Evolution of Proteins of the DNA Photolyase/Cryptochrome Family.

Proteins of the cryptochrome/DNA photolyase family (CPF) are phylogenetically related and structurally conserved flavoproteins that perform various functions. DNA photolyases repair DNA damage caused by UV-B radiation by exposure to UV-A/blue light simultaneously or subsequently. Cryptochromes are photoreceptor proteins regulating circadian clock, morphogenesis, phototaxis, and other responses to UV and blue light in various organisms. The review describes the structure and functions of CPF proteins, their evolutionary relationship, and possible functions of the CPF ancestor protein.

Understanding the Role of Photolyases: Photoprotection and Beyond.

The limitations of photoprotection modalities have been the inability to arrest the progression of photodamage. Chemoprevention strategies involving a sunscreen has been incomplete because of the need to induce sustained repair of mutations and slow carcinogenesis. Photolyases, or photoreactivation enzymes, serve the role of repairing mutations and damage to DNA induced by ultraviolet (UV) radiation and therefore influence the initiation phases of carcinogenesis. As these enzymes are absent in humans, exogenous forms have been manufactured and are now utilized in topical agents to supplement and augment the innate repair mechanisms that are mostly inefficient. J Drugs Dermatol. 2017;16(5 Suppl):61-66.

Identification of the molecular target for the suppression of contact hypersensitivity by ultraviolet radiation.

This study was conducted to explore the involvement of DNA damage in the suppression of contact hypersensitivity (CHS) by UV irradiation. The opossum, Monodelphis domestica, was used because cells of these marsupials have an enzyme that is activated by visible light (photoreactivating enzyme) and repairs ultraviolet radiation (UVR)-induced pyrimidine dimers in DNA. A single dose of 1,500 J/m2 of UVB (280-320 nm) radiation, representing 2 minimal erythema doses, was administered to the dorsal skin of opossums. This treatment prevented the opossums from developing a CHS response to dinitrofluorobenze (DNFB) applied either at the site of irradiation or an unirradiated site. In addition, this dose of UVR decreased the number of ATPase+ epidermal Langerhans cells in the dorsal epidermis to approximately 3% of that in unirradiated skin at the time of DNFB application. Treatment of the animals with wavelengths that activate the repair enzyme (320-500 nm, photoreactivating light, PRL) for 120 min immediately after UV irradiation inhibited the UVR-induced suppression of CHS almost completely. Exposure to PRL before UVR did not prevent UVR-induced suppression of CHS. PRL treatment after UV irradiation also prevented the decrease in the number of ATPase+ Langerhans cells. Measurements of lesions in DNA indicated that PRL treatment removed around 85% of the UVR-induced pyrimidine dimers. These data provide direct evidence that DNA, and most likely, the pyrimidine dimer, is the primary molecular target for the UVB-induced suppression of contact hypersensitivity to haptens applied to irradiated or unexposed skin.

DNA photolyases of Chrysodeixis chalcites nucleopolyhedrovirus are targeted to the nucleus and interact with chromosomes and mitotic spindle structures.

Cyclobutane pyrimidine dimer (CPD) photolyases convert UV-induced CPDs in DNA into monomers using visible light as the energy source. Two phr genes encoding class II CPD photolyases PHR1 and PHR2 have been identified in Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV). Transient expression assays in insect cells showed that PHR1-EGFP fusion protein was localized in the nucleus. Early after transfection, PHR2-EGFP was distributed over the cytoplasm and nucleus but, over time, it became localized predominantly in the nucleus. Immunofluorescence analysis with anti-PHR2 antiserum showed that, early after transfection, non-fused PHR2 was already present mainly in the nucleus, suggesting that the fusion of PHR2 to EGFP hindered its nuclear import. Both PHR-EGFP fusion proteins strongly colocalized with chromosomes and spindle, aster and midbody structures during host-cell mitosis. When PHR2-EGFP-transfected cells were superinfected with Autographa californica multiple-nucleocapsid NPV (AcMNPV), the protein colocalized with virogenic stroma, the replication factories of baculovirus DNA. The collective data support the supposition that the PHR2 protein plays a role in baculovirus DNA repair.

Photoreactivation is the main repair pathway for UV-induced DNA damage in coral planulae.

The larvae of most coral species spend some time in the plankton, floating just below the surface and hence exposed to high levels of ultraviolet radiation (UVR). The high levels of UVR are potentially stressful and damaging to DNA and other cellular components, such as proteins, reducing survivorship. Consequently, mechanisms to either shade (prevent) or repair damage potentially play an important role. In this study, the role of photoreactivation in the survival of coral planulae was examined. Photoreactivation is a light-stimulated response to UV-damaged DNA in which photolyase proteins repair damaged DNA. Photoreactivation rates, as well as the localization of photolyase, were explored in planulae under conditions where photoreactivation was or was not inhibited. The results indicate that photoreactivation is the main DNA repair pathway in coral planulae, repairing UV-induced DNA damage swiftly (K=1.75 h(-1) and a half-life of repair of 23 min), with no evidence of any light-independent DNA repair mechanisms, such as nucleotide excision repair (NER), at work. Photolyase mRNA was localized to both the ectoderm and endoderm of the larvae. The amount of cell death in the coral planulae increased significantly when photoreactivation was inhibited, by blocking photoreactivating light. We found that photoreactivation, along with additional UV shielding in the form of five mycosporine-like amino acids, are sufficient for survival in surface tropical waters and that planulae do not accumulate DNA damage despite being exposed to high UVR.

Photolyases and cryptochromes: common mechanisms of DNA repair and light-driven signaling?

DNA photolyases are extremely efficient light-driven DNA repair enzymes that use the energy of a blue-light photon to 'inject' an electron onto UV-damaged DNA, catalyzing the splitting of mutagenic pyrimidine dimers. By contrast, cryptochromes use blue light to trigger signaling cascades in multicellular organisms, fungi and several prokaryotes. Despite these functional differences, both protein families arose from a common ancestor and share many similarities, such as the overall protein fold, the presence of antenna chromophores and the use of flavin adenine dinucleotide (FAD) as the primary reactive group. Several significant advances in the biophysical and structural characterization of photolyases and cryptochromes are now revealing the details of how light-driven redox reactions can be used for such seemingly different purposes.

RNA polymerase I transcription factors in active yeast rRNA gene promoters enhance UV damage formation and inhibit repair.

UV photofootprinting and repair of pyrimidine dimers by photolyase was used to investigate chromatin structure, protein-DNA interactions, and DNA repair in the spacer and promoter of Saccharomyces cerevisiae rRNA genes. Saccharomyces cerevisiae contains about 150 copies of rRNA genes separated by nontranscribed spacers. Under exponential growth conditions about half of the genes are transcribed by RNA polymerase I (RNAP-I). Initiation of transcription requires the assembly of the upstream activating factor (UAF), the core factor (CF), TATA binding protein, and RNAP-I with Rrn3p on the upstream element and core promoter. We show that UV irradiation of wild-type cells and transcription factor mutants generates photofootprints in the promoter elements. The core footprint depends on UAF, while the UAF footprint was also detected in absence of the CFs. Fractionation of active and inactive promoters showed the core footprint mainly in the active fraction and similar UAF footprints in both fractions. DNA repair by photolyase was strongly inhibited in active promoters but efficient in inactive promoters. The data suggest that UAF is present in vivo in active and inactive promoters and that recruitment of CF and RNAP-I to active promoters generates a stable complex which inhibits repair.

Base flipping of the thymine dimer in duplex DNA.

Exposure of two adjacent thymines in DNA to UV light of 260-320 nm can result in the formation of the cis,syn-cyclobutane pyrimidine dimer (CPD). The structure of DNA containing an intrahelical CPD lesion has been previously studied experimentally and computationally. However, the structure of the extrahelical, flipped-out, CPD lesion, which has been shown to be the structure that binds to the CPD repair enzyme, DNA photolyase, has yet to be reported. In this work the structure of both the flipped-in and the flipped-out CPD lesions in duplex DNA is reported. These structures were calculated using 8 ns molecular dynamics (MD) simulations. These structures are then used to define the starting and ending points for the base-flipping process for the CPD lesion. Using a complex, two-dimensional pseudodihedral coordinate, the potential of mean force (PMF) for the base-flipping process was calculcated using novel methodology. The free energy of the flipped-out CPD is roughly 6.5 kcal/mol higher than that of the flipped-in state, indicating that the barrier to flipping out is much lower for CPD than for undamaged DNA. This may indicate that the flipped-out CPD lesion may be recognized by its repair enzyme, DNA photolyase, whereas previous studies of other damaged, as well as nondamaged, bases indicate that they are recognized by enzymes in the intrahelical, flipped-in state.

PHR2 of Candida albicans encodes a functional homolog of the pH-regulated gene PHR1 with an inverted pattern of pH-dependent expression.

Deletion of PHR1, a pH-regulated gene of Candida albicans, results in pH-conditional defects in growth, morphogenesis, and virulence evident at neutral to alkaline pH but absent at acidic pH. Consequently, we searched for a functional homolog of PHR1 active at low pH. This resulted in the isolation of a second pH-regulated gene, designated PHR2. The expression of PHR2 was inversely related to that of PHR1, being repressed at pH values above 6 and progressively induced at more acidic pH values. The predicted amino acid sequence of the PHR2 protein, Phr2p, was 54% identical to that of Phr1p. A PHR2 null mutant exhibited pH-conditional defects in growth and morphogenesis analogous to those of PHR1 mutants but manifest at acid rather than alkaline pH values. Engineered expression of PHR1 at acid pH in a PHR2 mutant strain and PHR2 at alkaline pH in a PHR1 mutant strain complemented the defects in the opposing mutant. Deletion of both PHR1 and PHR2 resulted in a strain with pH-independent, constitutive growth and morphological defects. These results indicate that PHR1 and PHR2 represent a novel pH-balanced system of functional homologs required for C. albicans to adapt to environments of diverse pH.

A novel role for DNA photolyase: binding to DNA damaged by drugs is associated with enhanced cytotoxicity in Saccharomyces cerevisiae.

DNA photolyase binds to and repairs cyclobutane pyrimidine dimers induced by UV radiation. Here we demonstrate that in the yeast Saccharomyces cerevisiae, photolyase also binds to DNA damaged by the anticancer drugs cis-diamminedichloroplatinum (cis-DDP) and nitrogen mustard (HN2) and by the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Surprisingly, mutations in photolyase were associated with resistance of yeast cells to cis-DDP, MNNG, 4-nitroquinoline oxide (4NQO), and HN2. Transformation of yeast photolyase mutants with the photolyase gene increased sensitivity to these agents. Thus, while the binding of photolyase to DNA damaged by UV radiation aids survival of the cell, binding to DNA damaged by other agents may interfere with cell survival, perhaps by making the lesions inaccessible to the nucleotide excision repair system.

Identification and expression of the gene product encoding a CPD photolyase from Dunaliella salina.

Ultraviolet light induces photoproducts, cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs), in cellular DNA, which cause cytotoxic and genotoxic effects on the cells. Cells have several DNA repair mechanisms to repair the damage and to maintain genetic information of the cells. Photoreactivation is one of the DNA repair mechanism to remove UV-induced DNA damage from cellular DNA catalyzed by photolyase under visible light. Two types of photolyase, CPD photolyase and (6-4) photolyase, are specific for CPDs and for (6-4)PPs. We have isolated a gene product encoding CPD photolyase, named PHR2, from Dunaliella salina which is a kind of unicellular alga. Sequence analysis showed that PHR2 encodes a protein that has 529 amino acids and is similar to other Class II CPD photolyase. The complementation assay of the photoreactivation deficiency of the Escherichia coli SY2 by PHR2 cDNA showed a significant increase in survival rate when cells were irradiated with UV-C. Real-time PCR analysis indicated that the transcription of PHR2 was induced by UV-C, white light, high salinity, and H(2)O(2).

The cryptochrome-photolyase protein family in diatoms.

The cryptochrome - photolyase family (CPF) consists of homologous flavoproteins having completely different functions involving DNA repair, circadian rhythm and/or photoreception. From the original photolyases, working either as (6-4) or cyclobutane pyrimidine dimer photolyases, the animal- and plant-type cryptochromes, respectively, evolved and also the more intermediate DASH cryptochromes. Whereas animal cryptochromes work mostly in clock-related functions, plant cryptochromes are also directly involved in developmental processes such as hypocotyl elongation or flower induction. In diatoms, all types of cryptochromes and photolyases were predicted from genome sequences. However, up to now only two proteins have been characterised in more detail, CPF1 and CryP. CPF1 is related to animal-type cryptochromes, but works as a (6-4) photolyase in addition to having photoreceptor functions. It was shown to interact with the CLOCK:Bmal1 heterodimer in a heterologous system, and thus is probably involved in clock-related processes. Moreover, CPF1 directly influences transcription. The latter was also true for CryP, which is a cryptochrome distantly related to plant-type cryptochromes. In addition, CryP influences light-harvesting protein accumulation. For all diatom cryptochromes, down-stream signalling has to proceed via interaction partners different from the classical proteins involved in cryptochrome signalling in higher plants, because these candidates are missing in diatoms.

Structural and evolutionary aspects of algal blue light receptors of the cryptochrome and aureochrome type.

Blue-light reception plays a pivotal role for algae to adapt to changing environmental conditions. In this review we summarize the current structural and mechanistic knowledge about flavin-dependent algal photoreceptors. We especially focus on the cryptochrome and aureochrome type photoreceptors in the context of their evolutionary divergence. Despite similar photochemical characteristics to orthologous photoreceptors from higher plants and animals the algal blue-light photoreceptors have developed a set of unique structural and mechanistic features that are summarized below.

Repair of UV lesions in nucleosomes--intrinsic properties and remodeling.

Nucleotide excision repair and reversal of pyrimidine dimers by photolyase (photoreactivation) are two major pathways to remove UV-lesions from DNA. Here, it is discussed how lesions are recognized and removed when the DNA is condensed into nucleosomes. During the recent years it was shown that nucleosomes inhibit photolyase and excision repair in vitro and slow down repair in vivo. The correlation of DNA-repair rates with nucleosome positions in yeast suggests that intrinsic properties of nucleosomes such as mobility and transient unwrapping of nucleosomal DNA facilitate damage recognition. Moreover, it was shown that nucleosome remodeling activities can act on UV-damaged DNA in vitro and facilitate repair suggesting that random remodeling of chromatin might contribute to damage recognition in vivo. Recent work on nucleosome structure and mobility is included to evaluate how nucleosomes accommodate DNA lesions and how nucleosome mobility and remodeling can take place on damaged DNA.

The augmentation of melanocytic nevi in guinea pigs by solar-simulated light: an animal model for human melanocytic nevi.

Strong epidemiological evidence confirms the role of sunlight in human melanoma induction. Furthermore, the frequency of melanocytic nevi is a good indicator of future development of melanoma and a short-term marker of adverse reactions to melanoma-inducing sun exposure in humans. Thus, the aim of this study was to develop and define an animal model for sunlight-induced nevi that can be used as a surrogate model for sunlight-induced melanoma. Five treatment groups of 30-40 Hartley albino guinea pigs/group were treated with topical 7,12-dimethylbenzanthracene at a dose range of 6-240 mg on the dorsum of the skin. At week 20, half of the animals in each group were given a 12-month regimen of minimal erythemal solar-simulated light, 3 times/week, increased weekly to maintain erythema. These regimes induced epidermally derived pigmented melanocytic nevi clinically and histologically similar to human nevi (junctional, compound, and dermal). S100 and HMB45 staining was also consistent with the patterns seen in human nevi. In contrast to the high-dose 7,12-dimethylbenzanthracene-treated animals (60 and 240 mg), where solar-simulated light had no effect on nevi multiplicity, those groups treated with low doses (24, 12, and 6 mg) had a significant increase in nevi multiplicity after 12 months of solar-simulated light treatment (24 mg, 0.5 nevi/animal unirradiated versus 1.4 nevi/animal irradiated, P = 0.03; 12 mg, 0.2 unirradiated versus 1.2 irradiated, P = 0.02; 6 mg, 0 unirradiated versus 1.9 irradiated, P = 0.008). UVB-induced minimal erythemal dose was unaltered after exposure to photoreactivating light, consistent with the observation of others that placental mammals lack the DNA photolyase responsible for strong photoreactivation seen in nonplacental mammals and lower metazoans. Thus, our guinea pig model has some of the essential elements required to be a robust animal model for human nevi and a surrogate model for melanoma. These nevi are augmented by solar-simulated light, are histologically similar, occupy the same level within the skin, have the same natural history as human nevi, and are produced in an animal lacking strong photoreactivation. These features are not found in any previously described small laboratory animal model.

The mechanism of action of DNA photolyases.

Structural analysis, biochemistry and model studies have provided new insights into the mechanism of action of photolyases. The light-driven electron and energy transfer events that lead to the photolyase-catalyzed repair of lethal, mutagenic and carcinogenic UV-light-induced DNA lesions have all been examined in the past few years.

Cyclobutane pyrimidine dimers photolyase from extremophilic microalga: remarkable UVB resistance and efficient DNA damage repair.

Bacteria living in the Antarctic region have developed several adaptive features for growth and survival under extreme conditions. Chlamydomonas sp. ICE-Lis well adapted to high levels of solar UV radiation. A putative photolyase was identified in the Chlamydomonas sp. ICE-L transcriptome. The complete cDNA sequence was obtained by RACE-PCR. This PHR encoding includes a polypeptide of 579 amino acids with clear photolyase signatures belonging to class II CPD-photolyases, sharing a high degree of homology with Chlamydomonas reinhardtii (68%). Real-time PCR was performed to investigate the potential DNA damage and responses following UVB exposure. CPD photolyase mRNA expression level increased over 50-fold in response to UVB radiation for 6h. Using photolyase complementation assay, we demonstrated that DNA photolyase increased photo-repair more than 116-fold in Escherichia coli strain SY2 under 100µw/cm(2) UVB radiation. To determine whether photolyase is active in vitro, CPD photolyase was over-expressed. It was shown that pyrimidine dimers were split by the action of PHR2. This study reports the unique structure and high activity of the enzyme. These findings are relevant for further understanding of molecular mechanisms of photo-reactivation, and will accelerate the utilization of photolyase in the medical field.

Splitting of cis-syn cyclobutane thymine-thymine dimers by radiolysis and its relevance to enzymatic photoreactivation.

The 137Cs-gamma-irradiation of cis-syn thymine-thymine cyclobutane type dimers has been studied in aqueous solution. The mechanism of thymine dimer cleavage by eaq-, CO2.-, OH.,SO4.-,Br2.- and isopropanol radicals was studied using high pressure liquid chromatography (HPLC). Evidence that the one-electron reductants studied induce dimer cleavage partially by a chain reaction is presented. Approximate values for the one-electron reduction potential of thymine-thymine are obtained and thermodynamic calculations are presented in order to predict the direction of electron transfer in the case of enzymatic photoreactivation.

Enhanced survival by photoreactivation and liquid holding following UV damage of TN-368 insect cells.

These studies demonstrate that the TN-368 lepidopteran insect cell line, which is extremely resistant to the lethal effects of ionizing radiation, is also quite resistant to 254-nm ultraviolet light. While resistance to ionizing radiation in TN-368 cells has been associated with superior DNA repair processes, previous findings have indicated no correlation between survival ability and amount of unscheduled DNA synthesis in response to ultraviolet light. The present studies were undertaken to define the TN-368 ultraviolet light survival response, the ability of the cells to repair UV-induced damage by photoreactivation, the capacity of the cells to undergo UV repair during liquid holding in the dark, and the relationship between photoreactivation and liquid-holding recovery. Survival was assayed by colony formation. 254-nm irradiations were performed using germicidal lamps and photoreactivation was accomplished using black lights. Photoreactivable sectors of UV damage at 50 and 10% survival are 0.65 and 0.68, respectively. Survival responses, both with and without photoreactivation, have a small initial shoulder followed by an exponential region, and finally the curves continue to decrease but with decreasing slope. F0, Fq, and extrapolation number for the exponential portion of the curves are 77.5 J/m2, 16.8 J/m2, and 1.7 for non-photoreactivated cells and 234 J/m2, 56.1 J/m2, and 1.7 for those exposed to photoreactivating light. In the primarily exponential survival region, the fluences required to produce equivalent levels of survival in photoreactivated cells range from approximately 10.8 to 23.3 times as great as cells receiving UV light alone. The maximum survival enhancement of cells maintained under liquid-holding conditions over cells plated immediately following 100-400 J/m2 irradiations appears to be about 2-fold and occurs at 3-6 h of holding. Photoreactivation alone has a greater enhancement of survival than when photoreactivation follows liquid holding, but when liquid holding follows photoreactivation, the enhancement surpasses that of photoreactivation alone.