to multicellular plants and mammals. It has been proposed to modulate DNA structure and transcription, transmit information across generations and have a role in disease, among other functions. However, its existence in more recently evolved eukaryotes remains a topic of debate. Recent technological advancements have facilitated the identification and quantification of 6mA even when the modification is exceptionally rare, but each approach has limitations. Critical assessment of existing data, rigorous design of future studies and further development of methods will be required to confirm the presence and biological functions of 6mA in multicellular eukaryotes.

A quantitative model predicts how m6A reshapes the kinetic landscape of nucleic acid hybridization and conformational transitions

N6-methyladenosine (m6A) is a post-transcriptional modification that controls gene expression by recruiting proteins to RNA sites. The modification also slows biochemical processes through mechanisms that are not understood. Using temperature-dependent (20°C-65°C) NMR relaxation dispersion, we show that m6A pairs with uridine with the methylamino group in the anti conformation to form a Watson-Crick base pair that transiently exchanges on the millisecond timescale with a singly hydrogen-bonded low-populated (1%) mismatch-like conformation in which the methylamino group is syn. This ability to rapidly interchange between Watson-Crick or mismatch-like forms, combined with different syn:anti isomer preferences when paired (~1:100) versus unpaired (~10:1), explains how m6A robustly slows duplex annealing without affecting melting at elevated temperatures via two pathways in which isomerization occurs before or after duplex annealing. Our model quantitatively predicts how m6A reshapes the kinetic landscape of nucleic acid hybridization and conformational transitions, and provides an explanation for why the modification robustly slows diverse cellular processes.

Effects of Adenine Methylation on the Structure and Thermodynamic Stability of a DNA Minidumbbell

DNA methylation is a prevalent regulatory modification in prokaryotes and eukaryotes. N1-methyladenine (m1A) and N6-methyladenine (m6A) have been found to be capable of altering DNA structures via disturbing Watson-Crick base pairing. However, little has been known about their influences on non-B DNA structures, which are associated with genetic instabilities. In this work, we investigated the effects of m1A and m6A on both the structure and thermodynamic stability of a newly reported DNA minidumbbell formed by two TTTA tetranucleotide repeats. As revealed by the results of nuclear magnetic resonance spectroscopic studies, both m1A and m6A favored the formation of a T·m1A and T·m6A Hoogsteen base pair, respectively. More intriguingly, the m1A and m6A modifications brought about stabilization and destabilization effects on the DNA minidumbbell, respectively. This work provides new biophysical insights into the effects of adenine methylation on the structure and thermodynamic stability of DNA.

Thermal Stability Changes in Telomeric G-Quadruplex Structures Due to N6-Methyladenine Modification

N⁶-methyladenine modification (m⁶dA) has recently been identified in eukaryote genomic DNA. The methylation destabilizes the duplex structure when the adenine forms a Watson-Crick base pair, whereas the methylation on a terminal unpaired adenine stabilizes the duplex structure by increasing the stacking interaction. In this study, the effects of m⁶dA modification on the thermal stability of four distinct telomeric G-quadruplex (G4) structures were investigated. The m⁶dA-modified telomeric oligonucleotide d[AGGG(TTAGGG)₃] that forms a basket-type G4 in Na⁺, d[(TTAGGG)₄TT] that forms a hybrid-type G4 in K⁺ (Form-2), d[AAAGGG(TTAGGG)₃AA] that forms a hybrid-type G4 in K⁺ (Form-1), and d[GGG(TTAGGG)₃T] that forms a basket-type G4 with two G-tetrads in K⁺ (Form-3) were analyzed. Circular dichroism melting analysis demonstrated that (1) A7- and A19-methylation destabilized the basket-type G4 structure that formed in Na⁺, whereas A13-methylation stabilized the structure; (2) A15-methylation stabilized the Form-2 G4 structure; (3) A15- and A21-methylations stabilized the Form-1 G4 structure; and (4) A12-methylation stabilized the Form-3 G4 structure. These results suggest that m⁶dA modifications may affect the thermal stability of human telomeric G4 structures in regulating the biological functions.

Effects of CpG methylation on the thermal stability of c-kit2, c-kit*, and c-kit1 G-quadruplex structures

In genomic DNA, G-quadruplex (G4)-forming DNA can form either a duplex or G4 structure, suggesting that understanding the factors regulating G4 formation is important for revealing the cellular functions controlled by G4 formation. Cytosine DNA methylation in the CpG islands is known to play an important role in transcriptional regulation. Additionally, CpG methylation increases the thermal stability of G4 structures such as BCL2 and VEGF G4. In this study, we evaluated the effects of CpG methylation in three G4 structures (c-kit2, c-kit*, and c-kit1) produced by the c-KIT promoter. Each was analyzed using circular dichroism (CD) melting analysis. The results demonstrate that CpG methylation does not alter the thermal stability of c-kit2 G4 structure when formed in the presence of K⁺; a single-CpG methylation at C1 or C11 decreases the thermal stability of any c-kit2 G4 structure formed in the presence of Na⁺ and Mg²⁺ while methylation at C5 increases the thermal stability; CpG methylation does not alter the thermal stability of c-kit1 or c-kit* G4 structures formed in the presence of K⁺; and the c-kit1 and c-kit* G4-forming oligonucleotides do not form G4 structures in the presence of Na⁺ and Mg²⁺. These results provide important clues for understanding the regulatory mechanisms underlying the formation of CpG methylation-induced G4 structures.

DNA adenine methylation in eukaryotes: Enzymatic mark or a form of DNA damage?

6-methyladenine (6mA) is fairly abundant in nuclear DNA of basal fungi, ciliates and green algae. In these organisms, 6mA is maintained near transcription start sites in ApT context by a parental-strand instruction dependent maintenance methyltransferase and is positively associated with transcription. In animals and plants, 6mA levels are high only in organellar DNA. The 6mA levels in nuclear DNA are very low. They are attributable to nucleotide salvage and the activity of otherwise mitochondrial METTL4, and may be considered as a price that cells pay for adenine methylation in RNA and/or organellar DNA. Cells minimize this price by sanitizing dNTP pools to limit 6mA incorporation, and by converting 6mA that has been incorporated into DNA back to adenine. Hence, 6mA in nuclear DNA should be described as an epigenetic mark only in basal fungi, ciliates and green algae, but not in animals and plants.

Interactions of replication initiator RctB with single- and double-stranded DNA in origin opening of Vibrio cholerae chromosome 2

Studies of bacterial chromosomes and plasmids indicate that their replication initiator proteins bind to origins of replication at many double-stranded sites and also at AT-rich regions where single-stranded DNA is exposed during origin opening. Single-strand binding apparently promotes origin opening by stabilizing an open structure, but how the initiator participates in this process and the contributions of the several binding sites remain unclear. Here, we show that the initiator protein of Vibrio cholerae specific to chromosome 2 (Chr2) also has single-strand binding activity in the AT-rich region of its origin. Binding is strand specific, depends on repeats of the sequence 5'ATCA and is greatly stabilized in vitro by specific double-stranded sites of the origin. The stability derives from the formation of ternary complexes of the initiator with the single- and double-stranded sites. An IHF site lies between these two kinds of sites in the Chr2 origin and an IHF-induced looping out of the intervening DNA mediates their interaction. Simultaneous binding to two kinds of sites in the origin appears to be a common mechanism by which bacterial replication initiators stabilize an open origin.

Predictions and analyses of RNA nearest neighbor parameters for modified nucleotides

The most popular RNA secondary structure prediction programs utilize free energy (ΔG°₃₇) minimization and rely upon thermodynamic parameters from the nearest neighbor (NN) model. Experimental parameters are derived from a series of optical melting experiments; however, acquiring enough melt data to derive accurate NN parameters with modified base pairs is expensive and time consuming. Given the multitude of known natural modifications and the continuing use and development of unnatural nucleotides, experimentally characterizing all modified NNs is impractical. This dilemma necessitates a computational model that can predict NN thermodynamics where experimental data is scarce or absent. Here, we present a combined molecular dynamics/quantum mechanics protocol that accurately predicts experimental NN ΔG°₃₇ parameters for modified nucleotides with neighboring Watson–Crick base pairs. NN predictions for Watson-Crick and modified base pairs yielded an overall RMSD of 0.32 kcal/mol when compared with experimentally derived parameters. NN predictions involving modified bases without experimental parameters (N⁶-methyladenosine, 2-aminopurineriboside, and 5-methylcytidine) demonstrated promising agreement with available experimental melt data. This procedure not only yields accurate NN ΔG°₃₇ predictions but also quantifies stacking and hydrogen bonding differences between modified NNs and their canonical counterparts, allowing investigators to identify energetic differences and providing insight into sources of (de)stabilization from nucleotide modifications.

A m6 A Sensing Method by Its Impact on the Stability of RNA Double Helix

N⁶ -Methyladenosine (m⁶ A) is one of the most important RNA modifications in epigenetics. The development of detection method for m⁶ A is limited by its abundance and structure. Although it has been previously reported that its presence has an impact on the complementary pairing of RNA, few assays have been developed using this finding. We used this discovery and designed a detection method based on Cas13a system, which has different fluorescence signals for target RNAs containing m⁶ A modification and target RNAs without m⁶ A modification. We verified the fact that the presence of m⁶ A could cause the instability of dsRNA using the Cas13a system and provided a new direction and strategy for the development of m⁶ A detection methods in the future.

Epigenetic Modulation of Chromatin States and Gene Expression by G-Quadruplex Structures

G-quadruplexes are four-stranded helical nucleic acid structures formed by guanine-rich sequences. A considerable number of studies have revealed that these noncanonical structural motifs are widespread throughout the genome and transcriptome of numerous organisms, including humans. In particular, G-quadruplexes occupy strategic locations in genomic DNA and both coding and noncoding RNA molecules, being involved in many essential cellular and organismal functions. In this review, we first outline the fundamental structural features of G-quadruplexes and then focus on the concept that these DNA and RNA structures convey a distinctive layer of epigenetic information that is critical for the complex regulation, either positive or negative, of biological activities in different contexts. In this framework, we summarize and discuss the proposed mechanisms underlying the functions of G-quadruplexes and their interacting factors. Furthermore, we give special emphasis to the interplay between G-quadruplex formation/disruption and other epigenetic marks, including biochemical modifications of DNA bases and histones, nucleosome positioning, and three-dimensional organization of chromatin. Finally, epigenetic roles of RNA G-quadruplexes in post-transcriptional regulation of gene expression are also discussed. Undoubtedly, the issues addressed in this review take on particular importance in the field of comparative epigenetics, as well as in translational research.

A methyl-TROSY approach for NMR studies of high-molecular-weight DNA with application to the nucleosome core particle

The development of methyl-transverse relaxation-optimized spectroscopy (methyl-TROSY)-based NMR methods, in concert with robust strategies for incorporation of methyl-group probes of structure and dynamics into the protein of interest, has facilitated quantitative studies of high-molecular-weight protein complexes. Here we develop a one-pot in vitro reaction for producing NMR quantities of methyl-labeled DNA at the C5 and N6 positions of cytosine (5mC) and adenine (6mA) nucleobases, respectively, enabling the study of high-molecular-weight DNA molecules using TROSY approaches originally developed for protein applications. Our biosynthetic strategy exploits the large number of naturally available methyltransferases to specifically methylate DNA at a desired number of sites that serve as probes of structure and dynamics. We illustrate the methodology with studies of the 153-base pair Widom DNA molecule that is simultaneously methyl-labeled at five sites, showing that high-quality 13C-1H spectra can be recorded on 100 μM samples in a few minutes. NMR spin relaxation studies of labeled methyl groups in both DNA and the H2B histone protein component of the 200-kDa nucleosome core particle (NCP) establish that methyl groups at 5mC and 6mA positions are, in general, more rigid than Ile, Leu, and Val methyl probes in protein side chains. Studies focusing on histone H2B of NCPs wrapped with either wild-type DNA or DNA methylated at all 26 CpG sites highlight the utility of NMR in investigating the structural dynamics of the NCP and how its histone core is affected through DNA methylation, an important regulator of transcription.

A general strategy exploiting m5C duplex-remodelling effect for selective detection of RNA and DNA m5C methyltransferase activity in cells

RNA:5-methylcytosine (m⁵C) methyltransferases are currently the focus of intense research following a series of high-profile reports documenting their physiological links to several diseases. However, no methods exist which permit the specific analysis of RNA:m⁵C methyltransferases in cells. Herein, we described how a combination of biophysical studies led us to identify distinct duplex-remodelling effects of m⁵C on RNA and DNA duplexes. Specifically, m⁵C induces a C3′-endo to C2′-endo sugar-pucker switch in CpG RNA duplex but triggers a B-to-Z transformation in CpG DNA duplex. Inspired by these different ‘structural signatures’, we developed a m⁵C-sensitive probe which fluoresces spontaneously in response to m⁵C-induced sugar-pucker switch, hence useful for sensing RNA:m⁵C methyltransferase activity. Through the use of this probe, we achieved real-time imaging and flow cytometry analysis of NOP2/Sun RNA methyltransferase 2 (NSUN2) activity in HeLa cells. We further applied the probe to the cell-based screening of NSUN2 inhibitors. The developed strategy could also be adapted for the detection of DNA:m⁵C methyltransferases. This was demonstrated by the development of DNA m⁵C-probe which permits the screening of DNA methyltransferase 3A inhibitors. To our knowledge, this study represents not only the first examples of m⁵C-responsive probes, but also a new strategy for discriminating RNA and DNA m⁵C methyltransferase activity in cells.

Destabilisation of the c-kit1 G-quadruplex structure by N6-methyladenosine modification

N⁶-methyladenine (m⁶dA) has been recently discovered in eukaryotic genomic DNA. However, there have been few reports on its biological roles. G-quadruplex (G4) is a non-canonical nucleic acid structure formed by the stacking of G-tetrads. G4-forming sequences are enriched with cis-regulatory elements in genomic DNA and the G4 structures have important roles in various cellular functions. We previously reported that CpG methylation stabilized vascular endothelial growth factor (VEGF) G4 structure. Here we report that m⁶dA modification destabilizes the human c-kit1 G4 structure. These results suggest that epigenetic modifications may affect G4 formation in order to regulate the biological functions.

RNA modifications and the link to human disease

Ribonucleic acid (RNA) is involved in translation and transcription, which are the mechanisms in which cells express genes (Alberts et al., 2002). The three classes of RNA discussed are transfer RNA (tRNA), messenger RNA (mRNA), and ribosomal RNA (rRNA). mRNA is the transcript encoded from DNA, rRNA is associated with ribosomes, and tRNA is associated with amino acids and is used to read mRNA transcripts to make proteins (Lodish, Berk, Zipursky, et al., 2000). Interestingly, the function of tRNA, rRNA, and mRNA can be significantly altered by chemical modifications at the co-transcriptional and post-transcriptional levels, and there are over 171 of these modifications identified thus far (Boccaletto et al., 2018; Modomics-Modified bases, 2017). Several of these modifications are linked to diseases such as cancer, diabetes, and neurological disorders. In this review, we will introduce a few RNA modifications with biological functions and how dysregulation of these RNA modifications is linked to human disease.

Identification of a DNA N6-Adenine Methyltransferase Complex and Its Impact on Chromatin Organization

DNA N6-adenine methylation (6mA) has recently been described in diverse eukaryotes, spanning unicellular organisms to metazoa. Here, we report a DNA 6mA methyltransferase complex in ciliates, termed MTA1c. It consists of two MT-A70 proteins and two homeobox-like DNA-binding proteins and specifically methylates dsDNA. Disruption of the catalytic subunit, MTA1, in the ciliate Oxytricha leads to genome-wide loss of 6mA and abolishment of the consensus ApT dimethylated motif. Mutants fail to complete the sexual cycle, which normally coincides with peak MTA1 expression. We investigate the impact of 6mA on nucleosome occupancy in vitro by reconstructing complete, full-length Oxytricha chromosomes harboring 6mA in native or ectopic positions. We show that 6mA directly disfavors nucleosomes in vitro in a local, quantitative manner, independent of DNA sequence. Furthermore, the chromatin remodeler ACF can overcome this effect. Our study identifies a diverged DNA N6-adenine methyltransferase and defines the role of 6mA in chromatin organization.

The chemical diversity of RNA modifications

Nucleic acid modifications in DNA and RNA ubiquitously exist among all the three kingdoms of life. This trait significantly broadens the genome diversity and works as an important means of gene transcription regulation. Although mammalian systems have limited types of DNA modifications, over 150 different RNA modification types have been identified, with a wide variety of chemical diversities. Most modifications occur on transfer RNA and ribosomal RNA, however many of the modifications also occur on other types of RNA species including mammalian mRNA and small nuclear RNA, where they are essential for many biological roles, including developmental processes and stem cell differentiation. These post-transcriptional modifications are enzymatically installed and removed in a site-specific manner by writer and eraser proteins respectively, while reader proteins can interpret modifications and transduce the signal for downstream functions. Dysregulation of mRNA modifications manifests as disease states, including multiple types of human cancer. In this review, we will introduce the chemical features and biological functions of these modifications in the coding and non-coding RNA species.

A fluorescent methylation-switchable probe for highly sensitive analysis of FTO N 6-methyladenosine demethylase activity in cells

N 6-Methyladenosine (m6A) is one of the most abundant epigenetic modifications on mRNA. It is dynamically regulated by the m6A demethylases FTO and ALKBH5, which are currently attracting intense medical interest because of their strong association with several human diseases. Despite their clinical significance, the molecular mechanisms of m6A demethylases remain unclear, hence there is tremendous interest in developing analytical tools to facilitate their functional studies, with a longer term view of validating their therapeutic potentials. To date, no method exists which permits the analysis of m6A-demethylase activity in cells. To overcome this challenge, herein, we describe the first example of a fluorescent m6A-switchable oligonucleotide probe, which enables the direct detection of FTO demethylase activity both in vitro and in living cells. The m6A probe provides a simple, yet powerful visual tool for highly sensitive detection of demethylase activity. Through the use of m6A-probe, we were able to achieve real-time imaging and single-cell flow cytometry analyses of FTO activity in HepG2 cells. We also successfully applied the probe to monitor dynamic changes in FTO activity and m6A methylation levels during 3T3-L1 pre-adipocyte differentiation. The strategy outlined here is highly versatile and may, in principle, be adapted to the study of a range of RNA demethylases and, more widely, other RNA modifying enzymes. To the best of our knowledge, the present study represents not only the first assay for monitoring FTO activity in living cells, but also a new strategy for sensing m6A methylation dynamics.

Wet-dry cycles enable the parallel origin of canonical and non-canonical nucleosides by continuous synthesis

The molecules of life were created by a continuous physicochemical process on an early Earth. In this hadean environment, chemical transformations were driven by fluctuations of the naturally given physical parameters established for example by wet-dry cycles. These conditions might have allowed for the formation of (self)-replicating RNA as the fundamental biopolymer during chemical evolution. The question of how a complex multistep chemical synthesis of RNA building blocks was possible in such an environment remains unanswered. Here we report that geothermal fields could provide the right setup for establishing wet-dry cycles that allow for the synthesis of RNA nucleosides by continuous synthesis. Our model provides both the canonical and many ubiquitous non-canonical purine nucleosides in parallel by simple changes of physical parameters such as temperature, pH and concentration. The data show that modified nucleosides were potentially formed as competitor molecules. They could in this sense be considered as molecular fossils.

Epigenetic DNA Modification N6-Methyladenine Causes Site-Specific RNA Polymerase II Transcriptional Pausing

N6-Methyladenine (N6-mA or 6 mA) is an epigenetic DNA modification in eukaryotic genomes. In contrast to the well-established roles of 5-methylcytosine for epigenetic regulation of gene expression, the functional roles of N6-mA remain elusive. In particular, the impact of N6-mA modification of the DNA template on RNA polymerase II (pol II) transcription elongation is not known. In this work, using the Saccharomyces cerevisiae pol II transcriptional elongation system as a model, we investigated the molecular mechanism of pol II recognition and processing of N6-mA sites via both biochemical and structural approaches. We found that N6-mA causes site-specific pol II pausing/stalling. Structural analysis revealed that while N6-mA can reach the +1 template position, the stability of the N6-mA and UTP base pairing is compromised. Taken together, we reveal that the presence of the 6-methyl group on adenine reduces incorporation efficiency and promotes backtracking translocation. Our studies with yeast pol II provide molecular insights into understanding the impacts of N6-mA on pol II transcription dynamics in different organisms.

Chemical Modifications to RNA: A New Layer of Gene Expression Regulation

The first chemical modification to RNA was discovered nearly 60 years ago; to date, more than 100 chemically distinct modifications have been identified in cellular RNA. With the recent development of novel chemical and/or biochemical methods, dynamic modifications to RNA have been identified in the transcriptome, including N⁶-methyladenosine (m⁶A), inosine (I), 5-methylcytosine (m⁵C), pseudouridine (Ψ), 5-hydroxymethylcytosine (hm⁵C), and N¹-methyladenosine (m¹A). Collectively, the multitude of RNA modifications are termed epitranscriptome, leading to the emerging field of epitranscriptomics. In this review, we primarily focus on recently reported chemical modifications to mRNA; we discuss their chemical properties, biological functions, and mechanisms with an emphasis on their high-throughput detection methods. We also envision that future tools, particularly novel chemical biology methods, could further facilitate and enable studies in the field of epitranscriptomics.

Update: Mechanisms Underlying N6-Methyladenosine Modification of Eukaryotic mRNA

Eukaryotic mRNA undergoes chemical modification both at the 5' cap and internally. Among internal modifications, N⁶-methyladensone (m⁶A), by far the most abundant, is present in all eukaryotes examined so far, including mammals, flies, plants, and yeast. m⁶A modification has an essential role in diverse biological processes. Over the past few years, our knowledge relevant to the establishment and function of this modification has grown rapidly. In this review, we focus on technologies that have facilitated m⁶A detection in mRNAs, the identification of m⁶A methylation enzymes and binding proteins, and potential functions of the modification at the molecular level.

N(6)-Methyladenosine: a conformational marker that regulates the substrate specificity of human demethylases FTO and ALKBH5

N⁶-Methyladenosine (m6A) is currently one of the most intensively studied post-transcriptional modifications in RNA. Due to its critical role in epigenetics and physiological links to several human diseases, it is also of tremendous biological and medical interest. The m6A mark is dynamically reversed by human demethylases FTO and ALKBH5, however the mechanism by which these enzymes selectively recognise their target transcripts remains unclear. Here, we report combined biophysical and biochemical studies on the specificity determinants of m6A demethylases, which led to the identification of an m6A-mediated substrate discrimination mechanism. Our results reveal that m6A itself serves as a ‘conformational marker’, which induces different conformational outcomes in RNAs depending on sequence context. This critically impacts its interactions with several m6A-recognising proteins, including FTO and ALKBH5. Remarkably, through the RNA-remodelling effects of m6A, the demethylases were able to discriminate substrates with very similar nucleotide sequences. Our findings provide novel insights into the biological functions of m6A modifications. The mechanism identified in this work is likely of significance to other m6A-recognising proteins.

A methylation-switchable conformational probe for the sensitive and selective detection of RNA demethylase activity

We describe a novel methylation-sensitive nucleic acid (RNA) probe which switches conformation according to its methylation status. When combined with a differential scanning fluorimetry technique, it enables highly sensitive and selective detection of demethylase activity at a single methylated-base level. The approach is highly versatile and may be adapted to a broad range of RNA demethylases.

N6-Methyladenine: A Conserved and Dynamic DNA Mark

Chromatin, consisting of deoxyribonucleic acid (DNA) wrapped around histone proteins, facilitates DNA compaction and allows identical DNA codes to confer many different cellular phenotypes. This biological versatility is accomplished in large part by posttranslational modifications to histones and chemical modifications to DNA. These modifications direct the cellular machinery to expand or compact specific chromatin regions and mark regions of the DNA as important for cellular functions. While each of the four bases that make up DNA can be modified (Iyer et al. 2011), this chapter will focus on methylation of the sixth position on adenines (6mA), as this modification has been poorly characterized in recently evolved eukaryotes, but shows promise as a new conserved layer of epigenetic regulation. 6mA was previously thought to be restricted to unicellular organisms, but recent work has revealed its presence in metazoa. Here, we will briefly describe the history of 6mA, examine its evolutionary conservation, and evaluate the current methods for detecting 6mA. We will discuss the enzymes that bind and regulate this mark and finally examine known and potential functions of 6mA in eukaryotes.

N6-methyldeoxyadenosine marks active transcription start sites in Chlamydomonas

N(6)-methyldeoxyadenosine (6mA or m(6)A) is a DNA modification preserved in prokaryotes to eukaryotes. It is widespread in bacteria and functions in DNA mismatch repair, chromosome segregation, and virulence regulation. In contrast, the distribution and function of 6mA in eukaryotes have been unclear. Here, we present a comprehensive analysis of the 6mA landscape in the genome of Chlamydomonas using new sequencing approaches. We identified the 6mA modification in 84% of genes in Chlamydomonas. We found that 6mA mainly locates at ApT dinucleotides around transcription start sites (TSS) with a bimodal distribution and appears to mark active genes. A periodic pattern of 6mA deposition was also observed at base resolution, which is associated with nucleosome distribution near the TSS, suggesting a possible role in nucleosome positioning. The new genome-wide mapping of 6mA and its unique distribution in the Chlamydomonas genome suggest potential regulatory roles of 6mA in gene expression in eukaryotic organisms.

Structure and thermodynamics of N6-methyladenosine in RNA: a spring-loaded base modification

N(6)-Methyladenosine (m(6)A) modification is hypothesized to control processes such as RNA degradation, localization, and splicing. However, the molecular mechanisms by which this occurs are unclear. Here, we measured structures of an RNA duplex containing m(6)A in the GGACU consensus, along with an unmodified RNA control, by 2D NMR. The data show that m(6)A-U pairing in the double-stranded context is accompanied by the methylamino group rotating from its energetically preferred syn geometry on the Watson-Crick face to the higher-energy anti conformation, positioning the methyl group in the major groove. Thermodynamic measurements of m(6)A in duplexes reveal that it is destabilizing by 0.5-1.7 kcal/mol. In contrast, we show that m(6)A in unpaired positions base stacks considerably more strongly than the unmodified base, adding substantial stabilization in single-stranded locations. Transcriptome-wide nuclease mapping of methylated RNA secondary structure from human cells reveals a structural transition at methylated adenosines, with a tendency to single-stranded structure adjacent to the modified base.

DNA methylation impacts gene expression and ensures hypoxic survival of Mycobacterium tuberculosis

DNA methylation regulates gene expression in many organisms. In eukaryotes, DNA methylation is associated with gene repression, while it exerts both activating and repressive effects in the Proteobacteria through largely locus-specific mechanisms. Here, we identify a critical DNA methyltransferase in M. tuberculosis, which we term MamA. MamA creates N⁶-methyladenine in a six base pair recognition sequence present in approximately 2,000 copies on each strand of the genome. Loss of MamA reduces the expression of a number of genes. Each has a MamA site located at a conserved position relative to the sigma factor −10 binding site and transcriptional start site, suggesting that MamA modulates their expression through a shared, not locus-specific, mechanism. While strains lacking MamA grow normally in vitro, they are attenuated in hypoxic conditions, suggesting that methylation promotes survival in discrete host microenvironments. Interestingly, we demonstrate strikingly different patterns of DNA methyltransferase activity in different lineages of M. tuberculosis, which have been associated with preferences for distinct host environments and different disease courses in humans. Thus, MamA is the major functional adenine methyltransferase in M. tuberculosis strains of the Euro-American lineage while strains of the Beijing lineage harbor a point mutation that largely inactivates MamA but possess a second functional DNA methyltransferase. Our results indicate that MamA influences gene expression in M. tuberculosis and plays an important but strain-specific role in fitness during hypoxia.

Tuberculosis is a disease with a devastating impact on public health, killing over 1.5 million people each year around the globe. Tuberculosis is caused by the bacterium Mycobacterium tuberculosis, which over millennia has evolved the ability to survive and persist for decades in the harsh environment inside its human host. Regulation of gene expression is critical for adaptation to stressful conditions. To successfully tackle M. tuberculosis, we therefore need to understand how it regulates its genes and responds to environmental stressors. In this work, we report the first investigation of the role of DNA methylation in gene regulation and stress response in M. tuberculosis. We have found that DNA methylation is important for survival of hypoxia, a stress condition present in human infections, and furthermore that DNA methylation affects the expression of several genes. In contrast to methylation-regulation systems reported in other bacteria, in which the effects of methylation vary from one gene to the next, M. tuberculosis appears to use a concerted mechanism to influence multiple genes. Our findings identify a novel mechanism by which M. tuberculosis modulates gene expression in response to stress.

N6-methyl-adenosine (m6A) in RNA: an old modification with a novel epigenetic function

N⁶-methyl-adenosine (m⁶A) is one of the most common and abundant modifications on RNA molecules present in eukaryotes. However, the biological significance of m⁶A methylation remains largely unknown. Several independent lines of evidence suggest that the dynamic regulation of m⁶A may have a profound impact on gene expression regulation. The m⁶A modification is catalyzed by an unidentified methyltransferase complex containing at least one subunit methyltransferase like 3 (METTL3). m⁶A modification on messenger RNAs (mRNAs) mainly occurs in the exonic regions and 3′-untranslated region (3′-UTR) as revealed by high-throughput m⁶A-seq. One significant advance in m⁶A research is the recent discovery of the first two m⁶A RNA demethylases fat mass and obesity-associated (FTO) gene and ALKBH5, which catalyze m⁶A demethylation in an α-ketoglutarate (α-KG)- and Fe²⁺-dependent manner. Recent studies in model organisms demonstrate that METTL3, FTO and ALKBH5 play important roles in many biological processes, ranging from development and metabolism to fertility. Moreover, perturbation of activities of these enzymes leads to the disturbed expression of thousands of genes at the cellular level, implicating a regulatory role of m⁶A in RNA metabolism. Given the vital roles of DNA and histone methylations in epigenetic regulation of basic life processes in mammals, the dynamic and reversible chemical m⁶A modification on RNA may also serve as a novel epigenetic marker of profound biological significances.

Theoretical study on the binding mechanism between N6-methyladenine and natural DNA bases

N6-methyladenine (m(6)A) is a rare base naturally occurring in DNA. It is different from the base adenine due to its N-CH(3). Therefore, the base not only pairs with thymine, but also with other DNA bases (cytosine, adenine and guanine). In this work, Møller-Plesset second-order (MP2) method has been used to investigate the binding mechanism between m(6)A and natural DNA bases in gas phase and in aqueous solution. The results show that N-CH(3) changed the way of N6-methyladenine binding to natural DNA bases. The binding style significantly influences the stability of base pairs. The trans-m(6)A:G and trans-m(6)A:C conformers are the most stable among all the base pairs. The existence of solvent can remarkably reduce the stability of the base pairs, and the DNA bases prefer pairing with trans-m(6)A to cis-m(6)A. Besides, the properties of these hydrogen bonds have been analyzed by atom in molecules (AIM) theory, natural bond orbital (NBO) analysis and Wiberg bond indexes (WBI). In addition, pairing with m(6)A decreases the binding energies compared to the normal Watson-Crick base pairs, it may explain the instability of the N6 site methylated DNA in theory.

DNA methyltransferases: mechanistic models derived from kinetic analysis

The sequence-specific transfer of methyl groups from donor S-adenosyl-L-methionine (AdoMet) to certain positions of DNA-adenine or -cytosine residues by DNA methyltransferases (MTases) is a major form of epigenetic modification. It is virtually ubiquitous, except for some notable exceptions. Site-specific methylation can be regarded as a means to increase DNA information capacity and is involved in a large spectrum of biological processes. The importance of these functions necessitates a deeper understanding of the enzymatic mechanism(s) of DNA methylation. DNA MTases fall into one of two general classes; viz. amino-MTases and [C5-cytosine]-MTases. Amino-MTases, common in prokaryotes and lower eukaryotes, catalyze methylation of the exocyclic amino group of adenine ([N6-adenine]-MTase) or cytosine ([N4-cytosine]-MTase). In contrast, [C5-cytosine]-MTases methylate the cyclic carbon-5 atom of cytosine. Characteristics of DNA MTases are highly variable, differing in their affinity to their substrates or reaction products, their kinetic parameters, or other characteristics (order of substrate binding, rate limiting step in the overall reaction). It is not possible to present a unifying account of the published kinetic analyses of DNA methylation because different authors have used different substrate DNAs and/or reaction conditions. Nevertheless, it would be useful to describe those kinetic data and the mechanistic models that have been derived from them. Thus, this review considers in turn studies carried out with the most consistently and extensively investigated [N6-adenine]-, [N4-cytosine]- and [C5-cytosine]-DNA MTases.

Functional analysis of an acid adaptive DNA adenine methyltransferase from Helicobacter pylori 26695

HP0593 DNA-(N(6)-adenine)-methyltransferase (HP0593 MTase) is a member of a Type III restriction-modification system in Helicobacter pylori strain 26695. HP0593 MTase has been cloned, overexpressed and purified heterologously in Escherichia coli. The recognition sequence of the purified MTase was determined as 5'-GCAG-3'and the site of methylation was found to be adenine. The activity of HP0593 MTase was found to be optimal at pH 5.5. This is a unique property in context of natural adaptation of H. pylori in its acidic niche. Dot-blot assay using antibodies that react specifically with DNA containing m6A modification confirmed that HP0593 MTase is an adenine-specific MTase. HP0593 MTase occurred as both monomer and dimer in solution as determined by gel-filtration chromatography and chemical-crosslinking studies. The nonlinear dependence of methylation activity on enzyme concentration indicated that more than one molecule of enzyme was required for its activity. Analysis of initial velocity with AdoMet as a substrate showed that two molecules of AdoMet bind to HP0593 MTase, which is the first example in case of Type III MTases. Interestingly, metal ion cofactors such as Co(2+), Mn(2+), and also Mg(2+) stimulated the HP0593 MTase activity. Preincubation and isotope partitioning analyses clearly indicated that HP0593 MTase-DNA complex is catalytically competent, and suggested that DNA binds to the MTase first followed by AdoMet. HP0593 MTase shows a distributive mechanism of methylation on DNA having more than one recognition site. Considering the occurrence of GCAG sequence in the potential promoter regions of physiologically important genes in H. pylori, our results provide impetus for exploring the role of this DNA MTase in the cellular processes of H. pylori.

DNA adenine methylation is required to replicate both Vibrio cholerae chromosomes once per cell cycle

DNA adenine methylation is widely used to control many DNA transactions, including replication. In Escherichia coli, methylation serves to silence newly synthesized (hemimethylated) sister origins. SeqA, a protein that binds to hemimethylated DNA, mediates the silencing, and this is necessary to restrict replication to once per cell cycle. The methylation, however, is not essential for replication initiation per se but appeared so when the origins (oriI and oriII) of the two Vibrio cholerae chromosomes were used to drive plasmid replication in E. coli. Here we show that, as in the case of E. coli, methylation is not essential for oriI when it drives chromosomal replication and is needed for once-per-cell-cycle replication in a SeqA-dependent fashion. We found that oriII also needs SeqA for once-per-cell-cycle replication and, additionally, full methylation for efficient initiator binding. The requirement for initiator binding might suffice to make methylation an essential function in V. cholerae. The structure of oriII suggests that it originated from a plasmid, but unlike plasmids, oriII makes use of methylation for once-per-cell-cycle replication, the norm for chromosomal but not plasmid replication.

Bacteria usually have one chromosome but can have extrachromosomal replicons, called plasmids. Although normally dispensable, plasmids can confer adaptive advantage to cells in stressful environments. Bacteria can also have multiple chromosomes, each carrying essential genes, as in eukaryotes. In all organisms, chromosomes duplicate once before the cells divide so that the daughter cells can receive equal genetic dowry, but this is not usually the case with bacterial plasmids. Vibrio cholerae, the causative agent for the disease cholera, has a typical bacterial chromosome like the chromosome of the well-studied bacterium Escherichia coli and has a second chromosome with many signatures indicating its origin from a plasmid. Here we show that, in spite of the distinct nature of the two chromosomes, they both duplicate once per cell cycle, and they both require DNA adenine methylation for this purpose. Our study suggests that once-per-cell-cycle replication is a necessary feature of a chromosome in multichromosome bacteria, and provides a paradigm of how methylation could endow extrachromosomal replicons with the capacity to duplicate like chromosomes.

Multifactor dimensionality reduction analysis identifies specific nucleotide patterns promoting genetic polymorphisms

Background

The fidelity of DNA replication serves as the nidus for both genetic evolution and genomic instability fostering disease. Single nucleotide polymorphisms (SNPs) constitute greater than 80% of the genetic variation between individuals. A new theory regarding DNA replication fidelity has emerged in which selectivity is governed by base-pair geometry through interactions between the selected nucleotide, the complementary strand, and the polymerase active site. We hypothesize that specific nucleotide combinations in the flanking regions of SNP fragments are associated with mutation.

Results

We modeled the relationship between DNA sequence and observed polymorphisms using the novel multifactor dimensionality reduction (MDR) approach. MDR was originally developed to detect synergistic interactions between multiple SNPs that are predictive of disease susceptibility. We initially assembled data from the Broad Institute as a pilot test for the hypothesis that flanking region patterns associate with mutagenesis (n = 2194). We then confirmed and expanded our inquiry with human SNPs within coding regions and their flanking sequences collected from the National Center for Biotechnology Information (NCBI) database (n = 29967) and a control set of sequences (coding region) not associated with SNP sites randomly selected from the NCBI database (n = 29967). We discovered seven flanking region pattern associations in the Broad dataset which reached a minimum significance level of p ≤ 0.05. Significant models (p << 0.001) were detected for each SNP type examined in the larger NCBI dataset. Importantly, the flanking region models were elongated or truncated depending on the nucleotide change. Additionally, nucleotide distributions differed significantly at motif sites relative to the type of variation observed. The MDR approach effectively discerned specific sites within the flanking regions of observed SNPs and their respective identities, supporting the collective contribution of these sites to SNP genesis.

Conclusion

The present study represents the first use of this computational methodology for modeling nonlinear patterns in molecular genetics. MDR was able to identify distinct nucleotide patterning around sites of mutations dependent upon the observed nucleotide change. We discovered one flanking region set that included five nucleotides clustered around a specific type of SNP site. Based on the strongly associated patterns identified in this study, it may become possible to scan genomic databases for such clustering of nucleotides in order to predict likely sites of future SNPs, and even the type of polymorphism most likely to occur.