D. V. Bugreev

Formation of nucleoprotein RecA filament on single-stranded DNA. Analysis by stepwise increase in ligand complexity.

RecA protein plays a pivotal role in homologous recombination in Escherichia coli. RecA polymerizes on single‐stranded (ss) DNA forming a nucleoprotein filament. Then double‐stranded (ds) DNA is bound and searched for segments homologous to the ssDNA. Finally, homologous strands are exchanged, a new DNA duplex is formed, and ssDNA is displaced. We report a quantitative analysis of RecA interactions with ss d(pN)n of various structures and lengths using these oligonucleotides as inhibitors of RecA filamentation on d(pT)20. DNA recognition appears to be mediated by weak interactions between its structural elements and RecA monomers within a filament. Orthophosphate and dNMP are minimal inhibitors of RecA filamentation (I50 = 12–20 mm). An increase in homo‐d(pN)2−40 length by one unit improves their affinity for RecA (f factor) approximately twofold through electrostatic contacts of RecA with internucleoside phosphate DNA moieties (f≈ 1.56) and specific interactions with T or C bases (f≈ 1.32); interactions with adenine bases are negligible. RecA affinity for d(pN)n containing normal or modified nucleobases depends on the nature of the base, features of the DNA structure. The affinity considerably increases if exocyclic hydrogen bond acceptor moieties are present in the bases. We analyze possible reasons underlying RecA preferences for DNA sequence and length and propose a model for recognition of ssDNA by RecA.

INTERACTION OF HUMAN DNA TOPOISOMERASE I WITH SPECIFIC SEQUENCE OLIGODEOXYNUCLEOTIDES

The interaction of human DNA topoisomerase I (topo I) with specific sequence oligodeoxynucleotides (ODNs) of different length and structure has been investigated. All the ODNs used were shown to be effective enzyme inhibitors and to inhibit the topo I catalyzed relaxation of scDNA in a competitive manner. Among two DNA regions (A and B) required for topo I-mediated DNA cleavage, the former was found to display the higher affinity for the enzyme. The enzymes affinity for ODNs corresponding to the scissile strand (five and nine nucleotide units in length) is about 2-4 orders of magnitude higher than that for non-specific ODNs of the same length. Topo I can efficiently recognize even extremely short specific ODNs containing only two or three bases (AGA and pAG, Ki = 15 and 60 microM, respectively): the sequence AAGA (Ki = 10 microM) is essential for tight DNA binding to topo I. The affinities of ODNs corresponding to the non-scissile strand are significantly lower. The ligands affinity increases with its length. Additionally, about a ten-fold enhancement of specific sequence affinity occurs due to stable duplex formation during enzyme preincubation with ligands before addition of scDNA. We believe the possibility of using the short specific oligonucleotides and its derivatives as topoisomerase I-targeting drugs could not be excluded.

The Mechanism of Specific Cleavage of Supercoiled DNA by Human DNA Topoisomerase I: The Effect of Ligand Structure on the Catalytic Step of Cleavage

Eukaryotic DNA topoisomerase I (Topo) regulates the topological state of cell DNA and plays an important part in replication, transcription, repair, and recombination. Factors affecting the specific recognition of topologically stressed DNA were analyzed on the basis of the thermodynamic and kinetic data on the Topo–DNA interaction and the X-ray data on human Topo. A model was advanced for possible structural changes occurring in the ligand after initial recognition. The effect of conformational changes in specific DNA on the catalytic step of the reaction was analyzed.

High affinity interaction of mammalian DNA topoisomerase I with short single- and double-stranded oligonucleotides

The interaction of DNA topoisomerase I (topo I) with a set of single‐ and double‐stranded oligonucleotides containing 5–27 mononucleotides was investigated. All single‐ and double‐stranded oligonucleotides were found to inhibit competitively the supercoiled DNA relaxation reaction catalyzed by topo I. The enzyme affinity for specific sequence pentanucleotides of the scissile (GACTT, K i = 2 μM) and non‐cleaved chain (AAGTC, K i = 110 μM) is about 2–4 orders of magnitude higher than that for non‐specific oligonucleotides. This specific sequence affinity increases in several cases: lengthening of single‐stranded oligonucleotides, formation of stable duplexes between complementary oligonucleotides and preincubation of the enzyme with ligands before addition of supercoiled DNA. We assume that oligonucleotides having a high affinity to the enzyme can offer a unique opportunity for rational design of topoisomerase‐targeting drugs.

High affinity interaction of mouse DNA topoisomerase I with di- and trinucleotides corresponding to specific sequences of supercoiled DNA cleaved chain

Recently mouse DNA topoisomerase I (topo) was shown to possess high affinity for a single‐stranded AAGACTTAG nonanucleotide (K i=2.0 μM) corresponding to the scissile strand of the minimal DNA duplex, which is necessary for cleavage of supercoiled DNA. In order to determine the most important part of the above sequence for the DNA recognition by topo, the interactions of the enzyme with a set of extremely short (2–5 nucleotides in length) oligonucleotides corresponding to different parts of the nonanucleotide have been investigated. The affinities of different oligonucleotides corresponding to the CTTAG part of the sequence (K i=0.13–0.92 mM) were shown to be significantly lower than that for the AAGA tetranucleotide (K i=9.0 μM). Topo effectively recognized even short oligonucleotides containing only two or three bases (AGA and pAG, K i=20 and 50 μM). We suppose that oligonucleotides having a high affinity to the enzyme can offer a unique opportunity for the rational design of topoisomerase‐targeting drugs.

The Mechanism of the Supercoiled DNA Recognition by the Eukaryotic Type I Topoisomerases: I. The Enzyme Interaction with Nonspecific Oligonucleotides

Interaction of the DNA type I topoisomerases from the murine and human placenta cells with nonspecific oligonucleotides was analyzed. The contributions of strong and week nonspecific electrostatic, van der Waalss, and hydrophobic interactions, and hydrogen bonding of the enzymes to the complex formation with the single- and double-stranded DNAs were determined. The factors that determine the top-priority recognition of the topologically stressed DNA were revealed. The results were interpreted in comparison with the X-ray analysis data for human DNA topoisomerase I.

Interaction of single-stranded DNA with the second DNA-binding site of the RecA nucleoprotein filament

RecA first forms a filament on single-stranded DNA (ssDNA), thereby forming the first site for ssDNA binding and, simultaneously, the second site for binding double-stranded DNA (dsDNA). Then, the nucleoprotein filament interacts with dsDNA, although it can bind ssDNA as well. The resulting complex searches for homology sites and performs strand exchange between homologous DNA molecules. The interaction of various ssDNAs with the second DNA-recognizing site of RecA was studied by gradually increasing the structural complexity of the DNA ligand. Recognizing ssDNA with the second site, the protein interacts with each nucleotide of the ligand, forming contacts with both internucleotide phosphate groups and nitrogen bases. Pyrimidine oligonucleotides d(pC)n and d(pT)n interacted with the second site of the RecA filament more efficiently than d(pA)n did. This was due to a more efficient interaction of the RecA filament with the 5′-terminal nucleotide of pyrimidinic DNA and to the difference in specific conformational changes of the nucleoprotein filament in the presence of purinic and pyrimidinic DNAs. A comparison of thermodynamic characteristics of DNA recognition at the first and second DNA-binding sites of the filament showed that, at n > 10, d(pC)n and d(pN)n were bound at the second site less tightly than at the first site. At n > 20, the second site bound d(pA)n more efficiently than the first site. The difference in d(pN)n affinity for the first and second sites increased monotonically with increasing n. Possible mechanisms of a RecA-dependent search for homology and DNA strand exchange are discussed.

Physicochemical Basis of RecA Filamentation on Single-Stranded DNA

Quantitative analysis was performed for the first time for the interaction of Escherichia coli RecA, which plays the central role in homologous recombination, and ssDNA varying in length and structure. DNA recognition was shown to depend on weak additive interactions between RecA monomers of the filament and various structural elements of DNA. Orthophosphate and dNMPs acted as minimal inhibitors of RecA filamentation on d(pN)20. A stepwise increase in homooligonucleotide length by one nucleotide (from 2 to 20 nt) monotonically increased the affinity approximately twofold (factor f) due to weak additive contacts of RecA with each internucleoside phosphate (f ≈ 1.56) and specific interactions with T and C (f ≈ 1.32). In the case of d(pA)n, the RecA filament showed virtually no interaction with bases and interacted more efficiently with internucleoside phosphates of the first than of the next helix turn (n < 10, f ≈ 2.1 vs. n > 10, f ≈ 1.3). The affinity of RecA for d(pN)n and various modified bases proved to depend on the base, the DNA structure, and the conformation of the sugar-phosphate backbone. The affinity considerably increased with bases containing exocyclic proton-accepting groups. Possible causes of the preferential binding of RecA with DNAs of a particular length and composition are considered. Mechanisms are proposed for ssDNA recognition by RecA and for homologous strand exchange.

The mechanism of supercoiled DNA recognition by eukaryotic type I topoisomerases. II. A comparison of the enzyme interaction with specific and nonspecific oligonucleotides

Data on the interaction of DNA type I topoisomerases from the murine and human placenta cells with specific and nonspecific oligonucleotides of various structures and lengths are summarized. The relative contributions of various contacts between the enzymes and DNA that have previously been detected by X-ray analysis to the total affinity of the topoisomerases for DNA substrates are estimated. Factors that determine the differences in the enzyme interactions with specific and nonspecific single- and double-stranded DNAs are revealed. The results of the X-ray analysis of human DNA topoisomerase I are interpreted taking into account data on the comprehensive thermodynamic and kinetic analysis of the enzyme interaction with the specific and nonspecific DNAs.

Inhibition of Human DNA Topoisomerase I by New DNA Minor Groove Ligands: Derivatives of Oligo-1,3-Thiazolecarboxamides

A series of novel thiazole-containing oligopeptides (oligo-1,3-thiazolecarboxamides) interesting specifically with the minor groove of DNA was shown to inhibit human DNA topoisomerase I (topo I). Inhibitory effects of thiazole-containing oligopeptides (TCO) increase with the number of thiazole units in such compounds. Inhibitory properties of TCO containing 3 or 4 thiazole units were shown to be 3-10 times better than those of the well-known natural antibiotic, distamycin A containing pyrrole rings. The structure of various additional groups attached to the N-terminus and C-terminus of TCO had no significant effect on TCO interaction with the complex of DNA and topo I. TCO were shown to be capable of binding with double-stranded DNA (dsDNA), and the majority of TCO analyzed were more effective in binding with dsDNA than distamycin A. Possible reasons for the different effects of distamycin A and TCO on the reaction of relaxation catalyzed by topo I are discussed.