Modulate DNA Binding Research Articles

The long non-coding RNA GAS5 contributes to the suppression of inflammatory responses by inhibiting NF-κB activity.

Nuclear factor kappa B (NF-κB) is a key regulator of immune and inflammatory responses. Glucocorticoid drugs (GC) act through the glucocorticoid receptor (GR) as immunosuppressant also in pediatric patients inhibiting NF-κB activity. The long non-coding RNA GAS5 interacts with the GR, influencing GC activity. No data on the role of GAS5 on GR-dependent inhibition of NF-κB activity have been published. This study investigated the impact of GAS5 on NF-κB activity in HeLa cells overexpressing GAS5, both under basal conditions and during GC treatment. The study used EMSA, RNA-immunoprecipitation (RIP), Western blotting, and bioinformatic analyses to assess NF-κB DNA binding, GAS5-p65 interaction, and NF-κB signaling pathway modulation. GAS5 overexpression increased NF-κB DNA binding activity in untreated cells. RNA-IP confirmed a direct interaction between GAS5 and the NF-κB subunit p65, suggesting a potential regulatory mechanism. GAS5 overexpression led to downregulation of NF-κB target genes, TNF-α, and NR3C1. GC treatment reduced NF-κB DNA binding activity in GAS5-overexpressing cells, indicating a potential synergistic effect. Furthermore, GAS5 overexpression increased IκB levels and reduced p-p65/pan-p65 levels during GC treatment. GAS5 appears to modulate NF-κB activity in a complex manner, influencing both basal and GC-induced signaling. The interaction between GAS5, GCs, and NF-κB is multi-faceted, and further research is needed to fully elucidate the underlying mechanisms. These findings suggest that GAS5 could be a potential target for personalized therapy, particularly in pediatric patients with inflammatory conditions.

Molecular analysis of changes in DNA binding affinity and bending extent induced by the mutations on the aromatic amino residues in Cren7.

Proteins from Crenarchaeal organisms exhibit remarkable thermal stability. The aromatic amino acids in Cren7, a Crenarchaeal protein, regulate protein stability and further modulate DNA binding and its compaction. Specific aromatic amino acids were mutated, and using spectroscopic and theoretical approaches, we have examined the structure, DNA binding affinity, and DNA bending ability of mutants. and compared with wild-type (WT) Cren7. The reverse titration profiles were analysed by a noncooperativeMcGhee-von Hippel model to estimate affinity constant (Ka) and site size (n) associated with binding to the DNA. Biolayer interferometry (BLI) measurements showed that the binding affinity decreased at higher salt concentrations. For theoretical analysis of extent of DNA bending, radius of gyration and bending angle were compared for WT and mutants. Time evolution of order parameters based on translational and rotational motion of tryptophan residue (W26) was used for qualitative detection of stacking interactions between W26 of Cren7 and DNA nucleobases. It was observed that orientation of W26 in F41A favoredformation of a new lone pair-lone pair interaction between DNA and Cren7. Consequently, in thermostable proteins, the aromatic residues at the terminus maintain structural stability, whereas the residues at the core optimize the degree of DNA bending and compaction.

Intrinsically Disordered Regions in the Transcription Factor MYC:MAX Modulate DNA Binding via Intramolecular Interactions.

The basic helix-loop-helix leucine zipper (bHLH-LZ) transcription factor (TF) MYC is in large part an intrinsically disordered oncoprotein. In complex with its obligate heterodimerization partner MAX, MYC preferentially binds E-Box DNA sequences (CANNTG). At promoters containing these sequence motifs, MYC controls fundamental cellular processes such as cell cycle progression, metabolism, and apoptosis. A vast network of proteins in turn regulates MYC function via intermolecular interactions. In this work, we establish another layer of MYC regulation by intramolecular interactions. We used nuclear magnetic resonance (NMR) spectroscopy to identify and map multiple binding sites for the C-terminal MYC:MAX DNA-binding domain (DBD) on the intrinsically disordered regions (IDRs) in the MYC N-terminus. We find that these binding events in trans are driven by electrostatic attraction, that they have distinct affinities, and that they are competitive with DNA binding. Thereby, we observe the strongest effects for the N-terminal MYC box 0 (Mb0), a conserved motif involved in MYC transactivation and target gene induction. We prepared recombinant full-length MYC:MAX complex and demonstrate that the interactions identified in this work are also relevant in cis, i.e., as intramolecular interactions. These findings are supported by surface plasmon resonance (SPR) experiments, which revealed that intramolecular IDR:DBD interactions in MYC decelerate the association of MYC:MAX complexes to DNA. Our work offers new insights into how bHLH-LZ TFs are regulated by intramolecular interactions, which open up new possibilities for drug discovery.

TetR and OmpR family regulators in natural product biosynthesis and resistance.

This article provides a comprehensive review and sequence-structure analysis of transcription regulator (TR) families, TetR and OmpR/PhoB, involved in specialized secondary metabolite (SSM) biosynthesis and resistance. Transcription regulation is a fundamental process, playing a crucial role in orchestrating gene expression to confer a survival advantage in response to frequent environmental stress conditions. This process, coupled with signal sensing, enables bacteria to respond to a diverse range of intra and extracellular signals. Thus, major bacterial signaling systems use a receptor domain to sense chemical stimuli along with an output domain responsible for transcription regulation through DNA-binding. Sensory and output domains on a single polypeptide chain (one component system, OCS) allow response to stimuli by allostery, that is, DNA-binding affinity modulation upon signal presence/absence. On the other hand, two component systems (TCSs) allow cross-talk between the sensory and output domains as they are disjoint and transmit information by phosphorelay to mount a response. In both cases, however, TRs play a central role. Biosynthesis of SSMs, which includes antibiotics, is heavily regulated by TRs as it diverts the cell's resources towards the production of these expendable compounds, which also have clinical applications. These TRs have evolved to relay information across specific signals and target genes, thus providing a rich source of unique mechanisms to explore towards addressing the rapid escalation in antimicrobial resistance (AMR). Here, we focus on the TetR and OmpR family TRs, which belong to OCS and TCS, respectively. These TR families are well-known examples of regulators in secondary metabolism and are ubiquitous across different bacteria, as they also participate in a myriad of cellular processes apart from SSM biosynthesis and resistance. As a result, these families exhibit higher sequence divergence, which is also evident from our bioinformatic analysis of 158 389 and 77 437 sequences from TetR and OmpR family TRs, respectively. The analysis of both sequence and structure allowed us to identify novel motifs in addition to the known motifs responsible for TR function and its structural integrity. Understanding the diverse mechanisms employed by these TRs is essential for unraveling the biosynthesis of SSMs. This can also help exploit their regulatory role in biosynthesis for significant pharmaceutical, agricultural, and industrial applications.

Regulation of eosinophils by the transcription factor Ikaros

Abstract Immune cell development is a tightly controlled process that enables a homeostatic level of innate and adaptive immune cells of the myeloid and lymphoid lineages, respectively. Imbalance in immune cells can lead to diseases due to immune defects or over-activation. such as immunodeficiency, allergies or autoimmunity. Ikaros is a transcription factor known to be critically important for lymphoid development, with recurrent mutations in different human diseases, but less is known about the role of Ikaros in myeloid development. Ikaros is encoded by the Ikzf1gene, and contains four DNA-binding zinc fingers (ZnF) and two ZnFs mediating protein dimerization. The four DNA-binding ZnFs (ZnF1 through ZnF4) are encoded by three exons, with the two central ZnF2 and ZnF3 encoded by exon 5 being required for binding to the core DNA recognition site. The two flanking ZnFs 1 and 4 are encoded by exon 4 and exon 6, and can modulate DNA binding by contacting the flanking DNA. Interestingly, Ikaros is highly alternatively spliced, creating multiple isoforms with different combinations of the DNA-binding ZnFs. We study the role of Ikaros in immune cell development using Ikzf1-mutant mouse models, and analysis of mice with deletions of exon 4 or exon 6 (encoding ZnF1 and ZnF4, respectively) has revealed differential phenotypes in the two mutant mouse models. Here, we describe a new role of Ikaros in regulation of eosinophils. We have found that Ikaros is required for limiting eosinophil production in bone marrow (BM), as both mutants show increased levels of BM eosinophils. Interestingly, the levels of peripheral tissue eosinophils differ between the two mutants, suggesting a second role of Ikaros in regulating migration, tissue recruitment or retention.

The role of replication-induced chromosomal copy numbers in spatio-temporal gene regulation and evolutionary chromosome plasticity.

For a coherent response to environmental changes, bacterial evolution has formed a complex transcriptional regulatory system comprising classical DNA binding proteins sigma factors and modulation of DNA topology. In this study, we investigate replication-induced gene copy numbers - a regulatory concept that is unlike the others not based on modulation of promoter activity but on replication dynamics. We show that a large fraction of genes are predominantly affected by transient copy numbers and identify cellular functions and central pathways governed by this mechanism in Escherichia coli. Furthermore, we show quantitatively that the previously observed spatio-temporal expression pattern between different growth phases mainly emerges from transient chromosomal copy numbers. We extend the analysis to the plant pathogen Dickeya dadantii and the biotechnologically relevant organism Vibrio natriegens. The analysis reveals a connection between growth phase dependent gene expression and evolutionary gene migration in these species. A further extension to the bacterial kingdom indicates that chromosome evolution is governed by growth rate related transient copy numbers.

Mechanistic Investigation of the Androgen Receptor DNA-Binding Domain and Modulation via Direct Interactions with DNA Abasic Sites: Understanding the Mechanisms Involved in Castration-Resistant Prostate Cancer.

The androgen receptor (AR) is an important drug target in prostate cancer and a driver of castration-resistant prostate cancer (CRPC). A significant challenge in designing effective drugs lies in targeting constitutively active AR variants and, most importantly, nearly all AR variants lacking the ligand-binding domain (LBD). Recent findings show that an AR's constitutive activity may occur in the presence of somatic DNA mutations within non-coding regions, but the role of these mutations remains elusive. The discovery of new drugs targeting CRPC is hampered by the limited molecular understanding of how AR binds mutated DNA sequences, frequently observed in prostate cancer, and how mutations within the protein and DNA regulate AR-DNA interactions. Using atomistic molecular dynamics (MD) simulations and quantum mechanical calculations, we focused our efforts on (i) rationalising the role of several activating DBD mutations linked to prostate cancer, and (ii) DBD interactions in the presence of abasic DNA lesions, which frequently occur in CRPC. Our results elucidate the role of mutations within DBD through their modulation of the intrinsic dynamics of the DBD-DNA ternary complex. Furthermore, our results indicate that the DNA apurinic lesions occurring in the androgen-responsive element (ARE) enhance direct AR-DNA interactions and stabilise the DBD homodimerisation interface. Moreover, our results strongly suggest that those abasic lesions may form reversible covalent crosslinks between DNA and lysine residues of an AR via a Schiff base. In addition to providing an atomistic model explaining how protein mutations within the AR DNA-binding domain affect AR dimerisation and AR-DNA interactions, our findings provide insight into how somatic mutations occurring in DNA non-coding regions may activate ARs. These mutations are frequently observed in prostate cancer and may contribute to disease progression by enhancing direct AR-DNA interactions.

Coupling of binding and differential subdomain folding of the intrinsically disordered transcription factor CREB.

The cyclic AMP response element binding protein (CREB) contains a basic leucine zipper motif (bZIP) that forms a coiled coil structure upon dimerization and specific DNA binding. Although this state is well characterized, key features of CREB bZIP binding and folding are not well understood. We used single-molecule Förster resonance energy transfer (smFRET) to probe conformations of CREB bZIP subdomains. We found differential folding of the basic region and leucine zipper in response to different binding partners; a strong and previously unreported DNA-independent dimerization affinity; folding upon binding to nonspecific DNA; and evidence of long-range interdomain interactions in full-length CREB that modulate DNA binding. These studies provide new insights into DNA binding and dimerization and have implications for CREB function.

The helicase core accessory regions of the phage BFK20 DnaB-like helicase gp43 significantly affect its activity, oligomeric state and DNA binding properties

The multifunctional phage replication protein gp43 is composed of an N-terminal prim-pol domain and a C-terminal domain similar to the SF4-type replicative helicases. We prepared four mutants all missing the prim-pol domain with the helicase core flanked by accessory N- and C-terminal regions truncated to varying extents. The shortest fragment still possessing strong ssDNA-dependent ATPase activity and helicase activity was gp43HEL519-983. The other proteins tested were gp43HEL557-983, gp43HEL519-855 and gp43HEL519-896. Removal of the 38 N-terminal residues in gp43HEL557-983, or the 128 and 87 C-terminal residues in gp43HEL519-855 and gp43HEL519-896, resulted in a significant decrease in the ATPase activities. The 38-amino acid N-terminal region has probably a function in modulating DNA binding and protein oligomerization. Deletion of the 87 C-terminal residues resulted in a twofold increase in the unwinding rate. This region is likely indispensable for binding to DNA substrates.

Structural insights into the inhibition of bacterial RecA by naphthalene polysulfonated compounds.

SummaryAs a promising target for alternative antimicrobials, bacterial recombinase A (RecA) protein has attracted much attention for its roles in antibiotic-driven SOS response and mutagenesis. Naphthalene polysulfonated compounds (NPS) such as suramin have previously been explored as antibiotic adjuvants targeting RecA, although the underlying structural bases for RecA-ligand interactions remain obscure. Based on our in silico predictions and documented activity of NPS in vitro, we conclude that the analyzed NPS likely interact with Tyr103 (Y103) and other key residues in the ATPase activity center (pocket A). For validation, we generated recombinant RecA proteins (wild-type versus Y103 mutant) to determine the binding affinities for RecA protein interactions with suramin and underexamined NPS in isothermal titration calorimetry. The corresponding dissociation constants (Kd) ranged from 11.5 to 18.8 μM, and Y103 was experimentally shown to be critical to RecA-NPS interactions.

Structural mechanism for replication origin binding and remodeling by a metazoan origin recognition complex and its co-loader Cdc6

Eukaryotic DNA replication initiation relies on the origin recognition complex (ORC), a DNA-binding ATPase that loads the Mcm2–7 replicative helicase onto replication origins. Here, we report cryo-electron microscopy (cryo-EM) structures of DNA-bound Drosophila ORC with and without the co-loader Cdc6. These structures reveal that Orc1 and Orc4 constitute the primary DNA binding site in the ORC ring and cooperate with the winged-helix domains to stabilize DNA bending. A loop region near the catalytic Walker B motif of Orc1 directly contacts DNA, allosterically coupling DNA binding to ORC’s ATPase site. Correlating structural and biochemical data show that DNA sequence modulates DNA binding and remodeling by ORC, and that DNA bending promotes Mcm2–7 loading in vitro. Together, these findings explain the distinct DNA sequence-dependencies of metazoan and S. cerevisiae initiators in origin recognition and support a model in which DNA geometry and bendability contribute to Mcm2–7 loading site selection in metazoans.

Abstract 2468: Phosphorylation of Thr55 mediates the tumor suppression activities of p53

Abstract P53 is a transcription factor that plays a central role in the regulation of cell cycle, apoptosis and senescence. In unstressed cells, p53 is kept at low level under the strict control of its negative regulators. Upon DNA damage or oncogenic stress, p53 is activated via post-translational modifications. The phosphorylation of serine and threonine, such as Ser15 and Ser46, play important roles in p53 regulation. However, data for Thr55 (T55) phosphorylation is limited and functions of T55 phosphorylation remain elusive. We have previously defined an intramolecular interaction in p53 which modulates DNA binding. The observation of altered intramolecular interactions in T55A and T55D mutants leads to the hypothesis that T55 phosphorylation might affect p53 functions. We generate a T55 phospho-specific antibody to investigate the regulation through this site. The results show T55 phosphorylation is induced by DNA damage compounds. We induce the expression of wild type and T55A mutant p53 in p53 null H1299 cells via the transduction of doxycycline-inducible lentiviral gene expression plasmids. T55A mutant p53 exhibits significantly weaker inhibition of cell viability compared with wild type p53 containing cell in the existence of DNA damaging compounds. Consistently, T55A mutant p53 induces lower mRNA expression level of several important apoptosis related genes and higher MDM2 level in comparison with wild type p53. Overall, our results reveal that phosphorylation of Thr55 is induced by several DNA damaging compounds and alters the expression of p53 target genes, leading to an enhanced tumor suppression. Citation Format: Fan He, Lihong Chen, Jiandong Chen. Phosphorylation of Thr55 mediates the tumor suppression activities of p53 [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2468.

Electron and Proton Transfers Modulate DNA Binding by the Transcription Regulator RsrR.

The [Fe2S2]-RsrR gene transcription regulator senses the redox status in bacteria by modulating DNA binding, while its cluster cycles between +1 and +2 states-only the latter binds DNA. We have previously shown that RsrR can undergo remarkable conformational changes involving a 100° rotation of tryptophan 9 between exposed (Out) and buried (In) states. Here, we have used the chemical modification of Trp9, site-directed mutagenesis, and crystallographic and computational chemical studies to show that (i) the Out and In states correspond to oxidized and reduced RsrR, respectively, (ii) His33 is protonated in the In state due to a change in its pKa caused by cluster reduction, and (iii) Trp9 rotation is conditioned by the response of its dipole moment to environmental electrostatic changes. Our findings illustrate a novel function of protonation resulting from electron transfer.

The metal centre in salen-acridine dyad N2O2 ligand–metal complexes modulates DNA binding and photocleavage efficiency

Metal centre in a coordination complex modulates DNA binding.

Crystal Structure of TetR Family Repressor AlkX from Dietzia sp. Strain DQ12-45-1b Implicated in Biodegradation of n-Alkanes.

n-Alkanes are ubiquitous in nature and are widely used by microorganisms as carbon sources. Alkane hydroxylation by alkane monooxygenases is a critical step in the aerobic biodegradation of n-alkanes, which plays important roles in natural alkane attenuation and is used in industrial and environmental applications. The alkane oxidation operon, alkW1-alkX, in the alkane-degrading strain Dietzia sp. strain DQ12-45-1b is negatively autoregulated by the TetR family repressor AlkX via a product positive feedback mechanism. To predict the gene regulation mechanism, we determined the 3.1-Å crystal structure of an AlkX homodimer in a non-DNA-bound state. The structure showed traceable long electron density deep inside a hydrophobic cavity of each monomer along the long axis of the helix bundle, and further gas chromatography-mass spectrometry analysis of AlkX revealed that it contained the Escherichia coli-derived long-chain fatty acid molecules as a ligand. Moreover, an unusual structural feature of AlkX is an extra helix, α6', forming a lid-like structure with α6 covering the inducer-binding pocket and occupying the space between the two symmetrical DNA-binding motifs in one dimer, indicating a distinct conformational transition mode in modulating DNA binding. Sequence alignment of AlkX homologs from Dietzia strains showed that the residues involved in DNA and inducer binding are highly conserved, suggesting that the regulation mechanisms of n-alkane hydroxylation are possibly a common characteristic of Dietzia strains.IMPORTANCE With n-alkanes being ubiquitous in nature, many bacteria from terrestrial and aquatic environments have evolved n-alkane oxidation functions. Alkane hydroxylation by alkane monooxygenases is a critical step in the aerobic biodegradation of n-alkanes, which plays important roles in natural alkane attenuation and petroleum-contaminating environment bioremediation. The gene regulation of the most common alkane hydroxylase, AlkB, has been studied widely in Gram-negative bacteria but has been less explored in Gram-positive bacteria. Our previous study showed that the TetR family regulator (TFR) AlkX negatively autoregulated the alkane oxidation operon, alkW1-alkX, in the Gram-positive strain Dietzia sp. strain DQ12-45-1b. Although TFRs are one of the most common transcriptional regulator families in bacteria, the TFR involved in n-alkane metabolism has been reported only recently. In this study, we determined the crystal structure of AlkX, which implies a distinct DNA/ligand binding mode. Our results shed light upon the regulation mechanism of the common alkane degradation process in nature.

Leaderless secreted peptide signaling molecule alters global gene expression and increases virulence of a human bacterial pathogen

Successful pathogens use complex signaling mechanisms to monitor their environment and reprogram global gene expression during specific stages of infection. Group A Streptococcus (GAS) is a major human pathogen that causes significant disease burden worldwide. A secreted cysteine protease known as streptococcal pyrogenic exotoxin B (SpeB) is a key virulence factor that is produced abundantly during infection and is critical for GAS pathogenesis. Although identified nearly a century ago, the molecular basis for growth phase control of speB gene expression remains unknown. We have discovered that GAS uses a previously unknown peptide-mediated intercellular signaling system to control SpeB production, alter global gene expression, and enhance virulence. GAS produces an eight-amino acid leaderless peptide [SpeB-inducing peptide (SIP)] during high cell density and uses the secreted peptide for cell-to-cell signaling to induce population-wide speB expression. The SIP signaling pathway includes peptide secretion, reimportation into the cytosol, and interaction with the intracellular global gene regulator Regulator of Protease B (RopB), resulting in SIP-dependent modulation of DNA binding and regulatory activity of RopB. Notably, SIP signaling causes differential expression of ∼14% of GAS core genes. Several genes that encode toxins and other virulence genes that enhance pathogen dissemination and infection are significantly up-regulated. Using three mouse infection models, we show that the SIP signaling pathway is active during infection and contributes significantly to GAS pathogenesis at multiple host anatomic sites. Together, our results delineate the molecular mechanisms involved in a previously undescribed virulence regulatory pathway of an important human pathogen and suggest new therapeutic strategies.

Comprehensive mapping of the Helicobacter pylori NikR regulon provides new insights in bacterial nickel responses

Nickel homeostasis is important for pathogenic and ureolytic bacteria, which use this metal ion as enzymatic cofactor. For example, in the human pathogen Helicobacter pylori an optimal balance between nickel uptake and incorporation in metallo-enzymes is fundamental for colonization of the host. Nickel is also used as cofactor to modulate DNA binding of the NikR regulator, which controls transcription of genes involved in nickel trafficking or infection in many bacteria. Accordingly, there is much interest in a systematic characterization of NikR regulation. Herein we use H. pylori as a model to integrate RNA-seq and ChIP-seq data demonstrating that NikR not only regulates metal-ion transporters but also virulence factors, non-coding RNAs, as well as toxin-antitoxin systems in response to nickel stimulation. Altogether, results provide new insights into the pathobiology of H. pylori and contribute to understand the responses to nickel in other bacteria.

Crystal structures of the NO sensor NsrR reveal how its iron-sulfur cluster modulates DNA binding

NsrR from Streptomyces coelicolor (Sc) regulates the expression of three genes through the progressive degradation of its [4Fe–4S] cluster on nitric oxide (NO) exposure. We report the 1.95 Å resolution crystal structure of dimeric holo-ScNsrR and show that the cluster is coordinated by the three invariant Cys residues from one monomer and, unexpectedly, Asp8 from the other. A cavity map suggests that NO displaces Asp8 as a cluster ligand and, while D8A and D8C variants remain NO sensitive, DNA binding is affected. A structural comparison of holo-ScNsrR with an apo-IscR-DNA complex shows that the [4Fe–4S] cluster stabilizes a turn between ScNsrR Cys93 and Cys99 properly oriented to interact with the DNA backbone. In addition, an apo ScNsrR structure suggests that Asn97 from this turn, along with Arg12, which forms a salt-bridge with Asp8, are instrumental in modulating the position of the DNA recognition helix region relative to its major groove.

A Polymerase With Potential: The Fe-S Cluster in Human DNA Primase.

Replication of DNA in eukaryotes is primarily executed by the combined action of processive DNA polymerases δ and ɛ. These enzymes cannot initiate synthesis of new DNA without the presence of a primer on the template ssDNA. The primers on both the leading and lagging strands are generated by DNA polymerase α-primase (pol-prim). DNA primase is a DNA-dependent RNA polymerase that synthesizes the first ~10 nucleotides and then transfers the substrate to polymerase α to complete primer synthesis. The mechanisms governing the coordination and handoff between primase and polymerase α are largely unknown. Isolated DNA primase contains a [4Fe-4S]2+ cluster that has been shown to serve as a redox switch modulating DNA binding affinity. This discovery suggests a mechanism for modulating the priming activity of primase and handoff to polymerase α. In this chapter, we briefly discuss the current state of knowledge of primase structure and function, including the role of its iron-sulfur cluster. This is followed by providing the methods for expressing, purifying, and biophysically/structurally characterizing primase and its iron-sulfur cluster-containing domain, p58C.

Optimizing Protonation States for Selective Double-Strand DNA Photocleavage in Hypoxic Tumors: pH-Gated Transitions of Lysine Dipeptides.

We report pH-switching properties of the new family of dipeptide-acetylene conjugates where pH-gated light-activated double-strand (ds) DNA cleavage is controlled by variations in electronic and geometric parameters. The conjugates have higher activities at the slightly acidic pH values that separate normal and cancerous tissue (pH < 7). This favorable pH dependence originates from several elements of structural design. Basicities of the two amines determine the threshold pH range where the changes in binding and reactivity are observed, whereas the distance between the two amino groups and the hydrophobic aryl alkyne moiety can further modulate DNA binding. The changes of the protonation state from a neutral molecule to a dication results in dramatically increased efficiency of ds DNA photocleavage, the most therapeutically valuable type of DNA cleavage.