DNA cleavage reactions by type II restriction enzymes that require two copies of their recognition sites

smFRET Detection of Cis and Trans DNA Interactions by the BfiI Restriction Endonuclease

Protein-DNA interactions are fundamental to many biological processes. Proteins must find their target site on a DNA molecule to perform their function, and mechanisms for target search differ across proteins. Especially challenging phenomena to monitor and understand are transient binding events that occur across two DNA target sites, whether occurring in cis or trans. Type IIS restriction endonucleases rely on such interactions. They play a crucial role in safeguarding bacteria against foreign DNA, including viral genetic material. BfiI, a type IIS restriction endonuclease, acts upon a specific asymmetric sequence, 5-ACTGGG-3, and precisely cuts both upper and lower DNA strands at fixed locations downstream of this sequence. Here, we present two single-molecule Förster resonance energy-transfer-based assays to study such interactions in a BfiI-DNA system. The first assay focuses on DNA looping, detecting both "Phi"- and "U"-shaped DNA looping events. The second assay only allows in trans BfiI-target DNA interactions, improving the specificity and reducing the limits on observation time. With total internal reflection fluorescence microscopy, we directly observe on- and off-target binding events and characterize BfiI binding events. Our results show that BfiI binds longer to target sites and that BfiI rarely changes conformations during binding. This newly developed assay could be employed for other DNA-interacting proteins that bind two targets and for the dsDNA substrate BfiI-PAINT, a useful strategy for DNA stretch assays and other super-resolution fluorescence microscopy studies.

Structures, activity and mechanism of the Type IIS restriction endonuclease PaqCI

Type IIS restriction endonucleases contain separate DNA recognition and catalytic domains and cleave their substrates at well-defined distances outside their target sequences. They are employed in biotechnology for a variety of purposes, including the creation of gene-targeting zinc finger and TAL effector nucleases and DNA synthesis applications such as Golden Gate assembly. The most thoroughly studied Type IIS enzyme, FokI, has been shown to require multimerization and engagement with multiple DNA targets for optimal cleavage activity; however, details of how it or similar enzymes forms a DNA-bound reaction complex have not been described at atomic resolution. Here we describe biochemical analyses of DNA cleavage by the Type IIS PaqCI restriction endonuclease and a series of molecular structures in the presence and absence of multiple bound DNA targets. The enzyme displays a similar tetrameric organization of target recognition domains in the absence or presence of bound substrate, with a significant repositioning of endonuclease domains in a trapped DNA-bound complex that is poised to deliver the first of a series of double-strand breaks. PaqCI and FokI share similar structural mechanisms of DNA cleavage, but considerable differences in their domain organization and quaternary architecture, facilitating comparisons between distinct Type IIS enzymes.

Structure, subunit organization and behavior of the asymmetric Type IIT restriction endonuclease BbvCI

BbvCI, a Type IIT restriction endonuclease, recognizes and cleaves the seven base pair sequence 5'-CCTCAGC-3', generating 3-base, 5'-overhangs. BbvCI is composed of two protein subunits, each containing one catalytic site. Either site can be inactivated by mutation resulting in enzyme variants that nick DNA in a strand-specific manner. Here we demonstrate that the holoenzyme is labile, with the R1 subunit dissociating at low pH. Crystallization of the R2 subunit under such conditions revealed an elongated dimer with the two catalytic sites located on opposite sides. Subsequent crystallization at physiological pH revealed a tetramer comprising two copies of each subunit, with a pair of deep clefts each containing two catalytic sites appropriately positioned and oriented for DNA cleavage. This domain organization was further validated with single-chain protein constructs in which the two enzyme subunits were tethered via peptide linkers of variable length. We were unable to crystallize a DNA-bound complex; however, structural similarity to previously crystallized restriction endonucleases facilitated creation of an energy-minimized model bound to DNA, and identification of candidate residues responsible for target recognition. Mutation of residues predicted to recognize the central C:G base pair resulted in an altered enzyme that recognizes and cleaves CCTNAGC (N = any base).

Direct AFM Visualization of the Nanoscale Dynamics of Biomolecular Complexes

High-speed AFM (HS-AFM) is an advanced technique with numerous applications in biology, particularly in molecular biophysics. Developed as a time-lapse AFM technique for direct imaging fully hydrated biological molecules, HS-AFM is currently capable of visualizing the dynamics of biological molecules and their complexes at a video-data acquisition rate. Spatial resolution at the nanometer level is another important characteristic of HS-AFM. This review focuses on examples of primarily protein-DNA complexes to illustrate the high temporal and spatial resolution capabilities of HS-AFM that have resulted in novel models and/or the functional mechanisms of these biological systems.

CgII cleaves DNA using a mechanism distinct from other ATP-dependent restriction endonucleases

The restriction endonuclease CglI from Corynebacterium glutamicum recognizes an asymmetric 5′-GCCGC-3′ site and cleaves the DNA 7 and 6/7 nucleotides downstream on the top and bottom DNA strands, respectively, in an NTP-hydrolysis dependent reaction. CglI is composed of two different proteins: an endonuclease (R.CglI) and a DEAD-family helicase-like ATPase (H.CglI). These subunits form a heterotetrameric complex with R₂H₂ stoichiometry. However, the R₂H_2·CglI complex has only one nuclease active site sufficient to cut one DNA strand suggesting that two complexes are required to introduce a double strand break. Here, we report studies to evaluate the DNA cleavage mechanism of CglI. Using one- and two-site circular DNA substrates we show that CglI does not require two sites on the same DNA for optimal catalytic activity. However, one-site linear DNA is a poor substrate, supporting a mechanism where CglI complexes must communicate along the one-dimensional DNA contour before cleavage is activated. Based on experimental data, we propose that adenosine triphosphate (ATP) hydrolysis by CglI produces translocation on DNA preferentially in a downstream direction from the target, although upstream translocation is also possible. Our results are consistent with a mechanism of CglI action that is distinct from that of other ATP-dependent restriction-modification enzymes.

Synthetic temperature-inducible lethal gene circuits in Escherichia coli

Temperature sensitivity is often used as a way to attenuate micro-organisms to convert them into live vaccines. In this work, we explore the use of temperature-sensitive (TS) genetic circuits that express lethal genes as a widely applicable approach to TS attenuation. We tested different combinations of TS repressors and cognate promoters controlling the expression of genes encoding restriction endonucleases inserted at four different non-essential sites in the Escherichia coli chromosome. We found that the presence of the restriction endonuclease genes did not affect the viability of the host strains at the permissive temperature, but that expression of the genes at elevated temperatures killed the strains to varying extents. The chromosomal insertion site of the lethal cassettes affected their functionality, and insertion at one site, ycgH, rendered them ineffective at inducing death at high temperature. Induction of a TS circuit in a growing culture led to a reduced cell mass and a reduction of the number of cells that could exclude a dye that indicated viability. Incubation of cells carrying a TS lethal gene circuit initially grown at low temperature and then suspended in phosphate buffered saline at high temperature led to about 100-fold loss of cell viability per day, compared to a minimal loss of viability for the parental strain. Strains carrying either one or two TS lethal circuits could generate mutants that survived at high temperature. These mutants included complete deletions of the lethal gene circuits.

Imaging of DNA and Protein-DNA Complexes with Atomic Force Microscopy

This article reviews atomic force microscopy (AFM) studies of DNA structure and dynamics and protein-DNA complexes, including recent advances in the visualization of protein-DNA complexes with the use of cutting-edge, high-speed AFM. Special emphasis is given to direct nanoscale visualization of dynamics of protein-DNA complexes. In the area of DNA structure and dynamics, structural studies of local non-B conformations of DNA and the interplay of local and global DNA conformations are reviewed. The application of time-lapse AFM nanoscale imaging of DNA dynamics is illustrated by studies of Holliday junction branch migration. Structure and dynamics of protein-DNA interactions include problems related to site-specific DNA recombination, DNA replication, and DNA mismatch repair. Studies involving the structure and dynamics of chromatin are also described.

An Engineered SS Bridge Blocks the Conformational Change Required for the Nuclease Activity of BfiI

The type IIS restriction endonuclease BfiI is a homodimer, and each monomer is composed of the N-terminal catalytic and C-terminal DNA recognition domains connected by a 28-residue linker segment. In the crystal in the absence of cognate DNA, BfiI exists in a "closed" conformation, in which an interdomain linker occludes a putative DNA binding surface at the catalytic domain and sterically hinders access to the active site. Cognate DNA binding presumably triggers a conformational change from the inactive "closed" state to the catalytically competent "open" state. Here we show that the disulfide SS bridge engineered at the domain interface locks the enzyme in the "closed" state. In the "closed" SS-linked state, BfiI binds cognate DNA with the same affinity as the wild-type enzyme but does not cut it, indicating that cross-linking introduces a restraint on the conformational transition, which couples DNA recognition and cleavage. Disruption of the interdomain SS bridge by the reducing agent restores the DNA cleavage ability of BfiI.

Functional significance of protein assemblies predicted by the crystal structure of the restriction endonuclease BsaWI

Type II restriction endonuclease BsaWI recognizes a degenerated sequence 5′-W/CCGGW-3′ (W stands for A or T, ‘/’ denotes the cleavage site). It belongs to a large family of restriction enzymes that contain a conserved CCGG tetranucleotide in their target sites. These enzymes are arranged as dimers or tetramers, and require binding of one, two or three DNA targets for their optimal catalytic activity. Here, we present a crystal structure and biochemical characterization of the restriction endonuclease BsaWI. BsaWI is arranged as an ‘open’ configuration dimer and binds a single DNA copy through a minor groove contacts. In the crystal primary BsaWI dimers form an indefinite linear chain via the C-terminal domain contacts implying possible higher order aggregates. We show that in solution BsaWI protein exists in a dimer-tetramer-oligomer equilibrium, but in the presence of specific DNA forms a tetramer bound to two target sites. Site-directed mutagenesis and kinetic experiments show that BsaWI is active as a tetramer and requires two target sites for optimal activity. We propose BsaWI mechanism that shares common features both with dimeric Ecl18kI/SgrAI and bona fide tetrameric NgoMIV/SfiI enzymes.

Single molecular investigation of DNA looping and aggregation by restriction endonuclease BspMI

DNA looping and aggregation induced by restriction endonuclease BspMI are studied by atomic force microscopy (AFM) and magnetic tweezers (MT). With Ca(2+) substituted for the normal enzyme cofactor Mg(2+) and enzyme concentration below the critical concentration of 6 units/mL, AFM images of DNA-BspMI complex show that the number of binding and looping events increases with enzyme concentration. At the critical concentration 6 of units/mL, all the BspMI binding sites are saturated. It is worth noting that nonspecific BspMI binding to DNA at saturation concentration represents more than 8% of the total BspMI-DNA complexes directly observed in AFM images. Furthermore, we used MT to prove that additional loops can form when enzyme concentration is higher than its saturation valueand the complex is incubated for a long time (>2 hrs). We ascribe this phenomenon to the aggregation of enzymes. The force spectroscopy of the BspMI-DNA complex shows that the pulling force required to open the loop of the complex at less than saturation concentration has a peak at about 3 pN, which is lower than the force required to open additional loops due to enzyme aggregation at higher than saturation concentration (>6 pN).

Type II restriction endonucleases--a historical perspective and more

This article continues the series of Surveys and Summaries on restriction endonucleases (REases) begun this year in Nucleic Acids Research. Here we discuss 'Type II' REases, the kind used for DNA analysis and cloning. We focus on their biochemistry: what they are, what they do, and how they do it. Type II REases are produced by prokaryotes to combat bacteriophages. With extreme accuracy, each recognizes a particular sequence in double-stranded DNA and cleaves at a fixed position within or nearby. The discoveries of these enzymes in the 1970s, and of the uses to which they could be put, have since impacted every corner of the life sciences. They became the enabling tools of molecular biology, genetics and biotechnology, and made analysis at the most fundamental levels routine. Hundreds of different REases have been discovered and are available commercially. Their genes have been cloned, sequenced and overexpressed. Most have been characterized to some extent, but few have been studied in depth. Here, we describe the original discoveries in this field, and the properties of the first Type II REases investigated. We discuss the mechanisms of sequence recognition and catalysis, and the varied oligomeric modes in which Type II REases act. We describe the surprising heterogeneity revealed by comparisons of their sequences and structures.

Structure of 5-hydroxymethylcytosine-specific restriction enzyme, AbaSI, in complex with DNA

AbaSI, a member of the PvuRts1I-family of modification-dependent restriction endonucleases, cleaves deoxyribonucleic acid (DNA) containing 5-hydroxymethylctosine (5hmC) and glucosylated 5hmC (g5hmC), but not DNA containing unmodified cytosine. AbaSI has been used as a tool for mapping the genomic locations of 5hmC, an important epigenetic modification in the DNA of higher organisms. Here we report the crystal structures of AbaSI in the presence and absence of DNA. These structures provide considerable, although incomplete, insight into how this enzyme acts. AbaSI appears to be mainly a homodimer in solution, but interacts with DNA in our structures as a homotetramer. Each AbaSI subunit comprises an N-terminal, Vsr-like, cleavage domain containing a single catalytic site, and a C-terminal, SRA-like, 5hmC-binding domain. Two N-terminal helices mediate most of the homodimer interface. Dimerization brings together the two catalytic sites required for double-strand cleavage, and separates the 5hmC binding-domains by ∼70 Å, consistent with the known activity of AbaSI which cleaves DNA optimally between symmetrically modified cytosines ∼22 bp apart. The eukaryotic SET and RING-associated (SRA) domains bind to DNA containing 5-methylcytosine (5mC) in the hemi-methylated CpG sequence. They make contacts in both the major and minor DNA grooves, and flip the modified cytosine out of the helix into a conserved binding pocket. In contrast, the SRA-like domain of AbaSI, which has no sequence specificity, contacts only the minor DNA groove, and in our current structures the 5hmC remains intra-helical. A conserved, binding pocket is nevertheless present in this domain, suitable for accommodating 5hmC and g5hmC. We consider it likely, therefore, that base-flipping is part of the recognition and cleavage mechanism of AbaSI, but that our structures represent an earlier, pre-flipped stage, prior to actual recognition.

TstI, a Type II restriction-modification protein with DNA recognition, cleavage and methylation functions in a single polypeptide

Type II restriction–modification systems cleave and methylate DNA at specific sequences. However, the Type IIB systems look more like Type I than conventional Type II schemes as they employ the same protein for both restriction and modification and for DNA recognition. Several Type IIB proteins, including the archetype BcgI, are assemblies of two polypeptides: one with endonuclease and methyltransferase roles, another for DNA recognition. Conversely, some IIB proteins express all three functions from separate segments of a single polypeptide. This study analysed one such single-chain protein, TstI. Comparison with BcgI showed that the one- and the two-polypeptide systems differ markedly. Unlike the heterologous assembly of BcgI, TstI forms a homotetramer. The tetramer bridges two recognition sites before eventually cutting the DNA in both strands on both sides of the sites, but at each site the first double-strand break is made long before the second. In contrast, BcgI cuts all eight target bonds at two sites in a single step. TstI also differs from BcgI in either methylating or cleaving unmodified sites at similar rates. The site may thus be modified before TstI can make the second double-strand break. TstI MTase acts best at hemi-methylated sites.

Organization of the BcgI restriction-modification protein for the cleavage of eight phosphodiester bonds in DNA

Type IIB restriction-modification systems, such as BcgI, feature a single protein with both endonuclease and methyltransferase activities. Type IIB nucleases require two recognition sites and cut both strands on both sides of their unmodified sites. BcgI cuts all eight target phosphodiester bonds before dissociation. The BcgI protein contains A and B polypeptides in a 2:1 ratio: A has one catalytic centre for each activity; B recognizes the DNA. We show here that BcgI is organized as A₂B protomers, with B at its centre, but that these protomers self-associate to assemblies containing several A₂B units. Moreover, like the well known FokI nuclease, BcgI bound to its site has to recruit additional protomers before it can cut DNA. DNA-bound BcgI can alternatively be activated by excess A subunits, much like the activation of FokI by its catalytic domain. Eight A subunits, each with one centre for nuclease activity, are presumably needed to cut the eight bonds cleaved by BcgI. Its nuclease reaction may thus involve two A₂B units, each bound to a recognition site, with two more A₂B units bridging the complexes by protein–protein interactions between the nuclease domains.

DNA looping by FokI: the impact of twisting and bending rigidity on protein-induced looping dynamics

Protein-induced DNA looping is crucial for many genetic processes such as transcription, gene regulation and DNA replication. Here, we use tethered-particle motion to examine the impact of DNA bending and twisting rigidity on loop capture and release, using the restriction endonuclease FokI as a test system. To cleave DNA efficiently, FokI bridges two copies of an asymmetric sequence, invariably aligning the sites in parallel. On account of the fixed alignment, the topology of the DNA loop is set by the orientation of the sites along the DNA. We show that both the separation of the FokI sites and their orientation, altering, respectively, the twisting and the bending of the DNA needed to juxtapose the sites, have profound effects on the dynamics of the looping interaction. Surprisingly, the presence of a nick within the loop does not affect the observed rigidity of the DNA. In contrast, the introduction of a 4-nt gap fully relaxes all of the torque present in the system but does not necessarily enhance loop stability. FokI therefore employs torque to stabilise its DNA-looping interaction by acting as a ‘torsional’ catch bond.

Visual analysis of concerted cleavage by type IIF restriction enzyme SfiI in subsecond time region

Many DNA regulatory factors require communication between distantly separated DNA sites for their activity. The type IIF restriction enzyme SfiI is often used as a model system of site communication. Here, we used fast-scanning atomic force microscopy to monitor the DNA cleavage process with SfiI and the changes in the single SfiI-DNA complex in the presence of either Mg²⁺ or Ca²⁺ at a scan rate of 1-2 fps. The increased time resolution allowed us to visualize the concerted cleavage of the protein at two cognate sites. The four termini generated by the cleavage were released in a multistep manner. The high temporal resolution enabled us to visualize the translocation of a DNA strand on a looped complex and intersegmental transfer of the SfiI protein in which swapping of the site is performed without protein dissociation. On the basis of our results, we propose that the SfiI tetramer can remain bound to one of the sites even after cleavage, allowing the other site on the DNA molecule to fill the empty DNA-binding cleft by combining a one-dimensional diffusion-mediated sliding and a segment transfer mechanism.

The reaction mechanism of FokI excludes the possibility of targeting zinc finger nucleases to unique DNA sites

The FokI endonuclease is a monomeric protein with discrete DNA-recognition and catalytic domains. The latter has only one active site so, to cut both strands, the catalytic domains from two monomers associate to form a dimer. The dimer involving a monomer at the recognition site and another from free solution is less stable than that from two proteins tethered to the same DNA. FokI thus cleaves DNA with two sites better than one-site DNA. The two sites can be immediately adjacent, but they can alternatively be many hundreds of base pairs apart, in either inverted or repeated orientations. The catalytic domain of FokI is often a component of zinc finger nucleases. Typically, the zinc finger domains of two such nucleases are designed to recognize two neighbouring DNA sequences, with the objective of cutting the DNA exclusively between the target sequences. However, this strategy fails to take account of the fact that the catalytic domains of FokI can dimerize across distant sites or even at a solitary site. Additional copies of either target sequence elsewhere in the chromosome must elicit off-target cleavages.

Concerted action at eight phosphodiester bonds by the BcgI restriction endonuclease

The BcgI endonuclease exemplifies a subset of restriction enzymes, the Type IIB class, which make two double-strand breaks (DSBs) at each copy of their recognition sequence, one either side of the site, to excise the sequence from the remainder of the DNA. In this study, we show that BcgI is essentially inactive when bound to a single site and that to cleave a DNA with one copy of its recognition sequence, it has to act in trans, bridging two separate DNA molecules. We also show that BcgI makes the two DSBs at an individual site in a highly concerted manner. Intermediates cut on one side of the site do not accumulate during the course of the reaction: instead, the DNA is converted straight to the final products cut on both sides. On DNA with two sites, BcgI bridges the sites in cis and then generally proceeds to cut both strands on both sides of both sites without leaving the DNA. The BcgI restriction enzyme can thus excise two DNA segments together, by cleaving eight phosphodiester bonds within a single-DNA binding event.

Unraveling DNA dynamics using atomic force microscopy

The elucidation of structure-function relationships of biological samples has become important issue in post-genomic researches. In order to unveil the molecular mechanisms controlling gene regulations, it is essential to understand the interplay between fundamental DNA properties and the dynamics of the entire molecule. The wide range of applicability of atomic force microscopy (AFM) has allowed us to extract physicochemical properties of DNA and DNA-protein complexes, as well as to determine their topographical information. Here, we review how AFM techniques have been utilized to study DNA and DNA-protein complexes and what types of analyses have accelerated the understanding of the DNA dynamics. We begin by illustrating the application of AFM to investigate the fundamental feature of DNA molecules; topological transition of DNA, length dependent properties of DNA molecules, flexibility of double-stranded DNA, and capability of the formation of non-Watson-Crick base pairing. These properties of DNA are critical for the DNA folding and enzymatic reactions. The technical advancement in the time-resolution of AFM and sample preparation methods enabled visual analysis of DNA-protein interactions at sub-second time region. DNA tension-dependent enzymatic reaction and DNA looping dynamics by restriction enzymes were examined at a nanoscale in physiological environments. Contribution of physical properties of DNA to dynamics of nucleosomes and transition of the higher-order structure of reconstituted chromatin are also reviewed.

DNA synapsis through transient tetramerization triggers cleavage by Ecl18kI restriction enzyme

To cut DNA at their target sites, restriction enzymes assemble into different oligomeric structures. The Ecl18kI endonuclease in the crystal is arranged as a tetramer made of two dimers each bound to a DNA copy. However, free in solution Ecl18kI is a dimer. To find out whether the Ecl18kI dimer or tetramer represents the functionally important assembly, we generated mutants aimed at disrupting the putative dimer–dimer interface and analysed the functional properties of Ecl18kI and mutant variants. We show by atomic force microscopy that on two-site DNA, Ecl18kI loops out an intervening DNA fragment and forms a tetramer. Using the tethered particle motion technique, we demonstrate that in solution DNA looping is highly dynamic and involves a transient interaction between the two DNA-bound dimers. Furthermore, we show that Ecl18kI cleaves DNA in the synaptic complex much faster than when acting on a single recognition site. Contrary to Ecl18kI, the tetramerization interface mutant R174A binds DNA as a dimer, shows no DNA looping and is virtually inactive. We conclude that Ecl18kI follows the association model for the synaptic complex assembly in which it binds to the target site as a dimer and then associates into a transient tetrameric form to accomplish the cleavage reaction.

A putative mobile genetic element carrying a novel type IIF restriction-modification system (PluTI)

Genome comparison and genome context analysis were used to find a putative mobile element in the genome of Photorhabdus luminescens, an entomopathogenic bacterium. The element is composed of 16-bp direct repeats in the terminal regions, which are identical to a part of insertion sequences (ISs), a DNA methyltransferase gene homolog, two genes of unknown functions and an open reading frame (ORF) (plu0599) encoding a protein with no detectable sequence similarity to any known protein. The ORF (plu0599) product showed DNA endonuclease activity, when expressed in a cell-free expression system. Subsequently, the protein, named R.PluTI, was expressed in vivo, purified and found to be a novel type IIF restriction enzyme that recognizes 5′-GGCGC/C-3′ (/ indicates position of cleavage). R.PluTI cleaves a two-site supercoiled substrate at both the sites faster than a one-site supercoiled substrate. The modification enzyme homolog encoded by plu0600, named M.PluTI, was expressed in Escherichia coli and shown to protect DNA from R.PluTI cleavage in vitro, and to suppress the lethal effects of R.PluTI expression in vivo. These results suggested that they constitute a restriction–modification system, present on the putative mobile element. Our approach thus allowed detection of a previously uncharacterized family of DNA-interacting proteins.

Bacteriophage T4 endonuclease II, a promiscuous GIY-YIG nuclease, binds as a tetramer to two DNA substrates

The oligomerization state and mode of binding to DNA of the GIY-YIG endonuclease II (EndoII) from bacteriophage T4 was studied using gel filtration and electrophoretic mobility shift assays with a set of mutants previously found to have altered enzyme activity. At low enzyme/DNA ratios all mutants except one bound to DNA only as tetramers to two DNA substrates. The putatively catalytic E118 residue actually interfered with DNA binding (possibly due to steric hindrance or repulsion between the glutamate side chain and DNA), as shown by the ability of E118A to bind stably also as monomer or dimer to a single substrate. The tetrameric structure of EndoII in the DNA-protein complex is surprising considering the asymmetry of the recognized sequence and the predominantly single-stranded nicking. Combining the results obtained here with those from our previous in vivo studies and the recently obtained crystal structure of EndoII E118A, we suggest a model where EndoII translocates DNA between two adjacent binding sites and either nicks one strand of one or both substrates bound by the tetramer, or nicks both strands of one substrate. Thus, only one or two of the four active sites in the tetramer is catalytically active at any time.

Differences between Ca2+ and Mg2+ in DNA binding and release by the SfiI restriction endonuclease: implications for DNA looping

Many enzymes acting on DNA require Mg(2+) ions not only for catalysis but also to bind DNA. Binding studies often employ Ca(2+) as a substitute for Mg(2+), to promote DNA binding whilst disallowing catalysis. The SfiI endonuclease requires divalent metal ions to bind DNA but, in contrast to many systems where Ca(2+) mimics Mg(2+), Ca(2+) causes SfiI to bind DNA almost irreversibly. Equilibrium binding by wild-type SfiI cannot be conducted with Mg(2+) present as the DNA is cleaved so, to study the effect of Mg(2+) on DNA binding, two catalytically-inactive mutants were constructed. The mutants bound DNA in the presence of either Ca(2+) or Mg(2+) but, unlike wild-type SfiI with Ca(2+), the binding was reversible. With both mutants, dissociation was slow with Ca(2+) but was in one case much faster with Mg(2+). Hence, Ca(2+) can affect DNA binding differently from Mg(2+). Moreover, SfiI is an archetypal system for DNA looping; on DNA with two recognition sites, it binds to both sites and loops out the intervening DNA. While the dynamics of looping cannot be measured with wild-type SfiI and Ca(2+), it becomes accessible with the mutant and Mg(2+).

Dissecting protein-induced DNA looping dynamics in real time

Many proteins that interact with DNA perform or enhance their specific functions by binding simultaneously to multiple target sites, thereby inducing a loop in the DNA. The dynamics and energies involved in this loop formation influence the reaction mechanism. Tethered particle motion has proven a powerful technique to study in real time protein-induced DNA looping dynamics while minimally perturbing the DNA-protein interactions. In addition, it permits many single-molecule experiments to be performed in parallel. Using as a model system the tetrameric Type II restriction enzyme SfiI, that binds two copies of its recognition site, we show here that we can determine the DNA-protein association and dissociation steps as well as the actual process of protein-induced loop capture and release on a single DNA molecule. The result of these experiments is a quantitative reaction scheme for DNA looping by SfiI that is rigorously compared to detailed biochemical studies of SfiI looping dynamics. We also present novel methods for data analysis and compare and discuss these with existing methods. The general applicability of the introduced techniques will further enhance tethered particle motion as a tool to follow DNA-protein dynamics in real time.

The MmeI family: type II restriction-modification enzymes that employ single-strand modification for host protection

The type II restriction endonucleases form one of the largest families of biochemically-characterized proteins. These endonucleases typically share little sequence similarity, except among isoschizomers that recognize the same sequence. MmeI is an unusual type II restriction endonuclease that combines endonuclease and methyltransferase activities in a single polypeptide. MmeI cuts DNA 20 bases from its recognition sequence and modifies just one DNA strand for host protection. Using MmeI as query we have identified numerous putative genes highly similar to MmeI in database sequences. We have cloned and characterized 20 of these MmeI homologs. Each cuts DNA at the same distance as MmeI and each modifies a conserved adenine on only one DNA strand for host protection. However each enzyme recognizes a unique DNA sequence, suggesting these enzymes are undergoing rapid evolution of DNA specificity. The MmeI family thus provides a rich source of novel endonucleases while affording an opportunity to observe the evolution of DNA specificity. Because the MmeI family enzymes employ modification of only one DNA strand for host protection, unlike previously described type II systems, we propose that such single-strand modification systems be classified as a new subgroup, the type IIL enzymes, for Lone strand DNA modification.

Cloning and analysis of a bifunctional methyltransferase/restriction endonuclease TspGWI, the prototype of a Thermus sp. enzyme family

Background

Restriction-modification systems are a diverse class of enzymes. They are classified into four major types: I, II, III and IV. We have previously proposed the existence of a Thermus sp. enzyme family, which belongs to type II restriction endonucleases (REases), however, it features also some characteristics of types I and III. Members include related thermophilic endonucleases: TspGWI, TaqII, TspDTI, and Tth111II.

Results

Here we describe cloning, mutagenesis and analysis of the prototype TspGWI enzyme that recognises the 5'-ACGGA-3' site and cleaves 11/9 nt downstream. We cloned, expressed, and mutagenised the tspgwi gene and investigated the properties of its product, the bifunctional TspGWI restriction/modification enzyme. Since TspGWI does not cleave DNA completely, a cloning method was devised, based on amino acid sequencing of internal proteolytic fragments. The deduced amino acid sequence of the enzyme shares significant sequence similarity with another representative of the Thermus sp. family – TaqII. Interestingly, these enzymes recognise similar, yet different sequences in the DNA. Both enzymes cleave DNA at the same distance, but differ in their ability to cleave single sites and in the requirement of S-adenosylmethionine as an allosteric activator for cleavage. Both the restriction endonuclease (REase) and methyltransferase (MTase) activities of wild type (wt) TspGWI (either recombinant or isolated from Thermus sp.) are dependent on the presence of divalent cations.

Conclusion

TspGWI is a bifunctional protein comprising a tandem arrangement of Type I-like domains; particularly noticeable is the central HsdM-like module comprising a helical domain and a highly conserved S-adenosylmethionine-binding/catalytic MTase domain, containing DPAVGTG and NPPY motifs. TspGWI also possesses an N-terminal PD-(D/E)XK nuclease domain related to the corresponding domains in HsdR subunits, but lacks the ATP-dependent translocase module of the HsdR subunit and the additional domains that are involved in subunit-subunit interactions in Type I systems. The MTase and REase activities of TspGWI are autonomous and can be uncoupled. Structurally and functionally, the TspGWI protomer appears to be a streamlined 'half' of a Type I 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.

Tetrameric restriction enzymes: expansion to the GIY-YIG nuclease family

The GIY-YIG nuclease domain was originally identified in homing endonucleases and enzymes involved in DNA repair and recombination. Many of the GIY-YIG family enzymes are functional as monomers. We show here that the Cfr42I restriction endonuclease which belongs to the GIY-YIG family and recognizes the symmetric sequence 5′-CCGC/GG-3′ (‘/’ indicates the cleavage site) is a tetramer in solution. Moreover, biochemical and kinetic studies provided here demonstrate that the Cfr42I tetramer is catalytically active only upon simultaneous binding of two copies of its recognition sequence. In that respect Cfr42I resembles the homotetrameric Type IIF restriction enzymes that belong to the distinct PD-(E/D)XK nuclease superfamily. Unlike the PD-(E/D)XK enzymes, the GIY-YIG nuclease Cfr42I accommodates an extremely wide selection of metal-ion cofactors, including Mg²⁺, Mn²⁺, Co²⁺, Zn²⁺, Ni²⁺, Cu²⁺ and Ca²⁺. To our knowledge, Cfr42I is the first tetrameric GIY-YIG family enzyme. Similar structural arrangement and phenotypes displayed by restriction enzymes of the PD-(E/D)XK and GIY-YIG nuclease families point to the functional significance of tetramerization.

The restriction endonuclease R.NmeDI from Neisseria meningitidis that recognizes a palindromic sequence and cuts the DNA on both sides of the recognition sequence

The restriction endonuclease Type II R.NmeDI from Neisseria meningitidis 2120 (serogroup C, ST-11 complex) was characterized. The cloned nmeDIR gene was expressed in Escherichia coli cells, and the endonucleolytic and restriction activities of R.NmeDI were then observed in vitro and in vivo. The nmeDIR gene consists of 1056 bp coding 351 aa protein with a calculated molecular weight of M_(r) = 39 000 ± 1000 Da. The R.NmeDI enzyme was purified to apparent homogeneity following overexpression, using metal affinity chromatography. This enzyme recognizes a palindrome sequence and cleaves double-stranded DNA upstream and downstream of its recognition sequence (12/7) RCCGGY (7/12) (R = A/G, Y = C/T) cutting out a 25-bp fragment. R.NmeDI cleaves in two steps. The enzyme cleaves the first strand randomly on either side of the recognition sequence generating an intermediate, and the second cleavage occurs more slowly and results in the production of a final reaction product. The R.NmeDI endonuclease requires two recognition sequences for effective cleavage. The tetramer is an active form of the R.NmeDI enzyme.

Direct visualization of the EcoRII-DNA triple synaptic complex by atomic force microscopy

Interactions between distantly separated DNA regions mediated by specialized proteins lead to the formation of synaptic protein-DNA complexes. This is a ubiquitous phenomenon which is critical in various genetic processes. Although such interactions typically occur between two sites, interactions among three specific DNA regions have been identified, and a corresponding model has been proposed. Atomic force microscopy was used to test this model for the EcoRII restriction enzyme and provide direct visualization and characterization of synaptic protein-DNA complexes involving three DNA binding sites. The complex appeared in the images as a two-loop structure, and the length measurements proved the site specificity of the protein in the complex. The protein volume measurements showed that an EcoRII dimer is the core of the three-site synaptosome. Other complexes were identified and analyzed. The protein volume data showed that the dimeric form of the protein is responsible for the formation of other types of synaptic complexes as well. The applications of these results to the mechanisms of the protein-DNA interactions are discussed.

A switch in the mechanism of communication between the two DNA-binding sites in the SfiI restriction endonuclease

While many Type II restriction enzymes are dimers with a single DNA-binding cleft between the subunits, SfiI is a tetramer of identical subunits. Two of its subunits (a dimeric unit) create one DNA-binding cleft, and the other two create a second cleft on the opposite side of the protein. The two clefts bind specific DNA cooperatively to give a complex of SfiI with two recognition sites. This complex is responsible for essentially all of the DNA-cleavage reactions by SfiI: virtually none is due to the complex with one site. The communication between the DNA-binding clefts was examined by disrupting one of the very few polar interactions in the otherwise hydrophobic interface between the dimeric units: a tyrosine hydroxyl was removed by mutation to phenylalanine. The mutant protein remained tetrameric in solution and could bind two DNA sites. But instead of being activated by binding two sites, like wild-type SfiI, it showed maximal activity when bound to a single site and had a lower activity when bound to two sites. This interaction across the dimer interface thus enforces in wild-type SfiI a cooperative transition between inactive and active states in both dimers, but without this interaction as in the mutant protein, a single dimer can undergo the transition to give a stable intermediate with one inactive dimer and one active dimer.

An EM view of the FokI synaptic complex by single particle analysis

FokI is a type IIS restriction endonuclease that recognizes the 5'-GGATG-3' sequence and cleaves non-specifically at 9 and 13 base-pairs away on the top and bottom strands, respectively, to produce a 5' overhang. FokI is a bipartite endonuclease with separate recognition and cleavage domains. Because of its bipartite nature, FokI has received considerable interest in generating chimeric nucleases for use in biotechnology, and recently as possible therapeutic agents in gene therapy by initiating homologous gene recombination and repair. Here we show, using single-particle electron microscopic studies, that the FokI active complex prefers a single conformation in which the subunits are arranged in a doughnut shape complex with protein-protein and possibly protein-DNA interactions stabilizing the cleavage complex. Our electron microscopy (EM) model provides new insights into the activation mechanism of FokI and how non-specific cleavage is avoided.

Dynamics of synaptic SfiI-DNA complex: single-molecule fluorescence analysis

A single-molecule analysis was applied to study the dynamics of synaptic and presynaptic DNA-protein complexes (binding of two DNA and one DNA duplex, respectively). In the approach used in this study, the protein was tethered to a surface, allowing a freely diffusing fluorescently labeled DNA to bind to the protein, thus forming a presynaptic complex. The duration of fluorescence burst is the measure of the characteristic lifetime of the complex. To study the formation of the synaptic complex, the two SfiI-bound duplexes with the labeled donor and acceptor were used. The synaptic complex formation by these duplexes was detected by the fluorescence resonance energy transfer approach. The duration of the fluorescence resonance energy transfer burst is the measure of the characteristic lifetime of the synaptic complex. We showed that both synaptic and presynaptic complexes have characteristic dissociation times in the range of milliseconds, with the synaptic SfiI-DNA complex having the shorter dissociation time. Comparison of the off-rate data for the synaptic complex with the rate of DNA cleavage led to the hypothesis that the complex is very dynamic, so the formation of an enzymatically active synaptic complex is a rather rare event in these series of conformational transitions.

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.

DNA strand arrangement within the SfiI-DNA complex: atomic force microscopy analysis

The SfiI restriction enzyme binds to DNA as a tetramer holding two usually distant DNA recognition sites together before cleavage of the four DNA strands. To elucidate structural properties of the SfiI-DNA complex, atomic force microscopy (AFM) imaging of the complexes under noncleaving conditions (Ca2+ instead of Mg2+ in the reaction buffer) was performed. Intramolecular complexes formed by protein interaction between two binding sites in one DNA molecule (cis interaction) as well as complexes formed by the interaction of two sites in different molecules (trans interaction) were analyzed. Complexes were identified unambiguously by the presence of a tall spherical blob at the DNA intersections. To characterize the path of DNA within the complex, the angles between the DNA helices in the proximity of the complex were systematically analyzed. All the data show clear-cut bimodal distributions centered around peak values corresponding to 60 degrees and 120 degrees. To unambiguously distinguish between the crossed and bent models for the DNA orientation within the complex, DNA molecules with different arm lengths flanking the SfiI binding site were designed. The analysis of the AFM images for complexes of this type led to the conclusion that the DNA recognition sites within the complex are crossed. The angles of 60 degrees or 120 degrees between the DNA helices correspond to a complex in which one of the helices is flipped with respect to the orientation of the other. Complexes formed by five different recognition sequences (5'-GGCCNNNNNGGCC-3'), with different central base pairs, were also analyzed. Our results showed that complexes containing the two possible orientations of the helices were formed almost equally. This suggests no preferential orientation of the DNA cognate site within the complex, suggesting that the central part of the DNA binding site does not form strong sequence specific contacts with the protein.

Protein assembly and DNA looping by the FokI restriction endonuclease

The FokI restriction endonuclease recognizes an asymmetric DNA sequence and cuts both strands at fixed positions upstream of the site. The sequence is contacted by a single monomer of the protein, but the monomer has only one catalytic centre and forms a dimer to cut both strands. FokI is also known to cleave DNA with two copies of its site more rapidly than DNA with one copy. To discover how FokI acts at a single site and how it acts at two sites, its reactions were examined on a series of plasmids with either one recognition site or with two sites separated by varied distances, sometimes in the presence of a DNA-binding defective mutant of FokI. These experiments showed that, to cleave DNA with one site, the monomer bound to that site associates via a weak protein–protein interaction with a second monomer that remains detached from the recognition sequence. Nevertheless, the second monomer catalyses phosphodiester bond hydrolysis at the same rate as the DNA-bound monomer. On DNA with two sites, two monomers of FokI interact strongly, as a result of being tethered to the same molecule of DNA, and sequester the intervening DNA in a loop.

Loops in DNA: an overview of experimental and theoretical approaches

DNA loop formation plays a central role in many cellular processes. The aim of this paper is to present the state of the art and open problems regarding the experimental and theoretical approaches to DNA looping. A particular attention is devoted to the effects of the protein bridge size and of protein induced sharp DNA bending on DNA loop formation enhancement.

Simultaneous binding of three recognition sites is necessary for a concerted plasmid DNA cleavage by EcoRII restriction endonuclease

According to the current paradigm type IIE restriction endonucleases are homodimeric proteins that simultaneously bind to two recognition sites but cleave DNA at only one site per turnover: the other site acts as an allosteric locus, activating the enzyme to cleave DNA at the first. Structural and biochemical analysis of the archetypal type IIE restriction enzyme EcoRII suggests that it has three possible DNA binding interfaces enabling simultaneous binding of three recognition sites. To test if putative synapsis of three binding sites has any functional significance, we have studied EcoRII cleavage of plasmids containing a single, two and three recognition sites under both single turnover and steady state conditions. EcoRII displays distinct reaction patterns on different substrates: (i) it shows virtually no activity on a single site plasmid; (ii) it yields open-circular DNA form nicked at one strand as an obligatory intermediate acting on a two-site plasmid; (iii) it cleaves concertedly both DNA strands at a single site during a single turnover on a three site plasmid to yield linear DNA. Cognate oligonucleotide added in trans increases the reaction velocity and changes the reaction pattern for the EcoRII cleavage of one and two-site plasmids but has little effect on the three-site plasmid. Taken together the data indicate that EcoRII requires simultaneous binding of three rather than two recognition sites in cis to achieve concerted DNA cleavage at a single site. We show that the orthodox type IIP enzyme PspGI which is an isoschisomer of EcoRII, cleaves different plasmid substrates with equal rates. Data provided here indicate that type IIE restriction enzymes EcoRII and NaeI follow different mechanisms. We propose that other type IIE restriction enzymes may employ the mechanism suggested here for EcoRII.

Real-time observation of DNA looping dynamics of Type IIE restriction enzymes NaeI and NarI

Many restriction enzymes require binding of two copies of a recognition sequence for DNA cleavage, thereby introducing a loop in the DNA. We investigated looping dynamics of Type IIE restriction enzymes NaeI and NarI by tracking the Brownian motion of single tethered DNA molecules. DNA containing two endonuclease recognition sites spaced a few 100 bp apart connect small polystyrene beads to a glass surface. The position of a bead is tracked through video microscopy. Protein-mediated looping and unlooping is then observed as a sudden specific change in Brownian motion of the bead. With this method we are able to directly follow DNA looping kinetics of single protein–DNA complexes to obtain loop stability and loop formation times. We show that, in the absence of divalent cations, NaeI induces DNA loops of specific size. In contrast, under these conditions NarI mainly creates non-specific loops, resulting in effective DNA compaction for higher enzyme concentrations. Addition of Ca²⁺ increases the NaeI-DNA loop lifetime by two orders of magnitude and stimulates specific binding by NarI. Finally, for both enzymes we observe exponentially distributed loop formation times, indicating that looping is dominated by (re)binding the second recognition site.

A view of consecutive binding events from structures of tetrameric endonuclease SfiI bound to DNA

Many reactions in cells proceed via the sequestration of two DNA molecules in a synaptic complex. SfiI is a member of a growing family of restriction enzymes that can bind and cleave two DNA sites simultaneously. We present here the structures of tetrameric SfiI in complex with cognate DNA. The structures reveal two different binding states of SfiI: one with both DNA-binding sites fully occupied and the other with fully and partially occupied sites. These two states provide details on how SfiI recognizes and cleaves its target DNA sites, and gives insight into sequential binding events. The SfiI recognition sequence (GGCCNNNN[downward arrow]NGGCC) is a subset of the recognition sequence of BglI (GCCNNNN[downward arrow]NGGC), and both enzymes cleave their target DNAs to leave 3-base 3' overhangs. We show that even though SfiI is a tetramer and BglI is a dimer, and there is little sequence similarity between the two enzymes, their modes of DNA recognition are unusually similar.

Long-range communications between DNA sites by the dimeric restriction endonuclease SgrAI

The SgrAI endonuclease displays its maximal activity on DNA with two copies of its recognition sequence, cleaving both sites concertedly. While most restriction enzymes that act concurrently at two sites are tetramers, SgrAI is a dimer in solution. Its reaction at two cognate sites involves the association of two DNA-bound dimers. SgrAI can also bridge cognate and secondary sites, the latter being certain sequences that differ from the cognate by one base-pair. The mechanisms for cognate-cognate and cognate-secondary communications were examined for sites in the following topological relationships: in cis, on plasmids with two sites in a single DNA molecule; on catenanes containing two interlinked rings of DNA with one site in each ring; and in trans, on oligoduplexes carrying either a single site or the DNA termini generated by SgrAI. Both cognate-cognate and cognate-secondary interactions occur through 3-D space and not by 1-D tracking along the DNA. Both sorts of communication arise more readily when the sites are tethered to each other, either in cis on the same molecule of DNA or by the interlinking of catenane rings, than when released from the tether. However, the dimer bound to an oligoduplex carrying either a cognate or a secondary site could be activated to cleave that duplex by interacting with a second dimer bound to the recognition site, provided both duplexes are at least 30 base-pairs long: the second dimer could alternatively be bound to the two duplexes that correspond to the products of DNA cleavage by SgrAI.

Cleavage of individual DNA strands by the different subunits of the heterodimeric restriction endonuclease BbvCI

BbvCI cleaves an asymmetric DNA sequence, 5'-CC downward arrow TCAGC-3'/5'-GC downward arrow TGAGG-3', as indicated. While many Type II restriction enzymes consist of identical subunits, BbvCI has two different subunits: R(1), which acts at GC downward arrow TGAGG; and R(2), which acts at CC downward arrow TCAGC. Some mutants of BbvCI with defects in one subunit, either R(1)(-)R(2)(+) or R(1)(+)R(2)(-), cleave only one strand, that attacked by the native subunit. In analytical ultracentrifugation at various concentrations of protein, wild-type and mutant BbvCI enzymes aggregated extensively, but are R(1)R(2) heterodimers at the concentrations used in DNA cleavage reactions. On a plasmid with one recognition site, wild-type BbvCI cleaved both strands before dissociating from the DNA, while the R(1)(-)R(2)(+) and R(1)(+)R(2)(-) mutants acted almost exclusively on their specified strands, albeit at relatively slow rates. During the wild-type reaction, the DNA is cleaved initially in one strand, mainly that targeted by the R(1) subunit. The other strand is then cleaved slowly by R(2) before the enzyme dissociates from the DNA. Hence, the nicked form accumulates as a transient intermediate. This behaviour differs from that of many other restriction enzymes, which cut both strands at equal rates. However, the activities of the R(1)(+) and R(2)(+) subunits in the wild-type enzyme can differ from their activities in the R(1)(+)R(2)(-) and R(1)(-)R(2)(+) mutants. Each active site in BbvCI therefore influences the other.

Conversion of the Tetrameric Restriction Endonuclease Bse634I into a Dimer: Oligomeric Structure–Stability–Function Correlations

The Bse634I restriction endonuclease is a tetramer and belongs to the type IIF subtype of restriction enzymes. It requires two recognition sites for its optimal activity and cleaves plasmid DNA with two sites much faster than a single-site DNA. We show that disruption of the tetramerisation interface of Bse634I by site-directed mutagenesis converts the tetrameric enzyme into a dimer. Dimeric W228A mutant cleaves plasmid DNA containing one or two sites with the same efficiency as the tetramer cleaves the two-site plasmid. Hence, the catalytic activity of the Bse634I tetramer on a single-site DNA is down-regulated due to the cross-talking interactions between the individual dimers. The autoinhibition within the Bse634I tetramer is relieved by bridging two DNA copies into the synaptic complex that promotes fast and concerted cleavage at both sites. Cleavage analysis of the oligonucleotide attached to the solid support revealed that Bse634I is able to form catalytically competent synaptic complexes by bridging two molecules of the cognate DNA, cognate DNA-miscognate DNA and cognate DNA-product DNA. Taken together, our data demonstrate that a single W228A mutation converts a tetrameric type IIF restriction enzyme Bse634I into the orthodox dimeric type IIP restriction endonuclease. However, the stability of the dimer towards chemical denaturants, thermal inactivation and proteolytic degradation are compromised.

DNA communications by Type III restriction endonucleases--confirmation of 1D translocation over 3D looping

DNA cleavage by Type III restriction enzymes is governed strictly by the relative arrangement of recognition sites on a DNA substrate--endonuclease activity is usually only triggered by sequences in head-to-head orientation. Tens to thousands of base pairs can separate these sites. Long distance communication over such distances could occur by either one-dimensional (1D) DNA translocation or 3D DNA looping. To distinguish between these alternatives, we analysed the activity of EcoPI and EcoP15I on DNA catenanes in which the recognition sites were either on the same or separate rings. While substrates with a pair of sites located on the same ring were cleaved efficiently, catenanes with sites on separate rings were not cleaved. These results exclude a simple 3D DNA-looping activity. To characterize the interactions further, EcoPI was incubated with plasmids carrying two recognition sites interspersed with two 21res sites for site-specific recombination by Tn21 resolvase; inhibition of recombination would indicate the formation of stable DNA loops. No inhibition was observed, even under conditions where EcoPI translocation could also occur.

Enzyme-mediated DNA looping

Most reactions on DNA are carried out by multimeric protein complexes that interact with two or more sites in the DNA and thus loop out the DNA between the sites. The enzymes that catalyze these reactions usually have no activity until they interact with both sites. This review examines the mechanisms for the assembly of protein complexes spanning two DNA sites and the resultant triggering of enzyme activity. There are two main routes for bringing together distant DNA sites in an enzyme complex: either the proteins bind concurrently to both sites and capture the intervening DNA in a loop, or they translocate the DNA between one site and another into an expanding loop, by an energy-dependent translocation mechanism. Both capture and translocation mechanisms are discussed here, with reference to the various types of restriction endonuclease that interact with two recognition sites before cleaving DNA.

Finite-element analysis of the displacement of closed DNA loops under torsional stress

Closed DNA loops that contain intrinsic curvature occur in biologically important structures that are formed by bringing together proteins attached at distinct sites. Such loops constitute topological domains that are characterized by a linking number Delta Lk. We calculate, using finite-element analysis, the structural changes induced by small changes in this linking number, Delta Lk. Because of the intrinsic curvature, the slightest change in linking number induces writhe and the loop begins to fold in space. We previously studied the case in which the initial curvature is uniformly distributed along the DNA rod. We found that there are two different folding modes, depending on the amount of intrinsic curvature and the Poisson ratio, a quantity that measures the ratio of bending stiffness to torsional rigidity. For combinations of the Poisson ratio and curvature that lie below a critical curve, called the Fickel curve, the folding is monotonic in the sense that the writhe uniformly increases as Delta Lk increases, until self-contact occurs. For combinations below this curve, the folding is non-monotonic in the sense that as Delta Lk increases the writhe first increases, then decreases back to essentially zero, and then increases uniformly until self-contact occurs. The folding behaviour and the self-contact points in the two folding modes are completely different. In this paper we first review this previous work. We then extend those results to more-complex situations in which the curvature is initially distributed non-uniformly along the DNA rod. We show that the location of the Fickel curve depends upon both the extent of the initial curvature and upon its distribution along the rod. We also show that two DNAs with the same total intrinsic curvature will fold differently depending upon the distribution of that curvature along the DNA axis, and upon the point of the loop at which the applied rotation or change in Delta Lk is introduced.

One recognition sequence, seven restriction enzymes, five reaction mechanisms

The diversity of reaction mechanisms employed by Type II restriction enzymes was investigated by analysing the reactions of seven endonucleases at the same DNA sequence. NarI, KasI, Mly113I, SfoI, EgeI, EheI and BbeI cleave DNA at several different positions in the sequence 5'-GGCGCC-3'. Their reactions on plasmids with one or two copies of this sequence revealed five distinct mechanisms. These differ in terms of the number of sites the enzyme binds, and the number of phosphodiester bonds cleaved per turnover. NarI binds two sites, but cleaves only one bond per DNA-binding event. KasI also cuts only one bond per turnover but acts at individual sites, preferring intact to nicked sites. Mly113I cuts both strands of its recognition sites, but shows full activity only when bound to two sites, which are then cleaved concertedly. SfoI, EgeI and EheI cut both strands at individual sites, in the manner historically considered as normal for Type II enzymes. Finally, BbeI displays an absolute requirement for two sites in close physical proximity, which are cleaved concertedly. The range of reaction mechanisms for restriction enzymes is thus larger than commonly imagined, as is the number of enzymes needing two recognition sites.

Dynamics of DNA loop capture by the SfiI restriction endonuclease on supercoiled and relaxed DNA

The SfiI endonuclease is a prototype for DNA looping. It binds two copies of its recognition sequence and, if Mg(2+) is present, cuts both concertedly. Looping was examined here on supercoiled and relaxed forms of a 5.5 kb plasmid with three SfiI sites: sites 1 and 2 were separated by 0.4 kb, and sites 2 and 3 by 2.0 kb. SfiI converted this plasmid directly to the products cut at all three sites, though DNA species cleaved at one or two sites were formed transiently during a burst phase. The burst revealed three sets of doubly cut products, corresponding to the three possible pairings of sites. The equilibrium distribution between the different loops was evaluated from the burst phases of reactions initiated by adding MgCl(2) to SfiI bound to the plasmid. The short loop was favored over the longer loops, particularly on supercoiled DNA. The relative rates for loop capture were assessed after adding SfiI to solutions containing the plasmid and MgCl(2). On both supercoiled and relaxed DNA, the rate of loop capture across 0.4 kb was only marginally faster than over 2.0 kb or 2.4 kb. The relative strengths and rates of looping were compared to computer simulations of conformational fluctuations in DNA. The simulations concurred broadly with the experimental data, though they predicted that increasing site separations should cause a shallower decline in the equilibrium constants than was observed but a slightly steeper decline in the rates for loop capture. Possible reasons for these discrepancies are discussed.