{"sourceUrl":null,"slug":"ddrc-a-unique-dna-repair-factor-from-d-r-0cf847","apex":{"label":"DdrC DNA repair factor mechanism","id":"n1","text":"DdrC, a unique DNA repair factor from Deinococcus radiodurans, senses and stabilizes DNA breaks through a novel lesion-recognition mechanism involving protein asymmetry and dynamic structural changes.","children":[{"parentId":"n1","id":"n2","label":"D. radiodurans DNA damage resistance","type":"CONC","text":"Deinococcus radiodurans bacteria survive high doses of DNA damaging agents, including UV-C radiation, ionizing radiation, and desiccation.","children":[{"text":"Factors include robust antioxidant systems and highly efficient DNA repair mechanisms unique to Deinococcus species.","children":[{"text":"An atypically high intracellular concentration of Mn2+-based antioxidant species protects the proteome from oxidative damage enzymatically and non-enzymatically.","children":[],"type":"DETL","label":"Antioxidant system","parentId":"n3","id":"n4"},{"children":[],"text":"Shielding of the proteome enables D. radiodurans to respond rapidly to DNA damage via extremely efficient DNA repair mechanisms.","type":"DETL","label":"Rapid DNA repair response","parentId":"n3","id":"n5"}],"type":"DETL","label":"Factors contributing to resistance","parentId":"n2","id":"n3"},{"children":[{"label":"Upregulated DNA repair proteins","parentId":"n6","id":"n7","text":"The RDR cascade triggers the upregulation of proteins such as RecA, UvrA/B, GyrA/B, and SSB.","children":[],"type":"DETL"},{"label":"Deinococcus-unique genes","parentId":"n6","id":"n8","children":[],"text":"Five highly upregulated genes (ddrA, ddrB, ddrC, ddrD, and pprA) are unique to Deinococcus and lack identifiable sequence homologs outside this genus.","type":"DETL"}],"text":"The RDR is activated when the bacterium senses conditions leading to DNA damage, triggering upregulation of several DNA repair proteins.","type":"CONC","label":"Radiation-Desiccation Response (RDR)","parentId":"n2","id":"n6"},{"type":"CONC","text":"The protein DdrC has been identified as an important component of the DNA repair machinery.","children":[{"parentId":"n9","id":"n10","label":"Previously known DdrC activities","type":"DETL","text":"DdrC is known to bind DNA in vitro, circularize, and compact DNA fragments, but its mechanism and biological relevance were poorly understood.","children":[]}],"parentId":"n2","id":"n9","label":"DdrC protein as repair component"}]},{"type":"CONC","children":[{"label":"DdrC shifts DNA mobility","parentId":"n11","id":"n12","children":[{"label":"Intermolecular complex formation","parentId":"n12","id":"n13","children":[],"text":"The shift in mobility suggests the formation of a large intermolecular complex, though its biological relevance is unclear.","type":"DETL"},{"label":"Smaller complex formation","parentId":"n12","id":"n14","text":"At lower DdrC concentrations (<300 nM DdrC per nM DNA), DdrC forms smaller complexes that migrate into the gel.","children":[],"type":"DETL"}],"text":"Incubation of DdrC with supercoiled, relaxed, or linear PX174 plasmid dsDNA results in DNA mobility shifted into the gel well at high DdrC concentrations.","type":"FIND"},{"label":"Differential mobility shift by plasmid topology","parentId":"n11","id":"n15","text":"DdrC binding shifts the electrophoretic mobility of DNA differently depending on the starting plasmid topology.","children":[{"parentId":"n15","id":"n16","label":"Linear and supercoiled PX174 shifts","type":"DETL","text":"Both linear and supercoiled ΦX174 are shifted to a discrete species, but the shift for supercoiled DNA is progressive, while linear DNA is sudden.","children":[]},{"type":"INSG","text":"Relaxed ΦX174 plasmid shows an unexpected increase in mobility upon DdrC binding, which is unusual for DNA binding proteins.","children":[],"parentId":"n15","id":"n17","label":"Increased mobility of relaxed PX174"},{"type":"JUST","children":[{"text":"The increased mobility is not due to a more negative net charge as DdrC has a theoretical pI of 9.7, expecting a net charge of +5.","children":[],"type":"DETL","label":"Not due to charge","parentId":"n18","id":"n19"},{"parentId":"n18","id":"n20","label":"Not due to nuclease activity","type":"DETL","children":[],"text":"Proteinase K addition to a pre-formed DdrC-DNA complex restores plasmid mobility, showing it is not due to nuclease degradation."},{"text":"The fast-moving species formed upon DdrC addition is in fact compacted plasmid, as previously shown by TEM.","children":[],"type":"INSG","label":"Compacted plasmid as fast-moving species","parentId":"n18","id":"n21"}],"text":"Increased DNA mobility could be due to the complex being more negatively charged, DNA shortening by nuclease, or topological changes reducing radius of gyration.","parentId":"n15","id":"n18","label":"Reasons for increased mobility"},{"label":"Compaction depends on nicks","parentId":"n15","id":"n22","children":[],"text":"Compaction of ΦX174 plasmid by DdrC is much greater when the plasmid has many randomly generated single-strand breaks compared to a single enzymatically-produced nick.","type":"FIND"}],"type":"FIND"}],"text":"DdrC compacts circular dsDNA through specific interactions with single-strand (ss) breaks and circularizes linear DNA by binding to double-strand (ds) breaks.","parentId":"n1","id":"n11","label":"DdrC binding and DNA compaction"},{"parentId":"n1","id":"n23","label":"DdrC mechanism of compaction","type":"CONC","text":"DdrC induces DNA compaction by recognizing and binding directly to DNA nicks, with the degree of compaction dependent on the number of available nicks.","children":[{"children":[{"parentId":"n24","id":"n25","label":"Stabilization of nicked DNA","type":"DETL","children":[],"text":"DdrC stabilizes the nicked DNA duplex, transforming a smeared, diffuse band into a sharp, discrete bound state."},{"type":"STAT","text":"The DNA band fully shifted at a ratio of 2 DdrC monomers per nick, suggesting DdrC binds to DNA as a dimer.","children":[],"parentId":"n24","id":"n26","label":"DdrC binding stoichiometry"},{"type":"INSG","children":[],"text":"Binding to the 22-mer duplex results in an upwards band shift because the fragment is too short to become compacted.","parentId":"n24","id":"n27","label":"Fragment length and compaction"}],"text":"DdrC recognizes and binds directly to ss-breaks, evidenced by a discrete band shift only when a nick is present in a 22-mer DNA duplex.","type":"FIND","label":"Direct binding to ss-breaks","parentId":"n23","id":"n24"},{"type":"FIND","children":[{"label":"Single nick binding sufficiency","parentId":"n28","id":"n29","text":"A single nick is sufficient for DdrC binding but insufficient for DNA compaction, leaving the plasmid in a circular, relaxed topology.","children":[],"type":"INSG"},{"parentId":"n28","id":"n30","label":"DdrC protects DNA from digestion","type":"FIND","text":"DdrC protects DNA from SNM1a exonuclease digestion, but not BglI endonuclease digestion, indicating direct binding at the ss-break site.","children":[]}],"text":"A plasmid with only one nick showed no compaction, but a plasmid with three nicks was compacted to a single, discrete species.","parentId":"n23","id":"n28","label":"Compaction requires multiple nicks"},{"parentId":"n23","id":"n31","label":"5' phosphate independent binding","type":"FIND","text":"Removing terminal 5' phosphates from nicked pUC19 with Shrimp Alkaline Phosphatase has no effect on subsequent DdrC-mediated DNA compaction.","children":[{"label":"Mechanism independent of 5' phosphate","parentId":"n31","id":"n32","children":[],"text":"Nick binding and plasmid compaction by DdrC do not rely on interactions with a terminal 5' phosphate.","type":"JUST"},{"parentId":"n31","id":"n33","label":"Mechanical sensing of lesions","type":"INSG","text":"DdrC may rely on a mechanical sensing mechanism, exploiting the higher conformational freedom of a DNA duplex at ss-break sites.","children":[]}]},{"type":"FIND","text":"The degree of compaction of nicked plasmids increases with the number of available nicks.","children":[{"label":"DdrC-bound pUC19 plasmids: Circular","parentId":"n34","id":"n35","table":{"cols":["DNA sample","Unbound (-DdrC) Normalized mobility","Unbound (-DdrC) Linking number ()","Bound (+DdrC) Normalized mobility","Bound (+DdrC) Linking number ()"],"rows":[{"label":"Topo marker","cells":["0.000","0.00","0.000","0.00"]},{"cells":["0.219","1.00","0.262","1.00"],"label":"Topo marker"},{"cells":["0.499","2.00","0.538","2.00"],"label":"Topo marker"},{"label":"Topo marker","cells":["0.765","3.00","0.800","3.00"]},{"cells":["1.000","4.00","1.000","4.00"],"label":"Topo marker"},{"label":"0-nick","cells":["-0.063","-0.19","0.091","0.32"]},{"label":"0-nick","cells":["0.157","0.67","0.245","0.92"]},{"cells":["0.452","1.82","0.538","2.07"],"label":"0-nick"},{"label":"0-nick","cells":["0.702","2.81","0.769","2.98"]}]},"text":"The mobility of DdrC-compacted species appears 'quantized', matching specific pUC19 topoisomers in the marker.","children":[],"type":"CMPR"},{"type":"STAT","children":[],"text":"A single nick results in an apparent ∆Lk of 0, two and three nicks both result in an apparent ∆Lk of 1, and four nicks result in a ∆Lk of 2.","parentId":"n34","id":"n36","label":"Apparent ∆Lk values"},{"text":"Each DdrC unit recognizes and binds to two nicks, bridging two distal nick sites into close spatial proximity to cause plasmid compaction.","children":[],"type":"INSG","label":"Mechanism of compaction","parentId":"n34","id":"n37"}],"parentId":"n23","id":"n34","label":"Compaction degree correlates with nicks"},{"children":[{"type":"INSG","text":"DdrC likely binds to writhe points of supercoiled plasmids, inducing structures similar to DdrC-compacted DNA.","children":[],"parentId":"n38","id":"n39","label":"Binding to writhe points"},{"label":"Active supercoiling vs writhe induction","parentId":"n38","id":"n40","children":[],"text":"It is unclear whether DdrC actively supercoils DNA or simply induces a writhe point-like structure without over- or under-winding the DNA duplex.","type":"JUST"}],"text":"DdrC shows significant binding to supercoiled pUC19 and ΦX174 plasmid, with a progressive band shift suggesting multiple binding sites.","type":"FIND","label":"DdrC binding to supercoiled DNA","parentId":"n23","id":"n38"}]},{"parentId":"n1","id":"n41","label":"DdrC circularizes linear dsDNA","type":"CONC","text":"DdrC circularizes linear dsDNA and compacts it in the presence of ssDNA breaks, likely by binding at double-strand break sites.","children":[{"label":"Binding to linear ΦX174 plasmid","parentId":"n41","id":"n42","text":"DdrC binding to linear ΦX174 plasmid results in an upward band shift to a single, discrete position, indicating a fixed stoichiometric ratio of DdrC to DNA.","children":[],"type":"FIND"},{"label":"Preference for blunt ds-breaks","parentId":"n41","id":"n43","children":[],"text":"DdrC has a significantly higher affinity for blunt ds-breaks compared to overhangs, showing ~2-fold higher affinity than 2 nt overhangs and ~7-fold higher than 4 nt overhangs.","type":"FIND"},{"text":"The gel migration position of bound linear pUC19 matches unbound, relaxed circular pUC19, suggesting DdrC circularizes linear DNA.","children":[],"type":"INSG","label":"DdrC-induced circularization","parentId":"n41","id":"n44"},{"children":[{"table":{"rows":[{"label":"Topo marker","cells":["0.000","0.00","0.000","0.00"]},{"label":"Topo marker","cells":["0.193","1.00","0.164","1.00"]},{"cells":["0.496","2.00","0.474","2.00"],"label":"Topo marker"},{"cells":["0.773","3.00","0.759","3.00"],"label":"Topo marker"},{"cells":["1.000","4.00","1.000","4.00"],"label":"Topo marker"},{"cells":["0.210","0.84","0.009","0.04"],"label":"0-nick"},{"cells":["0.210","0.84","0.198","0.81"],"label":"1-nick"},{"cells":["0.176","0.71","0.284","1.16"],"label":"2-nick"},{"cells":["0.244","0.97","0.336","1.37"],"label":"3-nick"},{"cells":["0.261","1.04","0.491","2.00"],"label":"4-nick"},{"cells":["—","—","0.647","2.63"],"label":"4-nick"}],"cols":["DNA sample","Unbound (-DdrC) Normalized mobility","Unbound (-DdrC) Linking number ()","Bound (+DdrC) Normalized mobility","Bound (+DdrC) Linking number ()"]},"parentId":"n45","id":"n46","label":"DdrC-bound pUC19 plasmids: Linear","type":"CMPR","children":[],"text":"The degree of compaction of linear plasmids incubated with DdrC scales with the number of available nicks, as seen in the apparent linking numbers."},{"label":"Nick-bridging model of circularization","parentId":"n45","id":"n47","children":[],"text":"DdrC circularizes linear DNA by bridging ds-breaks, then compacts the circularized plasmid by bridging ss-breaks.","type":"INSG"},{"text":"The nick-bridging model implies each functional DdrC unit has two DNA binding sites capable of recognizing and binding either an ss-break or a ds-break.","children":[],"type":"JUST","label":"DdrC binding sites","parentId":"n45","id":"n48"}],"text":"DdrC induces compaction of blunt-end, linear pUC19 harboring nicks, with the degree of compaction scaling with the number of available nicks.","type":"FIND","label":"Linear plasmid compaction with nicks","parentId":"n41","id":"n45"},{"type":"FIND","children":[{"parentId":"n49","id":"n50","label":"Apparent preference for ssDNA","type":"JUST","text":"The apparent preference for ssDNA may be a symptom of selective DNA circularization.","children":[]},{"text":"A 67-mer ssDNA may contact both DdrC binding sites due to flexibility, while a dsDNA of the same length may only contact one due to its rigidity.","children":[],"type":"JUST","label":"DNA length and binding sites","parentId":"n49","id":"n51"},{"children":[],"text":"Re-testing with shorter 48-mer ssDNA and dsDNA ligands showed no preference for either, suggesting the 48-mer ssDNA is too short or rigid to contact both binding sites.","type":"FIND","label":"No preference with shorter DNA","parentId":"n49","id":"n52"}],"text":"Previous reports indicated higher DdrC affinity for 67-mer ssDNA than dsDNA, with binding to DNA fragments at the termini.","parentId":"n41","id":"n49","label":"DdrC affinity for ssDNA vs dsDNA"}]},{"label":"DdrC homodimer structural domains","parentId":"n1","id":"n53","children":[{"label":"DdrC structure determination","parentId":"n53","id":"n54","text":"The structure of DdrC was solved by X-ray crystallography to gain insight into the molecular mechanism of nick detection.","children":[{"type":"DETL","text":"Initial selenomethionyl (SeMet)-derivatized DdrC crystals yielded high-quality diffraction data but insufficient anomalous signal for phasing.","children":[],"parentId":"n54","id":"n55","label":"Initial crystallization challenges"},{"text":"Additional SeMet residues (L131M, L184M) were introduced at conserved positions to facilitate phasing, yielding the 7UDI crystal structure.","children":[],"type":"DCSN","label":"Improved phasing strategy","parentId":"n54","id":"n56"}],"type":"DCSN"},{"children":[{"parentId":"n57","id":"n58","label":"Flexible linker region","type":"DETL","children":[],"text":"The first α-helix in the CTD (P111-A126) contains a short stretch of 17 residues with an alternate conformation, identified as a flexible 'linker'."},{"label":"DSF analysis confirms domains","parentId":"n57","id":"n59","children":[],"text":"Full-length DdrC exhibits two melt peaks (40°C and 73°C), and isolated NTD/CTD constructs show single melt peaks matching these temperatures, indicating independent folding.","type":"FIND"},{"parentId":"n57","id":"n60","label":"Structural homology analysis","type":"FIND","children":[],"text":"NTD and CTD regions independently align to different structures in the PDB databank, further supporting their distinction as domains."}],"text":"The crystal structure reveals DdrC is composed of an N-terminal domain (NTD, M1-E110) and a C-terminal domain (CTD, P111-G231).","type":"FIND","label":"Two distinct structural domains","parentId":"n53","id":"n57"},{"parentId":"n53","id":"n61","label":"DdrC exists as a dimer in solution","type":"FIND","text":"SEC-MALS measurements confirmed that DdrC exists as a dimer in solution, with the common interaction interface identified by crystallography likely being the dimerization interface.","children":[{"parentId":"n61","id":"n62","label":"Full-length DdrC oligomeric state","type":"STAT","text":"Full-length DdrC (1-231 FL) has a measured MW of 47.99 ± 4.14 kDa and an oligomeric state of 1.90 ± 0.16 (n-mer).","children":[]}]},{"type":"SUBC","children":[{"label":"NTD dimerization in degraded sample","parentId":"n63","id":"n64","children":[],"text":"A proteolytically degraded DdrC sample yielded a crystal structure (8U1J) composed solely of residues 1–97 from the NTD, demonstrating independent NTD dimerization.","type":"FIND"},{"parentId":"n63","id":"n65","label":"NTD dimerization in solution","type":"FIND","children":[{"type":"STAT","text":"The NTD (1-98) has a measured MW of 19.67 ± 1.15 kDa and an oligomeric state of 1.84 ± 0.11 (n-mer).","children":[],"parentId":"n65","id":"n66","label":"NTD oligomeric state"}],"text":"Explicitly expressed NTD (1–98 truncation) purified and measured by SEC-MALS also dimerizes in solution."},{"children":[{"text":"The CTD (99-231) has a measured MW of 28.63 ± 5.06 kDa and an oligomeric state of 1.96 ± 0.35 (n-mer).","children":[],"type":"STAT","label":"CTD oligomeric state","parentId":"n67","id":"n68"}],"text":"Despite low stability and initial insolubility, the CTD domain (99–231) expressed with a fusion tag also dimerizes independently in solution after cleavage.","type":"FIND","label":"CTD dimerization in solution","parentId":"n63","id":"n67"}],"text":"Both the NTD and CTD domains can dimerize independently, contributing to the overall DdrC homodimer structure.","parentId":"n53","id":"n63","label":"NTD and CTD independent dimerization"}],"text":"DdrC exists as a homodimer in solution, composed of two distinct domains, NTD and CTD, which fold and dimerize independently.","type":"CONC"},{"type":"CONC","children":[{"type":"INSG","children":[],"text":"The same asymmetric structure is seen in both crystal forms despite different protein contacts and chemical environments, indicating it's an endogenous structural feature, not a crystallographic artifact.","parentId":"n69","id":"n70","label":"Asymmetry as endogenous feature"},{"type":"FIND","text":"The asymmetry is attributed to five residues (120–125) within the interdomain region, where an α6 helix is deformed in one DdrC chain.","children":[{"children":[],"text":"The broken α6 helix appears to be under tension, similar to a bent spring.","type":"INSG","label":"Deformed α6 helix tension","parentId":"n71","id":"n72"}],"parentId":"n69","id":"n71","label":"Residues causing asymmetry"},{"label":"Loaded mousetrap mechanism","parentId":"n69","id":"n73","text":"Static forces within the DdrC homodimer counteract each other in a loaded mousetrap mechanism, where a deformed α6 helix acts as a loaded spring.","children":[{"text":"A strong network of salt-bridges and H-bonds between the NTD and CTD on one face forms a 'holding clasp' counteracting the tension.","children":[],"type":"DETL","label":"Holding clasp network","parentId":"n73","id":"n74"},{"parentId":"n73","id":"n75","label":"Potential energy storage","type":"INSG","text":"The mousetrap mechanism may store potential energy used by DdrC for its biological functions, triggered by specific biochemical signals like DNA strand ends.","children":[]}],"type":"CONC"}],"text":"The full DdrC homodimer is inherently asymmetric, with its NTD and CTD C2 axes offset by 46°, a feature confirmed across different crystal forms.","parentId":"n1","id":"n69","label":"DdrC homodimer asymmetry"},{"children":[{"type":"INSG","text":"The DdrC apo-structure reveals two large patches of partial positive charge, hinting at the location of two possible DNA binding sites.","children":[],"parentId":"n76","id":"n77","label":"Apo-structure hints at binding sites"},{"parentId":"n76","id":"n78","label":"Binding site conformations","type":"DETL","text":"Despite involving the same residues, one potential binding site appears in an 'open' conformation while the other is in a 'closed' conformation due to asymmetry.","children":[]},{"type":"FIND","text":"Computational docking shows dsDNA binds to the 'open' site, with protein-DNA interactions primarily mediated by the CTD clasp, which deforms DNA into the NTD.","children":[{"parentId":"n79","id":"n80","label":"Lesion scanning state","type":"INSG","text":"In this conformation, the α6 helices remain 'loaded', suggesting this binding mode represents a lesion scanning state where DdrC interrogates the DNA.","children":[]}],"parentId":"n76","id":"n79","label":"Unbroken DNA binding model"},{"label":"Nicked DNA binding model","parentId":"n76","id":"n81","text":"RF2NA prediction shows nicked DNA docks to the 'closed' pocket, where a single nick increases DNA flexibility allowing this binding.","children":[{"label":"Duplex deformation by CTD","parentId":"n81","id":"n82","children":[],"text":"CTD clasp residues (Lys-170) deform the DNA duplex, disrupting base-pair contacts and forming a π-cation interaction with a DNA base.","type":"DETL"},{"label":"Nick-specific NTD contacts","parentId":"n81","id":"n83","children":[],"text":"DNA duplex deformation allows for nick-specific contacts within the NTD, with Arg-14 interacting with the 5' terminal base and Arg-81 binding 3' DNA backbone atoms.","type":"DETL"},{"label":"5' phosphate not important","parentId":"n81","id":"n84","children":[],"text":"The terminal 5' Phosphate group on the nick is not predicted to form polar contacts with DdrC, consistent with experimental observations.","type":"JUST"},{"type":"FIND","children":[{"parentId":"n85","id":"n86","label":"CTD interface disruption","type":"INSG","children":[],"text":"Disruption of the CTD dimer interface is necessary for both binding sites to be closed, suggesting the weak CTD dimer contacts plausibly break during nick detection."}],"text":"RF2NA predicts a symmetric binding conformation for nicked DNA, with both binding sites in a 'closed' conformation and α6 helices in a relaxed state.","parentId":"n81","id":"n85","label":"Symmetric binding for nicked DNA"}],"type":"FIND"},{"type":"SUBC","children":[{"label":"Topological writhe point mimicry","parentId":"n87","id":"n88","children":[],"text":"Trapping two DNA nicks in a symmetric conformation places duplexes perpendicularly, topologically mimicking a supercoiling writhe point.","type":"DETL"},{"parentId":"n87","id":"n89","label":"Compaction proportional to damage","type":"INSG","text":"DdrC progressively compacts circular DNA, with the degree of compaction proportional to the amount of DNA damage (more ss-breaks lead to more compaction).","children":[]},{"text":"DdrC likely recognizes similar duplex deformations (wrinkles, bubbles, kinks) that arise spontaneously in supercoiled DNA to alleviate torsional strain.","children":[],"type":"INSG","label":"Binding to supercoiled DNA deformations","parentId":"n87","id":"n90"},{"type":"DETL","text":"The CTD forms sequence-independent contacts, pushing the ds-break into the NTD, which then makes end-specific contacts with the 5' and 3' terminal ends.","children":[{"type":"JUST","children":[],"text":"A 5' overhang disrupts the NTD's ability to properly engage the 3' end of the duplex, thereby reducing DdrC affinity to ds-breaks.","parentId":"n91","id":"n92","label":"Impact of 5' overhang"}],"parentId":"n87","id":"n91","label":"Mechanism for ds-break circularization"}],"text":"DdrC binds unbroken DNA via its open face, scans for nicks by deforming DNA, and upon finding a ss-break, triggers a conformational change that opens the second binding site to trap a second nick.","parentId":"n76","id":"n87","label":"Proposed dual nick detection mechanism"}],"text":"Dimer asymmetry and the two structurally different binding sites form the basis of DdrC's DNA nick detection mechanism.","type":"CONC","label":"DdrC nick detection mechanism basis","parentId":"n1","id":"n76"},{"label":"Functional analysis of DdrC mutants","parentId":"n1","id":"n93","text":"DdrC mutations disrupting DNA binding or lesion recognition match computational predictions, highlighting the essentiality of both activities for DdrC function and UV-C resistance.","children":[{"parentId":"n93","id":"n94","label":"Two distinct binding events","type":"FIND","children":[{"children":[],"text":"The HA binding mode corresponds to DdrC localized to specific DNA damage sites, while the LA binding mode corresponds to DdrC scanning non-specifically.","type":"INSG","label":"HA vs LA binding modes","parentId":"n94","id":"n95"}],"text":"Gel-based DdrC-DNA binding assays resolve two distinct events: high-affinity (HA) binding, lesion-dependent and fixed stoichiometric, and low-affinity (LA) binding, non-specific and variable stoichiometric."},{"children":[],"text":"Targeted mutagenesis was used to selectively disrupt non-lesion-specific (LA) and lesion-specific (HA) binding activities based on computational models.","type":"DCSN","label":"Targeted mutagenesis strategy","parentId":"n93","id":"n96"},{"label":"Disruption of non-lesion-specific interactions","parentId":"n93","id":"n97","text":"CTD-mut and ΔCTD mutants, targeting conserved CTD residues (Arg-128, Arg-142, Arg-164, Lys-170), disrupted all DNA binding activity (LA and HA) across all pUC19 topologies.","children":[{"type":"JUST","text":"This finding supports computational models predicting the CTD domain harbors 'core' DNA binding residues essential for binding any DNA ligand.","children":[],"parentId":"n97","id":"n98","label":"CTD as core binding domain"}],"type":"FIND"},{"type":"FIND","children":[{"children":[],"text":"Disrupting nick-recognition residues (Arg-14, Arg-81) in NTD (NTD-mut) completely lost HA binding for nicked pUC19 but had milder effects for linear and supercoiled pUC19.","type":"FIND","label":"NTD-mut effects on HA binding","parentId":"n99","id":"n100"},{"parentId":"n99","id":"n101","label":"R14/R81 ss-break specificity","type":"JUST","text":"This suggests Arg-14 and/or Arg-81 specifically bind ss-breaks over ds-breaks, supported by computational models where only Arg-14 recognizes ds-breaks.","children":[]},{"label":"CTD-NTD cooperation in detection","parentId":"n99","id":"n102","children":[],"text":"Lesion recognition in NTD requires proper DNA duplex positioning by the CTD to make proper contact, as the NTD alone cannot stabilize the DdrC-DNA complex.","type":"INSG"}],"text":"ΔNTD removal completely lost HA binding but retained detectable LA binding, indicating the NTD is responsible for lesion recognition while the CTD suffices for DNA binding.","parentId":"n93","id":"n99","label":"Disruption of lesion-specific interactions"},{"children":[{"type":"FIND","text":"D. radiodurans ΔuvsE harboring an empty expression vector shows a 94% loss in UV-resistance at 200 J/m².","children":[],"parentId":"n103","id":"n104","label":"Baseline UV-C sensitivity"},{"parentId":"n103","id":"n105","label":"WT DdrC restores UV resistance","type":"FIND","text":"Complementary expression of WT ddrC restores UV resistance, making the strain 210% more UV resistant than the ΔuvsE baseline at 200 J/m².","children":[]},{"text":"Neither NTD-mut nor CTD-mut ddrC variants can restore UV resistance to a ddrC knockout strain.","children":[],"type":"FIND","label":"Mutant DdrC fails to restore resistance","parentId":"n103","id":"n106"},{"label":"Mutant UV resistance loss","parentId":"n103","id":"n107","children":[],"text":"NTD-mut and CTD-mut variants exhibit 99.8% and 99.7% loss of UV resistance, respectively, at a UV dose of 200 J/m².","type":"STAT"},{"label":"Essentiality of DdrC functions","parentId":"n103","id":"n108","children":[],"text":"In vitro observations show NTD-mut lacks ss-break binding and CTD-mut is deficient in overall DNA binding, confirming both functions are essential for DdrC.","type":"INSG"},{"label":"Dominant-negative phenotype","parentId":"n103","id":"n109","children":[{"label":"Possible additional DdrC functions","parentId":"n109","id":"n110","children":[],"text":"The dominant-negative phenotype suggests DdrC has additional functions beyond DNA binding and lesion recognition, possibly recruiting other factors via protein-protein interactions.","type":"JUST"}],"text":"NTD-mut and CTD-mut strains exhibit significantly lower UV resistance compared to the ddrC knockout strain, indicating a dominant-negative effect.","type":"FIND"},{"parentId":"n103","id":"n111","table":{"cols":["UV-C dose (J/m²)","ΔuvsE+ empty vector","ΔuvsEΔddrC+ empty vector","ΔuvsEΔddrC+ ddrC (WT)","ΔuvsEΔddrC+ ddrC (NTD-mut)"],"rows":[{"cells":["1.00 ± 0.09 x 10^0","1.00 ± 0.13 × 10^0","1.00 ± 0.06 x 10^0","1.00 ± 0.12 × 10^0"],"label":"0"},{"cells":["5.19±0.48 x 10^-1","7.52 ± 1.35 × 10^-1","6.07 ± 1.12 x 10^-1","2.68 ± 0.44 x 10^-1"],"label":"28.7"},{"cells":["3.09 ± 0.70 × 10^-1","5.09 ± 1.02 x 10^-1","4.39 ± 1.22 x 10^-1","6.89 ± 0.85 x 10^-2"],"label":"57.4"},{"label":"86.1","cells":["1.04 ± 0.17 × 10^-1","2.68 ± 0.46 × 10^-1","2.11 ± 0.39 × 10^-1","6.36 ± 3.47 x 10^-3"]},{"label":"114.8","cells":["6.63 ± 1.08 × 10^-2","9.22 ± 0.97 x 10^-2","2.10 ± 0.38 × 10^-1","2.02 ± 1.42 x 10^-3"]},{"cells":["9.37 ± 5.01 × 10^-3","6.26 ± 1.51 x 10^-3","5.33 ± 2.48 × 10^-2","6.19 ± 5.59 x 10^-4"],"label":"143.5"},{"cells":["3.77 ± 1.00 × 10^-3","7.26 ± 3.27 x 10^-4","1.92 ± 0.24 x 10^-2","8.16 ± 3.52 x 10^-5"],"label":"172.2"},{"cells":["1.71 ± 0.32 x 10^-3","1.07 ± 0.75 x 10^-4","5.37 ± 0.86 x 10^-3","1.13 ± 0.53 x 10^-5"],"label":"200.9"}]},"label":"D. radiodurans UV-C survival","type":"CMPR","text":"WT ddrC significantly restores UV-C resistance compared to mutant or empty vector strains, highlighting its critical role in DNA repair.","children":[]}],"text":"DdrC's DNA binding and lesion recognition capabilities are essential for UV-C resistance in Deinococcus bacteria.","type":"CONC","label":"DdrC function in UV-C resistance","parentId":"n93","id":"n103"}],"type":"CONC"},{"children":[{"text":"DdrC mechanism is similar to PARP-1 and Rad4/XPC in utilizing conformational changes or scanning for lesions, but DdrC uniquely traps two lesions.","children":[],"type":"CMPR","label":"DdrC vs. PARP-1 vs. Rad4/XPC","parentId":"n112","id":"n113","table":{"rows":[{"label":"Lesion binding","cells":["Rapidly binds single-strand breaks","Binds DNA in scanning conformation","Senses and traps two DNA lesions per structural unit"]},{"cells":["Loosely associated domains with high potential energy on un-nicked DNA","Attempts lower-energy protein-DNA conformation","Stored tension forces in dimer trigger conformational change"],"label":"Energy state / conformation"},{"cells":["Interrogates DNA via F1/F2 dimerization, F2 twists DNA upon ss-break","Interrogates DNA for lesions by flipping out DNA bases","Scans for nicks by deforming duplex via loaded α6 helix"],"label":"Detection process"},{"label":"Outcome/Consequence","cells":["Initiates 'structure collapse' for high affinity interaction, recruits repair factors","Kinetic barrier lower for damaged DNA (e.g., thymine dimer)","Circularization of linear DNA, compaction of nicked DNA"]}],"cols":["Mechanism Aspect","PARP-1 (Human)","Rad4/XPC","DdrC"]}}],"text":"The proposed DdrC mechanism for nick detection shares similarities with PARP-1 and Rad4/XPC, but uniquely senses and traps two DNA lesions per unit.","type":"CONC","label":"Similarities to other nick detection mechanisms","parentId":"n1","id":"n112"},{"label":"Biological utility of DdrC in Deinococcus","parentId":"n1","id":"n114","children":[{"parentId":"n114","id":"n115","label":"Prevention of ds-breaks","type":"JUST","children":[],"text":"Immobilization and compaction of ss-breaks by DdrC could prevent ss-breaks from becoming ds-breaks as they accumulate."},{"label":"Prevention of end diffusion","parentId":"n114","id":"n116","children":[],"text":"In the event of a ds-break, DdrC bridges DNA ends to prevent end diffusion.","type":"JUST"},{"parentId":"n114","id":"n117","label":"Genome supercoiling control","type":"JUST","text":"Trapping DNA lesions allows Deinococcus to control its genome's supercoiling state, even with ss- and ds-breaks.","children":[]},{"type":"FIND","children":[{"type":"JUST","text":"Our results support the claim that DdrC is a novel nucleoid-associated protein (NAP) maintaining compact nucleoid structure after extreme DNA damage.","children":[],"parentId":"n118","id":"n119","label":"Support for NAP claim"},{"children":[],"text":"DdrC acts as a lesion-specific NAP, neutralizing ss-breaks and compacting the nucleoid proportionally to damage, aiding other NAPs in maintaining a compact nucleoid.","type":"INSG","label":"Role as lesion-specific NAP","parentId":"n118","id":"n120"}],"text":"Previous TEM/AFM showed DdrC promotes plasmid compaction and circularization in vitro, and co-localizes with compact nucleoid DNA in vivo after γ-radiation.","parentId":"n114","id":"n118","label":"DdrC as nucleoid-associated protein (NAP)"},{"label":"DdrC interaction with other NAPs","parentId":"n114","id":"n121","children":[],"text":"Dominant-negative behavior of DdrC mutants suggests DdrC may interact with other NAPs or recruit repair factors via protein-protein interactions.","type":"INSG"}],"text":"DdrC's observed behavior of immobilizing ss-breaks and compacting nicked DNA is useful for Deinococcus under DNA damaging conditions.","type":"INSG"}],"type":"APEX"},"contentType":"Explainer","sourceType":"file","sharedAt":{"_seconds":1780342262,"_nanoseconds":633000000},"title":"DdrC DNA repair factor mechanism"}