Structure of the metal-independent restriction enzyme BfiI reveals fusion of a specific DNA-binding domain with a nonspecific nuclease

ClsC protein encoded by a stress-responsive operon in Escherichia coli functions as a trans-acting activator of RNase III

RNase III, an endoribonuclease that cleaves double-stranded RNAs (dsRNAs), significantly impacts Escherichia coli (E. coli) adaptation by regulating global RNA gene expression. YmdB from E. coli was characterized as a trans-acting regulator of RNase III. However, no protein encoded in E. coli has been characterized as an activator of RNase III. This study reports the discovery of ClsC protein, a phospholipase D (PLD) superfamily enzyme previously known as the third cardiolipin synthase (Cls) and a biofilm inhibitor in E. coli, as a novel RNase III activator. Overexpression of clsC in vivo stimulated the cleavage of RNase III-targeted lacZ fusions and antagonized the inhibition of RNase III by YmdB. Additional in vitro cleavage assays of RNase III-targeted RNAs using RNase III and ClsC confirmed this activity. Moreover, we identified multiple RNAs targeted by RNase III that are regulated dependently on cellular ClsC levels. Mechanistic investigations revealed that ClsC interacts with RNase III. Moreover, the isoleucine residue at the 466th position from the N-terminus of ClsC was identified as crucial for ClsC function. This study is the first to demonstrate that the ymdAB-clsC operon serves as an unexpected source for RNase III regulation in E. coli.

Crystal structure of Alzheimer's disease phospholipase D3 provides a molecular basis for understanding its normal and pathological functions

Human 5'-3' exonuclease PLD3, a member of the phospholipase D family of enzymes, has been validated as a therapeutic target for treating Alzheimer's disease. Here, we have determined the crystal structure of the luminal domain of the enzyme at 2.3 Å resolution, revealing a bilobal structure with a catalytic site located between the lobes. We then compared the structure with published crystal structures of other human PLD family members which revealed that a number of catalytic and lipid recognition residues, previously shown to be key for phospholipase activity, are not conserved or, are absent. This led us to test whether the enzyme is actually a phospholipase. We could not measure any phospholipase activity but the enzyme shows robust nuclease activity. Finally, we have mapped key single nucleotide polymorphisms onto the structure which reveals plausible reasons as to why they have an impact on Alzheimer's disease.

Catalytically active prokaryotic Argonautes employ phospholipase D family proteins to strengthen immunity against different genetic invaders

Prokaryotic Argonautes (pAgos) provide bacteria and archaea with immunity against plasmids and viruses. Catalytically active pAgos utilize short oligonucleotides as guides to directly cleave foreign nucleic acids, while inactive pAgos lacking catalytic residues employ auxiliary effectors, such as nonspecific nucleases, to trigger abortive infection upon detection of foreign nucleic acids. Here, we report a unique group of catalytically active pAgo proteins that frequently associate with a phospholipase D (PLD) family protein. We demonstrate that this particular system employs the catalytic center of the associated PLD protein rather than that of pAgo to restrict plasmid DNA, while interestingly, its immunity against a single-stranded DNA virus relies on the pAgo catalytic center and is enhanced by the PLD protein. We also find that this system selectively suppresses viral DNA propagation without inducing noticeable abortive infection outcomes. Moreover, the pAgo protein alone enhances gene editing, which is unexpectedly inhibited by the PLD protein. Our data highlight the ability of catalytically active pAgo proteins to employ auxiliary proteins to strengthen the targeted eradication of different genetic invaders and underline the trend of PLD nucleases to participate in host immunity.

Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4

Endolysosomal exonucleases PLD3 and PLD4 (phospholipases D3 and D4) are associated with autoinflammatory and autoimmune diseases. We report structures of these enzymes, and the molecular basis of their catalysis. The structures reveal an intra-chain dimer topology forming a basic active site at the interface. Like other PLD superfamily members, PLD3 and PLD4 carry HxKxxxxD/E motifs and participate in phosphodiester-bond cleavage. The enzymes digest ssDNA and ssRNA in a 5'-to-3' manner and are blocked by 5'-phosphorylation. We captured structures in apo, intermediate, and product states and revealed a "link-and-release" two-step catalysis. We also unexpectedly demonstrated phosphatase activity via a covalent 3-phosphohistidine intermediate. PLD4 contains an extra hydrophobic clamp that stabilizes substrate and could affect oligonucleotide substrate preference and product release. Biochemical and structural analysis of disease-associated mutants of PLD3/4 demonstrated reduced enzyme activity or thermostability and the possible basis for disease association. Furthermore, these findings provide insight into therapeutic design.

Characterization of winged helix domain fusion endonucleases as N6-methyladenine-dependent type IV restriction systems

Winged helix (wH) domains, also termed winged helix-turn-helix (wHTH) domains, are widespread in all kingdoms of life and have diverse roles. In the context of DNA binding and DNA modification sensing, some eukaryotic wH domains are known as sensors of non-methylated CpG. In contrast, the prokaryotic wH domains in DpnI and HhiV4I act as sensors of adenine methylation in the 6mApT (N6-methyladenine, 6mA, or N6mA) context. DNA-binding modes and interactions with the probed dinucleotide are vastly different in the two cases. Here, we show that the role of the wH domain as a sensor of adenine methylation is widespread in prokaryotes. We present previously uncharacterized examples of PD-(D/E)XK-wH (FcyTI, Psp4BI), PUA-wH-HNH (HtuIII), wH-GIY-YIG (Ahi29725I, Apa233I), and PLD-wH (Aba4572I, CbaI) fusion endonucleases that sense adenine methylation in the Dam+ Gm6ATC sequence contexts. Representatives of the wH domain endonuclease fusion families with the exception of the PLD-wH family could be purified, and an in vitro preference for adenine methylation in the Dam context could be demonstrated. Like most other modification-dependent restriction endonucleases (MDREs, also called type IV restriction systems), the new fusion endonucleases except those in the PD-(D/E)XK-wH family cleave close to but outside the recognition sequence. Taken together, our data illustrate the widespread combinatorial use of prokaryotic wH domains as adenine methylation readers. Other potential 6mA sensors in modified DNA are also discussed.

Structural and mechanistic insights into disease-associated endolysosomal exonucleases PLD3 and PLD4

Endolysosomal exonucleases PLD3 and PLD4 (phospholipases D3 and D4) are associated with autoinflammatory and autoimmune diseases. We report structures of these enzymes, and the molecular basis of their catalysis. The structures reveal an intra-chain dimer topology forming a basic active site at the interface. Like other PLD superfamily members, PLD3 and PLD4 carry HxKxxxxD/E motifs and participate in phosphodiester-bond cleavage. The enzymes digest ssDNA and ssRNA in a 5'-to-3' manner and are blocked by 5'-phosphorylation. We captured structures in apo, intermediate, and product states and revealed a 'link-and-release' two-step catalysis. We also unexpectedly demonstrated phosphatase activity via a covalent 3' phosphistidine intermediate. PLD4 contains an extra hydrophobic clamp that stabilizes substrate and could affect oligonucleotide substrate preference and product release. Biochemical and structural analysis of disease-associated mutants of PLD3/4 demonstrated reduced enzyme activity or thermostability and the possible basis for disease association. Furthermore, these findings provide insight into therapeutic design.

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.

Structural basis for broad anti-phage immunity by DISARM

In the evolutionary arms race against phage, bacteria have assembled a diverse arsenal of antiviral immune strategies. While the recently discovered DISARM (Defense Island System Associated with Restriction-Modification) systems can provide protection against a wide range of phage, the molecular mechanisms that underpin broad antiviral targeting but avoiding autoimmunity remain enigmatic. Here, we report cryo-EM structures of the core DISARM complex, DrmAB, both alone and in complex with an unmethylated phage DNA mimetic. These structures reveal that DrmAB core complex is autoinhibited by a trigger loop (TL) within DrmA and binding to DNA substrates containing a 5′ overhang dislodges the TL, initiating a long-range structural rearrangement for DrmAB activation. Together with structure-guided in vivo studies, our work provides insights into the mechanism of phage DNA recognition and specific activation of this widespread antiviral defense system.

DISARM (Defense Island System Associated with Restriction Modification) systems can provide bacteria with protection against a wide range of phage. Here, Bravo et al. determine cryo-EM structures of the core DISARM complex that shed light onto phage DNA recognition and activation of this widespread defense system.

Phospholipids (PLs) know-how: exploring and exploiting phospholipase D for its industrial dissemination

Owing to their numerous nutritional and bioactive functions, phospholipids (PLs), which are major components of biological membranes in all living organisms, have been widely applied as nutraceuticals, food supplements, and cosmetic ingredients. To date, PLs are extracted solely from soybean or egg yolk, despite the diverse market demands and high cost, owing to a tedious and inefficient manufacturing process. A microbial-based manufacturing process, specifically phospholipase D (PLD)-based biocatalysis and biotransformation process for PLs, has the potential to address several challenges associated with the soybean- or egg yolk-based supply chain. However, poor enzyme properties and inefficient microbial expression systems for PLD limit their wide industrial dissemination. Therefore, sourcing new enzyme variants with improved properties and developing advanced PLD expression systems are important. In the present review, we systematically summarize recent achievements and trends in the discovery, their structural properties, catalytic mechanisms, expression strategies for enhancing PLD production, and its multiple applications in the context of PLs. This review is expected to assist researchers to understand current advances in this field and provide insights for further molecular engineering efforts toward PLD-mediated bioprocessing.

Crystal Structure of a Phospholipase D from the Plant-Associated Bacteria Serratia plymuthica Strain AS9 Reveals a Unique Arrangement of Catalytic Pocket

Phospholipases D (PLDs) play important roles in different organisms and in vitro phospholipid modifications, which attract strong interests for investigation. However, the lack of PLD structural information has seriously hampered both the understanding of their structure-function relationships and the structure-based bioengineering of this enzyme. Herein, we presented the crystal structure of a PLD from the plant-associated bacteria Serratia plymuthica strain AS9 (SpPLD) at a resolution of 1.79 Å. Two classical HxKxxxxD (HKD) motifs were found in SpPLD and have shown high structural consistence with several PLDs in the same family. While comparing the structure of SpPLD with the previous resolved PLDs from the same family, several unique conformations on the C-terminus of the HKD motif were demonstrated to participate in the arrangement of the catalytic pocket of SpPLD. In SpPLD, an extented loop conformation between β9 and α9 (aa228-246) was found. Moreover, electrostatic surface potential showed that this loop region in SpPLD was positively charged while the corresponding loops in the two Streptomyces originated PLDs (PDB ID: 1F0I, 2ZE4/2ZE9) were neutral. The shortened loop between α10 and α11 (aa272-275) made the SpPLD unable to form the gate-like structure which existed specically in the two Streptomyces originated PLDs (PDB ID: 1F0I, 2ZE4/2ZE9) and functioned to stabilize the substrates. In contrast, the shortened loop conformation at this corresponding segment was more alike to several nucleases (Nuc, Zuc, mZuc, NucT) within the same family. Moreover, the loop composition between β11 and β12 was also different from the two Streptomyces originated PLDs (PDB ID: 1F0I, 2ZE4/2ZE9), which formed the entrance of the catalytic pocket and were closely related to substrate recognition. So far, SpPLD was the only structurally characterized PLD enzyme from Serratia. The structural information derived here not only helps for the understanding of the biological function of this enzyme in plant protection, but also helps for the understanding of the rational design of the mutant, with potential application in phospholipid modification.

Mechanism of DNA cleavage by the endonuclease SauUSI: a major barrier to horizontal gene transfer and antibiotic resistance in Staphylococcus aureus

Acquisition of foreign DNA by Staphylococcus aureus, including vancomycin resistance genes, is thwarted by the ATP-dependent endonuclease SauUSI. Deciphering the mechanism of action of SauUSI could unravel the reason how it singularly plays a major role in preventing horizontal gene transfer (HGT) in S. aureus. Here, we report a detailed biochemical and structural characterization of SauUSI, which reveals that in the presence of ATP, the enzyme can cleave DNA having a single or multiple target site/s. Remarkably, in the case of multiple target sites, the entire region of DNA flanked by two target sites is shred into smaller fragments by SauUSI. Crystal structure of SauUSI reveals a stable dimer held together by the nuclease domains, which are spatially arranged to hydrolyze the phosphodiester bonds of both strands of the duplex. Thus, the architecture of the dimeric SauUSI facilitates cleavage of either single-site or multi-site DNA. The structure also provides insights into the molecular basis of target recognition by SauUSI. We show that target recognition activates ATP hydrolysis by the helicase-like ATPase domain, which powers active directional movement (translocation) of SauUSI along the DNA. We propose that a pile-up of multiple translocating SauUSI molecules against a stationary SauUSI bound to a target site catalyzes random double-stranded breaks causing shredding of the DNA between two target sites. The extensive and irreparable damage of the foreign DNA by shredding makes SauUSI a potent barrier against HGT.

Crystal structure and DNA cleavage mechanism of the restriction DNA glycosylase R.CcoLI from Campylobacter coli

While most restriction enzymes catalyze the hydrolysis of phosphodiester bonds at specific nucleotide sequences in DNA, restriction enzymes of the HALFPIPE superfamily cleave N-glycosidic bonds, similar to DNA glycosylases. Apurinic/apyrimidinic (AP) sites generated by HALFPIPE superfamily proteins are cleaved by their inherent AP lyase activities, other AP endonuclease activities or heat-promoted β-elimination. Although the HALFPIPE superfamily protein R.PabI, obtained from a hyperthermophilic archaea, Pyrococcus abyssi, shows weak AP lyase activity, HALFPIPE superfamily proteins in mesophiles, such as R.CcoLI from Campylobacter coli and R. HpyAXII from Helicobacter pylori, show significant AP lyase activities. To identify the structural basis for the AP lyase activity of R.CcoLI, we determined the structure of R.CcoLI by X-ray crystallography. The structure of R.CcoLI, obtained at 2.35-Å resolution, shows that a conserved lysine residue (Lys71), which is stabilized by a characteristic β-sheet structure of R.CcoLI, protrudes into the active site. The results of mutational assays indicate that Lys71 is important for the AP lyase activity of R.CcoLI. Our results help to elucidate the mechanism by which HALFPIPE superfamily proteins from mesophiles efficiently introduce double-strand breaks to specific sites on double-stranded DNA.

Distortion of double-stranded DNA structure by the binding of the restriction DNA glycosylase R.PabI

R.PabI is a restriction DNA glycosylase that recognizes the sequence 5′-GTAC-3′ and hydrolyses the N-glycosidic bond of adenine in the recognition sequence. R.PabI drastically bends and unwinds the recognition sequence of double-stranded DNA (dsDNA) and flips the adenine and guanine bases in the recognition sequence into the catalytic and recognition sites on the protein surface. In this study, we determined the crystal structure of the R.PabI-dsDNA complex in which the dsDNA is drastically bent by the binding of R.PabI but the base pairs are not unwound. This structure is predicted to be important for the indirect readout of the recognition sequence by R.PabI. In the complex structure, wedge loops of the R.PabI dimer are inserted into the minor groove of dsDNA to stabilize the deformed dsDNA structure. A base stacking is distorted between the two wedge-inserted regions. R.PabI is predicted to utilize the distorted base stacking for the detection of the recognition sequence.

Bacterial non-specific nucleases of the phospholipase D superfamily and their biotechnological potential

Bacterial non-specific nucleases are ubiquitously distributed and involved in numerous intra- and extracellular processes. Although all nucleases share the basic chemistry for the hydrolysis of phosphodiester bonds in nucleic acid molecules, the catalysis comprises diverse modes of action, which offers great potential for versatile biotechnological applications. A major criterium for their differentiation is substrate specificity. Specific endonucleases are widely used as restriction enzymes in molecular biology approaches, whereas the main applications of non-specific nucleases (NSNs) are the removal of nucleic acids from crude extracts in industrial downstream processing and the prevention of cell clumping in microfabricated channels. In nature, the predominant role of NSNs is the acquisition of nutrient sources such as nucleotides and phosphates. The number of extensively characterized NSNs and available structures is limited. Moreover, their applicability is mostly challenged by the presence of metal chelators that impede the hydrolysis of nucleic acids in a metal ion-dependent manner. However, a few metal ion-independent NSNs that tolerate the presence of metal chelators have been characterized in recent years with none being commercially available to date. The classification and biotechnological potential of bacterial NSNs with a special focus on metal ion-independent nucleases are presented and discussed.Key Points • Bacterial phospholipases (PLD-family) exhibit nucleolytic activity. • Bacterial nucleases of the PLD-family are metal ion-independent. • NSNs can be used in downstream processing approaches.

Characterization of Single Amino Acid Variations in an EDTA-Tolerating Non-specific Nuclease from the Ice-Nucleating Bacterium Pseudomonas syringae

Non-specific nuclease (NSN) can be applied in industrial downstream processing to remove nucleic acids from crude protein extracts or in cell-sorting systems to degrade nucleic acids derived from lysed cells. PsNuc from the ice-nucleating bacterium Pseudomonas syringae has the ability to decompose double- and single-stranded DNA in linear or circular form and RNA. It is not affected by the presence of metal-ion chelators such as EDTA and tolerates several protease inhibitors and reducing agents. A multiple sequence alignment of PsNuc with closely related enzymes (97-99% identity on the protein level) within the family Pseudomonaceae revealed the presence of only six amino acid residues that are variable in putative NSN from different members of the genus Pseudomonas. Single amino acid variants were produced in recombinant form in Escherichia coli, purified, and characterized. They showed similar activity compared to PsNuc, but a single variant even displayed an improved performance with an activity of > 20,000 U/mg at 35 °C, while amino acid residues S148 and V161 were found to be essential for enzymatic functionality. These results suggest that homologous nucleases from Pseudomonaceae display high activity levels in a metal-ion-independent manner and are therefore of interest for applications in biotechnology.

Decoding Essential Amino Acid Residues in the Substrate Groove of a Non-Specific Nuclease from Pseudomonas syringae

Non-specific nucleases (NSN) are of interest for biotechnological applications, including industrial downstream processing of crude protein extracts or cell-sorting approaches in microfabricated channels. Bacterial nucleases belonging to the superfamily of phospholipase D (PLD) are featured for their ability to catalyze the hydrolysis of nucleic acids in a metal-ion-independent manner. In order to gain a deeper insight into the composition of the substrate groove of a NSN from Pseudomonas syringae, semi-rational mutagenesis based on a structure homology model was applied to identify amino acid residues on the protein’s surface adjacent to the catalytic region. A collection of 12 mutant enzymes each with a substitution to a positively charged amino acid (arginine or lysine) was produced in recombinant form and biochemically characterized. Mutations in close proximity to the catalytic region (inner ring) either dramatically impaired or completely abolished the enzymatic performance, while amino acid residues located at the border of the substrate groove (outer ring) only had limited or no effects. A K119R substitution mutant displayed a relative turnover rate of 112% compared to the original nuclease. In conclusion, the well-defined outer ring of the substrate groove is a potential target for modulation of the enzymatic performance of NSNs belonging to the PLD superfamily.

Fixed DNA Molecule Arrays for High-Throughput Single DNA-Protein Interaction Studies

The DNA Curtains assay is a recently developed experimental platform for protein-DNA interaction studies at the single-molecule level that is based on anchoring and alignment of DNA fragments. The DNA Curtains so far have been made by using chromium barriers and fluid lipid bilayer membranes, which makes such a specialized assay technically challenging and relatively unstable. Herein, we report on an alternative strategy for DNA arraying for analysis of individual DNA-protein interactions. It relies on stable DNA tethering onto nanopatterned protein templates via high affinity molecular recognition. We describe fabrication of streptavidin templates (line features as narrow as 200 nm) onto modified glass coverslips by combining surface chemistry, atomic force microscopy (AFM), and soft lithography techniques with affinity-driven assembly. We have employed such chips for arraying single- and double-tethered DNA strands, and we characterized the obtained molecular architecture: we evaluated the structural characteristics and specific versus nonspecific binding of fluorescence-labeled DNA using AFM and total internal reflection fluorescence microscopy. We demonstrate the feasibility of our DNA molecule arrays for short single-tethered as well as for lambda single- and double-tethered DNA. The latter type of arrays proved very suitable for localization of single DNA-protein interactions employing restriction endonucleases. The presented molecular architecture and facile method of fabrication of our nanoscale platform does not require clean room equipment, and it offers advanced functional studies of DNA machineries and the development of future nanodevices.

Comparative analysis of two non-specific nucleases of the phospholipase D family from the plant pathogen competitor bacterium Pantoea agglomerans

Bacterial non-specific nucleases of the phospholipase D family are widely distributed among the members of the Enterobacteriaceae. Each genome mainly contains a single copy of a gene encoding a phospholipase D family protein. However, two distantly related isozymes (< 40% identity at the protein level) were identified by BLAST-analyses in the plant pathogenic competitor enterobacterium Pantoea agglomerans. The two nucleases PaNuc-1 and PaNuc-2 were produced in Escherichia coli. Identical gene constructs and expression conditions resulted in the production of PaNuc-1 in soluble form, while PaNuc-2 remained insoluble in inclusion bodies. PaNuc-2 was refolded and both proteins were purified by a combination of affinity and ion exchange chromatography. Proteolytic removal of the HIS-tag allowed the characterization of pure and mature tag-less proteins. Enzymatic properties of both isozymes revealed that they are non-specific nucleases, displaying activities against RNA, single- and double-stranded genomic DNA as well as circular plasmids. However, their biochemical activity profiles were clearly different, with PaNuc-1 being optimally active at 70 °C and pH 7.0, while PaNuc-2 was most active at 45 °C and pH 7.0. The enzymes retained > 90% nuclease activity at EDTA concentrations of 4 mM (PaNuc-2) and 20 mM (PaNuc-1), respectively. Different enzymatic properties suggest that the roles of PaNuc-1 and PaNuc-2 differ in the cell and might be the result of functional diversification after an ancient gene duplication event took place. The fact that both enzymes could be easily produced in recombinant form and their tolerance against metal ion chelators in combination with a broad substrate promiscuity might pave the way to versatile biotechnological applications.

The crystal structure of the Helicobacter pylori LlaJI.R1 N-terminal domain provides a model for site-specific DNA binding

Restriction modification systems consist of an endonuclease that cleaves foreign DNA site-specifically and an associated methyltransferase that protects the corresponding target site in the host genome. Modification-dependent restriction systems, in contrast, specifically recognize and cleave methylated and/or glucosylated DNA. The LlaJI restriction system contains two 5-methylcytosine (5mC) methyltransferases (LlaJI.M1 and LlaJI.M2) and two restriction proteins (LlaJI.R1 and LlaJI.R2). LlaJI.R1 and LlaJI.R2 are homologs of McrB and McrC, respectively, which in Escherichia coli function together as a modification-dependent restriction complex specific for 5mC-containing DNA. Lactococcus lactis LlaJI.R1 binds DNA site-specifically, suggesting that the LlaJI system uses a different mode of substrate recognition. Here we present the structure of the N-terminal DNA-binding domain of Helicobacter pylori LlaJI.R1 at 1.97-Å resolution, which adopts a B3 domain fold. Structural comparison to B3 domains in plant transcription factors and other restriction enzymes identifies key recognition motifs responsible for site-specific DNA binding. Moreover, biochemistry and structural modeling provide a rationale for how H. pylori LlaJI.R1 may bind a target site that differs from the 5-bp sequence recognized by other LlaJI homologs and identify residues critical for this recognition activity. These findings underscore the inherent structural plasticity of B3 domains, allowing recognition of a variety of substrates using the same structural core.

DISARM is a widespread bacterial defence system with broad anti-phage activities

The evolutionary pressure imposed by phage predation on bacteria and archaea has resulted in the development of effective anti-phage defence mechanisms, including restriction-modification and CRISPR-Cas systems. Here, we report on a new defence system, DISARM (defence island system associated with restriction-modification), which is widespread in bacteria and archaea. DISARM is composed of five genes, including a DNA methylase and four other genes annotated as a helicase domain, a phospholipase D (PLD) domain, a DUF1998 domain and a gene of unknown function. Engineering the Bacillus paralicheniformis 9945a DISARM system into Bacillus subtilis has rendered the engineered bacteria protected against phages from all three major families of tailed double-stranded DNA phages. Using a series of gene deletions, we show that four of the five genes are essential for DISARM-mediated defence, with the fifth (PLD) being redundant for defence against some of the phages. We further show that DISARM restricts incoming phage DNA and that the B. paralicheniformis DISARM methylase modifies host CCWGG motifs as a marker of self DNA akin to restriction-modification systems. Our results suggest that DISARM is a new type of multi-gene restriction-modification module, expanding the arsenal of defence systems known to be at the disposal of prokaryotes against their viruses.

Tetrameric structure of the restriction DNA glycosylase R.PabI in complex with nonspecific double-stranded DNA

R.PabI is a type II restriction enzyme that recognizes the 5′-GTAC-3′ sequence and belongs to the HALFPIPE superfamily. Although most restriction enzymes cleave phosphodiester bonds at specific sites by hydrolysis, R.PabI flips the guanine and adenine bases of the recognition sequence out of the DNA helix and hydrolyzes the N-glycosidic bond of the flipped adenine in a similar manner to DNA glycosylases. In this study, we determined the structure of R.PabI in complex with double-stranded DNA without the R.PabI recognition sequence by X-ray crystallography. The 1.9 Å resolution structure of the complex showed that R.PabI forms a tetrameric structure to sandwich the double-stranded DNA and the tetrameric structure is stabilized by four salt bridges. DNA binding and DNA glycosylase assays of the R.PabI mutants showed that the residues that form the salt bridges (R70 and D71) are essential for R.PabI to find the recognition sequence from the sea of nonspecific sequences. R.PabI is predicted to utilize the tetrameric structure to bind nonspecific double-stranded DNA weakly and slide along it to find the recognition sequence.

A sequence-specific DNA glycosylase mediates restriction-modification in Pyrococcus abyssi

Restriction-modification systems consist of genes that encode a restriction enzyme and a cognate methyltransferase. Thus far, it was believed that restriction enzymes are sequence-specific endonucleases that introduce double-strand breaks at specific sites by catalysing the cleavages of phosphodiester bonds. Here we report that based on the crystal structure and enzymatic activity, one of the restriction enzymes, R.PabI, is not an endonuclease but a sequence-specific adenine DNA glycosylase. The structure of the R.PabI-DNA complex shows that R.PabI unwinds DNA at a 5'-GTAC-3' site and flips the guanine and adenine bases out of the DNA helix to recognize the sequence. R.PabI catalyses the hydrolysis of the N-glycosidic bond between the adenine base and the sugar in the DNA and produces two opposing apurinic/apyrimidinic (AP) sites. The opposing AP sites are cleaved by heat-promoted β elimination and/or by endogenous AP endonucleases of host cells to introduce a double-strand break.

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.

The evolutionary journey of Argonaute proteins

Argonaute proteins are conserved throughout all domains of life. Recently characterized prokaryotic Argonaute proteins (pAgos) participate in host defense by DNA interference, whereas eukaryotic Argonaute proteins (eAgos) control a wide range of processes by RNA interference. Here we review molecular mechanisms of guide and target binding by Argonaute proteins, and describe how the conformational changes induced by target binding lead to target cleavage. On the basis of structural comparisons and phylogenetic analyses of pAgos and eAgos, we reconstruct the evolutionary journey of the Argonaute proteins through the three domains of life and discuss how different structural features of pAgos and eAgos relate to their distinct physiological roles.

DNA cleavage by CgII and NgoAVII requires interaction between N- and R-proteins and extensive nucleotide hydrolysis

The stress-sensitive restriction-modification (RM) system CglI from Corynebacterium glutamicum and the homologous NgoAVII RM system from Neisseria gonorrhoeae FA1090 are composed of three genes: a DNA methyltransferase (M.CglI and M.NgoAVII), a putative restriction endonuclease (R.CglI and R.NgoAVII, or R-proteins) and a predicted DEAD-family helicase/ATPase (N.CglI and N.NgoAVII or N-proteins). Here we report a biochemical characterization of the R- and N-proteins. Size-exclusion chromatography and SAXS experiments reveal that the isolated R.CglI, R.NgoAVII and N.CglI proteins form homodimers, while N.NgoAVII is a monomer in solution. Moreover, the R.CglI and N.CglI proteins assemble in a complex with R2N2 stoichiometry. Next, we show that N-proteins have ATPase activity that is dependent on double-stranded DNA and is stimulated by the R-proteins. Functional ATPase activity and extensive ATP hydrolysis (∼170 ATP/s/monomer) are required for site-specific DNA cleavage by R-proteins. We show that ATP-dependent DNA cleavage by R-proteins occurs at fixed positions (6-7 nucleotides) downstream of the asymmetric recognition sequence 5'-GCCGC-3'. Despite similarities to both Type I and II restriction endonucleases, the CglI and NgoAVII enzymes may employ a unique catalytic mechanism for DNA cleavage.

Crystal structure of the R-protein of the multisubunit ATP-dependent restriction endonuclease NgoAVII

The restriction endonuclease (REase) NgoAVII is composed of two proteins, R.NgoAVII and N.NgoAVII, and shares features of both Type II restriction enzymes and Type I/III ATP-dependent restriction enzymes (see accompanying paper Zaremba et al., 2014). Here we present crystal structures of the R.NgoAVII apo-protein and the R.NgoAVII C-terminal domain bound to a specific DNA. R.NgoAVII is composed of two domains: an N-terminal nucleolytic PLD domain; and a C-terminal B3-like DNA-binding domain identified previously in BfiI and EcoRII REases, and in plant transcription factors. Structural comparison of the B3-like domains of R.NgoAVII, EcoRII, BfiI and the plant transcription factors revealed a conserved DNA-binding surface comprised of N- and C-arms that together grip the DNA. The C-arms of R.NgoAVII, EcoRII, BfiI and plant B3 domains are similar in size, but the R.NgoAVII N-arm which makes the majority of the contacts to the target site is much longer. The overall structures of R.NgoAVII and BfiI are similar; however, whilst BfiI has stand-alone catalytic activity, R.NgoAVII requires an auxiliary cognate N.NgoAVII protein and ATP hydrolysis in order to cleave DNA at the target site. The structures we present will help formulate future experiments to explore the molecular mechanisms of intersubunit crosstalk that control DNA cleavage by R.NgoAVII and related endonucleases.

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.

Cofactor analogue-induced chemical reactivation of endonuclease activity in a DNA cleavage/methylation deficient TspGWI N₄₇₃A variant in the NPPY motif

We reported previously that TspGWI, a prototype enzyme of a new Thermus sp. family of restriction endonucleases-methyltransferases (REases-MTases), undergoes the novel phenomenon of sinefungin (SIN)-caused specificity transition. Here we investigated mutant TspGWI N₄₇₃A, containing a single amino acid (aa) substitution in the NPPY motif of the MTase. Even though the aa substitution is located within the MTase polypeptide segment, DNA cleavage and modification are almost completely abolished, indicating that the REase and MTase are intertwined. Remarkably, the TspGWI N₄₇₃A REase functionality can be completely reconstituted by the addition of SIN. We hypothesize that SIN binds specifically to the enzyme and restores the DNA cleavage-competent protein tertiary structure. This indicates the significant role of allosteric effectors in DNA cleavage in Thermus sp. enzymes. This is the first case of REase mutation suppression by an S-adenosylmethionine (SAM) cofactor analogue. Moreover, the TspGWI N₄₇₃A clone strongly affects E. coli division control, acting as a ‘selfish gene’. The mutant lacks the competing MTase activity and therefore might be useful for applications in DNA manipulation. Here we present a case study of a novel strategy for REase activity/specificity alteration by a single aa substitution, based on the bioinformatic analysis of active motif locations, combining (a) aa sequence engineering (b) the alteration of protein enzymatic properties, and (c) the use of cofactor–analogue cleavage reconstitution and stimulation.

Structural insight into the specificity of the B3 DNA-binding domains provided by the co-crystal structure of the C-terminal fragment of BfiI restriction enzyme

The B3 DNA-binding domains (DBDs) of plant transcription factors (TF) and DBDs of EcoRII and BfiI restriction endonucleases (EcoRII-N and BfiI-C) share a common structural fold, classified as the DNA-binding pseudobarrel. The B3 DBDs in the plant TFs recognize a diverse set of target sequences. The only available co-crystal structure of the B3-like DBD is that of EcoRII-N (recognition sequence 5'-CCTGG-3'). In order to understand the structural and molecular mechanisms of specificity of B3 DBDs, we have solved the crystal structure of BfiI-C (recognition sequence 5'-ACTGGG-3') complexed with 12-bp cognate oligoduplex. Structural comparison of BfiI-C-DNA and EcoRII-N-DNA complexes reveals a conserved DNA-binding mode and a conserved pattern of interactions with the phosphodiester backbone. The determinants of the target specificity are located in the loops that emanate from the conserved structural core. The BfiI-C-DNA structure presented here expands a range of templates for modeling of the DNA-bound complexes of the B3 family of plant TFs.

DNA-binding domains of plant-specific transcription factors: structure, function, and evolution

The families of the plant-specific transcription factors (TFs) are defined by their characteristic DNA-binding domains (DBDs), such as AP2/ERF, B3, NAC, SBP, and WRKY. Recently, three-dimensional structures of the DBDs, including those in complexes with DNA, were determined by NMR spectroscopy and X-ray crystallography. In this review we summarize the functional and evolutionary implications arising from structure analyses. The unexpected structural similarity between B3 and the noncatalytic DBD of the restriction endonuclease EcoRII allowed us to build structural models of the B3/DNA complex. Most of the DBDs of plant-specific TFs are likely to have originated from endonucleases associated with transposable elements. After the DBDs have been established in unicellular eukaryotes, they experienced extensive plant-specific expansion, by acquiring new functions.

Phospholipase D as a catalyst: application in phospholipid synthesis, molecular structure and protein engineering

Phospholipase D (PLD) is a useful enzyme for its transphosphatidylation activity, which enables the enzymatic synthesis of various phospholipids (PLs). Many reports exist on PLD-mediated synthesis of natural and tailor-made PLs with functional head groups, from easily available lecithin or phosphatidylcholine. Early studies on PLD-mediated synthesis mainly employed enzymes of plant origin, which were later supplanted by ones from microorganisms, especially actinomycetes. Many PLDs are members of the PLD superfamily, having one or two copies of a signature sequence, HxKxxxxD or HKD motif, in the primary structures. PLD superfamily members share a common core structure, and thereby, a common catalytic mechanism. The catalysis proceeds via two-step reaction with the formation of phosphatidyl-enzyme intermediate. Both of the two catalytic His residues are critical in the reaction course, where one acts as a nucleophile, while the other functions as a general acid/base. PLD is being engineered to improve its activity and stability, alter head group specificity and further identify catalytically important residues. Since the knowledge on PLD enzymology is constantly expanding, this review focuses on recent advances in the field, regarding PLD-catalyzed synthesis of bioactive PLs, deeper understanding of substrate recognition and binding mechanism, altering substrate specificity, and improving thermostability. We introduced some of our recent results in combination with existing facts to further deepen the story on the nature of this useful enzyme.

Mechanisms of catalytic action and chemical modifications of endonucleases WEN1 and WEN2 from wheat seedlings

Hydrolysis of DNA catalyzed by wheat endonucleases WEN1 and WEN2 is pronouncedly processive. A correlation has been revealed between appearance of new products of DNA hydrolysis with different length and conformational changes in the enzymes. The first conformational conversion of the endonucleases is associated with appearance of large fragments of DNA hydrolysis with length longer than 500 bp, and the second conversion is associated with formation of oligonucleotides with length of 120-140 bp, and the third conversion is associated with formation of short oligonucleotides and mononucleotides. Formation of the DNA-enzyme complex is accompanied by appearance of fluorescence at λ = 410-440 nm. The intensity, positions, and numbers of maximums of the fluorescence spectra of DNA-WEN1 and DNA-WEN2 complexes are different and depend on the methylation status of the DNA and on the presence of Mg2+. The endonucleases hydrolyze DNA by two mechanisms: one is metal-independent, and the other depends on one or two Mg2+ ions. One Mg2+ ion is located inside the catalytic center of WEN1, whereas the WEN2 center contains two Mg2+ ions. The first (site-specific) stage of DNA hydrolysis does not depend on Mg2+. Mg2+ ions evoke changes in the site specificity of the endonuclease action (WEN1) and abolish their ability to recognize the methylation status of DNA. Products of DNA hydrolysis by endonucleases WEN1 and WEN2 in the presence of Mg2+ are similar in length (120-140 bp). The endonucleases have at least two centers (domains) - catalytic and substrate-binding. Two histidine and apparently two lysine plus two dicarboxylic amino acid residues are present inside the catalytic center of WEN1. The catalytic center of WEN2 contains at least one histidine residue and apparently two residues of aspartic or glutamic acid, which are involved in coordination of the metal ions. The catalytic centers of WEN1 and WEN2 seem to be formed, respectively, by HD/E(D/EK)KH and HD/ED/E amino acid residues. The site-specificity of the endonuclease action is due to the DNA-binding domain. This domain contains dicarboxylic amino acid residues, which seem to be responsible for sensitivity of the enzymes to the methylation status of DNA. The hydroxyl groups of tyrosine residues in the enzymes also seem to contribute to recognizing methylated bases in DNA.

The Arabidopsis B3 domain protein VERNALIZATION1 (VRN1) is involved in processes essential for development, with structural and mutational studies revealing its DNA-binding surface

The B3 DNA-binding domain is a plant-specific domain found throughout the plant kingdom from the alga Chlamydomonas to grasses and flowering plants. Over 100 B3 domain-containing proteins are found in the model plant Arabidopsis thaliana, and one of these is critical for accelerating flowering in response to prolonged cold treatment, an epigenetic process called vernalization. Despite the specific phenotype of genetic vrn1 mutants, the VERNALIZATION1 (VRN1) protein localizes throughout the nucleus and shows sequence-nonspecific binding in vitro. In this work, we used a dominant repressor tag that overcomes genetic redundancy to show that VRN1 is involved in processes beyond vernalization that are essential for Arabidopsis development. To understand its sequence-nonspecific binding, we crystallized VRN1(208-341) and solved its crystal structure to 1.6 Å resolution using selenium/single-wavelength anomalous diffraction methods. The crystallized construct comprises the second VRN1 B3 domain and a preceding region conserved among VRN1 orthologs but absent in other B3 domains. We established the DNA-binding face using NMR and then mutated positively charged residues on this surface with a series of 16 Ala and Glu substitutions, ensuring that the protein fold was not disturbed using heteronuclear single quantum correlation NMR spectra. The triple mutant R249E/R289E/R296E was almost completely incapable of DNA binding in vitro. Thus, we have revealed that although VRN1 is sequence-nonspecific in DNA binding, it has a defined DNA-binding surface.

Structure and cleavage activity of the tetrameric MspJI DNA modification-dependent restriction endonuclease

The MspJI modification-dependent restriction endonuclease recognizes 5-methylcytosine or 5-hydroxymethylcytosine in the context of CNN(G/A) and cleaves both strands at fixed distances (N₁₂/N₁₆) away from the modified cytosine at the 3′-side. We determined the crystal structure of MspJI of Mycobacterium sp. JLS at 2.05-Å resolution. Each protein monomer harbors two domains: an N-terminal DNA-binding domain and a C-terminal endonuclease. The N-terminal domain is structurally similar to that of the eukaryotic SET and RING-associated domain, which is known to bind to a hemi-methylated CpG dinucleotide. Four protein monomers are found in the crystallographic asymmetric unit. Analytical gel-filtration and ultracentrifugation measurements confirm that the protein exists as a tetramer in solution. Two monomers form a back-to-back dimer mediated by their C-terminal endonuclease domains. Two back-to-back dimers interact to generate a tetramer with two double-stranded DNA cleavage modules. Each cleavage module contains two active sites facing each other, enabling double-strand DNA cuts. Biochemical, mutagenesis and structural characterization suggest three different monomers of the tetramer may be involved respectively in binding the modified cytosine, making the first proximal N₁₂ cleavage in the same strand and then the second distal N₁₆ cleavage in the opposite strand. Both cleavage events require binding of at least a second recognition site either in cis or in trans.

Crystal structure and mechanism of action of the N6-methyladenine-dependent type IIM restriction endonuclease R.DpnI

DNA methylation-dependent restriction enzymes have many applications in genetic engineering and in the analysis of the epigenetic state of eukaryotic genomes. Nevertheless, high-resolution structures have not yet been reported, and therefore mechanisms of DNA methylation-dependent cleavage are not understood. Here, we present a biochemical analysis and high-resolution DNA co-crystal structure of the N⁶-methyladenine (m6A)-dependent restriction enzyme R.DpnI. Our data show that R.DpnI consists of an N-terminal catalytic PD-(D/E)XK domain and a C-terminal winged helix (wH) domain. Surprisingly, both domains bind DNA in a sequence- and methylation-sensitive manner. The crystal contains R.DpnI with fully methylated target DNA bound to the wH domain, but distant from the catalytic domain. Independent readout of DNA sequence and methylation by the two domains might contribute to R.DpnI specificity or could help the monomeric enzyme to cut the second strand after introducing a nick.

Characterization of LlaKI, a New Metal Ion-Independent Restriction Endonuclease from Lactococcus lactis KLDS4

Requirement of divalent cations for DNA cleavage is a general feature of type II restriction enzymes with the exception of few members of this group. A new type II restriction endonuclease has been partially purified from Lactococcus lactis KLDS4. The enzyme was denoted as LlaKI and showed to recognize and cleave the same site as FokI. The enzyme displayed a denatured molecular weight of 50 kDa and behaved as a dimer in solution as evidenced by the size exclusion chromatography. To investigate the role of divalent cations in DNA cleavage by LlaKI, digestion reactions were carried out at different Mg(2+), Mn(2+), and Ca(2+) concentrations. Unlike most of type II restriction endonucleases, LlaKI did not require divalent metal ions to cleave DNA and is one of the few metal-independent restriction endonucleases found in bacteria. The enzyme showed near-maximal levels of activity in 10 mM Tris-HCl pH 7.9, 50 mM NaCl, 10 mM MgCl2, and 1 mM dithiothreitol at 30°C. The presence of DNA modification was also determined and was correlated with the correspondent restriction enzyme.

Characterization of type II and III restriction-modification systems from Bacillus cereus strains ATCC 10987 and ATCC 14579

The genomes of two Bacillus cereus strains (ATCC 10987 and ATCC 14579) have been sequenced. Here, we report the specificities of type II/III restriction (R) and modification (M) enzymes. Found in the ATCC 10987 strain, BceSI is a restriction endonuclease (REase) with the recognition and cut site CGAAG 24-25/27-28. BceSII is an isoschizomer of AvaII (G/GWCC). BceSIII cleaves at ACGGC 12/14. The BceSIII C terminus resembles the catalytic domains of AlwI, MlyI, and Nt.BstNBI. BceSIV is composed of two subunits and cleaves on both sides of GCWGC. BceSIV activity is strongly stimulated by the addition of cofactor ATP or GTP. The large subunit (R1) of BceSIV contains conserved motifs of NTPases and DNA helicases. The R1 subunit has no endonuclease activity by itself; it strongly stimulates REase activity when in complex with the R2 subunit. BceSIV was demonstrated to hydrolyze GTP and ATP in vitro. BceSIV is similar to CglI (GCSGC), and homologs of R1 are found in 11 sequenced bacterial genomes, where they are paired with specificity subunits. In addition, homologs of the BceSIV R1-R2 fusion are found in many sequenced microbial genomes. An orphan methylase, M.BceSV, was found to modify GCNGC, GGCC, CCGG, GGNNCC, and GCGC sites. A ParB-methylase fusion protein appears to nick DNA nonspecifically. The ATCC 14579 genome encodes an active enzyme Bce14579I (GCWGC). BceSIV and Bce14579I belong to the phospholipase D (PLD) family of endonucleases that are widely distributed among Bacteria and Archaea. A survey of type II and III restriction-modification (R-M) system genes is presented from sequenced B. cereus, Bacillus anthracis, and Bacillus thuringiensis strains.

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.

Characterization and crystal structure of the type IIG restriction endonuclease RM.BpuSI

A type IIG restriction endonuclease, RM.BpuSI from Bacillus pumilus, has been characterized and its X-ray crystal structure determined at 2.35Å resolution. The enzyme is comprised of an array of 5-folded domains that couple the enzyme's N-terminal endonuclease domain to its C-terminal target recognition and methylation activities. The REase domain contains a PD-x₁₅-ExK motif, is closely superimposable against the FokI endonuclease domain, and coordinates a single metal ion. A helical bundle domain connects the endonuclease and methyltransferase (MTase) domains. The MTase domain is similar to the N6-adenine MTase M.TaqI, while the target recognition domain (TRD or specificity domain) resembles a truncated S subunit of Type I R–M system. A final structural domain, that may form additional DNA contacts, interrupts the TRD. DNA binding and cleavage must involve large movements of the endonuclease and TRD domains, that are probably tightly coordinated and coupled to target site methylation status.

Hpy188I-DNA pre- and post-cleavage complexes--snapshots of the GIY-YIG nuclease mediated catalysis

The GIY-YIG nuclease domain is present in all kingdoms of life and has diverse functions. It is found in the eukaryotic flap endonuclease and Holliday junction resolvase Slx1–Slx4, the prokaryotic nucleotide excision repair proteins UvrC and Cho, and in proteins of ‘selfish’ genetic elements. Here we present the structures of the ternary pre- and post-cleavage complexes of the type II GIY-YIG restriction endonuclease Hpy188I with DNA and a surrogate or catalytic metal ion, respectively. Our structures suggest that GIY-YIG nucleases catalyze DNA hydrolysis by a single substitution reaction. They are consistent with a previous proposal that a tyrosine residue (which we expect to occur in its phenolate form) acts as a general base for the attacking water molecule. In contrast to the earlier proposal, our data identify the general base with the GIY and not the YIG tyrosine. A conserved glutamate residue (Glu149 provided in trans in Hpy188I) anchors a single metal cation in the active site. This metal ion contacts the phosphate proS oxygen atom and the leaving group 3′-oxygen atom, presumably to facilitate its departure. Taken together, our data reveal striking analogy in the absence of homology between GIY-YIG and ββα-Me nucleases.

Nucleases: diversity of structure, function and mechanism

Nucleases cleave the phosphodiester bonds of nucleic acids and may be endo or exo, DNase or RNase, topoisomerases, recombinases, ribozymes, or RNA splicing enzymes. In this review, I survey nuclease activities with known structures and catalytic machinery and classify them by reaction mechanism and metal-ion dependence and by their biological function ranging from DNA replication, recombination, repair, RNA maturation, processing, interference, to defense, nutrient regeneration or cell death. Several general principles emerge from this analysis. There is little correlation between catalytic mechanism and biological function. A single catalytic mechanism can be adapted in a variety of reactions and biological pathways. Conversely, a single biological process can often be accomplished by multiple tertiary and quaternary folds and by more than one catalytic mechanism. Two-metal-ion-dependent nucleases comprise the largest number of different tertiary folds and mediate the most diverse set of biological functions. Metal-ion-dependent cleavage is exclusively associated with exonucleases producing mononucleotides and endonucleases that cleave double- or single-stranded substrates in helical and base-stacked conformations. All metal-ion-independent RNases generate 2',3'-cyclic phosphate products, and all metal-ion-independent DNases form phospho-protein intermediates. I also find several previously unnoted relationships between different nucleases and shared catalytic configurations.

Unusual target site disruption by the rare-cutting HNH restriction endonuclease PacI

The crystal structure of the rare-cutting HNH restriction endonuclease PacI in complex with its eight-base-pair target recognition sequence 5'-TTAATTAA-3' has been determined to 1.9 A resolution. The enzyme forms an extended homodimer, with each subunit containing two zinc-bound motifs surrounding a betabetaalpha-metal catalytic site. The latter is unusual in that a tyrosine residue likely initiates strand cleavage. PacI dramatically distorts its target sequence from Watson-Crick duplex DNA base pairing, with every base separated from its original partner. Two bases on each strand are unpaired, four are engaged in noncanonical A:A and T:T base pairs, and the remaining two bases are matched with new Watson-Crick partners. This represents a highly unusual DNA binding mechanism for a restriction endonuclease, and implies that initial recognition of the target site might involve significantly different contacts from those visualized in the DNA-bound cocrystal structures.

Characterization of nucleases involved in seedling development of cauliflower

The ability of cells to control the degradation of their own DNA is a common feature of most living organisms. In plants, extensive hydrolysis of nuclear DNA occurs during different forms of programmed cell death (PCD). In addition to the removal of unwanted cells, the PCD process allows for the remobilization of cellular constituents, including the products of DNA hydrolysis. Although programmed cell death occurs widely during normal development and plant defense responses to pathogens, only one class of deoxyribonucleases, the S1 type, involved in these processes, has been well characterized. Using DNA-SDS-PAGE, we identified the activities of 14 deoxyribonucleases expressed in different organs of cauliflower seeds, seedlings and the flower head. These enzymes represent several classes based on their substrate specificity and ion dependency. In addition to four Zn(2+)-dependent enzymes, we identified five Ca(2+)-dependent, two Mg(2+)-dependent, three Ca(2+)/Mg(2+)-dependent and one nuclease whose activities seem to be independent of any divalent cations. We also identified a set of DNases whose expression seems to be common for different organs and different stages of development, as well as a few highly tissue-specific nucleases. Expression of three nucleases was inducible by drought stress and hydrogen peroxide.

Natural and engineered nicking endonucleases--from cleavage mechanism to engineering of strand-specificity

Restriction endonucleases (REases) are highly specific DNA scissors that have facilitated the development of modern molecular biology. Intensive studies of double strand (ds) cleavage activity of Type IIP REases, which recognize 4–8 bp palindromic sequences, have revealed a variety of mechanisms of molecular recognition and catalysis. Less well-studied are REases which cleave only one of the strands of dsDNA, creating a nick instead of a ds break. Naturally occurring nicking endonucleases (NEases) range from frequent cutters such as Nt.CviPII (^CCD; ^ denotes the cleavage site) to rare-cutting homing endonucleases (HEases) such as I-HmuI. In addition to these bona fida NEases, individual subunits of some heterodimeric Type IIS REases have recently been shown to be natural NEases. The discovery and characterization of more REases that recognize asymmetric sequences, particularly Types IIS and IIA REases, has revealed recognition and cleavage mechanisms drastically different from the canonical Type IIP mechanisms, and has allowed researchers to engineer highly strand-specific NEases. Monomeric LAGLIDADG HEases use two separate catalytic sites for cleavage. Exploitation of this characteristic has also resulted in useful nicking HEases. This review aims at providing an overview of the cleavage mechanisms of Types IIS and IIA REases and LAGLIDADG HEases, the engineering of their nicking variants, and the applications of NEases and nicking HEases.

Structure and activity of the metal-independent fructose-1,6-bisphosphatase YK23 from Saccharomyces cerevisiae

Fructose-1,6-bisphosphatase (FBPase), a key enzyme of gluconeogenesis and photosynthetic CO(2) fixation, catalyzes the hydrolysis of fructose 1,6-bisphosphate (FBP) to produce fructose 6-phosphate, an important precursor in various biosynthetic pathways. All known FBPases are metal-dependent enzymes, which are classified into five different classes based on their amino acid sequences. Eukaryotes are known to contain only the type-I FBPases, whereas all five types exist in various combinations in prokaryotes. Here we demonstrate that the uncharacterized protein YK23 from Saccharomyces cerevisiae efficiently hydrolyzes FBP in a metal-independent reaction. YK23 is a member of the histidine phosphatase (phosphoglyceromutase) superfamily with homologues found in all organisms. The crystal structure of the YK23 apo-form was solved at 1.75-A resolution and revealed the core domain with the alpha/beta/alpha-fold covered by two small cap domains. Two liganded structures of this protein show the presence of two phosphate molecules (an inhibitor) or FBP (a substrate) bound to the active site. FBP is bound in its linear, open conformation with the cleavable C1-phosphate positioned deep in the active site. Alanine replacement mutagenesis of YK23 identified six conserved residues absolutely required for activity and suggested that His(13) and Glu(99) are the primary catalytic residues. Thus, YK23 represents the first family of metal-independent FBPases and a second FBPase family in eukaryotes.

A novel mechanism for the scission of double-stranded DNA: BfiI cuts both 3'-5' and 5'-3' strands by rotating a single active site

Metal-dependent nucleases that generate double-strand breaks in DNA often possess two symmetrically-equivalent subunits, arranged so that the active sites from each subunit act on opposite DNA strands. Restriction endonuclease BfiI belongs to the phospholipase D (PLD) superfamily and does not require metal ions for DNA cleavage. It exists as a dimer but has at its subunit interface a single active site that acts sequentially on both DNA strands. The active site contains two identical histidines related by 2-fold symmetry, one from each subunit. This symmetrical arrangement raises two questions: first, what is the role and the contribution to catalysis of each His residue; secondly, how does a nuclease with a single active site cut two DNA strands of opposite polarities to generate a double-strand break. In this study, the roles of active-site histidines in catalysis were dissected by analysing heterodimeric variants of BfiI lacking the histidine in one subunit. These variants revealed a novel mechanism for the scission of double-stranded DNA, one that requires a single active site to not only switch between strands but also to switch its orientation on the DNA.

Engineering Nt.BtsCI and Nb.BtsCI nicking enzymes and applications in generating long overhangs

Type IIS restriction endonuclease BtsCI (GGATG 2/0) is a neoschizomer of FokI (GGATG 9/13) and cleaves closer to the recognition sequence. Although M.BtsCI shows 62% amino acid sequence identity to M.FokI, BtsCI and FokI restriction endonucleases do not share significant amino acid sequence similarity. BtsCI belongs to a group of Type IIS restriction endonucleases, BsmI, Mva1269I and BsrI, that carry two different catalytic sites in a single polypeptide. By inactivating one of the catalytic sites through mutagenesis, we have generated nicking variants of BtsCI that specifically nick the bottom-strand or the top-strand of the target site. By treating target DNA sequentially with the appropriate combinations of FokI and BtsCI nicking variants, we are able to generate long overhangs suitable for fluorescent labeling through end-filling or other techniques based on annealing of complementary DNA sequences.