Journal of Photochemistry and Photobiology B: Biology 92 (2008) 110–116

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Journal of Photochemistry and Photobiology B: Biology

journal homepage: www.elsevier .com/locate / jphotobiol

Biological responses to the simulated Martian UV radiation of bacteriophagesand isolated DNA

Andrea Fekete a,*, Gáspár Kovács b, Márton Hegedüs a, Károly Módos a, Helmut Lammer c

a Institute of Biophysics and Radiation Biology, Semmelweis University, H-1088 Budapest, Puskin u. 9, Hungaryb MTA-SE Research Group for Biophysics, Associated to TKI, H-1088 Budapest, Puskin u. 9, Hungaryc Department for Extraterrestrial Physics, Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042 Graz, Austria

a r t i c l e i n f o a b s t r a c t

Article history:Received 31 January 2008Received in revised form 13 May 2008Accepted 13 May 2008Available online 22 May 2008

Keywords:Simulated Martian UV radiationDNABacteriophageUV damageQPCRUV photoproducts in bacteriophage/isolatedDNA

1011-1344/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.jphotobiol.2008.05.007

* Corresponding author. Tel.: +36 1 267 6261; fax:E-mail address: fekete@puskin.sote.hu (A. Fekete).

Mars is considered as a main target for astrobiologically relevant exploration programmes. In this workthe effect of simulated Martian solar UV radiation was examined on bacteriophage T7 and on isolated T7DNA. A decrease of the biological activity of phages, characteristic changes in the absorption spectrumand in the electrophoretic pattern of isolated DNA/phage and the decrease of the amount of PCR productswere detected indicating damage of isolated and intraphage T7 DNA by UV radiation. Further mechanisticinsights into the UV-induced formation of intraphage/isolated T7 DNA photoproducts were gained fromthe application of appropriate enzymatic digestion and neutral/alkaline agarose gel electrophoresis. Ourresults showed that intraphage DNA was about ten times more sensitive to simulated Martian UV radi-ation than isolated T7 DNA indicating the role of phage proteins in the DNA damage. Compared to solarUV radiation the total amount of DNA damage determined by QPCR was about ten times larger in isolatedDNA and phage T7 as well, and the types of the DNA photoproducts were different, besides cyclobutanepyrimidine dimers (CPD), double-strand breaks (dsb), and single-strand breaks (ssb), DNA-protein cross-links were produced as well. Surprisingly, energy deposition as low as 4–6 eV corresponding to 200–400 nm range could induce significant amount of ssb and dsb in phage/isolated DNA (in phage the ratioof ssb/dsb was �23%/12% and �32%/19% in isolated DNA). 5–8% of the CPD, 3–5% of the AP (apurinic/apy-rimidinic) sites were located in clusters in DNA/phage, suggesting that clustering of damage occur in theform of multiple damaged sites and these can have a high probability to produce strand breaks. Theamount of total DNA damage in samples which were irradiated in Tris buffer was reduced by a factor�2, compared to samples in phosphate buffer, suggesting that some of the photoproducts were producedvia radicals.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The study of solar ultraviolet (UV) radiation is of extremeimportance in a wide range of scientific disciplines and the effectof UV radiation on different organisms has received increasingattention [1]. Among the different hypotheses put forward to ex-plain the origin of terrestrial life, the delivery of exogenous organicmolecules to the primitive Earth is now considered as the mostpromising theory. Recently the panspermia theory was rejuve-nated since bacteria and bacterial spores are able to resist to spaceconditions [2–4], their transfer between planets via meteoritesseems possible. Another alternative to this theory is the transferof informational macromolecules such as DNA or RNA in place ofliving organisms [5]. Until life is demonstrated outside Earth, thereis no way to directly test the panspermia hypothesis. It is plausible,

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+36 1 266 6656.

however, to accumulate circumstantial evidence by studying theeffect of space conditions on nucleic acids and simple microorgan-isms. For example, the current candidate planet for possibly har-boring microbial life is Mars. Owing to the oxygen-poor Martianatmosphere, the total solar UV flux at the surface of Mars is similarto that of Earth, but the solar UV spectrum at the Martian surface isrelatively enriched in the shorter, more biologically damaging UVwavelengths (200–315 nm) [6]. UV radiation is damaging to anumber of key macromolecules, particularly to DNA. Microbesare always a consideration for missions to Mars, from concernsabout contaminating Mars with Earth (terrestrial) organisms, tosearching for extinct or extant life on Mars, to concerns that theremight be life on Mars which would subsequently contaminateEarth [7,8].

To explain the non-detection of organic material to a detectionlevel of several parts per billion (ppb) by the Viking Landers [9,10],several hypotheses have been suggested, including the intenseMartian UV flux. The Martian solar UV environment has previously

A. Fekete et al. / Journal of Photochemistry and Photobiology B: Biology 92 (2008) 110–116 111

been investigated theoretically revealing a surface environmentexposed to UV radiation at wavelengths as short as 200 nm [11–13]. Therefore the biological response of different organisms inthe 200–400 nm regions needs to be known to evaluate accuratelythe true biological effect of the solar UV environment on Mars.

In intact cells the most important target for UV radiation is theDNA folded into the compact structure of chromatin; hence it isimportant to understand the relationship between DNA damageand DNA-protein interaction. In this paper the effect of simulatedMartian solar radiation was examined on phage T7 and isolatedT7 DNA. Phage T7 contains a double-stranded DNA packed in asuperhelical structure between a protein-shell and core [14,15]that resembles the chromosomal DNA arrangement in mammaliancells. Thus, it can be used as a chromosome model from the point ofview of UV damage induction. Phage T7 was used as a biologicaldosimeter which weights the different components of UV radiationaccording to their efficacy and records the accumulated dose [16–18]. The biological activity of phage T7, characteristic changes inthe absorption spectrum and in the electrophoretic pattern ofDNA/phage and the decrease of the amount of PCR products havebeen detected indicating the UV damage of isolated and intraphageT7 DNA. The type of DNA damage was determined by using a com-bination of enzymatic probes and neutral/alkaline agarose gelelectrophoresis.

2. Methods

The preparation of bacteriophage T7 was carried out accordingto the method described in [19], while the isolation of T7 DNA ispublished in [18]. Absorption spectra and difference spectra wererecorded with a UNICAM UV/VIS spectrophotometer (equippedwith Vision 3.4 software). The absorption spectra were correctedfor the scattered light applying the relation of E = kk�n with empir-ical fitting for k and n, where E is the extinction, (OD), of phage sus-pension/isolated DNA solution at given wavelengths, k and n arefitting parameters obtained from the least square method [20].

The Martian UV simulator, which was constructed in the Insti-tute of Space Research in Graz, simulates the anticipated present-day Martian surface spectrum between 200 and 400 nm [21]. Thephage T7 suspension and isolated T7 DNA solution (20–50 ng/ll)in M9 (20 mM NH4Cl, 40 mM Na2HPO4, 20 mM KH2PO4, 1 mMMgSO4, pH 7.0) or Tris (10 mM Tris, 1 mM ethylenediaminotetra-acetic acid (EDTA), pH 7.4) buffer was irradiated in 3 ml quartzcuvettes with continuous mixing, after each dose the differencespectrum was measured and 200 ll samples were removed andanalyzed for photoproduct formation, in each irradiation seriesthree parallel measurements were performed at each dose.

The viability of T7 phage was determined in M9/Tris buffer bymeasuring the number of active phages in treated/untreated sam-ples using E. coli B host cells on agar plates [22]. In order to set freeintraphage DNA from the phage capsid, bacteriophage T7 washeated to 65 �C for 30 min before digestion/electrophoresis [14].The presence of double-strand (ds) breaks, single-strand (ss)breaks, cyclobutane pyrimidine dimers (CPD) and apurinic/apyrim-idinic (AP) sites in intraphage/isolated T7 DNA was determinedusing a combination of enzymatic probes and alkaline-agarosegel electrophoresis [23]. To detect CPD, aliquots of isolated or lib-erated T7 DNA was first digested with phage T4 endonuclease V(Endo V, Epicentre Technologies), which cleaves the phosphodies-ter backbone 50 to CPD. To detect apurinic/apyrimidinic (AP) sites,DNA was first digested with Endonuclease IV (Endo IV, EpicentreTechnologies); the ratio of CPD and AP that are in clusters wasdetermined by neutral agarose gel electrophoresis. Isolated DNA/liberated DNA were treated with an endonuclease that induces asingle-strand cleavage at a CPD or abasic site. If there are two clo-

sely spaced damages on opposing strands, such cleavage will re-duce the size of the DNA on a nondenaturing gel. Uponelectrophoresis in nondenaturing conditions, dsb decrease theaverage length of a population of DNA molecules; such changesin average molecular length can be quantified by number averagelength analysis [24]. Neutral agarose gel electrophoresis of DNAwas performed by standard techniques in TBE buffer (89 mMTris-HCl, 89 mM boric acid, 2 mM EDTA, pH = 8), 1 kB DNA ladderor k HindIII fragments were used as size standards [25]. In orderto detect ss breaks generated in DNA/phage either directly by UVradiation or as a result of Endo V or Endo IV cleavage at CPD orAP sites, DNA was denatured with 300 mM NaOH (final concentra-tion) and electrophoresed through 1% alkaline agarose gels. The gelwas soaked in a solution of 30 mM NaOH, 1 mM EDTA to convertthe DNA to single-strand form; this alkaline solution was also usedas running buffer [25]. Gels were stained with ethidium bromide,the fluorescence was excited using a Foto UV transilluminatorand the gel was photographed with a Canon Powershot PRO1 dig-ital camera under UVA irradiation. The camera was equipped witha Gelb orange B + W filter absorbing >90% of the exciting light. Theamount of ss, ds breaks, CPDs and AP sites was calculated from theintegrated areas under the peaks. The digital image was processedby our own program developed on the base of [23]. This analysisprogram converts darkening to DNA concentration by comparisonwith the mobility of DNA standards of known molecular length andcalculates the number average molecular length. By comparingthese values of the treated and untreated DNAs, the frequency ofbreaks can be calculated [23]. The amount of DNA-protein cross-links in the dissolved phages was determined by the method ofZhitkovich and Costa [26] using sodium dodecyl sulphate (SDS)and potassium chloride. The amount of SDS-precipitable DNA rep-resented a measure of protein cross-links.

To quantify isolated and intraphage DNA lesions as a result ofsimulated Martian UV radiation, a quantitative PCR method (QPCR)developed earlier was used based on the amplification of 555 bp(base pair) and 3826 bp segments of T7 DNA, the details of theamplification method and the quantitation of the DNA lesions aredescribed in Hegedüs et al., [16].

3. Results

3.1. Spectroscopic results

The effect of simulated Martian UV radiation on T7 phage andisolated T7 DNA was followed by changes in the UV absorptionspectrum; in order to enhance the relatively small effect in ODwe measured the difference spectra of the irradiated solutionagainst the unirradiated solution. As an example Fig. 1 shows theUV difference spectra of isolated T7 DNA (A) and phage T7 solution(B) under the effect of different doses of simulated Martian UVradiation. The decrease in OD near 270 nm due to the saturationof the 5, 6-bond of nucleotide bases indicates the production ofCPD, (6-4) PD and photohydrates, while in the DNA differencespectra the increase in the OD at 315 nm indicates the productionof (6-4) PD [27]. This effect cannot be seen in the spectrum ofphage; most likely because the much larger decrease near270 nm covers it. To compare the changes occurring in DNA andphage solution we choose the change of OD at 270 nm for the char-acterization of the DNA damage by UV irradiation. Fig. 2A showsthe OD changes at 270 nm (in%) as a function of incident UV dosein phage T7 suspension and in isolated T7 DNA solution, bothcurves show a saturation tendency. The explanation for the photo-steady state could be that under the influence of the short wave-length UV radiation present in the polychromatic spectrum notonly the bipyrimidine photoproducts (mostly CPD and (6-4) PD)

Fig. 1. (A) Difference spectra of isolated T7 DNA under the effect of simulatedMartian atmosphere, unirradiated DNA solution was used as reference. The data arethe averages of four measurements. (B) Difference spectra of phage T7 under theeffect of simulated Martian atmosphere, unirradiated phage T7 solution was used asreference. The data are the averages of four measurements.

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Fig. 2. (A) The change in OD at 270 nm (in%) as a function of the incident dose ofsimulated Martian radiation in phage T7 and in isolated T7 DNA in Tris buffer. (B)Photoinactivation of phage T7 by simulated Martian radiation in M9 and Tris buffer.

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are induced, but the reversion of them can be also provoked by thesame photons. Nevertheless, in nucleic acids/nucleoproteins themeasured OD change at 270 nm originated from the competinghypochromicity due to the decrease of pyrimidine monomers,and characteristic for the formation of CPD and (6-4) PD, and fromhyperchromicity due to the photoreversal of dimers to monomersand to the partial denaturation of the DNA structure as a result ofthe photoproducts. Therefore, the OD changes are not the directmeasure of the photoproduct production; they reflect the changesin the nucleotide base stacking that is the resultant of the twoopposite effects. It must be mentioned, that the measured spectralchanges in irradiated isolated DNA and in phage T7 were signifi-cantly larger in M9 buffer, than in Tris buffer.

3.2. Biological activity measurement

Fig. 2B shows the decrease of biological activity (ln N/No) ofphage T7 in M9 and Tris buffer; respectively, the decrease in bio-logical activity in Tris buffer is significantly smaller. The initial partof the curves is exponential in function of dose in both cases, indi-cating that biological inactivation is a single hit process. Bacterio-phage T7 can be used as a biological dosimeter and thebiologically effective dose (BED) is expressed in HT7 units (ln N/No) [22]. As phage T7 comprise only about 10% noncoding se-

quences [28], the inactivation curve can be interpreted on the basisof one-hit principle: each lesion is a lethal one, following Poissondistribution [22]. The difference in the inactivating effect in thetwo buffers suggests that some of the lethal photoproducts areproduced via radicals. The 10 mM Tris–HCl buffer which was usedhere represent moderate radical scavenging conditions (1.5 � 107/sOH� scavenging capacity) [29], and definitely reduces the amountof photoproducts. The explanation of the observed saturation ten-dency in the inactivation curves could be the photoreversion of di-mers as well as the clustering of the damages.

3.3. Total DNA damage determination by QPCR

The total DNA damage caused by UV radiation in phage T7 andisolated DNA was determined by QPCR, as we demonstrated earlierthat potentially every UV induced lesion in T7 DNA inhibits theDNA synthesis in phage T7 [16]. As an example, Fig. 3A showsthe effect of the simulated Martian UV radiation on the amplifica-tion of the 555 bp fragments in phage T7, while Fig. 3B shows theaverage number of damage related to 1000 nucleic acid bases (kB)in isolated T7 DNA and phage T7 in function of the incident UVdose in M9 and Tris buffer, respectively. As Fig. 3B indicates, intra-phage T7 DNA is about ten times more sensitive to simulated Mar-tian UV radiation at a given dose than isolated T7 DNA and thescavenging effect of Tris buffer can be detected in the total numberof damage as well. Earlier we measured the increased UV damageof intraphage T7 DNA under the effect of five different UV-C andUV-B radiation sources in isolated T7 DNA/phage T7 solutions bylesion specific antibodies and by QPCR as well [16,18,30]. Further-more, intraphage T7 DNA is more sensitive to UV-C and polychro-

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Fig. 3. (A) The effect of simulated Martian UV radiation on the amplification of the 555 bp fragment of intraphage T7 DNA. Lanes: 1: DNA standard 2: control DNA 3: after270 kJ/m2 irradiation 4: 180 kJ/m2 5: 120 kJ/m2 6: 60 kJ/m2 7: 30 kJ/m2 irradiation 8: negative control. (B) The average number of damage/kB caused by UV irradiation inisolated T7 DNA and intraphage DNA as a function of the UV dose determined by QPCR in M9 and Tris buffer, respectively. All the data are relative to the PCR product fromuntreated DNA or phage expressed as mean ± SD from eight individual PCR reactions.

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matic UV radiation in thin layers as well; we obtained a significantdecrease in the yield of photoproducts in thin layers of isolatedDNA compared to T7 thin layers of phage T7 [20,31–33].

3.4. Determination of the type of DNA photoproducts

The type and distribution of photoproducts in isolated DNA/phage depends strongly on the spectral composition of UV source.The presence of double-strand breaks (dsb) and single strandbreaks (ssb) was detected by neutral and alkaline agarose gel elec-trophoresis. Among the total damage in phage T7 the ratio of ssb/dsb was 24%/12%, in isolated T7 DNA the ratio of ssb/dsb was 32%/19% determined in M9 buffer, the presence of Tris decreased thesevalues by 6–8%. Earlier we found that UV-C (254 nm) and UV-Bradiation did not produce significant amount of dsb/ssb in phageT7 and in isolated T7 DNA in spite of the large number of totalDNA photoproducts caused by UV irradiation [16,32,33].

To determine the nature of the other DNA damage than strandbreaks, lesion-specific endonuclease digestion followed by alkalineagarose gel electrophoresis was used to resolve the amount ofCPDs and apurinic/apyrimidinic (AP) sites. On the basis of methodsdeveloped for quantization of damage in DNA by lesion-specific

endonucleases clustered DNA damage can be detected as well byquantitative nondenaturing agarose gel electrophoresis [24,29].Clustered DNA damage are two or more closely spaced damage(CPD, abasic sites, strand breaks etc.) on opposite strands. Wedetermined the frequency of induced dsb (each resulting from adamage cluster) and the number average molecular length (Ln) ofthe resulting DNA population. Fig. 4A shows an example for clusteranalysis, it can be seen, that unirradiated DNA retains its size, andtreatment with lesion-specific enzyme does not affect the size ofunirradiated DNA (lanes 2 and 3 in both gels). UV irradiation in-duces frank double strand breaks (lanes 4, 6, 8), the size of DNAmolecules decreases with the UV dose and treatment with EndoVinduces de novo dsb at the site of each clustered damages (lanes5, 7, 9) the size of DNA molecules decreases further upon cleavageby Endo IV. We found that 5–8% of the CPD were in clusters, while3–5% of the AP sites were located in clusters in isolated T7 DNA/phage.

Both in phage T7 and in isolated T7 DNA the number of ssb, dsband AP sites was larger by 5–8% in M9 buffer, than in Tris buffer,indicating that Tris participates chemically in these reactions andcompetes effectively for photons. Additionally, in phage T7 DNA-protein cross-links were produced by Martian UV radiation as well,

Fig. 4. (A) Electronic image of a neutral agarose gel for damage cluster analysis. Lanes 2–9 (Gel A) contain T7 DNA samples exposed to 0, 30 kJ/m2, 60 kJ/m2, 120 kJ/m2

simulated Martian UV radiation and lanes 2–7 (Gel B) contain samples following irradiation with 0, 180 kJ/m2 and 270 kJ/m2 simulated Martian UV irradiation. The samplesare paired, with the first of each pair not treated with enzyme, and the second treated with T4 endonuclease V. Lanes 1 contain k DNA Hind III digest molecular lengthstandard. (B) Correlation between the dose of simulated Martian UV irradiation (kJ/m2) and the amount of different type of damage caused by UV irradiation in phage T7(damage/kB) in Tris and M9 buffer, respectively.

114 A. Fekete et al. / Journal of Photochemistry and Photobiology B: Biology 92 (2008) 110–116

the amount of cross-links was 3.2%/4.5% of the total damage inTris/M9 buffer. The difference in M9/Tris buffer could be explainedby the fact that OH� radical was found to be the most effectiveaqueous radical intermediate for promotion of crosslinking of pro-teins to DNA [34]. Fig. 4B represents the distribution of differentUV photoproducts in phage T7 determined in M9 buffer. CPD rep-resent �38%, ssb �23%, AP �16%, dsb �12%, DNA-protein cross-links �4.5% of the total damage in phage T7, in isolated DNA�21.5 CPD, �32% ssb, �19% dsb, �14% AP were of the total numberof damage. Assuming that QPCR detects the total damage, 83–97%of the different Martian UV photoproducts was identified and de-tected in isolated T7 DNA and phage T7.

4. Discussion

The effect of simulated Martian UV radiation (200–400 nm) wasexamined on bacteriophage T7 and on isolated T7 DNA. Our results

show that intraphage T7 DNA is about ten times more sensitive tosimulated Martian UV radiation, than isolated T7 DNA. Therefore, itseems that DNA damage formation caused by UV radiation is influ-enced by the superhelical packing of intraphage T7 DNA, or by theinteraction with phage proteins (or both), which can pose thenucleotide bases favorably for photoproduct formation. Anotherpossibility for the explanation of the decreased photosensitivityof isolated T7 DNA is the escape of excitation energy along the axisof the double helix in isolated DNA [35], a process which is stalledin phages T7 owing to the protein binding. Alterations in DNAstructure resulting from protein-DNA contacts can produce hyper-susceptibiliy to, or protection from, UV-damage formation [36]. Inphage T7 the most likely explanation is that the bound phage pro-teins change the geometry of the intraphage T7 DNA in a way thatfacilitates UV photoproduct production, as we excluded DNA con-formation as a decisive factor in DNA damage caused by UV radia-tion [16,30].

A. Fekete et al. / Journal of Photochemistry and Photobiology B: Biology 92 (2008) 110–116 115

At comparable doses we detected by QPCR �10 and �100 dam-age/phage under the effect of solar simulator (SOL 2000) and Mar-tian UV radiation simulator, respectively. The damage in isolatedDNA/phage depends not only on the intensity of UV radiation,but on the spectral composition of the UV source as well, as theDNA action spectrum peaks in the UV-B and UV-C range, below280 nm the UV radiation becomes energetically very damaging toDNA. Both in phage T7 and in isolated DNA the amount of totalDNA damage is significantly smaller in Tris buffer than in phos-phate buffer, suggesting that some of the photoproducts were pro-duced via radicals.

Not only the amount of total damage, but the type and distribu-tion of the isolated DNA/intraphage DNA photoproducts producedby simulated Martian UV radiation are different than those pro-duced by solar radiation. Besides the major photoproduct, CPD pro-duced by solar UV, double-strand breaks (dsb), single-strand breaks(ssb), DNA-protein cross-links are produced as well under the effectof simulated Martian radiation. Surprisingly, energy deposition aslow as 4–6 eV corresponding to 200–400 nm range can induce sig-nificant amount ssb and dsb in phageT7/isolated DNA. Earlierworks, based on mainly theoretical considerations had indicatedthat substantially higher energies are required to induce these bio-logically important lesions [37,38]. The gas phase ionization poten-tials of the nucleotide bases vary from 7.77 eV for guanine to8.87 eV for thymine. These values are much higher than 6 eV, theenergy of 200 nm that is the most energetic wavelength of the sim-ulated Martian UV radiation. Folkard et al. [39] recently demon-strated that photons with energy as low as 7 eV were capable ofproducing ssb as well as dsb in plasmid DNA. In bacterial sporesUV-C and UV-A radiation caused accumulated dsb and ssb [40].

The comparison of solar simulator – UV-C – simulated MartianUV radiation caused damage provides useful information abouthow energy absorbing modes can affect the type and amount ofDNA damage. UV-C photochemistry has been intensively studied,less is known about exposure of DNA to high energy irradiation(shorter UV wavelengths, which can lead, in part to other photo-processes) [41]. It is well known that the typical DNA damagesare CPD and (6-4) PD in the far UV region, whereas strand breaksand base alterations exists in the X-ray region. At a certain wave-length one of these two types of the damage should replace theother as a major type of the damage. The border photon energy be-tween both types of damages has been studied in films of nucleo-tide bases, and was found P9 eV [37,38]. Our results suggests, thatin nucleic acid and nucleoprotein solutions this border photon en-ergy is much smaller (�6 eV) probably due to the hydration and/orpresence of proteins, and the interaction among different wave-lengths cannot be excluded. In the simulated Martian UV radiationUV-C, UV-B and UV-A components can be implicated individuallyor synergistically. It is already known for some cases that the bio-logical effects might not necessarily be additive, but can be syner-getic or antagonistic. A prominent example is the synergetic effectof vacuum and UV radiation with DNA being the critical target[33,42,43].

According to our results,�5% of the CPD and AP sites are locatedin clusters in isolated DNA/phage, suggesting that clustering ofdamage occur in the form of multiple damaged sites and thesecan have a high probability to produce strand breaks. The explana-tion of photoproduct clustering can be a combination of short-range energy migration into pyrimidine tracts and preferentialphotoproduct formation within protein-bound regions of DNA.Clustered damage can be critical lesion that induce biological dam-age, as the biological inactivation of phage is a single hit process,the clustering of damage can lead to the saturation tendency ofthe inactivation curve. Assuming these photoinduced reactions asstatistical events, the chance for a number of phage/DNA moleculesto survive the radiation damage can be a reality.

The UV climate of early Earth and present-day Mars may havebeen similar, thus the study of the biological effects of UV radiationon Mars will not only help us to understand the potential fate ofbiological evolution on Mars or the fate of organisms transferredto Mars on spacecraft, but could also be applied to understandthe potential of UV stress on molecules essential for life on the sur-face of early Earth.

Acknowledgements

The authors thank Professor Györgyi Rontó for stimulating dis-cussions, Mónika Drabbant and Edit Völgyi for their excellent tech-nical assistance and Christoph Kolb for providing the Mars UVlamp. This work was supported by Grants ESA-PECS No. 98006:SSIOUX, by the bilateral grant of the Austrian – Hungarian ÖADproject A – 20/2000. Spectral measurements of the UV source wereperformed by a spectroradiometer granted by OTKA M-04581.

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