SENSITIVITY OF HemA MUTANT Escherichia coli CELLS TO INACTIVATION BY NEAR-UV LIGHT DEPENDS ON THE LEVEL OF SUPPLEMENTATION WITH δ-AMINOLEVULINIC ACID

Abstract— Four strains carrying all four possible combinations of the alleles nur, nur⁺, uvr A6 and uvr A ⁺ were transduced to hemA8 . The hemA8 mutation blocks the synthesis of δ-aminolevulinic acid (δ-ALA), one of the first steps in the synthesis of porphyrin and, ultimately, cytochromes essential for aerobic respiration. The cells were grown either with or without δ-ALA and treated with broad-spectrum near-ultraviolet light (NUV; 300–400 nm). hemA8 defective cells grown without δ-ALA were resistant to inactivation by NUV while hemA8 cells were sensitive to such inactivation when supplemented with δ-ALA. The sensitivity to NUV inactivation conferred by the nur gene was retained in the hemA8 derivatives. The sensitivity of such cells to NUV inactivation can be controlled by varying the level of δ-ALA supplementation. The level of δ-ALA supplementation did not influence the sensitivity of the cells to inactivation by far-UV light (FUV; 200–300 nm). The near-UV sensitivity of hemA⁺ cells was not significantly altered when grown with δ-ALA suppiementation suggesting that endogenously formed δ-ALA supports the normal, regulated level of porphyrin synthesis. These results can be interpreted to mean that porphyrin components of the respiratory chain in E. coli represent chromophores involved specifically in broad-spectrum NUV inactivating events.     

NONRECIPROCAL SYNERGISTIC LETHAL INTERACTION BETWEEN 365-nm and 405-nm RADIATION IN WILD TYPE and uvrA STRAINS OF ESCHERICHIA COLI*

Abstract— Lethality by 405-nm radiation in three repair-proficient and two uvrA strains of Escherichia coli that belong to two isogenic series was greatly enhanced by prior exposures to 365-nm radiation at fluences greater than 1 times 10₆Jm_-2. Fluences at 365 nm that yielded a surviving fraction of 0.10 (>1 times 10₆ Jm_-2) in the 5 strains tested resulted in the following 405-nm fluence enhancement factors (FEF, ratio of the 405-nm F₃₇ in the absence of a prior 365-nm irradiation to that in the presence): strain K.12 AB1157 (wild type), 8.7; strain B/r (wild type), 52; strain WP2 (wild type), 25; strain WP2s (uvrA), 13; strain K.12 AB1886 (uvrA), 15. The maximal 405-nm FEF value obtained after a prior 365-nm irradiation at greater fluences was 83 in the wild-type strain B/r. Enhancement of anoxic 405-nm radiation after a prior aerobic 365-nm exposure was not detectable, suggesting that prior aerobic irradiation at 365-nm increased the effects of damage produced at 405 nm by means of an oxygen-dependent process. Single-strand breaks (or alkali-labile bonds) were produced by 405-nm radiation at 3.0 times 10_-5 breaks per 2.5 times 10₉ daltons per Jm_-2 in the polA strain P3478; pyrimidine dimers were not detected by biological assay (photoreactivation) at 405 nm. Although the introduction of different DNA lesions produced by 365- and 405-nm radiations cannot be ruled out, we propose that the strong synergistic effect of 365-nm irradiation on 405-nm lethality is the consequence of pronounced inhibition by 365-nm radiation of components of the DNA-repair systems that can mend or bypass damage produced by 405-nm radiation.     

PHOTOSENSITIZED INACTIVATION OF DNA BY MONOCHROMATIC 334-nm RADIATION IN THE PRESENCE OF 2-THIOURACIL: GENETIC ACTIVITY AND BACKBONE BREAKS

Abstract Monochromatic 334-nm radiation delivered under aerobic conditions inactivates the genetic activity (ability to transform auxotrophic recipient cells to nutritional prototrophy) of isolated transforming Bacillus subtilis DNA. The presence of superoxide dismutase (SOD), catalase, and mannitol reduces the 334-nm inactivation. The rate of inactivation of the genetic activity by 334-nm radiation is enhanced fivefold by the sensitizer 2-thiouracil (s²Ura). This enhancement is substantially reversed when the irradiations are performed in the presence of mannitol, and, to a lesser extent, SOD. Catalase slightly reduces the s²Ura enhancement of 334-nm inactivation of transforming activity. Backbone breaks induced in the same DNA by aerobic 334-nm radiation were also enhanced markedly by the presence of s²Ura; this enhancement was reversed by the presence of mannitol and, to a lesser extent, SOD during irradiation. Catalase had no effect upon s²Ura-enhanced, 334-nm-induced SSBs. Whereas DNA breakage may be responsible for a portion of the inactivation of the DNA by the photosensitized reaction between s²-Ura and 334-nm radiation, it is not the only inactivating lesion, because the yield of SSBs per lethal hit per unit length of DNA is not constant for all the irradiation conditions studied. The results support a complex role for active oxygen species in inactivation of transforming activity and DNA breakage by s²Ura-enhanced 334-nm radiation. They are also consistent with the formation of superoxide anion, hydroxyl radical, and possibly also singlet molecular oxygen, generated from ground-state molecular oxygen by reactive s²Ura in both Type I and II reactions.     

INDUCTION OF DIRECT AND INDIRECT SINGLE-STRAND BREAKS IN HUMAN CELL DNA BY FAR- AND NEAR-ULTRAVIOLET RADIATIONS: ACTION SPECTRUM AND MECHANISMS

Abstract— An action spectrum for the immediate induction in DNA of single-strand breaks (SSBs, frank breaks plus alkali-labile sites) in human P3 teratoma cells in culture by monochromatic 254-, 270-, 290-, 313-, 334-, 365-, and 405-nm radiation is described. The cells were held at +0.5C during irradiation and were Iysed immediately for alkaline sedimentation analysis following the irradiation treatments. Linear fluence responses were observed over the fluence ranges studied for all energies. Irradiation of the cells in a D₂O environment (compared with the normal H₂O environment) did not alter the rate of induction of SSBs by 290-nm radiation, whereas the D₂O environment enhanced the induction of SSBs by 365- and 405-nm irradiation. Analysis of the relative efficiencies for the induction of SSBs, corrected for quantum efficiency and cellular shielding, revealed a spectrum that coincided closely with nucleic acid absorption below 313 nm. At longer wavelengths, the plot of relative efficiency vs . wavelength contained a minor shoulder in the same wavelength region as that observed in a previously obtained action spectrum for stationary phase Bacillus subtilis cells. Far-UV radiation induced few breaks relative to pyrimidine dimers, whereas in the near-UV region of radiation, SSBs account for a significant proportion of the lesions relative to dimers, with a maximum number of SSBs per lethal event occurring at 365-nm radiation.     

LETHAL ACTION OF ULTRAVIOLET AND VISIBLE (BLUE-VIOLET) RADIATIONS AT DEFINED WAVELENGTHS ON HUMAN LYMPHOBLASTOID CELLS: ACTION SPECTRA AND INTERACTION SITES

Abstract— The repair proficient human lymphoblastoid line (TK6) has been employed to construcr an action spectrum for the lethal action of ultraviolet (UV) radiation in the range254–434 nm and to examine possible interactions between longer (334, 365 and 405 nm) and shorter wavelength (254 and 313 nm) radiations. The action spectrum follows a DNA absorption spectrum fairly closely out to 360 nm. As in previously determined lethal action spectra for procaryotic and eucaryotic cell populations, there is a broad shoulder in the334–405 nm region which could reflect the existence of either (a) a non-DNA chromophore or (b) a unique photochemical reaction in the DNA over this region. Pre-treatment with radiation at 334 or 365 nm causes either a slight sensitivity to (low fluences) or protection from (higher fluences) subsequent exposure to radiation at a shorter wavelength (254 or 313 nm). Pre-irradiation at a visible wavelength (405 nm) at all fluence levels employed sensitizes the populations to treatment with 254 or 313 nm radiations. These interactions will influence the lethal outcome of cellular exposure to broad-band radiation sources.     

MUTATION INDUCTION BY AND MUTATIONAL INTERACTION BETWEEN MONOCHROMATIC WAVELENGTH RADIATIONS IN THE NEAR-ULTRAVIOLET AND VISIBLE RANGES

Abstract— The induction of mutations (reversion to tryptophan independence) by various UV (254, 313, 334 and 365 nm) and visible (405 and 434 nm) wavelengths was measured in exponential phase populations of Escherichia coli B/r thy trp and B/r thy trp uvrA by assay of irradiated populations on semi-enriched media. No mutations were induced in the repair proficient strain at wavelengths longer than 313 nm. Mutations were induced in the excisionless strain at wavelengths as long as 405 nm but less than expected from the known amount of DNA damage induced. Irradiation at the longer wavelengths (434, 405, 365 and 334 nm) suppressed the appearance of 254- or 313-nm-induced mutations in the repair competent strain but not in the excision deficient strain. The relative dose-requirement for mutation suppression was related to the relative efficiency of these wavelengths in inducing growth delay. These results suggest that the growth delay induced by near-UV and visible wavelengths allows more time for the 'error-free" excision repair process to act on the potentially mutagenic lesions induced by 254- and 313-nm radiations, thereby reducing the mutation frequency observed in the repair-proficient strain. The level of near-UV mutation induced in the excision deficient strain is lower than expected from the DNA damage known to be induced. It is possible that near-UV radiation induces a class of lethal lesions that are not susceptible to error-prone repair.     

CONVEX CURVATURES OF ALKALINE ELUTION PROFILES OF DNA FROM HUMAN CELLS IRRADIATED WITH 405 nm UVA: EVIDENCE FOR INDUCTION OF SLOWLY DEVELOPING ALKALI-LABILE SITES

The alkaline (pH 12.1) elution profiles of DNA from human P3 cells exposed to monochromatic 405 nm UVA radiation deviate from exponential: on a logarithmic plot of eluted fraction of DNA vs time of elution, the rate of elution accelerates for the first 6 h. Following this period, the profiles become exponential. In contrast, the elution profiles of DNA after 520 nm green light or ionizing radiation exposures (x- and gamma rays, and fission spectrum neutrons) are always strictly exponential, evidence that the convex profiles were not due to an artifact caused by elution technique. Holding the DNA at pH 12.1 for 6 h after 405-nm exposures before initiating elution resulted in profiles that were close to exponential, with slopes similar to the final slopes observed following the 6-h elution period in the original experiments. This is evidence that some DNA breaks develop slowly during the first 6 h of elution, as a result of exposure to alkali. Therefore, the DNA lesions induced by 405-nm light as measured by the alkaline elution technique are apparently heterogeneous and include a major class of alkali-labile sites that develop slowly during incubation at pH 12.1. Convex profiles also occur following exposure of the cells to visible light at 434 and 512 nm.     

ULTRAVIOLET LIGHT INDUCES DOUBLE-STRAND BREAKS IN DNA OF CULTURED HUMAN P3 CELLS AS MEASURED BY NEUTRAL FILTER ELUTION

Neutral filter elution at pH 7.2 and 9.6 was used to measure the induction of DNA lesions in human P3 teratocarcinoma cells by monochromatic 254-, 270-, 313-, 334-, 365-, and 405-nm radiation and by 60 gamma rays. In this assay DNA double-strand breaks (dsb) increase the rate of elution of DNA from cell lysates on a filter. Yields of dsb as measured by this procedure were determined by using a calibration of the assay that correlates elution parameters with number of dsb caused by disintegration of 125I incorporated into the DNA. Analysis of fluence responses obtained by using the calibrated assay indicated that the number of dsb induced per dalton of DNA as measured by this assay is proportional to the square of the fluence at all the energies of radiation studied, implying that the induction of these lesions may be a two-hit event. Analysis of the relative efficiencies for the induction of dsb by ultraviolet radiation, corrected for quantum efficiency, revealed a spectrum that coincided closely with that for the induction of single-strand breaks (ssb) in the same cells, having a close fit with the spectrum of nucleic acid in the UVC and UVB region below 313 nm, and a shoulder in the UVA region. It was calculated, however, that there may be too few ssb for dsb to result from randomly distributed closely opposed ssb.     

OBSERVATIONS ON THE PHOTOSENSITIZED BREAKAGE OF DNA BY 2-THIOURACIL AND 334-nm ULTRAVIOLET RADIATION

Abstract— Breaks induced in purified DNA by 334-nm ultraviolet (UV) radiation are enhanced 30 times when 2-thiouracil (s²Ura) is present during aerobic irradiation. This enhancement by s²Ura is maximally effective at a concentration of about 1 m M. Anoxic irradiation reduces the s²Ura-enhanced breakage by 90%, indicating a Type II photosensitization. Benzoate, glycerol, diazabicyclo[2.2.2.]octane (DABCO) and histidine all inhibit formation of s²Ura photosensitized breaks, unlike diethylenetriaminepenta-acetic acid (DETAPAC) and catalase, which do not. The relationships between the concentration of DABCO. benzoate and histidine and their protection against induction of single strand breaks (SSBs) were similar, with little inhibition below 10 m M and maximal inhibition near 0.1 M for all compounds. Irradiation of the DNA-s²Ura mixture dissolved in D₂O instead of H₂O enhanced the rate of induction of SSBs in DNA by 334-nm light almost five times. Addition of superoxide dismutase (40, 80 and 200 μg/ml) decreased the rate of induction of breaks in DNA by 334-nm radiation plus s²Ura (in H₂O) by about 40%. Boiled superoxide dismutase had no effect.     

THE ROLE OF 4-THIOURIDINE IN LETHAL EFFECTS AND IN DNA BACKBONE BREAKAGE CAUSED BY 334 nm ULTRAVIOLET LIGHT IN Escherichia coli

Strains of Escherichia coli that lack 4-thiouridine (⁴Srd) are killed by monochromatic 334 nm UV light (UV) less efficiently than their wild-type parents, which contain ⁴Srd. Oxygen enhancement ratios (OER) at 10% survival are 3.3 for a strain that possesses ⁴Srd, and 2.6 for one that lacks ⁴Srd. Single-strand breaks in DNA caused by 334 nm UV accumulate more than twice as fast in the wild-type strains than in the strains lacking ⁴Srd. The results suggest that ⁴Srd is an important chromophore in some near-UV lethal effects. The results also suggest that the excitation energy from 334 nm UV light may be passed from RNA to DNA, resulting in single-strand breaks.     

REPAIR OF NEAR-VISIBLE- and BLUE-LIGHT-INDUCED DNA SINGLE-STRAND BREAKS BY THE CHO CELL LINES AA8 and EM9

The induction of single-strand breaks (SSB) and the kinetics of SSB repair were measured in two Chinese hamster ovary cell lines irradiated with monochromatic photons of near-visible radiation (405 nm) and blue light (434 nm). The radiosensitive and UV-A-sensitive mutant line EM9 is known to repair SSB induced by ionizing radiation or 365-nm UV-A more slowly than the parent line AA8. At the 10% survival level, EM9 cells were 1.7- and 1.6-fold more sensitive than AA8 cells to 405 and 434 nm radiation, respectively. This sensitivity was not due to differences in induction of SSB because AA8 and EM9 cells accumulated the same number of initial breaks when irradiated at 0.5 degrees C with either 405 nm (5.9 SSB per MJ/m2) or 434 nm (5.1 SSB per MJ/m2), as measured by alkaline elution. When the cells repaired these SSB at 37 degrees C in full culture medium, biphasic repair kinetics were observed for both cell lines. In both phases of repair, EM9 cells repaired breaks induced by both wavelengths more slowly than did AA8 cells. The t1/2 values for the repair phases for 405-nm-induced SSB were 3.8 and 150 min for EM9, and 1.5 and 52 min for AA8; the corresponding values for repair of 434 nm breaks were 3.7 and 39 min for EM9, and 2.0 and 30 min for AA8. Because of this slower repair, EM9 cells left more SSB unrepaired after 90 min than did AA8 cells for both wavelengths.(ABSTRACT TRUNCATED AT 250 WORDS)     

Oxygen-independent inactivation of Haemophilus influenzae transforming DNA by monochromatic radiation: action spectrum, effect of histitine and repair.

Abstract— The action spectrum for the oxygen-independent inactivation of native transforming DNA from Haemophilus influenzae with near-UV radiation revealed a shoulder beginning at 334 and extending to 460 nm. The presence of 0.2 M histidine during irradiation produced a small increase in inactivation at 254, 290 and 313 nm, a large increase at 334 nm and a decrease in inactivation at 365, 405 and 460 nm. Photoreactivation did not reverse the DNA damage produced at pH 7.0 at 334, 365, 405 and 460 nm, but did reactivate the DNA after irradiation at 254, 290 and 313 nm. The inactivation of DNA irradiated at 254, 290 and 313 nm was considerably greater when the transforming ability was assayed in an excision-defective mutant compared with the wild type, although DNA irradiated at 334, 365, 405 and 460 nm showed smaller differences. These results suggest that the oxygen-independent inactivation of H. influenzae DNA at pH 7 by irradiation at 334, 365, 405 and 460 nm is caused by lesions other than pyrimidine dimers.     

EFFECTS OF GLYCEROL UPON THE BIOLOGICAL ACTIONS OF NEAR-ULTRAVIOLET LIGHT: SPECTRA AND CONCENTRATION DEPENDENCE FOR TRANSFORMING DNA AND FOR ESCHERICHIA COLI B/r

The concentration dependence for the protection of isolated transforming DNA and Escherichia coli by glycerol against 365-nm monochromatic near-ultraviolet light (UV) was measured. Glycerol protection saturates at a concentration of about 0.1 M for DNA and 1.0 M for E. coli. Action spectra for glycerol protection of transforming DNA (tryptophan and histidine markers) are similar to those obtained previously for diazobicyclo[2.2.2.˜octane (DABCO) protection, with protection reaching a maximum near 350-nm UV and decreasing rapidly at wavelengths above and below 350 nm. However, glycerol protects against near-UV about twice as efficiently as DABCO. The action spectrum for protection of E. coli by glycerol against the lethal effects of near-UV was not the same as the spectrum for DNA since glycerol sensitized the cells, but not the DNA, at wavelengths longer than about 380 nm. A possible role of hydroxyl or other radicals was supported by the observation that benzoate also protected DNA against inactivation by 334-nm UV.     

DNA BREAKAGE CAUSED BY 334-nm ULTRAVIOLET LIGHT IS ENHANCED BY NATURALLY OCCURRING NUCLEIC ACID COMPONENTS AND NUCLEOTIDE COENZYMES

Abstract— The induction of breaks in DNA in vitro caused by 334-nm UV radíation is enhanced by the following compounds (fluence enhancement factors and concentrations used in parentheses): 4-thiouridine (6.9, 1 m M ), 5-methylamino-2-thiouridine (7.5, 1 m M ), 2-thiouracil (41.0, 1 m M ), riboflavin (14.4.0.1 m M ), and the oxidized (6.8, 1 m M ) and reduced (3.4, 1 m M ) forms of nicotinamide adenine dinucleotide. Anoxia and diazobicyclo(2.2.2)octane reduce the number of DNA breaks caused by 334-nm radiation plus 4-thiouridine by 70 and 76%, respectively.     

ACTION SPECTRUM FOR THE OXYGEN-INDEPENDENT INACTIVATION OF HAEMOPHILUS INFLUENZAE TRANSFORMING DNA WITH NEAR ULTRA-VIOLET LIGHT*

Abstract— The oxygen-independent inactivation of Haemophilis influenzae transforming DNA by near UV light (300–380 nm) has an action spectrum in which the efficiency of inactivation drops rapidly between 313 and 334 nm and more slowly between 334 and 405 nm, with a shoulder between 334 and 365 nm.     

TOXICITY, DNA DAMAGE AND INHIBITION OF DNA REPAIR SYNTHESIS IN HUMAN MELANOMA CELLS BY CONCENTRATED SUNLIGHT

A 1 m diameter water lens was used to focus solar radiation, giving an 8-fold concentration of the total spectrum and a cytocidal flux similar to that of laboratory UV sources. Survival curves for human melanoma cells were similar for sunlight and 254 nm UV, in that D _q, was usually larger than D _o. An xeroderma pigmentosum lymphoblastoid line was equally sensitive to both agents and human cell lines sensitive to ionizing radiation (lymphoblastoid lines), crosslinking agents or monofunctional alkylating agents (melanoma lines) had the same 254 nm UV and solar survival responses as appropriate control lines. Two melanoma sublines derived separately by 16 cycles of treatment with sunlight or 254 nm UV were crossresistant to both agents. In one melanoma cell line used for further studies, DNA strand breaks and DNA-protein crosslinking were induced in melanoma cells by sunlight but pyrimidine dimers (paper chromatography) and DNA interstrand crosslinking (alkaline elution) could not be detected. The solar fiuence response of DNA repair synthesis was much less than that from equitoxic 254 nm UV, reaching a maximum near the D _o value and then declining; semiconservative DNA synthesis on the other hand remained high. These effects were not due to changes in thymidine pool sizes. Solar exposure did not have a major effect on 254 nm UV-induced repair synthesis.     

ENDOGENOUS GLUTATHIONE PROTECTS HUMAN SKIN FIBROBLASTS AGAINST THE CYTOTOXIC ACTION OF UVB, UVA AND NEAR-VISIBLE RADIATIONS

Both the UVB (290-320 nm) and UVA (320-380 nm) regions of sunlight damage human skin cells but, particularly at the longer wavelengths, information is scant concerning the mechanism(s) of damage induction and the roles of cellular defense mechanisms. Following extensive glutathione depletion of cultured human skin fibroblasts, the cells become strongly sensitized to the cytotoxic action of near-visible (405 nm), UVA (334 nm, 365 nm) and UVB (313 nm) but not UVC (254 nm) radiations. In the critical UVB region, the magnitude of the protection afforded by endogenous glutathione approaches that of the protection provided by excision repair. The results suggest that a significant fraction of even UVB damage can be mediated by free radical attack and that a major role of glutathione in human skin cells is to protect them from the cytotoxic action of sunlight.     

LETHAL INTERACTION BETWEEN ULTRAVIOLET-VIOLET RADIATIONS AND METHYL METHANE SULPHONATE IN REPAIR PROFICIENT AND REPAIR DEFICIENT STRAINS OF ESCHERICHIA COLI

Abstract— The lethal interaction between monochromatic radiation at various wavelengths and methyl methane sulphonate was tested in strains of Escherichia coli proficient and deficient in DNA repair. In the repair proficient wild-type strain K12 AB1157, the efficiency of sensitization to MMS as a function of dose (at 334 nm, 365 nm and 405 nm) was found to be directly correlated with the dose necessary to remove the shoulder from the survival curve at the wavelength employed. The 365 nm: MMS interaction was also observed in other repair proficient E. coli strains (W3110 and B/r) but was absent in a recA and a polA strain. Pre-treatment of AB1157 with MMS leads to a much larger interaction than pre-irradiation with 365 nm. It is concluded that dose-dependent damage to DNA repair by the near-UV radiation is involved in the interaction and possibly that MMS causes irreversible damage 10 repair enzymes.     

EVIDENCE AGAINST DNA AS THE TARGET FOR 334 NM-INDUCED GROWTH DELAY IN ESCHERICHIA COLI*

Abstract— Growth delay was induced with near-UV (334 nm) radiation in Escherichia coli K12 bacterial strains followed by attempts at photoreactivation (PR) of this effect at 405 nm. In the UV-sensitive strain AB2480, a small PR of the observed population growth delay occurred after 334 nm irradiation at 35°C and a much larger PR after 334 nm irradiation at 5°C. However, much of the population growth delay in this strain can be explained as being due to killing, and all or most of the observed PR pertains only to this killed fraction of the population. The true cell growth delay (i.e. that of surviving cells) thus appears to be only slightly, if at all, photoreactivable. This conclusion is supported by studies with a wild-type strain KW8, which shows growth delay at non-lethal doses; this growth delay shows no PR, regardless of the temperature during 334 nm irradiation. These findings indicate that photoreactivable lesions (cyclo-butyl pyrimidine dimers) are not an important cause of near-UV-induced growth delay. Strain AB2480 lacks known dark-repair systems for DNA damage induced by far-UV (below 300 nm) radiation, yet shows the same efficiency for 334-nm-induced growth delay as the wild type, which possesses these dark repair systems. This indicates that lesions in DNA that are dark-repairable by the systems not tunctional in AB2480are not responsible for 334-nm-induced growth delay. It is possible, however, that fragmentary repair systems in AB2480 can operate on some DNA lesion that might cause growth delay. Spontaneously decaying lesions are unlikely, since growth-delay damage decays at a very low rate in non-nutrient medium. Since most of the known types of DNA damage and repair are thus eliminated, these considerations suggest that DNA damage is not involved in near-UV-induced growth delay.     

PHOTOPROTECTION OF E. COLI B/r: RESPIRATION,GROWTH, MACROMOLECULAR SYNTHESIS AND REPAIR OF DNA*

Abstract— Photoprotecting effects of near UV radiations (300–400 nm, maximum at 360 nm) against far UV radiations (primarily 254 nm) have been studied in Escherichia coli B/r cells in minimal medium with glycerol as a carbon source. Near UV light (10⁵ Jm^-2) has a negligible effect on survival, but causes transitory inhibition of respiration, growth, DNA, RNA, and protein syntheses and cell division. Far UV (52 J m^-2) reduces survival to about 0.5 per cent; respiration, growth and RNA and protein syntheses proceed for about 60 min, after which they nearly cease for several hours. Near UV given before this fluence of far UV increases survival 10-fold and the above processes resume at times and with kinetics characteristic of those produced by lower fluences of far UV. Single-strand breaks appear in the DNA of both unprotected and photoprotected cells; repair of the breaks is essentially complete in protected but not unprotected cells. The viability kinetics for far-UV-irradiated cells with and without photoprotecting treatment are identical except that the curve for the latter is displaced upward about 1 log; exponential increases (cell division) begin at 120 min in each case. The data suggest that, in B/r cells grown under our particular conditions, namely in minimal medium with glycerol, photoprotection is not the result of growth or division delays, but reflects an increased repair capability due to continued respiration.