Structure-function relationships of HIV-1 reverse transcriptase determined using monoclonal antibodies

Targeting the Structural Maturation Pathway of HIV-1 Reverse Transcriptase

Formation of active HIV-1 reverse transcriptase (RT) proceeds via a structural maturation process that involves subdomain rearrangements and formation of an asymmetric p66/p66' homodimer. These studies were undertaken to evaluate whether the information about this maturation process can be used to identify small molecule ligands that retard or interfere with the steps involved. We utilized the isolated polymerase domain, p51, rather than p66, since the initial subdomain rearrangements are largely limited to this domain. Target sites at subdomain interfaces were identified and computational analysis used to obtain an initial set of ligands for screening. Chromatographic evaluations of the p51 homodimer/monomer ratio support the feasibility of this approach. Ligands that bind near the interfaces and a ligand that binds directly to a region of the fingers subdomain involved in subunit interface formation were identified, and the interactions were further characterized by NMR spectroscopy and X-ray crystallography. Although these ligands were found to reduce dimer formation, further efforts will be required to obtain ligands with higher binding affinity. In contrast with previous ligand identification studies performed on the RT heterodimer, subunit interface surfaces are solvent-accessible in the p51 and p66 monomers, making these constructs preferable for identification of ligands that directly interfere with dimerization.

Characterization of monoclonal antibodies that recognize the amino- and carboxy-terminal epitopes of the pseudorabies virus UL42 protein

The pseudorabies virus (PRV) UL42 protein, known as the DNA polymerase processivity factor, is an essential protein required for viral replication. The in vitro function of UL42 has been characterized; however, there is little information concerning the linear B cell epitopes of UL42 that are recognized during humoral immune responses. We generated and characterized six UL42-reactive monoclonal antibodies (mAbs) from mice that had been immunized with a recombinant form of UL42. Through western blotting analysis, we identified two regions of UL42 (amino acids 39-148 and 302-384) that reacted with these mAbs. We then synthesized a panel of UL42-derived peptides spanning the two regions and screened the six mAbs. We were able to identify three linear epitopes ((116)SGGVLDALK(124), (354)KRPAAPR(360), and (360)RMYTPIAK(367)) by enzyme-linked immunosorbent assays. The (116)SGGVLDALK(124) epitope was located at the amino-terminus, while the other two epitopes were at the carboxy-terminus. Using these mAbs, we found that UL42 localized to the nucleus during viral replication and could be immunoprecipitated from PRV-infected PK-15 cells. We also established a UL42 mAb-based immunoperoxidase monolayer assay for the determination of PRV titers. Sequence analysis showed that the linear epitopes of UL42 were highly conserved among PRV strains. Taken together, our results indicate that the six generated mAbs could be useful tools for investigating the structure and function of UL42 during viral replication. In addition, these mAbs could be applied to diagnostic and therapeutic approaches for the effective control of PRV infections.

Efavirenz stimulates HIV-1 reverse transcriptase RNase H activity by a mechanism involving increased substrate binding and secondary cleavage activity

Efavirenz is a non-nucleoside reverse transcriptase inhibitor used for treating HIV/AIDS. We found that polymerization activity of a reverse transcriptase (RT) with the E478Q mutation that inactivates the RNase H catalytic site is much more sensitive to efavirenz than wild-type RT, indicating that a functional RNase H attenuates the effectiveness of efavirenz. Moreover, efavirenz actually stimulated wild-type RNase H binding and catalytic functions, indicating another link between efavirenz action and RNase H function. During reverse transcription in vivo, the RT that is extending the DNA primer also periodically cleaves the genomic RNA. The RNase H makes primary template cuts ~18 nucleotides from the growing DNA 3'-end, and when the RT pauses synthesis, it shifts to make secondary cuts ~9 nucleotides from the DNA 3'-end. After synthesis, RTs return to bind the remaining template RNA segments at their 5'-ends and make primary and secondary cuts, 18 and 9 nucleotides in, respectively. We found that efavirenz stimulates both 3'- and 5'-directed RNase H activity. Use of specific substrates revealed a particular acceleration of secondary cuts. Efavirenz specifically promoted binding of the RT to RNase H substrates, suggesting that it stabilizes the shifting of RTs to make the secondary cuts. We further showed that efavirenz similarly stimulates the RNase H of an RT from a patient-derived virus that is highly resistant and grows more rapidly in the presence of low concentrations of efavirenz. We suggest that for efavirenz-resistant RTs, stimulated RNase H activity contributes to increased viral fitness.

HIV-1 nucleocapsid traps reverse transcriptase on nucleic acid substrates

Conversion of the genomic RNA of human immunodeficiency virus (HIV) into full-length viral DNA is a complex multistep reaction catalyzed by the reverse transcriptase (RT). Numerous studies have shown that the viral nucleocapsid (NC) protein has a vital impact on various steps during reverse transcription, which is crucial for virus infection. However, the exact molecular details are poorly defined. Here, we analyzed the effect of NC on RT-catalyzed single-turnover, single-nucleotide incorporation using different nucleic acid substrates. In the presence of NC, we observed an increase in the amplitude of primer extension of up to 3-fold, whereas the transient rate of nucleotide incorporation ( k pol) dropped by up to 50-fold. To unravel the underlying molecular mechanism, we carefully analyzed the effect of NC on RT-nucleic acid substrate dissociation. The studies revealed that NC considerably enhances the stability of RT-substrate complexes by reducing the observed dissociation rate constants, which more than compensates for the observed drop in k pol. In conclusion, our data strongly support the concept that NC not only indirectly assists the reverse transcription process by its nucleic acid chaperoning activity but also positively affects the RT-catalyzed nucleotide incorporation reaction by increasing polymerase processivity presumably via a physical interaction of the two viral proteins.

Small molecule inhibitors targeting HIV-1 reverse transcriptase dimerization

The enzymatic activities of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) are strictly correlated with the dimeric forms of this vital retroviral enzyme. Accordingly, the development of inhibitors targeting the dimerization of RT represents a promising alternative antiviral strategy. Based on mutational studies, we applied a structure-based ligand design approach generating pharmacophoric models of the large subunit connection subdomain to possibly identify small molecules from the ASINEX database, which might interfere with the RT subunit interaction. Docking studies of the selected compounds identified several candidates, which were initially tested in an in vitro subunit association assay. One of these molecules (MAS0) strongly reduced the association of the two RT subunits p51 and p66. Most notably, the compound simultaneously inhibited both the polymerase as well as the RNase H activity of the retroviral enzyme, following preincubation with t(1/2) of about 2 h, indicative of a slow isomerization step. This step most probably represents a shift of the RT dimer equilibrium from an active to an inactive conformation. Taken together, to the best of our knowledge, this study represents the first successful rational screen for a small molecule HIV RT dimerization inhibitor, which may serve as attractive hit compound for the development of novel therapeutic agents.

Mapping epitopes of monoclonal antibodies against HIV-1 integrase with limited proteolysis and matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Monoclonal antibodies (mAbs) have been used extensively in the biochemical analysis of proteins. Molecular identification of a specific epitope can enhance our understanding of the relationship between the structure and function of a protein. We recently developed a protein footprint technique for mapping mAb epitopes that employs limited proteolysis followed by peptide analysis with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Here we describe the rational for the technique and illustrate its use in mapping the epitopes of two mAbs that bind to the C-terminal domain of human immunodeficiency virus type-1 integrase. The results provide a plausible explanation for the fact that one mAb inhibits enzyme activity while the second does not.

Generation of HIV-1/HIV-2 cross-reactive peptide antisera by small sequence changes in HIV-1 reverse transcriptase and integrase immunizing peptides

We have generated peptide antisera against selected regions in HIV-1 and HIV-2 reverse transcriptase (RT) and integrase (IN) to investigate the specificity of determinants governing the immune response. Peptides representing homologous regions (>50%) in the N- and C-termini and central portions of these proteins were synthesized and injected into rabbits. HIV-1 and HIV-2 IN peptide antisera inhibited IN-mediated cleavage of an HIV-1 DNA oligonucleotide substrate in a 3' processing assay, while anti-RT or normal sera had no effect. None of the RT sera inhibited RT activity. In Western blots, HIV-2 antisera directed against RT or IN peptides recognized HIV-2 RT and IN proteins, respectively, as expected, but also cross-reacted with the corresponding HIV-1 proteins. By contrast, corresponding HIV-1 antisera were type-specific. In some cases, HIV-1 cross-reactive antisera could be generated by immunization with HIV-1 chimeric peptides with as few as two residues in the HIV-1 sequence changed to the corresponding HIV-2 amino acids. The finding that a type-specific response can be converted to a cross-reactive response suggests alternate strategies for developing new diagnostic reagents which detect HIV-1 and HIV-2. In addition, our results provide a general model for generating HIV peptide vaccines with dual specificity against HIV-1 and HIV-2.

Immunogenic properties of reverse transcriptase of HIV type 1 assessed by DNA and protein immunization of rabbits

Genetic immunization may be one way to prime individuals for a subsequent broad anti-HIV-1 immune response. Reverse transcriptase of HIV-1 (RT) presents a selective target for attempts to arrest replication of HIV-1. Rabbits immunized with a plasmid carrying the gene for reverse transcriptase HIV-1 (RT DNA) developed potent antibody and cellular responses to the gene product. The immunogenic properties of RT DNA and recombinant reverse transcriptase were compared in rabbits. The specific immune responses were similar to those reported previously for HIV-1 infected humans. The array of B and T cell epitopes recognized in RT DNA-immunized rabbits was broader than in rabbits immunized with the recombinant RT. We localized seven novel B and T cell epitopes and concordance between B cell and helper T cell epitopes was observed. B cell epitopes of RT induced proliferation of peripheral blood mononuclear cells and were active as helper T cell epitopes. T cell-proliferative responses to the epitopes of RT preceded or paralleled the production of antibodies of the same specificity. Subdomains of reverse transcriptase involved in the enzymatic activity of RT were highly immunogenic. Anti-RT IgG partially inhibited reverse transcription in vitro.

Monoclonal antibodies to native HIV type 1 reverse transcriptase and their interaction with enzymes from different subtypes

Recombinant reverse transcriptase (RT) from HIV-1 subtype B was used to produce mouse anti-RT monoclonal antibodies (MAbs). Immunization was done by mixing RT with the ISCOM matrix-forming adjuvant saponin (Quil A). Two different assays, both based on the interaction of native RT and antibodies, were used to monitor the immune response in mice and for screening, selection, and characterization of the MAbs. The first assay measures the capacity of antibodies to inhibit the polymerase activity of the RT and the second assay measures the ability of antibodies to capture enzymatically active RT. Twelve clones with the capacity to inhibit at least 50% of the RT activity and 34 clones with high RT-capturing capacity were found. The MAb panel was utilized to evaluate the immunological properties of 18 different RTs representing 9 different HIV1 subtypes. The RT-inhibitory MAbs could be divided into two groups based on their pattern of cross-reactivity toward the different HIV-1 RTs. The degree of diversity recorded among MAbs with RT-capturing capacity was larger. At least seven groups of MAbs with distinct cross-reactivity patterns were identified. Thus, the degree of isoenzyme specificity varied greatly, from MAbs that were quite specific for subtype B RT to one MAb that was able to capture the RTs from all HIV-1 isolates tested except one of the two group O isolates. In conclusion, our study revealed that there exist surprisingly large immunological differences between RTs from different HIV-1 subtypes as well as from the same subtype.

Eukaryotic expression of enzymatically active human immunodeficiency virus type 1 reverse transcriptase

Reverse transcriptase of human immunodeficiency virus type I is a vitalenzyme in the HIV-1 replication cycle and an attractive target of attempts to arrest a primary viral infection. We designed a vector for eukaryotic expression of the 66 kDa subunit of reverse transcriptase under the control of the immediate early cytomegalovirus promoter. Efficient transient expression of the 66 kDa subunit of reverse transcriptase was achieved in a variety of cells. Immunostaining of the transfected cells revealed the cytoplasmatic localization of reverse transcriptase. Reverse transcriptase activity was detected in all transfected cell lines. Injection of this plasmid encoding the 66 kDa subunit of reverse transcriptase into mice resulted in strong reverse transcriptase-specific immune responses indicating that the 66 kDa subunit of reverse transcriptase is expressed in vivo. Sera from DNA-immunized mice inhibited reverse transcription in vitro.

Epitope mapping of SHP-1 monoclonal antibodies using peptide phage display

We have characterized the binding epitopes of four monoclonal antibodies for SHP-1, an SH2 domain containing protein tyrosine phosphatase, using two phage displayed random peptide libraries. Three of the antibodies are directed against the phosphatase domain of the molecule and the fourth is toward the NH2-terminal part of the second SH2 domain. The first two antibodies recognize the sequence NANY, amino acid 305 to amino acid 308, numbered in the non haematopoietic form of human SHP-1 sequence. The third antibody binds the sequence P Y W P (amino acids 365 to 368) located toward the middle of the phosphatase domain of the enzyme. The fourth antibody is directed against the first two amino acids, W Y (amino acids 112 and 113), of the second SH2 domain. The specificities of these antibodies are demonstrated by ELISA and western blot using different protein constructs expressed in bacteria. All the antibodies can detect wild type SHP-1, expressed in 293 cells, by western blot analysis, both under denaturing conditions as well as following renaturation. The data presented here show that the antibodies characterized in this study are raised against linear epitopes and suggest that these epitopes are accessible from the outside in the native SHP-1 molecule.

The p51 subunit of human immunodeficiency virus type 1 reverse transcriptase is essential in loading the p66 subunit on the template primer

Human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) is a dimeric enzyme consisting of p66 and p51 subunits. The functional role of the p51 subunit remains elusive since all the catalytic functions appear to be executed through the p66 subunit. We report here that the p51 subunit, in addition to providing structural support to the p66 subunit, may be involved in facilitating the loading of the p66 subunit on to the template-primer (TP). This possibility is supported by following observations: (i) Upon binding to the TP, the p51 subunit can be dissociated by acetonitrile treatment and the template-primer-bound p66 monomer alone is capable of catalyzing DNA synthesis. (ii) Photo-cross-linking of template-primer to HIV-1 RT is abolished by dissociation of the p51 subunit prior to the TP binding but remains unaffected after the TP binding step. (iii) The p66-TP covalent complex selectively generated by UV irradiation and separated by gel electrophoresis can incorporate a single nucleotide in situ upon its renaturation in the gel. (iv) Treatment of HIV-1 RT with (tert-butyldimethylsilyl)spiroaminooxathioledioside (TSAO), an inhibitor that specifically binds to the beta7 beta8 loop of p51, destabilizes the heterodimeric enzyme, resulting in the subsequent loss of DNA binding.

Generation of neutralizing antibody to the reverse transcriptase of human immunodeficiency virus type 1 by immunizing of mice with an infectious vaccinia virus recombinant

Antibodies inhibiting the reverse transcriptase (RT) of human immunodeficiency virus type-1 (HIV-1) were found to be generated in the serum of mice repeatedly infected with a vaccinia virus recombinant, WRRT, expressing the enzyme. A monoclonal antibody (mAb), 7C4, which specifically and almost completely inhibits the RNA-dependent DNA polymerase activity of HIV-1 RT was produced from a mouse repeatedly immunized with WRRT. 7C4 seems to be specific for HIV-1 among retroviruses: 7C4 inhibited RT activity of three strains of HIV-1 (IIIB, Bru, and IMS-1) but not of two strains of HIV-2 (GH-1 and LAV-2) or two strains of SIV (MAC and MND). The immunoglobulin isotype of three out of four mAbs produced from spleen cells of the immunized mouse were IgG2a. This immunization method that avoids protein denaturation may preferentially induce a T helper type-1 immune response and increase the chances of producing the only occasionally obtainable mAb capable of recognizing a conformational epitope and completely inhibiting enzyme activity.

Reverse transcriptase and corresponding activity-blocking antibody for monitoring SIVsm infection in macaques

A nonradioactive reverse transcriptase (RT) assay was used to measure RT activity in serum during the viremia peak associated with primary infection and for measuring the generation and maintenance of RT activity-blocking antibody (RTb-ab) titers during and after seroconversion in SIV-infected macaques. The RT assay was compared to an antigen capture immunoassay designed for HIV-2/SIVsm and was found to be approximately 40 times more sensitive in detecting SIVsm in serum from infected macaques. The RT assay detected RT activity in serum corresponding to levels from 3 pg/ml. Earliest detection of viral replication using the RT assay was on day 6-8, with a peak at day 10 (up to 8000 pg/ml). The earliest detection of RTb-ab was seen on day 17-23, with established RTb-ab titers by day 29, followed by increasing titers of 15,000-120,000 by day 62-77. The usefulness of RT and RTb-ab for monitoring the course of SIV infection in monkey models is discussed.

Human recombinant antibody fragments neutralizing human immunodeficiency virus type 1 reverse transcriptase provide an experimental basis for the structural classification of the DNA polymerase family

We describe in this paper the binding and biochemical properties of two human antibody fragments directed against the human immunodeficiency virus type 1 reverse transcriptase (RT). These fragments were isolated from a synthetic combinatorial library of human Fab antibody fragments displayed on the surface of filamentous phage. The antibody fragments were selected by using recombinant heterodimeric human immunodeficiency virus type 1 RT purified from insect cells as a solid-phase selector. This procedure led to the isolation of two antibody fragments that completely neutralize the RNA-dependent DNA polymerase activity of RT at nanomolar concentrations. Both antibody fragments bind only to the enzymatically active form of the RT. The inhibitory activity of the anti-RT antibody fragments is competitive with respect to the template primer. The antibody fragments also neutralize the activities of RTs from avian and murine retroviruses and of DNA polymerases of prokaryotic origin as well as human DNA polymerase alpha. Thus, the antibody fragments selected and characterized in this study appear to recognize a structural fold that is common to the different DNA polymerases and necessary for their activity. The results provide an immunological experimental basis for a purely structural and evolutionary classification of the polymerase family.

Characterization of monoclonal antibodies recognizing amino- and carboxy-terminal epitopes of the herpes simplex virus UL42 protein

A panel of monoclonal antibodies (MAbs) directed against the herpes simplex virus type 1 (HSV-1) DNA polymerase (Pol) accessory protein, UL42, was developed and characterized. Thirteen different MAbs were isolated which exhibited varied affinities for the protein. All MAbs reacted with UL42 in ELISA, Western blot and immunoprecipitation analyses. Competitive ELISA was used to show that 6 different epitopes within UL42 were recognized by the MAbs. Immunoprecipitation of amino- and carboxy-terminal truncations of UL42 mapped the epitopes to regions containing amino acids 1-10, 10-108, 338-402, 402-460, and 460-477. All but one of these epitopes were outside the minimal active portion of the protein previously mapped to amino acids 20-315. None of these MAbs, alone or in combination, specifically neutralized the ability of UL42 to stimulate Pol activity in vitro. These results are consistent with structure-function studies that showed that N- and C-terminal regions of the UL42 protein, those recognized by the MAbs, are not involved in UL42 function in vitro.

Monoclonal antibodies against HIV type 1 integrase: clues to molecular structure

Eleven murine hybridoma clones were selected for their ability to produce anti-HIV-1 integrase (IN) antibodies. Competition and epitope mapping studies allowed segregation of the monoclonal antibodies (MAbs) into four distinct classes. The five MAbs that comprise the first class showed high affinity for epitopes within an N-terminal domain of 58 amino acids that includes a conserved zinc finger motif. The second class, with two MAbs, showed high affinity for epitopes within 29 amino acids at the C terminus. Another two MAbs, which constitute the third class, displayed moderate affinities for epitopes that mapped to regions within the highly conserved catalytic core referred to as the D,D(35)E domain. One of these MAbs showed significant cross-reactivity with HIV-2 IN and weak, but detectable, cross-reactivity with RSV IN. The remaining two MAbs, which comprise the fourth class, exhibited fairly low binding affinities and appeared to recognize epitopes in the zinc finger motif domain as well as the C-terminal half of the IN protein. The MAbs can be used for immunoprecipitation and immunoblotting procedures as well as for purification of HIV-1 IN protein by affinity chromatography. We show that several can also be used to immunostain viral IN sequences in HIV-1-infected T cells, presumably as a component of Gag-Pol precursors. Finally, analysis of our mapping and competition data suggests a structure for mature IN in which the C terminus approaches the central core domain, and the N and C termini touch or are proximal to each other. These MAbs should prove useful for further analyses of the structure and function of IN both in vitro and in vivo.

Mechanism of enzyme inhibition mediated by anti-reverse transcriptase antibodies from HIV type 1-infected individuals

We have examined the mechanisms of reverse transcriptase (RT) inhibition mediated by anti-RT antibodies, isolated by affinity chromatography, from four HIV-1-positive individuals. In kinetics assays, anti-RT immunoglobulin (Ig) obtained from three of the sera mediated a noncompetitive type of inhibition against template primer; two of these three also mediated noncompetitive inhibition with respect to deoxyribonucleoside triphosphate. Such inhibition did not require that the Ig be preincubated with RT prior to the addition of reaction components. In contrast, a more complicated pattern of inhibition was exhibited by anti-RT Ig from the fourth serum. Preincubation of this Ig with enzyme markedly enhanced the inhibition. The results demonstrate that the specificities of RT-inhibiting antibodies vary among HIV-1-infected individuals, but that one prevalent mechanism of inhibition involves interactions with epitopes outside of the enzyme active site.

Characterization of HIV-1 reverse transcriptase with antibodies indicates conformational differences between the RNAse H domains of p 66 and p 15

Antibody binding to the p 66 and p 15 RNase H regions of HIV-1 reverse transcriptase was compared using a polyclonal rabbit immune serum raised against a synthetic peptide from the RNase H region of reverse transcriptase (aa 511-527) and six monoclonal antibodies binding to discontinuous epitopes in the RNase H region of p 66. The antigens used in Western blot analysis included recombinantly expressed homodimeric p 66 digested with the HIV-1 protease for generation of the p 51 and p 15 polypeptides and two different length RNase H domains expressed as Trp E fusion proteins (aa 410-560 and aa 441-560). The polyclonal rabbit antibody binding to a continuous epitope recognized both the Trp E-fusion proteins and also the polypeptides p 66 and p 15 generated by processing of homodimeric p 66 with the viral protease. Two additional cleavage products with estimated molecular weights of 9 and 11 kDa were also detected. The anti-RNase H MAbs binding to discontinuous epitopes recognized only the RNase H domain of the p 66 polypeptide and the Trp E-RNase H fusion protein when this was expressed together with the C-terminal part of the polymerase domain. The results indicate conformational differences between the RNase H domain of the p 66 subunit and the RNase H p 15 polypeptide.

Purification and characterization of an active human immunodeficiency virus type 1 RNase H domain

We have expressed and purified from Escherichia coli a human immunodeficiency virus type 1 (HIV-1) RNase H domain consisting of amino acids 400 to 560 of reverse transcriptase with either an N- or C-terminal polyhistidine tag. The native protease cleavage site of HIV-1 reverse transcriptase is between amino acids 440 and 441. Purification on Ni(2+)-nitrilotriacetate agarose resulted in a highly active RNase H domain dependent on MnCl2 rather than MgCl2. Activity was unambiguously attributed to the purified proteins by an in situ RNase H gel assay. Residues 400 to 426, which include a stretch of tryptophans, did not contribute to RNase H activity, and the polyhistidine tag was essential for activity. Despite the requirement for a histidine tag, the recombinant RNase H proteins retained characteristics of the wild-type heterodimer, as determined by examining activity in the presence of several known inhibitors of HIV-1 RNase H, including ribonucleoside vanadyl complexes, dAMP, and a monoclonal antibody. Importantly, the isolated RNase H domain produced the same specific cleavage in tRNA(3Lys) removal as HIV-1 heterodimer, leaving the 3'-rA (adenosine 5' phosphate) residue of a model tRNA attached to the adjacent U5 sequence. This HIV-1 RNase H domain sedimented as a monomer in a glycerol gradient.

Characterization of the dimerization process of HIV-1 reverse transcriptase heterodimer using intrinsic protein fluorescence

Intrinsic protein fluorescence has been used to study dimerization of the HIV-1 reverse transcriptase (RT). We observed a 25% increase of the tryptophan fluorescence of the enzyme during dissociation of the subunits induced by the addition of acetonitrile. Upon reassociation of the separated subunits, the original fluorescence emission of the heterodimer is restored. A two-state transition model for the RT dimerization process in which the dimers are in equilibrium with folded monomers is proposed. The free energy of dissociation was determined to be 12.2 (+/- 0.2) kcal/mol. In the absence of Mg2+ ions a decrease of this value was observed, whereas the addition of a synthetic primer/template (18/36mer) results in an increase of dimer stability. Analyzing the effect of Mg2+ on the establishment of the binding equilibrium, a dramatic effect with a 100-fold acceleration of the association by the divalent ion was observed.