Recognition of chemically damaged DNA by the gene 32 protein from bacteriophage T4

We have used fluorescence spectroscopy to investigate the binding of gene 32 protein from bacteriophage T4 to DNA which has been chemically modified with carcinogens or antitumor drugs. This protein exhibits a high specificity for single-stranded nucleic acids and binds more efficiently to DNA modified either with cis-diaminodichloroplatinum(II) or with aminofluorene derivatives than to native DNA. This increased affinity is related to the formation of locally unpaired regions which are strong binding sites for the single-strand binding protein. In contrast, gene 32 protein has the same affinity for native DNA, DNA containing methylated purines and DNA that has reacted with trans-diaminodichloroplatinum(II) or with chlorodiethylenetriaminoplatinum(II) chloride. These types of damage do not induce a sufficient structural change to allow gene 32 protein binding. Depurination of DNA does not create binding sites for the T4 gene 32 protein but nicked apurinic sites are strong ligands for the protein. This T4 single-strand binding protein does not exhibit a significantly increased affinity for nicked DNA as compared with native DNA. These results are discussed with respect to the recognition of DNA damage by proteins involved in DNA repair and to the possible role of single-strand binding proteins in DNA repair mechanisms.     

Kinetics of protein-nucleic acid interactions: use of salt effects to probe mechanisms of interaction

The kinetics of protein-nucleic acid interactions are discussed with particular emphasis on the effects of salt concentration and valence on the observed rate constants. A general review is given of the use of experimentally determined salt dependences of observed kinetic parameters as a tool to probe the mechanism of interaction. Quantitative analysis of these salt dependences, through the application of polyelectrolyte theory, can be used to distinguish reactions which occur in a single step from those reactions which involve distinct intermediates. For those rate constants which display a large salt dependence, in either the association or dissociation reaction, this is due to the high concentration of counterions (e.g., Na+) in the vicinity of the nucleic acid which are subsequently released (or bound in the case of dissociation) at some point before the rate limiting step of the reaction. A general discussion of other features which affect protein-nucleic acid kinetics, such as nucleic acid length and the ratio of nonspecific to specific DNA binding sites (in the case of sequence specific binding proteins), is also given. The available data on the nucleic acid binding kinetics of small ligands (ions, dyes, oligopeptides), nonspecific binding proteins (T4 gene 32 protein, fd gene 5 and Escherichia coli SSB), and sequence specific binding proteins (lac repressor, RNA polymerase, Eco RI restriction endonuclease) are discussed with emphasis on the interpretation of the experimentally determined salt dependences.     

Structural and enzymatic studies of the T4 DNA replication system. II. ATPase properties of the polymerase accessory protein complex

In this paper we report a detailed enzymatic characterization of the interaction of the polymerase accessory protein complex of the T4 DNA replication system with the various nucleic acid cofactors that activate the ATPase of the complex. We show that the ATPase activity of the T4 coded gene 44/62 protein complex is stimulated synergistically by binding of DNA and T4 gene 45 protein and that the level of ATPase activation appears to be directly correlated with the binding of nucleic acid cofactor. Binding of any partially or completely single-stranded DNA to the complete accessory protein complex increases the catalytic activity (as measured by Vmax) while decreasing the binding affinity for the ATP substrate. While single-stranded DNA is a moderately effective cofactor, we find that the optimal nucleic acid-binding site for the complex is the primer-template junction, rather than single-stranded DNA ends as previously reported in the literature. Gene 45 protein plays an essential role in directing the specificity of binding to primer-template sites, lowering the Km for primer-template sites almost 1000-fold, and increasing Vmax 100-fold, compared with the analogous values for gene 44/62 protein alone. The most effective primer-template site for binding and enzymatic activation has the physiologically relevant recessed 3'-OH configuration and an optimal size in excess of 18 base pairs of duplex DNA. We find that the chemical nature of the primer terminus (i.e. 3'-OH or 3'-H) does not affect the extent of ATPase activation and that binding of the polymerase accessory protein complex to DNA cofactors is salt concentration dependent but appreciably less so when the activating DNA is a primer-template junction. Finally, we show that the gene 32 protein (T4 coded single-stranded DNA-binding protein) can compete with the polymerase accessory protein complex for single-stranded DNA but not for the primer-template junction activation sites. The implications of these results for the structure and function of the polymerase accessory protein complex within the T4 DNA replication system are discussed.     

Interactions of aromatic residues of proteins with nucleic acids. Fluorescence studies of the binding of oligopeptides containing tryptophan and tyrosine residues to polynucleotides.

The binding of oligopeptides of general structure Lys-X-Lys (where X is an aromatic residue) to several polynucleotides has been studied by fluorescence spectroscopy. Two types of complexes are formed, both involving electrostatic interactions between lysyl residues and phosphate groups as shown by the ionic strength and pH dependence of binding. The fluorescence quantum yield of the first complex is identical with that of the free peptide. The other complex involves a stacking of the nucleic acid bases with the aromatic amino acid whose fluorescence is quenched. Fluorescence data have been quantitatively analyzed according to a model involving these two types of complexes. Association constants and the size of binding sites have been determined. Stacking interactions are favored in single-stranded polynucleotides as compared to double-stranded ones. A short oligopeptide such as Lys-X-Lys is thus able to distinguish between single-stranded and double-stranded nucleic acids. Fluorescence results are compared to those obtained by proton magnetic resonance and circular dichroism.     

Theory of electrostatically regulated binding of T4 gene 32 protein to single- and double-stranded DNA

Bacteriophage T4 gene 32 protein (gp32) is a single-stranded DNA binding protein, which is essential for DNA replication, recombination, and repair. In a recent article, we described a new method using single DNA molecule stretching measurements to determine the noncooperative association constants K(ds) to double-stranded DNA for gp32 and *I, a truncated form of gp32. In addition, we developed a single molecule method for measuring K(ss), the association constant of these proteins to single-stranded DNA. We found that in low salt both K(ds) and K(ss) have a very weak salt dependence for gp32, whereas for *I the salt dependence remains strong. In this article we propose a model that explains the salt dependence of gp32 and *I binding to single-stranded nucleic acids. The main feature of this model is the strongly salt-dependent removal of the C-terminal domain of gp32 from its nucleic acid binding site that is in pre-equilibrium to protein binding to both double-stranded and single-stranded nucleic acid. We hypothesize that unbinding of the C-terminal domain is associated with counterion condensation of sodium ions onto this part of gp32, which compensates for sodium ion release from the nucleic acid upon its binding to the protein. This results in the salt-independence of gp32 binding to DNA in low salt. The predictions of our model quantitatively describe the large body of thermodynamic and kinetic data from bulk and single molecule experiments on gp32 and *I binding to single-stranded nucleic acids.     

Real-time monitoring of nucleic acid ligation in homogenous solutions using molecular beacons

Nucleic acids ligation is a vital process in the repair, replication and recombination of nucleic acids. Traditionally, it is assayed by denatured gel electrophoresis and autoradiography, which are not sensitive, and are complex and discontinuous. Here we report a new approach for ligation monitoring using molecular beacon DNA probes. The molecular beacon, designed in such a way that its sequence is complementary with the product of the ligation process, is used to monitor the nucleic acid ligation in a homogeneous solution and in real-time. Our method is fast and simple. We are able to study nucleic acids ligation kinetics conveniently and to determine the activity of DNA ligase accurately. We have studied different factors that influence DNA ligation catalyzed by T4 DNA ligase. The major advantages of our method are its ultrasensitivity, excellent specificity, convenience and real-time monitoring in homogeneous solution. This method will be widely useful for studying nucleic acids ligation process and other nucleic acid interactions.     

Two sides of the coin: affinity and specificity of nucleic acid interactions

During the past decade, synthetic nucleobase oligomers have found wide use in biochemical sciences, biotechnology and molecular medicine, both as research and/or diagnostic tools and as therapeutics. Numerous applications of common and modified oligonucleotides and oligonucleotide mimics rely on their ability to sequence-specifically recognize nucleic acid targets (DNA or RNA) by forming duplexes or triplexes. In general, these applications would benefit significantly from enhanced binding affinities of nucleobase oligomers in the formation of various secondary structures. However, for high-affinity probes, the selectivity of sequence recognition must also be improved to avoid undesirable associations with mismatched DNA and RNA sites. Here, we review recent progress in understanding the molecular mechanisms of nucleic acid interactions and the development of new high-affinity plus high-specificity oligonucleotides and their mimics, with particular emphasis on peptide nucleic acids.     

Libraries for genomic SELEX.

An increasing number of proteins are being identified that regulate gene expression by binding specific nucleic acidsin vivo. A method termed genomic SELEX facilitates the rapid identification of networks of protein-nucleic acid interactions by identifying within the genomic sequences of an organism the highest affinity sites for any protein of the organism. As with its progenitor, SELEX of random-sequence nucleic acids, genomic SELEX involves iterative binding, partitioning, and amplification of nucleic acids. The two methods differ in that the variable region of the nucleic acid library for genomic SELEX is derived from the genome of an organism. We have used a quick and simple method to construct Escherichia coli, Saccharomyces cerevisiae, and human genomic DNA PCR libraries that can be transcribed with T7 RNA polymerase. We present evidence that the libraries contain overlapping inserts starting at most of the positions within the genome, making these libraries suitable for genomic SELEX.     

Site-specific mutagenesis of T4 gene 32: the role of tyrosine residues in protein-nucleic acid interactions

Bacteriophage T4 gene 32 encodes a single-stranded DNA (ssDNA) binding protein (gp32) required for T4 DNA replication, recombination, and repair. Previous physicochemical studies on gp32 and other ssDNA binding proteins have suggested that binding may involve hydrophobic interactions that result from the close approach of several aromatic amino acid side chains with the nucleic acid bases. In the case of gp32, five tyrosines and two phenylalanines have previously been implicated in gp32.ssDNA complex formation. Site-directed mutagenesis of T4 gene 32 was employed to produce a set of eight gp32 mutant proteins, each of which encoded a single substitution at one of the eight tyrosine residues within gp32. The mutant gp32 proteins were then subjected to physicochemical analysis to evaluate the role of each tyrosine residue in gp32 structure and function. Oligonucleotide binding studies suggest that tyrosine residues 84, 99, 106, 115, and 186 each contribute from 0.3 to 0.7 kcal/mol to ssDNA binding, which corresponds to 3-7% of the overall binding energy for gp32.ssDNA complex formation. Replacement of tyrosine residues 73 and 92 appears to lead to large structural changes that may be the result of disrupting the zinc binding subdomain within gp32.     

DNA damage-inducible SUMOylation of HERC2 promotes RNF8 binding via a novel SUMO-binding Zinc finger

Nonproteolytic ubiquitylation of chromatin surrounding deoxyribonucleic acid (DNA) double-strand breaks (DSBs) by the RNF8/RNF168/HERC2 ubiquitin ligases facilitates restoration of genome integrity by licensing chromatin to concentrate genome caretaker proteins near the lesions. In parallel, SUMOylation of so-far elusive upstream DSB regulators is also required for execution of this ubiquitin-dependent chromatin response. We show that HERC2 and RNF168 are novel DNA damage-dependent SUMOylation targets in human cells. In response to DSBs, both HERC2 and RNF168 were specifically modified with SUMO1 at DSB sites in a manner dependent on the SUMO E3 ligase PIAS4. SUMOylation of HERC2 was required for its DSB-induced association with RNF8 and for stabilizing the RNF8-Ubc13 complex. We also demonstrate that the ZZ Zinc finger in HERC2 defined a novel SUMO-specific binding module, which together with its concomitant SUMOylation and T4827 phosphorylation promoted binding to RNF8. Our findings provide novel insight into the regulatory complexity of how ubiquitylation and SUMOylation cooperate to orchestrate protein interactions with DSB repair foci.     

Chemically modified DNA substrates implicate the importance of electrostatic interactions for DNA unwinding by Dda helicase

Helicase-catalyzed disruption of double-stranded nucleic acid is vital to DNA replication, recombination, and repair in all forms of life. The relative influence of specific chemical interactions between helicase and the substrate over a series of multistep catalytic events is still being defined. To this end, three modified DNA oligonucleotides were designed to serve as substrates for the bacteriophage T4 helicase, Dda. A 5'-DNA-PNA-DNA-3' chimera was synthesized, thereby, conferring both a loss of charge and altering the conformational flexibility of the oligonucleotide. The second modified oligonucleotide possessed a single methylphosphonate replacement on the phosphate backbone, creating a gap in the charge distribution of the substrate. The third modification introduced an abasic site into the oligonucleotide sequence. This abasic site retains the charge distribution of the normal DNA substrate yet alters the conformational flexibility of the oligonucleotide. The loss of a base also serves to disrupt the hydrogen-bonding lattice, the intramolecular base-stacking interactions, as well as the intermolecular base-stacking interactions between aromatic amino acid side chains and the substrate. Our results indicate that a gap in the charge distribution along the backbone of the substrate has a more pronounced effect upon helicase-catalyzed unwinding than does the loss of a single base. While all three substrates exhibited some degree of inhibition, analysis of both pre-steady-state and excess enzyme experiments places a greater value upon the electrostatic interactions between helicase and the substrate.     

A capture approach for supercoiled plasmid DNA using a triplex-forming oligonucleotide

Proteins that recognize and bind specific sites in DNA are essential for regulation of numerous biological functions. Such proteins often require a negative supercoiled DNA topology to function correctly. In current research, short linear DNA is often used to study DNA–protein interactions. Although linear DNA can easily be modified, for capture on a surface, its relaxed topology does not accurately resemble the natural situation in which DNA is generally negatively supercoiled. Moreover, specific binding sequences are flanked by large stretches of non-target sequence in vivo. Here, we present a straightforward method for capturing negatively supercoiled plasmid DNA on a streptavidin surface. It relies on the formation of a temporary parallel triplex, using a triple helix forming oligonucleotide containing locked nucleic acid nucleotides. All materials required for this method are commercially available. Lac repressor binding to its operator was used as model system. Although the dissociation constants for both the linear and plasmid-based operator are in the range of 4 nM, the association and dissociation rates of Lac repressor binding to the plasmid-based operator are ∼18 times slower than on a linear fragment. This difference underscores the importance of using a physiologically relevant DNA topology for studying DNA–protein interactions.     

Nucleic acid binding and chaperone properties of HIV-1 Gag and nucleocapsid proteins

The Gag polyprotein of HIV-1 is essential for retroviral replication and packaging. The nucleocapsid (NC) protein is the primary region for the interaction of Gag with nucleic acids. In this study, we examine the interactions of Gag and its NC cleavage products (NCp15, NCp9 and NCp7) with nucleic acids using solution and single molecule experiments. The NC cleavage products bound DNA with comparable affinity and strongly destabilized the DNA duplex. In contrast, the binding constant of Gag to DNA was found to be ~10-fold higher than that of the NC proteins, and its destabilizing effect on dsDNA was negligible. These findings are consistent with the primary function of Gag as a nucleic acid binding and packaging protein and the primary function of the NC proteins as nucleic acid chaperones. Also, our results suggest that NCp7's capability for fast sequence-nonspecific nucleic acid duplex destabilization, as well as its ability to facilitate nucleic acid strand annealing by inducing electrostatic attraction between strands, likely optimize the fully processed NC protein to facilitate complex nucleic acid secondary structure rearrangements. In contrast, Gag's stronger DNA binding and aggregation capabilities likely make it an effective chaperone for processes that do not require significant duplex destabilization.     

Helicase from hepatitis C virus,energetics of DNA binding

The ability of a helicase to bind single-stranded nucleic acid is critical for nucleic acid unwinding. The helicase from the hepatitis C virus, NS3 protein, binds to the 3'-DNA or the RNA strand during unwinding. As a step to understand the mechanism of unwinding, DNA binding properties of the helicase domain of NS3 (NS3h) were investigated by fluorimetric binding equilibrium titrations. The global analysis of the binding data by a combinatorial approach was done using MATLAB. NS3h interactions with single-stranded DNA (ssDNA) are 300-1000-fold tighter relative to duplex DNA. The NS3h protein binds to ssDNA less than 15 nt in length with a stoichiometry of one protein per DNA. The minimal ssDNA binding site of NS3h helicase was determined to be 8 nucleotides with the microscopic K(d) of 2-4 nm or an observed free energy of -50 kJ/mol. These NS3h-DNA interactions are highly sensitive to salt, and the K(d) increases 4 times when the NaCl concentration is doubled. Multiple HCV helicase proteins bind to ssDNA >15 nucleotides in length, with an apparent occluded site of 8-11 nucleotides. The DNA binding data indicate that the interactions of multiple NS3h protein molecules with long ssDNA are both noncooperative and sequence-independent. We discuss the DNA binding properties of HCV helicase in relation to other superfamily 1 and 2 helicases. These studies provide the basis to investigate the DNA binding interactions with the unwinding substrate and their modulation by the ATPase activity of HCV helicase.     

Real-time investigation of nucleic acids phosphorylation process using molecular beacons

Phosphorylation of nucleic acids is an indispensable process to repair strand interruption of nucleic acids. We have studied the process of phosphorylation using molecular beacon (MB) DNA probes in real-time and with high selectivity. The MB employed in this method is devised to sense the product of a ‘phosphorylation–ligation’ coupled enzyme reaction. Compared with the current assays, this novel method is convenient, fast, selective, highly sensitive and capable of real-time monitoring in a homogenous solution. The preference of T4 polynucleotide kinase (T4 PNK) has been investigated using this approach. The results revealed that a single-stranded oligonucleotide containing guanine at the 5′ termini is most preferred, while those utilizing cytosine in this location are least preferred. The preference of (T)₉ was reduced greatly when phosphoryl was modified at the 5′ end, implying that T4 PNK could discern the phosphorylated/unphosphorylated oligonucleotides. The increase of oligonucleotide DNA length leads to an enhancement in preference. A fast and accurate method for assaying the kinase activity of T4 PNK has been developed with a wide linear detection range from 0.002 to 4.0 U/ml in 3 min. The effects of certain factors, such as NTP, ADP, (NH₄)₂SO₄ and Na₂HPO₄, on phosphorylation have been investigated. This novel approach enables us to investigate the interactions between proteins and nucleic acids in a homogenous solution, such as those found in DNA repair or in drug development.     

Proteomic dissection of DNA polymerization

DNA polymerases replicate the genome by associating with a range of other proteins that enable rapid, high-fidelity copying of DNA. This complex of proteins and nucleic acids is termed the replisome. Proteins of the replisome must interact with other networks of proteins, such as those involved in DNA repair. Many of the proteins involved in DNA polymerization and the accessory proteins are known, but the array of proteins they interact with, and the spatial and temporal arrangement of these interactions, are current research topics. Mass spectrometry is a technique that can be used to identify the sites of these interactions and to determine the precise stoichiometries of binding partners in a functional complex. A complete understanding of the macromolecular interactions involved in DNA replication and repair may lead to discovery of new targets for antibiotics against bacteria and biomarkers for diagnosis of diseases, such as cancer, in humans.     

Proteomic dissection of DNA polymerization

DNA polymerases replicate the genome by associating with a range of other proteins that enable rapid, high-fidelity copying of DNA. This complex of proteins and nucleic acids is termed the replisome. Proteins of the replisome must interact with other networks of proteins, such as those involved in DNA repair. Many of the proteins involved in DNA polymerization and the accessory proteins are known, but the array of proteins they interact with, and the spatial and temporal arrangement of these interactions, are current research topics. Mass spectrometry is a technique that can be used to identify the sites of these interactions and to determine the precise stoichiometries of binding partners in a functional complex. A complete understanding of the macromolecular interactions involved in DNA replication and repair may lead to discovery of new targets for antibiotics against bacteria and biomarkers for diagnosis of diseases, such as cancer, in humans.