Recombination and Replication in DNA Repair of Heavily Irradiated Deinococcus radiodurans

Engineering tripartite gene editing machinery for highly efficient non-viral targeted genome integration

Non-viral DNA donor templates are commonly used for targeted genomic integration via homologous recombination (HR), with efficiency improved by CRISPR/Cas9 technology. Circular single-stranded DNA (cssDNA) has been used as a genome engineering catalyst (GATALYST) for efficient and safe gene knock-in. Here, we introduce enGager, an enhanced GATALYST associated genome editor system that increases transgene integration efficiency by tethering cssDNA donors to nuclear-localized Cas9 fused with single-stranded DNA binding peptide motifs. This approach further improves targeted integration and expression of reporter genes at multiple genomic loci in various cell types, showing up to 6-fold higher efficiency compared to unfused Cas9, especially for large transgenes in primary cells. Notably, enGager enables efficient integration of a chimeric antigen receptor (CAR) transgene in 33% of primary human T cells, enhancing anti-tumor functionality. This 'tripartite editor with ssDNA optimized genome engineering (TESOGENASE) offers a safer, more efficient alternative to viral vectors for therapeutic gene modification.

Biochemical characterization and functional insights into DNA substrate-specific activities of a unique radiation-inducible DR1143 protein from Deinococcus radiodurans

The highly radiation-resistant bacterium Deinococcus radiodurans R1 employs various pathways to respond to radiation and other genotoxic stresses, involving the upregulation of several genes. Despite decades of study, the functions of many D. radiodurans genes remain unknown. Among them, the DR1143 gene has been reported transiently induced during the early phase of post-irradiation recovery. The DR1143 protein has been detected in the interactomes of key DNA repair proteins, including Ssb, RecA, and DdrB, suggesting its involvement in DNA repair processes. Our study revealed that DR1143 exhibits structural similarity to structural maintenance proteins (SMC) and exists as a hexamer in its soluble form. The protein showed a preference for binding longer DNA molecules (≥2 kb) and interacted differentially with various DNA forms. It bound single-stranded DNA (ssDNA) with high affinity, compacted, and protected it, indicating a potential role in safeguarding ssDNA during repair. DR1143 also created a single-strand nick in circular double-stranded DNA (dsDNA) while simply binding to linear dsDNA. Its nickase activity likely facilitates DNA end resection and relaxation, a critical step in repair processes. Localization studies using GFP-tagged DR1143 showed its accumulation at cell poles in both Escherichia coli and D. radiodurans, hinting at a functional role in specific subcellular compartments.

Surviving the storm: exploring the role of natural transformation in nutrition and DNA repair of stressed Deinococcus radiodurans

Deinococcus radiodurans, a natural transformation (NT)-enabled bacterium renowned for its exceptional radiation resistance, employs unique DNA repair and oxidative stress mitigation mechanisms as a strategic response to DNA damage. This study excavates into the intricate roles of NT machinery in the stressed D. radiodurans, focusing on the genes comEA, comEC, endA, pilT, and dprA, which are instrumental in the uptake and processing of extracellular DNA (eDNA). Our data reveal that NT not only supports the nutritional needs of D. radiodurans under stress but also has roles in DNA repair. The study findings establish that NT-specific proteins (ComEA, ComEC, and endonuclease A [EndA]) may contribute to support the nutritional requirements in unstressed and heavily DNA-damaged cells, while DprA contributes differently and in a context-dependent manner to navigating through the DNA damage storm. Thus, this dual functionality of NT-specific genes is proposed to be a contributing factor in the remarkable ability of D. radiodurans to survive and thrive in environments characterized by high levels of DNA-damaging agents.IMPORTANCEDeinococcus radiodurans is a bacterium known for its extraordinary radiation resistance. This study explores the roles of NT machinery in the radiation-resistant bacterium Deinococcus radiodurans, focusing on the genes comEA, comEC, endA, pilT, and dprA. These genes are crucial for the uptake and processing of eDNA and contribute to the bacterium nutritional needs and DNA repair under stress. The findings suggest that the NT-specific proteins ComEA, ComEC, and EndA may help meet the nutritional needs of unstressed and heavily DNA-damaged cells, whereas DprA plays a distinct role that varies, depending on the context in aiding cells to cope with DNA damage. The functionality of NT genes is proposed to enhance D. radiodurans survival in environments with high levels of DNA-damaging agents.

Effect of gamma irradiation on the proteogenome of cold-acclimated Kocuria rhizophila PT10

The effects of ionizing radiation (IR) on the protein dynamics of cold-stressed cells of a radioresistant actinobacterium, Kocuria rhizophila PT10, isolated from the rhizosphere of the desert plant Panicum turgidum were investigated using a shotgun methodology based on nanoflow liquid chromatography coupled to tandem mass spectrometry. Overall, 1487 proteins were certified, and their abundances were compared between the irradiated condition and control. IR of cold-acclimated PT10 triggered the over-abundance of proteins involved in (1) a strong transcriptional regulation, (2) amidation of peptidoglycan and preservation of cell envelope integrity, (3) detoxification of reactive electrophiles and regulation of the redox status of proteins, (4) base excision repair and prevention of mutagenesis and (5) the tricarboxylic acid (TCA) cycle and production of fatty acids. Also, one of the more significant findings to emerge from this study is the SOS response of stressed PT10. Moreover, a comparison of top hits radio-modulated proteins of cold-acclimated PT10 with proteomics data from gamma-irradiated Deinococcus deserti showed that stressed PT10 has a specific response characterised by a high over-abundance of NemA, GatD, and UdgB.

The Construction of an Extreme Radiation-Resistant Perchlorate-Reducing Bacterium Using Deinococcus deserti Promoters

Perchlorate is one of the major inorganic pollutants in the natural environment and the living environment, which is toxic to organisms and difficult to degrade due to its special structure. As previously reported, the Phoenix Mars lander detected approximately 0.6% perchlorate in the Martian soil, indicating challenges for Earth-based life to survive there. Currently, biological approaches using dissimilatory perchlorate-reducing bacteria (DPRB) are the most promising methods for perchlorate degradation. However, the majority of DPRB exhibit limited radiation resistance, rendering them unsuitable for survival on Mars. In this study, we obtained the transcriptome data of Deinococcus deserti, and predicted and identified multiple constitutive expression promoters of D. deserti with varying activities. The top-five most active promoters were separately fused to specific genes involved in the degradation of perchlorate from DPRB Dechloromonas agitata CKB, and transformed into Deinococcus radiodurans R1, forming a novel dissimilatory perchlorate-reducing bacterium, R1-CKB. It exhibited both efficient perchlorate degradation capability and strong radiation resistance, potentially offering a valuable tool for the further enhancement of the Martian atmosphere in the future.

DdrC, a unique DNA repair factor from D. radiodurans, senses and stabilizes DNA breaks through a novel lesion-recognition mechanism

The bacterium Deinococcus radiodurans is known to survive high doses of DNA damaging agents. This resistance is the result of robust antioxidant systems which protect efficient DNA repair mechanisms that are unique to Deinococcus species. The protein DdrC has been identified as an important component of this repair machinery. DdrC is known to bind to DNA in vitro and has been shown to circularize and compact DNA fragments. The mechanism and biological relevance of this activity is poorly understood. Here, we show that the DdrC homodimer is a lesion-sensing protein that binds to two single-strand (ss) or double-strand (ds) breaks. The immobilization of DNA breaks in pairs consequently leads to the circularization of linear DNA and the compaction of nicked DNA. The degree of compaction is directly proportional with the number of available nicks. Previously, the structure of the DdrC homodimer was solved in an unusual asymmetric conformation. Here, we solve the structure of DdrC under different crystallographic environments and confirm that the asymmetry is an endogenous feature of DdrC. We propose a dynamic structural mechanism where the asymmetry is necessary to trap a pair of lesions. We support this model with mutant disruption and computational modeling experiments.

Flying microbes-survival in the extreme conditions of the stratosphere during a stratospheric balloon flight experiment

Earth's stratosphere is characterized by hypobaric conditions, low temperatures, and high intensities of ultraviolet (UV) and cosmic radiation as well as low water and nutrient availability. While it is not considered a permanent habitat for microorganisms, they can be transported to the stratosphere by storms, volcanic action, or human activity. The impact of those extreme conditions on microorganisms and their survival were tested by sending a sample gondola to the stratosphere. The sample gondola was built to allow exposure of Bacillus subtilis endospores at different angles to the sun. It moreover had holders for three environmental samples to test the effect of stratospheric conditions on complex microbial communities. The gondola attached to a stratospheric balloon was launched near Kiruna, Sweden, ascended to ~25 km, and drifted eastward for ~200 km. Samples were exposed to pressures as low as 2 kPa and temperatures as low as -50°C as well as high UV radiation. Survival rates of B. subtilis were determined by comparing the numbers of colony-forming units (CFUs) for the different exposure angles. Survival was negatively correlated with exposure angle, indicating the significant impact of UV radiation. The effect of stratospheric conditions on environmental samples was assessed by comparing most probable numbers, microbial community composition, and substrate-use profiles to controls that had stayed on the ground. Cultivation was possible from all samples with survival rates of at least 1%, and differences in community composition were observed. Survival of environmental microorganisms might have been supported by the sample matrix, which provided protection from radiation and desiccation.

IMPORTANCE

Earth's stratosphere is a hostile environment that has challenged microbial survival. We set out to test the effect of stratosphere exposure on survival of single species (Bacillus subtilis) and complex microbial communities from soils and sediment. B. subtilis survival was strongly impacted by sun exposure, i.e., ultraviolet (UV) radiation, with only 1% survival at full sun exposure. Complex microbial communities had high survival rates, and the soil or sediment matrix may have provided protection against radiation and desiccation, supporting the survival of environmental microorganisms.

Precise CRISPR/Cpf1 genome editing system in the Deinococcus radiodurans with superior DNA repair mechanisms

Deinococcus radiodurans, with its high homologous recombination (HR) efficiency of double-stranded DNA breaks (DSBs), is a model organism for studying genome stability maintenance and an attractive microbe for industrial applications. Here, we developed an efficient CRISPR/Cpf1 genome editing system in D. radiodurans by evaluating and optimizing double-plasmid strategies and four Cas effector proteins from various organisms, which can precisely introduce different types of template-dependent mutagenesis without off-target toxicity. Furthermore, the role of DNA repair genes in determining editing efficiency in D. radiodurans was evaluated by introducing the CRISPR/Cpf1 system into 13 mutant strains lacking various DNA damage response and repair factors. In addition to the crucial role of RecA-dependent HR required for CRISPR/Cpf1 editing, D. radiodurans showed higher editing efficiency when lacking DdrB, the single-stranded DNA annealing (SSA) protein involved in the RecA-independent DSB repair pathway. This suggests a possible competition between HR and SSA pathways in the CRISPR editing of D. radiodurans. Moreover, off-target effects were observed during the genome editing of the pprI knockout strain, a master DNA damage response gene in Deinococcus species, which suggested that precise regulation of DNA damage response is critical for a high-fidelity genome editing system.

Natural transformation-specific DprA coordinate DNA double-strand break repair pathways in heavily irradiated D. radiodurans

Deinococcus radiodurans exhibits remarkable survival under extreme conditions, including ionizing radiation, desiccation, and various DNA-damaging agents. It employs unique repair mechanisms, such as single-strand annealing (SSA) and extended synthesis-dependent strand annealing (ESDSA), to efficiently restore damaged genome. In this study, we investigate the role of the natural transformation-specific protein DprA in DNA repair pathways following acute gamma radiation exposure. Our findings demonstrate that the absence of DprA leads to rapid repair of gamma radiation-induced DNA double-strand breaks primarily occur through SSA repair pathway. Additionally, our findings suggest that the DprA protein may hinder both the SSA and ESDSA repair pathways, albeit in distinct manners. Overall, our results highlight the crucial function of DprA in the selection between SSA and ESDSA pathways for DNA repair in heavily irradiated D. radiodurans.IMPORTANCEDeinococcus radiodurans exhibits an extraordinary ability to endure and thrive in extreme environments, including exposure to radiation, desiccation, and damaging chemicals, as well as intense UV radiation. The bacterium has evolved highly efficient repair mechanisms capable of rapidly mending hundreds of DNA fragments in its genome. Our research indicates that natural transformation (NT)-specific dprA genes play a pivotal role in regulating DNA repair in response to radiation. Remarkably, we found that DprA is instrumental in selecting DNA double-strand break repair pathways, a novel function that has not been reported before. This unique regulatory mechanism highlights the indispensable role of DprA beyond its native function in NT and underscores its ubiquitous presence across various bacterial species, regardless of their NT proficiency. These findings shed new light on the resilience and adaptability of Deinococcus radiodurans, opening avenues for further exploration into its exceptional survival strategies.

Differential cellular localization of DNA gyrase and topoisomerase IB in response to DNA damage in Deinococcus radiodurans

Topoisomerases are crucial enzymes in genome maintenance that modulate the topological changes during DNA metabolism. Deinococcus radiodurans, a Gram-positive bacterium is characterized by its resistance to many abiotic stresses including gamma radiation. Its multipartite genome encodes both type I and type II topoisomerases. Time-lapse studies using fluorescently tagged topoisomerase IB (drTopoIB-RFP) and DNA gyrase (GyrA-RFP) were performed to check the dynamics and localization with respect to DNA repair and cell division under normal and post-irradiation growth conditions. Results suggested that TopoIB and DNA gyrase are mostly found on nucleoid, highly dynamic, and show growth phase-dependent subcellular localization. The drTopoIB-RFP was also present at peripheral and septum regions but does not co-localize with the cell division protein, drFtsZ. On the other hand, DNA gyrase co-localizes with PprA a pleiotropic protein involved in radioresistance, on the nucleoid during the post-irradiation recovery (PIR). The topoIB mutant was found to be sensitive to hydroxyurea treatment, and showed more accumulation of single-stranded DNA during the PIR, compared to the wild type suggesting its role in DNA replication stress. Together, these results suggest differential localization of drTopoIB-RFP and GyrA-RFP in D. radiodurans and their interaction with PprA protein, emphasizing the functional significance and role in radioresistance.

Desiccation induces varied responses within a soil bacterial genus

Desiccation impacts a suite of physiological processes in microbes by elevating levels of damaging reactive oxygen species and inducing DNA strand breaks. In response to desiccation-induced stress, microbes have evolved specialized mechanisms to help them survive. Here, we performed a 128-day lab desiccation experiment on nine strains from three clades of an abundant soil bacterium, Curtobacterium. We sequenced RNA from each strain at three time points to investigate their response. Curtobacterium was highly resistant to desiccation, outlasting both Escherichia coli and a famously DNA damage-resistant bacterium, Deinococcus radiodurans. However, within the genus, there were also 10-fold differences in survival rates among strains. Transcriptomic profiling revealed responses shared within the genus including up-regulation of genes involved in DNA damage repair, osmolyte production, and efflux pumps, but also up-regulation of pathways and genes unique to the three clades. For example, trehalose synthesis gene otsB, the chaperone groEL, and the oxygen scavenger katA were all found in either one or two clades but not the third. Here, we provide evidence of considerable variation in closely related strains, and further elucidation of the phylogenetic conservation of desiccation tolerance remains an important goal for microbial ecologists.

Single-molecule tracking of PprI in D. radiodurans without interference of autoblinking

Autoblinking is a widespread phenomenon and exhibits high level of intensity in some bacteria. In Deinococcus radiodurans (D. radiodurans), strong autoblinking was found to be indistinguishable from PAmCherry and greatly prevented single-molecule tracking of proteins of interest. Here we employed the bright photoswitchable fluorescent protein mMaple3 to label PprI, one essential DNA repair factor, and characterized systematically the fluorescence intensity and bleaching kinetics of both autoblinking and PprI-mMaple3 molecules within cells grown under three different conditions. Under minimal media, we can largely separate autoblinking from mMaple3 molecules and perform reliably single-molecule tracking of PprI in D. radiodurans, by means of applying signal-to-noise ratio and constraining the minimal length for linking the trajectories. We observed three states of PprI molecules, which bear different subcellular localizations and distinct functionalities. Our strategy provides a useful means to study the dynamics and distributions of proteins of interest in bacterial cells with high level of autoblinking.

The scientific revolution that unraveled the astonishing DNA repair capacity of the Deinococcaceae: 40 years on

The family Deinococcaceae exhibits exceptional radiation resistance and possesses all the necessary traits for surviving in radiation-exposed environments. Their survival strategy involves the coupling of metabolic and DNA repair functions, resulting in an extraordinarily efficient homologous repair of DNA double-strand breaks (DSBs) caused by radiation or desiccation. The keys to their survival lie in the hyperaccumulation of manganous (Mn2+)-metabolite antioxidants that protect their DNA repair proteins under extreme oxidative stress and the persistent structural linkage by Holliday junctions of their multiple genome copies per cell that facilitates DSB repair. This coupling of metabolic and DNA repair functions has made polyploid Deinococcus bacteria a useful tool in environmental biotechnology, radiobiology, aging, and planetary protection. The review highlights the groundbreaking contributions of the late Robert G.E. Murray to the field of Deinococcus research and the emergent paradigm-shifting discoveries that revolutionized our understanding of radiation survivability and oxidative stress defense, demonstrating that the proteome, rather than the genome, is the primary target responsible for survivability. These discoveries have led to the commercial development of irradiated vaccines using Deinococcus Mn-peptide antioxidants and have significant implications for various fields.

New Insights into Radio-Resistance Mechanism Revealed by (Phospho)Proteome Analysis of Deinococcus Radiodurans after Heavy Ion Irradiation

Deinococcus radiodurans (D. radiodurans) can tolerate various extreme environments including radiation. Protein phosphorylation plays an important role in radiation resistance mechanisms; however, there is currently a lack of systematic research on this topic in D. radiodurans. Based on label-free (phospho)proteomics, we explored the dynamic changes of D. radiodurans under various doses of heavy ion irradiation and at different time points. In total, 2359 proteins and 1110 high-confidence phosphosites were identified, of which 66% and 23% showed significant changes, respectively, with the majority being upregulated. The upregulated proteins at different states (different doses or time points) were distinct, indicating that the radio-resistance mechanism is dose- and stage-dependent. The protein phosphorylation level has a much higher upregulation than protein abundance, suggesting phosphorylation is more sensitive to irradiation. There were four distinct dynamic changing patterns of phosphorylation, most of which were inconsistent with protein levels. Further analysis revealed that pathways related to RNA metabolism and antioxidation were activated after irradiation, indicating their importance in radiation response. We also screened some key hub phosphoproteins and radiation-responsive kinases for further study. Overall, this study provides a landscape of the radiation-induced dynamic change of protein expression and phosphorylation, which provides a basis for subsequent functional and applied studies.

One Advantage of Being Polyploid: Prokaryotes of Various Phylogenetic Groups Can Grow in the Absence of an Environmental Phosphate Source at the Expense of Their High Genome Copy Numbers

Genomic DNA has high phosphate content; therefore, monoploid prokaryotes need an external phosphate source or an internal phosphate storage polymer for replication and cell division. For two polyploid prokaryotic species, the halophilic archaeon Haloferax volcanii and the cyanobacterium Synechocystis PCC 6803, it has been reported that they can grow in the absence of an external phosphate source by reducing the genome copy number per cell. To unravel whether this feature might be widespread in and typical for polyploid prokaryotes, three additional polyploid prokaryotic species were analyzed in the present study, i.e., the alphaproteobacterium Zymomonas mobilis, the gammaproteobacterium Azotobacter vinelandii, and the haloarchaeon Halobacterium salinarum. Polyploid cultures were incubated in the presence and in the absence of external phosphate, growth was recorded, and genome copy numbers per cell were quantified. Limited growth in the absence of phosphate was observed for all three species. Phosphate was added to phosphate-starved cultures to verify that the cells were still viable and growth-competent. Remarkably, stationary-phase cells grown in the absence or presence of phosphate did not become monoploid but stayed oligoploid with about five genome copies per cell. As a negative control, it was shown that monoploid Escherichia coli cultures did not exhibit any growth in the absence of phosphate. Taken together, all five polyploid prokaryotic species that have been characterized until now can grow in the absence of environmental phosphate by reducing their genome copy numbers, indicating that cell proliferation outperforms other evolutionary advantages of polyploidy.

Genomic Mysteries of Giant Bacteria: Insights and Implications

Bacteria and Archaea are traditionally regarded as organisms with a simple morphology constrained to a size of 2-3 µm. Nevertheless, the history of microbial research is rich in the description of giant bacteria exceeding tens and even hundreds of micrometers in length or diameter already from its early days, for example, Beggiatoa spp., to the present, for example, Candidatus Thiomargarita magnifica. While some of these giants are still being studied, some were lost to science, with merely drawings and photomicrographs as evidence for their existence. The physiology and biogeochemical role of giant bacteria have been studied, with a large focus on those involved in the sulfur cycle. With the onset of the genomic era, no special emphasis has been given to this group, in an attempt to gain a novel, evolutionary, and molecular understanding of the phenomenon of bacterial gigantism. The few existing genomic studies reveal a mysterious world of hyperpolyploid bacteria with hundreds to hundreds of thousands of chromosomes that are, in some cases, identical and in others, extremely different. These studies on giant bacteria reveal novel organelles, cellular compartmentalization, and novel mechanisms to combat the accumulation of deleterious mutations in polyploid bacteria. In this perspective paper, we provide a brief overview of what is known about the genomics of giant bacteria and build on that to highlight a few burning questions that await to be addressed.

Deinococcus lineage and Rad52 family-related protein DR0041 is involved in DNA protection and compaction

DR0041 ORF encodes an uncharacterized Deinococcus lineage protein. We earlier reported presence of DR0041 protein in DNA repair complexes of Ssb and RecA in Deinococcus radiodurans. Here, we systematically examined the role of DR0041 in DNA metabolism using various experimental methodologies including electrophoretic mobility assays, nuclease assays, strand exchange assays and transmission electron microscopy. Interaction between DR0041 and the C-terminal acidic tail of Ssb was assessed through co-expression and in vivo cross-linking studies. A knockout mutant was constructed to understand importance of DR0041 ORF for various physiological processes. Results highlight binding of DR0041 protein to single-stranded and double-stranded DNA, interaction with Ssb-coated single-stranded DNA without interference with RecA-mediated strand exchange, protection of DNA from exonucleases, and compaction of high molecular weight DNA molecules into tightly condensed forms. Bridging and compaction of sheared DNA by DR0041 protein might have implications in the preservation of damaged DNA templates to maintain genome integrity upon exposure to gamma irradiation. Our results suggest that DR0041 protein is dispensable for growth under standard growth conditions and following gamma irradiation but contributes to protection of DNA during transformation. We discuss the role of DR0041 protein from the perspective of protection of broken DNA templates and functional redundancy.

TESOGENASE, An Engineered Nuclease Editor for Enhanced Targeted Genome Integration

Non-viral DNA donor template has been widely used for targeted genomic integration by homologous recombination (HR). This process has become more efficient with RNA guided endonuclease editor system such as CRISPR/Cas9. Circular single stranded DNA (cssDNA) has been harnessed previously as a g enome engineering c atalyst (GATALYST) for efficient and safe targeted gene knock-in. However, the engineering efficiency is bottlenecked by the nucleoplasm trafficking and genomic tethering of cssDNA donor, especially for extra-large transgene integration. Here we developed enGager, en hanced G ATALYST a ssociated g enome e ditor system by fusion of nucleus localization signal (NLS) peptide tagged Cas9 with various single stranded DNA binding protein modules through a GFP reporter Knock-in screening. The enGager system assembles an integrative genome integration machinery by forming tripartite complex for engineered nuclease editors, sgRNA and ssDNA donors, thereby facilitate the nucleus trafficking of DNA donors and increase their active local concentration at the targeted genomic site. When applied for genome integration with cssDNA donor templates to diverse genomic loci in various cell types, these enGagers outperform unfused editors. The enhancement of integration efficiency ranges from 1.5- to more than 6-fold, with the effect being more prominent for > 4Kb transgene knock-in in primary cells. We further demonstrated that enGager mediated enhancement for genome integration is ssDNA, but less dsDNA dependent. Using one of the mini-enGagers, we demonstrated large chimeric antigen receptor (CAR) transgene integration in primary T cells with exceptional efficiency and anti-tumor function. These tripartite e ditors with s sDNA o ptimized g enome en gineering system (TESOGENASE TM ) add a set of novel endonuclease editors into the gene-editing toolbox for potential cell and gene therapeutic development based on ssDNA mediated non-viral genome engineering.

Highlight

A reporter Knock-in screening establishes enGager system to identify TESOGENASE editor to improving ssDNA mediated genome integrationMini-TESOGENASEs developed by fusing Cas9 nuclease with novel ssDNA binding motifsmRNA mini-TESOGENASEs enhance targeted genome integration via various non-viral delivery approachesEfficient functional CAR-T cell engineering by mini-TESOGENASE.

Antioxidant defense of Deinococcus radiodurans: how does it contribute to extreme radiation resistance?

PURPOSE

Deinococcus radiodurans is an extremely radioresistant bacterium characterized by D10 of 10 kGy, and able to grow luxuriantly under chronic ionizing radiation of 60 Gy/h. The aim of this article is to review the antioxidant system of D. radiodurans and its possible role in the unusual resistance of this bacterium to ionizing radiation.

CONCLUSIONS

The unusual radiation resistance of D. radiodurans has apparently evolved as a side effect of the adaptation of this extremophile to other damaging environmental factors, especially desiccation. The antioxidant proteins and low-molecular antioxidants (especially low-molecular weight Mn2+ complexes and carotenoids, in particular, deinoxanthin), as well as protein and non-protein regulators, are important for the antioxidant defense of this species. Antioxidant protection of proteins from radiation inactivation enables the repair of DNA damage caused by ionizing radiation.

Generation and Repair of Postreplication Gaps in Escherichia coli

When replication forks encounter template lesions, one result is lesion skipping, where the stalled DNA polymerase transiently stalls, disengages, and then reinitiates downstream to leave the lesion behind in a postreplication gap. Despite considerable attention in the 6 decades since postreplication gaps were discovered, the mechanisms by which postreplication gaps are generated and repaired remain highly enigmatic. This review focuses on postreplication gap generation and repair in the bacterium Escherichia coli. New information to address the frequency and mechanism of gap generation and new mechanisms for their resolution are described. There are a few instances where the formation of postreplication gaps appears to be programmed into particular genomic locations, where they are triggered by novel genomic elements.

SLICER: A Seamless Gene Deletion Method for Deinococcus radiodurans

Deinococcus radiodurans' high resistance to various stressors combined with its ability to utilize sustainable carbon sources makes it an attractive bacterial chassis for synthetic biology and industrial bioproduction. However, to fully harness the capabilities of this microbe, further strain engineering and tool development are required. Methods for creating seamless genome modifications are an essential part of the microbial genetic toolkit to enable strain engineering. Here, we report the development of the SLICER method, which can be used to create seamless gene deletions in D. radiodurans. This process involves (a) integration of a seamless deletion cassette replacing a target gene, (b) introduction of the pSLICER plasmid to mediate cassette excision by I-SceI endonuclease cleavage and homologous recombination, and (c) curing of the helper plasmid. We demonstrate the utility of SLICER for creating multiple gene deletions in D. radiodurans by sequentially targeting 5 putative restriction-modification system genes, recycling the same selective and screening markers for each subsequent deletion. While we observed no significant increase in transformation efficiency for most of the knockout strains, we demonstrated SLICER as a promising method to create a fully restriction-minus strain to expand the synthetic biology applications of D. radiodurans, including its potential as an in vivo DNA assembly platform.

Bacillus subtilis RadA/Sms-Mediated Nascent Lagging-Strand Unwinding at Stalled or Reversed Forks Is a Two-Step Process: RadA/Sms Assists RecA Nucleation, and RecA Loads RadA/Sms

Replication fork rescue requires Bacillus subtilis RecA, its negative (SsbA) and positive (RecO) mediators, and fork-processing (RadA/Sms). To understand how they work to promote fork remodeling, reconstituted branched replication intermediates were used. We show that RadA/Sms (or its variant, RadA/Sms C13A) binds to the 5'-tail of a reversed fork with longer nascent lagging-strand and unwinds it in the 5'→3' direction, but RecA and its mediators limit unwinding. RadA/Sms cannot unwind a reversed fork with a longer nascent leading-strand, or a gapped stalled fork, but RecA interacts with and activates unwinding. Here, the molecular mechanism by which RadA/Sms, in concert with RecA, in a two-step reaction, unwinds the nascent lagging-strand of reversed or stalled forks is unveiled. First, RadA/Sms, as a mediator, contributes to SsbA displacement from the forks and nucleates RecA onto single-stranded DNA. Then, RecA, as a loader, interacts with and recruits RadA/Sms onto the nascent lagging strand of these DNA substrates to unwind them. Within this process, RecA limits RadA/Sms self-assembly to control fork processing, and RadA/Sms prevents RecA from provoking unnecessary recombination.

Comprehensive Temporal Protein Dynamics during Postirradiation Recovery in Deinococcus radiodurans

Deinococcus radiodurans (D. radiodurans) is an extremophile that can tolerate ionizing radiation, ultraviolet radiation, and oxidation. How D. radiodurans responds to and survives high levels of ionizing radiation is still not clear. In this study, we performed label-free proteomics to explore the proteome dynamics during postirradiation recovery (PIR). Surprisingly, proteins involved in translation were repressed during the initial hours of PIR. D. radiodurans also showed enhanced DNA repair and antioxidative response after 6 kGy of gamma irradiation. Moreover, proteins involved in sulfur metabolism and phenylalanine metabolism were enriched at 1 h and 12 h, respectively, indicating different energy and material needs during PIR. Furthermore, based on these findings, we proposed a novel model to elucidate the possible molecular mechanisms of robust radioresistance in D. radiodurans, which may serve as a reference for future radiation repair.

Culturomics of Bacteria from Radon-Saturated Water of the World's Oldest Radium Mine

Balneotherapeutic water springs, such as those with thermal, saline, sulfur, or any other characteristics, have recently been the subject of phylogenetic studies with a closer focus on the description and/or isolation of phylogenetically novel or biotechnologically interesting microorganisms. Generally, however, most such microorganisms are rarely obtained in pure culture or are even, for now, unculturable under laboratory conditions. In this culture-dependent study of radioactive water springs of Jáchymov (Joachimstahl), Czech Republic, we investigated a combination of classical cultivation approaches with those imitating sampling source conditions. Using these environmentally relevant cultivation approaches, over 1,000 pure cultures were successfully isolated from 4 radioactive springs. Subsequent dereplication yielded 121 unique taxonomic units spanning 44 genera and 9 taxonomic classes, ~10% of which were identified as hitherto undescribed taxa. Genomes of the latter were sequenced and analyzed, with a special focus on endogenous defense systems to withstand oxidative stress and aid in radiotolerance. Due to their origin from radioactive waters, we determined the resistance of the isolates to oxidative stress. Most of the isolates were more resistant to menadione than the model strain Deinococcus radiodurans DSM 20539T. Moreover, isolates of the Deinococcacecae, Micrococcaceae, Bacillaceae, Moraxellaceae, and Pseudomonadaceae families even exhibited higher resistance in the presence of hydrogen peroxide. In summary, our culturomic analysis shows that subsurface water springs contain diverse bacterial populations, including as-yet-undescribed taxa and strains with promising biotechnological potential. Furthermore, this study suggests that environmentally relevant cultivation techniques increase the efficiency of cultivation, thus enhancing the chance of isolating hitherto uncultured microorganisms. IMPORTANCE The mine Svornost in Jáchymov (Joachimstahl), Czech Republic is a former silver-uranium mine and the world's first and for a long time only radium mine, nowadays the deepest mine devoted to the extraction of water which is saturated with radon and has therapeutic benefits given its chemical properties. This healing water, which is approximately 13 thousand years old, is used under medical supervision for the treatment of patients with neurological and rheumatic disorders. Our culturomic approach using low concentrations of growth substrates or the environmental matrix itself (i.e., water filtrate) in culturing media combined with prolonged cultivation time resulted in the isolation of a broad spectrum of microorganisms from 4 radioactive springs of Jáchymov which are phylogenetically novel and/or bear various adaptive or coping mechanisms to thrive under selective pressure and can thus provide a wide spectrum of capabilities potentially exploitable in diverse scientific, biotechnological, or medical disciplines.

Lysine Acetylome Profiling Reveals Diverse Functions of Acetylation in Deinococcus radiodurans

Lysine acetylation is a highly conserved posttranslational modification that plays essential roles in multiple biological functions in a variety of organisms. Deinococcus radiodurans (D. radiodurans) is famous for its extreme resistance to radiation. However, few studies have focused on the lysine acetylation in D. radiodurans. In the present study, antibody enrichment technology and high-resolution liquid chromatography mass spectrometry are used to perform a global analysis of lysine acetylation of D. radiodurans. We create the largest acetylome data set in D. radiodurans to date, totally identifying 4,364 lysine acetylation sites on 1,410 acetylated proteins. Strikingly, of the 3,085 proteins annotated by the uniport database, 45.7% of proteins are acetylated in D. radiodurans. In particular, the glutamate (G) preferentially appears at the -1 and +1 positions of acetylated lysine residues by motif analysis. The acetylated proteins are involved in metabolic pathways, propanoate metabolism, carbon metabolism, fatty acid metabolism, and the tricarboxylic acid cycle. Protein-protein interaction networks demonstrate that four clusters are involved in DNA damage repair, including homologous recombination, mismatch repair, nucleotide excision repair, and base excision repair, which suggests that acetylation plays an indispensable role in the extraordinary capacity to survive high levels of ionizing radiation. Taken together, we report the most comprehensive lysine acetylation in D. radiodurans for the first time, which is of great significance to reveal its robust resistance to radiation. IMPORTANCE D. radiodurans is distinguished by the most radioresistant organism identified to date. Lysine acetylation is a highly conserved posttranslational modification that plays an essential role in the regulation of many cellular processes and may contribute to its extraordinary radioresistance. We integrate acetyl-lysine enrichment strategy, high-resolution mass spectrometry, and bioinformatics to profile the lysine acetylated proteins for the first time. It is striking that almost half of the total annotated proteins are identified as acetylated forms, which is the largest acetylome data set reported in D. radiodurans to date. The acetylated proteins are involved in metabolic pathways, propanoate metabolism, carbon metabolism, fatty acid metabolism, and the tricarboxylic acid cycle. The results of this study reinforce the notion that acetylation plays critical regulatory roles in diverse aspects of the cellular process, especially in DNA damage repair and metabolism. It provides insight into the roles of lysine acetylation in the robust resistance to radiation.

Genomic attributes of thermophilic and hyperthermophilic bacteria and archaea

Thermophiles and hyperthermophiles are immensely useful in understanding the evolution of life, besides their utility in environmental and industrial biotechnology. Advancements in sequencing technologies have revolutionized the field of microbial genomics. The massive generation of data enhances the sequencing coverage multi-fold and allows to analyse the entire genomic features of microbes efficiently and accurately. The mandate of a pure isolate can also be bypassed where whole metagenome-assembled genomes and single cell-based sequencing have fulfilled the majority of the criteria to decode various attributes of microbial genomes. A boom has, therefore, been seen in analysing the extremophilic bacteria and archaea using sequence-based approaches. Due to extensive sequence analysis, it becomes easier to understand the gene flow and their evolution among the members of bacteria and archaea. For instance, sequencing unveiled that Thermotoga maritima shares around 24% of genes of archaeal origin. Comparative and functional genomics provide an analytical view to understanding the microbial diversity of thermophilic bacteria and archaea, their interactions with other microbes, their adaptations, gene flow, and evolution over time. In this review, the genomic features of thermophilic bacteria and archaea are dealt with comprehensively.

RADA-dependent branch migration has a predominant role in plant mitochondria and its defect leads to mtDNA instability and cell cycle arrest

Mitochondria of flowering plants have large genomes whose structure and segregation are modulated by recombination activities. The post-synaptic late steps of mitochondrial DNA (mtDNA) recombination are still poorly characterized. Here we show that RADA, a plant ortholog of bacterial RadA/Sms, is an organellar protein that drives the major branch-migration pathway of plant mitochondria. While RadA/Sms is dispensable in bacteria, RADA-deficient Arabidopsis plants are severely impacted in their development and fertility, correlating with increased mtDNA recombination across intermediate-size repeats and accumulation of recombination-generated mitochondrial subgenomes. The radA mutation is epistatic to recG1 that affects the additional branch migration activity. In contrast, the double mutation radA recA3 is lethal, underlining the importance of an alternative RECA3-dependent pathway. The physical interaction of RADA with RECA2 but not with RECA3 further indicated that RADA is required for the processing of recombination intermediates in the RECA2-depedent recombination pathway of plant mitochondria. Although RADA is dually targeted to mitochondria and chloroplasts we found little to no effects of the radA mutation on the stability of the plastidial genome. Finally, we found that the deficient maintenance of the mtDNA in radA apparently triggers a retrograde signal that activates nuclear genes repressing cell cycle progression.

In flowering plants, the mitochondrial genome is very large and dynamic, and its stability influences plant fitness and development. Rearrangements by recombination drive its very rapid evolution and can lead to valuable agronomic traits such as cytoplasmic sterility, used by breeders for the production of hybrid seeds. Here we describe RADA, a DNA helicase essential for the stability of the mitochondrial DNA in Arabidopsis. We demonstrate that RADA has branch migrating activity, accelerating the processing of recombination intermediates. radA mutants are severely affected in development and fertility. They display mitochondrial genome instability that results in uncoordinated replication of subgenomes created by recombination. Furthermore, we found that an important component of the growth defects of radA mutants is apparently a cellular response triggered by the sensing of damages to the mitochondrial genome, resulting in the activation of genes that suppress the progression of the cell cycle. Our results underline the importance of better understanding the plant mitochondrial recombination pathways and their cross-talk with nuclear gene expression.

The Effects of Freeze-Thaw and UVC Radiation on Microbial Survivability in a Selected Mars-like Environment

The purpose of this study was to determine survivability of Escherichia coli, Deinococcus radiodurans and Paraburkholderia fungorum under Mars-simulated conditions for freeze-thawing (−80 °C to +30 °C) and UV exposure alone and in combination. E. coli ATCC 25922, D. radiodurans and P. fungorum remained viable following 20 successive freeze-thaw cycles, exhibiting viabilities of 2.3%, 96% and 72.6%, respectively. E. coli ATCC 9079 was non-recoverable by cycle 9. When exposed to UV irradiation, cells withstood doses of 870 J/m² (E. coli ATCC 25922), 200 J/m² (E. coli ATCC 9079), 50,760 J/m² (D. radiodurans) and 44,415 J/m² (P. fungorum). Data suggests P. fungorum is highly UV-resistant. Combined freeze-thawing with UV irradiation showed freezing increased UV resistance in E. coli ATCC 25922, E. coli DSM 9079 and D. radiodurans by 6-fold, 30-fold and 1.2-fold, respectively. Conversely, freezing caused P. fungorum to exhibit a 1.75-fold increase in UV susceptibility. Strain-dependent experimentation demonstrated that freezing increases UV resistance and prolongs survival. These findings suggest that exposure to short wavelength UV rays (254 nm) and temperature cycles resembling the daily fluctuating conditions on Mars do not significantly affect survival of D. radiodurans, P. fungorum and E. coli ATCC 25922 following 20 days of exposure.

Bacillus subtilis RecA, DisA, and RadA/Sms Interplay Prevents Replication Stress by Regulating Fork Remodeling

Reviving Bacillus subtilis spores require the recombinase RecA, the DNA damage checkpoint sensor DisA, and the DNA helicase RadA/Sms to prevent a DNA replication stress. When a replication fork stalls at a template lesion, RecA filaments onto the lesion-containing gap and the fork is remodeled (fork reversal). RecA bound to single-strand DNA (ssDNA) interacts with and recruits DisA and RadA/Sms on the branched DNA intermediates (stalled or reversed forks), but DisA and RadA/Sms limit RecA activities and DisA suppresses its c-di-AMP synthesis. We show that RecA, acting as an accessory protein, activates RadA/Sms to unwind the nascent lagging-strand of the branched intermediates rather than to branch migrate them. DisA limits the ssDNA-dependent ATPase activity of RadA/Sms C13A, and inhibits the helicase activity of RadA/Sms by a protein-protein interaction. Finally, RadA/Sms inhibits DisA-mediated c-di-AMP synthesis and indirectly inhibits cell proliferation, but RecA counters this negative effect. We propose that the interactions among DisA, RecA and RadA/Sms, which are mutually exclusive, contribute to generate the substrate for replication restart, regulate the c-di-AMP pool and limit fork restoration in order to maintain cell survival.

Characterization of the Radiation Desiccation Response Regulon of the Radioresistant Bacterium Deinococcus radiodurans by Integrative Genomic Analyses

Numerous genes are overexpressed in the radioresistant bacterium Deinococcus radiodurans after exposure to radiation or prolonged desiccation. It was shown that the DdrO and IrrE proteins play a major role in regulating the expression of approximately twenty genes. The transcriptional repressor DdrO blocks the expression of these genes under normal growth conditions. After exposure to genotoxic agents, the IrrE metalloprotease cleaves DdrO and relieves gene repression. At present, many questions remain, such as the number of genes regulated by DdrO. Here, we present the first ChIP-seq analysis performed at the genome level in Deinococcus species coupled with RNA-seq, which was achieved in the presence or not of DdrO. We also resequenced our laboratory stock strain of D. radiodurans R1 ATCC 13939 to obtain an accurate reference for read alignments and gene expression quantifications. We highlighted genes that are directly under the control of this transcriptional repressor and showed that the DdrO regulon in D. radiodurans includes numerous other genes than those previously described, including DNA and RNA metabolism proteins. These results thus pave the way to better understand the radioresistance pathways encoded by this bacterium and to compare the stress-induced responses mediated by this pair of proteins in diverse bacteria.

Studies of multifunctional DNA polymerase I from the extremely radiation resistant Deinococcus radiodurans: Recombinant expression, purification and characterization of the full-length protein and its large fragment

Deinococcus radiodurans is a bacterium with extreme resistance to desiccation and radiation. Although the origins of this extreme resistance have not been fully elucidated, an efficient DNA repair machinery that includes the enzyme DNA polymerase I, is potentially crucial as part of a protection mechanism. Here we have cloned and performed small, medium, and large-scale expression of full-length D. radiodurans DNA polymerase I (DrPolI) as well as the large/Klenow fragment (DrKlenow). We then carried out functional characterization of 5' exonuclease, DNA strand displacement and polymerase activities of these proteins using gel-based and molecular beacon-based biochemical assays. With the same expression and purification strategy, we got higher yield in the production of DrKlenow than of the full-length protein, approximately 2.5 mg per liter of culture. Moreover, we detected a prominent 5' exonuclease activity of DrPolI in vitro. This activity and, DrKlenow strand displacement and DNA polymerase activities are preferentially stimulated at pH 8.0-8.5 and are reduced by addition of NaCl. Interestingly, both protein variants are more thermostable at pH 6.0-6.5. The characterization of DrPolI's multiple functions provides new insights into the enzyme's role in DNA repair pathways, and how the modulation of these functions is potentially used by D. radiodurans as a survival strategy.

Characterization of gross genome rearrangements in Deinococcus radiodurans recA mutants

Genome stability in radioresistant bacterium Deinococcus radiodurans depends on RecA, the main bacterial recombinase. Without RecA, gross genome rearrangements occur during repair of DNA double-strand breaks. Long repeated (insertion) sequences have been identified as hot spots for ectopic recombination leading to genome rearrangements, and single-strand annealing (SSA) postulated to be the most likely mechanism involved in this process. Here, we have sequenced five isolates of D. radiodurans recA mutant carrying gross genome rearrangements to precisely characterize the rearrangements and to elucidate the underlying repair mechanism. The detected rearrangements consisted of large deletions in chromosome II in all the sequenced recA isolates. The mechanism behind these deletions clearly differs from the classical SSA; it utilized short (4-11 bp) repeats as opposed to insertion sequences or other long repeats. Moreover, it worked over larger linear DNA distances from those previously tested. Our data are most compatible with alternative end-joining, a recombination mechanism that operates in eukaryotes, but is also found in Escherichia coli. Additionally, despite the recA isolates being preselected for different rearrangement patterns, all identified deletions were found to overlap in a 35 kb genomic region. We weigh the evidence for mechanistic vs. adaptive reasons for this phenomenon.

Revival of Anhydrobiotic Cyanobacterium Biofilms Exposed to Space Vacuum and Prolonged Dryness: Implications for Future Missions beyond Low Earth Orbit

Dried biofilms of Chroococcidiopsis sp. CCMEE 029 were revived after a 672-day exposure to space vacuum outside the International Space Station during the EXPOSE-R2 space mission. After retrieval, they were air-dried stored for 3.5 years. Space vacuum reduced cell viability and increased DNA damage compared to air-dried storage for 6 years under laboratory conditions. Long exposure times to space vacuum and extreme dryness decrease the changes of survival that ultimately depend on DNA damage repair upon rehydration, and hence, an in silico analysis of Chroococcidiopsis sp. CCMEE 029's genome was performed with a focus on DNA repair pathways. The analysis identified a high number of genes that encode proteins of the homologous recombination RecF pathway and base excision repair that were over-expressed during 1 and 6 h rehydration of space-vacuum exposed biofilms. This suggests that Chroococcidiopsis developed a survival strategy against desiccation, with DNA repair playing a key role, which allowed the revival of biofilms exposed to space vacuum. Unravelling how long anhydrobiotic cyanobacteria can persist under space vacuum followed by prolonged air-dried storage is relevant to future astrobiological experiments that use space platforms and might require prolonged air-dried storage of the exposed samples before retrieval to Earth.

An in vivo Interaction Network of DNA-Repair Proteins: A Snapshot at Double Strand Break Repair in Deinococcus radiodurans

An extremophile Deinococcus radiodurans survives massive DNA damage by efficiently mending hundreds of double strand breaks through homology-dependent DNA repair pathways. Although DNA repair proteins that contribute to its impressive DNA repair capacity are fairly known, interactions among them or with proteins related to other relevant pathways remain unexplored. Here, we report in vivo cross-linking of the interactomes of key DNA repair proteins DdrA, DdrB, RecA, and Ssb (baits) in D. radiodurans cells recovering from gamma irradiation. The protein-protein interactions were systematically investigated through co-immunoprecipitation experiments coupled to mass spectrometry. From a total of 399 proteins co-eluted with the baits, we recovered interactions among diverse biological pathways such as DNA repair, transcription, translation, chromosome partitioning, cell division, antioxidation, protein folding/turnover, metabolism, cell wall architecture, membrane transporters, and uncharacterized proteins. Among these, about 80 proteins were relevant to the DNA damage resistance of the organism based on integration of data on inducible expression following DNA damage, radiation sensitive phenotype of deletion mutant, etc. Further, we cloned ORFs of 23 interactors in heterologous E. coli and expressed corresponding proteins with N-terminal His-tag, which were used for pull-down assays. A total of 95 interactions were assayed, in which we confirmed 25 previously unknown binary interactions between the proteins associated with radiation resistance, and 2 known interactions between DdrB and Ssb or DR_1245. Among these, five interactions were positive even under non-stress conditions. The confirmed interactions cover a wide range of biological processes such as DNA repair, negative regulation of cell division, chromosome partitioning, membrane anchorage, etc., and their functional relevance is discussed from the perspective of DNA repair. Overall, the study substantially advances our understanding on the cross-talk between different homology-dependent DNA repair pathways and other relevant biological processes that essentially contribute to the extraordinary DNA damage repair capability of D. radiodurans. The data sets generated and analyzed in this study have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier PXD021822.

Proteome Damage Inflicted by Ionizing Radiation: Advancing a Theme in the Research of Miroslav Radman

Oxidative proteome damage has been implicated as a major contributor to cell death and aging. Protein damage and aging has been a particular theme of the recent research of Miroslav Radman. However, the study of how cellular proteins are damaged by oxidative processes is still in its infancy. Here we examine oxidative changes in the proteomes of four bacterial populations—wild type E. coli, two isolates from E. coli populations evolved for high levels of ionizing radiation (IR) resistance, and D. radiodurans—immediately following exposure to 3000 Gy of ionizing radiation. By a substantial margin, the most prominent intracellular oxidation events involve hydroxylation of methionine residues. Significant but much less frequent are carbonylation events on tyrosine and dioxidation events on tryptophan. A few proteins are exquisitely sensitive to targeted oxidation events, notably the active site of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in E. coli. Extensive experimental evolution of E. coli for IR resistance has decreased overall proteome sensitivity to oxidation but not to the level seen in D. radiodurans. Many observed oxidation events may reflect aspects of protein structure and/or exposure of protein surfaces to water. Proteins such as GAPDH and possibly Ef-Tu may have an evolved sensitivity to oxidation by H₂O₂.

PprA Protein Inhibits DNA Strand Exchange and ATP Hydrolysis of Deinococcus RecA and Regulates the Recombination in Gamma-Irradiated Cells

DrRecA and PprA proteins function are crucial for the extraordinary resistance to γ-radiation and DNA strand break repair in Deinococcus radiodurans. DrRecA mediated homologous recombination help in DNA strand break repair and cell survival, while the PprA protein confers radio-resistance via its roles in DNA repair, genome maintenance, and cell division. Genetically recA and pprA genes interact and constitute an epistatic group however, the mechanism underlying their functional interaction is not clear. Here, we showed the physical and functional interaction of DrRecA and PprA protein both in solution and inside the cells. The absence of the pprA gene increases the recombination frequency in gamma-irradiated D. radiodurans cells and genomic instability in cells growing under normal conditions. PprA negatively regulates the DrRecA functions by inhibiting DrRecA mediated DNA strand exchange and ATPase function in vitro. Furthermore, it is shown that the inhibitory effect of PprA on DrRecA catalyzed DNA strand exchange was not due to sequestration of homologous dsDNA and was dependent on PprA oligomerization and DNA binding property. Together, results suggest that PprA is a new member of recombination mediator proteins (RMPs), and able to regulate the DrRecA function in γ-irradiated cells by protecting the D. radiodurans genome from hyper-recombination and associated negative effects.

Learning Yeast Genetics from Miro Radman

Miroslav Radman's far-sighted ideas have penetrated many aspects of our study of the repair of broken eukaryotic chromosomes. For over 35 years my lab has studied different aspects of the repair of chromosomal breaks in the budding yeast, Saccharomyces cerevisiae. From the start, we have made what we thought were novel observations that turned out to have been predicted by Miro's extraordinary work in the bacterium Escherichia coli and then later in the radiation-resistant Dienococcus radiodurans. In some cases, we have been able to extend some of his ideas a bit further.

Coexistence of SOS-Dependent and SOS-Independent Regulation of DNA Repair Genes in Radiation-Resistant Deinococcus Bacteria

Deinococcus bacteria are extremely resistant to radiation and able to repair a shattered genome in an essentially error-free manner after exposure to high doses of radiation or prolonged desiccation. An efficient, SOS-independent response mechanism to induce various DNA repair genes such as recA is essential for radiation resistance. This pathway, called radiation/desiccation response, is controlled by metallopeptidase IrrE and repressor DdrO that are highly conserved in Deinococcus. Among various Deinococcus species, Deinococcus radiodurans has been studied most extensively. Its genome encodes classical DNA repair proteins for error-free repair but no error-prone translesion DNA polymerases, which may suggest that absence of mutagenic lesion bypass is crucial for error-free repair of massive DNA damage. However, many other radiation-resistant Deinococcus species do possess translesion polymerases, and radiation-induced mutagenesis has been demonstrated. At least dozens of Deinococcus species contain a mutagenesis cassette, and some even two cassettes, encoding error-prone translesion polymerase DnaE2 and two other proteins, ImuY and ImuB-C, that are probable accessory factors required for DnaE2 activity. Expression of this mutagenesis cassette is under control of the SOS regulators RecA and LexA. In this paper, we review both the RecA/LexA-controlled mutagenesis and the IrrE/DdrO-controlled radiation/desiccation response in Deinococcus.

Biochemical characterization of a unique DNA polymerase A from the extreme radioresistant organism Deinococcus radiodurans

Deinococcus radiodurans survives extraordinary doses of ionizing radiation and desiccation that cause numerous DNA strand breaks. D. radiodurans DNA polymerase A (DrPolA) is essential for reassembling the shattered genome, while its biochemical property has not been fully demonstrated. In this study, we systematically examined the enzymatic activities of DrPolA and characterized its unique features. DrPolA contains an N-terminal nuclease domain (DrPolA-NTD) and a C-terminal Klenow fragment (KlenDr). Compared with the Klenow fragment of E. coli Pol I, KlenDr shows higher fidelity despite the lacking of 3'-5' exonuclease proofreading activity and prefers double-strand DNA rather than Primer-Template substrates. Apart from the well-annotated 5'-3' exonuclease and flap endonuclease activities, DrPolA-NTD displays approximately 140-fold higher gap endonuclease activity than its homolog in E. coli and Human FEN1. Its 5'-3' exonuclease activity on ssDNA, gap endonuclease, and Holliday junction cleavage activities are greatly enhanced by Mn²⁺. The DrPolA-NTD deficient strain shows increased sensitivity to UV and gamma-ray radiation. Collectively, our results reveal distinct biochemical characteristics of DrPolA during DNA degradation and re-synthesis, which provide new insight into the outstanding DNA repair capacity of D. radiodurans.

Effects of Conserved Wedge Domain Residues on DNA Binding Activity of Deinococcus radiodurans RecG Helicase

Deinococcus radiodurans is extremely resistant to ionizing radiation and has an exceptional ability to repair DNA damage caused by various DNA-damaging agents. D. radiodurans uses the same DNA-repair strategies as other prokaryotes, but certain proteins involved in the classical DNA repair machinery have characteristics different from their counterparts. RecG helicase, which unwinds a variety of branched DNA molecules, such as Holliday junctions (HJ) and D-loops, plays important roles in DNA repair, recombination, and replication. Primary sequence analysis of RecG from a number of bacterial species revealed that three amino acids (QPW) in the DNA-binding wedge domain (WD) are well-conserved across the Deinococcus RecG proteins. Interactions involving these conserved residues and DNA substrates were predicted in modeled domain structures of D. radiodurans RecG (DrRecG). Compared to the WD of Escherichia coli RecG protein (EcRecG) containing FSA amino acids corresponding to QPW in DrRecG, the HJ binding activity of DrRecG-WD was higher than that of EcRecG-WD. Reciprocal substitution of FSA and QPW increased and decreased the HJ binding activity of the mutant WDs, EcRecG-WD_QPW, and DrRecG-WD_FSA, respectively. Following γ-irradiation treatment, the reduced survival rate of DrRecG mutants (ΔrecG) was fully restored by the expression of DrRecG, but not by that of EcRecG. EcRecG_QPW also enhanced γ-radioresistance of ΔrecG, whereas DrRecG_FSA did not. ΔrecG cells complemented in trans by DrRecG and EcRecG_QPW reconstituted an intact genome within 3 h post-irradiation, as did the wild-type strain, but ΔrecG with EcRecG and DrRecG_FSA exhibited a delay in assembly of chromosomal fragments induced by γ-irradiation. These results suggested that the QPW residues facilitate the association of DrRecG with DNA junctions, thereby enhancing the DNA repair efficiency of DrRecG.

N6-methyladenine and epigenetic immunity of Deinococcus radiodurans

DNA methylation is ubiquitously found in all three domains of life. This epigenetic modification on adenine or cytosine residues serves to regulate gene expression or to defend against invading DNA in bacteria. Here, we report the significance of N6-methyladenine (6mA) to epigenetic immunity in Deinococcus radiodurans. Putative protein encoded by DR_2267 ORF (Dam2_DR) contributed 35% of genomic 6mA in D. radiodurans but did not influence gene expression or radiation resistance. Dam2_DR was characterized to be a functional S-adenosyl methionine (SAM)-dependent N6-adenine DNA methyltransferase (MTase) but with no endonuclease activity. Adenine methylation from Dam2_DR or Dam1_DR (N6-adenine MTase encoded by DR_0643) improved DNA uptake during natural transformation. To the contrary, methylation from Escherichia coli N6-adenine MTase (Dam_EC that methylates adenine in GATC sequence) on donor plasmid drastically reduced DNA uptake in D. radiodurans, even in presence of Dam2_DR or Dam1_DR methylated adenines. With these results, we conclude that self-type N6-adenine methylation on donor DNA had a protective effect in absence of additional foreign methylation, a separate methylation-dependent Restriction Modification (R-M) system effectively identifies and limits uptake of G6mATC sequence containing donor DNA. This is the first report demonstrating presence of epigenetic immunity in D. radiodurans.

Draft genome sequence of Promicromonospora panici sp. nov., a novel ionizing-radiation-resistant actinobacterium isolated from roots of the desert plant Panicum turgidum

A novel strain of the genus Promicromonospora, designated PT9^T, was recovered from irradiated roots of the xerophyte Panicum turgidum collected from the Ksar Ghilane oasis in southern Tunisia. Strain PT9^T is aerobic, non-spore-forming, Gram- positive actinomycete that produces branched hyphae and forms white to yellowish-white colonies. Chemotaxonomic features, including fatty acids, whole cell sugars and polar lipid profiles, support the assignment of PT9^T to the genus Promicromonospora. The genomic relatedness indexes based on DNA-DNA hybridization and average nucleotide identity values revealed a significant genomic divergence between strain PT9^T and all sequenced type strains of the taxon. Phylogenomic analysis showed that isolate PT9^T was most closely related to Promicromonospora soli CGMCC 4.7398^T. Phenotypic and phylogenomic analyses suggest that isolate PT9^T represents a novel species of the genus Promicromonospora, for which the name Promicromonospora panici sp. nov. is proposed. The type strain is PT9^T (LMG 31103^T = DSM 108613^T).The isolate PT9^T is an ionizing-radiation-resistant actinobacterium (D₁₀ value = 2.6 kGy), with resistance to desiccation and hydrogen peroxide. The complete genome sequence of PT9^T consists of 6,582,650 bps with 71.2% G+C content and 6291 protein-coding sequences. This genome will help to decipher the microbial genetic bases for ionizing-radiation resistance mechanisms including the response to oxidative stress.

Structural features and functional implications of proteins enabling the robustness of Deinococcus radiodurans

Deinococcus radiodurans can survive under extreme conditions, including high doses of DNA damaging agents and ionizing radiation, desiccation, and oxidative stress. Both the efficient cellular DNA repair machinery and antioxidation systems contribute to the extreme resistance of this bacterium, making it an ideal organism for studying the cellular mechanisms of environmental adaptation. The number of stress-related proteins identified in this bacterium has mushroomed in the past two decades. The newly identified proteins reveal both commonalities and diversity of structure, mechanism, and function, which impact a wide range of cellular functions. Here, we review the unique and general structural features of these proteins and discuss how these studies improve our understanding of the environmental stress adaptation mechanisms of D. radiodurans.

Single-Molecule Insights into ATP-Dependent Conformational Dynamics of Nucleoprotein Filaments of Deinococcus radiodurans RecA

Deinococcus radiodurans (Dr) has one of the most robust DNA repair systems, which is capable of withstanding extreme doses of ionizing radiation and other sources of DNA damage. DrRecA, a central enzyme of recombinational DNA repair, is essential for extreme radioresistance. In the presence of ATP, DrRecA forms nucleoprotein filaments on DNA, similar to other bacterial RecA and eukaryotic DNA strand exchange proteins. However, DrRecA catalyzes DNA strand exchange in a unique reverse pathway. Here, we study the dynamics of DrRecA filaments formed on individual molecules of duplex and single-stranded DNA, and we follow conformational transitions triggered by ATP hydrolysis. Our results reveal that ATP hydrolysis promotes rapid DrRecA dissociation from duplex DNA, whereas on single-stranded DNA, DrRecA filaments interconvert between stretched and compressed conformations, which is a behavior shared by E. coli RecA and human Rad51. This indicates a high conservation of conformational switching in nucleoprotein filaments and suggests that additional factors might contribute to an inverse pathway of DrRecA strand exchange.

WD40 domain of RqkA regulates its kinase activity and role in extraordinary radioresistance of D. radiodurans

RqkA, a DNA damage responsive serine/threonine kinase, is characterized for its role in DNA repair and cell division in D. radiodurans. It has a unique combination of a kinase domain at N-terminus and a WD40 type domain at C-terminus joined through a linker. WD40 domain is comprised of eight β-propeller repeats held together via 'tryptophan-docking motifs' and forming a typical 'velcro' closure structure. RqkA mutants lacking the WD40 region (hereafter referred to as WD mutant) could not complement RqkA loss in γ radiation resistance in D. radiodurans and lacked γ radiation-mediated activation of kinase activity in vivo. WD mutants failed to phosphorylate its cognate substrate (e.g. DrRecA) in surrogate E. coli cells. Unlike wild-type enzyme, the kinase activity of its WD40 mutants was not stimulated by pyrroloquinoline quinine (PQQ) indicating the role of the WD motifs in PQQ interaction and stimulation of its kinase activity. Together, results highlighted the importance of the WD40 domain in the regulation of RqkA kinase signaling functions in vivo, and thus, the role of WD40 domain in the regulation of any STPK is first time demonstrated in bacteria.Communicated by Ramaswamy H. Sarma.

Purification from Deinococcus radiodurans of a 66 kDa ABC transporter acting on peptides containing at least 3 amino acids

Deinococcus radiodurans is a Gram positive bacterium the capability of which to withstand high doses of ionizing radiations is well known. Physiologically speaking, D. radiodurans is a proteolytic prokaryote able to express and secrete quite a number of proteases, and to use amino acids as an energy source. When considering this, it is surprising that little information is available on the biochemical components responsible for the uptake of peptides in D. radiodurans. Here we report on the purification and characterization of an ABC peptide transporter, isolated from D. radiodurans cells grown in tryptone-glucose-yeast extract (TGY) medium. In particular, we show here that the action of this transporter (denoted DR1571, SwissProt data bank accession number Q9RU24 UF71_DEIRA) is exerted on peptides containing at least 3 amino acids. Further, using tetra-peptides as model systems, we were able to observe that the DR1571 protein does not bind to peptides containing phenylalanine or valine, but associates with high efficiency to tetra-glycine, and with moderate affinity to tetra-peptides containing arginine or aspartate.