The extreme radioresistance of Deinococcus radiodurans depends on a dedicated transcriptional program, the radiation/desiccation response (RDR) regulon, in which the XRE-family repressor DdrO binds a conserved 17-bp palindromic promoter motif (the RDRM) and is irreversibly cleaved by the IrrE metalloprotease after genotoxic stress. A 2021 integrative ChIP-seq and RNA-seq study mapped this regulon in vivo for the first time, roughly doubling it to 37 direct targets (42 counting operons) and listing 47 further candidate genes, while leaving unresolved what most of the newly identified members do and how the regulon's induction is organized. That study leaves open: the functional roles of the uncharacterized new targets (the GNAT-family acetyltransferases DdrN and DR2255 and their substrates, the transposases DRC0017 and DR1287, the WGR-domain protein DR2173, and the DRA0165-DRA0166 operon) in the radiation/desiccation response and radioresistance; validation of the 47 two-criteria candidate targets (including whether uvrA and uvrD are co-regulated or induced within the first hour of stress); the molecular basis of the ordered early-versus-late induction of regulon genes, proposed to reflect different DdrO affinities for divergent RDRM sequences; identification of the RDR gene(s) whose sustained expression triggers apoptotic-like cell death upon prolonged DdrO depletion; the in vivo interplay of DdrO with other regulators, notably the CRP-family protein DdrI and LexA-type SOS regulation; and the conservation and variation of the regulon across Deinococcus species and other bacteria encoding IrrE/DdrO-like pairs.

Background

The bacterium Deinococcus radiodurans survives doses of ionizing radiation and prolonged desiccation that fragment its genome into hundreds of pieces, rebuilding it essentially error-free. Unusually for a bacterium, its main genotoxic-stress response is not the classical RecA/LexA-gated SOS system but a dedicated transcriptional program called the radiation/desiccation response (RDR) regulon. RDR genes share a conserved 17-bp palindromic promoter motif, the RDRM (radiation/desiccation response motif), first identified by comparative genomics across radiation-resistant Deinococcus species (Makarova et al. 2007). The regulon is controlled by a two-protein switch: DdrO, an XRE-family (helix-turn-helix DNA-binding domain family) transcriptional repressor that binds RDRM sequences as a dimer and is essential for viability, and IrrE (also called PprI), a constitutively expressed zinc metallopeptidase that after genotoxic stress cleaves the C-terminal dimerization domain of DdrO, irreversibly relieving repression. Crystal structures of DdrO from D. radiodurans and D. geothermalis (de Groot et al. 2019; Lu et al. 2019), together with later work showing that IrrE is directly stimulated by single-stranded DNA (Lu et al. 2024) and forms a transient heterodimer with DdrO (Reuzeau et al. 2026), have established this activation mechanism in detail. Eugénie et al. (2021) then mapped the regulon in vivo at genome scale by combining ChIP-seq (chromatin immunoprecipitation followed by sequencing of a tagged DdrO; the method localizes protein-binding sites across a genome) with RNA-seq transcriptomics during conditional DdrO depletion and promoter-motif searches. They identified 37 genes meeting all three criteria (ChIP-seq peak, RDRM motif, DdrO-dependent expression; 42 counting operons, which are groups of co-transcribed genes) and 47 further genes meeting exactly two criteria as candidate targets. New members include genes with clear DNA-repair roles (the RecG helicase, the HelD-family helicase DR1572, the replicative DNA ligase LigA) but also genes of unknown or unexpected function: the predicted GNAT-family (Gcn5-related N-acetyltransferase) acetyltransferases DdrN and DR2255; the transposases DRC0017 (Tn3 family, part of the composite transposon TnDra1) and DR1287 (insertion sequence ISDra5; the paper's Discussion names DR1287, whereas its Results and Table 2 identify the same ISDra5 regulon member as DR1296/DRC0033, the two tags denoting one element); the WGR-domain protein DR2173 (WGR denotes a conserved Trp-Gly-Arg motif domain found in poly(ADP-ribose)-polymerase-like proteins and some bacterial regulators); and the DRA0165-DRA0166 operon of conserved hypothetical proteins. Several observations in that study define the open questions addressed here. Prolonged DdrO depletion, which forces sustained expression of the whole regulon, triggers an apoptotic-like cell death (ALD) with nucleoid condensation, DNA fragmentation, membrane blebbing, and vesicle formation, first described by Devigne et al. (2015); the gene(s) responsible were not identified. RDR genes are not induced simultaneously during DdrO depletion: ddrC, ddrD, and pprA rise early whereas ssb and ddrF rise late, suggesting DdrO binds divergent RDRM sequences with different affinities. Some regulon genes (ddrA, gyrA, dr1572) are also controlled by DdrI, a CRP-family (cAMP receptor protein family) regulator involved in stress adaptation (Meyer et al. 2018). Candidate targets such as the nucleotide-excision-repair genes uvrA and uvrD are induced by ionizing radiation but were not significantly upregulated in the DdrO-depletion time course — uvrD changed, if at all, only at a late stage, under the threshold for DdrO-dependent expression — a time course that did not sample the first hour of stress. Finally, although the IrrE/DdrO pair is conserved across Deinococcus and related genera, and several Deinococcus species additionally carry RecA/LexA-controlled mutagenesis cassettes alongside it (Blanchard and de Groot 2021), the regulon has been experimentally mapped only in D. radiodurans.

Problem Statement

Determine how the expanded DdrO-controlled RDR regulon operates in Deinococcus radiodurans, by resolving, with genetic, biochemical, and comparative-genomic evidence, the six questions left open by the 2021 genome-scale mapping of the regulon: (1) the functional roles in the radiation/desiccation response and in extreme radioresistance of the uncharacterized direct DdrO targets — the transposases DRC0017 (Tn3 family, within TnDra1) and DR1287 (ISDra5), the predicted GNAT-family acetyltransferases DdrN and DR2255 (including the identity of their acetylation substrates, and whether these include DNA repair or nucleoid-structure proteins), the WGR-domain protein DR2173, and the proteins of the DRA0165-DRA0166 operon — together with whether and how DdrO-controlled expression shapes transposition of DRC0017 and DR1287 and what this implies for genome stability under stress; (2) which of the 47 genes that met only two of the three mapping criteria (ChIP-seq peak, RDRM motif, DdrO-dependent expression) are genuine DdrO targets, classifying all 47 (with uvrA and uvrD as a mandatory named subcase: whether they are co-regulated by DdrO and other regulators or are induced within the first hour of stress, a window the original design did not sample); (3) the molecular basis of the ordered induction of the regulon during DdrO depletion — early genes such as ddrC, ddrD, and pprA versus late genes such as ssb and ddrF — that is, whether DdrO binds the divergent RDRM sequences with measurably different affinities, and what this differential timing contributes physiologically to the ordered repair process; (4) which RDR regulon gene(s), when their expression is sustained by prolonged DdrO depletion, trigger the apoptotic-like cell death pathway, or whether death instead requires a cascade of regulatory events; (5) how DdrO regulation integrates in vivo with other transcriptional regulators sharing its targets, notably the CRP-family regulator DdrI, and with the RecA/LexA-controlled regulation that coexists with IrrE/DdrO in several Deinococcus species; and (6) how the DdrO regulon is conserved and varied across Deinococcus species and in other bacteria encoding IrrE/DdrO-like protein pairs. These components jointly serve one objective: to convert the mapped regulon into a mechanistic account of how its member genes and their regulated, ordered induction produce the extraordinary radiation and desiccation tolerance of D. radiodurans. All six components are mandatory and jointly required; a complete answer resolves each to its named deliverable — (1) the functions and transposition consequences of the new targets, (2) a determination for all 47 two-criteria candidates, (3) the affinity basis and physiological role of the ordered induction, (4) the gene(s) or cascade triggering apoptotic-like cell death, (5) the in vivo integration with DdrI and RecA/LexA, and (6) the conserved and varied regulon across species — each as a specific mechanistic claim supported by mutant phenotypes, biochemical evidence, or comparative-genomic analysis sufficient for a specialist to judge that component's contribution to the response; neither a proper subset of the six components nor a survey that leaves any component without such a claim is a complete answer.

References

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