Poly(dA:dT) Tracts Differentially Modulate Nucleosome Remodeling Activity of RSC and ISW1a Complexes, Exerting Tract Orientation-Dependent and -Independent Effects.

The establishment and maintenance of nucleosome-free regions (NFRs) are prominent processes within chromatin dynamics. Transcription factors, ATP-dependent chromatin remodeling complexes (CRCs) and DNA sequences are the main factors involved. In Saccharomyces cerevisiae, CRCs such as RSC contribute to chromatin opening at NFRs, while other complexes, including ISW1a, contribute to NFR shrinking. Regarding DNA sequences, growing evidence points to poly(dA:dT) tracts as playing a direct role in active processes involved in nucleosome positioning dynamics. Intriguingly, poly(dA:dT)-tract-containing NFRs span asymmetrically relative to the location of the tract by a currently unknown mechanism. In order to obtain insight into the role of poly(dA:dT) tracts in nucleosome remodeling, we performed a systematic analysis of their influence on the activity of ISW1a and RSC complexes. Our results show that poly(dA:dT) tracts differentially affect the activity of these CRCs. Moreover, we found differences between the effects exerted by the two alternative tract orientations. Remarkably, tract-containing linker DNA is taken as exit DNA for nucleosome sliding catalyzed by RSC. Our findings show that defined DNA sequences, when present in linker DNA, can dictate in which direction a remodeling complex has to slide nucleosomes and shed light into the mechanisms underlying asymmetrical chromatin opening around poly(dA:dT) tracts.

Structural and thermodynamic insights into the Cren7 mediated DNA organization in Crenarchaeota.

Archaea have histone homologues and chromatin proteins to organize their DNA into a compact form. This allows them to survive in extreme climates. Cren7 is one such chromatin protein conserved in Crenarchaeota. When Cren7 binds to model natural DNA, calf thymus DNA (CTD, 58% AT content) and polynucleotides under adverse solution conditions (high temperature, ionic strength), CD bands at 275-290 nm shift to higher wavelengths indicating structural changes in DNA. It formed a strong complex with CTD and poly(dA-dT)·poly(dA-dT), via a combination of electrostatic and non-electrostatic interactions. A low binding enthalpy indicated that the process was driven by entropy. The interaction was independent of the nature of the anions present in the solution. On studying the variation in protein affinity with salt concentration, it was estimated that the electrostatic interaction at the interface involves 3 pairs of ions at the protein-DNA interface. The affinity and binding site size decreased on changing the pH of the solution (between pH 6 and 8), but temperature did not result in such effects. Cren7 bound to 10 bp of DNA, increasing its flexibility and thermal stability by more than 30 °C. Increasing the amount of Cren7 produces cooperative structural transitions in DNAs without any similar transition in the protein. These crucial binding parameters, energetics, and structural changes decipher the mystery of Cren7 mediated DNA organization in Crenarchaeota.

Agonistic Activation of Cytosolic DNA Sensing Receptors in Woodchuck Hepatocyte Cultures and Liver for Inducing Antiviral Effects.

Immune modulation for the treatment of chronic hepatitis B (CHB) has gained more traction in recent years, with an increasing number of compounds designed for targeting different host pattern recognition receptors (PRRs). These agonistic molecules activate the receptor signaling pathway and trigger an innate immune response that will eventually shape the adaptive immunity for control of chronic infection with hepatitis B virus (HBV). While definitive recognition of HBV nucleic acids by PRRs during viral infection still needs to be elucidated, several viral RNA sensing receptors, including toll-like receptors 7/8/9 and retinoic acid inducible gene-I-like receptors, are explored preclinically and clinically as possible anti-HBV targets. The antiviral potential of viral DNA sensing receptors is less investigated. In the present study, treatment of primary woodchuck hepatocytes generated from animals with CHB with HSV-60 or poly(dA:dT) agonists resulted in increased expression of interferon-gamma inducible protein 16 (IFI16) or Z-DNA-binding protein 1 (ZBP1/DAI) and absent in melanoma 2 (AIM2) receptors and their respective adaptor molecules and effector cytokines. Cytosolic DNA sensing receptor pathway activation correlated with a decline in woodchuck hepatitis virus (WHV) replication and secretion in these cells. Combination treatment with HSV-60 and poly(dA:dT) achieved a superior antiviral effect over monotreatment with either agonist that was associated with an increased expression of effector cytokines. The antiviral effect, however, could not be enhanced further by providing additional type-I interferons (IFNs) exogenously, indicating a saturated level of effector cytokines produced by these receptors following agonism. In WHV-uninfected woodchucks, a single poly(dA:dT) dose administered via liver-targeted delivery was well-tolerated and induced the intrahepatic expression of ZBP1/DAI and AIM2 receptors and their effector cytokines, IFN-ß and interleukins 1ß and 18. Receptor agonism also resulted in increased IFN-γ secretion of peripheral blood cells. Altogether, the effect on WHV replication and secretion following in vitro activation of IFI16, ZBP1/DAI, and AIM2 receptor pathways suggested an antiviral benefit of targeting more than one cytosolic DNA receptor. In addition, the in vivo activation of ZBP1/DAI and AIM2 receptor pathways in liver indicated the feasibility of the agonist delivery approach for future evaluation of therapeutic efficacy against HBV in woodchucks with CHB.

Cytokine secretion and pyroptosis of cholesteatoma keratinocytes mediated by AIM2 inflammasomes in response to cytoplasmic DNA.

Cholesteatoma constitutes an acquired benign epidermal non­permanent bone lesion that is locally destructive and patients often relapse. Inflammasomes, which mediate the maturation and production of IL­18 and IL­1ß, resulting in pyroptosis, have been documented to serve a core function in multiple inflammatory conditions. Absent in melanoma 2 (AIM2) is an inflammasome that identifies cytoplasmic DNA and has previously been reported as a pivotal modulator of inflammatory responses. Therefore, the present study aimed to determine the expression levels of AIM2 in human cholesteatoma tissues, and elucidate its function in modulating cytokine production. The expression levels of IL­18, apoptosis­associated speck­like protein containing a CARD (ASC), IL­1ß, AIM2 and caspase­1 were markedly elevated in cholesteatoma tissues. Protein expression levels of AIM2, caspase­1 and ASC were localized in the cellular cytoplasm, primarily in the granular and prickle­cell layers in the cholesteatoma epithelium. Induction using IFN­Î³, as well as cytoplasmic DNA markedly activated the AIM2 inflammasome and elevated the release of IL­18 and IL­1ß in human cholesteatoma keratinocytes. IFN­Î³ was found to enhance poly(dA:dT)­induced pyroptosis of cells and cytokine production. The results of the present study revealed that AIM2 expressed in human cholesteatoma serves a vital function in the inflammatory response by initiating the inflammasome signaling cascade in cholesteatoma.

Formation of SUMO3-conjugated chains of MAVS induced by poly(dA:dT), a ligand of RIG-I, enhances the aggregation of MAVS that drives the secretion of interferon-ß in human keratinocytes.

The retinoic-acid inducible gene (RIG)-I is a cytoplasmic pattern recognition receptor that senses single-stranded (ss) or double-stranded (ds) RNA. RIG-I also senses AT-rich dsDNA, poly(dA:dT), through the action of an RNA polymerase III-transcribed RNA intermediate. Upon the binding of an RNA ligand, RIG-I binds to the mitochondrial antiviral-signaling protein (MAVS) and induces the formation of filamentous aggregates of MAVS, leading to the formation of a signaling complex that drives Type I interferon (IFN) responses. In the current study, we investigated the issue of whether the SUMOylation of MAVS induced by poly(dA:dT) affects the aggregation of MAVS in the RIG-I/MAVS pathway in human keratinocytes. Our results show that the poly(dA:dT)-induced secretion of IFN-ß was dependent on RIG-I and MAVS. The inhibition of SUMOylation by Ginkgolic acid or Ubc9 siRNA was found to inhibit the poly(dA:dT)-induced secretion of IFN-ß, suggesting that the SUMOylation is required for the poly(dA:dT)-activated RIG-I/MAVS pathway, which drives the secretion of IFN-ß. In addition, treatment with poly(dA:dT) enhanced the formation of polymeric chains of small-ubiquitin like modifiers (SUMO)3, but not SUMO1 and SUMO2, on MAVS. Our results also show that the conjugation of SUMO3 to MAVS induced by poly (dA:dT) enhanced the aggregation of MAVS. These collective results show that the formation of SUMO3-conjugated chains of MAVS induced by poly (dA:dT), a ligand of RIG-I, enhances the aggregation of MAVS which, in turn, drives the secretion of IFN-ß in human keratinocytes.

Poly(dA:dT) Suppresses HSV-2 Infection of Human Cervical Epithelial Cells Through RIG-I Activation.

Epithelial cells of the female reproductive tract (FRT) participate in the initial innate immunity against viral infections. Poly(dA:dT) is a synthetic analog of B form double-stranded (ds) DNA which can activate the interferon (IFN) signaling pathway-mediated antiviral immunity through DNA-dependent RNA Polymerase III. Here we investigated whether poly(dA:dT) could inhibit herpes simplex virus type 2 (HSV-2) infection of human cervical epithelial cells (End1/E6E7). We demonstrated that poly(dA:dT) treatment of End1/E6E7 cells could significantly inhibit HSV-2 infection. Mechanistically, poly(dA:dT) treatment of the cells induced the expression of the intracellular IFNs and the multiple antiviral IFN-stimulated genes (ISGs), including IFN-stimulated gene 15 (ISG15), IFN-stimulated gene 56 (ISG56), 2'-5'-oligoadenylate synthetase 1 (OAS1), 2'-5'-oligoadenylate synthetase 2 (OAS2), myxovirus resistance protein A (MxA), myxovirus resistance protein B (MxB), virus inhibitory protein, endoplasmic reticulum-associated, IFN-inducible (Viperin), and guanylate binding protein 5 (GBP5). Further investigation showed that the activation of RIG-I was largely responsible for poly(dA:dT)-mediated HSV-2 inhibition and IFN/ISGs induction in the cervical epithelial cells, as RIG-I knockout abolished the poly(dA:dT) actions. These observations demonstrate the importance for design and development of AT-rich dsDNA-based intervention strategies to control HSV-2 mucosal transmission in FRT.

Sequence-Dependent Deviations of Constrained DNA from Canonical B-Form.

Decades of crystallographic and NMR studies have produced canonical structural models of short DNA. However, no experimental method so far has been able to test these models in vivo, where DNA is long and constrained by interactions with membranes, proteins, and other molecules. Here, we employ high-resolution frequency-modulation AFM to image single long poly(dA)-poly(dT), poly(dG)-poly(dC), and lambda DNA molecules interacting with an underlying substrate that emulates the effect of biological constraints on molecular structure. We find systematic sequence-dependent variations in groove dimensions, indicating that the structure of DNA subject to realistic interactions may differ profoundly from canonical models. These findings highlight the value of AFM as a unique, single molecule characterization tool.

Independent Generation of Reactive Intermediates Leads to an Alternative Mechanism for Strand Damage Induced by Hole Transfer in Poly(dA-T) Sequences.

Purine radical cations (dA•+ and dG•+) are the primary hole carriers of DNA hole migration due to their favorable oxidation potential. Much less is known about the reactivity of higher energy pyrimidine radical cations. The thymidine radical cation (T•+) was produced at a defined position in DNA from a photochemical precursor for the first time. T•+ initiates hole transfer to dGGG triplets in DNA. Hole localization in a dGGG sequence accounts for ∼26% of T•+ formed under aerobic conditions in 9. Reduction to yield thymidine is also quantified. 5-Formyl-2'-deoxyuridine is formed in low yield in DNA when T•+ is independently generated. This is inconsistent with mechanistic proposals concerning product formation from electron transfer in poly(dA-T) sequences, following hole injection by a photoexcited anthraquinone. Additional evidence that is inconsistent with the original mechanism was obtained using hole injection by a photoexcited anthraquinone in DNA. Instead of requiring the intermediacy of T•+, the strand damage patterns observed in those studies, in which thymidine is oxidized, are reproduced by independent generation of 2'-deoxyadenosin- N6-yl radical (dA•). Tandem lesion formation by dA• provides the basis for an alternative mechanism for thymidine oxidation ascribed to hole migration in poly(dA-T) sequences. Overall, these experiments indicate that the final products formed following DNA hole transfer in poly(dA-T) sequences do not result from deprotonation or hydration of T•+, but rather from deprotonation of the more stable dA•+, to form dA•, which produces tandem lesions in which 5'-flanking thymidines are oxidized.

Dual Roles of Poly(dA:dT) Tracts in Replication Initiation and Fork Collapse.

Replication origins, fragile sites, and rDNA have been implicated as sources of chromosomal instability. However, the defining genomic features of replication origins and fragile sites are among the least understood elements of eukaryote genomes. Here, we map sites of replication initiation and breakage in primary cells at high resolution. We find that replication initiates between transcribed genes within nucleosome-depleted structures established by long asymmetrical poly(dA:dT) tracts flanking the initiation site. Paradoxically, long (>20 bp) (dA:dT) tracts are also preferential sites of polar replication fork stalling and collapse within early-replicating fragile sites (ERFSs) and late-replicating common fragile sites (CFSs) and at the rDNA replication fork barrier. Poly(dA:dT) sequences are fragile because long single-strand poly(dA) stretches at the replication fork are unprotected by the replication protein A (RPA). We propose that the evolutionary expansion of poly(dA:dT) tracts in eukaryotic genomes promotes replication initiation, but at the cost of chromosome fragility.

Molecular cloning and functional characterization of duck DDX41.

DEAD (Asp-Glu-Ala-Asp) box polypeptide 41 (DDX41), a receptor belonging to DExD/H-box helicase family, acts as an intracellular DNA sensor and induces type I IFN production in mammals and fish. However, the function of avian DDX41 in innate immune response is still unknown. In this study, the full-length duck DDX41 (duDDX41) cDNA sequence was cloned for the first time and encoded a putative protein of 618 amino acid residues which showed the high sequence similarity with both zebra finch and chicken DDX41s. The duDDX41 mRNA was widely distributed in all tested tissues, especially the cerebrum, cerebellum, and liver. Overexpression of duDDX41 triggered the activation of transcription factors IRF1 and NF-κB, as well as IFN-ß expression in DEFs. The DEADc domain of duDDX41 played an extremely vital role in duck type I IFN signaling pathway. Knockdown of duDDX41 by siRNA silencing dramatically decreased IFN-ß expression stimulated by poly(dA:dT) or duck enteritis virus (DEV). In addition, the replication of DEV was significantly inhibited in duDDX41-expressed DEFs and was enhanced in DDX41 knockdown DEFs. These results suggest that DDX41 is an important cytosolic DNA sensor and plays a crucial role in duck antiviral innate immune response.

Non-dissociative structural transitions of the Watson-Crick and reverse Watson-Crick А·Ð¢ DNA base pairs into the Hoogsteen and reverse Hoogsteen forms.

In this study it was theoretically shown that discovered by us recently (Brovarets' et al., Frontiers in Chemistry, 2018, 6:8; doi: 10.3389/fchem.2018.00008) high-energetical, significantly non-planar (symmetry C1), short-lived wobbled conformers of the classical Watson-Crick А·Ð¢(WC), reverse Watson-Crick А·Ð¢(rWC), Hoogsteen А·Ð¢(Н) and reverse Hoogsteen А·Ð¢(rН) DNA base pairs are the intermediates of their pairwise А∙Т(WC)/А∙Т(rWC) ↔ А∙Т(H)/А∙Т(rH) conformational transformations. These transitions do not require for their realization the energy-consumable anisotropic rotation of the amino group of A around the exocyclic C6-N6 bond. They are controlled by the non-planar transition states with quasi-orthogonal geometry (symmetry C1) joined by the single intermolecular (Т)N3H···N6(А) H-bond (~4 kcal∙mol-1). The Gibbs free energies of activation for these non-dissociative, dipole-active conformational transitions consist 7.33 and 7.81 kcal∙mol-1, accordingly. Quantum-mechanical (QM) calculations in combination with Bader's quantum theory of "Atoms in Molecules" (QTAIM) have been performed at the MP2/aug-cc-pVDZ//B3LYP/6-311++G(d,p) level of QM theory in the continuum with ε = 4 under normal conditions.

Quercetin inhibits the poly(dA:dT)-induced secretion of IL-18 via down-regulation of the expressions of AIM2 and pro-caspase-1 by inhibiting the JAK2/STAT1 pathway in IFN-γ-primed human keratinocytes.

Quercetin, a polyphenol, belongs to a class of flavonoids that exerts anti-inflammatory effects. Interleukin (IL)-18 is a member of the IL-1 family cytokine that regulates immune responses and is implicated in various inflammatory skin diseases. Absent in melanoma 2 (AIM2) is a cytosolic double-stranded (ds) DNA sensor that recognizes the dsDNA of a microbial or host origin. Binding of dsDNA to AIM2 simulates caspase-1-dependent inflammasome activity, which leads to the production of IL-1ß and IL-18. Increased levels of AIM2 have been observed in patients with inflammatory skin diseases. In the current study, we investigated the issue of whether or how Quercetin attenuates poly (dA:dT), a synthetic analog of microbial dsDNA, -induced IL-18 secretion in IFN-γ-primed human keratinocytes. Treatment with 5 and 10 µM of Quercetin inhibited the poly (dA:dT)-induced secretion of IL-18 after IFN-γ priming and before poly (dA:dT)-induced AIM2 activation. In addition, treatment with Quercetin at 10 µM, significantly inhibited the phosphorylation of JAK2 and STAT1, and the nuclear translocation of phosphorylated STAT1 in poly (dA:dT)-treated and IFN-γ-primed keratinocytes. These results suggest that treatment with Quercetin inhibits the poly (dA:dT)-induced secretion of IL-18 via down-regulation of the expressions of AIM2 and pro-caspase-1 by inhibiting the JAK2/STAT1 pathway in IFN-γ-primed keratinocytes.

Differential Inflammatory-Response Kinetics of Human Keratinocytes upon Cytosolic RNA- and DNA-Fragment Induction.

Keratinocytes are non-professional immune cells contributing actively to innate immune responses partially by reacting to a wide range of molecular patterns by activating pattern recognition receptors. Cytosolic nucleotide fragments as pathogen- or self-derived trigger factors are activating inflammasomes and inducing anti-viral signal transduction pathways as well as inducing expression of inflammatory cytokines. We aimed to compare the induced inflammatory reactions in three keratinocyte cell types-normal human epidermal keratinocytes, the HaCaT cell line and the HPV-KER cell line-upon exposure to the synthetic RNA and DNA analogues poly(I:C) and poly(dA:dT) to reveal the underlying signaling events. Both agents induced the expression of interleukin-6 and tumor necrosis factor α in all cell types; however, notable kinetic and expression level differences were found. Western blot analysis revealed rapid activation of the nuclear factor κB (NF-κB), mitogen activated protein kinase and signal transducers of activator of transcription (STAT) signal transduction pathways in keratinocytes upon poly(I:C) treatment, while poly(dA:dT) induced slower activation. Inhibition of NF-κB, p38, STAT-1 and STAT-3 signaling resulted in decreased cytokine expression, whereas inhibition of mitogen-activated protein kinase kinase 1/2 (MEK1/2) signaling showed a negative feedback role in both poly(I:C)- and poly(dA:dT)-induced cytokine expression. Based on our in vitro results nucleotide fragments are able to induce inflammatory reactions in keratinocytes, but with different rate and kinetics of cytokine expression, explained by faster activation of signaling routes by poly(I:C) than poly(dA:dT).

Binding, sliding, and function of cohesin during transcriptional activation.

The ring-shaped cohesin complex orchestrates long-range DNA interactions to mediate sister chromatid cohesion and other aspects of chromosome structure and function. In the yeast Saccharomyces cerevisiae, the complex binds discrete sites along chromosomes, including positions within and around genes. Transcriptional activity redistributes the complex to the 3' ends of convergently oriented gene pairs. Despite the wealth of information about where cohesin binds, little is known about cohesion at individual chromosomal binding sites and how transcription affects cohesion when cohesin complexes redistribute. In this study, we generated extrachromosomal DNA circles to study cohesion in response to transcriptional induction of a model gene, URA3. Functional cohesin complexes loaded onto the locus via a poly(dA:dT) tract in the gene promoter and mediated cohesion before induction. Upon transcription, the fate of these complexes depended on whether the DNA was circular or not. When gene activation occurred before DNA circularization, cohesion was lost. When activation occurred after DNA circularization, cohesion persisted. The presence of a convergently oriented gene also prevented transcription-driven loss of functional cohesin complexes, at least in M phase-arrested cells. The results are consistent with cohesin binding chromatin in a topological embrace and with transcription mobilizing functional complexes by sliding them along DNA.

Biases in the SMART-DNA library preparation method associated with genomic poly dA/dT sequences.

Avoiding biases in next generation sequencing (NGS) library preparation is crucial for obtaining reliable sequencing data. Recently, a new library preparation method has been introduced which has eliminated the need for the ligation step. This method, termed SMART (switching mechanism at the 5' end of the RNA transcript), is based on template switching reverse transcription. To date, there has been no systematic analysis of the additional biases introduced by this method. We analysed the genomic distribution of sequenced reads prepared from genomic DNA using the SMART methodology and found a strong bias toward long (≥12bp) poly dA/dT containing genomic loci. This bias is unique to the SMART-based library preparation and does not appear when libraries are prepared with conventional ligation based methods. Although this bias is obvious only when performing paired end sequencing, it affects single end sequenced samples as well. Our analysis demonstrates that sequenced reads originating from SMART-DNA libraries are heavily skewed toward genomic poly dA/dT tracts. This bias needs to be considered when deciding to use SMART based technology for library preparation.

HTLV-1 Tax impairs K63-linked ubiquitination of STING to evade host innate immunity.

The cellular antiviral innate immune system is essential for host defense and viruses have evolved a variety of strategies to evade the innate immunity. Human T lymphotropic virus type 1 (HTLV-1) belongs to the deltaretrovirus family and it can establish persistent infection in human beings for many years. However, how this virus evades the host innate immune responses remains unclear. Here we report a new strategy used by HTLV-1 to block innate immune responses. We observed that stimulator of interferon genes (STING) limited HTLV-1 protein expression and was critical to HTLV-1 reverse transcription intermediate (RTI) ssDNA90 triggered interferon (IFN)-ß production in phorbol12-myristate13-acetate (PMA)-differentiated THP1 (PMA-THP1) cells. The HTLV-1 protein Tax inhibited STING overexpression induced transcriptional activation of IFN-ß. Tax also impaired poly(dA:dT), interferon stimulatory DNA (ISD) or cyclic GMP-AMP (cGAMP) -stimulated IFN-ß production, which was dependent on STING activation. Coimmunoprecipitation assays and confocal microscopy indicated that Tax was associated with STING in the same complex. Mechanistic studies suggested that Tax decreased the K63-linked ubiquitination of STING and disrupted the interactions between STING and TANK-binding kinase 1 (TBK1). These findings may shed more light on the molecular mechanisms underlying HTLV-1 infection.

Genomic Nucleosome Organization Reconstituted with Pure Proteins.

Chromatin remodelers regulate genes by organizing nucleosomes around promoters, but their individual contributions are obfuscated by the complex in vivo milieu of factor redundancy and indirect effects. Genome-wide reconstitution of promoter nucleosome organization with purified proteins resolves this problem and is therefore a critical goal. Here, we reconstitute four stages of nucleosome architecture using purified components: yeast genomic DNA, histones, sequence-specific Abf1/Reb1, and remodelers RSC, ISW2, INO80, and ISW1a. We identify direct, specific, and sufficient contributions that in vivo observations validate. First, RSC clears promoters by translating poly(dA:dT) into directional nucleosome removal. Second, partial redundancy is recapitulated where INO80 alone, or ISW2 at Abf1/Reb1sites, positions +1 nucleosomes. Third, INO80 and ISW2 each align downstream nucleosomal arrays. Fourth, ISW1a tightens the spacing to canonical repeat lengths. Such a minimal set of rules and proteins establishes core mechanisms by which promoter chromatin architecture arises through a blend of redundancy and specialization.

Ground and excited state interactions of metalloporphyrin PtTMPyP4 with polynucleotides [poly(dG-dC)]2 and [poly(dA-dT)]2.

The ground- and excited-state interactions of Pt(ii) meso-tetrakis(4-N-methylpyridyl)porphyrin (PtTMPyP4) with polynucleotides [poly(dG-dC)]2 and [poly(dA-dT)]2 have been investigated using UV/visible, circular dichroism, and steady-state and time-resolved emission spectroscopy. PtTMPyP4 intercalates into [poly(dG-dC)]2 with K∼ 10(6) M(-1). When bound to [poly(dG-dC)]2 in aerated solution there is a six-fold emission enhancement with 18 nm red-shift in emission maximum. Emission lifetimes are biexponential. In the presence of [poly(dA-dT)]2 at least two distinct groove-binding modes are observed, depending on the binding ratio. In [poly(dA-dT)]2 the emission intensity increases by a maximum factor of 17 with no shift in the emission spectrum. Three exponentials were required for lifetime fitting. The lower extent of emission enhancement in the presence of [poly(dG-dC)]2 suggests that a slow electron transfer may take place to guanine, which is significantly less efficient than that previously observed for PtTMPyP4 in the presence of guanosine 5'-monophosphate (GMP). The results are compared to those previously recorded with free base H2TMPyP4.

Investigating the impacts of DNA binding mode and sequence on thermodynamic quantities and water exchange values for two small molecule drugs.

Doxorubicin and nogalamycin are antitumor antibiotics that interact with DNA via intercalation and threading mechanisms, respectively. Because the importance of water, particularly its impact on entropy changes, has been established in other biological processes, we investigated the role of water in these two drug-DNA binding events. We used the osmotic stress method to calculate the number of water molecules exchanged (Δnwater), and isothermal titration calorimetry to measure Kbinding, ΔH, and ΔS for two synthetic DNAs, poly(dA·dT) and poly(dG·dC), and calf thymus DNA (CT DNA). For nogalamycin, Δnwater<0 for CT DNA and poly(dG·dC). For doxorubicin, Δnwater>0 for CT DNA and Δnwater<0 for poly(dG·dC). For poly(dA·dT), Δnwater~0 with both drugs. Net enthalpy changes were always negative, but net entropy changes depended on the drug. The effect of water exchange on the overall sign of entropy change appears to be smaller than other contributions.

Regulation of Poly(A) Tail and Translation during the Somatic Cell Cycle.

Poly(A) tails are critical for mRNA stability and translation. However, recent studies have challenged this view, showing that poly(A) tail length and translation efficiency are decoupled in non-embryonic cells. Using TAIL-seq and ribosome profiling, we investigate poly(A) tail dynamics and translational control in the somatic cell cycle. We find dramatic changes in poly(A) tail lengths of cell-cycle regulatory genes like CDK1, TOP2A, and FBXO5, explaining their translational repression in M phase. We also find that poly(A) tail length is coupled to translation when the poly(A) tail is <20 nucleotides. However, as most genes have >20 nucleotide poly(A) tails, their translation is regulated mainly via poly(A) tail length-independent mechanisms during the cell cycle. Specifically, we find that terminal oligopyrimidine (TOP) tract-containing transcripts escape global translational suppression in M phase and are actively translated. Our quantitative and comprehensive data provide a revised view of translational control in the somatic cell cycle.