Optical properties of ZnMgO films grown by spray pyrolysis and their application to UV photodetection

This work presents a comprehensive optical characterization of Zn1−xMgxO thin films grown by spray pyrolysis (SP). Absorption measurements show the high potential of this technique to tune the bandgap from 3.30 to 4.11 eV by changing the Mg acetate content in the precursor solution, leading to a change of the Mg-content ranging from 0 up to 35%, as measured by transmission electron microscopy-energy dispersive x-ray spectroscopy. The optical emission of the films obtained by cathodoluminescence and photoluminescence spectroscopy shows a blue shift of the peak position from 3.26 to 3.89 eV with increasing Mg incorporation, with a clear excitonic contribution even at high Mg contents. The linewidth broadening of the absorption and emission spectra as well as the magnitude of the observed Stokes shift are found to significantly increase with the Mg content. This is shown to be related to both potential fluctuations induced by pure statistical alloy disorder and the presence of a tail of band states, the latter dominating for medium Mg contents. Finally, metal–semiconductor–metal photodiodes were fabricated showing a high sensitivity and a blue shift in the cut-off energy from 3.32 to 4.02 eV, i.e., down to 308 nm. The photodiodes present large UV/dark contrast ratios (102 − 107), indicating the viability of SP as a growth technique to fabricate low cost (Zn, Mg)O-based UV photodetectors reaching short wavelengths.

Oceanic protection of prebiotic organic compounds from UV radiation

It is frequently stated that UV light would cause massive destruction of prebiotic organic compounds because of the absence of an ozone layer. The elevated UV flux of the early sun compounds this problem. This applies to organic compounds of both terrestrial and extraterrestrial origin. Attempts to deal with this problem generally involve atmospheric absorbers. We show here that prebiotic organic polymers as well as several inorganic compounds are sufficient to protect oceanic organic molecules from UV degradation. This aqueous protection is in addition to any atmospheric UV absorbers and should be a ubiquitous planetary phenomenon serving to increase the size of planetary habitable zones.

Protein kinase mutants of human ATR increase sensitivity to UV and ionizing radiation and abrogate cell cycle checkpoint control

In fission yeast, the rad3 gene product plays a critical role in sensing DNA structure defects and activating damage response pathways. A structural homologue of rad3 in humans (ATR) has been identified based on sequence similarity in the protein kinase domain. General information regarding ATR expression, protein kinase activity, and cellular localization is known, but its function in human cells remains undetermined. In the current study, the ATR protein was examined by gel filtration of protein extracts and was found to exist predominantly as part of a large protein complex. A kinase-inactivated form of the ATR gene was prepared by site-directed mutagenesis and was used in transfection experiments to probe the function of this complex. Introduction of this kinase-dead ATR into a normal fibroblast cell line, an ATM-deficient fibroblast line derived from a patient with ataxia–telangiectasia, or a p53 mutant cell line all resulted in significant losses in cell viability. Clones expressing the kinase-dead ATR displayed increased sensitivity to x-rays and UV and a loss of checkpoint control. We conclude that ATR functions as a critical part of a protein complex that mediates responses to ionizing and UV radiation in human cells. These responses include effects on cell viability and cell cycle checkpoint control.

Structural kinetics of transcription activation at the malT promoter of Escherichia coli by UV laser footprinting

We have studied the kinetics of transcriptional initiation and activation at the malT and malTp1 promoters of Escherichia coli using UV laser footprinting. Contrary to previous studies and because of the very rapid signal acquisition by this technique, we can obtain structural information about true reaction intermediates of transcription initiation. The consequences of adding a transcriptional activator, the cAMP receptor protein/cAMP complex (CRP), are monitored in real time, permitting us to assign specific interactions to the activation of discrete steps in transcription initiation. Direct protein–protein contacts between CRP and the RNA polymerase appeared very rapidly, followed by DNA melting around the −10 hexamer. CRP slightly increased the rate of this isomerization reaction but, more importantly, favored the establishment of additional contacts between the DNA upstream of the CRP binding site and RNA polymerase subsequent to open complex formation. These contacts make a major contribution to transcriptional activation by stabilizing open forms of the promoter complex, thereby indirectly accelerating promoter escape. The ensemble of the kinetic, structural signals demonstrated directly that CRP exerts most of its activating effects on the late stages of transcriptional initiation at the malT promoter.

Lack of cell cycle regulation of telomerase activity in human cells

Conflicting reports have appeared concerning the cell cycle regulation of telomerase activity and its possible repression during quiescence and cell differentiation. We have reexamined these issues in an attempt to uncover the basis for the discrepancies. Variations in extracted telomerase activity during the cell cycle are not observed in cells sorted on the basis of DNA content. Variations are observed in cells synchronized using some biochemical cell cycle inhibitors, but only with those agents where cellular toxicity is evident. A progressive decline in telomerase activity is observed in cells whose growth rate is reduced from seven to eight population doublings per week to one to two doublings per week. Telomerase is largely absent in cells that truly exit the cell cycle and do not divide over the 7-day period. Although it is not necessary for all cell types to regulate telomerase in the same way, we conclude that in the immortal cultured cell lines examined, extracted telomerase activity does not change significantly during progression through the stages of the cell cycle. Telomerase activity generally correlates with growth rate and is repressed in cells that exit the cell cycle and become quiescent.

Down-regulation of interferon γ-activated STAT1 by UV light

STAT1 is a cytoplasmic transcription factor that is phosphorylated by Janus kinases (Jak) in response to interferon-γ (IFNγ). Phosphorylated STAT1 translocates to the nucleus, where it turns on specific sets of IFNγ-inducible genes. Here, we show that UV light interferes with tyrosine phosphorylation of STAT1, thereby hindering IFNγ from exerting its biological effects. This effect is not due to a down-regulation of the IFNγ receptor because phosphorylation of upstream-located Jak1 and Jak2 was not suppressed by UV light. In contrast, UV light had no effect on the phosphorylation of STAT3, which is activated by the proinflammatory cytokine interleukin 6. The UV light effect on STAT1 phosphorylation could be antagonized by vanadate, indicating at least partial involvement of a protein tyrosine phosphatase. Therefore, this study indicates a mechanism by which UV light can inhibit gene activation and suggests STAT1 as a new extranuclear UV target closely located to the membrane.

Proteinase inhibitors I and II from potatoes specifically block UV-induced activator protein-1 activation through a pathway that is independent of extracellular signal-regulated kinases, c-Jun N-terminal kinases, and P38 kinase

Solar UV irradiation is the causal factor for the increasing incidence of human skin carcinomas. The activation of the transcription factor activator protein-1 (AP-1) has been shown to be responsible for the tumor promoter action of UV light in mammalian cells. We demonstrate that proteinase inhibitor I (Inh I) and II (Inh II) from potato tubers, when applied to mouse epidermal JB6 cells, block UV-induced AP-1 activation. The inhibition appears to be specific for UV-induced signal transduction for AP-1 activation, because these inhibitors did not block UV-induced p53 activation nor did they exhibit any significant influence on epidermal growth factor-induced AP-1 transactivation. Furthermore, the inhibition of UV-induced AP-1 activity occurs through a pathway that is independent of extracellular signal-regulated kinases and c-Jun N-terminal kinases as well as P38 kinases. Considering the important role of AP-1 in tumor promotion, it is possible that blocking UV-induced AP-1 activity by Inh I or Inh II may be functionally linked to irradiation-induced cell transformation.

Ionizing radiation and short wavelength UV activate NF-κB through two distinct mechanisms

We examined the mechanisms by which two different types of photonic radiation, short wavelength UV (UV-C) and γ radiation, activate transcription factor NF-κB. Exposure of mammalian cells to either form of radiation resulted in induction with similar kinetics of NF-κB DNA binding activity, nuclear translocation of its p65(RelA) subunit, and degradation of the major NF-κB inhibitor IκBα. In both cases, induction of NF-κB activity was attenuated by proteasome inhibitors and a mutation in ubiquitin-activating enzyme, suggesting that both UV-C and γ radiation induce degradation of IκBs by means of the ubiquitin/proteasome pathway. However, although the induction of IκBα degradation by γ rays was dependent on its phosphorylation at Ser-32 and Ser-36, UV-C-induced IκBα degradation was not dependent on phosphorylation of these residues. Even the “super repressor” IκBα mutant, which contains alanines at positions 32 and 36, was still susceptible to UV-C-induced degradation. Correspondingly, we found that γ radiation led to activation of IKK, the protein kinase that phosphorylates IκBα at Ser-32 and Ser-36, whereas UV-C radiation did not. Furthermore, expression of a catalytically inactive IKKβ mutant prevented NF-κB activation by γ radiation, but not by UV-C. These results indicate that γ radiation and UV-C activate NF-κB through two distinct mechanisms.

Interaction between replication protein A and p53 is disrupted after UV damage in a DNA repair-dependent manner

Replication protein A (RPA) is required for both DNA replication and nucleotide excision repair. Previous studies have shown that RPA interacts with the tumor suppressor p53. Herein, we have mapped a 20-amino acid region in the N-terminal part of p53 that is essential for its binding to RPA. This region is distinct from the minimal activation domain of p53 previously identified. We also demonstrate that UV radiation of cells greatly reduces the ability of RPA to bind to p53. Interestingly, damage-induced hyperphosphorylated RPA does not associate with p53. Furthermore, down-regulation of the RPA/p53 interaction is dependent upon the capability of cells to perform global genome repair. On the basis of these data, we propose that RPA may participate in the coordination of DNA repair with the p53-dependent checkpoint control by sensing UV damage and releasing p53 to activate its downstream targets.

A CDKN2-like polymorphism in Xiphophorus LG V is associated with UV-B-induced melanoma formation in platyfish–swordtail hybrids

The genetic basis of spontaneous melanoma formation in spotted dorsal (Sd) Xiphophorus platyfish–swordtail hybrids has been studied for decades, and is adequately explained by a two-gene inheritance model involving a sex-linked oncogene, Xmrk, and an autosomal tumor suppressor, DIFF. The Xmrk oncogene encodes a receptor tyrosine kinase related to EGFR; the nature of the DIFF tumor suppressor gene is unknown. We analyzed the genetic basis of UV-B-induced melanoma formation in closely related, spotted side platyfish–swordtail hybrids, which carry a different sex-linked pigment pattern locus, Sp. We UV-irradiated spotted side Xiphophorus platyfish–swordtail backcross hybrids to induce melanomas at frequencies 6-fold higher than occur spontaneously in unirradiated control animals. To identify genetic determinants of melanoma susceptibility in this UV-inducible Xiphophorus model, we genotyped individual animals from control and UV-irradiated experimental regimes using allozyme and DNA restriction fragment length polymorphisms and tested for joint segregation of genetic markers with pigmentation phenotype and UV-induced melanoma formation. Joint segregation results show linkage of a CDKN2-like DNA polymorphism with UV-B-induced melanoma formation in these hybrids. The CDKN2-like polymorphism maps to Xiphophorus linkage group V and exhibits recombination fractions with ES1 and MDH2 allozyme markers consistent with previous localization of the DIFF tumor suppressor locus. Our results indicate that the CDKN2-like sequence we have cloned and mapped is a candidate for the DIFF tumor suppressor gene.

The inhibition of antigen-presenting activity of dendritic cells resulting from UV irradiation of murine skin is restored by in vitro photorepair of cyclobutane pyrimidine dimers

Exposing skin to UVB (280–320 nm) radiation suppresses contact hypersensitivity by a mechanism that involves an alteration in the activity of cutaneous antigen-presenting cells (APC). UV-induced DNA damage appears to be an important molecular trigger for this effect. The specific target cells in the skin that sustain DNA damage relevant to the immunosuppressive effect have yet to be identified. We tested the hypothesis that UV-induced DNA damage in the cutaneous APC was responsible for their impaired ability to present antigen after in vivo UV irradiation. Cutaneous APC were collected from the draining lymph nodes of UVB-irradiated, hapten-sensitized mice and incubated in vitro with liposomes containing a photolyase (Photosomes; Applied Genetics, Freeport, NY), which, upon absorption of photoreactivating light, splits UV-induced cyclobutane pyrimidine dimers. Photosome treatment followed by photoreactivating light reduced the number of dimer-containing APC, restored the in vivo antigen-presenting activity of the draining lymph node cells, and blocked the induction of suppressor T cells. Neither Photosomes nor photoreactivating light alone, nor photoreactivating light given before Photosomes, restored APC activity, and Photosome treatment did not reverse the impairment of APC function when isopsoralen plus UVA (320–400 nm) radiation was used instead of UVB. These controls indicate that the restoration of APC function matched the requirements of Photosome-mediated DNA repair for dimers and post-treatment photoreactivating light. These results provide compelling evidence that it is UV-induced DNA damage in cutaneous APC that leads to reduced immune function.

The Drosophila melanogaster Homologue of the Xeroderma Pigmentosum D Gene Product Is Located in Euchromatic Regions and Has a Dynamic Response to UV Light-induced Lesions in Polytene Chromosomes

The XPD/ERCC2/Rad3 gene is required for excision repair of UV-damaged DNA and is an important component of nucleotide excision repair. Mutations in the XPD gene generate the cancer-prone syndrome, xeroderma pigmentosum, Cockayne’s syndrome, and trichothiodystrophy. XPD has a 5′- to 3′-helicase activity and is a component of the TFIIH transcription factor, which is essential for RNA polymerase II elongation. We present here the characterization of the Drosophila melanogaster XPD gene (DmXPD). DmXPD encodes a product that is highly related to its human homologue. The DmXPD protein is ubiquitous during development. In embryos at the syncytial blastoderm stage, DmXPD is cytoplasmic. At the onset of transcription in somatic cells and during gastrulation in germ cells, DmXPD moves to the nuclei. Distribution analysis in polytene chromosomes shows that DmXPD is highly concentrated in the interbands, especially in the highly transcribed regions known as puffs. UV-light irradiation of third-instar larvae induces an increase in the signal intensity and in the number of sites where the DmXPD protein is located in polytene chromosomes, indicating that the DmXPD protein is recruited intensively in the chromosomes as a response to DNA damage. This is the first time that the response to DNA damage by UV-light irradiation can be visualized directly on the chromosomes using one of the TFIIH components.

Nuclear Ferritin Protects DNA From UV Damage in Corneal Epithelial Cells

Previously, we identified the heavy chain of ferritin as a developmentally regulated nuclear protein of embryonic chicken corneal epithelial cells. The nuclear ferritin is assembled into a supramolecular form indistinguishable from the cytoplasmic form of ferritin found in other cell types and thus most likely has iron-sequestering capabilities. Free iron, via the Fenton reaction, is known to exacerbate UV-induced and other oxidative damage to cellular components, including DNA. Since corneal epithelial cells are constantly exposed to UV light, we hypothesized that the nuclear ferritin might protect the DNA of these cells from free radical damage. To test this possibility, primary cultures of cells from corneal epithelium and stroma, and from skin epithelium and stroma, were UV irradiated, and DNA strand breaks were detected by an in situ 3′-end labeling method. Corneal epithelial cells without nuclear ferritin were also examined. We observed that the corneal epithelial cells with nuclear ferritin had significantly less DNA breakage than other cell types examined. Furthermore, increasing the iron concentration of the culture medium exacerbated the generation of UV-induced DNA strand breaks in corneal and skin fibroblasts, but not in the corneal epithelial cells. Most convincingly, corneal epithelial cells in which the expression of nuclear ferritin was inhibited became much more susceptible to UV-induced DNA damage. Therefore, it seems that corneal epithelial cells have evolved a novel, nuclear ferritin-based mechanism for protecting their DNA against UV damage.

UV light selectively coinduces supply pathways from primary metabolism and flavonoid secondary product formation in parsley

The UV light-induced synthesis of UV-protective flavonoids diverts substantial amounts of substrates from primary metabolism into secondary product formation and thus causes major perturbations of the cellular homeostasis. Results from this study show that the mRNAs encoding representative enzymes from various supply pathways are coinduced in UV-irradiated parsley cells (Petroselinum crispum) with two mRNAs of flavonoid glycoside biosynthesis, encoding phenylalanine ammonia-lyase and chalcone synthase. Strong induction was observed for mRNAs encoding glucose 6-phosphate dehydrogenase (carbohydrate metabolism, providing substrates for the shikimate pathway), 3-deoxyarabinoheptulosonate 7-phosphate synthase (shikimate pathway, yielding phenylalanine), and acyl-CoA oxidase (fatty acid degradation, yielding acetyl-CoA), and moderate induction for an mRNA encoding S-adenosyl-homocysteine hydrolase (activated methyl cycle, yielding S-adenosyl-methionine for B-ring methylation). Ten arbitrarily selected mRNAs representing various unrelated metabolic activities remained unaffected. Comparative analysis of acyl-CoA oxidase and chalcone synthase with respect to mRNA expression modes and gene promoter structure and function revealed close similarities. These results indicate a fine-tuned regulatory network integrating those functionally related pathways of primary and secondary metabolism that are specifically required for protective adaptation to UV irradiation. Although the response of parsley cells to UV light is considerably broader than previously assumed, it contrasts greatly with the extensive metabolic reprogramming observed previously in elicitor-treated or fungus-infected cells.

Cattle lack vascular receptors for Escherichia coli O157:H7 Shiga toxins

Escherichia coli O157:H7 causes Shiga toxin (Stx)-mediated vascular damage, resulting in hemorrhagic colitis and the hemolytic uremic syndrome in humans. These infections are often foodborne, and healthy carrier cattle are a major reservoir of E. coli O157:H7. We were interested in knowing why cattle are tolerant to infection with E. coli O157:H7. Cattle tissues were examined for the Stx receptor globotriaosylceramide (Gb3), for receptivity to Stx binding in vitro, and for susceptibility to the enterotoxic effects of Stx in vivo. TLC was used to detect Gb3 in tissues from a newborn calf. Gb3 was detected by TLC in kidney and brain, but not in the gastrointestinal tract. Immunohistochemistry was used to define binding of Stx1 and Stx2 overlaid onto sections from cattle tissues. Stx1 and Stx2 bound to selected tubules in the cortex of the kidney of both newborn calves (n = 3) and adult cattle (n = 3). Stx did not bind to blood vessels in any of the six gastrointestinal and five extraintestinal organs examined. The lack of Gb3 and of Stx receptivity in the gastrointestinal tract raised questions about the toxicity of Stx in bovine intestine. We found that neither viable E. coli O157:H7 nor Stx-containing bacterial extracts were enterotoxic (caused fluid accumulation) in ligated ileal loops in newborn calves. The lack of vascular receptors for Stx provides insight into why cattle are tolerant reservoir hosts for E. coli O157:H7.