Heavily irradiated Deinococcus radiodurans reassembles its shattered genome mainly through extended synthesis-dependent strand annealing (ESDSA), a repair route that begins with massive 5'-to-3' exonucleolytic resection of hundreds of double-strand-break ends, uses the liberated 3' single-stranded overhangs to prime extensive repair synthesis on overlapping fragments, and restores chromosomal contiguity by annealing the elongated complementary strands. The in vivo dissection of this process (Slade et al., 2009) left its molecular mechanism open: which nuclease(s) execute the resection and how they are regulated, what triggers the transition from DNA polymerase III to DNA polymerase I during elongation, how elongation is quantitatively partitioned between ESDSA-type strand-displacement synthesis and break-induced-replication-like migrating D-loops, and how annealing among hundreds of fragments is coordinated in time and space while a balance between DNA degradation and preservation is maintained. Later literature (2009–2026) has largely resolved the formerly open identity of the RecA-independent repair pathway (mainly DdrB-mediated single-strand annealing, plus alternative-end-joining-like events) and has biochemically and structurally characterized candidate resection enzymes (RecJ, NurA–HerA) and polymerases, but no study assigns the in vivo resection machinery, demonstrates the polymerase-switch trigger, quantifies the synthesis-mode partition, or provides direct in vivo evidence for the coordination of fragment annealing. The objective is to complete this molecular-mechanistic account.

Background

Deinococcus radiodurans is a bacterium famous for surviving ionizing radiation doses (thousands of grays) that fragment its genome into hundreds of double-strand-break (DSB) pieces, whereas most bacteria are killed by doses a hundredfold smaller. Its survival depends on an extraordinarily efficient homologous repair that reassembles the shattered chromosomes within hours, aided by polyploidy (many genome copies per cell) and by manganese-based antioxidant protection of repair proteins. The central repair route, discovered by Zahradka et al. (2006), is ESDSA — extended synthesis-dependent strand annealing. In ESDSA, DNA ends are first extensively eroded by 5'-to-3' exonucleases ('resection'), generating long 3' single-stranded DNA (ssDNA) overhangs; these overhangs prime DNA synthesis on homologous intact or partially intact fragments; and the newly made complementary strands, which can grow to lengths exceeding the chromosome, restore chromosomal contiguity by annealing to each other. This differs from canonical bacterial recombinational repair (RecBCD-mediated resection followed by RecA-mediated strand invasion and crossing over) because D. radiodurans naturally lacks the RecBCD enzyme complex and relies on the RecFOR pathway, a set of proteins (RecF, RecO, RecR) that load the recombinase RecA onto ssDNA; resection in ESDSA must also be genome-wide and coordinated with DNA synthesis rather than limited to single break sites. Break-induced replication (BIR) denotes a repair mode in which an invading 3' end primes a replication fork that migrates along a template through a moving displacement loop (D-loop); Slade et al. (2009) could not exclude that such BIR-like migrating D-loops, rather than ESDSA-specific strand-displacement synthesis, contribute to the observed elongation. Slade et al. (2009) established in vivo that priming of repair synthesis requires RecA (the bacterial recombinase that catalyzes homologous pairing and strand exchange) and RadA (a RecA paralog implicated in branch migration, the ATP-driven movement of the crossover point at which two recombining DNA strands exchange partners), that DNA polymerase III (the replicative polymerase) is essential for initiating repair synthesis, that efficient elongation requires both DNA polymerase III and DNA polymerase I (Pol I, otherwise known for gap filling during base excision repair, the pathway that repairs oxidative base lesions), and that inactivating both polymerases causes degradation of the DNA fragments and rapid cell death. Their discussion explicitly acknowledged the mechanistic gaps that define this problem: the exonuclease(s) performing the initial resection were not identified; the relative contributions of BIR-like processes versus ESDSA-specific modes were not quantified; the mechanistic basis of Pol I's assistance in elongation versus protection and the trigger of the Pol III-to-Pol I switch were unknown; the RecA-independent repair observed in recA mutants was of unclear molecular nature (single-strand annealing (SSA), nonhomologous end joining (NHEJ), or another process); the coordination of annealing among hundreds of fragments — including a proposed role of 'bridging' fragments joining distant homologous regions — lacked direct evidence; and the balance between exonucleolytic degradation and polymerase-mediated preservation under varying damage loads was unexplained. Subsequent work has built up the parts list: the radiation-induced Deinococcus-specific proteins DdrA (a Rad52-like 3'-end protector), DdrB (a single-strand-annealing protein), and DdrC (a break-sensing compaction protein); the DHH-family exonuclease RecJ (a single-strand-specific 5'-to-3' exonuclease belonging to a phosphoesterase superfamily named for its conserved aspartate-histidine-histidine active-site motif) and the archaeal-type NurA nuclease–HerA helicase pair as candidate resection machines; PprA and DprA as accessory regulators; and detailed biochemical characterizations of the DnaE (Pol III alpha subunit) and PolA (Pol I) polymerases. What is still missing is the integration of these parts into a demonstrated in vivo mechanism.

Problem Statement

Provide a complete molecular-mechanistic account of ESDSA-mediated genome reassembly in heavily irradiated Deinococcus radiodurans, as left open by Slade et al. (2009) and not closed by subsequent literature. The single overarching objective is a demonstrated in vivo mechanism in which each step of the pathway is assigned to specific molecular actors with evidence for their regulation and coordination. The five determinations below are jointly required: the account is complete only when every step of the pathway is assigned to specific molecular actors with the supporting in vivo evidence stated, so an answer that resolves only some of (1)–(5) is a partial contribution rather than a resolution of the task. Concretely, determine: (1) which exonuclease(s) execute the genome-wide 5'-to-3' resection that erodes hundreds of double-strand-break ends and generates the long 3' single-stranded overhangs that prime repair synthesis, and how that activity is regulated and targeted in coordination with RecA/RadA and the repair polymerases (the candidate enzymes RecJ and the NurA–HerA complex are biochemically characterized but their in vivo assignment and regulation are unresolved); (2) what triggers the transition from DNA polymerase III to DNA polymerase I during repair elongation, and the mechanistic basis of DNA polymerase I's contribution to elongation as distinct from its protective gap-filling role; (3) the quantitative partition of repair synthesis between ESDSA-specific strand-displacement modes and break-induced-replication-like migrating D-loops in reassembling the genome; (4) how the annealing of complementary long single strands is coordinated in time and space among hundreds of fragments, including whether a bridging-fragment mechanism operates in vivo; and (5) how the balance between exonucleolytic degradation and polymerase-mediated preservation of DNA fragments is maintained under varying levels and types of DNA damage, including the residual question of the fidelity of RecA-independent rejoining products. The earlier-listed question of the molecular nature of the RecA-independent repair pathway is already largely resolved (mainly DdrB-mediated single-strand annealing, with alternative-end-joining-like events producing rearrangements) and is retained only through its fidelity residual.

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