Impact of ultraviolet radiation on cell structure, UV-absorbing compounds, photosynthesis, DNA damage, and germination in zoospores of Arctic Saccorhiza dermatodea

Stratospheric ozone depletion leads to enhanced UV-B radiation. Therefore, the capacity of reproductive cells to cope with different spectral irradiance was investigated in the laboratory. Zoospores of the upper sublittoral kelp Saccorhiza dermatodea were exposed to varying fluence of spectral irradiance consisting of photosynthetically active radiation (PAR, 400-700 nm; =P), PAR+UV-A radiation (UV-A, 320-400 nm; =PA), and PAR+UV-A+UV-B radiation (UV-B, 280-320 nm; =PAB). Structural changes, localization of phlorotannin-containing physodes, accumulation of UV-absorbing phlorotannins, and physiological responses of zoospores were measured after exposure treatments as well as after 2-6 d recovery in dim white light (8 mumol photon m(-2) s(-1)). Physodes increased in size under PAB treatment. Extrusion of phlorotannins into the medium and accumulation of physodes was induced not only under UVR treatment but also under PAR. UV-B radiation caused photodestruction indicated by a loss of pigmentation. Photosynthetic efficiency of spores was photoinhibited after 8 h exposure to 22 and 30 mumol photon m(-2) s(-1) of PAR, while supplement of UVR had a significant additional effect on photoinhibition. A relatively low recovery of photosystem II function was observed after 2 d recovery in spores exposed to 1.7 x 10(4) J m(-2) of UV-B, with a germination rate of only 49% of P treatment after 6 d recovery. The amount of UV-B-induced DNA damage measured as cyclobutane-pyrimidine dimers (CPDs) increased with the biologically effective UV-B dose (BED(DNA)). Significant removal of CPDs indicating repair of DNA damage was observed after 2 d in low white light. The protective function of phlorotannins has restricted efficiency for a single cell. Within a plume of zoospores, however, each cell can buffer each other and protect the lower layer of spores from excessive radiation. Exudation of phlorotannins into the water can also reduce the impact of UV-B radiation on UV-sensitive spores. The results of this study showed that the impact of UVR on reproductive cells can be mitigated by protective and repair mechanisms.     

Acclimation of Maximal Quantum Yield of Photosynthesis in the Brown Alga Alaria esculenta under High Light and UV Radiation

Abstract: Macroalgae of the upper sublittoral zone of Arctic coastal ecosystems are subjected to darkness or low light for several months during winter and have to withstand large changes in irradiance after the breakup of sea ice in the Arctic spring. Changes in photosynthetic response to high PAR (pho-tosynthetically active radiation) and UV-B radiation (UV-B) in the cold temperate brown alga A/aria esculenta were monitored with a PAM fluorometer to study photoinhibition, recovery and acclimation of maximal quantum yield of photochemistry. Plants collected in the field, as well as specimens raised in the laboratory, were exposed to various radiation conditions including different levels of PAR and UV radiation (UV-A + UV-B). Measurements of variable chlorophyll fluorescence of photosystem II revealed that the photosynthetic apparatus in A. esculenta was able to acclimate to the respective high light and UV treatments within several days. However, two different mechanisms of acclimation seem to be involved. Initially, the rate of recovery of maximal quantum yield increased after only a few exposures to high light or UV. Second, after several exposure cycles, the degree of inhibition was reduced. Data on fluorescence induction kinetics and quenching analysis showed that exposure to the respective UV radiation resulted in an increase of non-photochemical quenching, while effective quantum yield of photochemistry was hardly affected.     

Survival of subsurface microorganisms exposed to UV radiation and hydrogen peroxide.

Aerobic and microaerophilic subsurface bacteria were screened for resistance to UV light. Contrary to the hypothesis that subsurface bacteria should be sensitive to UV light, the organisms studied exhibited resistance levels as efficient as those of surface bacteria. A total of 31% of the aerobic subsurface isolates were UV resistant, compared with 26% of the surface soil bacteria that were tested. Several aerobic, gram-positive, pigmented, subsurface isolates exhibited greater resistance to UV light than all of the reference bacterial strains tested except Deinococcus radiodurans. None of the microaerophilic, gram-negative, nonpigmented, subsurface isolates were UV resistant; however, these isolates exhibited levels of sensitivity similar to those of the gram-negative reference bacteria Escherichia coli B and Pseudomonas fluorescens. Photoreactivation activity was detected in three subsurface isolates, and strain UV3 exhibited a more efficient mechanism than E. coli B. The peroxide resistance of four subsurface isolates was also examined. The aerobic subsurface bacteria resistant to UV light tolerated higher levels of H2O2 than the microaerophilic organisms. The conservation of DNA repair pathways in subsurface microorganisms may be important in maintaining DNA integrity and in protecting the organisms against chemical insults, such as oxygen radicals, during periods of slow growth.     

羊栖菜幼孢子体对不同N水平生长条件和阳光紫外辐射的响应

探讨了太阳紫外辐射对两种N水平生长条件下羊栖菜幼孢子体光化学特性的影响及其恢复。结果显示,在高的光辐射下羊栖菜藻体的有效光化学效率和相对电子传递速率急剧下降,在全波长太阳辐射条件下它们的下降幅度要比仅在可见光处理下的藻体更大,2种N水平条件下藻体的光化学活性下降趋势相似,但是N加富的生长条件使得藻体具有更高抵御紫外辐射的能力,这可能是与N加富生长条件下的藻体中含有较高含量的紫外吸收物质和类胡萝卜素有关。结果表明羊栖菜的幼孢子体具有比成体更强抵御紫外线的能力,这主要体现在藻体受到紫外辐射损伤后的修复上。     

L-Arginine improves multiple physiological parameters in mice exposed to diet-induced metabolic disturbances

L-Arginine (L-Arg) is a conditionally essential amino acid and a natural constituent of dietary proteins. Studies in obese rats and type 2 diabetic humans have indicated that dietary supplementation with L-Arg can diminish gain in white adipose tissue (WAT) and improve insulin sensitivity. However, the effects of L-Arg on glucose homeostasis, body composition and energy metabolism remain unclear. In addition, no studies have, to our knowledge, examined whether L-Arg has beneficial effects as a dietary supplement in the mouse model. In the present study, we investigated the effects of L-Arg supplementation to male C57BL/6 mice on an array of physiological parameters. L-Arg supplemented mice were maintained on a low-protein diet and body composition, appetite regulation, glucose tolerance, insulin sensitivity and energy expenditure were evaluated. A significant reduction in epididymal WAT was observed in L-Arg supplemented mice compared with mice fed an isocaloric control diet. Surprisingly, the L-Arg supplemented animals were hyperphagic corresponding to a highly significant decrease in feed efficiency, as body weight developed in a similar pattern in both experimental groups. Glucose homeostasis experiments revealed a major effect of L-Arg supplementation on glucose tolerance and insulin sensitivity, interestingly, independent of a parallel regulation in whole-body adiposity. Increased L-Arg ingestion also raised energy expenditure; however, no concurrent effect on locomotor activity, substrate metabolism or expression of uncoupling proteins (UCP1 and UCP2) in adipose tissues was displayed. In conclusion, dietary L-Arg supplementation substantially affects an array of metabolic-associated parameters including a reduction in WAT, hyperphagia, improved insulin sensitivity and increased energy expenditure in mice fed a low-protein diet.     

Thallus morphology and optical characteristics affect growth and DNA damage by UV radiation in juvenile Arctic Laminaria sporophytes

Growth of young sporophytes of the brown algae Laminaria digitata, L. saccharina and L. solidungula from Spitsbergen were measured in the laboratory after being exposed for 21 days to either photosynthetically active radiation (PAR=P) or to full light spectrum (PAR + UV-A + UV-B=PAB) using of cutoff glass filters. The plants were grown at 8±2°C and 16 h light : 8 h dark cycles with 6 h additional ultraviolet radiation (UVR) exposure in the middle of the light period. Growth was measured every 10 min using growth chambers with online video measuring technique. Tissue morphology and absorption spectra were measured in untreated young sporophytes while chlorophyll (Chl) a content and DNA damage were measured in treated thalli at the end of the experiment. In all species, growth rates were significantly higher in sporophytes exposed to P alone compared to sporophytes exposed to PAB. Tissue DNA damage is dependent on thallus thickness and absorption spectra characteristics of pigments and UV-absorbing compounds. In sporophytes exposed to UVR, energy demands for repair of DNA damage and synthesis of UV-absorbing compounds for protection effectively diverts photosynthate at the expense of growth. Photosynthetic pigment was not significantly different between treatments suggesting a capacity for acclimation to moderate UVR fluence. The general growth pattern in sporophytes exposed to P alone showed an increasing growth rate from the onset of light (0500–0900 hours) to a peak at the middle of the light phase (0900–1500 hours), a decline towards the end of the light phase (1500–2100 hours) and a minimum “low” growth in the dark (2100–0500 hours) relative to growth during the entire light phase. Under PAB, different growth patterns were observed such as growth compensation at night in L. digitata, delayed growth recovery in L. saccharina and minimal but continuous growth in L. solidungula. Growth as an integrative parameter of all physiological processes showed that the effect of UVR is correlated to the depth distribution of these species.     

Growth parameters and photosynthetic pigments in leaf segments of Zea mays exposed to cadmium, as related to protection mechanisms

The influence of cadmium on growth parameters and photosynthetic pigment content was studied in maize leaf segments differing in tissue maturity. Experiments were carried out with maize seedlings Zea mays L. cv. Hidosil treated with 25, 50, 100, 150 and 200 microM Cd for 14 days under low light conditions. Tissue age-dependent decrease of fresh mass, dry mass and area of the leaf segments was correlated with Cd concentration in the growth medium. Cd induced changes in chlorophyll and carotenoid contents, and the specific areas and densities of the leaf segments were dependent on age and metal concentration. Results are discussed with respect to the protection mechanisms in the leaf segments.     

Morphological and physiological responses of bean plants to supplemental UV radiation in a Mediterranean climate

During the last few decades many experiments have been performed to evaluate the responses of plants to enhanced solar UV-B radiation (280–320 nm) that may occur because of stratospheric ozone depletion; most of them were performed in controlled environment conditions where plants were exposed to low photosynthetically active radiation (PAR) levels and high UV-B irradiance. Since environmental radiative regimes can play a role in the response of plants to UV-B enhancement, it appears doubtful whether it is valid to extrapolate the results from these experiments to plants grown in natural conditions. The objective of this work was to evaluate the effects on physiology and morphology of a bean (Phaseolus vulgaris L.) cultivar Nano Bobis, exposed to supplemental UV radiation in the open-air. UV-B radiation was supplied by fluorescent lamps to simulate a 20% stratospheric ozone reduction. Three groups of plants were grown: control (no supplemental UV), UV-A treatment (supplementation in the UV-A band) and UV-B treatment (supplemental UV-B and UV-A radiation). Each group was replicated three times. After 33 days of treatment plants grown under UV-B treatment had lower biomass, leaf area and reduced leaf elongation compared to UV-A treatment. No significant differences were detected in photosynthetic parameters, photosynthetic pigments and UV-B absorbing compounds among the three groups of plants. However, plants exposed to UV-A treatment showed a sort of 'stimulation' of their growth when compared to the control. The results of this experiment showed that plants may be sensitive to UV-A radiation, thus it is difficult to evaluate the specific effects of UV-B (280–320 nm) radiation from fluorescent lamps and it is important to choose the appropriate control. Environmental conditions strongly affect plant response to UV radiation so further field studies are necessary to assess the interaction between UV-B exposure and meteorological variability.     

Metabolomic and physiological responses reveal multi-phasic acclimation of Arabidopsis thaliana to chronic UV radiation

Biochemical changes in vivo and pathway interactions were investigated using integrated physiological and metabolic responses of Arabidopsis thaliana L. to ultraviolet (UV) radiation (280–400 nm) at 9.96 kJ m⁻² d⁻¹ over the entire life cycle from seed to seed (8 weeks). Columbia-0 (Col-0) and a UV-B sensitive accession ( fah-1 ) showed significant ( P  < 0.001) reductions in leaf growth after 6 weeks. Col-0 recovered growth after 8 weeks, with recovery corresponding to a switch from production of phenylpropanoids to flavonoids. fah-1 failed to recover, indicating that sinapate production is an essential component of recovery. Epidermal features show that UV radiation caused significant ( P  < 0.001) increases in trichome density, which may act as a structural defence response. Stomatal indices showed a significant ( P  < 0.0001) reduction in Col-0 and a significant ( P  < 0.001) increase in fah-1 . Epidermal cell density was significantly increased under UV radiation on the abaxial leaf surface, suggesting that that a fully functioning phenylpropanoid pathway is a requirement for cell expansion and leaf development. Despite wild-type acclimation, the costs of adaptation lead to reduced plant fitness by decreasing flower numbers and total seed biomass. A multi-phasic acclimation to UV radiation and the induction of specific metabolites link stress-induced biochemical responses to enhanced acclimation.     

Epidermal ornithine decarboxylase and polyamines in mice exposed to 50 Hz magnetic fields and UV radiation

We studied the influence of magnetic fields (MFs) and simulated solar radiation (SSR) on ornithine decarboxylase (ODC) and polyamines in mouse epidermis. Chronic exposure to combined MF and SSR did not cause persistent effects on ODC activity or polyamines compared to the animals exposed only to UV, although the same MF treatment was previously found to accelerate skin tumor development. In an acute 24-h experiment, an elevation of putrescine and down-regulation of ODC activity was observed in the animals exposed to a 100-μT MF. No effect was seen 24 h after a single 2-MED (minimal erythemal dose) exposure to SSR. The results indicate that acute exposure to 50 Hz MF does exert distinctive biological effects on epidermal polyamine synthesis. Bioelectromagnetics 19:388–391, 1998. © 1998 Wiley-Liss, Inc.     

高原鼢鼠对低氧高二氧化碳环境适应的相关血液生理指标的季节变化

高原鼢鼠是终生生活于地下封闭洞道中的一种啮齿动物。为了探讨高原鼢鼠对低氧和高二氧化碳环境的适应机制,对春、夏和秋三季高原鼢鼠血液生理指标以及心肌和骨骼肌肌红蛋白含量进行了测定。春、夏和秋季高原鼢鼠动脉血和静脉血pH值分别为7.44±0.09、7.44±0.05、7.51±0.07和7.17±0.05、7.29±0.07、7.35±0.08,动脉血pH在不同季节没有显著性差异,静脉血pH春季显著低于夏季和秋季。动脉和静脉血二氧化碳分压(Pco2)分别为47.81±14.50mmHg、55.10±13.50mmHg、53.00±10.81mmHg和75.17±31.09mmHg、75.40±20.21mmHg、80.00±17.44mmHg,没有季节性差异;动脉和静脉血氧分压(Po2)分别为76.17±33.24mmHg、78.90±16.61mmHg、88.67±24.94mmHg和7.34±4.07mmHg、10.60±2.76mmHg、10.33±3.56mmHg,没有季节性差异;动脉和静脉血血氧饱和度(SaO2)分别为92.71%±5.00%、94.70%±3.65%、96.00%±4.38%和4.27%±3.03%、9.01%±5.26%、9.67%±6.02%,动脉血SaO2没有季节性差异,而静脉血SaO2春季显著低于夏季和秋季。血液红细胞数(RBC)、血红蛋白(Hb)和红细胞压积(HCT),从春季到冬季依次增加,并具有显著性差异,血清诱导型一氧化氮合成酶(iNOS)活力随着夏季洞道中二氧化碳浓度的增加而显著提高。心肌和骨骼肌肌红蛋白(Mb)含量春季显著高于夏季和秋季,红细胞内2,3-二磷酸甘油酸(2,3-DPG)含量秋季显著高于春季和夏季。上述结果表明:随着洞道中氧气和二氧化碳浓度的季节波动,高原鼢鼠通过调节氧转运有关的功能蛋白含量以及氧代谢有关的生理机制维持组织的氧平衡。     

Modelling the kinetics of chromosome exchange formation in human cells exposed to ionising radiation

A biophysical model has been applied to study the kinetics of chromosome exchange formation in human cells. Chromosomal exchange induction (for example dicentrics) by ionising radiation was modelled by means of the Monte Carlo technique. This involved energy deposition by electrons, production of chromosomal breaks (assumed to be DNA double-strand breaks) and their repair and exchange. Exchanges were assumed to result from pairwise interaction between two DNA breaks in a distance-dependent manner. The rate at which exchanges are formed was found to depend upon how the exchange to no-exchange probability ratio varied with time. The assumption that this ratio did not alter with time produced a time constant for the formation of exchanges which was exactly half that of the repair time constant. Longer time constants could not be accommodated unless the probability ratio for exchange increases with time. Different time constants for inter- and intratrack exchanges could be achieved on the basis of DNA double-strand break separation.     

QTLs mapping of physiological traits related to salt tolerance in young rice seedlings

Oryza sativa L. F₂ population and F_2:3 derived from a cross between salt tolerance cv. Tarommahali and salt sensitive cv. Khazar were used in this study. A linkage map based on F₂ population was constructed (74 SSR markers on 192 individuals), which covered a total of 1231.50 cM with an average two locus interval of 19.83 cM. Two QTLs related to Na⁺/K⁺ ratio were found on chromosome 3 and 6. qDM-3 and qDM-8 (for dry mass of shoot) are major QTLs with very large effects explained 20.90 and 17.72 % of the total phenotypic variance, respectively. Major locus for DM (qDM-3) was bracketed by RM1022 — RM6283 spread over 13.6 cM on chromosome 3. Major part of the variability for standard tolerance ranking (STR) was explained by the qSTR-6 flanked by RM3727 — RM340 on chromosome 6, which exhibited phenotypic variance of 17.25 % and peak likelihood ratio (LR) of 17.51. The length of this QTL is 8.8 cM and identification of any tightly linked markers in this region will serve as a candidate gene for fine-mapping. qSTR-3 overlapped with qNA-3 and qNAK-3. The qSTR-3 may contain a new major gene for salt stress tolerance at seedling stage in rice. Major QTLs identified in this paper, after fine-mapping, could be used for marker assisted selection.     

Role of dipicolinic acid in survival of Bacillus subtilis spores exposed to artificial and solar UV radiation

Pyridine-2,6-dicarboxylic acid (dipicolinic acid [DPA]) constitutes approximately 10% of Bacillus subtilis spore dry weight and has been shown to play a significant role in the survival of B. subtilis spores exposed to wet heat and to 254-nm UV radiation in the laboratory. However, to date, no work has addressed the importance of DPA in the survival of spores exposed to environmentally relevant solar UV radiation. Air-dried films of spores containing DPA or lacking DPA due to a null mutation in the DPA synthetase operon dpaAB were assayed for their resistance to UV-C (254 nm), UV-B (290 to 320 nm), full-spectrum sunlight (290 to 400 nm), and sunlight from which the UV-B portion was filtered (325 to 400 nm). In all cases, air-dried DPA-less spores were significantly more UV sensitive than their isogenic DPA-containing counterparts. However, the degree of difference in UV resistance between the two strains was wavelength dependent, being greatest in response to radiation in the UV-B portion of the spectrum. In addition, the inactivation responses of DPA-containing and DPA-less spores also depended strongly upon whether spores were exposed to UV as air-dried films or in aqueous suspension. Spores lacking the gerA, gerB, and gerK nutrient germination pathways, and which therefore rely on chemical triggering of germination by the calcium chelate of DPA (Ca-DPA), were also more UV sensitive than wild-type spores to all wavelengths tested, suggesting that the Ca-DPA-mediated spore germination pathway may consist of a UV-sensitive component or components.     

Growth and physiological parameters of whiting (Sillaginodes punctata) in relation to salinity

This study assessed the impact of salinity on whiting (Sillaginodes punctata) in an attempt to understand the mechanisms by which salinity could potentially influence habitat selection and growth of King George whiting in southern Australia. The experiment included whiting of two age classes, young of the year (YOY) and 2⁺ year, at three salinities (30, 40, 50 ppt). YOY whiting showed no significant difference in length or weight gain, specific growth rate, feed intake, food conversion ratio or condition factor when exposed to the three salinities for 72 day. Plasma osmolality of YOY whiting was not significantly different at any salinity, although it was significantly lower than that of 2⁺ year whiting. The 2⁺ year whiting showed significantly higher plasma osmolality than the YOY. Blood plasma potassium and chloride levels of 2⁺ year fish at 50 ppt were significantly higher than those at 30 ppt and 40 ppt. Blood sodium levels at 50 ppt were significantly higher than at 30 ppt but the sodium level at 40 ppt was not different from 30 ppt or 50 ppt. Haematocrit of 2⁺ whiting was significantly higher at 30 than at 50 ppt while haematocrit at 40 ppt was not different from 30 or 50 ppt. The 2⁺ year‐old whiting had a more pronounced increase in plasma osmolality and plasma ions at high salinities, indicating poorer osmoregulatory capacity in older fish. This study provides physiological evidence to partially explain habitat occupancy and growth in relation to salinity of different age groups of whiting in southern Australia.     

Ecophysiological and morphological parameters related to survival in grass species exposed to an extreme climatic event

An experiment was performed to elucidate interspecific differences in survival time of grass species subjected to an extreme climatic event. We exposed eight grass species to a simulated heat wave in the field ('free air' temperature increase at 11°C above ambient) combined with drought. We determined whether interspecific differences in survival time were related to the responses of the species to the imposed stress or could be explained by their ecophysiological or morphological characteristics in unstressed conditions. Surprisingly, there was no effect of specific leaf area, but species with a higher total leaf area survived longer. This may arise from a greater water reserve in the plant as a whole, which could delay the desiccation of the meristem, or from reduced evaporation due to a higher leaf area index. Species in which the decrease in light-saturated stomatal conductance ( g _s ) and photosynthetic CO₂ uptake rate ( A _max ) was strongly related to the decrease in soil water availability (measured as soil relative water content and stress duration) survived longer than species in which g _s and A _max likewise declined but responded more to daily fluctuations in irradiance, temperature, and vapor pressure deficit during the heat wave. We, therefore, hypothesize that interspecific differences in stress survival time might be related to the extent to which stomata react to changes in soil water conditions relatively to changes in other environmental and physiological factors. The results suggest that resistance to extremes is governed by other mechanisms than resistance to moderate drought.     

Growth related changes to functional parameters in the bovine lens.

Dimensions, volumes and protein contents were measured for bovine lenses with wet weights ranging from 0.17-3.07 g (2 months gestation to 19 years post-natal). All increase in a non-linear fashion. The lens becomes flatter with age due to a more rapid increase in the equatorial plane, but the ratio of anterior to posterior sagittal distances remains constant (1.19). The radius of curvature increases from 4.9 to 15 for the anterior surface and from 4.4 to 13 for the posterior. Protein content increases more rapidly than volume resulting in an increased average protein concentration from around 18% in the early prenatal lens to nearly 50% in the 19 year old. Total protein content (TPC) was found to be related to wet weight (We) according to the equation, TPC = 0.3We1.33. It is suggested that TPC is a better parameter for describing growth than wet weight or age. The refractive index, in the equatorial plane, increases towards the centre, from 1.38 at the edge of the lens. The maximum index, in the centre, increases with lens size up to 1.474 in the largest lens studied. This corresponds to a protein concentration of 70%. In all lenses, refractive index and protein concentration gradients were superimposable when plotted from the outside towards the centre. The optical performance of the lenses was assessed by measuring the back focal length which increases gradually from 24 to 51.5 mm over the 0.17 to 3.07 g size range. This was attributed to the increased radii of curvature.     

The structure and properties of different types of starch exposed to UV radiation: A comparative study

The effect of UV-irradiation on four different types of native starch (corn, waxy corn, wheat and potato) have been investigated. Although the changes in the chemical structure of starch specimens were small, indicating good photostability, the samples lost adsorbed water and their crystallinity degree decreased after irradiation. Moreover, a drop in average molecular weight occurred in samples (with the exception of potato starch) as a result of main chain scission. The variations in the properties of investigated specimens of various origin were related to the differences in their structure and macromolecular arrangement. The lowest photostability among the four starches was exhibited by potato starch.     

Alteration of the immune response to Mycobacterium bovis BCG in mice exposed chronically to low doses of UV radiation

BALB/c mice were exposed on shaved dorsal skin to 1 minimal erythemal dose (MED) of UVB radiation (2.25 kJ/m2) from a bank of six FS-40 sunlamps three times per week. The total number of irradiations ranged from 1 to 27. At regular intervals, groups of mice were injected in the left hind foot pad with 1 x 10(6) live mycobacteria (Mycobacterium bovis BCG) 3 days after the last UVB exposure. The mice were tested 21 and 42 days after infection for a delayed type hypersensitivity (DTH) response to the purified protein derivative (PPD) of tubercle bacilli by injecting PPD into the right hind foot pad and measuring the foot pad swelling 24 hr later. The course of infection was followed by assessing the number of bacterial colony forming units in the lymph node draining the site of BCG infection and the spleen. Mice exposed from 1 to 15 times to 1 MED of UV radiation showed a significant suppression in their DTH response to PPD compared with the unirradiated mice. At the same time, the number of bacterial colony-forming units in the lymph node and spleen of the UV-irradiated mice was greater than in control mice. With continued exposure to UVB, however, the DTH response recovered to a normal level, and there was no longer an increase in the number of viable bacteria in the lymphoid organs. These results indicate that early in the course of chronic UV irradiation, mice were impaired in their ability to mount a DTH response to BCG and to clear these bacteria from their lymphoid organs; later the mice recovered from these effects of UV, with continued treatment. A dose-response study using single doses of UV radiation indicated that a dose of 2.7 kJ/m2 suppressed the DTH response by 50%. Thus, exposure of mice to a single or multiple low doses of UV radiation prior to infection can interfere with systemic immunity to mycobacteria.