The role of the Zn(II) binding domain in the mechanism of E. coli DNA topoisomerase I
© Ahumada and Tse-Dinh; licensee BioMed Central Ltd. 2002
Received: 11 March 2002
Accepted: 29 May 2002
Published: 29 May 2002
Escherichia coli DNA topoisomerase I binds three Zn(II) with three tetracysteine motifs which, together with the 14 kDa C-terminal region, form a 30 kDa DNA binding domain (ZD domain). The 67 kDa N-terminal domain (Top67) has the active site tyrosine for DNA cleavage but cannot relax negatively supercoiled DNA. We analyzed the role of the ZD domain in the enzyme mechanism.
Addition of purified ZD domain to Top67 partially restored the relaxation activity, demonstrating that covalent linkage between the two domains is not necessary for removal of negative supercoils from DNA. The two domains had similar affinities to ssDNA. However, only Top67 could bind dsDNA with high affinity. DNA cleavage assays showed that the Top67 had the same sequence and structure selectivity for DNA cleavage as the intact enzyme. DNA rejoining also did not require the presence of the ZD domain.
We propose that during relaxation of negatively supercoiled DNA, Top67 by itself can position the active site tyrosine near the junction of double-stranded and single-stranded DNA for cleavage. However, the interaction of the ZD domain with the passing single-strand of DNA, coupled with enzyme conformational change, is needed for removal of negative supercoils.
Removal of negative supercoils from DNA by bacterial type IA topoisomerase involves the following steps: (1) binding of the enzyme to the junction of double-stranded and single-stranded DNA ; (2) cleavage of a single-strand of DNA near the junction with cleavage sequence preference of a cytosine in the -4 position to form the covalent intermediate [9, 10]; (3) conformational change of the covalent enzyme-DNA complex to result in physical separation of the 5' phosphate covalently linked to the active tyrosine, and the 3' hydroxyl of the cleaved DNA; (4) passage of the complementary single strand through the break; (5) enzyme conformational change to bring the 5' phosphoryl end back into the proximity of the 3' hydroxyl group of the cleaved DNA; (6) religation of the phosphodiester bond. Although it is known that the ZD domain can function as a DNA binding domain, its exact role in these individual steps of removal of a negative superhelical turn from DNA by E. coli topoisomerase I remains to be defined. Using purified 67 kDa transesterification domain and 30 kDa ZD domain, results from experiments described here provide new insight into the action of these two individual domains in the enzyme mechanism.
Partial restoration of relaxation activity from mixing of Top67 and ZD domains
Top67 and ZD domains have comparable binding affinities to single-stranded DNA but significantly different affinities for double-stranded DNA
Top67 can recognize cleavage sites preferred by E. coli DNA topoisomerase I
Top67 cleavage sites are religated upon addition of high salt and Mg2+
The ZD domain is not required for catenation of double-stranded DNA circles
There are two homologous type IA topoisomerases present in E. coli. Topoisomerase III has potent DNA decatenating activity for resolution of plasmid DNA replication intermediates, but much weaker relaxation activity than topoisomerase I . To exhibit maximal relaxation activity, topoisomerase III requires high temperature (52°C) along with low magnesium and monovalent ion [17, 18]. In contrast, E. coli topoisomerase I was not active in the in vitro assay for resolution of plasmid DNA replication intermediates . Removal of the C-terminal 49 amino acids from the 653 amino acid topoisomerase III protein resulted in drastic reduction of catalytic activity . Fusion of the carboxyl-terminal 312 amino acid residues of E. coli topoisomerase I, which includes the entire ZD domain, onto the 605 N-terminal amino acids of topoisomerase III generated a hybrid topoisomerase that has relaxation activity resembling topoisomerase III along with weak decatenating activity . Although preferring single-stranded DNA as binding substrate, topoisomerase I had been shown to also bind double-stranded DNA , but there is no data available to indicate which domain in the enzyme is responsible for this interaction.
The experiments described here measured directly the interaction of the ZD domain with both double-stranded and single-stranded DNA substrates. ZD domain was found to bind to single-stranded DNA, but not double-stranded DNA, with high affinity. This result indicated that with regard to the mechanism of E. coli topoisomerase I, the ZD domain was likely to function as a single-stranded DNA binding domain instead of having double-stranded DNA binding function as previously suggested . Even though Zn(II) binding transcription factors that recognise specific double-stranded DNA are well represented in eukaryotes [23, 24], there are also numerous examples of Zn(II) coordination being required for interaction with single-stranded nucleic acid or damaged DNA with single-strand characteristics [24–27].
The effect of removal of the ZD domain on the individual step of enzyme action was also investigated using Top67. The results indicated that Top67 was effective in binding to both double-stranded and single-stranded DNA. As a result, Top67 could position itself in the absence of ZD domain at the junction of double- and single-stranded DNA for subsequent DNA cleavage, as previously observed for intact topoisomerase I . Reversal of DNA cleavage also took place readily with Top67 upon addition of 1 M NaCl and 4 mM MgCl2. The ZD domain also was not required for selectivity of a cytosine in the -4 position relative to the cleavage sites.
Despite its ability to recognise the DNA substrate and carry out DNA cleavage-religation, Top67 by itself cannot catalyze change of linking number in the relaxation of supercoiled DNA. The single-strand DNA substrate designated for the ZD domain in the catalytic mechanism of the enzyme may be the strand of DNA complementary to the strand first cleaved by the enzyme to form the covalent complex. This interaction with the passing strand of DNA would not be needed for the first two steps of enzyme mechanism up to the formation of the covalent complex. Our results showed that adding the purified ZD domain partially restored the relaxation activity. Therefore the ZD domain can supply the function that is missing in Top67 even when the two domains are not covalently linked. However, the resulting relaxation activity is much less efficient than that of the intact enzyme, suggesting that coordinated actions of the two domains are required for efficient removal of negative supercoils from DNA. The requirement of specific protein-protein interactions between the two domains could also account for the weak relaxation activity observed for the hybrid topoisomerase with ZD linked to topoisomerase III sequence .
This proposed role for the ZD domain in interacting with the passing single-strand of DNA is also supported by the observation that there is no difference between Top67 and intact topoisomerase I in the formation of catenanes. This reaction involves passage of another double-stranded DNA circle, instead of the complementary DNA strand through the break generated by DNA cleavage so the ZD domain would not be expected to play any significant role. High concentration of DNA substrate is required to favor formation of catenanes catalyzed by topoisomerase I, and the enzyme also has to be present in higher concentration compared to the relaxation reaction. The double-stranded DNA-binding activity in E. coli topoisomerase III required for highly efficient decatenation activity is attributed to a 17-amino-acid residue with no counterpart in E. coli topoisomerase I [28, 29]. It may be required for interaction with the passing double-strand of DNA in the decatenation mechanism. The presence of this decatenation loop instead of the Zn(II) binding ZD domain in topoisomerase III may account for the dominance of the decatenation activity over the relaxation activity.
The hyperthermophilic topoisomerase I from Thermotoga maritima has been shown to coordinate one Zn(II) with a unique tetracysteine motif Cys-X-Cys-X16-Cys-X-Cys but Zn(II) binding is not required for relaxation activity . The sequence of this unique tetracysteine motifs is somewhat different from those present in other type IA topoisomerases in that the other tetracysteine motifs always had at least two amino acids separating the pairs of cysteines (Cys-X2-11-Cys), instead of just one amino acid (Cys-X-Cys) in T. maritima topoisomerase I . Therefore the structure and function of the single Zn(II) binding motif in T. maritima may differ from the multiple Zn(II) binding motifs in E. coli topoisomerase I. Direct interaction between DNA and the T. maritima Zn(II) binding motif has not been demonstrated. It has been suggested that the mechanisms of these two enzymes may be different . Direct interaction between the enzyme and the passing strand may not be necessary for the T. maritima topoisomerase I activity. The relaxation and decatenation activities of T. maritima topoisomerase I appear to be significantly more efficient than those of the E. coli topoisomerase I . Based on their primary sequences, a number of bacterial topoisomerase I enzymes do not appear to coordinate any Zn(II) with tetracysteines motifs while other type IA topoisomerase has up to 4 tetracysteine motifs . The topoisomerase I from Mycobacterium smegmatis has been demonstrated biochemically not to bind Zn(II) . In contrast, mutation disrupting the fourth Zn(II) motif of Helicobacter pylori topoisomerase I abolished enzyme function in vivo. Therefore there may be significant differences in the mechanisms of type IA topoisomerases from different organisms with respect to requirement of Zn(II) binding for relaxation activity.
There is also another possible explanation for the varied number of tetracysteine motifs and requirement of Zn(II) for relaxation activity found in different type IA topoisomerases. The 14 kDa C-terminal region of E. coli topoisomerase I has been classified based on its structure to be in the Zn-ribbon superfamily [SCOP release 1.50, 7] even though it does not bind Zn(II). It also has high affinity for binding to single-stranded DNA on its own when separated from the three tetracysteine motifs . Based on the structural and DNA-binding properties of the E. coli topoisomerase I 14 kDa domain, one can conclude that it is possible for a subdomain in topoisomerase I to lose the Zn(II) binding cysteines during evolution and still maintains the Zn-ribbon structure and single-strand DNA binding properties .
Finally, the in vivo catalytic activities of eukarytotic type IA topoisomerases, the topoisomerase III from various higher organisms may be related to their sequences. The transesterification domains of these eukaryotic enzymes have high degrees of identity to E. coli DNA topoisomerase III [7, 37]. However, the decatenation loop is not present in the eukaryotic topoisomerase III sequences and to date the decatenation activity has not been demonstrated for these enzymes. The number of potential Zn(II) binding cysteine motifs range from none in S. cerevisiae DNA topoisomerase III to four highly conserved tetracysteine motifs in the beta family of the topoisomerase III enzymes . The Zn(II) domain formed by these tetracysteine motifs may be required for interaction with single-strand DNA in removal of hypernegative supercoils  or disruption of early recombination intermediates between inappropriately paired DNA molecules .
We have shown that the ZD domain of E. coli DNA topoisomerase I is not required for the substrate recognition and DNA cleavage-religation action of the enzyme. We propose that the ZD domain interacts with the passing single-strand of DNA in the relaxation of negatively supercoiled DNA by this enzyme.
Materials and methods
Enzyme and DNA
E. coli DNA topoisomerase I and the ZD domain were expressed and purified as described [6, 41]. To express the 67 kDa N-terminal transesterification domain (Top67), a stop codon at amino acid 598 was introduced into plasmid pJW312  used for topoisomerase I expression by site-directed mutagenesis employing the Chameleon-Mutagenesis kit from Stratagene. Top67 was expressed and purified with the same procedures as topoisomerase I.
The oligonucleotides were custom synthesized by Genosys. The single-strand substrates and the top strand of the duplex substrates were labeled at the 5' termini with T4 polynucleotide kinase and γ32P-ATP. The labeled oligonucleotides were purified by electrophoresis in a 12 or 15% sequencing gel. After elution from the gel slice, the labeled single-stranded oligonucleotides were desalted by centrifugation through a Sephadex G10 spin column.
The duplex or heteroduplex substrates were prepared by mixing the labeled top strand with 4 fold excess of the unlabeled bottom strand, heating at 80°C for three minutes, cooling to room temperature and purified by electrophoresis in a 20% non-denaturing polyacrylamide gel with TBE buffer.
Plasmid pJW312 DNA used in relaxation assay was purified by CsCl centrifugation. Phage PM2 DNA was extracted from infected Pseudoalteromonas espejiana cells  and PM2 DNA with one or more single-chain scissions used in the catenation assay was prepared as described .
DNA relaxation assay
Top67 and the ZD domains at different concentrations were mixed and incubated at 37°C for 10 min before addition to the 0.3 μg of supercoiled plasmid DNA in 20 μl of 10 mM Tris-HCl pH 8.0, 2 mM MgCl2, 0.1 mg/ml gelatin. After incubation at 37°C for up to 1 h, the reaction was stopped by addition of 50 mM EDTA and electrophoresed in a 0.7% agarose gel and visualized by ethidium bromide staining as described .
Gel mobility shift assay
The proteins were mixed with the 1 pmole of the labeled DNA substrates in 10 μl of 20 mM Tris-HCl pH 8.0, 100 μg/ml BSA, 12% glycerol and 0.5 mM EDTA. The samples were incubated at 37°C for 5 min and then loaded onto a 6% polyacrylamide gel and electrophoresed with buffer of 45 mM Tris-borate pH 8.3, 1 mM EDTA. Electrophoresis was carried out at room temperature at 2 V/cm for 2 h. After drying of the gel, bands corresponding to the protein-bound oligonucleotides and unbound oligonucleotides were visualized by autoradiography, excised and counted in a Scintillation counter for quantitation.
DNA cleavage assay
The cleavage assays were carried out with 1 pmole of 5' 32P-end labeled DNA substrate and 5–10 pmoles of topoisomerase I or Top67 in 10 μl of the buffer used for the gel mobility shift assay. After incubation at 37°C for up to 20 min, an equal volume of 90% formamide, 10 mM KOH, 0.25% bromophenol blue and 0.25% xylene cyanol was added to stop the reactions. The samples were analyzed by electrophoresis in a 12% sequencing gel followed by autoradiography.
Salt and Mg2+ induced reversal of cleavage
The conditions were modified from those described previously . The cleavage reactions were incubated at 37°C for 5 min and then divided into three aliquots. The cleavage products were trapped in one aliquot by the addition of SDS to 1%. NaCl (1 M) alone or NaCl with MgCl2 (4 mM) were added to the other aliquots followed by further incubation at 37°C for up to 30 min before the addition of SDS. The products were analyzed as described for the cleavage reactions.
Catenation of nicked DNA circles
The catenation reaction was carried out with 1.4 μg of nicked PM2 phage DNA in 20 μl of 10 mM Tris-HCl, pH 8.0, 0.1 mM EDTA, 10 mM KCl, 10 mM MgCl2. After incubation at 37°C for up to1 h, the reactions were stopped with the addition of 1% SDS and 50 mM EDTA. The products were analyzed as described for the relaxation assay.
This work was supported by a grant (GM54226) to Y.T. and a predoctoral fellowship (GM17315) to A.A. from NIGMS, HHS. We thank Chang-Xi Zhu for preparation of topoisomerase I.
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