Fluorescence stopped-flow kinetics of the cleavage of synthetic oligodeoxynucleotides by the EcoRI restriction endonuclease

We have investigated in fluorescence stopped-flow and temperature-jump experiments the EcoRI-catalyzed cleavage of synthetic palindromic tridecadeoxynucleotides which contain the EcoRI site but differ in the flanking sequences. The overall reaction can be resolved in several reactions which were analyzed by a nonlinear least-squares fitting procedure on the experimental data. The result of this analysis is a minimal scheme that describes the overall reaction in terms of the rate constants of the individual reactions. According to this scheme EcoRI and the tridecadeoxynucleotide substrates associate in the presence of Mg2+ in a nearly diffusion-controlled process. This is followed by a reaction which is or includes the cleavage of the first phosphodiester bond. There is no indication for a time-resolved conformational transition prior to catalysis. After cleavage of the first strand, dissociation of the nicked double strand can occur, which then rearranges to the original palindromic double-stranded substrate and is bound again by the enzyme. Alternatively, the nicked double strand can be cleaved in the second strand. This reaction is followed by product release from the enzyme. The magnitude of the individual rate constants depends on the substrate used; the differences explain the preference of EcoRI for substrates that contain AT as compared to GC base pairs next to the recognition site.     

Chlorella virus DNA ligase: nick recognition and mutational analysis.

Chlorella virus PBCV-1 DNA ligase seals nicked DNA substrates consisting of a 5'-phosphate-terminated strand and a 3'-hydroxyl-terminated strand annealed to a bridging DNA template strand. The enzyme discriminates at the DNA binding step between substrates containing a 5'-phosphate versus a 5'-hydroxyl at the nick. Mutational analysis of the active site motif KxDGxR (residues 27-32) illuminates essential roles for the conserved Lys, Asp and Arg moieties at different steps of the ligase reaction. Mutant K27A is unable to form the covalent ligase-(Lys-straightepsilonN-P)-adenylate intermediate and hence cannot activate a nicked DNA substrate via formation of the DNA-adenylate intermediate. Nonetheless, K27A catalyzes phosphodiester bond formation at a pre-adenylated nick. This shows that the active site lysine is not required for the strand closure reaction. K27A binds to nicked DNA-adenylate, but not to a standard DNA nick. This suggests that occupancy of the AMP binding pocket of DNA ligase is important for nick recognition. Mutant D29A is active in enzyme-adenylate formation and binds readily to nicked DNA, but is inert in DNA-adenylate formation. R32A is unable to catalyze any of the three reactions of the ligation pathway and does not bind to nicked DNA.     

Characterization of the reaction product of the oriT nicking reaction catalyzed by Escherichia coli DNA helicase I.

DNA helicase I, encoded on the Escherichia coli F plasmid, catalyzes a site- and strand-specific nicking reaction within the F plasmid origin of transfer (oriT) to initiate conjugative DNA strand transfer. The product of the nicking reaction contains a single phosphodiester bond interruption as determined by single-nucleotide resolution mapping of both sides of the nick site. This analysis has demonstrated that the nick is located at precisely the same site previously shown to be nicked in vivo (T. L. Thompson, M. B. Centola, and R. C. Deonier, J. Mol. Biol. 207:505-512, 1989). In addition, studies with two oriT point mutants have confirmed the specificity of the in vitro reaction. Characterization of the nicked DNA product has revealed a modified 5' end and a 3' OH available for extension by E. coli DNA polymerase I. Precipitation of nicked DNA with cold KCl in the presence of sodium dodecyl sulfate suggests the existence of protein covalently attached to the nicked DNA molecule. The covalent nature of this interaction has been directly demonstrated by transfer of radiolabeled phosphate from DNA to protein. On the basis of these results, we propose that helicase I becomes covalently bound to the 5' end of the nicked DNA strand as part of the reaction mechanism for phosphodiester bond cleavage. A model is presented to suggest how helicase I could nick the F plasmid at oriT and subsequently unwind the duplex DNA to provide single-stranded DNA for strand transfer during bacterial conjugation.     

Target site cleavage by the monomeric restriction enzyme BcnI requires translocation to a random DNA sequence and a switch in enzyme orientation

Endonucleases that generate double-strand breaks in DNA often possess two identical subunits related by rotational symmetry, arranged so that the active sites from each subunit act on opposite DNA strands. In contrast to many endonucleases, Type IIP restriction enzyme BcnI, which recognizes the pseudopalindromic sequence 5'-CCSGG-3' (where S stands for C or G) and cuts both DNA strands after the second C, is a monomer and possesses a single catalytic center. We show here that to generate a double-strand break BcnI nicks one DNA strand, switches its orientation on DNA to match the polarity of the second strand and then cuts the phosphodiester bond on the second DNA strand. Surprisingly, we find that an enzyme flip required for the second DNA strand cleavage occurs without an excursion into bulk solution, as the same BcnI molecule acts processively on both DNA strands. We provide evidence that after cleavage of the first DNA strand, BcnI remains associated with the nicked intermediate and relocates to the opposite strand by a short range diffusive hopping on DNA.     

Interaction of EcoRII restriction and modification enzymes with synthetic DNA fragments. V. Study of single-strand cleavages.

Concatemer DNA duplexes which contain at the EcoRII restriction endonuclease cleavage sites (formula; see text) phosphodiester, phosphoamide or pyrophosphate internucleotide bonds have been synthesized. It has been shown that this enzyme did not cleave the substrate at phosphoamide bond. EcoRII endonuclease catalyzes single-strand cleavages both in dA- and dT-containing strands of the recognition site if the cleavage of the other strand has been blocked by modification of scissile bond or if the other strand has been cleaved. This enzyme interacts with both strands of the DNA recognition site, each of them being cleaved independently on the cleavage of another one. Nucleotide sequences flanking the EcoRII site on both sides are necessary for effective cleavage of the substrate.     

Strand specific cleavage of phosphorothioate-containing DNA by reaction with restriction endonucleases in the presence of ethidium bromide.

A method for achieving strand specific nicking of DNA has been developed. Phosphorothioate groups were incorporated enzymatically into the (-)strand of M13 RF IV DNA. When such DNA is reacted with restriction endonucleases in the presence of ethidium bromide nicked DNA (RF II) is produced. All of the restriction enzymes tested linearised phosphorothioate-containing DNA in the absence of this dye. The strand specificity of the reaction was investigated by employing the ethidium bromide mediated nicking reaction in the phosphorothioate-based oligonucleotide-directed mutagenesis method. The mutational efficiencies obtained were in the region of 64-89%, indicating that these restriction enzymes hydrolyse the phosphodiester bond at the cleavage site of the unsubstituted (+)strand.     

The time-resolved kinetics of superhelical DNA cleavage by BamHI restriction endonuclease

The kinetics of the cleavage of superhelical plasmid DNA (pBR322) by the restriction endonuclease, BamHI, have been analyzed in terms a compartmental model consistent with the chemistry first proposed by Rubin and Modrich (Rubin, R. A., and Modrich, P. (1978) Nucleic Acids Res. 5, 2991-2997) for analysis of the kinetics of the restriction endonuclease, EcoRI. The model was defined in terms of two compartments representing DNA substrate (bound and free), two compartments representing nicked intermediate (bound and free), one compartment representing linear product, and one compartment for free enzyme. A simultaneous analysis of concentration changes over time of the three DNA forms (superhelical, nicked, and linear) at six different enzyme concentrations was undertaken employing this compartmental model using SAAM (Simulation Analysis And Modeling) software. Results showed that rate constants characterizing the association of enzyme with superhelical DNA (6.0 x 10(5) M-1 s-1) and nicked DNA (2.8 x 10(5) M-1 s-1) were similar in magnitude and rate constants characterizing cleavage of the first (1.2 x 10(-2) s-1) and second phosphodiester bonds (3.1 x 10(-2) s-1) were also similar. The analysis yields a kinetically determined equilibrium constant of 12.9 nM for the dissociation of nicked intermediate from the enzyme. The rate constant describing the release of the nicked intermediate from the enzyme has a value of 3.7 x 10(-3) s-1. By comparing the value of this release rate constant to the value of the constant describing the second cleavage event, it can be determined that only 10% of the nicked intermediate bound to the enzyme is released as free nicked DNA and that 90% of the nicked intermediate is processed to the linear form without being released. Hence, most of the DNA is cleaved as the result of a single enzyme-DNA recognition event. No steady state assumptions were made in the analysis. The approach was to directly solve the differential equations which described the kinetic processes using an interactive method. This study demonstrates the usefulness of this approach for the analysis of kinetics of protein-DNA interactions for the restriction endonucleases.     

Engineering variants of the I-SceI homing endonuclease with strand-specific and site-specific DNA-nicking activity

The number of strand-specific nicking endonucleases that are currently available for laboratory procedures and applications in vivo is limited, and none is sufficiently specific to nick single target sites within complex genomes. The extreme target specificity of homing endonucleases makes them attractive candidates for engineering high-specificity nicking endonucleases. I-SceI is a monomeric homing enzyme that recognizes an 18 bp asymmetric target sequence, and cleaves both DNA strands to leave 3′-overhangs of 4 bp. In single turnover experiments using plasmid substrates, I-SceI generates transient open circle intermediates during the conversion of supercoiled to linear DNA, indicating that the enzyme cleaves the two DNA strands sequentially. A novel hairpin substrate was used to demonstrate that although wild-type I-SceI cleaves either the top or bottom DNA strand first to generate two nicked DNA intermediates, the enzyme has a preference for cleaving the bottom strand. The kinetics data are consistent with a parallel sequential reaction mechanism. Substitution of two pseudo-symmetric residues, Lys122 and Lys223, markedly reduces top and bottom-strand cleavage, respectively, to generate enzymes with significant strand- and sequence-specific nicking activity. The two active sites are partially interdependent, since alterations to one site affect the second. The kinetics analysis is consistent with X-ray crystal structures of I-SceI/DNA complexes that reveal a role for the lysines in establishing important solvent networks that include nucleophilic water molecules thought to attack the scissile phosphodiester bonds.     

Nuclease S1 cleavage and the primary structure of mitochondrial DNA.

The single-strand-specific nuclease S1 from Aspergillus oryzae rapidly converts superhelical mitochondrial DNA (African Green Monkey cells, Vero ATCC; CCL 81) into nicked circular DNA. These nicked mitochondrial DNA molecules contain two nicks, one in each strand. The phosphodiester backbones are cleaved during this reaction at or near sites that are alkali-labile. In a second slow reaction the circular mitochondrial DNA is converted into a linear duplex DNA. Permutation tests indicate that this linear DNA represents a nonpermutated collection of DNA molecules. These results suggest that two of the alkai-labile sites in the phosphodiester backbones of the mitochondrial chromosome are closely spaced on opposite strands and at specific positions.     

Uncoupling of the DNA breaking and rejoining steps of Escherichia coli type I DNA topoisomerase. Demonstration of an active covalent protein-DNA complex

DNA topoisomerases have been shown to cleave DNA phosphodiester bond and simultaneously become linked to the DNA at the cleavage site via a phosphotyrosine linkage (Tse, Y.-C., Kirkegaard, K., and Wang, J. C. (1980) J. Biol. Chem. 255, 5560-5565). For prokaryotic DNA topoisomerases, this is observed only when denaturant or protease is added to the topoisomerase-DNA incubation mixture. Previous attempts to reform DNA phosphodiester bonds from the covalent protein-DNA complex have been unsuccessful. Using oligonucleotides as substrates, the cleavage reaction of Escherichia coli DNA topoisomerase I occurs spontaneously (Tse-Dinh, Y.-C., McCarron, B. G. H., Arentzen, R., and Chowdhry, V. (1983) Nucleic Acids Res. 11, 8691-8701). Upon reaction with oligo(dA) labeled with 32P using terminal transferase and [alpha-32P]dATP, the enzyme becomes covalently linked to the 32P-labeled oligonucleotide. This 32P label can then be transferred to the 3'-OH end of a linear or nicked duplex DNA molecule subsequently added to the reaction mixture. This phosphodiester bond rejoining reaction can occur at a recessed, blunt, or protruding 3'-end of double-stranded DNA. It requires magnesium ions. These observations suggest that the covalent protein-DNA complex is a true intermediate during topoisomerization. Implications on the structure of prokaryotic type I DNA topoisomerases as compared to their eukaryotic counterparts are discussed.     

Crystal structures of I-SceI complexed to nicked DNA substrates: snapshots of intermediates along the DNA cleavage reaction pathway

I-SceI is a homing endonuclease that specifically cleaves an 18-bp double-stranded DNA. I-SceI exhibits a strong preference for cleaving the bottom strand DNA. The published structure of I-SceI bound to an uncleaved DNA substrate provided a mechanism for bottom strand cleavage but not for top strand cleavage. To more fully elucidate the I-SceI catalytic mechanism, we determined the X-ray structures of I-SceI in complex with DNA substrates that are nicked in either the top or bottom strands. The structures resemble intermediates along the DNA cleavage reaction. In a structure containing a nick in the top strand, the spatial arrangement of metal ions is similar to that observed in the structure that contains uncleaved DNA, suggesting that cleavage of the bottom strand occurs by a common mechanism regardless of whether this strand is cleaved first or second. In the structure containing a nick in the bottom strand, a new metal binding site is present in the active site that cleaves the top strand. This new metal and a candidate nucleophilic water molecule are correctly positioned to cleave the top strand following bottom strand cleavage, providing a plausible mechanism for top strand cleavage.     

Single strand DNA cleavage reaction of duplex DNA by Drosophila topoisomerase II

A unique reaction for type II DNA topoisomerase is its cleavage of a pair of DNA strands in concert. We show however, that in a reaction mixture containing a molar excess of EDTA over Mg2+, or when Mg2+ is substituted by Ca2+, Mn2+, or Co2+, the enzyme cleaves only one rather than both strands. These results suggest that the divalent cations may play an important role in coordinating the two subunits of DNA topoisomerase II during the strand cleavage reaction. The single strand and the double strand cleavage reactions are similar in the following aspects: both require the addition of a protein denaturant, can be reversed by low temperature or high salt, and a topoisomerase II molecule is attached covalently to the 5' phosphoryl end of each broken DNA strand. Furthermore, the single strand cleavage sites share a similar sequence preference with double strand cleavage sites. There is, however, a strand bias for the single strand cleavage reaction. We show also that under single strand cleavage conditions, topoisomerase II still possesses a low level of double strand passage activity: it can introduce topological knots into both covalently closed or nicked DNA rings, and change the linking number of a plasmid DNA by steps of two. The implication of this observation on the sequential cleavage of the two strands of the DNA duplex during the normal DNA double strand passage process catalyzed by type II DNA topoisomerases is discussed.     

The use of phosphorothioate-modified DNA in restriction enzyme reactions to prepare nicked DNA

The RF IV form of M13 DNA was synthesized enzymatically in vitro, using the viral (+)strand as template, to contain phosphorothioate-modified internucleotidic linkages of the Rp configuration on the 5' side of every base of a particular type in the newly-synthesized (-)strand. Twenty nine restriction enzymes were then tested for their reactions with the appropriate modified DNA types having a phosphorothioate linkage placed exactly at the cleavage site(s) of these enzymes in the (-)strand. Eleven of the seventeen restriction enzymes tested that had recognition sequences of five bases or more could be used to convert the phosphorothioate DNA entirely into the nicked form, either by simply allowing the reaction to go to completion with excess enzyme (Ava I, Ava II, Ban II, Hind II, Nci I, Pst I or Pvu I) or by stopping the reaction at the appropriate time before the nicked DNA is linearized (Bam HI, Bgl I, Eco RI or Hind III). Only modification of the exact cleavage site in the (-)strand could block linearization by the first class of enzymes. The results presented imply that the restriction enzyme-directed nicking of phosphorothioate M13 DNA occurs exclusively in the (+)strand.     

Properties of circular dumbbell RNA/DNA chimeric oligonucleotides containing antisense phosphodiester oligonucleotides.

We have designed a new class of oligonucleotides, "dumbbell RNA/DNA chimeric phosphodiesters", containing two alkyl loop structures with RNA/DNA base pairs (sense (RNA) and antisense (DNA) in the double helical stem. The reaction of nicked (NDRDON) and circular (CDRDON) dumbbell RNA/DNA chimeric oligonucleotides with RNaseH gave the corresponding antisense phosphodiester oligonucleotide together with the sense RNA cleavage products. The liberated antisense phosphodiester oligodeoxynucleotide was bound to the target 35mer RNA, which gave 35mer RNA cleavage products by treatment with RNaseH. The circular dumbbell RNA/DNA chimeric oligonucleotide showed more nuclease resistance than the linear antisense phosphodiester oligodeoxynucleotide(anti-ODN) and the nicked dumbbell RNA/DNA chimeric oligonucleotide.     

Nick sensing by vaccinia virus DNA ligase requires a 5' phosphate at the nick and occupancy of the adenylate binding site on the enzyme.

Vaccinia virus DNA ligase has an intrinsic nick-sensing function. The enzyme discriminates at the substrate binding step between a DNA containing a 5' phosphate and a DNA containing a 5' hydroxyl at the nick. Further insights into nick recognition and catalysis emerge from studies of the active-site mutant K231A, which is unable to form the covalent ligase-adenylate intermediate and hence cannot activate a nicked DNA substrate via formation of the DNA-adenylate intermediate. Nonetheless, K231A does catalyze phosphodiester bond formation at a preadenylated nick. Hence, the active-site lysine of DNA ligase is not required for the strand closure step of the ligation reaction. The K231A mutant binds tightly to nicked DNA-adenylate but has low affinity for a standard DNA nick. The wild-type vaccinia virus ligase, which is predominantly ligase-adenylate, binds tightly to a DNA nick. This result suggests that occupancy of the AMP binding pocket of DNA ligase is essential for stable binding to DNA. Sequestration of an extrahelical nucleotide by DNA-bound ligase is reminiscent of the base-flipping mechanism of target-site recognition and catalysis used by other DNA modification and repair enzymes.     

Purification of the HhaII restriction endonuclease from an overproducer Escherichia coli clone

An Escherichia coli K12 strain carrying the HhaII methylase and restriction genes on two separate compatible plasmids, pSK5 and pSK7, is used to overproduce the restriction endonuclease. Plasmid pSK5 expresses the methylase gene constitutively from its chloramphenicol resistance gene promoter, and plasmid pSK7 expresses the restriction endonuclease under control of the lacUV5 promoter. Induction of the two-plasmid clone with 1 mM isopropyl-1-thio-beta-D-galactopyranoside results in a 15-fold increase in HhaII endonuclease activity. The enzyme has been purified to apparent homogeneity. It migrates as a 23-kilodalton polypeptide on denaturing sodium dodecyl sulfate-polyacrylamide electrophoretic gels and as a 52-kilo-dalton native protein dimer on a high pressure liquid chromatography sizing column.     

Oligonucleotides with bistranded abasic sites interfere with substrate binding and catalysis by human apurinic/apyrimidinic endonuclease.

Apurinic/apyrimidinic endonuclease (AP endo) is a key enzyme in oxidative damage DNA repair. The enzyme, which repairs abasic sites, makes a single nick 5' to the phosphodeoxyribose, leaving a free 3'-hydroxyl. We recently described single turnover kinetics for human recombinant AP endo acting on an oligonucleotide with a single abasic site. We hypothesized that the structural changes induced by the presence of a second abasic site might provide insight into how AP endo recognizes the first abasic site. Here we performed steady state and single turnover experiments using bistranded abasic site substrates, with the second site located on the complementary strand to the one being followed and either opposite to the first or displaced in the 5' direction. All sites on the complementary strand were within half a helical turn of the first. The catalytic efficiency was reduced 80 to 96% and the Kd for substrate binding and dissociation was elevated 40- to 125-fold. The smaller changes occurred when the second site was opposite the first site or displaced by four nucleotides. In addition, if the second abasic site was directly across the helix or displaced by 1 or 3 nucleotides from the first abasic site, cleavage of the first abasic site was subject to apparent substrate inhibition, which did not occur if the second abasic site was displaced by four nucleotides from the first. While a substrate containing a nick without a phosphodeoxyribose on the contralateral strand abasic site did not inhibit nicking of the first strand, a substrate with a nicked abasic site on the contralateral strand was an even stronger inhibitor of enzyme action than an oligonucleotide containing the corresponding abasic site on each strand. Consequently, the inhibitory effect of the second abasic site is probably the result of prior cleavage of the abasic site on the contralateral strand with resulting distortions to the DNA helix that interfere with enzyme binding and/or cleavage.     

Substrate specificity and adenosine triphosphatase activity of the ATP-dependent deoxyribonuclease of Bacillus subtilis

Studies on the specificity of the ATP-dependent DNase of Bacillus subtilis 168, carried out with pure enzyme at the optimal conditions for its action, have shown that the substrate is double-stranded linear DNA. Linear single-stranded DNA (separated strands of B. subtilis DNA and linear phage fd DNA) is not attacked, neither are there any circular forms (supercoiled or nicked simian virus 40 and circular single-stranded fd DNAs). The double-stranded DNA can be completely hydrolysed, the limit products being, almost exclusively, mononucleotides. The presence of terminal phosphate residues in the substrate (either at the 3' or the 5' end) is not necessary for enzyme action. This DNase appears therefore to be an exonuclease processively liberating mononucleotides from both strands of the native linear DNA. ATP (indispensable for the DNase reaction) is also hydrolysed by the enzyme, to ADP and inorganic orthophosphate (Pi) in the presence of DNA. The apparent Km for ATP, in the ATPase reaction, is 0.15 mM. At high ATP concentrations, which inhibit the DNase activity, there is activation of the ATPase reaction. Three molecules of ATP are consumed for each DNA phosphodiester bond split, at optimal conditions for DNase activity.     

Characterization of steady state, single-turnover, and binding kinetics of the TaqI restriction endonuclease.

The TaqI restriction endonuclease recognizes and cleaves the duplex DNA sequence T decreases CGA. Steady state kinetic analysis with a small oligodeoxyribonucleotide substrate showed that the enzyme obeyed Michaelis-Menten kinetics (Km = 53 nM, kcat = 1.3 min-1 at 50 degrees C and Km = 0.5 nM, kcat = 2.9 min-1 at 60 degrees C). At 0 degree C, the enzyme was completely inactive, while at 15 degrees C, turnover produced nicked substrate as the major product in excess of enzyme indicating dissociation between nicking events. Above 37 degrees C, both strands in the duplex were cleaved prior to dissociation. In contrast to the tight, temperature-dependent binding of substrate, binding of the Mg2+ cofactor was weak (Kd = 2.5 mM) and the same at either 50 degrees C or 60 degrees C. Single-turnover experiments using oligonucleotide substrate showed that hydrolysis of duplex DNA occurred via two independent nicking events, each with a first order rate constant (kst) of 5.8 min-1 at 60 degrees C and 3.5 min-1 at 50 degrees C. The pH dependence of Km (pKa = 9) and kst (pKa = 7) suggests Lys/Arg and His, respectively, as possible amino acids influencing these constants. Moreover, although kst increased significantly with pH, kcat did not, indicating that at least two steps can be rate-controlling in the reaction pathway. Binding of protein to canonical DNA in the presence of Mg2+ at 0 degree C or in the absence of Mg2+ at 50 degrees C was weak (Kd = 2.5 microM or 5,000-fold weaker than the optimal measured Km) and equal to the binding of noncanonical DNA as judged by retention on nitrocellulose. Similar results were seen in gel retardation assays. These results suggest that both Mg2+ and high temperature are required to attain the correct protein conformation to form the tight complex seen in the steady state analysis. In the accompanying paper (Zebala, J. A., Choi, J., Trainor, G. L., and Barany, F. (1992) J. Biol. Chem. 267, 8106-8116), we report how these kinetic constants are altered using substrate analogues and propose a model of functional groups involved in TaqI endonuclease recognition.