Low Doses Of Ultraviolet Light Research Articles

Evidence for the dark repair of ultraviolet damage in Saccharomyces cerevisiae mitochondrial DNA.

Evidence for the dark repair of ultraviolet damage to yeast mitochondrial DNA has been observed. The ultraviolet dose necessary to inflict significant damage to both nuclear and mitochondrial DNA was determined. Cell survival at large doses of ultraviolet light was observed after immediate and delayed plating of yeast onto 1% pyruvate and 1% glucose media. In the highly lethal dose ranges of irradiation an increase in the number of normal colonies appeared after a period of liquid holding and delayed plating. This increase, demonstrated separately on 1% glucose and 1% pyruvate media suggested that the repair of both mitochondrial and nuclear DNA had occurred. After low doses of ultraviolet light an actual decrease in the number of petite survivors was seen after delayed plating, even though the total number of survivors increased. When a known repair inhibitor, caffeine, was added to the liquid holding buffer prior to the delayed plating of yeast, a marked decrease in the number of petites did not occur after delayed plating. Therefore, the decrease in the number of petite survivors after delayed plating following low doses of ultraviolet light is attributed to the repair of yeast mitochondrial DNA.

Repair of pyrimidine dimers in nuclear and mitochondrial DNA of yeast irradiated with low doses of ultraviolet light

The repair of damage induced by ultraviolet light has been examined in both the nuclear and mitochondrial DNA of the yeast Saccharomyces cerevisiae . The sensitive assay used in this study is based on the capacity of the bacteriophage T4 u.v. endonuclease to produce single-strand breaks in DNA that contains pyrimidine dimers, thus permitting the use of low fluences (doses) of u.v. The results demonstrate that virtually all of the dimers induced in the nuclear DNA of a repair-proficient strain ( RAD +) are removed following dark incubation for four hours in growth medium. In contrast, the dimers induced in mitochondrial DNA by the same u.v. fluence are retained under the same conditions. In the excision-deficient mutant, rad1-2 , no evidence was obtained for removal of pyrimidine dimers from nuclear DNA. Photoreactivation of both RAD + and rad1-2 cultures resulted in decrease of dimers from both nuclear and mitochondrial DNA. We conclude that an excision-repair mechanism operates on nuclear but not mitochondrial DNA in repair-proficient yeast, and that the rad1-2 mutant is defective in this process.

Induction of prophage lambda during the division cycle of Escherichia coli.

When synchronous populations of Escherichia coli B/r (lambda) were exposed to low doses of ultraviolet light, the yield of infective centres varied with cell age. The yield was highest if the lysogenic bacteria were irradiated at a time which coincides approximately with the termination of rounds of DNA replication and it was lowest when dividing cells were irradiated. No such variation was detected following either irradiation of excision-defective lysogenic cells or thermal induction of lambdacI857 prophage in irradiated bacteria. It is suggested that the variation reflects a relationship between prophage induction and inhibition of cell division. This hypothesis is supported by data showing that irradiation promoted induction and curtailed division in E. coli K12 dnaA mutants which were dividing in the absence of DNA replication.

Transcription and translation of the sex message in the smut fungus, Ustilago violacea. I. The effect of ultraviolet light.

ABSTRACT Cells of opposite mating type, a1 and a2, of the anther smut, Ustilago violácea assemble a conjugation tube after about 3–4 h of mating on nutrient-free media. Low doses of ultraviolet light (u.v.) delay but do not prevent conjugation in wild-type strains if given in the first 2–3 h of mating. However, irradiation later than this period has little effect and conjugation proceeds normally. The u.v. effect is photoreactivable and we conclude that u.v. induces dimers which affect transcription of specific messenger RNA molecules needed for conjugation (‘sex message ‘). Our evidence suggests that the dimers may cause mistranscription rather than the complete prevention of transcription. The effect of u.v. on conjugation in reciprocal crosses of u.v.-sensitive and wild-type strains indicates clearly that both partners must complete transcription of sex messages in order to conjugate. Inactivation of either partner before transcription prevents conjugation, but conjugation proceeds when either cell is inactivated after transcription of the sex messages has occurred. These results suggest that a mutual and reciprocal exchange of information between the 2 mating-types occurs prior to the assembly of the conjugation tube.

Induction of R Bodies by Ultraviolet Light in Killer Paramecia

SUMMARY: Kappa, a bacterial endosymbiont present in killer stocks of Paramecium aurelia, exists in two forms - brights, which contain a refractile (R) body, and non-brights which lack the R body. Non-brights are self-reproducing and give rise to brights, which cannot reproduce and are toxic to sensitive paramecia which ingest them. Associated with the R body are structures resembling bacteriophages in form and chemical composition. Animals of stock 51, syngen 4 of P. aurelia were given low doses of ultraviolet light. The percentage of brights increased markedly, beginning 6 h after irradiation. In three experiments, absolute numbers of kappas were determined 13 to 14 h after exposure to u.v.: the number of brights increased and non-brights decreased significantly, while the total number of kappas remained the same. At 24 h, after irradiation the numbers of kappa particles had decreased considerably; after 3 days they were completely gone. The results are consistent with the hypothesis that the phage-like structures exist as u.v.-inducible prophages in non-brights, and that when a non-bright becomes a bright, the prophage is spontaneously induced forming, at the same time, mature phage-like elements and the R body. These events constitute the lytic cycle which ends with the destruction of the bright.

A method for detecting pyrimidine dimers in the DNA of bacteria irradiated with low doses of ultraviolet light.

Cells of Escherichia coli treated with lysozyme and Brij-58 become permeable to proteins, but do not release their DNA. I incubated permeable cells with an endonuclease that produces single-strand breaks in DNA-containing pyrimidine dimers. The enzyme entered the permeable cells, and, if they had been irradiated with ultraviolet light, caused breaks in their DNA. The frequency of breaks was estimated from the sedimentation pattern of the DNA in alkaline sucrose gradients. The procedure is sensitive enough to detect the dimers produced by a dose of 10 erg/mm(2) at 254 nm, or about 50 dimers per E. coli genome. This method exemplifies and extends the use of permeabilized cells for examining biological processes at the molecular level.

A specific photoreaction in polydeoxyadenylic acid.

Ultraviolet photosensitivity of the single-stranded forms of poly(A) and poly(dA) is compared. In contrast to poly(A), poly(dA) is rather sensitive to ultraviolet irradiation. Relatively low doses of ultraviolet light lead to characteristic changes of the ultraviolet and circular dichroism spectra of poly(dA), suggesting formation of a specific photoproduct in poly(dA) that cannot be formed in poly(A). The quantum yields in poly(dA) (varphi = 2.5 x 10(-3)) are about a factor of 10 higher than in poly(A), indicating a high structure specificity. The base-pairing capacity of irradiated probes of poly(dA) to poly(dT) is inhibited. These results demonstrate a relatively high probability of DNA photolesions, that cannot be repaired by simple repair enzyme systems.

Loss of lysis inhibition in filamentous Escherichia coli infected with wild-type bacteriophage T4.

The plaque enlargement of wild-type T4 bacteriophage observed when assayed in the presence of low concentrations of mitomycin C or after exposure to very low doses of ultraviolet light was studied by using solid as well as liquid culture media. It was found that the filamentous cell formed by the treatment with the agents is responsible for the phenomenon. The filamentous cell was also shown to be characterized not only by the loss of capacity of lysis inhibition but also by a shortening of the latent period. No difference in cellular rigidity could be seen between the filamentous cell and normal cell as far as the analysis from the outside of the cell was concerned, whereas the former cell was shown to be more readily susceptible to phage-induced lysozyme from the inside of the cell. A possible change in the membrane of the filamentous cell and a possible mechanism for lysis inhibition are discussed.

Effects of ultraviolet light irradiation and heat-shocks on cell division in synchronized Tetrahymena

The division of synchronized Tetrahymena pyriformis (strain W) irradiated with low doses of ultraviolet light is delayed if the irradiation is administered during the first 30 min after the end of the synchronizing treatment. After a short period of transition, division in most cells is not delayed even by relatively high doses. The delay induced by a particular low dose administered at any time up to the transition to insensitivity is essentially constant. The division of synchronized cells is delayed by heat-shocks (34 ± 0.1 °C, for 20 min) applied at any time up to approximately 60 min after the end of the synchronizing treatment. After this time, for any single cell in a sample, there is an instantaneous transition to heat-shock insensitivity. The delays in cell division induced by heat shocks increase with time from the end of the synchronizing treatment, attaining a maximum just before the transition to insensitivity. The two different transition times in the responses of synchronized cells to UV on the one hand and to heat shocks on the other are interpreted as representing the completion of two different processes in the final preparation of the cells for division. The qualitative and quantitative differences between the responses to UV and to heat shock suggest that two different division-essential substances are involved, one sensitive to UV and one to heat. Some evidence is presented that suggests that the completion of the heat-sensitive process depends upon the UV-sensitive process.

Mutational studies onPhytophthora infestans

Attempts were made to induce and isolate auxotrophic mutants ofPhytophthora infestans, using ultra-violet light and ethyl methane sulphonate as mutagens. Low doses of ultra-violet light had little effect on the germination of the spore but markedly reduced the number of spores forming colonies and the growth rate of these colonies. Ethyl methane sulphonate did not affect the growth rate of the surviving colonies. No auxotrophic mutants were isolated but differences were noted in the ability of some survivors to initiate growth on the minimal medium, although once growth was established the colonies grew quite readily.

COMPARISON OF THE EFFECTS OF VISIBLE, ULTRAVIOLET, AND X-RADIATION ON RAT THYMOCYTES.

Ultraviolet and X-irradiation produce many of the same effects on rat thymocytes in vitro. Exposure of the cells to low doses results in a latent lethal effect, which requires incubation at 37 °C for its expression, and in an apparent increase in the rate at which deoxyribonucleoprotein from the cells dissolves to form a gel in 2 M NaCl. As the doses are increased, a decrease in the viscosity of the deoxyribonucleoprotein gel, an immediate "death" and swelling of the cells, and an immediate destruction of nucleotide bases become evident. For either radiation, doses which do not appear to have any immediate effect on the intact cell lead to loss of soluble materials (potassium ion, ribonucleotides) from the cells after incubation at 37 °C for several hours. The amount of ultraviolet energy required to produce most of the above effects is nearly 1000 times greater than the amount of X-ray energy required to produce the same effect.Three differences between the effects of ultraviolet and X-irradiation were observed: Ultraviolet light was relatively inefficient in breaking down the long deoxyribonucleoprotein chains but appeared to destroy hydrogen-bonding in the native nucleic acid structure at the same time as it destroyed the nucleotide basis. Moreover, the lethal effects of low doses of ultraviolet light on thymus cells require a longer time to develop than do the effects of X-irradiation.Thymocytes can also be killed by high intensities of visible light, particularly in the presence of photoreducible dyes. However, visible light produces little or no latent damage to the cells, nor was any evidence of nucleic acid damage observed.

MUTATIONAL SYNERGISM BETWEEN RADIATIONS AND METHYLATED PURINES IN ESCHERICHIA COLI.

Doneson, Ira N. (University of Kansas, Lawrence), and Delbert M. Shankel. Mutational synergism between radiations and methylalted purines in Escherichia coli. J. Bacteriol. 87:61-67. 1964.-A synergistic mutational effect was demonstrated between low doses of ultraviolet light and the methylated purines caffeine, theophylline, and theobromine. Caffeine produced the greatest effect and theobromine the least effect. The magnitude of the synergism was inversely related to the ultraviolet dosage. A large percentage of the synergistic effect could be "photoprevented" by exposure of the ultraviolet-treated cells to white light prior to exposure to the analogues. The consequence of the combined treatment occurred only when the chemical treatment followed the ultraviolet treatment. Furthermore, it was necessary to administer the chemical treatment soon after the ultraviolet treatment or the mutants were "lost." When cells were treated with low dosages of ultraviolet light and of X irradiation (X ray), the result was merely additive, and combinations of X ray and chemical treatment yielded no synergism. Synchronous growth studies indicated that a particular growth stage of the organisms was most susceptible to the synergistic effect. The mutation studied was that of Escherichia coli B/r to high-level streptomycin resistance.

Inhibition of cell division of Escherichia coli by low doses of ultraviolet light.

Inhibition of cell division of Escherichia coli by low doses of ultraviolet light.

EVIDENCE INDICATING THAT THE MUTATION RATE INDUCED IN DROSOPHILA BY LOW DOSES OF ULTRAVIOLET LIGHT IS AN EXPONENTIAL FUNCTION OF THE DOSE

EVIDENCE INDICATING THAT THE MUTATION RATE INDUCED IN DROSOPHILA BY LOW DOSES OF ULTRAVIOLET LIGHT IS AN EXPONENTIAL FUNCTION OF THE DOSE