The radioresistant bacterium Deinococcus radiodurans lacks a canonical LexA/RecA SOS response, yet stabilizing guanine quadruplex (G4) DNA structures with ligands such as N-methyl mesoporphyrin IX (NMM) during post-irradiation recovery renders it radiosensitive, represses DNA damage-inducible repair genes, and selectively lowers the copy number of its small plasmid. Subsequent work established that G4 structures genuinely form and resolve in vivo during the damage response, that the RecQ helicase is the primary G4-resolving enzyme, and that manganese ions destabilize G4s. What remains unresolved is the molecular-mechanistic layer: at which steps of the extended synthesis-dependent strand annealing (ESDSA) repair pathway G4 structures act and which proteins are recruited to them; why G4 stabilization selectively reduces small-plasmid copy number although replication origins lack G4 motifs; whether replication initiation through DnaA is directly G4-controlled in vivo; how G4 stabilization indirectly alters genes lacking G4 motifs such as the repressor gene ddrO; and which further native G4-resolving or G4-binding proteins maintain G4 homeostasis during the radiation response. The problem asks for these mechanisms to be established with genetic and biochemical evidence.

Background

Guanine quadruplexes (G4s) are non-canonical DNA secondary structures in which four guanines are hydrogen-bonded into a planar G-quartet and stacked quartets are stabilized by monovalent cations; G-rich single-stranded DNA exposed during replication, transcription, recombination, and repair can fold into G4s, which can either impede or regulate these processes. Potential G4-forming sequences (PQS) are sequence motifs, typically runs of three or more guanines, predicted to adopt such structures. N-methyl mesoporphyrin IX (NMM) is a small ligand that binds and stabilizes G4 structures, and is widely used to arrest G4 dynamics experimentally. Deinococcus radiodurans is a bacterium famous for surviving gamma-radiation doses that shatter its genome into hundreds of fragments. Its genome is multipartite — two chromosomes plus a megaplasmid and a small plasmid — and GC-rich, carrying a high density of PQS that are overrepresented near DNA repair genes. Radioresistance relies on extended synthesis-dependent strand annealing (ESDSA), a double-strand-break repair pathway in which long single-stranded stretches synthesized from fragmented ends anneal and recombine to reassemble the genome during the post-irradiation recovery (PIR) period; because ESDSA generates extensive single-stranded DNA, it is intrinsically prone to G4 formation. Unlike most bacteria, D. radiodurans has no LexA/RecA-mediated canonical SOS response; known alternative regulatory layers include the DdrO repressor, which is cleaved by the PprI protease upon DNA damage to derepress radiation-desiccation response genes, and damage-responsive protein phosphorylation. A series of studies showed that promoter PQS near repair genes (recA, recQ, mutL and others) fold into G4s in vitro and modulate promoter activity, and that NMM present during PIR sensitizes cells to ionizing radiation, represses damage-inducible expression of repair proteins (RecA, PprA, SSB, DdrA/B/C, DNA gyrase, and others), slows DNA synthesis and translation of G4-motif mRNAs, and selectively lowers the copy number of the small plasmid — which encodes PQQ synthase implicated in oxidative-stress resistance — while barely affecting the chromosomes. The 2019 study that framed this problem inferred a regulatory role for G4 dynamics in radioresistance from these ligand-based experiments, and its authors listed the mechanistic questions they could not answer. Subsequent work partially closed the list: the RecQ helicase of D. radiodurans was characterized as an ATP-independent G4-unwinding enzyme with roughly 400-fold higher affinity for G4 DNA than for double-stranded DNA (dissociation constants of about 12 nM versus 5 µM), and its loss causes G4 accumulation in vivo; topoisomerase IB, the other characterized G4-resolving enzyme, interacts with genome-segregation proteins and DnaA and contributes to multipartite genome maintenance; and in 2025 the first direct in vivo visualization of G4s (using the fluorescent dye thioflavin T and G4-specific antibodies) showed that G4s are scarce during normal growth, peak about one hour after gamma irradiation, are destabilized by the divalent manganese cations that accumulate in this bacterium as antioxidants, and, when locked by ligands during PIR, raise 8-oxoguanine levels and delay double-strand-break repair. Manganese- and RecQ-dependent control therefore explains when G4s form, but not how they act.

Problem Statement

Establish the molecular mechanisms by which guanine quadruplex (G4) DNA structure dynamics regulate genome functions during the gamma-radiation damage response of Deinococcus radiodurans. The overarching objective is a causal, experimentally supported account that replaces the current ligand-arrest correlations, and it comprises five linked mechanistic questions: (1) ESDSA integration — identify the molecular steps of the extended synthesis-dependent strand annealing repair pathway at which G4 structures form, impede, or assist repair during post-irradiation recovery, and which proteins are recruited to radiation-induced G4s to mediate these effects; (2) replicon-selective copy-number control — identify the mechanism by which G4-stabilizing ligand treatment selectively reduces the copy number of the small plasmid while barely affecting the two chromosomes and the megaplasmid, given that the replication-origin regions of all four genome elements lack G4 motifs; (3) replication-initiation control — determine whether replication initiation is directly regulated by G4 structures in vivo, in particular whether expression of the initiator protein DnaA (whose coding sequence carries a G4-forming 3G-run motif and whose neighboring intergenic region carries a PQS) is G4-controlled under normal and post-irradiation conditions; (4) indirect regulation of genes lacking G4 motifs — identify the pathway by which G4 stabilization alters the expression of genes without G4 motifs, exemplified by the repressor gene ddrO (DR_2574); and (5) the native G4-management machinery — identify, beyond RecQ and topoisomerase IB, the further G4-resolving and G4-binding proteins and regulatory pathways that maintain G4 homeostasis during the DNA damage response, including how the destabilizing action of manganese ions on G4s is itself regulated and integrated with these factors. A complete answer addresses all five questions and, for each, comprises a mechanistic account — grounded in and consistent with the cited literature — that names the molecular step, pathway, or protein factor at issue, proposes how it produces the observed effect, distinguishes direct G4-mediated effects from secondary consequences of G4 arrest, and states a concrete genetic or biochemical prediction by which the proposed mechanism could be confirmed or refuted.

References

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