Structural basis for sequence-dependent DNA cleavage by nonspecific endonucleases

Hydrolytic Mechanism of a Metalloenzyme Is Modified by the Nature of the Coordinated Metal Ion

The nuclease domain of colicin E7 cleaves double-strand DNA non-specifically. Zn²⁺ ion was shown to be coordinated by the purified NColE7 as its native metal ion. Here, we study the structural and catalytic aspects of the interaction with Ni²⁺, Cu²⁺ and Cd²⁺ non-endogenous metal ions and the consequences of their competition with Zn²⁺ ions, using circular dichroism spectroscopy and intact protein mass spectrometry. An R447G mutant exerting decreased activity allowed for the detection of nuclease action against pUC119 plasmid DNA via agarose gel electrophoresis in the presence of comparable metal ion concentrations. It was shown that all of the added metal ions could bind to the apoprotein, resulting in a minor secondary structure change, but drastically shifting the charge distribution of the protein. Zn²⁺ ions could not be replaced by Ni²⁺, Cu²⁺ and Cd²⁺. The nuclease activity of the Ni²⁺-bound enzyme was extremely high in comparison with the other metal-bound forms, and could not be inhibited by the excess of Ni²⁺ ions. At the same time, this activity was significantly decreased in the presence of equivalent Zn²⁺, independent of the order of addition of each component of the mixture. We concluded that the Ni²⁺ ions promoted the DNA cleavage of the enzyme through a more efficient mechanism than the native Zn²⁺ ions, as they directly generate the nucleophilic OH^- ion.

HK97 gp74 Possesses an α-Helical Insertion in the ββα Fold That Affects Its Metal Binding, cos Site Digestion, and In Vivo Activities

The last gene in the genome of the bacteriophage HK97 encodes gp74, an HNH endonuclease. HNH motifs contain two conserved His residues and an invariant Asn residue, and they adopt a ββα structure. gp74 is essential for phage head morphogenesis, likely because gp74 enhances the specific endonuclease activity of the HK97 terminase complex. Notably, the ability of gp74 to enhance the terminase-mediated cleavage of the phage cos site requires an intact HNH motif in gp74. Mutation of H82, the conserved metal-binding His residue in the HNH motif, to Ala abrogates gp74-mediated stimulation of terminase activity. Here, we present nuclear magnetic resonance (NMR) studies demonstrating that gp74 contains an α-helical insertion in the Ω-loop, which connects the two β-strands of the ββα fold, and a disordered C-terminal tail. NMR data indicate that the Ω-loop insert makes contacts to the ββα fold and influences the ability of gp74 to bind divalent metal ions. Further, the Ω-loop insert and C-terminal tail contribute to gp74-mediated DNA digestion and to gp74 activity in phage morphogenesis. The data presented here enrich our molecular-level understanding of how HNH endonucleases enhance terminase-mediated digestion of the cos site and contribute to the phage replication cycle.IMPORTANCE This study demonstrates that residues outside the canonical ββα fold, namely, the Ω-loop α-helical insert and a disordered C-terminal tail, regulate the activity of the HNH endonuclease gp74. The increased divalent metal ion binding when the Ω-loop insert is removed compared to reduced cos site digestion and phage formation indicates that the Ω-loop insert plays multiple regulatory roles. The data presented here provide insights into the molecular basis of the involvement of HNH proteins in phage DNA packing.

Systematic classification of the His-Me finger superfamily

The His-Me finger endonucleases, also known as HNH or ββα-metal endonucleases, form a large and diverse protein superfamily. The His-Me finger domain can be found in proteins that play an essential role in cells, including genome maintenance, intron homing, host defense and target offense. Its overall structural compactness and non-specificity make it a perfectly-tailored pathogenic module that participates on both sides of inter- and intra-organismal competition. An extremely low sequence similarity across the superfamily makes it difficult to identify and classify new His-Me fingers. Using state-of-the-art distant homology detection methods, we provide an updated and systematic classification of His-Me finger proteins. In this work, we identified over 100 000 proteins and clustered them into 38 groups, of which three groups are new and cannot be found in any existing public domain database of protein families. Based on an analysis of sequences, structures, domain architectures, and genomic contexts, we provide a careful functional annotation of the poorly characterized members of this superfamily. Our results may inspire further experimental investigations that should address the predicted activity and clarify the potential substrates, to provide more detailed insights into the fundamental biological roles of these proteins.

Structure-based domain assignment in Leishmania infantum EndoG: characterization of a pH-dependent regulatory switch and a C-terminal extension that largely dictates DNA substrate preferences

Mitochondrial endonuclease G from Leishmania infantum (LiEndoG) participates in the degradation of double-stranded DNA (dsDNA) during parasite cell death and is catalytically inactive at a pH of 8.0 or above. The presence, in the primary sequence, of an acidic amino acid-rich insertion exclusive to trypanosomatids and its spatial position in a homology-built model of LiEndoG led us to postulate that this peptide stretch might act as a pH sensor for self-inhibition. We found that a LiEndoG variant lacking residues 145–180 is indeed far more active than its wild-type counterpart at pH values >7.0. In addition, we discovered that (i) LiEndoG exists as a homodimer, (ii) replacement of Ser211 in the active-site SRGH motif with the canonical aspartate from the DRGH motif of other nucleases leads to a catalytically deficient enzyme, (iii) the activity of the S211D variant can be restored upon the concomitant replacement of Ala247 with Arg and (iv) a C-terminal extension is responsible for the observed preferential cleavage of single-stranded DNA (ssDNA) and ssDNA–dsDNA junctions. Taken together, our results support the view that LiEndoG is a multidomain molecular machine whose nuclease activity can be subtly modulated or even abrogated through architectural changes brought about by environmental conditions and interaction with other binding partners.

Efficient, ultra-high-affinity chromatography in a one-step purification of complex proteins

Protein purification is an essential primary step in numerous biological studies. It is particularly significant for the rapidly emerging high-throughput fields, such as proteomics, interactomics, and drug discovery. Moreover, purifications for structural and industrial applications should meet the requirement of high yield, high purity, and high activity (HHH). It is, therefore, highly desirable to have an efficient purification system with a potential to meet the HHH benchmark in a single step. Here, we report a chromatographic technology based on the ultra-high-affinity (K_d ∼ 10^-14-10^-17 M) complex between the Colicin E7 DNase (CE7) and its inhibitor, Immunity protein 7 (Im7). For this application, we mutated CE7 to create a CL7 tag, which retained the full binding affinity to Im7 but was inactivated as a DNase. To achieve high capacity, we developed a protocol for a large-scale production and highly specific immobilization of Im7 to a solid support. We demonstrated its utility with one-step HHH purification of a wide range of traditionally challenging biological molecules, including eukaryotic, membrane, toxic, and multisubunit DNA/RNA-binding proteins. The system is simple, reusable, and also applicable to pulldown and kinetic activity/binding assays.

Intrinsic protein disorder could be overlooked in cocrystallization conditions: An SRCD case study

X-ray diffractometry dominates protein studies, as it can provide 3D structures of these diverse macromolecules or their molecular complexes with interacting partners: substrates, inhibitors, and/or cofactors. Here, we show that under cocrystallization conditions the results could reflect induced protein folds instead of the (partially) disordered original structures. The analysis of synchrotron radiation circular dichroism spectra revealed that the Im7 immunity protein stabilizes the native-like solution structure of unfolded NColE7 nuclease mutants via complex formation. This is consistent with the fact that among the several available crystal structures with its inhibitor or substrate, all NColE7 structures are virtually the same. Our results draw attention to the possible structural consequence of protein modifications, which is often hidden by compensational effects of intermolecular interactions. The growing evidence on the importance of protein intrinsic disorder thus, demands more extensive complementary experiments in solution phase with the unligated form of the protein of interest.

Structural characterization of the virulence factor Sda1 nuclease from Streptococcus pyogenes

Infection by Group A Streptococcus pyogenes (GAS) is a leading cause of severe invasive disease in humans, including streptococcal toxic shock syndrome and necrotizing fasciitis. GAS infections lead to nearly 163,000 annual deaths worldwide. Hypervirulent strains of S. pyogenes have evolved a plethora of virulence factors that aid in disease—by promoting bacterial adhesion to host cells, subsequent invasion of deeper tissues and blocking the immune system's attempts to eradicate the infection. Expression and secretion of the extracellular nuclease Sda1 is advantageous for promoting bacterial dissemination throughout the host organism, and evasion of the host's innate immune response. Here we present two crystal structures of Sda1, as well as biochemical studies to address key structural features and surface residues involved in DNA binding and catalysis. In the active site, Asn211 is observed to directly chelate a hydrated divalent metal ion and Arg124, on the putative substrate binding loop, likely stabilizes the transition state during phosphodiester bond cleavage. These structures provide a foundation for rational drug design of small molecule inhibitors to be used in prevention of invasive streptococcal disease.

DNA Structural Correlation in Short and Long Ranges

Recent single-molecule measurements have revealed the DNA allostery in protein/DNA binding. MD simulations showed that this allosteric effect is associated with the deformation properties of DNA. In this study, we used MD simulations to further investigate the mechanism of DNA structural correlation, its dependence on DNA sequence, and the chemical modification of the bases. Besides a random sequence, poly d(AT) and poly d(GC) are also used as simpler model systems, which show the different bending and twisting flexibilities. The base-stacking interactions and the methyl group on the 5-carbon site of thymine causes local structures and flexibility to be very different for the two model systems, which further lead to obviously different tendencies of the conformational deformations, including the long-range allosteric effects.

Structural characterization of the virulence factor nuclease A from Streptococcus agalactiae

The group B pathogen Streptococcus agalactiae commonly populates the human gut and urogenital tract, and is a major cause of infection-based mortality in neonatal infants and in elderly or immunocompromised adults. Nuclease A (GBS_NucA), a secreted DNA/RNA nuclease, serves as a virulence factor for S. agalactiae, facilitating bacterial evasion of the human innate immune response. GBS_NucA efficiently degrades the DNA matrix component of neutrophil extracellular traps (NETs), which attempt to kill and clear invading bacteria during the early stages of infection. In order to better understand the mechanisms of DNA substrate binding and catalysis of GBS_NucA, the high-resolution structure of a catalytically inactive mutant (H148G) was solved by X-ray crystallography. Several mutants on the surface of GBS_NucA which might influence DNA substrate binding and catalysis were generated and evaluated using an imidazole chemical rescue technique. While several of these mutants severely inhibited nuclease activity, two mutants (K146R and Q183A) exhibited significantly increased activity. These structural and biochemical studies have greatly increased our understanding of the mechanism of action of GBS_NucA in bacterial virulence and may serve as a foundation for the structure-based drug design of antibacterial compounds targeted to S. agalactiae.

Design of a colicin E7 based chimeric zinc-finger nuclease

Colicin E7 is a natural bacterial toxin. Its nuclease domain (NColE7) enters the target cell and kills it by digesting the nucleic acids. The HNH-motif as the catalytic centre of NColE7 at the C-terminus requires the positively charged N-terminal loop for the nuclease activity-offering opportunities for allosteric control in a NColE7-based artificial nuclease. Accordingly, four novel zinc finger nucleases were designed by computational methods exploiting the special structural features of NColE7. The constructed models were subjected to MD simulations. The comparison of structural stability and functional aspects showed that these models may function as safely controlled artificial nucleases. This study was complemented by random mutagenesis experiments identifying potentially important residues for NColE7 function outside the catalytic region.

Fine tuning of the catalytic activity of colicin E7 nuclease domain by systematic N-terminal mutations

The nuclease domain of colicin E7 (NColE7) promotes the nonspecific cleavage of nucleic acids at its C-terminal HNH motif. Interestingly, the deletion of four N-terminal residues (446-449 NColE7 = KRNK) resulted in complete loss of the enzyme activity. R447A mutation was reported to decrease the nuclease activity, but a detailed analysis of the role of the highly positive and flexible N-terminus is still missing. Here, we present the study of four mutants, with a decreased activity in the following order: NColE7  >> KGNK > KGNG ∼ GGNK > GGNG. At the same time, the folding, the metal-ion, and the DNA-binding affinity were unaffected by the mutations as revealed by linear and circular dichroism spectroscopy, isothermal calorimetric titrations, and gel mobility shift experiments. Semiempirical quantum chemical calculations and molecular dynamics simulations revealed that K446, K449, and/or the N-terminal amino group are able to approach the active centre in the absence of the other positively charged residues. The results suggested a complex role of the N-terminus in the catalytic process that could be exploited in the design of a controlled nuclease.

A new insight into the zinc-dependent DNA-cleavage by the colicin E7 nuclease: a crystallographic and computational study

The crystal structure of a colicin E7 metallonuclease mutant complemented by QM/MM calculations suggests an alternative catalytic mechanism of Zn²⁺-containing HNH nucleases.

Crystallization and preliminary crystallographic analysis of an Escherichia coli-selected mutant of the nuclease domain of the metallonuclease colicin E7

The metallonuclease colicin E7 is a member of the HNH family of endonucleases. It serves as a bacterial toxin in Escherichia coli, protecting the host cell from other related bacteria and bacteriophages by degradation of their chromosomal DNA under environmental stress. Its cell-killing activity is attributed to the nonspecific nuclease domain (NColE7), which possesses the catalytic ββα-type metal ion-binding HNH motif at its C-terminus. Mutations affecting the positively charged amino acids at the N-terminus of NColE7 (444-576) surprisingly showed no or significantly reduced endonuclease activity [Czene et al. (2013), J. Biol. Inorg. Chem. 18, 309-321]. The necessity of the N-terminal amino acids for the function of the C-terminal catalytic centre poses the possibility of allosteric activation within the enzyme. Precise knowledge of the intramolecular interactions of these residues that affect the catalytic activity could turn NColE7 into a novel platform for artificial nuclease design. In this study, the N-terminal deletion mutant ΔN4-NColE7-C* of the nuclease domain of colicin E7 selected by E. coli was overexpressed and crystallized at room temperature by the sitting-drop vapour-diffusion method. X-ray diffraction data were collected to 1.6 Å resolution and could be indexed and averaged in the trigonal space group P3121 or P3221, with unit-cell parameters a = b = 55.4, c = 73.1 Å. Structure determination by molecular replacement is in progress.

The protein gp74 from the bacteriophage HK97 functions as a HNH endonuclease

The last gene in the genome of the bacteriophage HK97 encodes the protein gp74. We present data in this article that demonstrates, for the first time, that gp74 possesses HNH endonuclease activity. HNH endonucleases are small DNA binding and digestion proteins characterized by two His residues and an Asn residue. We demonstrate that gp74 cleaves lambda phage DNA at multiple sites and that gp74 requires divalent metals for its endonuclease activity. We also present intrinsic tryptophan fluorescence data that show direct binding of Ni(2+) to gp74. The activity of gp74 in the presence of Ni(2+) is significantly decreased below neutral pH, suggesting the presence of one or more His residues in metal binding and/or DNA digestion. Surprisingly, this pH-dependence of activity is not seen with Zn(2+) , suggesting a different mode of binding of Zn(2+) and Ni(2+) . This difference in activity may result from binding of a second Zn(2+) ion by a putative zinc finger in gp74 in addition to binding of a Zn(2+) ion by the HNH motif. These studies define the biochemical function of gp74 as an HNH endonuclease and provide a platform for determining the role of gp74 in life cycle of the bacteriophage HK97.

The binding process of a nonspecific enzyme with DNA

Protein-DNA recognition of a nonspecific complex is modeled to understand the nature of the transient encounter states. We consider the structural and energetic features and the role of water in the DNA grooves in the process of protein-DNA recognition. Here we have used the nuclease domain of colicin E7 (N-ColE7) from Escherichia coli in complex with a 12-bp DNA duplex as the model system to consider how a protein approaches, encounters, and associates with DNA. Multiscale simulation studies using Brownian dynamics and molecular-dynamics simulations were performed to provide the binding process on multiple length- and timescales. We define the encounter states and identified the spatial and orientational aspects. For the molecular length-scales, we used molecular-dynamics simulations. Several intermediate binding states were found, which have different positions and orientations of protein around DNA including major and minor groove orientations. The results show that the contact number and the hydrated interfacial area are measures that facilitate better understanding of sequence-independent protein-DNA binding landscapes and pathways.

Crystal structure of the chromodomain helicase DNA-binding protein 1 (Chd1) DNA-binding domain in complex with DNA

Chromatin remodelers are ATP-dependent machines that dynamically alter the chromatin packaging of eukaryotic genomes by assembling, sliding, and displacing nucleosomes. The Chd1 chromatin remodeler possesses a C-terminal DNA-binding domain that is required for efficient nucleosome sliding and believed to be essential for sensing the length of DNA flanking the nucleosome core. The structure of the Chd1 DNA-binding domain was recently shown to consist of a SANT and SLIDE domain, analogous to the DNA-binding domain of the ISWI family, yet the details of how Chd1 recognized DNA were not known. Here we present the crystal structure of the Saccharomyces cerevisiae Chd1 DNA-binding domain in complex with a DNA duplex. The bound DNA duplex is straight, consistent with the preference exhibited by the Chd1 DNA-binding domain for extranucleosomal DNA. Comparison of this structure with the recently solved ISW1a DNA-binding domain bound to DNA reveals that DNA lays across each protein at a distinct angle, yet contacts similar surfaces on the SANT and SLIDE domains. In contrast to the minor groove binding seen for Isw1 and predicted for Chd1, the SLIDE domain of the Chd1 DNA-binding domain contacts the DNA major groove. The majority of direct contacts with the phosphate backbone occur only on one DNA strand, suggesting that Chd1 may not strongly discriminate between major and minor grooves.

Mechanistic insight into the catalytic activity of ββα-metallonucleases from computer simulations: Vibrio vulnificus periplasmic nuclease as a test case

Using information from wild-type and mutant Vibrio vulnificus nuclease (Vvn) and I-PpoI homing endonuclease co-crystallized with different oligodeoxynucleotides, we have built the complex of Vvn with a DNA octamer and carried out a series of simulations to dissect the catalytic mechanism of this metallonuclease in a stepwise fashion. The distinct roles played in the reaction by individual active site residues, the metal cation and water molecules have been clarified by using a combination of classical molecular dynamics simulations and quantum mechanical calculations. Our results strongly support the most parsimonious catalytic mechanism, namely one in which a single water molecule from bulk solvent is used to cleave the phosphodiester bond and protonate the 3'-hydroxylate leaving group.

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.

Redesign of high-affinity nonspecific nucleases with altered sequence preference

It is of crucial importance to elucidate the underlying principles that govern the binding affinity and selectivity between proteins and DNA. Here we use the nuclease domain of Colicin E7 (nColE7) as a model system to generate redesigned nucleases with improved DNA-binding affinities. ColE7 is a bacterial toxin, bearing a nonspecific endonuclease domain with a preference for hydrolyzing DNA phosphodiester bonds at the 3'O-side after thymine and adenine; i.e., it prefers Thy and Ade at the -1 site. Using systematic computational screening, six nColE7 mutants were predicted to bind DNA with high affinity. Five of the redesigned single-point mutants were constructed and purified, and four mutants had a 3- to 5-fold higher DNA binding affinity than wild-type nColE7 as measured by fluorescence kinetic assays. Moreover, three of the designed mutants, D493N, D493Q, and D493R, digested DNA with an increased preference for guanine at +3 sites compared to the wild-type enzyme, as shown by DNA footprint assays. X-ray structure determination of the ColE7 mutant D493Q-DNA complex in conjunction with structural and free energy decomposition analyses provides a physical basis for the improved protein-DNA interactions: Replacing D493 at the protein-DNA interface with an amino acid residue that can maintain the native hydrogen bonds removes the unfavorable electrostatic repulsion between the negatively charged carboxylate and DNA phosphate groups. These results show that computational screening combined with biochemical, structural, and free energy analyses provide a useful means for generating redesigned nucleases with a higher DNA-binding affinity and altered sequence preferences in DNA cleavage.

Separate production of single-stranded DNA is not necessary: circuit denaturation of double-stranded DNA followed by hybridization of single strands on oligonucleotide microchips

An approach to circuit renaturation-hybridization of dsDNA on oligonucleotide microchips is described. A close circuit cycling device has been developed, and the feasibility of the proposed technique was demonstrated on two platforms. First, a commercial microchip for detection of rifampicin resistance in Mycobacterium tuberculosis was used. Hybridization of a 126 nt long single-stranded DNA (ssDNA) fragment of the rpoB gene according to manufacturer's protocol has been compared to hybridization of the same double-stranded DNA (dsDNA) fragment using the developed approach. Hybridization signals obtained by both methods were comparable in intensity and correlated closely. Second, a 22 nt long hairpin-forming oligonucleotide was designed and hybridized with a custom microchip containing probes complementary to both strands of the oligonucleotide. Conventional hybridization of this oligonucleotide did not yield any significant signals. Cleavage of the hairpin loop resulted in the formation of a 9 bp long intermolecular duplex. Hybridization of the duplex using the suggested technique yielded strong signals. The proposed approach allows analyzing target DNA in double-stranded form bypassing the preparation of single-stranded targets. Moreover, both complementary chains could be analyzed simultaneously, providing a reliable internal control. Being combined with fragmentation this method opens new possibilities in analyzing ssDNA with complex secondary structure.

Identification of intrinsic dynamics in a DNA sequence preferentially cleaved by topoisomerase II enzyme

Topoisomerase II enzymes are essential enzymes that modulate DNA topology and play a role in chromatin compaction. While these enzymes appear to recognize and cleave the DNA in a nonrandom fashion, factors that underlie enzyme specificity remain an enigma. To gain new insights on these topics, we undertake, using NMR and molecular dynamics methods, studies of the structural and dynamic features of a 21 bp DNA segment preferentially cleaved by topoisomerases II. The large size of the oligonucleotide did not hamper the determination of structures of sufficient quality, and numerous interesting correlations between helicoidal parameters already depicted in crystals and molecular dynamics simulations are recovered here. The main feature of the sequence is the occurrence of a large opening of the base pairs in a four-residue AT-rich region located immediately at the 5' end of one of the cleaved sites. This opening seems to be largely dependent on sequence context, since a similar opening is not found in the other AT base pairs of the sequence. Furthermore, two adenine nucleotides of the same portion of the oligonucleotide present slow internal motions at the NMR timescale, revealing particular base dynamics. In conclusion, this AT-rich region presents the most salient character in the sequence and could be involved in the preferential cleavage by topoisomerase II. The examination of preferred sites in the literature pointed out the frequent occurrence of AT-rich sequences, namely matrix attachment region and scaffold attachment region sequences, at the sites cleaved by topoisomerase II. We could infer that the particular flexibility of these sequences plays an important role in enabling the formation of a competent cleavage complex. The sequences could then be selected based on their facility to undertake conformational change during the complex formation, rather than purely based on binding affinity.

The Conserved Asparagine in the HNH Motif Serves an Important Structural Role in Metal Finger Endonucleases

The HNH motif is a small nucleic acid binding and cleavage module, widespread in metal finger endonucleases in all life kingdoms. Here we studied a non-specific endonuclease, the nuclease domain of ColE7 (N-ColE7), to decipher the role of the conserved asparagine and histidine residues in the HNH motif. We found, using fluorescence resonance energy transfer (FRET) assays, that the DNA hydrolysis activity of H545 N-ColE7 mutants was completely abolished while activities of N560 and H573 mutants varied from 6.9% to 83.2% of the wild-type activity. The crystal structures of three N-ColE7 mutants in complex with the inhibitor Im7, N560A-Im7, N560D-Im7 and H573A-Im7, were determined at a resolution of 1.9 A to 2.2 A. H573 is responsible for metal ion binding in the wild-type protein, as the zinc ion is still partially associated in the structure of H573A, suggesting that H573 plays a supportive role in metal binding. Both N560A and N560D contain a disordered loop in the HNH motif due to the disruption of the hydrogen bond network surrounding the side-chain of residue 560, and as a result, the imidazole ring of the general base residue H545 is tilted slightly and the scissile phosphate is shifted, leading to the large reductions in hydrolysis activities. These results suggest that the highly conserved asparagine in the HNH motif, in general, plays a structural role in constraining the loop in the metal finger structure and keeping the general base histidine and scissile phosphate in the correct position for DNA hydrolysis.