MECHANISMS OF RADIATION RESISTANCE IN THERMOPHILES

Thennophiles deal with high temperature as well as radiations simultaneously which led to conclude that these microbes may have been present, and at tunes predominant throughout Earth’s geologic history or represent lineages descendent from first living microorganisms on the planet. The mechanisms by which radiation-resistant thennophiles deal with str esses will give new insights into evolution and astrobiology (Ranawat and Rawat, 2017a). The microbes are subjected to different stresses like an extreme vacuum, desiccation, solar, and cosmic radiation, microgravity, and both extreme hot and cold temperatures in outer space environments (Nicholson et al., 2000). The spores of Bacillus subtilis, vegetative cells of Deinococcus and some halophilic archaea like Halobacterium spp. are the model organisms for study due to then high resistance to extreme conditions (Hecker and Volker, 1998; De Vera et al., 2003; Rampelotto et al., 2007,2009; Nicholson, 2009; Rosa et al., 2009). Radiation resistant thennophiles include archaea like Archaeoglobus fulgidus, Methanocaldococcus jannaschii, Pyrococcus furiosus, Thermococcus gammatolerans and Thermococcus radiotolerans, actinobacteria like Rubrobacter radiotolerans, Rubrobacter taiwanensis, Rubrobacter xylanophilus and bacteria like Deinococcus geothermalis, Deinococcus murrayi and Truepera radiovictrix (Caireto et al., 1996; Di Ruggiero et al., 1997; Ferreira et al., 1997; Jolivet et al., 2003; Chen et al., 2004; Jolivet et al., 2004; Albuquerque et al., 2005; Beblo et al., 2011). It is very interesting to explore the science behind the processes that define radiation resistance and recovery from damage caused by IR in radiation- resistant thermophilic microorganisms. Thus, the upcoming sections present an overview of the DNA repair system, ROS detoxification mechanism, and other mechanisms that enable radiation resistance in thermophiles.

10.3.1 DNA REPAIR SYSTEM

IR exposure causes extensive damage to cellular macromolecules, especially DNA. The direct or indirect damage induces single-stranded and double- stranded DNA breaks. Hence, different radiation-resistant microorganisms respond in different ways to cope up with DNA damage and initiate an event of repair mechanisms, which are summarized in Figure 10.2.

FIGURE 10.2 DNA repair system in radiation-resistant thermophiles.

Most of the members of the genus Demococcus are resistant to radiation like D. radiodurans can resist a high dose of IR [>12,000 Gy (gray; absorbed radiation dose)]. D. geothemialis is the thermophilic radiation- resistant bacteria which belonged to the genus Deinococcus and recovered from geothermal spring (Ferreira et al., 1997). The other radiation-resistant species of genus Dewococctis which can tolerate an average dose of lOkGy are D. apachensis, D. grandis, D. hopiensis, D. indicus, D. moricopensis, D. murryai, D. pimensis, D. piscis, D. papagonensis, D. radiodurans, D. radiophilus, D. radiopugnans, D. sonorensis, and D. yumveiensis (Ferreira et al., 1997; Pavlopoulou et al., 2016). Genus Triiepera represents a distinct lineage of Phylum “Deinococcus/Thermus” and includes Triiepera radiovictrix, which is also a radiation-resistant thennophile and 60% cells can survive 5 kGy. It also shares the trait of radiation resistance with genus Deinococcus (Ferreira et al., 1999).

Ferreira et al. (1997) reported that when/), geothermalis (DSM 11300) and D. radiodurans (ATCC BAA-816) were exposed to ⁶⁰Co and ultraviolet (UV) radiations (254 mn) at 50°C and 32°C, respectively both were observed to be equally resistant to radiations but since D. geothermalis is a thermophilic radiation-resistant bacteria its recovery was 1000 times greater at high temperature (50°C) than at mesophilic temperature (32°C). It was observed that D. geothermalis and D. radiodurans contain a conserved set of genes that confer extreme radiation resistance to these microorganisms. DR1289, a protein of RecQ helicase family present inZ). radiodurans, contains three Helicase and RNAse D C-terminal (HRDC) domains while there is a single HRDC domain in other bacterial RecQ protein (Huang et al., 2007). In D. geothermalis no ortholog of D. radiodurans RecQ is reported, but it possesses another protein, Dgeo_1226, which has resemblance with corresponding domains of DR1289 and contains a helicase superfamily II C-terminal domain and a second HDRC domain. Thus, it is speculated that both proteins (DR 12 89 and D_ geo 1226) are the part of predicted resistance regulon (Huang et al., 2007). It was assumed that D. radiodurans possess non-homologous end joining (NHEJ) but later, studies reported that instead of NHEJ, D. radiodurans has DRB0100 which encode a ATP-dependent ligase (a phosphatase of H, macro superfamily), a polynucleotide kinase and a HD family phosphatase (Makarova et al., 2001,2007; Levin-Zaidman et al., 2003; Liu et al., 2003; Englander et al., 2004; Nanuni et al., 2004; Tanaka et al., 2004; Bowater and Doherty, 2006). In D. geothermalis, no orthologs of DRB0100 are reported but inZ). radiodurans the induction of DRB0100 is induced by IR exposure. But it was also observed that DRB0100 does not exhibit DNA or RNA ligase function in vitro (Blasius et al., 2007).

hr D. radiodurans, the exposure of cells to IR for 3 hours leads to increase in transcriptional induction by 2 folds (Liu et al., 2003). However,

• 45% genes induced in D. radiodurans following an exposure with IR have no orthologs in D. geothennalis. The plausible reason for this could be either all genes are not expressed in D. radiodurans or D geothennalis possess a different set of radiation resistance factors (Makarova et al., 2007). D. radiodurans own a unique set of genes which code for DNAheli- case RecQ (DR1289) and a transcriptional regulator (DR0171) but DNA single-strand annealing (SSA) protein, /y»A-involved in DNA damage resistance mechanism and other 4 genes (<7<7rC-DR003/Dgeo_0047; Deinococcus lineages which suggests that novel radiation resistance mechanisms are present in Deinococcus spp. (Liu et al., 2003; Tanaka et al., 2004). RecA is the crucial protein necessary for homologous DNA recombination repair following an exposure to IR (Cox and Battista, 2005). RecA of D. radiodurans first binds with DNA duplex and then with homologous single-stranded DNA while in other bacteria RecA first binds with single-stranded DNA and is a key protein involved in reassembly of damaged chromosome (Kim et al.,
• 2002). Wanarska et al. (2011) reported that Rec A protein of Deinococcus geothennalis (DgeRecA) and of Deinoccoccus murrayi (DnniRecA) are slightly thermostable which bind to ssDNA more readily as compared to dsDNA in the presence of Mg²⁺ ions and hydrolyze ATP and dATP in the presence of ssDNA (Kim et al., 2002).

RecA of Deinococcus radiodurans (DraRecA) can enhance DNA strand exchange through an inverse pathway which is not present in RecA of D. geothennalis {DgeRecA) and RecA of D. murryai (DmuRecA) (Kim et al., 2002). This suggests a unique property of DraRecA which is not observed in other RecA protein (Wanarska et al., 2011). Makarova et al. (2007) reported that D. radiodurans and D. geothennalis encode recJ (a putative 5’-3’ exonuclease) instead of recBCE. It has been speculated that recJ provides nuclease activity, which is not present in Klenow polymerase. In D. radiodurans, the regulation of gene expression while the cells are recovering from an exposure to IR has been deeply investigated. The irradiation leads to induction of RecA, which is regulated by IrrE/PprI protein. This protein is mainly comprised of two domains, a Xre-like HTH domain and a Zn-dependent protease (Earl et al., 2002; Hua et al., 2003). Gao et al. (2006) reported that gene irrE is present in upstream of folate biosynthesis operon and regulated individually in D geothennalis and D. radiodurans. D. geothennalis has one lex A gene (DG1366), whereas there are two lex A paralogs (DRA0344, DRA0074) inD. radiodurans, but

lex A genes are not induced after irradiation and therefore do not play any role in the induction of RecA in D. radiodurans (Naraumi et al., 2001). DdrO, a Xre family protein, is exclusively Deinococcus gene for predicted regulator and preceded by RDRM site. The arrangement of DdrO is identical to various stress response regulators like I ex A genes of many species (Little et al., 1981). The DdrO putative regulator inD. radiodurans is DR2574, expressed by microarray experiment at lower doses (3000 Gy) while Dgeo_0336 is an ortholog of DdrO in D geothermalis (Tanaka et al., 2004).

Radiation-desiccation response (RDR) regulon is present in both D. geothennalis and D. radiodurans. There are 20 operons in RDR regulon, which comprised of 25 genes inD. geothermalis and 29 genes inD. radiodurans. The dominating genes present in RDR regulon are DNA repair genes which also include recombination repair proteins RecA and RecQ, MutS, and MutL proteins (involved in mismatch repair (MMR) located in one operon in D. geothermalis) and UvrB and UvrC proteins (involved in nucleotide excision repair) (Kuzminov, 1999; Kunkel and Erie, 2005). Touati et al. (1996) reported that a transketolase gene is also present in RDR regulon, which is induced in stress like exposure to cold shock and mutagenic agents, which induces SOS response. In D. radiodurans, the DNA excision repair is reported to be facilitated by pentose-phosphate pathway (Zhang et al., 2003). The upstream regions of various genes, viz., DR0326, ddrD: DR0423, ddrA: DRA0346, pprA DR0070, ddrB exhibited the presence of a strong palindromic motif known as radiation/ desiccation response motif (RDRM). Liu et al. (2003) and Tanaka et al. (2004) reported that RDR regulon is comprised of two groups: (i) ortholo- gous genes present in D. geothermalis and D. radiodurans which contain RDRM; (ii) a exclusive set of genes present in D. radiodurans which contain RDRM and this gene set is upregulated when there is recovery of cells following an irradiation

Deinococcus spp. and Thermits spp. are members of group Deino- coccus-Thermus which besides sharing various similarities like pigmentation and chemoorganotrophism varies in their responses to IR as Deinococcus genus has both radiation-resistant mesophile like D. radiodurans and thennophile like D. geothermalis while Therm us spp. are thermophilic and radiation-sensitive (Weisburg et al., 1989). D. radiodurans has two LexA, a SOS- response transcriptional repressor while T ther- mopliilus is devoid of that and also does not contain endonuclease VII (XseAB). However, it possesses photolyase (PhrB) and endonuclease

IV DNA polymerase III, indicating a very few similarities between the DNA repair systems of D radiodurans and I thennophilus (Omelchenko et al., 2005). D. radiodurans has an extensive repertoire of enzymes or proteins involved in DNA repair, which are necessary for IR resistance. It include DNA ligase, RNA ligase, double-strand break repair complex, a protein of HD family phosphatase, polynucleotide kinase, phosphatase of H, macro superfamily, double-stranded DNA binding protein, PprA, and DNA single-strand annealing protein. DdrA among all these proteins Thermus has only an ortholog of DdrA (Makarova et al., 2001; Liu et al., 2003; Harris et al., 2004; Martins and Shuman, 2004; Nammi et al.,

2004). The megaplasmid of I thennophilus, encodes a putative thermophilic-specific repair system involved in uncharacterized DNA repair. It was postulated that this complex is functionally analogous to the bacterial-eukaryotic system of translesion and mutagenic repair whose components are DNA polymerases of UmuC-DinB-Rad20-Revl superfamily, not present in thermophiles (Makarova et al., 2002). Heime et al. (2004) reported that megaplasmid of I thennophilus HB27 encodes UY endonuclease (TTP0052), which repair DNA damages caused by UY. I ther- mophilus HB27 encodes two-uracil-DNA glycosylases that remove uracil residues from U:G mispairs formed in DNA (Ohta et al., 2006).

Rubrobacter spp. is radiation-resistant thermophilic actinobacteria which can tolerate 8-12 kGy doses of radiations except R. brocarensis which is radiation-sensitive (Jurado et al., 2012). The complete genome of R. rodiotolerons strain RSPS-4 was investigated for the genes which play a crucial role in DNA repair pathways and encode proteins required for homologous recombination, SSA, extended synthesis-dependent strand annealing or NHEJ (Egas et al., 2014). RSPS-4 contains all genes for RecFOR pathway, recF (RradSPS_0004), recR (RradSPS_0466), recO (RradSPS_1511), and recJ (RradSPS_0780). RecA, protein responsible for strand invasion and exchange, was encoded as a single copy (RradSPS_1428). Genes encoding the branch migration and resolution of Holliday junction proteins RuvA (RradSPS_l 317), RuvB (RradSPS_l 318), RuvC (RradSPS_1316), and RecG (RradSPS_1377) were detected, as were the homologs of the genes encoding the SbcD {mre 11) (RradSPS_2355), and SbcC (Rad50) (RradSPS_2356) proteins. Egas et al. (2014) reported that R. rodiotolerons strain RSPS-4 lacks lex A, which suggests the presence of alternative DNA repair mechanisms. The presence of gene copies of rnuiL (RradSPS_0036 and _0159) and nnitS (RradSPS_0158) predict that MMR is probably active in RSPS-4 (Nowosielska and Marinus, 2008).

Thermophilic archaea like Pyrococcus furiosus and Thermococcus gammatolerans can resist 6kGy and 3kGy dose of radiations (Webb and Di Ruggiero, 2012). T. gammatolerans posses nucleases Tg0864, Tgll77, Tgl631, ligases (Tgl718, Tg2005), endonucleases, AP-lyases, and glyco- sylases Tg0271, Tg0543, Tgll92, Tgl446, Tgl637, Tgl814 which are involved in base excision repair and Tg0130, Tg0280, Tgl742, Tgl743, Tgl744 and Tg2074 are presumed to be involved in double-strand break repair (Zivanovic et al., 2009) while in Pyrococcus furiosus RadA and RadB proteins (homologs of RecA and Rad51 found in bacteria and eukaryotes) are involved in homologous recombination and also produce a putative Dps-like iron chelating protein (Komori et al., 2000; Gerard et al., 2001). In Sulfolobus spp., UV stress causes upregulation of U’-inducible pili operon of Sulfolobus (ups operon), which leads to the formation of type IV pili (T4P) involved in UV induced pili assembly, cellular aggregation and subsequent exchange of DNA between cells. The gene deletion analysis explained that UpsX (a membrane-localized protein encoded by ups operon) is involved in DNA transfer (via unknown mechanism) which is assumed to be the repair mechanism of the cell for UV induced DNA damage (Frols et al., 2007, 2008; Gotz et al., 2007). Ajon et al. (2011) reported that ups cluster encode UpsX, UpsE, a secretion ATPase; UpsF, a membrane protein; and UpsA and B, two-class III signal peptide- containing pilin subunits which are essentially required in DNA transfer and repair mechanism during UV stress.

10.3.2 ROS DETOXIFICA TION SYSTEM

The exposure to IR leads to the formation of ROS, viz., superoxide (0₂~), hydrogen peroxide (H,0,), or hydroxyl radical (OH^-). These ROS cause extreme damage to the cell as they have a high affinity for nucleic acids and proteins (Mosteitz et al., 2004). The cells evade from ROS by a class of metalloenzymes, Superoxide dismutases (SODs), which detoxify oxygen radicals by breaking them to oxygen and hydrogen peroxide then catalase or peroxidase act on H,0, and reduce it to oxygen and water (Fridovich, 1995). Fridovich (1995) and Youn et al. (1996) reported that on the basis of metal co-factors, SODs are classified into four types: manganese co-factored (Mn-SOD or SodA), iron co-factored (Fe-SOD or SodB), copper-zinc co-factored (Cu/Zn-SOD or SodC) and nickel co-factored (Ni-SOD or SodN). Another class of SODs from Fe/Mn SOD family is cambialistic SODs that can use either manganese or iron cata- lytically (Gabbianelli et al., 1995). These SODs have been reported from thermophilic photosynthetic bacterium ChJoroflextis aurantiacus, which protects the bacterium from hypoxic environment subjected to high UV radiation fluxes (Lancaster et al., 2004). Streptococcus thermophilus and Thermus filiformis are other thermophiles having cambilalistic SODs while no such SODs have been reported in Deinococcus spp. (Mandelli et al., 2013; Rrauss et al., 2015). D. radioduraus encodes Mil dependent SOD DR1279 and Cu/Zn dependent SODs, viz., DR1546, DRA0202 and DR0644 (Makarova et al., 2001) while D. geothermalis contain Mn dependent SOD Dgeo_830 and Cu/Zn dependent SOD Dgeo_0284 an ortholog of DR0644 (Makarova et al., 2007). The ROS detoxification mechanisms adopted by radiation-resistant thermophiles are described in Figure 10.3.

FIGURE 10.3 Reactive oxygen species (ROS) detoxification system in radiation resistant thermophiles.

In D. geothermalis, sugars like xylose are metabolized by a cluster of genes on megaplasmid DG574 (Ferreira et al., 1997; Makarova et al.,

2007). The plausible resistance-related functions that can be proposed for expanded families of Deinococci include hydrolases-degrade oxidized lipids; Yfit/DinB proteins-involved in cell damage related pathways (Makarova et al., 2001); subtilisn like proteases which degrade oxidized proteins (Makarova et al., 2000; Daly et al., 2007) nudixrelated hydrolases- Apn, a diadenosine polyphosphotases, form adenosine which is crucial for protection of cell from radiation and oxidative stress (Makarova et al., 2000; Daly et al., 2007). hi comparison to D. geothermalis, D. radiodurans possess extra set of genes which code for Cu-Zn SOD, a peroxidase, HslJ- like heat shock proteins and other proteins that confer antibiotic resistance (Makarova et al., 2007). D. radiodurans lacks nadABCD genes while D. geothermalis, possess orthologs of nadABCD genes, essential for biosynthesis of nicotinamide adenine dinucleotide (NAD) (Venkateswaran et al., 2000; Makarova et al., 2001). InDeinococcus geothermalis, during oxidative stress the NADPH homeostasis is modulated by FprA (Dgeo_1014), a NADP-ferredoxin reductase. This similar defense barrier is known to be present in E. coli during oxidative stress (Krapp et al., 2002). Zhang et al. (2000) reported that the formation of ROS inZ). geothermalis leads to retrieval of NADPH from carbon substrate by rechanneling central carbon metabolism. It can be done by channeling glucose to pentose phosphate pathway so that it forms NADPH instead of NADH. The NADPH is then used to accelerate the ROS scavenging activity of superoxide dismutase (SodA), KatA, and thioredoxin which ultimately lead to detoxification of ROS. D. geothermalis also produces Mil dependent superoxide dismutase, catalase, several protein repair enzymes and chaperones and also convert carbon substrates to succinate for scavenging of ROS. Thus, D. geothermalis deals with ROS by using manganese dependent enzymes. However, in the absence of manganese other ROS neutralizing metabolites, produced by carbon substrates through central carbon metabolism are used to neutralize ROS (Liedert et al., 2012).

Mn(II) ions play a crucial role in detoxification of ROS by scavenging О A protecting proteins from oxidative stress and acting as cofactors of ROS scavenging enzymes like catalases and SODs (Jakubovich and Jenkinson, 2001; Daly et al., 2007). Fe-S clusters are the most prolific and versatile enzyme cofactors, but these clusters are also a prime target of ROS (Imlay, 2008). Nachin et al. (2003) reported that there is induction of Suf Fe-S assembly protein system in nongrowing cells of D geothermalis. In oxidative stress, the proteins of this family enhance ATP- coupled assembly and repair of proteins with exposed [Fe-S]x clusters.

Liedert et al. (2012) reported that in the presence of manganese a new round of cell division is initiated in nongrowing cells of D. geother- malis. D. geothermalis, under oxidative stress expresses one class II fumarase and peptidylprolyl isomerase (Dgeo_0070), which is a protein repair enzyme and helps in reversing covalent damage to proline residues (Visick and Clarke, 1995). Thus, in nongrowing cells with limited synthesis capacity, maintenance, and repair of damaged proteins is an important metabolic activity which helps in the revival of damaged cells. In Deinococcus spp., the radiation resistance is primarily determined by protecting proteins from oxidative damage (Daly et al., 2007). Lesniak et al. (2003) reported that in D. geothermalis, the protection against oxidative stress is provided by Dgeo_0526, which is a homolog of osmotically inducible protein (OsmC). Moreover, it has been observed that following an oxidative stress the genes which encode for pyridoxine biosynthesis protein (PdxS) are upregulated. PdxS is important in synthesis of vitamin В 6 which is an effective antioxidant and a quencher of singlet oxygen (Bilski et al., 2001; Liedert et al., 2012). Deinococcus and Thermits and possess different ROS detoxification mechanisms (Omelchenko et al.,

2005). In T thermophilus, there is one SodA and one Mn dependent catalase while D. radioduraiis has Mn dependent SOD DR1279 and Cu/Zn dependent SODs, viz., DR 1546, DRA0202 and DR0644 (Makarova et al., 2001). Peptide methionine sulfoxide reductases (PMSRs) are present in D radioduraiis but absent in T. thermophilus which suggests that in T. thermophilus PMSRs are exchanged with analogous enzymes which help the organism to survive at high temperatures. T. thermophilus also lacks the proteins of Dps/Ferritin family and desiccation resistance proteins (Omelchenko et al., 2005).

In Rubrobacter spp., the damage induced by ROS is prevented by manganese-containing catalase (RradSPS_2184) which is encoded by a single gene and SOD is encoded by sodA (RradSPS_327) (Makarova et al., 2001; Terato et al., 2011; Basu and Apte, 2012). Peroxiredoxins are encoded by six copies of alkyl hydroperoxide reductase subunit С/ Thiol specific antioxidant (AhpC/TSA) (RradSPS_0148, _0515, _988, _1124, _2530, _2650) (Basu and Apte, 2012). In strain RSPS-4, the genes encoding for catalase and SODs are predicted to be encoding manganese- containing enzymes and two ABC-type Mn²7Zn²⁺ transport systems (Yuan et al., 2012). The archaeal members Tliermococcus spp. and Pyrococcus spp. are anaerobic hyperthennophiles which possess low Mn/Fe ratios as compared to their thermophilic radiation-resistant counterparts (Daly et al., 2007). This is in contrast with the model of Mn²⁺-dependent ROS scavenging for aerobic bacteria and archaea (Daly et al., 2007; Robinson et al., 2011; Slade andRadman, 2011).

However, few proteins in anaerobic microorganisms need iron, such as dehydrogenases and ferredoxin, an electron carrier that P furiosus uses instead of NAD (Jeimey and Adams, 2008; Lancaster et al., 2011; Schut et al., 2012). P furiosus contains peroxidases including rubrerythrin, alkyl hydroperoxide reductase I and II and a nonheme iron-containing enzyme called superoxide reductase (SOR) in place of SOD which catalyzes the reduction of 0,~ into H,0, (Jenney et al., 1999; Strand et al., 2010). The formation of 67 is faster in the absence of O, in comparison to its presence, which takes place in the one-step process as a free electron (e-) reacts with O, (Lin et al., 2005). Webb and Di Ruggiero (2012) reported that in anaerobic conditions, the ultrafilterates of P furiosus and T. gammatolerans displayed increased protection. I gammatolerans cope up with oxidative stress with the help of cascade of proteins or enzymes which include thioredoxin reductase (Tg0180), a glutaredoxin-like protein (Tgl302) and two peroxiredoxins (Tgl253, Tgl220) while DNA damage can be recovered by constitutively expressed nucleases Tg0864, Tgll77, Tgl631, ligases (Tgl718, Tg2005), endonucleases, AP-lyases, and glycosylases Tg0271, Tg0543, Tgll92, Tgl446, Tgl637, Tgl814 which are putatively involved in base excision repair and Tg0130, Tg0280, Tgl742, Tgl743, Tgl744 and Tg2074 are presumed to be involved in DSB repair (Zivanovic et al., 2009). In P furiosus, following an irradiation and formation of H,0, there is no increase in the expression of genes responsible for SOR pathway and these genes are normally expressed in anaerobic environments also which prove that SOR pathway can function efficiently all time (Williams et al., 2007; Strand et al., 2010; Schut et al., 2012). In anaerobic conditions, the low level of ROS formed during irradiation combine with constitutively expressed detoxification system. This system is efficiently used by hypeithennophiles like P furiosus and T. gammotoleraus to resist radiations. The dependence on this system is quite high as it circumvents the process of accumulation of Mn-antioxidant complexes in these hypeithennophilic archaea (Webb and Di Ruggiero, 2012).

Halobacterium salinarum NRC-1 (a haloarchaea) combat ROS with the help of metal halides, viz., NaBr, KC1, and KBr. These metal halides besides scavenging ROS also provide enhanced protection against carbon- ylation of protein residues and modification of nucleotides. H. salinarum

NRC-1 possess a high Mii/Fe ratio identical to that of Deinococcus radio- durans and other radiation-resistant microorganisms. Thus, radiation resistance in microorganisms is a combined function of mechanisms which provide cytoprotection, detoxification, and maintenance as well as repair of biomolecules (Kish et al., 2009).

10.3.3 OTHER MECHANISMS GOVERNING RADI A TION RESISTANCE

The other mechanisms employed by microorganisms to survive in stress conditions like osmotic shock and radiations include accumulation of compatible solutes. Trehalose and maimoglycerate are accumulated by Rubrobacter xylaiiophihis and R. radiotolerans RSPS-4 to cope up with radiations (Empadinhas et al., 2007; Nobre et al., 2008). It has been reported that there is a constitutive accumulation of compatible solutes in response to various stress conditions which effect the survival of cells (Empadinhas et al., 2007).

Cliroococcidiopsis thermalis has the ability to tolerate 5kGy dose of IR as well as it is resistant to desiccation (Billi et al., 2001). This cyanobacterium can survive in a range of habitats like water deficit conditions, microscopic fissures of weathering rocks and form biofilms at the stone- soil interface under pebbles in desert pavements (Friedmann, 1993). For survival in dehydrated and high ionizing radiation-exposed environments, it accumulates trehalose, sucrose, and replaces water of cellular components by non-reducing sugars. It also forms exocellular polysaccharides which play central role in desiccation tolerance of cells by regulating loss and uptake of water (Potts, 1999; Hoiczyk and Hansel, 2000). Dried cells in dehydrating conditions exhibited deposition of acid-, sulfate-, and beta- linked polysaccharides in cell envelope (Caiola et al., 1996a, b). Fe-super- oxide dismutase reduces risk of OH' formation in C. thermalis. It has been speculated that C. thermalis resist gamma radiation due to its ability to cope up in a desiccated enviromnent (Potts, 1999).