By Jhonata Lam
Extremophiles are organisms able to tolerate and survive in even the harshest environments on Earth. While such conditions can be attributed to their temperature, pH or pressure, different extremophiles’ tolerance to radiation remains a particularly great interest to the scientific community.1
Radiation itself is generated through the natural decay of radioactive elements – possibly emitting γ-rays, photons acting as a source of energy. Within the cell, γ-rays can strike water molecules to generate Reactive Oxygen Species (ROS), which facilitate the damage and subsequent loss of genetic information. A hydroxyl radical (•OH), for example, can chemically alter DNA in a variety of ways: by introducing single-strand breaks or double-strand breaks, by modifying the nitrogenous bases in DNA, and so forth.2 The progressive accumulation of these changes can consequently lead to diseases like cancer. It is therefore imperative to consider the mitigation of oxidative stress by extremophiles.
Deinococcus radiodurans is a highly radiation-resistant bacterium capable of tolerating up to 5000 grays (Gy) whilst still retaining viability.3 For comparison, it has been suggested in humans that 4.5 Gy begins to constitute a “lethal dose” of radiation.4 Historically, D. radiodurans is one of the most comprehensively categorised organisms of this category.5
Antioxidant systems in D. radiodurans confer a drastic reduction in ROS concentrations. There are several superoxide dismutases (SODs) encoded in the genome, converting the highly injurious superoxide radical (•O2-) into less toxic compounds like molecular oxygen and hydrogen peroxide. Of these is the enzyme MnSOD – differing from the E. coli and human equivalents by its extremely fast turnover rates.2 Another class of enzymes encoded in the genome is the catalases, of which there are also several. As before, the function of catalase is to attenuate hydrogen peroxide levels.2
Interestingly, D. radiodurans is dependent on the element manganese (Mn(II)) for its resistance properties. Accordingly, it has been shown that Mn transporters serve to augment the resting levels of Mn in the cell.2 It may also play a role in ROS scavenging by preventing the generation of hydroxyl radicals in the Fenton reaction.5
Transcriptomic analysis unveiled a global change in the bacterium’s gene expression as a result of ionizing radiation.6 72 genes were observed to have been upregulated in this treatment and characterised as important for resistance, including ddrA, ddrB, and pprA. DdrA is associated[LJ2] [HA3] [HA4] with single-stranded DNA ends; DdrB associates with single-stranded DNA; and PprA associates with double-stranded ends and facilitates ligation by ligases, meaning that PprA mutants suffer from a decreased viability in cell division.7 All of these functions serve to reduce the damage inflicted on DNA. It is also known that RecA proteins combine to form filaments with Deinococcus DNA duplexes and are necessary for genetic repair mechanisms such as homologous recombination (HR) or extended synthesis-dependent strand annealing (ESDSA).7
A natural corollary to this is that there must be a regulatory system in place to actuate this expression imbalance upon exposure to radiation. A DNA sequence residing upstream of radiation-induced genes, RDRM (radiation/desiccation response motif), has been shown to be involved in this regulation.8 Researchers have shown that DdrO, a transcription regulator that binds to the RDRM sequences, acts to repress radiation-induced genes.8 Others have presented data that the zinc-dependent metalloprotease IrrE (aka PprI), proteolytically degrades the DdrO transcriptional repressor as a result of augmented zinc concentrations with oxidative stress.9 Conclusively, a model was proposed to explain the alleviation of repression proportionate to irradiation. This, in some sense, mimics some radiation-resistant E. coli, whose usage of RecA-dependent proteolysis of Lex proteins attenuates the repression on SOS genes.8
In another study, a two-component system was elucidated in D. radiodurans to first sense radiation-induced oxidative stress and then encourage the expression of radiation-resistance genes.7 The anatomy of this system consists of a histidine kinase, which detects the irradiation stimuli. This leads to the transfer of a phosphoryl group onto the transcription regulator RR (Response Regulator), which activates the genes in question.7 Inevitably, this permits the bacterium to dynamically respond to fluctuating radiation signals.
These mechanisms are not merely fascinating but are useful therapeutically. For instance, optimal radiotherapy for the treatment of cancerous cells is hindered by the radiation-resistance dynamics described previously.10 Transcription factors affecting gene expression (e.g., NF-κB and so forth) have been demonstrated to prevent apoptosis in cancer cells by increasing the expression of anti-apoptotic genes and emphasising proliferative qualities.10 NF-κB, for instance, has been noted to induce the expression of the MnSOD enzyme.10 Hence, using such extremophiles as model organisms could advise existing biomedical knowledge and cancer therapies.
Biotechnologically, D. radiodurans provides unique opportunities within the realm of bioremediation.1 They have proved useful in the degradation of phthalate esters like DBP, owing to their sturdiness in a broad variety of environments.11 After being subjected to high concentrations of Uranium, recombinant D. radiodurans was further found to be highly effective in precipitating and thus neutralising the heavy metal, highlighting its potential in mitigating levels of radioactive contaminants in nature.12
Clearly, microbial systems have evolved incredibly sophisticated means to adapt to their environments. Thus, polyextremotolerant organisms like D. radiodurans, whose mechanisms for tolerance still need to be fully elaborated,7 may be exploited in the future for purposes in both the environmental and medical sectors.
References:
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2. Qi H zhou, Wang W zhou, He J yan, Ma Y, Xiao F zhu, He S ya. Antioxidative system of Deinococcus radiodurans. Research in Microbiology. 2020 Mar 1;171(2):45–54.
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9. Blanchard L, Guérin P, Roche D, Cruveiller S, Pignol D, Vallenet D, et al. Conservation and diversity of the IrrE/DdrO-controlled radiation response in radiation-resistant Deinococcus bacteria. MicrobiologyOpen. 2017;6(4):e00477.
10. Galeaz C, Totis C, Bisio A. Radiation Resistance: A Matter of Transcription Factors. Frontiers in Oncology [Internet]. 2021 [cited 2023 Feb 24];11. Available from: https://www.frontiersin.org/articles/10.3389/fonc.2021.662840
11. Liao CS, Chen LC, Chen BS, Lin SH. Bioremediation of endocrine disruptor di-n-butyl phthalate ester by Deinococcus radiodurans and Pseudomonas stutzeri. Chemosphere. 2010 Jan 1;78(3):342–6.
12. Misra CS, Appukuttan D, Kantamreddi VSS, Rao AS, Apte SK. Recombinant D. radiodurans cells for bioremediation of heavy metals from acidic/neutral aqueous wastes. Bioengineered. 2012 Jan 1;3(1):44–8.
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