No / Raman spectra of an extremophile cyanobacterial colony in hydromagnesite from Lake Salda in Turkey have revealed a biogeological modification which is manifest as aragonite in the stratum associated with the colony. The presence of key spectral biomarkers of organic protectant molecules such as (8-carotene and scytonemin indicate that the survival strategy of the cyanobacteria is significantly one of UV-radiation protection. The terrestrial location of this extremophile is worthy of consideration further because of its possible putative link with the White Rock formations in Sabaea Terra and Juventae Chasma on Mars.

Les rayons ultraviolets (UV) sont les principaux responsables de l’initiation des cancers cutanée, car ils induisent différents dommages à l’ADN, dont les dimères cyclobutyliques de pyrimidines (CPD). Ces dommages sont principalement induits par les UVB et sont responsables de la formation des mutations trouvées les cancers cutanés non mélanocytiques. Nos résultats ont démontré qu’une irradiation chronique à de faibles doses de rayons UVB (CLUV) induit la formation de CPD, qui persistent dans le temps. Ces dommages résiduels s’accumulent dans le génome et ne semblent pas être réparés. Nous nous sommes donc intéressés à ces dommages résiduels et à leur impact dans le génome. Premièrement, la distribution post-irradiation des CPD résiduels a été observée pour des fibroblastes dermiques humains soumis à une irradiation CLUV (75 J/m² aux 12h, durant 7,5 jours). Ces analyses ont démontré que les CPD résiduels sont tolérés et dilués dans le génome lors des divisions cellulaires. Deuxièmement, la localisation des CPD résiduels a été observée pour les cellules irradiées avec une CLUV et pour les cellules irradiées avec une dose unique et aigüe (400 J/m²). La quantification des CPD résiduels a permis d’observer que pour les cellules irradiées avec une CLUV, environ deux fois plus de CPD résiduels sont présents dans l’hétérochromatine par rapport à l’euchromatine. Pour les cellules irradiées avec une dose aigüe, les CPD sont répartis également entre les deux fractions de la chromatine. Ces résultats suggèrent que l’état de compaction de la chromatine a un impact sur l’accumulation et la réparation des CPD. Troisièmement, la fréquence de chaque type dipyrimidinique de CPD résiduels a été déterminée par la technique de LC-MS/MS. Une proportion plus faible de CPD contenant des cytosines a été observée dans les cellules irradiées avec la CLUV par rapport aux cellules irradiées avec la dose aigüe. De plus, l’analyse a démontré que les photoproduit de pyrimidine (6-4) pyrimidone (6-4 PP) étaient presque complètement absents des dommages résiduels. Finalement, afin d’observer l’impact des CPD résiduels sur la stabilité génomique, la quantité d’échanges de chromatides sœurs (SCE) a été comparée pour les cellules irradiées par la CLUV et les cellules non irradiées. La quantité de SCE observée était plus élevée dans les cellules irradiées que dans les cellules non irradiées. Globalement, nous avons observés que les CPD résiduels, majoritairement de type TT, s’accumulent principalement dans l’hétérochromatine, où ils sont tolérés. Ces dommages sont dilués dans le génome au fil des divisions cellulaires, en causant une augmentation de l’instabilité génomique. Globalement, mon projet a permis de mieux comprendre l’impact des CPD résiduels induits par une irradiation chronique dans l’initiation de la cancérogenèse cutanée. / Ultraviolet (UV) rays are known to be the main initiator of skin cancer, as they induce different types of DNA damage, including cyclobutane pyrimidine dimers (CPD). CPD are mostly produced by UVB rays and are the predominant premutagenic DNA damage responsible for non-melanoma skin cancers. While most CPD are repaired by the nucleotide excision repair (NER) pathway, some remain unrepaired and persist in the genome. We recently observed those residual CPD after exposure of human fibroblasts cells to chronic low dose of UVB (CLUV). Then, we aimed to observe the distribution of residual CPD occurring in dividing cells submitted to CLUV irradiation. Human dermal fibroblasts were irradiated with CLUV (75 J/m² every 12h for 7.5 days). Our results showed that residual CPD are tolerated and diluted in the genome by DNA replication. Then, localization of CLUV-induced residual CPD was observed and compared with residual CPD induced by a single and acute UVB irradiation (400 J/m²). Euchromatin and heterochromatin fraction were isolated and the amount of CPD was quantified in each fraction. The quantification showed that residual CPD accumulate mostly in the heterochromatin fraction of the genome, where the amount of CPD was two times greater than in the euchromatin. This suggests that DNA compaction has an impact on CPD accumulation and repair. Then, we measured the frequency of the different types of residual CPD by LC-MS/MS technique. A lower proportion of cytosine containing CPD was found in CLUV irradiated cells than in acute irradiated cells. The quantification of the different types of 6-4 photoproducts (6-4 PP) demonstrated that they were almost all absent after a CLUV irradiation, in the residual damage. Finally, genomic instability was investigated in CLUV irradiated cells by measuring the amount of SCE induced after the irradiation. A higher number of SCE was observed in CLUV-irradiated cells than in control cells, suggesting that residual CPD are responsible for an increase of genomic instability. Overall, we observed that residual CPD, mostly TT-CPD, accumulate in the heterochromatin where they are tolerated. These CPD are diluted during cellular division but they are causing genomic instability. Finally, my project aimed to characterise residual CPD induced by chronic irradiation and to gain more knowledge on their impact in skin carcinogenesis initiation.

Tableau d'honneur de la Faculté des études supérieures et postdoctorales, 2017-2018 / La poly(ADP-ribose) polymérase 1 (PARP1) est une enzyme nucléaire très abondante chez les eucaryotes supérieurs, humains compris, mais néanmoins absente chez les bactéries et les levures. En réponse aux dommages à l’ADN, elle utilise le substrat nicotinamide adénine dinucléotide (NAD+) pour former des polymères d’ADP-ribose (PAR) sur elle-même et sur d’autres protéines cibles. L’enzyme PARP1 et son activité catalytique sont impliquées dans la réparation des dommages à l’ADN contenant des cassures simple et double brin. Cependant, l’hypothèse que l’enzyme PARP1 joue un rôle dans la réparation de dommages sans cassures de brin a toujours rencontré des réticences. Par exemple, la PARP1 est activée rapidement par ces dommages, comme ceux induits par les radiations ultraviolettes (UV), mais son rôle dans leur réparation par excision de nucléotides (NER) n’était pas accepté généralement. Ainsi, ce projet de doctorat consiste à déterminer le mécanisme exact par lequel la PARP1 et son activité catalytique contribuent à la NER. Cette voie de réparation utilise plus de 30 protéines pour réparer une très grande variété de dommages. Bien que nous ayons une bonne connaissance des étapes de la NER grâce aux études in vitro chez les bactéries et les levures, les facteurs qui influencent le fonctionnement de la NER chez les eucaryotes supérieurs ne sont pas tous connus. Cependant, de récentes études ont montré que des complexes de remodelage de la chromatine et des modifications post-traductionnelles facilitent la NER dans la chromatine. Dans ce contexte, l’implication de la modification posttraductionnelle effectuée par la PARP1, dite PARylation, est encore inconnue dans la NER. Dans la NER, l’étape cruciale de la réparation globale du génome est la reconnaissance des quelques bases endommagées qui sont entourées de nombreuses bases non modifiées par la protéine «Xeroderma pigmentosum C» (XPC). Un autre facteur clé de cette phase est le facteur «UV-damaged DNA binding protein 2» (DDB2) qui fait partie du complexe ubiquitine-ligase UV-DDB. Ici, nous avons démontré que, après irradiation aux UVC, la PARP1 se lie asymétriquement à la photolésion et elle interagit avec le facteur DDB2. Ce dernier stimule l’activité catalytique de la PARP1 et est à son tour PARylé par la PARP1. Les polymères formés autour de la photolésion agissent comme signal de recrutement pour le complexe PARP1-XPC déjà présent dans le nucléoplasme. La confluence de ces facteurs de réparation au site de dommage assure la séparation de la protéine XPC de ce complexe suivi de son transfert et de sa stabilisation autour du dommage. Ainsi, la PARP1 n'est pas seulement l'une des premières protéines recrutées aux lésions induites par les UV, mais son activation rapide par ces dommages joue un rôle clé dans les étapes situées en aval de la phase de reconnaissance des dommages de la NER. En effet, nous avons montré que l’inhibition ou la déplétion de la PARP1 ralentit radicalement la réparation par la NER des dommages directs induits à l’ADN par les UV. Cette étude montre que la PARP1, en coopération avec les protéines DDB2 et XPC augmente l’efficacité de la voie NER dans les cellules des mammifères. / Poly(ADP-ribose) polymerase 1 (PARP1) is a highly abundant nuclear enzyme which is present in higher eukaryotes but absent in bacteria and yeasts. In response to DNA damage, it uses the nicotinamide adenine dinucleotide (NAD+) to form polymers of ADPribose (PAR) on itself and other target proteins. PARP1 and its catalytic activity are involved in the repair of DNA damages comprising of single and double strand breaks. However, the role of PARP1 in repairing DNA damage without strand breaks has not been readily accepted. For example, although PARP1 is rapidly activated in response to such damages caused by ultraviolet radiation (UV), its role in their repair by nucleotide excision repair pathway (NER) was not generally recognized. Thus, the project of my doctoral work is to determine the exact mechanism by which PARP1 and its catalytic activity influence NER. This pathway uses more than 30 proteins to repair a wide variety of DNA damages. Although we have a good understanding of NER steps through studies in vitro, bacteria and yeasts, we still do not know all the factors that influence the functioning of the NER in higher eukaryotes including humans. Recent studies have shown that chromatin remodelling complexes and post-translational modifications facilitate NER in the context of chromatin. However, the contribution of PARylation, the post-translational modification carried out by PARP1, in NER remains largely unknown. Xeroderma pigmentosum C protein (XPC) plays a crucial role in NER by recognizing the few UV induced lesions in the vast undamaged chromatin. Another key factor in damage recognition is the UV- damaged DNA binding protein (DDB2), which is part of the UV-DDB ubiquitin-ligase complex. Here, we have demonstrated that after UVC irradiation, PARP1 binds asymmetrically to the photolesions and interacts with DDB2. DDB2 stimulates the catalytic activity of PARP1 and in turn it is PARylated. The polymers formed around the photolesion act as recruitment signal for the PARP1-XPC complex already present in the nucleoplasm. The confluence of these repair factors at the damage site ensures the separation of the XPC protein from its complex with PARP1 followed by its transfer and stabilization at the site of damage. Thus, PARP1 is not only one of the first proteins to respond to UV induced DNA damage, but also its early rapid activation plays a key role in the downstream events of NER. Indeed, we have shown that both inhibition and depletion of PARP1 significantly delays the repair of these lesions. This study demonstrates that PARP1 increases the efficiency of NER in cooperation with the DDB2 and XPC proteins in mammalian cells.

Filtered Rayleigh scattering (FRS) is a non-intrusive, optical measurement technique that can currently provide time-averaged, simultaneous planar measurements of three-component velocity, static temperature, and static density of aerodynamic flows. Development of the FRS technique has typically employed 532 nm Nd:YAG lasers coupled with the use of iodine vapor cells as the molecular filter. One method to improve the effective signal-to-noise ratio (SNR), and therefore the performance of an FRS system, is to use shorter wavelengths. This takes advantage of the dependence of the Rayleigh scattering signal on the inverse of the wavelength of the incident laser light to the fourth power: even small shifts to shorter wavelengths can offer significant gains in SNR as a result. This study explores the implementation of an ultraviolet (UV) FRS system nominally at 387 nm with the use cesium vapor as the molecular filter. The cesium absorption lineshapes (corresponding to the 62S1/2 → 82P3/2 atomic transitions around 387 nm) are considered along with camera specifications to simulate an ultraviolet filtered Rayleigh scattering (UV FRS) measurement of aerodynamic flows. A signal model is developed using numerical functions for the cesium vapor cell transmission, camera specifications, signal-dependent shot noise, and signal-independent electronic detector read noise. Using this noise-inclusive model (over a 2.4 GHz scan bandwidth with a 7.5 cm long cesium vapor cell corresponding to current Virginia Tech FRS capabilities) velocity, static temperature, and static density measurement sensitivities for this proposed configuration are analyzed by evaluating and deriving the Cramér-Rao lower bound (CRLB) for each quantity. The effects of different flow conditions, Mie and geometric scattering levels, cesium vapor cell temperature, and spectral resolution are demonstrated. It is found that the best possible theoretical measurement results are obtained for high-speed wind tunnel flow conditions with high spectral resolution, and that the CRLB for velocity, static temperature, and static density for a 387 nm system approaches or exceeds that of a 532 nm system for a given signal-to-noise ratio (SNR). / Master of Science / One type of non-intrusive measurement technique that can be applied to aerodynamic flows is filtered Rayleigh scattering (FRS). Unlike other non-intrusive techniques such as particle image velocimetry (PIV) and Doppler global velocimetry (DGV), FRS does not require that the flow be seeded with particles and can provide simultaneous measurements of three-component velocity, static temperature, and static density. Current FRS measurement systems commonly use 532 nm green-light lasers and iodine cells for filtering. However, a stronger Rayleigh scattering signal (and therefore better measurement) can be attained by using shorter laser wavelengths as the strength of the Rayleigh scattering is related to the inverse of the incident wavelength to the fourth power. This study takes advantage of this fact to propose an FRS measurement system using ultraviolet laser light at nominally 387 nm. The implementation of a commercially available 387 nm laser system with the use of cesium cells for filtering is investigated. In order to simulate the performance of the system, a signal model is developed that includes both signal-dependent shot noise, and signal-independent electronic detector read noise. The signal model is combined with the transmission profile of cesium vapor, commercially available camera specifications, and typical FRS measurement parameters to simulate a 387 nm FRS system measurement. The measurement sensitives and performance of the proposed UV FRS system at 387 nm are investigated by deriving and evaluating the Cramér-Rao lower bound (CRLB) for velocity, static temperature, and static density. The effects of different flow conditions, Mie and geometric scattering levels, cesium vapor cell temperature, and scan resolution are demonstrated. The best performance is attained at high-speed conditions with high spectral resolution, and this approaches or exceeds the simulated performance of a 532 nm system with an iodine vapor cell over the same range of conditions.

This research investigated the efficacy of chlorine dioxide (ClO₂), ultraviolet (UV) irradiation, UV in combination with chlorine dioxide (UV/ClO₂), and UV in combination with hydrogen peroxide (UV/H₂O₂) for decolorizing three reactive azo dyes (sultan red, indigo blue and cypress green) and treated textile-manufacturing wastewater. The objective was to determine the best treatment for reducing color to the Virginia Pollutant Discharge Elimination System (VPDES) permit level of 300 American Dye Manufacturers Institute (ADMI) units. The effects of the three chemical oxidation treatments provided color reduction for all three dyes. The results suggested UV/H₂O₂ and UV/ClO₂2 treatments provided maximum color reduction of the red and blue dyes, and UV/H₂O₂ was the most effective for maximum reduction of the green dye. A research goal was to provide predictive models of the wastewater effluent for the treatment processes, including the UV exposure time required to reach the 300 ADMI permit value and the effective ClO₂ dose necessary to achieve the 300 units. The results of the investigations regarding the effluent indicated that UV/H₂O₂ and UV/ClO₂ (5 mg/L) provided reduction to 300 units in less than 10 minutes UV exposure when the initial effluent color was less than 500 ADMI units. Without the addition of oxidant, contact times longer than 10 minutes were required for UV to decolorize these effluents to 300 ADMI units. Chlorine dioxide dosages between 10 and 30 mg/L both with and without UV irradiation achieved the same results. / Master of Science

La lumière solaire constitue le principal facteur de risque des cancers de peau non mélanocytaires (NMSC). L'effet génotoxique de la lumière solaire est dû aux dommages dans l'ADN induits par les rayonnements ultraviolets (UV). Les rayons UVB longs (290-315 nm) sont les principaux responsables de l'initiation et de la promotion des NMSC qui prennent naissance dans les kératinocytes épidermiques. En effet, l’absorption directe des photons d’UVB par l’ADN conduit à la génération de deux principaux types de dommages, les dimères cyclobutyliques de pyrimidines (CPD) et les photoproduits de pyrimidine (6-4) pyrimidone (6-4PP). Les CPD sont les plus abondants et sont hautement mutagènes. Ils sont responsables des mutations de transitions C → T au niveau des sites dipyrimidiniques, les mutations signatures observées dans les cancers de peau. Les cellules possèdent différents mécanismes pour éviter la conversion des CPD en mutations, à savoir, l’arrêt du cycle cellulaire, la réparation des dommages dans l'ADN par le système de réparation par excision de nucléotides (NER) et la mort cellulaire par apoptose. L’importance de la NER dans la prévention des cancers de peau est bien démontrée par le fait qu’une déficience en protéines de la NER, comme chez les patients atteints de Xeroderma Pigmentosum (XP), entraîne une incidence jusqu’à 2000 fois plus élevée de cancers de peau. De nombreux facteurs influencent la NER et une meilleure compréhension de ces derniers pourrait conduire au développement de nouvelles stratégies de prévention contre les cancers de peau. La peau est un assemblage complexe de cellules et de matrice dans lequel la communication entre les composants épidermiques et dermiques est essentielle pour de nombreux mécanismes cutanés. En utilisant des peaux reconstruites dérivées uniquement de fibroblastes et de kératinocytes primaires humains, nous avons analysé l’impact des composants dermiques sur l’efficacité de réparation des CPD épidermiques. Nous avons montré que l’élimination des CPD dans les kératinocytes est positivement influencée par la présence d'un derme et nous avons déterminé que cet effet du derme sur les kératinocytes proviendrait de molécules sécrétées. En étudiant le sécrétome, nous avons découvert que la cytokine CXCL5 (ou ENA78 - Epithelial neutrophil-activating peptide 78) possède un patron d'expression unique : elle est pratiquement absente du milieu de culture des peaux reconstruites, comparativement au milieu de culture de fibroblastes et de kératinocytes seuls. En modulant les niveaux de CXCL5 dans les milieux de culture de kératinocytes, nous avons montré que CXCL5 était un inhibiteur de la réparation des CPD. Cette première étude décrit l'impact des molécules sécrétées par le derme sur la réparation iii des CPD épidermiques et met en lumière un nouveau rôle de CXCL5 dans la réparation des dommages induits par les rayons UV. L’environnement immédiat des kératinocytes n’est pas le seul facteur qui peut influencer la réparation des CPD, le régime d’irradiation a également un impact sur cette efficacité d’élimination des lésions. Jusqu’à présent, l'efficacité de la NER a été largement étudiée après une seule exposition aiguë aux rayons UV. Cependant, l'utilisation d’une irradiation unique n'est pas représentative de l'exposition solaire humaine, qui est plutôt constituée d’une multitude d'irradiations répétées. Dans ce travail, nous avons donc exposé des cellules épidermiques à un régime d’irradiation chronique composé de faibles doses d’UVB (CLUV) afin de déterminer l’impact de cette irradiation sur la réparation NER. Nous avons montré que le traitement CLUV entraîne l’accumulation de CPD résiduels, qui ne sont pas réparés mais plutôt tolérés et dilués lors de la réplication de l’ADN. Nous avons également constaté que le prétraitement CLUV réduisait la capacité d'élimination des nouveaux dommages sans induire de sensibilité accrue à la mort cellulaire. Enfin, en utilisant nos données expérimentales, nous avons élaboré un modèle théorique pour prédire l’induction, la dilution et la réparation des CPD épidermiques lors d’une irradiation chronique aux rayons UVB. Nos résultats montrant que les kératinocytes accumulent des dommages dans l'ADN après des irradiations chroniques, constituent un facteur important à prendre en compte, car l'accumulation de CPD non réparés pourrait entraîner une augmentation des mutations dans les kératinocytes. Dans l’ensemble, ces travaux soulignent l’importance d’utiliser des modèles plus complexes, visant une meilleure représentation physiologique, pour mieux comprendre les réponses de la peau à l’exposition solaire. / Skin exposure to solar light is the main risk factor for non-melanoma skin cancers (NMSC). The genotoxic effect of sunlight is attributed to DNA damage induced by ultraviolet (UV) radiations. Long UVB wavelengths (290-315 nm) are the main responsible for NMSC initiation and promotion that occur in epidermal keratinocytes. Indeed, the direct absorption of UVB photons by DNA leads to the generation of the two main types of UV-induced DNA damage, i.e. cyclobutane pyrimidine dimers (CPD) and (6-4) pyrimidine-pyrimidone photoproducts (6-4PP). CPD are the most abundant and are highly mutagenic. They are responsible for the C → T transition mutations at dipyrimidine sites, the signature mutation found in sun-related skin cancers. Skin cells use different mechanisms to avoid the conversion of UVB-induced CPD into skin cancer driver mutations, i.e. cell cycle arrest, DNA damage removal by nucleotide excision repair (NER) pathway and cell death by apoptosis. The importance of NER for skin cancer prevention is well demonstrated by the fact that a deficiency in NER proteins, such as in Xeroderma Pigmentosum (XP) patients, leads to an increase of up to 2,000-fold in skin cancer occurrence. Many factors influence NER and a better understanding of those factors might lead to new prevention strategies against skin cancer. Skin is a complex assembly of cells and matrix in which a crosstalk between epidermal and dermal components is essential for many cutaneous mechanisms. Using self-assembled tissue-engineered skin equivalents derived from human primary fibroblasts and keratinocytes, we have analyzed the impact of dermal components on epidermal CPD repair efficiency. We showed that CPD repair in keratinocytes is positively influenced by the presence of the dermis and we brought evidence that this dermal effect comes from secreted molecules. We then investigated the secretome and found that the cytokine CXCL5 (also known as ENA78 - Epithelial neutrophil-activating peptide 78) has a unique expression pattern, i.e. is virtually absent in the culture medium of reconstructed skin, when compared to the media from fibroblasts and keratinocytes alone. By modulating CXCL5 levels in keratinocytes culture medium, we have shown that CXCL5 is an inhibitor of CPD repair. This work outlines the impact of the secreted dermal components on epidermal UV-induced DNA damage repair and shed light on a novel role of CXCL5 in CPD repair. The immediate environment of the keratinocytes is not the only factor that can influence the CPD repair, the irradiation protocol also has an impact on this damage removal. Until now, v NER efficiency has been extensively studied after a single acute UVB exposure. However, the use of single UVB irradiation is not representative of the human solar exposure, which is rather a multitude of repeated irradiations than a single acute one. In this work, we thus exposed keratinocytes to a chronic low-dose of UVB (CLUV) protocol to determine the impact of this irradiation procedure on CPD removal. We showed that the CLUV treatment leads to the accumulation of residuals CPD. Those residual CPD are not repaired but rather tolerated and diluted through DNA replication. We also found that a CLUV pre-treatment reduces CPD removal rate of newly generated damage without inducing a higher sensitivity to UV-induced cell death. Finally, using our experimental data, we derived a theoretical model to predict CPD induction, dilution and repair that occur in keratinocytes when chronically irradiated with UVB. These results showing that keratinocytes accumulate DNA damage after chronic irradiations is an important factor to consider since the accumulation of unrepaired CPD might lead to an increase of skin cancer driver mutations formation. Taking together, this work outlines the importance of more relevant and physiological models to study the skin response to solar exposure.

Ready-to-eat meat products have been implicated in several foodborne listeriosis outbreaks. Microbial contamination of these products can occur after the product has been thermally processed and is being rapidly chilled using salt brines. The objective of this study was to determine the effect of ultraviolet irradiation on the inactivation of Listeria monocytogenes and lactic acid bacteria in a model brine chiller system. Two concentrations of brines (7.9%w/w or 13.2%w/w) were inoculated with a ~6.0 log10 CFU/ml cocktail of L. monocytogenes or lactic acid bacteria and passed through the ultraviolet (UV) treatment system for 60 minutes. Three replications of each bacteria and brine combination were performed and resulted in at least a 4.5 log reduction in microbial numbers in all treated brines after exposure to ultraviolet light. Bacterial populations were significantly reduced after five minutes exposure to UV light in the model brine chiller as compared to the control, which received no UV light exposure (P<0.05). The maximum rate of inactivation for both microorganisms in treated brines occurred between minute 1 and 15 of ultraviolet exposure. Overall, results indicate that inline treatment of chill brines with ultraviolet light (UVC) shows promise in inactivating L. monocytogenes and lactic acid bacteria. Due to the low capital involved in initiating a continuous inline UV system, the use of ultraviolet energy may prove to be beneficial for effectively controlling pathogens in recycled chill brines without interrupting the chilling operation. An inline ultraviolet system could be used in a hazard analysis and critical control points plan. / Master of Science

In contrast to the damage caused by far-UV, the damaging effects of UVA (320-400 nm) in living cells are not well understood. The damage caused by UVA irradiation is largely oxygen-dependent, suggesting UVA-mediated DNA damage involves reactive oxygen species produced through the action of an endogenous photosensitizer. Previous studies examining cellular responses to UVA irradiation in E. coli have been hindered by the fact that, at sublethal fluences, wild-type cells undergo a transient inhibition of cell growth termed a "growth delay". This effect is absent in nuvA⁻ strains, thereby facilitating the study of DNA repair factors required for the repair of UVA-mediated damage. Formamidopyrimidine (Fapy) glycosylase (encoded by fpg) and the UvrABC excinuclease are both capable of excising oxidatively damaged DNA bases. An fpg::kan mutation was placed into isogenic uvrA⁺ and uvrA⁻ strains of E. coli to evaluate the relative importance of these repair enzymes in the recovery from UVA-induced stress. In a nuvA⁻ background, the survival of fpg⁻ mutants exposed to UVA was significantly reduced relative to isogenic fpg⁺ control strains. This effect was enhanced in the absence of the UvrABC excinuclease, suggesting a role for both of these enzymes in repairing UVA-generated lesions. Survival of isogenic nuvA⁺ repair-deficient strains was significantly lower than nuvA⁻ strains, suggesting a role for the modified base 4-thiouridine in UVA-mediated lethality. An in vitro plasmid DNA irradiation assay in the presence and absence of 4-thiouridine was used to examine this possibility. When irradiated DNA was subsequently used to transform the fpg⁻ and uvrA⁻ mutant strains, no increase in DNA damage (as measured by a decrease in transformational efficiency) in the presence of 4-thiouridine was observed, suggesting that when present in solution this base does not play a photosensitizing role in UVA-mediated lethality. / Thesis / Master of Science (MSc)

The transmissions of some optical crystals were determined for the vacuum ultraviolet using a SeyaNamioka monochromator. The characteristic low wavelength cutoffs were determined for crystals transmitting in the region 1050-2000 A. The crystals so studied are: MgF₂, NaF, LaF₃, SrF₂, BaF₂, BeO, Al₂O₃, KD*P, KDP, ADP, NaCl, NdF₃, KBr, CdF₃ and PrF₃. The techniques employed in the experiment and the data obtained will be presented. / Master of Science

No / Ultraviolet radiation (UVR) in sunlight has deleterious effects on skin, while behavioural changes have resulted in people gaining more sun exposure. The clinical impact includes a year-on-year increase in skin cancer incidence, and topical sunscreens alone provide an inadequate measure to combat overexposure to UVR. Novel methods of photoprotection are being targeted as additional measures, with growing interest in the potential for systemic photoprotection through naturally sourced nutrients. Omega-3 polyunsaturated fatty acids (n-3 PUFA) are promising candidates, showing potential to protect the skin from UVR injury through a range of mechanisms. In this review, we discuss the biological actions of n-3 PUFA in the context of skin protection from acute and chronic UVR overexposure and describe how emerging new technologies such as nutrigenomics and lipidomics assist our understanding of the contribution of such nutrients to skin health.