DNA charge transport as a first step in coordinating the detection of lesions by repair proteins

Damaged bases in DNA are known to lead to errors in replication and transcription, compromising the integrity of the genome. We have proposed a model where repair proteins containing redox-active [4Fe-4S] clusters utilize DNA charge transport (CT) as a first step in finding lesions. In this model, the population of sites to search is reduced by a localization of protein in the vicinity of lesions. Here, we examine this model using single-molecule atomic force microscopy (AFM). XPD, a 5′-3′ helicase involved in nucleotide excision repair, contains a [4Fe-4S] cluster and exhibits a DNA-bound redox potential that is physiologically relevant. In AFM studies, we observe the redistribution of XPD onto kilobase DNA strands containing a single base mismatch, which is not a specific substrate for XPD but, like a lesion, inhibits CT. We further provide evidence for DNA-mediated signaling between XPD and Endonuclease III (EndoIII), a base excision repair glycosylase that also contains a [4Fe-4S] cluster. When XPD and EndoIII are mixed together, they coordinate in relocalizing onto the mismatched strand. However, when a CT-deficient mutant of either repair protein is combined with the CT-proficient repair partner, no relocalization occurs. These data not only indicate a general link between the ability of a repair protein to carry out DNA CT and its ability to redistribute onto DNA strands near lesions but also provide evidence for coordinated DNA CT between different repair proteins in their search for damage in the genome

ATP-Stimulated, DNA-Mediated Redox Signaling by XPD, a DNA Repair and Transcription Helicase

Using DNA-modified electrodes, we show DNA-mediated signaling by XPD, a helicase that contains a [4Fe-4S] cluster and is critical for nucleotide excision repair and transcription. The DNA-mediated redox signal resembles that of base excision repair proteins, with a DNA-bound redox potential of ~80 mV versus NHE. Significantly, this signal increases with ATP hydrolysis. Moreover, the redox signal is substrate-dependent, reports on the DNA conformational changes associated with enzymatic function, and may reflect a general biological role for DNA charge transport

Accurate SAXS Profile Computation and its Assessment by Contrast Variation Experiments

AbstractA major challenge in structural biology is to characterize structures of proteins and their assemblies in solution. At low resolution, such a characterization may be achieved by small angle x-ray scattering (SAXS). Because SAXS analyses often require comparing profiles calculated from many atomic models against those determined by experiment, rapid and accurate profile computation from molecular structures is needed. We developed fast open-source x-ray scattering (FoXS) for profile computation. To match the experimental profile within the experimental noise, FoXS explicitly computes all interatomic distances and implicitly models the first hydration layer of the molecule. For assessing the accuracy of the modeled hydration layer, we performed contrast variation experiments for glucose isomerase and lysozyme, and found that FoXS can accurately represent density changes of this layer. The hydration layer model was also compared with a SAXS profile calculated for the explicit water molecules in the high-resolution structures of glucose isomerase and lysozyme. We tested FoXS on eleven protein, one DNA, and two RNA structures, revealing superior accuracy and speed versus CRYSOL, AquaSAXS, the Zernike polynomials-based method, and Fast-SAXS-pro. In addition, we demonstrated a significant correlation of the SAXS score with the accuracy of a structural model. Moreover, FoXS utility for analyzing heterogeneous samples was demonstrated for intrinsically flexible XLF-XRCC4 filaments and Ligase III-DNA complex. FoXS is extensively used as a standalone web server as a component of integrative structure determination by programs IMP, Chimera, and BILBOMD, as well as in other applications that require rapidly and accurately calculated SAXS profiles

Translocation and deletion breakpoints in cancer genomes are associated with potential non-B DNA-forming sequences

Gross chromosomal rearrangements (including translocations, deletions, insertions and duplications) are a hallmark of cancer genomes and often create oncogenic fusion genes. An obligate step in the generation of such gross rearrangements is the formation of DNA double-strand breaks (DSBs). Since the genomic distribution of rearrangement breakpoints is non-random, intrinsic cellular factors may predispose certain genomic regions to breakage. Notably, certain DNA sequences with the potential to fold into secondary structures [potential non-B DNA structures (PONDS); e.g. triplexes, quadruplexes, hairpin/cruciforms, Z-DNA and single-stranded looped-out structures with implications in DNA replication and transcription] can stimulate the formation of DNA DSBs. Here, we tested the postulate that these DNA sequences might be found at, or in close proximity to, rearrangement breakpoints. By analyzing the distribution of PONDS-forming sequences within ±500 bases of 19 947 translocation and 46 365 sequence-characterized deletion breakpoints in cancer genomes, we find significant association between PONDS-forming repeats and cancer breakpoints. Specifically, (AT)n, (GAA)n and (GAAA)n constitute the most frequent repeats at translocation breakpoints, whereas A-tracts occur preferentially at deletion breakpoints. Translocation breakpoints near PONDS-forming repeats also recur in different individuals and patient tumor samples. Hence, PONDS-forming sequences represent an intrinsic risk factor for genomic rearrangements in cancer genomes

Implementation and performance of SIBYLS: a dual endstation small-angle X-ray scattering and macromolecular crystallography beamline at the Advanced Light Source.

The SIBYLS beamline (12.3.1) of the Advanced Light Source at Lawrence Berkeley National Laboratory, supported by the US Department of Energy and the National Institutes of Health, is optimized for both small-angle X-ray scattering (SAXS) and macromolecular crystallography (MX), making it unique among the world's mostly SAXS or MX dedicated beamlines. Since SIBYLS was commissioned, assessments of the limitations and advantages of a combined SAXS and MX beamline have suggested new strategies for integration and optimal data collection methods and have led to additional hardware and software enhancements. Features described include a dual mode monochromator [containing both Si(111) crystals and Mo/B(4)C multilayer elements], rapid beamline optics conversion between SAXS and MX modes, active beam stabilization, sample-loading robotics, and mail-in and remote data collection. These features allow users to gain valuable insights from both dynamic solution scattering and high-resolution atomic diffraction experiments performed at a single synchrotron beamline. Key practical issues considered for data collection and analysis include radiation damage, structural ensembles, alternative conformers and flexibility. SIBYLS develops and applies efficient combined MX and SAXS methods that deliver high-impact results by providing robust cost-effective routes to connect structures to biology and by performing experiments that aid beamline designs for next generation light sources

Human DNA ligase III bridges two DNA ends to promote specific intermolecular DNA end joining

Mammalian DNA ligase III (LigIII) functions in both nuclear and mitochondrial DNA metabolism. In the nucleus, LigIII has functional redundancy with DNA ligase I whereas LigIII is the only mitochondrial DNA ligase and is essential for the survival of cells de-pendent upon oxidative respiration. The unique LigIII zinc finger (ZnF) domain is not required for catalytic activity but senses DNA strand breaks and stimulates intermolecular ligation of two DNAs by an unknown mechanism. Consistent with this activity, LigIII acts in an alternative pathway of DNA double strand break repair that buttresses canonical non-homologous end joining (NHEJ) and is manifest in NHEJ-defective cancer cells, but how LigIII acts in joining inter-molecular DNA ends versus nick ligation is unclear. To investigate how LigIII efficiently joins two DNAs, we developed a real-time, fluorescence-based as-say of DNA bridging suitable for high-throughput screening. On a nicked duplex DNA substrate, the results reveal binding competition between the ZnF and the oligonucleotide/oligosaccharide-binding do-main, one of three domains constituting the LigIII cat-alytic core. In contrast, these domains collaborate and are essential for formation of a DNA-bridging in-termediate by adenylated LigIII that positions a pair of blunt-ended duplex DNAs for efficient and specific intermolecular ligation

Synthesis and characterization of oligonucleotides containing 2′-fluorinated thymidine glycol as inhibitors of the endonuclease III reaction

Endonuclease III (Endo III) is a base excision repair enzyme that recognizes oxidized pyrimidine bases including thymine glycol. This enzyme is a glycosylase/lyase and forms a Schiff base-type intermediate with the substrate after the damaged base is removed. To investigate the mechanism of its substrate recognition by X-ray crystallography, we have synthesized oligonucleotides containing 2′-fluorothymidine glycol, expecting that the electron-withdrawing fluorine atom at the 2′ position would stabilize the covalent intermediate, as observed for T4 endonuclease V (Endo V) in our previous study. Oxidation of 5′- and 3′-protected 2′-fluorothymidine with OsO(4) produced two isomers of thymine glycol. Their configurations were determined by NMR spectroscopy after protection of the hydroxyl functions. The ratio of (5R,6S) and (5S,6R) isomers was 3:1, whereas this ratio was 6:1 in the case of the unmodified sugar. Both of the thymidine glycol isomers were converted to the corresponding phosphoramidite building blocks and were incorporated into oligonucleotides. When the duplexes containing 2′-fluorinated 5R- or 5S-thymidine glycol were treated with Escherichia coli endo III, no stabilized covalent intermediate was observed regardless of the stereochemistry at C5. The 5S isomer was found to form an enzyme–DNA complex, but the incision was inhibited probably by the fluorine-induced stabilization of the glycosidic bond

Archaeal Genome Guardians Give Insights into Eukaryotic DNA Replication and Damage Response Proteins

As the third domain of life, archaea, like the eukarya and bacteria, must have robust DNA replication and repair complexes to ensure genome fidelity. Archaea moreover display a breadth of unique habitats and characteristics, and structural biologists increasingly appreciate these features. As archaea include extremophiles that can withstand diverse environmental stresses, they provide fundamental systems for understanding enzymes and pathways critical to genome integrity and stress responses. Such archaeal extremophiles provide critical data on the periodic table for life as well as on the biochemical, geochemical, and physical limitations to adaptive strategies allowing organisms to thrive under environmental stress relevant to determining the boundaries for life as we know it. Specifically, archaeal enzyme structures have informed the architecture and mechanisms of key DNA repair proteins and complexes. With added abilities to temperature-trap flexible complexes and reveal core domains of transient and dynamic complexes, these structures provide insights into mechanisms of maintaining genome integrity despite extreme environmental stress. The DNA damage response protein structures noted in this review therefore inform the basis for genome integrity in the face of environmental stress, with implications for all domains of life as well as for biomanufacturing, astrobiology, and medicine