DNA Gyrase of Deinococcus radiodurans is characterized as Type II bacterial topoisomerase and its activity is differentially regulated by PprA in vitro

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.

Guanine quadruplexes and their roles in molecular processes

The role of guanine quadruplexes (G4) in fundamental biological processes like DNA replication, transcription, translation and telomere maintenance is recognized. G4 structure dynamics is regulated by G4 structure binding proteins and is thought to be crucial for the maintenance of genome integrity in both prokaryotic and eukaryotic cells. Growing research over the last decade has expanded the existing knowledge of the functional diversity of G4 (DNA and RNA) structures across the working models. The control of G4 structure dynamics using G4 binding drugs has been suggested as the putative targets in the control of cancer and bacterial pathogenesis. This review has brought forth the collections of recent information that indicate G4 (mostly G4 DNA) roles in microbial pathogenesis, DNA damaging stress response in bacteria and mammalian cells. Studies in mitochondrial gene function regulation by G4s have also been underscored. Finally, the interdependence of G4s and epigenetic modifications and their speculated medical implications through G4 interacting proteins has been discussed.

Molecular insights into replication initiation in a multipartite genome harboring bacterium Deinococcus radiodurans

Deinococcus radiodurans harbors a multipartite ploid genome system consisting of two chromosomes and two plasmids present in multiple copies. How these discrete genome elements are maintained and inherited is not well understood. PprA, a pleiotropic protein involved in radioresistance, has been characterized for its roles in DNA repair, genome segregation, and cell division in this bacterium. Here, we show that PprA regulates ploidy of chromosome I and II and inhibits the activity of drDnaA, the initiator protein in D. radiodurans. We found that pprA deletion resulted in an increased genomic content and ploidy of both the chromosomal elements. Expression of PprA in trans rescued the phenotypes of the pprA mutant. To understand the molecular mechanism underlying these phenotypes, we characterized drDnaA and drDnaB. As expected for an initiator protein, recombinant drDnaA showed sequence-specific interactions with the putative oriC sequence in chromosome I (oriCI). Both drDnaA and drDnaB showed ATPase activity, also typical of initiator proteins, but only drDnaB exhibited 5'→3' dsDNA helicase activity in vitro. drDnaA and drDnaB showed homotypic and heterotypic interactions with each other, which were perturbed by PprA. Interestingly, PprA has inhibited the ATPase activity of drDnaA but showed no effect on the activity of drDnaB. Regulation of chromosome copy number and inhibition of the initiator protein functions by PprA strongly suggest that it plays a role as a checkpoint regulator of the DNA replication initiation in D. radiodurans perhaps through its interaction with the replication initiation machinery.

Topoisomerase IB interacts with genome segregation proteins and is involved in multipartite genome maintenance in Deinococcus radiodurans

Deinococcus radiodurans, an extremophile, resistant to many abiotic stresses including ionizing radiation, has 2 type I topoisomerases (drTopo IA and drTopo IB) and one type II topoisomerase (DNA gyrase). The role of drTopo IB in guanine quadruplex DNA (G4 DNA) metabolism was demonstrated earlier in vitro. Here, we report that D. radiodurans cells lacking drTopo IB (ΔtopoIB) show sensitivity to G4 DNA binding drug (NMM) under normal growth conditions. The activity of G4 motif containing promoters like mutL and recQ was reduced in the presence of NMM in mutant cells. In mutant, the percentage of anucleate cells was more while the copy number of genome elements were less as compared to wild type. Protein-protein interaction studies showed that drTopo IB interacts with genome segregation and DNA replication initiation (DnaA) proteins. The typical patterns of cellular localization of GFP-PprA were affected in the mutant cells. Microscopic examination of D. radiodurans cells expressing drTopo IB-RFP showed its localization on nucleoid forming a streak parallel to the old division septum and perpendicular to newly formed septum. These results together suggest the role of drTopo IB in genome maintenance in this bacterium.

N 4-Cytosine DNA Methylation Is Involved in the Maintenance of Genomic Stability in Deinococcus radiodurans

DNA methylation serves as a vital component of restriction-modification (R-M) systems in bacteria, where it plays a crucial role in defense against foreign DNA. Recent studies revealed that DNA methylation has a global impact on gene expression. Deinococcus radiodurans, an ideal model organism for studying DNA repair and genomic stability, possesses unparalleled resistance to DNA-damaging agents such as irradiation and strong oxidation. However, details on the methylome of this bacterium remain unclear. Here, we demonstrate that N ⁴-cytosine is the major methylated form (4mC) in D. radiodurans. A novel methylated motif, "C^4mCGCGG" was identified that was fully attributed to M.DraR1 methyltransferase. M.DraR1 can specifically bind and methylate the second cytosine at N ⁴ atom of "CCGCGG" motif, preventing its digestion by a cognate restriction endonuclease. Cells deficient in 4mC modification displayed higher spontaneous rifampin mutation frequency and enhanced DNA recombination and transformation efficiency. And genes involved in the maintenance of genomic stability were differentially expressed in conjunction with the loss of M.DraR1. This study provides evidence that N ⁴-cytosine DNA methylation contributes to genomic stability of D. radiodurans and lays the foundation for further research on the mechanisms of epigenetic regulation by R-M systems in bacteria.

Guanine Quadruplex DNA Regulates Gamma Radiation Response of Genome Functions in the Radioresistant Bacterium Deinococcus radiodurans

Guanine quadruplex (G4) DNA/RNA are secondary structures that regulate the various cellular processes in both eukaryotes and bacteria. Deinococcus radiodurans, a Gram-positive bacterium known for its extraordinary radioresistance, shows a genomewide occurrence of putative G4 DNA-forming motifs in its GC-rich genome. N-Methyl mesoporphyrin (NMM), a G4 DNA structure-stabilizing drug, did not affect bacterial growth under normal conditions but inhibited the postirradiation recovery of gamma-irradiated cells. Transcriptome sequencing analysis of cells treated with both radiation and NMM showed repression of gamma radiation-responsive gene expression, which was observed in the absence of NMM. Notably, this effect of NMM on the expression of housekeeping genes involved in other cellular processes was not observed. Stabilization of G4 DNA structures mapped at the upstream of recA and in the encoding region of DR_2199 had negatively affected promoter activity in vivo, DNA synthesis in vitro and protein translation in Escherichia coli host. These results suggested that G4 DNA plays an important role in DNA damage response and in the regulation of expression of the DNA repair proteins required for radioresistance in D. radiodurans IMPORTANCE Deinococcus radiodurans can recover from extensive DNA damage caused by many genotoxic agents. It lacks LexA/RecA-mediated canonical SOS response. Therefore, the molecular mechanisms underlying the regulation of DNA damage response would be worth investigating in this bacterium. D. radiodurans genome is GC-rich and contains numerous islands of putative guanine quadruplex (G4) DNA structure-forming motifs. Here, we showed that in vivo stabilization of G4 DNA structures can impair DNA damage response processes in D. radiodurans Essential cellular processes such as transcription, DNA synthesis, and protein translation, which are also an integral part of the double-strand DNA break repair pathway, are affected by the arrest of G4 DNA structure dynamics. Thus, the role of DNA secondary structures in DNA damage response and radioresistance is demonstrated.

Proteome dynamics during post-desiccation recovery reveal convergence of desiccation and gamma radiation stress response pathways in Deinococcus radiodurans

Deinococcus radiodurans is inherently resistant to both ionizing radiation and desiccation. Fifteen months of desiccation was found to be the LD₅₀ dose for D. radiodurans. Desiccated cells of D. radiodurans entered 6h of growth arrest during post-desiccation recovery (PDR). Proteome dynamics during PDR were mapped by resolving cellular proteins by 2-dimensional gel electrophoresis coupled with mass spectrometry. At least 41 proteins, represented by 51 spots on proteome profiles, were differentially expressed throughout PDR. High upregulation in expression was observed for DNA repair proteins involved in single strand annealing (DdrA and DdrB), nucleotide excision repair (UvrA and UvrB), homologous recombination (RecA) and other vital proteins that contribute to DNA replication, recombination and repair (Ssb, GyrA and GyrB). Expression of CRP/FNR family transcriptional regulator (Crp) remained high throughout PDR. Other pathways such as cellular detoxification, protein homeostasis and metabolism displayed both, moderately induced and repressed proteins. Functional relevance of proteomic modulations to surviving desiccation stress is discussed in detail. Comparison of our data with the published literature revealed convergence of radiation and desiccation stress responses of D. radiodurans. This is the first report that substantiates the hypothesis that the radiation stress resistance of D. radiodurans is incidental to its desiccation stress resistance.

A Genome-Wide Search for Ionizing-Radiation-Responsive Elements in Deinococcus radiodurans Reveals a Regulatory Role for the DNA Gyrase Subunit A Gene's 5' Untranslated Region in the Radiation and Desiccation Response

Tight regulation of gene expression is important for the survival of Deinococcus radiodurans, a model bacterium of extreme stress resistance. Few studies have examined the use of regulatory RNAs as a possible contributing mechanism to ionizing radiation (IR) resistance, despite their proffered efficient and dynamic gene expression regulation under IR stress. This work presents a transcriptome-based approach for the identification of stress-responsive regulatory 5' untranslated region (5'-UTR) elements in D. radiodurans R1 that can be broadly applied to other bacteria. Using this platform and an in vivo fluorescence screen, we uncovered the presence of a radiation-responsive regulatory motif in the 5' UTR of the DNA gyrase subunit A gene. Additional screens under H2O2-induced oxidative stress revealed the specificity of the response of this element to IR stress. Further examination of the sequence revealed a regulatory motif of the radiation and desiccation response (RDR) in the 5' UTR that is necessary for the recovery of D. radiodurans from high doses of IR. Furthermore, we suggest that it is the preservation of predicted RNA structure, in addition to DNA sequence consensus of the motif, that permits this important regulatory ability.IMPORTANCEDeinococcus radiodurans is an extremely stress-resistant bacterium capable of tolerating up to 3,000 times more ionizing radiation than human cells. As an integral part of the stress response mechanism of this organism, we suspect that it maintains stringent control of gene expression. However, understanding of its regulatory pathways remains incomplete to date. Untranslated RNA elements have been demonstrated to play crucial roles in gene regulation throughout bacteria. In this work, we focus on searching for and characterizing responsive RNA elements under radiation stress and propose that multiple levels of gene regulation work simultaneously to enable this organism to efficiently recover from exposure to ionizing radiation. The model we propose serves as a generic template to investigate similar mechanisms of gene regulation under stress that have likely evolved in other bacterial species.

Identification of distinctive molecular traits that are characteristic of the phylum "Deinococcus-Thermus" and distinguish its main constituent groups

The phylum "Deinococcus-Thermus" contains two heavily researched groups of extremophilic bacteria: the highly radioresistant order Deinococcales and the thermophilic order Thermales. Very few characteristics are known that are uniquely shared by members of the phylum "Deinococcus-Thermus". Comprehensive phylogenetic and comparative analyses of >65 "Deinococcus-Thermus" genomes reported here have identified numerous molecular signatures in the forms of conserved signature insertions/deletions (CSIs) and conserved signature proteins (CSPs), which provide distinguishing characteristics of the phylum "Deinococcus-Thermus" and its main groups. We have identified 58 unique CSIs and 155 unique CSPs that delineate different phylogenetic groups within the phylum. Of these identified traits, 24 CSIs and 29 CSPs are characteristic of the phylum "Deinococcus-Thermus" and they provide novel and reliable means to circumscribe/describe this phylum. An additional 3 CSIs and 3 CSPs are characteristic of the order Deinococcales, and 6 CSIs and 51 CSPs are characteristic of the order Thermales. The remaining 25 CSIs and 72 CSPs identified in this study are distinctive traits of genus level groups within the phylum "Deinococcus-Thermus". The molecular characteristics identified in this work provide novel and independent support for the common ancestry of the members of the phylum "Deinococcus-Thermus" and provide a new means to distinguish the main constituent clades of the phylum. Additionally, the CSIs and CSPs identified in this work may play a role in the unique extremophilic adaptations of the members of this phylum and further functional analyses of these characteristics could provide novel biochemical insights into the unique adaptations found within the phylum "Deinococcus-Thermus".