Low Doses Of UV Light Research Articles

Effect of chlorination and ultraviolet disinfection on tetA-mediated tetracycline resistance of Escherichia coli

Antibiotic-resistant bacteria are an emerging threat to public health during drinking water consumption and reclaimed water reuse. Several studies have shown that the proportions of antibiotic-resistant bacteria in waters may increase when exposed to low doses of UV light or chlorine. In this study, inactivation of tetracycline-resistant Escherichia coli and antibiotic-sensitive E. coli by UV disinfection and chlorination was compared to determine the tolerance of tetracycline-resistant E. coli to UV light and chlorine, and tetracycline resistance of a tetracycline-resistant E. coli population was studied under different doses of the disinfectants. Our results showed that relative to antibiotic-sensitive E. coli, tetracycline-resistant E. coli had the same tolerance to UV light and a potentially higher tolerance to chlorination. The mortality frequency distributions of tetracycline-resistant E. coli exposed to tetracycline were shifted by both chlorination and UV disinfection. When compared to the hemi-inhibitory concentrations (IC(50)) of tetracycline-resistant E. coli with no exposure to UV or chlorination, the IC(50) of tetracycline-resistant E. coli treated with tetracycline was 40% lower when inactivation by UV light or chlorination reached 3-log but was 1.18 times greater when inactivation by chlorination reached 4.3-log. Chlorination applied to drinking water or reclaimed water treatment may increase the risk of selection for highly tetracycline-resistant E. coli.

Effects of α- and β-Hydroxy Acids on the Edemal Response Induced in Female SKH-1 Mice by Simulated Solar Light

alpha- and beta-Hydroxy acids have been used extensively in cosmetic and dermatological formulations. At present, there is an inadequate amount of information with which to assess the safety of topical applications of alpha- and beta-hydroxy acids in conjunction with exposure to ultraviolet light. In the present study, we examined changes in the epidermal basal cell proliferation and the edemal response using skin thickness measurements elicited in SKH-1 mice following exposure to simulated solar light (SSL) with or without topical treatment with creams containing alpha- (glycolic) and beta-hydroxy (salicylic) acids. The dose of SSL light required to induce measurable edema (MED(BIOL)) in nai;ve, free-moving SKH-1 mice was determined to be 90 mJ. CIE/cm(2). Pretreating the mice with daily (5 days/week) exposures of 14 mJ. CIE/cm(2) for 6 weeks resulted in a doubling of the MED(BIOL) to 180 mJ. CIE/cm(2). Topical application of control cream (pH 3.5), or creams containing glycolic acid (10%, pH 3.5) or salicylic acid (4%, pH 3.5) for 6 weeks (5 days/week) increased the MED(BIOL) to 137 mJ. CIE/cm(2). Daily treatments with SSL (14 mJ. CIE/cm(2)) and control cream (pH 3.5), glycolic (10%, pH 3.5) or salicylic (4%, pH 3.5) acid-containing creams for 6 weeks (5 days/week) resulted in an MED(BIOL) value of 180 mJ. CIE/cm(2), which was the same as treatment with light alone for 6 weeks. These data indicate that a 6-week treatment of mouse skin with a representative skin cream, with or without representative alpha- and beta-hydroxy acids (glycolic and salicylic acid, respectively), changes the UV light sensitivity; however, treatment with the cream, with or without the acids, does not contribute to the UV sensitivity of mice cotreated with low doses of UV light.

Stress-Inducible DNA Repair in Saccharomyces Cerevisiae

Saccharomyces cerevisiae shows altered radiation response under various stress conditions, such as nutrition depletion, nitrogen starvation, osmotic shock, heat shock, and mild chemical treatments. In general, the cells show higher levels of UV or gamma radiation resistance under the stress. However, not all the stress conditions affect the repair system in the same manner. For example, depletion of nitrogen supply in the growth medium has been shown to enhance the repair of gamma ray-induced DNA damage without significantly affecting the UV response of the cells. On the other hand, a mild treatment with alkali or hydrogen peroxide improves the response to UV light but not to gamma radiation. It has further been shown that the effect of these stresses are not additive, e.g., the alkali and hydrogen peroxide treatments given simultaneously show the same effect as either of them alone. Low levels of gamma and UV radiation exposures are also treated as stress in the present context. Studies show that irradiation of low-dose gamma rays results in enhanced excision repair of UV-induced pyrimidine dimers. However, in all the wild-type strains tested, none showed any effect on gamma rays response. The exposure to low doses of UV light did not show any effect on either the gamma rays or the UV response. It is suggested that the stress-induced enhancement of DNA repair can be of two types: 1) A general response to stress, which prepares the organism to survive in adverse circumstances (some of the proteins produced during this response also take part in the DNA repair), and 2) a particular response involving DNA damage, such as that caused by gamma irradiation. In this case, the DNA damage may act as a signal for enhancement of the DNA repair.

Exposure to low dose of gamma radiation enhances the excision repair in Saccharomyces cerevisiae.

The effect of low doses of ionizing and nonionizing radiation on the radiation response of yeast Saccharomyces cerevisiae toward ionizing and nonionizing radiation was studied. The wild-type strain D273-10B on exposure to 54 Gy gamma radiation (resulting in about 10% cell killing) showed enhanced resistance to subsequent exposure to UV radiation. This induced UV resistance increased with the incubation time between the initial gamma radiation stress and the UV irradiation. Exposure to low doses of UV light on the other hand showed no change in gamma or UV radiation response of this strain. The strains carrying a mutation at rad52 behaved in a way similar to the wild type, but with slightly reduced induced response. In contrast to this, the rad3 mutants, defective in excision repair, showed no induced UV resistance. Removal of UV-induced pyrimidine dimers in wild-type yeast DNA after UV irradiation was examined by analyzing the sites recognized by UV endonuclease from Micrococcus luteus. The samples that were exposed to low doses of gamma radiation before UV irradiation were able to repair the pyrimidine dimers more efficiently than the samples in which low gamma irradiation was omitted. The nature of enhanced repair was studied by scoring the frequency of induced gene conversion and reverse mutation at trp and ilv loci respectively in strain D7, which showed similar enhanced UV resistance induced by low-dose gamma irradiation. The induced repair was found to be essentially error-free. These results suggest that irradiation of strain D273-10B with low doses of gamma radiation enhances its capability for excision repair of UV-induced pyrimidine dimers.

DNA strand breaks: the DNA template alterations that trigger p53-dependent DNA damage response pathways.

The tumor suppressor protein p53 serves as a critical regulator of a G1 cell cycle checkpoint and of apoptosis following exposure of cells to DNA-damaging agents. The mechanism by which DNA-damaging agents elevate p53 protein levels to trigger G1/S arrest or cell death remains to be elucidated. In fact, whether damage to the DNA template itself participates in transducing the signal leading to p53 induction has not yet been demonstrated. We exposed human cell lines containing wild-type p53 alleles to several different DNA-damaging agents and found that agents which rapidly induce DNA strand breaks, such as ionizing radiation, bleomycin, and DNA topoisomerase-targeted drugs, rapidly triggered p53 protein elevations. In addition, we determined that camptothecin-stimulated trapping of topoisomerase I-DNA complexes was not sufficient to elevate p53 protein levels; rather, replication-associated DNA strand breaks were required. Furthermore, treatment of cells with the antimetabolite N(phosphonoacetyl)-L-aspartate (PALA) did not cause rapid p53 protein increases but resulted in delayed increases in p53 protein levels temporally correlated with the appearance of DNA strand breaks. Finally, we concluded that DNA strand breaks were sufficient for initiating p53-dependent signal transduction after finding that introduction of nucleases into cells by electroporation stimulated rapid p53 protein elevations. While DNA strand breaks appeared to be capable of triggering p53 induction, DNA lesions other than strand breaks did not. Exposure of normal cells and excision repair-deficient xeroderma pigmentosum cells to low doses of UV light, under conditions in which thymine dimers appear but DNA replication-associated strand breaks were prevented, resulted in p53 induction attributable to DNA strand breaks associated with excision repair. Our data indicate that DNA strand breaks are sufficient and probably necessary for p53 induction in cells with wild-type p53 alleles exposed to DNA-damaging agents.

DNA Strand Breaks: the DNA Template Alterations That Trigger p53-Dependent DNA Damage Response Pathways

The tumor suppressor protein p53 serves as a critical regulator of a G1 cell cycle checkpoint and of apoptosis following exposure of cells to DNA-damaging agents. The mechanism by which DNA-damaging agents elevate p53 protein levels to trigger G1/S arrest or cell death remains to be elucidated. In fact, whether damage to the DNA template itself participates in transducing the signal leading to p53 induction has not yet been demonstrated. We exposed human cell lines containing wild-type p53 alleles to several different DNA-damaging agents and found that agents which rapidly induce DNA strand breaks, such as ionizing radiation, bleomycin, and DNA topoisomerase-targeted drugs, rapidly triggered p53 protein elevations. In addition, we determined that camptothecin-stimulated trapping of topoisomerase I-DNA complexes was not sufficient to elevate p53 protein levels; rather, replication-associated DNA strand breaks were required. Furthermore, treatment of cells with the antimetabolite N(phosphonoacetyl)-L-aspartate (PALA) did not cause rapid p53 protein increases but resulted in delayed increases in p53 protein levels temporally correlated with the appearance of DNA strand breaks. Finally, we concluded that DNA strand breaks were sufficient for initiating p53-dependent signal transduction after finding that introduction of nucleases into cells by electroporation stimulated rapid p53 protein elevations. While DNA strand breaks appeared to be capable of triggering p53 induction, DNA lesions other than strand breaks did not. Exposure of normal cells and excision repair-deficient xeroderma pigmentosum cells to low doses of UV light, under conditions in which thymine dimers appear but DNA replication-associated strand breaks were prevented, resulted in p53 induction attributable to DNA strand breaks associated with excision repair. Our data indicate that DNA strand breaks are sufficient and probably necessary for p53 induction in cells with wild-type p53 alleles exposed to DNA-damaging agents.

Paracetamol inhibits UV-induced DNA repair in resting human mononuclear blood cells in vitro.

The effects of paracetamol on the repair of DNA damage in resting human peripheral mononuclear blood cells (MNC) in vitro were investigated by means of the alkaline elution technique. Low doses of UV light (254 nm, 3 J/m2) caused a transient increase in the amount of DNA single-strand breaks and alkali-labile sites (SSBs). Paracetamol (0.1-1.0 mM) present during post-irradiation incubation approximately doubled the maximum level of UV-induced (1-3 J/m2) SSBs and delayed the completion of repair. Although there were considerable variations between cells prepared from different donors, the level of UV-induced DNA SSBs was always higher with paracetamol. Hydroxyurea (0.3 mM), an inhibitor of ribonucleotide reductase, caused a similar increased accumulation and slow removal of SSBs, whereas cytosine-1-beta-D-arabinofuranoside (Ara C) (10 microM), an inhibitor of DNA polymerases, led to a steady accumulation of DNA SSBs. The increased levels of SSBs caused by paracetamol or hydroxyurea were both completely suppressed by concomitant addition of deoxyribonucleosides; this supports the notion that paracetamol as well as hydroxyurea inhibits ribonucleotide reductase. About the same rates of formation and removal of UV-induced SSBs were observed in T lymphocytes, B lymphocytes and monocytes. In both isolated T lymphocytes and B lymphocytes, paracetamol (0.3 mM) markedly increased the level of DNA SSBs induced by UV, whereas monocytes seemed to be less sensitive to the effect of paracetamol. It is concluded that the inhibition of DNA repair may contribute to the clastogenic effects of paracetamol.

RNA synthesis and stability in UV-irradiated and nonirradiated mouse L cells.

In mouse L cells, relatively low doses of UV light (e.g., about 35 J/m2) induced the rapid breakdown of the molecules of many RNA species transcribed shortly before irradiation. This included 28S, 18S, 5.8S, and 5S rRNA, U1, U2, U3, U4, and U5 small nuclear RNA, but not the main band of transfer RNAs or 7SL RNA. At higher UV doses, an RNA band that contains tRNAleu was also degraded rapidly after UV irradiation. RNA molecules synthesized long before irradiation (e.g., 22 h for small RNAs, 4 h for large rRNAs) were not affected. Our results suggest that the maturation and/or assembly into fully mature ribonucleoprotein particles of several small RNA species is not completed 4 h after transcription. The effect of UV radiation occurred in mouse L cells, but not in human HeLa or KB cells. In a previous report, L cells were transformed by DNA transfection with two mouse U1b RNA genes, named U1.1 and U1.2. We observed now that, in L cells transformed with the U1.2 gene, the ratio of radioactivity in the apparent U1b and U1a RNA precursors after 5 min of labeling was about 20 times higher than a) this ratio in briefly labeled L cells that had been transformed with the U1.1 gene, and b) the ratio of radioactive mature U1b and U1a RNA after 20 h of chase in L cells transformed with the U1.2 gene. These results suggest that very high levels of U1b RNA are transcribed from the exogenous U1.2 gene copies, followed by the rapid degradation of most of these transcripts.

Two transcription products of the vesicular stomatitis virus genome may control L-cell protein synthesis.

When mouse L-cells are infected with vesicular stomatitis virus, there is a decrease in the rate of protein synthesis ranging from 20 to 85% of that in mock-infected cells. Vesicular stomatitis virus, irradiated with increasing doses of UV light, eventually loses this capacity to inhibit protein synthesis. The UV inactivation curve was biphasic, suggesting that transcription of two regions of the viral genome is necessary for the virus to become inactivated in this capacity. The first transcription product corresponded to about 373 nucleotides, and the second corresponded to about 42 nucleotides. Inhibition of transcription of the larger product by irradiating the virus with low doses of UV light left a residual inhibition of protein synthesis consisting of approximately 60 to 65% of the total inhibition. This residual inhibition could be obviated by irradiating the virus with a UV dose of greater than 20,000 ergs/mm(2) and was thus considered to represent the effect of the smaller transcription product. In the R1 mutant of C. P. Stanners et al. (Cell 11:273-281, 1977), inhibition of transcription of the larger product sufficed to restore protein synthesis to the mock-infected level, suggesting that the smaller transcription product is nonfunctional with respect to protein synthesis inhibition. It thus appears that the inhibition of protein synthesis by wild-type vesicular stomatitis virus involved at least two separate viral transcription products, and the inhibition by the R1 mutant involved only one. Extracts from cells infected with virus irradiated with low doses of UV light showed a protein synthesis capacity quite similar to that of their in vivo counterparts, indicating that these extracts closely reflect the in vivo effects of virus infection.

Effects of antibiotics on UV-stimulated tube growth of Pinus silvestris pollen.

Studies were made to investigate the effects of different antibiotics on unirradiated pollen and on pollen with enhanced tube growth, stimulated by low doses of UV-light. The antibiotics mitomycin, chloramphenicol, tetracyclin, penicillin, nystatin and carbony-cyanid phenylhydrazon were not able to suppress tube growth stimulation of pine pollen. The data obtained are discussed in view of the stimulation mechanism of low doses of UVP-light.

Localisation of the site of action of tube growth stimulation by micro UV-irradiation of pine pollen.

Micro-irradiation of pine pollen grains was carried out with different doses at four different dose rates and the tube growth was observed. The irradiation of the whole pollen grains in the dehydrated state and dorsal position and of those in the hydrated state and ventral position induced stimulated tube growth after receiving low doses of UV light. The effect of stimulation depended on the ratio between dose and dose rate. After partial irradiation of pollen grains at low doses, carried out with the technique of "blind shot", a stimulation effect could also be observed. It was calculated that the irradiation of the cytoplasm had a strong, the irradiation of the active nucleus (vegetative) had a moderate and the irradiation of the dormant nucleus (generative) had little or no dose rate dependance. The dose effect of the nuclei showed a reverse tendency to the dose effect of the cytoplasm. Experiments with different light filters suggested that the dose rate dependence of the cytoplasm is probably not caused by a repair mechanism. The vegetative nucleus showed an effect of photoreactivation and probably also of a dark repair. The generative nucleus exhibited only an effect of photoreactivation.

Effect of ultraviolet radiation on pinocytosis in Amoeba proteua.

Ultraviolet (UV) irradiation (4 000-10 000 erg X mm(-2) decreased membrane potential and input resistance of Amoeba proteus and induced formation of pinocytotic channels. Submaximal pinocytosis induced by UV light was additive to pinocytosis induced by K+ or Na+ and stimulated in the presence of EGTA. It was not inhibited by the presence of La+++ or by pretreatment with dibucaine. In these respects and with respect to optimum pH and pCa, UV induced pinocytosis. Accumulation of K+ in the amoeba membrane after a dose of radiation may explain the similarity between pinocytosis induced by UV light and potassium salts. Ca++ present during the period of irradiation inhibited the effect of UV light. Instead Ca++ applied after irradiation (1-20 mM) increased channel formation. This effect was stimulated the presence of local anesthetic drugs. It is suggested that high doses of UV light may induce channel formation by releasing Ca++ from the cell membrane into the cell (UV induced pinocytosis). Ca++ may be released at the moment of absorption of UV light in the membrane as well as during the period of depolarization which follows irradiation. Low doses of UV light may permit extracellular Ca++ to enter the cell and stimulate channel formation (calcium induced pinocytosis). Dithiotreitol (1 mM) applied after irradiation depressed both UV and calcium induced pinocytosis so these may be the result of the same structural change which involves the formation of disulphide bonds in the membrane.

Studies on mitotic gene conversion in Ustilago.

In order to develop a system for the study of the mechanism of intragenic recombination inUstilago, mutants lacking nitrate reductase activity were isolated, and five alleles were combined in pairs in ten vegetative heteroallelic diploids. The diploids have the mutant phenotype, i.e. inability to utilize nitrate as sole source of nitrogen, but they will recombine to produce wild-type cells much more frequently than the back-mutation rates of haploids or homoallelic diploids. The spontaneous rate of recombination can be enormously increased by low doses of UV light, particularly if treatment is during the period of DNA synthesis in the mitotic cycle. By means of half-tetrad analysis it has been shown that this process of intragenic recombination, as in other fungi, is due to gene conversion rather than reciprocal exchange. It has also been shown that the frequency of UV-induced conversion under standard conditions gives a rough measure of the distance between two mutant sites, since it was possible to use these frequencies to make a linear fine structure map of the gene. These results are discussed in relation to a hybrid DNA model for gene conversion slightly modified from that previously suggested for meiotic recombination.

Modification of mutagenesis initiated by ultraviolet light through postteatment of bacteria with basic dyes.

Post treatment with basic dyes, in concentrations that retard cell division, was found to influence the tnduction of mutations to prototrophy by UV light in a tyrosine-requirtng strain of E.Coli. Pyronin, which is unique among the dyes in tts selective affinity for RNA, was found to duplicate the effects of chloramphenicol or amino acfd deprtvatfon in causfng the rapid and irreversible loss of potential prototrophs (mutation frequency decline, or MFD). Acriflavtne, methyl green. crystal violet, methylene blue, and toluidine blue, all of which are known to combine with DNA, delay or retard the occurrence of MFD under conditions of amino acid deprivation. When acriflavine is removed from its combination with cellular components by the addition of an excess of sodium deoxyrtbonucleate, MFD begins promptly. The same basic dyes that delay MFD were also found to interfere with the fixation of mutations (MF) in an amino acid- enriched medium, and to cause marked enhancement of the mutagenic potency of low doses of UV light. While showing no independent mutagenic activity for unirradiated bacteria, all the dyes except pyronin increased the yield of induced mutations signtficantly when added to the enriched medium upon which trradiated bacteria were incubated.These results were interpreted asmore » evidence that UV light initiates mutagenesis by producing unstable changas directly in genic DNA. MFD is interpreted as a repair process, blocked by the machinery of RNA and protein synthesis and by the presence of certain basic dyes.« less