Characterization of the DdrD protein from the extremely radioresistant bacterium Deinococcus radiodurans.

Here, we report the in vitro and in vivo characterization of the DdrD protein from the extraordinary stress-resistant bacterium, D. radiodurans. DdrD is one of the most highly induced proteins following cellular irradiation or desiccation. We confirm that DdrD belongs to the Radiation Desiccation Response (RDR) regulon protein family whose expression is regulated by the IrrE/DdrO proteins after DNA damage. We show that DdrD is a DNA binding protein that binds to single-stranded DNA In vitro, but not to duplex DNA unless it has a 5' single-stranded extension. In vivo, we observed no significant effect of the absence of DdrD on the survival of D. radiodurans cells after exposure to γ-rays or UV irradiation in different genetic contexts. However, genome reassembly is affected in a ∆ddrD mutant when cells recover from irradiation in the absence of nutrients. Thus, DdrD likely contributes to genome reconstitution after irradiation, but only under starvation conditions. Lastly, we show that the absence of the DdrD protein partially restores the frequency of plasmid transformation of a ∆ddrB mutant, suggesting that DdrD could also be involved in biological processes other than the response to DNA damage.

The interplay between Mn and Fe in Deinococcus radiodurans triggers cellular protection during paraquat-induced oxidative stress.

The bacterium Deinococcus radiodurans is highly resistant to several stress conditions, such as radiation. According to several reports, manganese plays a crucial role in stress protection, and a high Mn/Fe ratio is essential in this process. However, mobilization of manganese and iron, and the role of DNA-binding-proteins-under-starved-conditions during oxidative-stress remained open questions. We used synchrotron-based X-ray fluorescence imaging at nano-resolution to follow element-relocalization upon stress, and its dependency on the presence of Dps proteins, using dps knockout mutants. We show that manganese, calcium, and phosphorus are mobilized from rich-element regions that resemble electron-dense granules towards the cytosol and the cellular membrane, in a Dps-dependent way. Moreover, iron delocalizes from the septum region to the cytoplasm affecting cell division, specifically in the septum formation. These mechanisms are orchestrated by Dps1 and Dps2, which play a crucial role in metal homeostasis, and are associated with the D. radiodurans tolerance against reactive oxygen species.

The absence of the RecN protein suppresses the cellular defects of Deinococcus radiodurans irradiated cells devoid of the PprA protein by limiting recombinational repair of DNA lesions.

The Deinococcus radiodurans bacterium is one of the most radioresistant organisms known. It can repair hundreds of radiation-induced DNA double-strand breaks without loss of viability and reconstitute an intact genome through RecA-dependent and RecA-independent DNA repair pathways. Among the Deinococcus specific proteins required for radioresistance, the PprA protein was shown to play a major role for accurate chromosome segregation and cell division after completion of DNA repair. Here, we analyzed the cellular role of the deinococcal RecN protein belonging to the SMC family and, surprisingly, observed that the absence of the RecN protein suppressed the sensitivity of cells devoid of the PprA protein to γ- and UV-irradiation and to treatment with MMC or DNA gyrase inhibitors. This suppression was not observed when ΔpprA cells were devoid of SMC or SbcC, two other proteins belonging to the SMC family. The absence of RecN also alleviated the DNA segregation defects displayed by ΔpprA cells recovering from γ-irradiation. When exposed to 5 kGy γ-irradiation, ΔpprA, ΔrecN and ΔpprA ΔrecN cells repaired their DNA with a delay of about one hour, as compared to the wild type cells. After irradiation, the absence of RecN reduced recombination between chromosomal and plasmid DNA, indicating that the deinococcal RecN protein is important for recombinational repair of DNA lesions. The transformation efficiency of genomic DNA was also reduced in the absence of the RecN protein. Here, we propose a model in which RecN, via its cohesin activity, might favor recombinational repair of DNA double strand breaks. This might increase, in irradiated cells, DNA constraints with PprA protein being required to resolve them via its ability to recruit DNA gyrase and to stimulate its decatenation activity.

Deinococcus radiodurans' SRA-HNH domain containing protein Shp (Dr1533) is involved in faithful genome inheritance maintenance following DNA damage.

BACKGROUND: Deinococcus radiodurans R1 (DR) survives conditions of extreme desiccation, irradiation and exposure to genotoxic chemicals, due to efficient DNA breaks repair, also through Mn2+ protection of DNA repair enzymes. METHODS: Possible annotated domains of the DR1533 locus protein (Shp) were searched by bioinformatic analysis. The gene was cloned and expressed as fusion protein. Band-shift assays of Shp or the SRA and HNH domains were performed on oligonucleotides, genomic DNA from E. coli and DR. shp knock-out mutant was generated by homologous recombination with a kanamycin resistance cassette. RESULTS: DR1533 contains an N-terminal SRA domain and a C-terminal HNH motif (SRA-HNH Protein, Shp). Through its SRA domain, Shp binds double-strand oligonucleotides containing 5mC and 5hmC, but also unmethylated and mismatched cytosines in presence of Mn2+. Shp also binds to Escherichia coli dcm+ genomic DNA, and to cytosine unmethylated DR and E. coli dcm- genomic DNAs, but only in presence of Mn2+. Under these binding conditions, Shp displays DNAse activity through its HNH domain. Shp KO enhanced >100 fold the number of spontaneous mutants, whilst the treatment with DNA double strand break inducing agents enhanced up to 3-log the number of survivors. CONCLUSIONS: The SRA-HNH containing protein Shp binds to and cuts 5mC DNA, and unmethylated DNA in a Mn2+ dependent manner, and might be involved in faithful genome inheritance maintenance following DNA damage. GENERAL SIGNIFICANCE: Our results provide evidence for a potential role of DR Shp protein for genome integrity maintenance, following DNA double strand breaks induced by genotoxic agents.

Comparative proteomics reveals key proteins recruited at the nucleoid of Deinococcus after irradiation-induced DNA damage.

The nucleoids of radiation-resistant Deinococcus species show a high degree of compaction maintained after ionizing irradiation. We identified proteins recruited after irradiation in nucleoids of Deinococcus radiodurans and Deinococcus deserti by means of comparative proteomics. Proteins in nucleoid-enriched fractions from unirradiated and irradiated Deinococcus were identified and semiquantified by shotgun proteomics. The ssDNA-binding protein SSB, DNA gyrase subunits GyrA and GyrB, DNA topoisomerase I, RecA recombinase, UvrA excinuclease, RecQ helicase, DdrA, DdrB, and DdrD proteins were found in significantly higher amounts in irradiated nucleoids of both Deinococcus species. We observed, by immunofluorescence microscopy, the subcellular localization of these proteins in D. radiodurans, showing for the first time the recruitment of the DdrD protein into the D. radiodurans nucleoid. We specifically followed the kinetics of recruitment of RecA, DdrA, and DdrD to the nucleoid after irradiation. Remarkably, RecA proteins formed irregular filament-like structures 1 h after irradiation, before being redistributed throughout the cells by 3 h post-irradiation. Comparable dynamics of DdrD localization were observed, suggesting a possible functional interaction between RecA and DdrD. Several proteins involved in nucleotide synthesis were also seen in higher quantities in the nucleoids of irradiated cells, indicative of the existence of a mechanism for orchestrating the presence of proteins involved in DNA metabolism in nucleoids in response to massive DNA damage. All MS data have been deposited in the ProteomeXchange with identifier PXD00196 (http://proteomecentral.proteomexchange.org/dataset/PXD000196).

The essential histone-like protein HU plays a major role in Deinococcus radiodurans nucleoid compaction.

The nucleoid of radioresistant bacteria, including D. radiodurans, adopts a highly condensed structure that remains unaltered after exposure to high doses of irradiation. This structure may contribute to radioresistance by preventing the dispersion of DNA fragments generated by irradiation. In this report, we focused our study on the role of HU protein, a nucleoid-associated protein referred to as a histone-like protein, in the nucleoid compaction of D. radiodurans. We demonstrate, using a new system allowing conditional gene expression, that HU is essential for viability in D. radiodurans. Using a tagged HU protein and immunofluorescence microscopy, we show that HU protein localizes all over the nucleoid and that when HU is expressed from a thermosensitive plasmid, its progressive depletion at the non-permissive temperature generates decondensation of DNA before fractionation of the nucleoid into several entities and subsequent cell lysis. We also tested the effect of the absence of Dps, a protein also involved in nucleoid structure. In contrast to the drastic effect of HU depletion, no change in nucleoid morphology and cell viability was observed in dps mutants compared with the wild-type, reinforcing the major role of HU in nucleoid organization and DNA compaction in D. radiodurans.

Mutational analysis of the helicase-like domain of Thermotoga maritima reverse gyrase.

Reverse gyrase is a unique type IA topoisomerase that is able to introduce positive supercoils into DNA in an ATP-dependent process. ATP is bound to the helicase-like domain of the enzyme that contains most of the conserved motifs found in helicases of the SF1 and SF2 superfamilies. In this paper, we have investigated the role of the conserved helicase motifs I, II, V, VI, and Q by generating mutants of the Thermotoga maritima reverse gyrase. We show that mutations in motifs I, II, V, and VI completely eliminate the supercoiling activity of reverse gyrase and that a mutation in the Q motif significantly reduces this activity. Further analysis revealed that for most mutants, the DNA binding and cleavage properties are not significantly changed compared with the wild type enzyme, whereas their ATPase activity is impaired. These results clearly show that the helicase motifs are tightly involved in the coupling of ATP hydrolysis to the topoisomerase activity. The zinc finger motif located at the N-terminal end of reverse gyrases was also mutated. Our results indicate that this motif plays an important role in DNA binding.

Type IA topoisomerases: a simple puzzle?

Type IA topoisomerases are enzymes that can modify DNA topology. They form a distinct family of proteins present in all domains of life, from bacteria to archaea and higher eukaryotes. They are composed of two domains: a core domain containing all the conserved motifs involved in the trans-esterification reactions, and a carboxyl-terminal domain that is highly variable in size and sequence. The latter appears to interact with other proteins, defining the physiological use of the topoisomerase activity. The evolutionary relevance of this topoisomerase-cofactor complex, also known as the "toposome", as well as its enzymatic consequences are discussed in this review.

Thermotoga maritima-Escherichia coli chimeric topoisomerases. Answers about involvement of the carboxyl-terminal domain in DNA topoisomerase I-mediated catalysis.

Bacterial topoisomerases I are generally composed of two domains as follows: a core domain, which contains all the conserved motifs involved in the trans-esterification reactions, and a carboxyl-terminal domain, highly variable in size and sequence. In the present work, we have addressed the question of the respective roles of the two domains in the different steps of the topoisomerization cycle. For this purpose, we prepared various recombinant topoisomerases from two model enzymes: topoisomerase I from the hyperthermophilic bacterium Thermotoga maritima and topoisomerase I from Escherichia coli. We compared the properties of the two core domains to that of the topoisomerases formed by combining the core domain of one enzyme to the carboxyl-terminal domain of the other. We found that, contrary to E. coli (Lima, C. D., Wang, J. C., and Mondragon, A. (1993) J. Mol. Biol. 232, 1213-1216), the core domain from T. maritima (TmTop65) is able to sustain by itself a complete topoisomerization cycle, although with low efficiency. Fusion of TmTop65 to the entire carboxyl-terminal domain from E. coli considerably increases binding efficiency, thermal stability, and DNA relaxation activity. Moreover, the chimera predominantly acquires the cleavage specificity of E. coli full-length topoisomerase. For the chimera obtained by fusion of the T. maritima carboxyl-terminal domain to the core EcTop67, very low DNA relaxation activity and binding are recovered, but formation of a covalent DNA adduct is impaired. Taken together, our results show that the presence and the nature of the carboxyl-terminal domain of bacterial topoisomerases I strongly determine their DNA binding efficiency and cleavage specificity but is not strictly required for strand passage.