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Introduction

Most of the organisms that are mesophiles thrive at

optimum growth temperature (OGT) of 24-40C.

Organisms living at higher OGT of 50-70C are

thermophiles and at OGT of greater than 80C are

hyperthermophiles which include members of

domains Archaea and Eubacteria. Archaea are

probably the earliest living organisms that occupy

diverse habitats and are accordingly identified as

thermophiles, halophiles or psychrophiles etc. (Woese

et al., 1990; Doolittle, 1995; Zlatanova, 1997;

Makarova and Koonin, 2003). Since most of the

Archaea and some Eubacteria (Kreil and Ouzounis,

2001; Bao et al., 2002) live under extreme conditions,

certain characteristic features might have enabled

them to survive in these environments. Some of these

features are modifications in the metabolic pathways

of synthesis of cofactors like heme, acetyl CoA, acyl

CoA, and folic acid which are either greatly reduced or

are eliminated in thermophiles because of constrains

of high temperature. This trend is seen in Archaea

61Journal of Cell and Molecular Biology 4: 61-69, 2005.

Hali University, Printed in Turkey.

Nucleic acid stability in thermophilic prokaryotes: a review

Seema Trivedi1*, Satyawada Rama Rao2 and Hukam Singh Gehlot2

1Department of Zoology, J N Vyas University, Jodhpur (Raj.), India 2Department of Botany, J N Vyas

University, Jodhpur (Raj.), India (*author for correspondence)

Received 09 February 2005; Accepted 11 May 2005

Abstract

In order to survive at temperatures of 60C, thermophilic prokaryotes (Archaea and Eubacteria) have adopted

different strategies. These strategies include high CG content in the coding sequences, nucleotide arrangement of

purine-purine and pyrimidine-pyrimidine, methylation of nucleotides, histone/histone like proteins, reverse gyrase,

cations, etc., also provide thermal stability to genome. Strategies adopted at the level of DNA are naturally, though

not universally, reflected in RNA in thermophiles. Increased purine load (particularly of adenine), preferential codon

usage, post-transcriptional modifications etc. provide thermal stability. All these factors may differ from taxa to taxa

as no single or all the factors together can be universally attributed for providing thermal stability to nucleic acids.

Key Words: Archaea, thermophiles, stability, CG content, reverse gyrase

Termofilik prokaryotlarda nkleik asit kararll

zet

600C ve zerindeki scaklklarda, termofilik prokaryotlar (Archaea ve Eubacteria) farkl stratejiler uygularlar.

Kodlayan dizinlerde CG ierii, purin purin ve pirimidin pirimidinlerin nkleotiotid dzenlenmesi, nkleotidlerin

metilasyonu, histon / histon benzeri proteinler, ters giraz, katyonlar v.b. stratejiler genoma termal kararllk salarlar.

DNA seviyesinde uygulanan stratejiler, doal olarak, niversal olmamasna ramen, termofillerde RNA ya yansr.

Artan purin yklemesi (zellikle adenin), tercihli kodon kullanm, post transkripsiyonel modifikasyonlar termal

kararll salarlar. Btn bu faktrler, tek olarak veya bir arada nkleik asitlere termal kararllk

salayabileceinden durumdan duruma farkllk gsterebilirler.

Anahtar szckler: Archae, termofiller, kararllk, CG ierii, ters giraz

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growing at OGT around 60C and enhances with the

increase in OGT 83-85C and 95-100C

(Kawashima et al., 2000). In this review we focus on

features which have rendered thermostability to

Archaeal and Eubacterial DNA and RNA with

emphasis on the former. This is because only a few

thermophilic Eubacteria have been studied in this

regard. Further, the term thermophile in this review is

being used of both thermophile and hyperthermophile

prokaryotes unless specified as thermophilic an

hyperthermophilic Archaea or Eubacteria respectively.

DNA

Protection of DNA would be essential, especially athigh temperature, to ensure not only integrity of

genetic information but also to carry out metabolic

functions. Thermophilic Archaea and Eubacteria may

have different nucleotide ratio, nucleotide preference,

flexibility, modification of bases, and association with

histone/histone like proteins as compared to the ones

living at more moderate temperatures. These strategies

may not necessarily work in isolation but may work in

synergistic way.

One school of thought believes that higher CG

content in genome particularly in the protein coding

regions would be an essential feature to protect DNAfrom thermal extremes even at 83-100C OGT or

beyond (Gorgan, 1998; Kreil and Ouzounis, 2001;

Bao et al., 2002; Singer and Hickey, 2003). However,

amongst prokaryotes living at high temperature, there

are differences in CG content. It has been found that

thermophiles have high CG content more than that of

mesophiles but it is less than that of

hyperthermophiles (Kreil and Ouzounis, 2001). On the

contrary, other investigations suggest that CG content

is not as high as expected in the thermophile genome

except for in t-RNA and r-RNA sequences (Gorgan,

1998; Galtier et al., 1999; Kawashima et al., 2000;

Nakashima et al., 2003). From the above reports itappears that though the CG content may be high in

coding regions of the genome it may not be high in the

intergenic regions and therefore possibly the total

genome does not show consistent pattern of high CG

content. At the same time there may be exceptions to

the high CG contents in the coding region, which is

evident from the report by McDonald et al., (1999)

where they do not find any symmetric and consistent

pattern of high CG content particularly in protein

coding genes in thermophiles. Moreover, it is

suggested that at high temperature, it is not the

question of CG content and denaturation but

thermodegradation that is important against which

thermophiles need protection (Lobry and Chessel,

2003). Possibly the report on marked differences in

relatively low CG content of Archaea living in cold

environment and high CG content i

hyperthermophiles tilts the balance in favor of high

CG content being an important factor in protection of

DNA (Saunders et al., 2003). However, there is no

uniform pattern of high CG content in the

thermophiles, it is apparent that it may be important

but CG content alone cannot be responsible for

providing thermal stability to DNA.So if it is not the CG content (at least, not in all

cases), there must be other features for thermal

protection of the genetic material. One such feature is

the effect of cations like potassium ions, and

polyvalent cations in particular that neutralize the

nucleic acid in order to maintain double stranded

structure and provide thermal stability irrespective of

the nucleotide ratio particularly in Archaea which

grow at 60C OGT or above (Gorgan, 1998;

Kawashima et al., 2000). Probably differences in

methylation of the nucleotides may also provide

thermo protection. This is evident in report where ithas been seen that in order to protect DNA from

temperature induced mutations, thermophiles

generally have N4-methylcytosine (m4C) instead of 5-

methylcytosine (m5C) and N6-methyladenine (m6A)

(Ehrlich et al., 1985). This study further point to the

fact, that if the methylation patterns are different in

thermophiles they may have a different type of DNA

methylase to enable this difference. This is evident

from the study that reports presence of DNA

(Cytosine-N4) methyltransferases only in prokaryotes

(Bujinicki and Radlinsks, 1999); however, the

evolutionary significance of this enzyme is still

illusive. Since numbers of methylated nucleotides arenot different in mesophiles and thermophiles (Gorgan,

1998) it is apparent that it is not the number of

methylated nucleotides but the position that may be

important for thermal stability.

In addition to the above factors, it is also possible

that arrangement of nucleotides may render

thermostability irrespective of CG content. DNA of

hyperthermophiles has higher frequencies of purine-

purine and pyrimidine-pyrimidine composition, which

62 Seema Trivedi et al.

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renders least flexibility to DNA and probably provides

thermostability as compared to other organisms. This

kind of nucleotide arrangement is reported to increase

with increasing OGT above 60 C (Kawashima et al.,

2000) and may also help in supercoiling of DNA

affecting thermostability of DNA (Nakashima et al.,

2003). Another feature that is common in most of the

hyperthermophile Archaea and Eubacteria and not in

mesophiles is the presence of reverse gyrase which

provides relaxed to positive supercoil to DNA. Its

presence probably helps in replication as well as in

gene regulation at high temperature (Napoli et al.,

2001; Bao et al., 2002). A nonspecific DNA-binding

protein, Smj12, has been found to additionally help in

positive supercoiling of DNA (Napoli et al., 2001).Contrary to this inPicrophilus torridus (Futterer et al.,

2004) and T. volcanium (Kawashima et al., 2000)

reverse gyrase gene has not been found. T. volcanium

has gyrase and topoisomerase I and does not have

chaperons like DnaK, DnaJ and GrpE. It also has a

negative supercoil unlike other thermophilic Archaea

(Kawashima et al., 2000). Absence of these proteins in

T. volcanium is an exception to other thermophilic

Archaea adopting strategies of alternate DNA folding.

This could also be because of the fact that T.

volcanium and Picrophilus torridus OGT 60C, is the

least amongst the Crenarchaeota. Therefore, it cannotbe categorically stated that reverse gyrase and

chaperone proteins are universally employed for

thermal protection of DNA.

It is possible that transient binding/association of

histone and non-histone proteins with DNA may

additionally provide thermostability (Peak et al., 1995,

also refer to review by Gorgan, 1998 and White,

2003). This gets support from studies on Archaeal

histones HMf and HTz that wrap DNA in positive

supercoil at high salt concentrations but induce

negative supercoiling at low salt concentrations,

whereas, MkaH induces negative supercoil under

different salt concentrations. Such alternate packingmay provide physiological advantage to Archaea even

if it is salt concentration dependent rather than

temperature dependent (Sandman et al., 1994;

Musgrave et al., 2000). Probably this transient type of

supercoil may vary at the time of gene activity/lack of

it and may protect DNA at high temperature as only

part of it would be exposed during gene activity. To

alter the salt concentrations, special control of ion

channels in the plasma membrane would be essential.

Further studies would probably reveal the precise

signal that would trigger this control. Irrespective of

differences in ion concentrations, histones or histone

like proteins (e.g. Crenarchaeota lack histone proteins

but have Alba protein) contribute in folding and

unfolding (Bailey et al., 2002) and thermal protection

of DNA (White, 2003; Weng et al., 2004). In this

regard, report suggest that six different histones inM.

jannaschii may be involved with different sequences

in transient manner, which would be helpful in gene

regulation especially at high temperature where there

is need for thermal protection of DNA (Bailey et al.,

2002). Besides, these proteins, single strand binding

proteins (SSB) have also been found to stabilize DNA

at elevated temperature. The roles of DNA stabilizingproteins have been extensively reviewed by White

(2003). The author further elaborates that though SSB

is present in all the three domains, the thermal stable

Archaeal SSB share partial homology with Eukaryotes

and Eubacteria. However, SSB is not found in

Pyrobaculum aerophilum and hence, the author

suggests possibility of other proteins that are similar in

function to SSB that may provide thermal protection

to DNA in this organism. These reports clearly

indicate that histones or histone like proteins are one

of the essential factors that provide thermal protection

to DNA irrespective of CG content of genome. Sincethese proteins are also present in mesophiles, the

difference in the protective mechanism of these DNA

associated proteins in thermophiles is not clear. These

reports focus on thermophilic and mesophilic Archaea

and Eubacteria and show differences in CG contents

of hyperthermophiles and mesophiles but since no

specific difference between Archaea and Eubacteria

have been indicated, it can be speculated that there are

no significant difference regarding this strategy

amongst the two domains.

Despite the protection provided by the aforesaid

factors, DNA in thermophiles is susceptible to

damages. DNA damage in hyperthermophiles may becaused at higher rate as compared to other organisms

due to hydrolytic deamination of cytosines and

adenosines, hydrolytic depurination or oxidation of

guanine, methylation of bases and strand break at high

temperature (Gorgan, 2000; Yang et al., 2000).

Therefore, it can be speculated that repair systems

should be taking care of these damages. Several

proteins employed for DNA damage repair in

mesophiles are also found in thermophiles with some

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exceptions. Homologues of Eukayal/Eubacterial

recombination repair, DNA polymerase and SSB

proteins associated with repair activities are reported

in thermophilic porkayotes (reviews by Gorgan, 1998,

2004; White, 2003). Crearchaeal SSB proteins are

unique in the sense that they share high level of

sequence similarity with Eukaryal SSB OB fold but

the tail is similar to Eubacterial SSB. However, in

pyrobaculum aerophilum genes for SSB have not been

found. It is speculated that if they are present, they

would be very different from SSB of other

thermophilic Archaea (White, 2003). Possibly some

unique proteins present in thermophiles and in

particular in hyperthermophilic Archaea are employed

for damage repair. In this regard unique repair-associated mysterious proteins (RAMPs) believed to

repair DNA damage have been found in only

thermophiles (Makarova et al., 2002) besides other

repair enzymes which are also found in eukarya

(Klenk et al., 1997). It is not known however whether

this unique repair protein enables DNA repair only

during stress conditions especially at elevated

temperatures. It is believed that horizontal gene

transfer may be responsible for similarities found in

thermophilic and hyperthermophilic Eubacteria and

Archaea regarding the RAMPs (Makarova et al.,

2002). Amongst several putative proteins predicted tobe involved in repair besides RAMP, two subunits of

nucleotidyltransferase in Archaea raise special interest

because of speculation of their being involved in

thermal adaptation besides DNA repair (Makarova and

Koonin, 2003). Besides these proteins, O6

alkylguanine-DNA alkyltransferase activity and not

alkylphosphotriester-DNAalkyltransferase activity for

DNA damage repair in the Archaea is reported. It may

provide thermal protection to DNA by binding

strongly and co-operatively to the double stranded

DNA. This enzyme is more efficient in thermophiles

than in mesophiles and the efficiency increases with

rise in temperature (60-100C) (Skorvaga et al.,1998). Information gathered form studies done so far

on DNA repair systems in thermophiles indicate

absences of nucleotide excision repair (NER) and

mismatch repair (MMR) system in thermophiles and

in particular in Archaea. However, in some

thermophiles, the repair pathways may be present that

may not be complete like mesophiles (Fitz-Gibbon et

al., 2002; Gorgan, 1998, 2000, 2004) with some

exceptions (White, 2003). In hyperthermophilic

archaeon Pyrobaculum aerophilum, putative U/G and

T/G mismatch-specific glycosylase (Mth-MIG) has

been found. Since it is not present universally, it is

believed that Archaea not having MIG would be

employing other methods to repair U/G and T/G

mismatch (Yang et al., 2000). In thermophiles where

nucleotide excision repair pathway is found, it

highlights the differences between the two domains

living in extreme conditions. This repair pathway

(UVrABC) is complete in thermophilic Eubacteria.

NER pathways in hyperthermophilic Archaea do not

share any homology with Eubacteria but do so with

Eukaryal pathway. However, thermophilic Archaea

Methanobacterium thrmoautotrophicum exhibits

intermediate situation where it shares homology of therepair system both with Eubacteria and Eukarya

(Gorgan, 2000; White 2003). In Crenarchaea (which

lack UVrABC), orthologues of Eukaryal helicase and

nuclease are implicated in NER (White, 2003; Gorgan,

2004). The latter author suggests that either

hyperthermophilic Archaea have systems to avoid

mutations or have efficient replication system which

reduces requirement for other repair systems eg.

Sulfolobus spp. In hyperthermophiles, B-family

polymerase can detect uracil in DNA and stalls. How

it repairs or recruits repair machinery is not known

(Gorgan, 2004). Since complete NER and MMRsystems are present in thermophilic Eubacteria, it

appears that hyprethermophilic Archaea are different

from thermophilic Eubacteria and have alternative

strategies for excision repair of DNA.

In addition to these factors, photolyase gene

activity amongst Archaea has been reported only in

Sulfolobus solfataricu, Methanobacterium

thermoautotrophicum (Gorgan, 2000) andPicrophilus

torridus (Futterer et al., 2004). This gene does not

share similarity with any other microbe (Skorvaga et

al., 1998) except forPicrophilus torridus photolyase

which shows similarity with bacterial homologues

(Futterer et al., 2004). Since photolyase appears to beuncommon amongst Archaea, it cannot be considered

as a possible universal means of DNA repair in

thermophile and hyperthermophilic Archaea but may

be universal in thermophilic Eubacteria. However,

photolyase mediated transleasion repair mechanism in

thermophilic Archaea remains illusive. In

hyperthermophile Methanopyrus kandleri unique type

IB topoisomerase with DNA repair activities has been

found. Additionally, in hyperthermophilic Eubacteria


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and Archaea (Thermotoga maritima and

Archaeoglobus fulgidus) respectively, uracilDNA

glycosylases (UDG) differ significantly from the other

Eubacterial and Eukaryal UDG enzymes at the amino

acid sequence level. It is believed that the

thermophiles make efficient usage of genetic material

by having multiple activities in one enzyme

(Sandigursky and Franklin 1999; Belova et al., 2001).

High number of the exonuclease family and the

accumulation of PIN-domain are reported in

thermophilic Archaea and Eubacteria. This

augmentation is believed to help in DNA and RNA

repair, which is required more due to damages caused

at elevated temperature. It is suggested that increase in

PIN doamains in thermophile prokaryotes is tocompensate and substitute lack of repairs systems in

them. However, reasons for increase in number of

exonuclease in mesophiles remain illusive (Arcus, et

al., 2004).

Recombination repair systems are also reported to

protect DNA at elevated temperatures as recA

homologous to radA have been found to repair double

stranded breaks inPyrococcus furiosus even at 95C

(Di Ruggiero, 1997, White, 2003; Gorgan, 1998,

2004). Additionally it is also possible that some of the

repair mechanisms are not unique in thermophiles.

This is evident from the report on endonuclease IV (anapurinic/apyrimidinic endonuclease and repair

diesterase) from Thermotoga maritima that shows

significant similarity in structure and function with

Escherichia coli enzyme. However, significant

difference is found in thermal stability where T.

maritima enzyme is stable at 88C butE. coli enzyme

begins to denature at 70C (Haas et al., 1999).

Therefore, it appears that it is the thermal stability of

repair systems that may be important at least in

thermophilic Eubacteria in addition to unique repair

systems (Gorgan, 1998). Many homologues of

Archaeal repair systems in eubacteria and eukaryotes

have been found (see Aravind et al., 1999 for details),it is still not known whether they function in the same

manner in thermophilic Archaea. We agree with the

proposal by Gorgan (1998) that possibly

hyperthermophiles do not need efficient repair

systems at the cost of vital cellular functions. The

author also suggests that mutations due to replication

or inefficient repair system may be beneficial to these

organisms living under stressful conditions.

The common underlying feature(s) believed to

provide thermostability to DNA in thermophilic

Archaea and Eubacteria thus appear to be high CG

content, difference in methylation of nucleotides,

cations, histones/ histone like proteins, SSB and

flexibility of DNA affected by nucleotide

arrangement. However, differences in thermophilic

Archaea and Eubacteria show that thermophilic

Archaea have yet unknown system to avoid DNA

damage specially by UV radiations, absence of

UVrABC system and MMR that is not present in

thermophilic Eubacteria and Archaea still keeps the

research area open to look features that are essential

for DNA repair.

RNA

In order to survive the high temperature conditions,

thermophiles have adopted strategies at the level of

transcription too. Investigations suggest that high CG

content and external factors like high temperature may

influence the usage of codons in RNA of thermophiles

(Lobry and Chessel, 2003; Singer and Hickey, 2003).

Generally thermophiles have purine rich genome

and it is speculated that purine load is preferred

probably for efficient transcription of m-RNA and to

avoid unnecessary double strand interaction. This is

evident from study where purine loading is stillpreferred even in those thermophiles which have low

C/G content which is not found in non-thermophiles

(Lao and Forsdyke, 2000). This is supported by

findings of purine richness in genomes of

thermophiles (Lobry and Chessel, 2003) particularly

in most of the m-RNA except for short m-RNA(Paz et

al., 2004). Comparison between thermophiles and

hyperthermophiles does not show significant

differences in purine richness except on excluding

Aeropyrum pernix. This may be due to high CG ratio

in this organism (Paz et al., 2004). However, in

hyperthermophilic Pyrobaculum aerophilum long

tracts of purines have been found in coding andnoncoding regions. Hence, it is suggested that purine

richness could not be of significant adaptive value, as

it is not preferably present in coding regions alone

(Fitz-Gibbon et al., 2002). Contrary to this, it is

suggested that purine rich RNA is preferred in

thermophiles and particularly adenine is preferred as

compared to guanine in t-RNA and r-RNA besides m-

RNA. This preference is in order to avoid undesirable

bonds with other RNA and specially to avoid any


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competition with t-RNA anticodon. Purine richness

may be helpful in high transcription rate of many m-

RNA from pyrimidine rich template and may be more

stable to spontaneous hydrolysis at high temperature

(Paz et al., 2004).

It is also evident that besides nucleotide content of

a genome, the second most important factor is the

optimal growth temperature, which decides the

preference for codon usage. It has been found that

thermophiles have a different preference for

synonymous codon usage from that of mesophiles

(Lobry and Chessel 2003) e.g. arginine (AGR and

CGN) and isoleucine (ATH) etc (Lynn et al., 2002,

Singer and Hickey, 2003). This must be primarily for

not only ensuring stable pre-mRNA transcript andmRNA but also for translation at high temperatures.

But additionally it appears that tRNA abundance may

also play an important role in preferential codon

usage. Possibly some elusive selective forces at higher

temperatures promote this kind of nucleotide content

in genome (Lynn et al., 2002, Singer and Hickey,

2003). Thermophiles are expected to avoid of 5-UpA-

3 and 5-CpA-3 phosphodiester bonds in RNAs but

this has not been observed in thermophilic

prokaryotes. Therefore, it appears that selective

pressures have not acted against these bonds even

though they are susceptible to hydrolysis. It is possiblethat these thermophilic prokaryotes either do not need

to avoid these bonds or there may be other mechanism

that prevents hydrolysis or stabilises RNA (Lobry and

Chessel, 2003).

Survival strategies extend to provide thermal

stability to t-RNA and in particular to aminoacyl t-

RNA which is labile at high temperatures but not at

lower temperatures (Stepanov and Nyborg, 2002), and

possibly several factors help in stabilizing RNA in

thermophilic Eubacteria. These factors are high CG

content, short length of RNA, increase in hydrogen

bonds in helices, minimizing alternative folding,

additional Watson-Crick base pairs at the base of stemloop and shortened connections between helices

(Brown et al., 1993). These factors may be important

in thermophilic Archaea also. However, it has been

observed that higher content of dihydrouridin is

present in thermostable t-RNA of other thermophiles

but is underrepresented in Archaea. Therefore, at least

in most thermophilic Archaea, dihydrouridin may not

be important for t-RNA stability at high temperature.

In fact, its absence is considered to provide

thermostability in Archaea. It has also been reported

that post-transcription modified nucleotides e.g. 2-

thiolation of T (54) and 5-Methylcytidine contribute

directly to the thermostability of tRNA species

(Edmonds et al., 1991; Yokoyama et al., 1987; Lobry

and Chessel, 2003). This type of post-transcriptional

modification may be universally employed by

thermophilic Archaea and Eubacteria. Besides this,

archaeosine, a derivative of 7-deazaguanosine is

exclusively present in the D-loop of Archaea t-RNA

(Watanabe et al., 1997) amongst other unique

derivatives (refer for details to Edmonds et al., 1991).

In addition to these factors, high number of ribose

methylated nucleotides and presence of Mg ions, help

in stabilization of t-RNA (Edmonds et al., 1991;Kowalak et al., 1994). Like t-RNA, thermostability in

r-RNA and other non-coding RNA (ncRNA), though

not universally, is probably achieved by double

stranded douplex, post-transcriptional modifications

and higher CG content, which leads to increased

hydrogen bonding (Bao et al., 2002; Klein et al., 2002;

Nakashima et al., 2003; Lobry and Chessel, 2003).

Reports on contribution of high CG content being

important for thermal stability is contradicted by the

finding that report no significant differences in the CG

content in t-RNA of cold-adapted Archaea and

thermophiles. Therefore, it appears that additionalfactors provide thermal stability to t-RNA at high

temperature (Saunders et al., 2003). It is also possible

that rigidity rendered to t-RNA in thermophiles

provides protection again thermal damages and hence

in some cases there are no differences in CG contents

of thermophiles and psychrophiles. In the studies

carried out so far, since only few thermophile

Eubacterial genomes have been included, it is not

possible at present to deduce whether all thermophilic

Eubacteria have adaptations that are similar to

Archaea.

Conclusion

From the forgoing information it is apparent that

thermophilic Archaea and Eubacteria have adapted to

high temperature conditions or possibly because of

their unique features they have been able to survive in

these conditions. The adaptations have happened in

DNA, RNA which is naturally reflected in translation.

Whether evolutionary forces induced changes in DNA

that are reflected in expression or whether it was the


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necessity of undergoing modification at the expression

level that the DNA had to undergo changes still

remains debatable. However, it is clear that factors like

CG content, histones/histone like proteins,

methylation, post-transcriptional modifications,

cations etc. either alone or in combination help in

nucleic acid stability even though CG content may or

may not be significantly different in mesophiles and

thermophiles. Differences in repair systems between

the two domains also suggest that both NER and

MMR systems are either absent or have not been

detected in hyperthermophilic Archaea. We agree with

Gorgan (1998) suggesting a possibility of

evolutionarily and functionally divergent repair

pathways in hyperthermophilic Archaea that do notshow similarity with well known established systems.

So far only few thermophilic Eubacteria have been

chosen for comparison with thermophilc Archcaea

which have revealed few differences between the two

domains regarding the strategies used for

thermoprotection. The situation is compounded by the

fact that the two thermophilc Eubacteria used for

studies, are considered close to Archaea in lineage and

appear to share many genes with Archaea possibly due

to horizontal gene transfer. In future, understanding

and discoveries in more thermophilic bacteria (if more

exist) would probably help in comparing the strategiesof the two domains. These differences and similarities

may help in understanding the specific strategies that

are common and necessary for surviving at high

temperature.

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