Functional mapping of the EcoRV DNA methyltransferase by random mutagenesis and screening for catalytically inactive mutants

Conversion of the CG specific M.MpeI DNA methyltransferase into an enzyme predominantly methylating CCA and CCC sites

We used structure guided mutagenesis and directed enzyme evolution to alter the specificity of the CG specific bacterial DNA (cytosine-5) methyltransferase M.MpeI. Methylation specificity of the M.MpeI variants was characterized by digestions with methylation sensitive restriction enzymes and by measuring incorporation of tritiated methyl groups into double-stranded oligonucleotides containing single CC, CG, CA or CT sites. Site specific mutagenesis steps designed to disrupt the specific contacts between the enzyme and the non-substrate base pair of the target sequence (5'-CG/5'-CG) yielded M.MpeI variants with varying levels of CG specific and increasing levels of CA and CC specific MTase activity. Subsequent random mutagenesis of the target recognizing domain coupled with selection for non-CG specific methylation yielded a variant, which predominantly methylates CC dinucleotides, has very low activity on CG and CA sites, and no activity on CT sites. This M.MpeI variant contains a one amino acid deletion (ΔA323) and three substitutions (N324G, R326G and E305N) in the target recognition domain. The mutant enzyme has very strong preference for A and C in the 3' flanking position making it a CCA and CCC specific DNA methyltransferase.

N6-Adenosine DNA Methyltransferase from H. pylori 98-10 Strain in Complex with DNA and AdoMet: Structural Insights from in Silico Studies

Helicobacter pylori is a primitive Gram-negative bacterium that resides in the acidic environment of the human gastrointestinal tract, and some strains of this bacterium cause gastric ulcers and cancer. DNA methyltransferases (MTases) are promising drug targets for the treatment of cancer and other diseases that are also caused by epigenetic alternations of the genome. The N6-adenine-specific DNA MTase from H. pylori (M. Hpy N6mA) catalyzes the transfer of a methyl group from the cofactor S-adenosyl-l-methionine (AdoMet) to the flipped adenine of the substrate DNA. In this work, we report the sequence analyses, three-dimensional structure modeling, and molecular dynamics simulations of M. Hpy N6mA, when complexed with AdoMet as well as DNA. We analyzed the protein-DNA interactions prominently established by the flipped cytosine and the interactions between protein cofactors in the active site. The comparable orientation of AdoMet in both systems confirms that AdoMet is in a catalytically competent orientation in the bimolecular system that is retained upon DNA binding in the termolecular system of M. Hpy N6mA. In both systems, AdoMet is stabilized in the binding pocket by hydrogen bonding (Thr84, Glu99, Asp122, and Phe123) as well as van der Waals (Ile100, Phe160, Arg104, and Cys76) interactions. We propose that the contacts made by flipped adenine DA6 with Asn138 (N6 and N1 atom of DA6) and Pro139 (N6) and π-stacking interactions with Phe141 and Phe219 play an important role in the methylation mechanism at the N6 position in our N6mA model. Specific recognition of DNA is mediated by residues 143-155, 183-189, 212-220, 280-293, and 308-325. These findings are further supported by alanine scanning mutagenesis studies. The conserved residues in distantly related sequences of the small domain are important in DNA binding. Results reported here elucidate the sequence, structure, and binding features necessary for the recognition between cofactor AdoMet and substrate DNA by the vital epigenetic enzyme, M. Hpy N6mA.

Engineering and Directed Evolution of DNA Methyltransferases

DNA methyltransferases (MTases) constitute an attractive target for protein engineering, thus opening the road to new ways of manipulating DNA in a unique and selective manner. Here, we review various aspects of MTase engineering, both methodological and conceptual, and also discuss future directions and challenges. Bacterial MTases that are part of restriction/modification (R/M) systems offer a convenient way for the selection of large gene libraries, both in vivo and in vitro. We review these selection methods, their strengths and weaknesses, and also the prospects for new selection approaches that will enable the directed evolution of mammalian DNA methyltransferases (Dnmts). We explore various properties of MTases that may be subject to engineering. These include engineering for higher stability and soluble expression (MTases, including bacterial ones, are prone to misfolding), engineering of the DNA target specificity, and engineering for the usage of S-adenosyl-L-methionine (AdoMet) analogs. Directed evolution of bacterial MTases also offers insights into how these enzymes readily evolve in nature, thus yielding MTases with a huge spectrum of DNA target specificities. Engineering for alternative cofactors, on the other hand, enables modification of DNA with various groups other than methyl and thus can be employed to map and redirect DNA epigenetic modifications.

Detection and identification of novel metabolomic biomarkers in preeclampsia

In a previous study, the ability of gas chromatography time-of-flight mass spectrometry to detect potential metabolic biomarkers in preeclampsia was demonstrated. In this study, the authors sought to validate their preliminary findings in an entirely different patient cohort using a complementary, novel, and powerful combination of analytical tools (ultra performance liquid chromatography and LTQ Orbitrap mass spectrometry system). Eight metabolites that appeared in the authors' previous patient cohort were identified as being statistically significant (P < .01) as discriminatory biomarkers. The chemical identities of these 8 metabolites were established using authentic chemical standards. They included uric acid, 2-oxoglutarate, glutamate, and alanine. This is the first study to identify, in an unbiased manner, a series of small-molecular-weight metabolites that effectively detect preeclampsia in plasma. The identity of these metabolites provides new insights into the pathology of this condition and raises the possibility of the development of a predictive test.

Structure, function and mechanism of exocyclic DNA methyltransferases

DNA MTases (methyltransferases) catalyse the transfer of methyl groups to DNA from AdoMet (S-adenosyl-L-methionine) producing AdoHcy (S-adenosyl-L-homocysteine) and methylated DNA. The C5 and N4 positions of cytosine and N6 position of adenine are the target sites for methylation. All three methylation patterns are found in prokaryotes, whereas cytosine at the C5 position is the only methylation reaction that is known to occur in eukaryotes. In general, MTases are two-domain proteins comprising one large and one small domain with the DNA-binding cleft located at the domain interface. The striking feature of all the structurally characterized DNA MTases is that they share a common core structure referred to as an 'AdoMet-dependent MTase fold'. DNA methylation has been reported to be essential for bacterial virulence, and it has been suggested that DNA adenine MTases (Dams) could be potential targets for both vaccines and antimicrobials. Drugs that block Dam could slow down bacterial growth and therefore drug-design initiatives could result in a whole new generation of antibiotics. The transfer of larger chemical entities in a MTase-catalysed reaction has been reported and this represents an interesting challenge for bio-organic chemists. In general, amino MTases could therefore be used as delivery systems for fluorescent or other reporter groups on to DNA. This is one of the potential applications of DNA MTases towards developing non-radioactive DNA probes and these could have interesting applications in molecular biology. Being nucleotide-sequence-specific, DNA MTases provide excellent model systems for studies on protein-DNA interactions. The focus of this review is on the chemistry, enzymology and structural aspects of exocyclic amino MTases.

Beyond Watson and Crick: DNA methylation and molecular enzymology of DNA methyltransferases

DNA methyltransferases catalyze the transfer of a methyl group from S-adenosyl-L-methionine to cytosine or adenine bases in DNA. These enzymes challenge the Watson/Crick dogma in two instances: 1) They attach inheritable information to the DNA that is not encoded in the nucleotide sequence. This so-called epigenetic information has many important biological functions. In prokaryotes, DNA methylation is used to coordinate DNA replication and the cell cycle, to direct postreplicative mismatch repair, and to distinguish self and nonself DNA. In eukaryotes, DNA methylation contributes to the control of gene expression, the protection of the genome against selfish DNA, maintenance of genome integrity, parental imprinting, X-chromosome inactivation in mammals, and regulation of development. 2) The enzymatic mechanism of DNA methyltransferases is unusual, because these enzymes flip their target base out of the DNA helix and, thereby, locally disrupt the B-DNA helix. This review describes the biological functions of DNA methylation in bacteria, fungi, plants, and mammals. In addition, the structures and mechanisms of the DNA methyltransferases, which enable them to specifically recognize their DNA targets and to induce such large conformational changes of the DNA, are discussed.

Beyond Watson and Crick: DNA methylation and molecular enzymology of DNA methyltransferases

DNA methyltransferases catalyze the transfer of a methyl group from S-adenosyl-L-methionine to cytosine or adenine bases in DNA. These enzymes challenge the Watson/Crick dogma in two instances: 1) They attach inheritable information to the DNA that is not encoded in the nucleotide sequence. This so-called epigenetic information has many important biological functions. In prokaryotes, DNA methylation is used to coordinate DNA replication and the cell cycle, to direct postreplicative mismatch repair, and to distinguish self and nonself DNA. In eukaryotes, DNA methylation contributes to the control of gene expression, the protection of the genome against selfish DNA, maintenance of genome integrity, parental imprinting, X-chromosome inactivation in mammals, and regulation of development. 2) The enzymatic mechanism of DNA methyltransferases is unusual, because these enzymes flip their target base out of the DNA helix and, thereby, locally disrupt the B-DNA helix. This review describes the biological functions of DNA methylation in bacteria, fungi, plants, and mammals. In addition, the structures and mechanisms of the DNA methyltransferases, which enable them to specifically recognize their DNA targets and to induce such large conformational changes of the DNA, are discussed.

Bacteriophage T4 Dam DNA-[N6-adenine]methyltransferase. Kinetic evidence for a catalytically essential conformational change in the ternary complex

We carried out a steady state kinetic analysis of the bacteriophage T4 DNA-[N6-adenine]methyltransferase (T4 Dam) mediated methyl group transfer reaction from S-adenosyl-l-methionine (AdoMet) to Ade in the palindromic recognition sequence, GATC, of a 20-mer oligonucleotide duplex. Product inhibition patterns were consistent with a steady state-ordered bi-bi mechanism in which the order of substrate binding and product (methylated DNA, DNA(Me) and S-adenosyl-l-homocysteine, AdoHcy) release was AdoMet downward arrow DNA downward arrow DNA(Me) upward arrow AdoHcy upward arrow. A strong reduction in the rate of methylation was observed at high concentrations of the substrate 20-mer DNA duplex. In contrast, increasing substrate AdoMet concentration led to stimulation in the reaction rate with no evidence of saturation. We propose the following model. Free T4 Dam (initially in conformational form E) randomly interacts with substrates AdoMet and DNA to form a ternary T4 Dam-AdoMet-DNA complex in which T4 Dam has isomerized to conformational state F, which is specifically adapted for catalysis. After the chemical step of methyl group transfer from AdoMet to DNA, product DNA(Me) dissociates relatively rapidly (k(off) = 1.7 x s(-1)) from the complex. In contrast, dissociation of product AdoHcy proceeds relatively slowly (k(off) = 0.018 x s(-1)), indicating that its release is the rate-limiting step, consistent with kcat = 0.015 x s(-1). After AdoHcy release, the enzyme remains in the F conformational form and is able to preferentially bind AdoMet (unlike form E, which randomly binds AdoMet and DNA), and the AdoMet-F binary complex then binds DNA to start another methylation cycle. We also propose an alternative pathway in which the release of AdoHcy is coordinated with the binding of AdoMet in a single concerted event, while T4 Dam remains in the isomerized form F. The resulting AdoMet-F binary complex then binds DNA, and another methylation reaction ensues. This route is preferred at high AdoMet concentrations.

A mathematical model for experimental gene evolution

The purpose of this paper is to determine the optimal mutation rate for random mutagenesis procedures used to make mutant libraries for subsequent screening. When the mutation rate is low, the probability of achieving a rare beneficial mutation is low. When the mutation rate is high, the probability of producing lethal mutations which result in loss of function is also high. We demonstrate that between these two extremes, an optimal mutation rate exists for experimental gene improvement. This rate depends strongly on the number of simultaneous mutations required for a beneficial change of the gene, but only weakly on the number of possible lethal mutations. This model predicts that when mutagenesis is performed at the optimum mutation rate, at least 63% (1--e(-1)) of the cloned genes in a mutant library will be non-functional.

Molecular enzymology of the EcoRV DNA-(Adenine-N (6))-methyltransferase: kinetics of DNA binding and bending, kinetic mechanism and linear diffusion of the enzyme on DNA

The EcoRV DNA-(adenine-N(6))-methyltransferase recognizes GATATC sequences and modifies the first adenine residue within this site. We show here, that the enzyme binds to the DNA and the cofactor S-adenosylmethionine (AdoMet) in an ordered bi-bi fashion, with AdoMet being bound first. M.EcoRV binds DNA in a non-specific manner and the enzyme searches for its recognition site by linear diffusion with a range of approximately 1800 bp. During linear diffusion the enzyme continuously scans the DNA for the presence of recognition sites. Upon specific M.EcoRV-DNA complex formation a strong increase in the fluorescence of an oligonucleotide containing a 2-aminopurine base analogue at the GAT-2AP-TC position is observed which, most likely, is correlated with DNA bending. In contrast to the GAT-2AP-TC substrate, a G-2AP-TATC substrate in which the target base is replaced by 2-aminopurine does not show an increase in fluorescence upon M.EcoRV binding, demonstrating that 2-aminopurine is not a general tool to detect base flipping. Stopped-flow experiments show that DNA bending is a fast process with rate constants >10 s(-1). In the presence of cofactor, the specific complex adopts a second conformation, in which the target sequence is more tightly contacted by the enzyme. M.EcoRV exists in an open and in a closed state that are in slow equilibrium. Closing the open state is a slow process (rate constant approximately 0.7 min(-1)) that limits the rate of DNA methylation under single turnover conditions. Product release requires opening of the closed complex which is very slow (rate constant approximately 0.05-0.1 min(-1)) and limits the rate of DNA methylation under multiple turnover conditions. M.EcoRV methylates DNA sequences containing more than one recognition sites in a distributive manner. Since the dissociation rate from non-specific DNA does not depend on the length of the DNA fragment, DNA dissociation does not preferentially occur at the ends of the DNA.

Biotin-avidin microplate assay for the quantitative analysis of enzymatic methylation of DNA by DNA methyltransferases

An assay is described to measure methylation of biotinylated oligonucleotide substrates by DNA methyltransferases using [methyl-3H]-AdoMet. After the methylation reaction the oligonucleotides are immobilized on an avidin-coated microplate. The incorporation of [3H] into the DNA is quenched by addition of unlabeled AdoMet to the binding buffer. Unreacted AdoMet and enzyme are removed by washing. To release the radioactivity incorporated into the DNA, the wells are incubated with a non-specific endonuclease and the radioactivity determined by liquid scintillation counting. As an example, we have studied methylation of DNA by the EcoRV DNA methyltransferase. The reaction progress curves measured with this assay are linear with respect to time. Methylation rates linearly increase with enzyme concentration. The rates are comparable to results obtained with the same enzyme using a different assay. The biotin-avidin assay is inexpensive, convenient, quantitative, fast and well suited to process many samples in parallel. The accuracy of the assay is high, allowing to reproduce results within +/- 10%. The assay is very sensitive as demonstrated by the detection of incorporation of 0.8 fmol methyl groups into the DNA. Under the experimental conditions, this corresponds to methylation of only 0.03% of all target sites of the substrate. Using this assay, the DNA methylation activity of some M.EcoRV variants could be detected that was not visible by other in vitro methylation assays.

Mutational analysis of target base flipping by the EcoRV adenine-N6 DNA methyltransferase

DNA methyltransferases flip their target base out of the DNA helix. Here, we have investigated base flipping by wild-type EcoRV DNA methyltransferase (M.EcoRV) and five M.EcoRV variants (D193A, Y196A, S229A, W231R and Y258A). These variants bind to DNA and S-adenosylmethionine but have a severely reduced catalytic efficiency or are catalytically inactive. To measure base flipping three different assays were used, viz. analysis of the yields of photocrosslinking reactions between the enzymes and a substrate in which the target base is replaced by 5-iodouracil, analysis of the binding constants to substrates containing a mismatch base-pair at the target position and analysis of the salt dependence of specific complex formation. Our data show that the Y196A, W231R and Y258A variants are not able to stabilize a flipped target base, suggesting that the aromatic amino acid residues (Tyr196, Trp231 and Tyr258) are involved in hydrophobic interactions with the flipped base. The D193A variant behaves like wild-type M.EcoRV with respect to base flipping. The fact that this variant is catalytically inactive indicates that Asp193 has a function in chemical catalysis. The S229A variant can better flip modified bases but does not tightly lock the flipped base into the adenine-binding pocket, suggesting that Ser229 could form a contact to the flipped adenine.

Functional roles of conserved amino acid residues in DNA methyltransferases investigated by site-directed mutagenesis of the EcoRV adenine-N6-methyltransferase

All DNA methyltransferases (MTases) have similar catalytic domains containing nine blocks of conserved amino acid residues. We have investigated by site-directed mutagenesis the function of 17 conserved residues in the EcoRV alpha-adenine-N6-DNA methyltransferase. The structure of this class of MTases has been predicted recently. The variants were characterized with respect to their catalytic activities and their abilities to bind to DNA and the S-adenosylmethionine (AdoMet) cofactor. Amino acids located in motifs X, I, and II are shown to be involved in AdoMet binding (Lys16, Glu37, Phe39, and Asp58). Some of the mutants defective in AdoMet binding are also impaired in DNA binding, suggesting allosteric interactions between the AdoMet and DNA binding site. Asp78 (motif III), which was supposed to form a hydrogen bond to the AdoMet on the basis of the structure predictions, turned out not to be important for AdoMet binding, suggesting that motif III has not been identified correctly. R128A and N130A, having mutations in the putative DNA binding domain, are unable to bind to DNA. Residues located in motifs IV, V, VI, and VIII are involved in catalysis (Asp193, Tyr196, Asp211, Ser229, Trp231, and Tyr258), some of them presumably in binding the flipped target base, because mutations at these residues fail to significantly interfere with DNA and AdoMet binding but strongly reduce catalysis. Our results are in substantial agreement with the structure prediction for EcoRV alpha-adenine-N6-methyltransferase and x-ray structures of other MTases.