Two intertwined methylation activities of the MmeI restriction-modification class-IIS system from Methylophilus methylotrophus

Structure of Type IIL Restriction-Modification Enzyme MmeI in Complex with DNA Has Implications for Engineering New Specificities

The creation of restriction enzymes with programmable DNA-binding and -cleavage specificities has long been a goal of modern biology. The recently discovered Type IIL MmeI family of restriction-and-modification (RM) enzymes that possess a shared target recognition domain provides a framework for engineering such new specificities. However, a lack of structural information on Type IIL enzymes has limited the repertoire that can be rationally engineered. We report here a crystal structure of MmeI in complex with its DNA substrate and an S-adenosylmethionine analog (Sinefungin). The structure uncovers for the first time the interactions that underlie MmeI-DNA recognition and methylation (5’-TCCRAC-3’; R = purine) and provides a molecular basis for changing specificity at four of the six base pairs of the recognition sequence (5’-TCCRAC-3’). Surprisingly, the enzyme is resilient to specificity changes at the first position of the recognition sequence (5’-TCCRAC-3’). Collectively, the structure provides a basis for engineering further derivatives of MmeI and delineates which base pairs of the recognition sequence are more amenable to alterations than others.

The structure of the bifunctional Type IIL restriction-and-modification enzyme MmeI provides a basis for understanding how such enzymes recognize their substrates and a framework for manipulating their specificities.

Type II restriction endonucleases (REases) are the bedrock of modern biotechnology. Type II REases were essential for the recombinant DNA revolution and the development of gene technology. However, despite the discovery of more than 4,000 REases, the DNA recognition specificities are limited to only ~365. The recently discovered Type IIL MmeI family of restriction-and-modification (RM) enzymes provides a framework for understanding and engineering new specificities. We report here a crystal structure of MmeI in complex with its DNA substrate and an S-adenosylmethionine analog (Sinefungin). The structure uncovers for the first time the interactions that underlie MmeI-DNA recognition and methylation. The results establish a platform for rationally engineering further derivatives from MmeI and its homologs that will possess new, intentionally chosen, specificities.

EXPRSS: an Illumina based high-throughput expression-profiling method to reveal transcriptional dynamics

Background

Next Generation Sequencing technologies have facilitated differential gene expression analysis through RNA-seq and Tag-seq methods. RNA-seq has biases associated with transcript lengths, lacks uniform coverage of regions in mRNA and requires 10–20 times more reads than a typical Tag-seq. Most existing Tag-seq methods either have biases or not high throughput due to use of restriction enzymes or enzymatic manipulation of 5’ ends of mRNA or use of RNA ligations.

Results

We have developed EXpression Profiling through Randomly Sheared cDNA tag Sequencing (EXPRSS) that employs acoustic waves to randomly shear cDNA and generate sequence tags at a relatively defined position (~150-200 bp) from the 3′ end of each mRNA. Implementation of the method was verified through comparative analysis of expression data generated from EXPRSS, NlaIII-DGE and Affymetrix microarray and through qPCR quantification of selected genes. EXPRSS is a strand specific and restriction enzyme independent tag sequencing method that does not require cDNA length-based data transformations. EXPRSS is highly reproducible, is high-throughput and it also reveals alternative polyadenylation and polyadenylated antisense transcripts. It is cost-effective using barcoded multiplexing, avoids the biases of existing SAGE and derivative methods and can reveal polyadenylation position from paired-end sequencing.

Conclusions

EXPRSS Tag-seq provides sensitive and reliable gene expression data and enables high-throughput expression profiling with relatively simple downstream analysis.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-341) contains supplementary material, which is available to authorized users.

Crystallization and preliminary crystallographic analysis of the type IIL restriction enzyme MmeI in complex with DNA

Type IIL restriction enzymes have rejuvenated the search for user-specified DNA binding and cutting. By aligning and contrasting the highly comparable amino-acid sequences yet diverse recognition specificities across the family of enzymes, amino acids involved in DNA binding have been identified and mutated to produce alternative binding specificities. To date, the specificity of MmeI (a type IIL restriction enzyme) has successfully been altered at positions 3, 4 and 6 of the asymmetric TCCRAC (where R is a purine) DNA-recognition sequence. To further understand the structural basis of MmeI DNA-binding specificity, the enzyme has been crystallized in complex with its DNA substrate. The crystal belonged to space group P1, with unit-cell parameters a = 61.73, b = 94.96, c = 161.24 Å, α = 72.79, β = 89.12, γ = 71.68°, and diffracted to 2.6 Å resolution when exposed to synchrotron radiation. The structure promises to reveal the basis of MmeI DNA-binding specificity and will complement efforts to create enzymes with novel specificities.

Kinetics of Methylation by EcoP1I DNA Methyltransferase

EcoP1I DNA MTase (M.EcoP1I), an N⁶-adenine MTase from bacteriophage P1, is a part of the EcoP1I restriction-modification (R-M) system which belongs to the Type III R-M system. It recognizes the sequence 5′-AGACC-3′ and methylates the internal adenine. M.EcoP1I requires Mg²⁺ for the transfer of methyl groups to DNA. M.EcoP1I is shown to exist as dimer in solution, and even at high salt concentrations (0.5 M) the dimeric M.EcoP1I does not dissociate into monomers suggesting a strong interaction between the monomer subunits. Preincubation and isotope partitioning studies with M.EcoP1I indicate a kinetic mechanism where the duplex DNA binds first followed by AdoMet. Interestingly, M.EcoP1I methylates DNA substrates in the presence of Mn²⁺ and Ca²⁺ other than Mg²⁺ with varying affinities. Amino acid analysis and methylation assays in the presence of metal ions suggest that M.EcoP1I has indeed two metal ion-binding sites [³⁵⁸ID(x)_n … ExK⁴⁰¹ and ⁶⁰⁰DxDxD⁶⁰⁴ motif]. EcoP1I DNA MTase catalyzes the transfer of methyl groups using a distributive mode of methylation on DNA containing more than one recognition site. A chemical modification of EcoP1I DNA MTase using N-ethylmaleimide resulted in an irreversible inactivation of enzyme activity suggesting the possible role of cysteine residues in catalysis.

Diversity of DNA methyltransferases that recognize asymmetric target sequences

DNA methyltransferases (MTases) are a group of enzymes that catalyze the methyl group transfer from S-adenosyl-L-methionine in a sequence-specific manner. Orthodox Type II DNA MTases usually recognize palindromic DNA sequences and add a methyl group to the target base (either adenine or cytosine) on both strands. However, there are a number of MTases that recognize asymmetric target sequences and differ in their subunit organization. In a bacterial cell, after each round of replication, the substrate for any MTase is hemimethylated DNA, and it therefore needs only a single methylation event to restore the fully methylated state. This is in consistent with the fact that most of the DNA MTases studied exist as monomers in solution. Multiple lines of evidence suggest that some DNA MTases function as dimers. Further, functional analysis of many restriction-modification systems showed the presence of more than one or fused MTase genes. It was proposed that presence of two MTases responsible for the recognition and methylation of asymmetric sequences would protect the nascent strands generated during DNA replication from cognate restriction endonuclease. In this review, MTases recognizing asymmetric sequences have been grouped into different subgroups based on their unique properties. Detailed characterization of these unusual MTases would help in better understanding of their specific biological roles and mechanisms of action. The rapid progress made by the genome sequencing of bacteria and archaea may accelerate the identification and study of species- and strain-specific MTases of host-adapted bacteria and their roles in pathogenic mechanisms.

Rational engineering of type II restriction endonuclease DNA binding and cleavage specificity

The type II restriction endonucleases are indispensible tools for molecular biology. Although enzymes recognizing nearly 300 unique sequences are known, the ability to engineer enzymes to recognize any sequence of choice would be valuable. However, previous attempts to engineer new recognition specificity have met limited success. Here we report the rational engineering of multiple new type II specificities. We recently identified a family of MmeI-like type II endonucleases that have highly similar protein sequences but different recognition specificity. We identified the amino-acid positions within these enzymes that determine position specific DNA base recognition at three positions within their recognition sequences through correlations between their aligned amino-acid residues and aligned recognition sequences. We then altered the amino acids at the identified positions to those correlated with recognition of a desired new base to create enzymes that recognize and cut at predictable new DNA sequences. The enzymes so altered have similar levels of endonuclease activity compared to the wild-type enzymes. Using simple and predictable mutagenesis in this family it is now possible to create hundreds of unique new type II restriction endonuclease specificities. The findings suggest a simple mechanism for the evolution of new DNA specificity in Nature.

Functional analysis of MmeI from methanol utilizer Methylophilus methylotrophus, a subtype IIC restriction-modification enzyme related to type I enzymes

MmeI from Methylophilus methylotrophus belongs to the type II restriction-modification enzymes. It recognizes an asymmetric DNA sequence, 5'-TCCRAC-3' (R indicates G or A), and cuts both strands at fixed positions downstream of the specific site. This particular feature has been exploited in transcript profiling of complex genomes (using serial analysis of gene expression technology). We have shown previously that the endonucleolytic activity of MmeI is strongly dependent on the presence of S-adenosyl-l-methionine (J. Nakonieczna, J. W. Zmijewski, B. Banecki, and A. J. Podhajska, Mol. Biotechnol. 37:127-135, 2007), which puts MmeI in subtype IIG. The same cofactor is used by MmeI as a methyl group donor for modification of an adenine in the upper strand of the recognition site to N(6)-methyladenine. Both enzymatic activities reside in a single polypeptide (919 amino acids [aa]), which puts MmeI also in subtype IIC of the restriction-modification systems. Based on a molecular model, generated with the use of bioinformatic tools and validated by site-directed mutagenesis, we were able to localize three functional domains in the structure of the MmeI enzyme: (i) the N-terminal portion containing the endonucleolytic domain with the catalytic Mg2+-binding motif D(70)-X(9)-EXK(82), characteristic for the PD-(D/E)XK superfamily of nucleases; (ii) a central portion (aa 310 to 610) containing nine sequence motifs conserved among N(6)-adenine gamma-class DNA methyltransferases; (iii) the C-terminal portion (aa 610 to 919) containing a putative target recognition domain. Interestingly, all three domains showed highest similarity to the corresponding elements of type I enzymes rather than to classical type II enzymes. We have found that MmeI variants deficient in restriction activity (D70A, E80A, and K82A) can bind and methylate specific nucleotide sequence. This suggests that domains of MmeI responsible for DNA restriction and modification can act independently. Moreover, we have shown that a single amino acid residue substitution within the putative target recognition domain (S807A) resulted in a MmeI variant with a higher endonucleolytic activity than the wild-type enzyme.

MmeI: a minimal Type II restriction-modification system that only modifies one DNA strand for host protection

MmeI is an unusual Type II restriction enzyme that is useful for generating long sequence tags. We have cloned the MmeI restriction-modification (R-M) system and found it to consist of a single protein having both endonuclease and DNA methyltransferase activities. The protein comprises an amino-terminal endonuclease domain, a central DNA methyltransferase domain and C-terminal DNA recognition domain. The endonuclease cuts the two DNA strands at one site simultaneously, with enzyme bound at two sites interacting to accomplish scission. Cleavage occurs more rapidly than methyl transfer on unmodified DNA. MmeI modifies only the adenine in the top strand, 5′-TCCRAC-3′. MmeI endonuclease activity is blocked by this top strand adenine methylation and is unaffected by methylation of the adenine in the complementary strand, 5′-GTYGGA-3′. There is no additional DNA modification associated with the MmeI R-M system, as is required for previously characterized Type IIG R-M systems. The MmeI R-M system thus uses modification on only one of the two DNA strands for host protection. The MmeI architecture represents a minimal approach to assembling a restriction-modification system wherein a single DNA recognition domain targets both the endonuclease and DNA methyltransferase activities.

Binding of MmeI restriction-modification enzyme to its specific recognition sequence is stimulated by S-adenosyl-L-methionine

Restriction endonucleases serve as a very good model for studying specific protein-DNA interaction. MmeI is a very interesting restriction endonuclease, but although it is useful in Serial Analysis of Gene Expression, still very little is known about the mechanism of its interaction with DNA. MmeI is a unique enzyme as besides cleaving DNA it also methylates specific sequence. For endonucleolytic activity MmeI requires Mg(II) and S-adenosyl-l-methionine (AdoMet). AdoMet is a methyl donor in the methylation reaction, but its requirement for DNA cleavage remains unclear. In the present article we investigated MmeI interaction with DNA with the use of numerous methods. Our electrophoretic mobility shift assay revealed formation of two types of specific protein-DNA complexes. We speculate that faster migrating complex consists of one protein molecule and one DNA fragment whereas, slower migrating complex, which appears in the presence of AdoMet, may be a dimer or multimer form of MmeI interacting with specific DNA. Additionally, using spectrophotometric measurements we showed that in the presence of AdoMet, MmeI protein undergoes conformational changes. We think that such change in the enzyme structure, upon addition of AdoMet, may enhance its specific binding to DNA. In the absence of AdoMet MmeI binds DNA to the much lower extent.

Restriction endonucleases that bridge and excise two recognition sites from DNA

Most restriction endonucleases bridge two target sites before cleaving DNA: examples include all of the translocating Type I and Type III systems, and many Type II nucleases acting at their sites. A subset of Type II enzymes, the IIB systems, recognise bipartite sequences, like Type I sites, but cut specified phosphodiester bonds near their sites, like Type IIS enzymes. However, they make two double-strand breaks, one either side of the site, to release the recognition sequence on a short DNA fragment; 34 bp long in the case of the archetype, BcgI. It has been suggested that BcgI needs to interact with two recognition sites to cleave DNA but whether this is a general requirement for Type IIB enzymes had yet to be established. Ten Type IIB nucleases were tested against DNA substrates with one or two copies of the requisite sequences. With one exception, they all bridged two sites before cutting the DNA, usually in concerted reactions at both sites. The sites were ideally positioned in cis rather than in trans and were bridged through 3-D space, like Type II enzymes, rather than along the 1-D contour of the DNA, as seen with Type I enzymes. The standard mode of action for the restriction enzymes that excise their recognition sites from DNA thus involves concurrent action at two DNA sites.

S-adenosyl methionine prevents promiscuous DNA cleavage by the EcoP1I type III restriction enzyme

DNA cleavage by the type III restriction endonuclease EcoP1I was analysed on circular and catenane DNA in a variety of buffers with different salts. In the presence of the cofactor S-adenosyl methionine (AdoMet), and irrespective of buffer, only substrates with two EcoP1I sites in inverted repeat were susceptible to cleavage. Maximal activity was achieved at a Res2Mod2 to site ratio of approximately 1:1 yet resulted in cleavage at only one of the two sites. In contrast, the outcome of reactions in the absence of AdoMet was dependent upon the identity of the monovalent buffer components, in particular the identity of the cation. With Na+, cleavage was observed only on substrates with two sites in inverted repeat at elevated enzyme to site ratios (>15:1). However, with K+ every substrate tested was susceptible to cleavage above an enzyme to site ratio of approximately 3:1, including a DNA molecule with two directly repeated sites and even a DNA molecule with a single site. Above an enzyme to site ratio of 2:1, substrates with two sites in inverted repeat were cleaved at both cognate sites. The rates of cleavage suggested two separate events: a fast primary reaction for the first cleavage of a pair of inverted sites; and an order-of-magnitude slower secondary reaction for the second cleavage of the pair or for the first cleavage of all other site combinations. EcoP1I enzymes mutated in either the ATPase or nuclease motifs did not produce the secondary cleavage reactions. Thus, AdoMet appears to play a dual role in type III endonuclease reactions: Firstly, as an allosteric activator, promoting DNA association; and secondly, as a "specificity factor", ensuring that cleavage occurs only when two endonucleases bind two recognition sites in a designated orientation. However, given the right conditions, AdoMet is not strictly required for DNA cleavage by a type III enzyme.

Identification and mutational analysis of Mg2+ binding site in EcoP15I DNA methyltransferase: involvement in target base eversion

EcoP15I DNA methyltransferase catalyzes the transfer of the methyl group of S-adenosyl-l-methionine to the N6 position of the second adenine within the double-stranded DNA sequence 5'-CAGCAG-3'. To achieve catalysis, the enzyme requires a magnesium ion. Binding of magnesium to the enzyme induces significant conformational changes as monitored by circular dichroism spectroscopy. EcoP15I DNA methyltransferase was rapidly inactivated by micromolar concentrations of ferrous sulfate in the presence of ascorbate at pH 8.0. The inactivated enzyme was cleaved into two fragments with molecular masses of 36 and 35 kDa. Using this affinity cleavage assay, we have located the magnesium binding-like motif to amino acids 355-377 of EcoP15I DNA methyltransferase. Sequence homology comparisons between EcoP15I DNA methyltransferase and other restriction endonucleases allowed us to identify a PD(X)n(D/E)XK-like sequence as the putative magnesium ion binding site. Point mutations generated in this region were analyzed for their role in methyltransferase activity, metal coordination, and substrate binding. Although the mutant methyltransferases bind DNA and S-adenosyl-l-methionine as well as the wild-type enzyme does, they are inactive primarily because of their inability to flip the target base. Collectively, these data are consistent with the fact that acidic amino acid residues of the region 355-377 in EcoP15I DNA methyltransferase are important for the critical positioning of magnesium ions for catalysis. This is the first example of metal-dependent function of a DNA methyltransferase. These findings provide impetus for exploring the role(s) of metal ions in the structure and function of DNA methyltransferases.

Characterization of the type IV restriction modification system BspLU11III from Bacillus sp. LU11

We report the characterization and cloning of the genes for an unusual type IV restriction-modification system, BspLU11III, from Bacillus sp. LU11. The system consists of two methyltransferases and one endonuclease, which also possesses methyltransferase activity. The three genes of the restriction-modification system, bsplu11IIIMa, bsplu11IIIMb and bsplu11IIIR, are closely linked and tandemly arranged. The corresponding enzymes recognize the dsDNA sequence 5'-GGGAC-3'/5'-GTCCC-3', with M.BspLU11IIIa modifying the A (underlined) of one strand and M.BspLU11IIIb the inner C (underlined) of the other strand. R.BspLU11III has both endonuclease and adenine-specific methyltransferase activities and is able to protect the DNA against cleavage by itself. In contrast to all type IV restriction-modification systems described so far, which have only one adenine-specific methyltransferase, BspLU11III is the first type IV restriction-modification system that includes two methyltransferases, one of them being cytosine specific.

Characterization of BseMII, a new type IV restriction-modification system, which recognizes the pentanucleotide sequence 5'-CTCAG(N)(10/8)/

We report the properties of the new BseMII restriction and modification enzymes from Bacillus stearothermophilus Isl 15-111, which recognize the 5'-CTCAG sequence, and the nucleotide sequence of the genes encoding them. The restriction endonuclease R.BseMII makes a staggered cut at the tenth base pair downstream of the recognition sequence on the upper strand, producing a two base 3'-protruding end. Magnesium ions and S:-adenosyl-L-methionine (AdoMet) are required for cleavage. S:-adenosylhomocysteine and sinefungin can replace AdoMet in the cleavage reaction. The BseMII methyltransferase modifies unique adenine residues in both strands of the target sequence 5'-CTCAG-3'/5'-CTGAG-3'. Monomeric R.BseMII in addition to endonucleolytic activity also possesses methyltransferase activity that modifies the A base only within the 5'-CTCAG strand of the target duplex. The deduced amino acid sequence of the restriction endonuclease contains conserved motifs of DNA N6-adenine methylases involved in S-adenosyl-L-methionine binding and catalysis. According to its structure and enzymatic properties, R.BseMII may be regarded as a representative of the type IV restriction endonucleases.