Bacterial genes involved in response to near-ultraviolet radiation

Effects of single and combined UV-LEDs on inactivation and subsequent reactivation of E. coli in water disinfection

Ultraviolet light emitting diodes (UV-LEDs) have shown a potential to replace traditional Ultraviolet (UV) pressure lamps for water disinfection. However, the research is not sufficient and hence, it is still difficult to make any logical conclusions. In this work, UV-LEDs with peak emissions at 267, 275, 310 nm and combined emissions at 267/275, 267/310 and 275/310 nm were applied to a batch water disinfection system. Under either single- or combined-wavelength situation, the inactivation efficiency, reactivation (due to photoreactivation and dark repair) after UV irradiation and electrical energy consumption were evaluated by way of the model bacterium Escherichia coli. It was found that, the 267 nm UV-LED had the highest inactivation efficiency than other UV-LEDs. Although reactivation occurred after 267, 275, 267/275 and 275/310 nm UV-LEDs' irradiations, it occurred to a lesser extent in dark repair than in photoreactivation, demonstrating that photo-effect is the dominant mechanism of reactivation. In addition, decay phase was more prominent than reactivation in dark repair. However, the irradiation by the 275 nm UV-LED showed a better persistence against reactivation which could be attributed to protein damage at 275 nm. No synergistic effect for combined wavelengths was observed in this study. The electrical energy consumption was lower for the 275 nm UV-LED than the other UV-LEDs which was attributed to its higher wall plug efficiency. This study showed the variation principle between the single and combined UVB/UVC-LEDs in inactivation efficiency, inhibition of reactivation, synergistic effect and electrical energy consumption in treatment of E. coli, which is useful for the reasonable exploitation of UV-LEDs in water disinfection systems.

A Geneticist's View of Prostate Cancer: Prostate Cancer Treatment Considerations

Prostate cancer remains a life-threatening disease of men. While early detection has been helpful to reduce the mortality rate, we currently do not have a desired therapy. In recent years, new strategies have been proposed to treat prostate cancers with poor prognosis by utilizing genetically modified bacteria, including Salmonella typhimurium that preferentially replicate within solid tumors (1000:1 and up to 10,000:1 compared to non-cancerous tissue) destroying cancer cells without causing septic shock that is typically associated with wild-type S. typhimurium infections. Furthermore, these bacteria have the potential to be utilized as drug delivery systems to more effectively target different subpopulations of prostate tumor cells. This chapter reviews progress in using genetically modified S. typhimurium for destruction of prostate tumors.

Sunlight mediated inactivation mechanisms of Enterococcus faecalis and Escherichia coli in clear water versus waste stabilization pond water

Escherichia coli and enterococci have been previously reported to differ in the mechanisms and conditions that affect their sunlight-mediated inactivation in waste stabilization ponds. This study was undertaken to further characterize these mechanisms, using simulated sunlight and single strains of laboratory-grown E. coli and Enterococcus faecalis, with a focus on characterizing the contribution of exogenous reactive oxygen species to the inactivation process. We found that direct damage by UVB light (280-320 nm) was not a significant inactivation mechanism for either organism. E. coli inactivation was strongly dependent on dissolved oxygen concentrations and the presence of UVB wavelengths but E. coli were not susceptible to inactivation by exogenous sensitizers present in waste stabilization pond water. In contrast, E. faecalis inactivation in pond water occurred primarily through exogenous mechanisms, with strong evidence that singlet oxygen is an important transient reactive species. The exogenous mechanism could utilize wavelengths into the visible spectrum and sensitizers were mainly colloidal, distributed between 0.2 and ∼1 μm in size. Singlet oxygen is likely an important endogenous species in both E. faecalis and E. coli inactivation due to sunlight. Although the two organisms had similar inactivation rates in buffered, clear water, the inactivation rate of E. faecalis was 7 times greater than that of E. coli in air-saturated pond water at circumneutral pH due to its susceptibility to exogenous sensitizers and longer wavelengths.

Reactive oxygen species mediate lethality induced by far-UV inEscherichia colicells

The involvement of reactive oxygen species (ROS) in the induction of DNA damage to Escherichia coli cells caused by UVC (254 nm) irradiation was studied. We verified the expression of the soxS gene induced by UVC (254 nm) and its inhibition by sodium azide, a singlet oxygen (1O2) scavenger. Additional results showed that a water-soluble carotenoid (norbixin) protects against the lethal effects of UVC. These results suggest that UVC radiation can also cause ROS-mediated lethality.

Stationary-Phase Gene Regulation in Escherichia coli §

In their stressful natural environments, bacteria often are in stationary phase and use their limited resources for maintenance and stress survival. Underlying this activity is the general stress response, which in Escherichia coli depends on the σS (RpoS) subunit of RNA polymerase. σS is closely related to the vegetative sigma factor σ70 (RpoD), and these two sigmas recognize similar but not identical promoter sequences. During the postexponential phase and entry into stationary phase, σS is induced by a fine-tuned combination of transcriptional, translational, and proteolytic control. In addition, regulatory "short-cuts" to high cellular σS levels, which mainly rely on the rapid inhibition of σS proteolysis, are triggered by sudden starvation for various nutrients and other stressful shift conditons. σS directly or indirectly activates more than 500 genes. Additional signal input is integrated by σS cooperating with various transcription factors in complex cascades and feedforward loops. Target gene products have stress-protective functions, redirect metabolism, affect cell envelope and cell shape, are involved in biofilm formation or pathogenesis, or can increased stationary phase and stress-induced mutagenesis. This review summarizes these diverse functions and the amazingly complex regulation of σS. At the molecular level, these processes are integrated with the partitioning of global transcription space by sigma factor competition for RNA polymerase core enzyme and signaling by nucleotide second messengers that include cAMP, (p)ppGpp, and c-di-GMP. Physiologically, σS is the key player in choosing between a lifestyle associated with postexponential growth based on nutrient scavenging and motility and a lifestyle focused on maintenance, strong stress resistance, and increased adhesiveness. Finally, research with other proteobacteria is beginning to reveal how evolution has further adapted function and regulation of σS to specific environmental niches.

Efficacy of solar disinfection of Escherichia coli, Shigella flexneri, Salmonella Typhimurium and Vibrio cholerae

AIMS

To determine the efficacy of solar disinfection (SODIS) for enteric pathogens and to test applicability of the reciprocity law.

METHODS AND RESULTS

Resistance to sunlight at 37 degrees C based on F99 values was in the following order: Salmonella Typhimurium>Escherichia coli>Shigella flexneri>Vibrio cholerae. While F90 values of Salm. Typhimurium and E. coli were similar, F99 values differed by 60% due to different inactivation curve shapes. Efficacy seemed not to be dependent on fluence rate for E. coli stationary cells. Sensitivity to mild heat was observed above a temperature of 45 degrees C for E. coli, Salm. Typhimurium and Sh. flexneri, while V. cholerae was already susceptible above 40 degrees C.

CONCLUSIONS

Salmonella Typhimurium was the most resistant and V. cholerae the least resistant enteric strain. The reciprocity law is applicable for stationary E. coli cells irradiated with sunlight or artificial sunlight.

SIGNIFICANCE AND IMPACT OF THE STUDY

Escherichia coli might not be the appropriate indicator bacterium to test the efficacy of SODIS on enteric bacteria and the physiological response to SODIS might be different among enteric bacteria. The applicability of the reciprocity law indicates that fluence rate plays a secondary role in SODIS efficacy. Stating inactivation efficacy with T90 or F90 values without showing original data is inadequate for SODIS studies.

Gene expression of Escherichia coli in continuous culture during adaptation to artificial sunlight

Escherichia coli growing in continuous culture under continuous UVA irradiation exhibits growth inhibition with a subsequent adaptation to the stress. Transcriptome analysis was performed during transient growth inhibition and in the UVA light-adapted growth state. The results indicate that UVA light induces stringent response and an additional response that includes the upregulation of the synthesis of valine, isoleucine, leucine, phenylalanine, histidine and glutamate. The induction of several SOS response-genes strongly points to DNA damage as a result of UVA exposure. The involvement of oxidative stress was observed with the induction of ahpCF. Taken together it supports the hypothesis of the production of reactive oxygen species by UVA light. In the UVA-adapted cell population strong repression of the acid tolerance response was found. We identified the enzyme chorismate mutase as a possible chromophore for UVA light-inactivation and found strong repression of the pyrBI operon and the gene mgtA encoding for an ATP-dependent Mg2+ transporter. Furthermore, our results indicate that the role of RpoS may not be as important in the adaptation of E. coli to UVA light as it was implicated by previous results with starved cells, but that RpoS might be of crucial importance for the resistance under transient light exposure.

Specific growth rate determines the sensitivity of Escherichia coli to thermal, UVA, and solar disinfection

Knowledge about the sensitivity of the test organism is essential for the evaluation of any disinfection method. In this work we show that sensitivity of Escherichia coli MG1655 to three physical stresses (mild heat, UVA light, and sunlight) that are relevant in the disinfection of drinking water with solar radiation is determined by the specific growth rate of the culture. Batch- and chemostat-cultivated cells from cultures with similar specific growth rates showed similar stress sensitivities. Generally, fast-growing cells were more sensitive to the stresses than slow-growing cells. For example, slow-growing chemostat-cultivated cells (D = 0.08 h(-1)) and stationary-phase bacteria from batch culture that were exposed to mild heat had very similar T(90) (time until 90% of the population is inactivated) values (T(90, chemostat) = 2.66 h; T(90, batch) = 2.62 h), whereas T(90) for cells growing at a mu of 0.9 h(-1) was 0.2 h. We present evidence that the stress sensitivity of E. coli is correlated with the intracellular level of the alternative sigma factor RpoS. This is also supported by the fact that E. coli rpoS mutant cells were more stress sensitive than the parent strain by factors of 4.9 (mild heat), 5.3 (UVA light), and 4.1 (sunlight). Furthermore, modeling of inactivation curves with GInaFiT revealed that the shape of inactivation curves changed depending on the specific growth rate. Inactivation curves of cells from fast-growing cultures (mu = 1.0 h(-1)) that were irradiated with UVA light showed a tailing effect, while for slow-growing cultures (mu = 0.3 h(-1)), inactivation curves with shoulders were obtained. Our findings emphasize the need for accurate reporting of specific growth rates and detailed culture conditions in disinfection studies to allow comparison of data from different studies and laboratories and sound interpretation of the data obtained.

Comparative analysis of differentially expressed genes in Shewanella oneidensis MR-1 following exposure to UVC, UVB, and UVA radiation

We previously reported that Shewanella oneidensis MR-1 is highly sensitive to UVC (254 nm), UVB (290 to 320 nm), and UVA (320 to 400 nm). Here we delineated the cellular response of MR-1 to UV radiation damage by analyzing the transcriptional profile during a 1-h recovering period after UVC, UVB, and UVA exposure at a dose that yields about a 20% survival rate. Although the SOS response was observed with all three treatments, the induction was more robust in response to short-wavelength UV radiation (UVB and UVC). Similarly, more prophage-related genes were induced by short-wavelength UV radiation. MR-1 showed an active detoxification mechanism in response to UVA, which included the induction of antioxidant enzymes and iron-sequestering proteins to scavenge reactive oxygen species. In addition, a great number of genes encoding multidrug and heavy metal efflux pumps were induced following UVA irradiation. Our data suggested that activation of prophages appears the major lethal factor in MR-1 following UVC or UVB irradiation, whereas oxidative damage contributes greatly to the high UVA sensitivity in MR-1.

Survival of Shewanella oneidensis MR-1 after UV radiation exposure

We systematically investigated the physiological response as well as DNA damage repair and damage tolerance in Shewanella oneidensis MR-1 following UVC, UVB, UVA, and solar light exposure. MR-1 showed the highest UVC sensitivity among Shewanella strains examined, with D37 and D10 values of 5.6 and 16.5% of Escherichia coli K-12 values. Stationary cells did not show an increased UVA resistance compared to exponential-phase cells; instead, they were more sensitive at high UVA dose. UVA-irradiated MR-1 survived better on tryptic soy agar than Luria-Bertani plates regardless of the growth stage. A 20% survival rate of MR-1 was observed following doses of 3.3 J of UVC m(-2), 568 J of UVB m(-2), 25 kJ of UVA m(-2), and 558 J of solar UVB m(-2), respectively. Photoreactivation conferred an increased survival rate to MR-1 of as much as 177- to 365-fold, 11- to 23-fold, and 3- to 10-fold following UVC, UVB, and solar light irradiation, respectively. A significant UV mutability to rifampin resistance was detected in both UVC- and UVB-treated samples, with the mutation frequency in the range of 10(-5) to 10(-6). Unlike in E. coli, the expression levels of the nucleotide excision repair (NER) component genes uvrA, uvrB, and uvrD were not damage inducible in MR-1. Complementation of Pseudomonas aeruginosa UA11079 (uvrA deficient) with uvrA of MR-1 increased the UVC survival of this strain by more than 3 orders of magnitude. Loss of damage inducibility of the NER system appears to contribute to the high sensitivity of this bacterium to UVR as well as to other DNA-damaging agents.

Genome-wide Examination of the Natural Solar Radiation Response in Shewanella oneidensis MR-1

We previously reported that Shewanella oneidensis MR-1 is extremely sensitive to natural solar radiation (NSR). Here we analyzed the global transcriptional profile of MR-1 during a 1-h recovering period after exposure to ambient solar light at a dose that yields about 20% survival rate on a Luria-Bertani (LB) plate. We observed the induction of DNA damage-repair genes, the SOS response as well as detoxification strategies that we previously observed in MR-1 following artificial UV-A irradiation. Few prophage-related genes were induced by natural solar UV radiation, however, in contrast to what was observed following artificial UV-B irradiation. Overall, the cellular response to NSR in MR-1 was more similar to that of UV-A than that of UV-B, but additional genes involved in detoxification were induced compared with induction by either UV-B or UV-A or their sum. Thus, oxidative stress appeared to contribute greatly to the NSR-induced cytotoxic effects in MR-1. A total of 29.1% of genome showed differential expression following NSR exposure, which is much greater than following exposure by UV-B (4.0%), UV-A (8.2%) or their sum (10.7%). Our data suggest that NSR may impact biological processes in a much more complex manner than previously thought.

Does UVB radiation induce SoxS gene expression in Escherichia coli cells?

The SoxRS regulon is induced when bacterial cells are exposed to redox-cycling agents such as menadione or paraquat. In this paper it is shown that a physical agent, such as ultraviolet radiation with a wavelength of 312 nm (UVB) can induce soxS gene expression. The results indicate that this induction involves the RpoS protein. Moreover, an unexpected increase of soxS gene expression independent of a functional soxR gene in UVB-irradiated cells has been verified. This increase could be explained by transcription of soxS gene in a rpoS-dependent pathway.

Sunlight inactivation of fecal indicator bacteria and bacteriophages from waste stabilization pond effluent in fresh and saline waters

Sunlight inactivation in fresh (river) water of fecal coliforms, enterococci, Escherichia coli, somatic coliphages, and F-RNA phages from waste stabilization pond (WSP) effluent was compared. Ten experiments were conducted outdoors in 300-liter chambers, held at 14C (mean river water temperature). Sunlight inactivation (k(S)) rates, as a function of cumulative global solar radiation (insolation), were all more than 10 times higher than the corresponding dark inactivation (k(D)) rates in enclosed (control) chambers. The overall k(S) ranking (from greatest to least inactivation) was as follows: enterococci > fecal coliforms greater-than-or-equal E. coli > somatic coliphages > F-RNA phages. In winter, fecal coliform and enterococci inactivation rates were similar but, in summer, enterococci were inactivated far more rapidly. In four experiments that included freshwater-raw sewage mixtures, enterococci survived longer than fecal coliforms (a pattern opposite to that observed with the WSP effluent), but there was little difference in phage inactivation between effluents. In two experiments which included simulated estuarine water and seawater, sunlight inactivation of all of the indicators increased with increasing salinity. Inactivation rates in freshwater, as seen under different optical filters, decreased with the increase in the spectral cutoff (50% light transmission) wavelength. The enterococci and F-RNA phages were inactivated by a wide range of wavelengths, suggesting photooxidative damage. Inactivation of fecal coliforms and somatic coliphages was mainly by shorter (UV-B) wavelengths, a result consistent with photobiological damage. Fecal coliform repair mechanisms appear to be activated in WSPs, and the surviving cells exhibit greater sunlight resistance in natural waters than those from raw sewage. In contrast, enterococci appear to suffer photooxidative damage in WSPs, rendering them susceptible to further photooxidative damage after discharge. This suggests that they are unsuitable as indicators of WSP effluent discharges to natural waters. Although somatic coliphages are more sunlight resistant than the other indicators in seawater, F-RNA phages are the most resistant in freshwater, where they may thus better represent enteric virus survival.

Role of spoT-dependent ppGpp accumulation in the survival of light-exposed starved bacteria

In bacteria, cytoplasmic levels of the effector nucleotide ppGpp are regulated in response to changes in growth conditions. This study describes the involvement of SpoT-mediated ppGpp accumulation in the survival of light-exposed bacteria during fatty acid starvation. In contrast to isogenic wild-type strains and relA mutants, the 'Vibrio angustum' S14 spoT and Escherichia coli relA spoT mutants displayed significant losses in viability in response to cerulenin-induced fatty acid starvation under cool-white fluorescent light. However, when starvation experiments were performed in complete darkness, or under light filtered through a UV-resistant perspex sheet, only a minor decline in viability was observed for the wild-type and mutant strains. This finding indicated that the lethal effect was mediated by weak UV emission. In contrast to the E. coli relA spoT mutant, which lacks ppGpp, the 'V. angustum' S14 spoT mutant exhibited higher ppGpp levels and lower RNA synthesis rates during fatty acid starvation, features that might be correlated with its lethality. In agreement with this finding, fatty acid starvation lethality also occurred upon induction of ppGpp overaccumulation in E. coli. These data suggest that the precise regulation of ppGpp levels in the stressed cell is crucial, and that both the absence and the overaccumulation of ppGpp impair fatty acid starvation survival of light-exposed cells. Moreover, the UV-induced lethal effect during fatty acid starvation was also observed for E. coli strains mutated in rpoS and dps, which, in the wild-type, are regulated directly or indirectly by ppGpp, respectively. The restoration of viability of fatty-acid-starved spoT mutant cells through the addition of exogenous catalase suggested that the observed light-dependent lethal effect was, at least in part, caused by UV-imposed oxidative stress. Based on these results, it is proposed that fatty acid starvation adaptation of light-exposed bacterial cells depends on the development of resistance to UV-induced oxidative stress. This stress resistance was found to require appropriate ppGpp levels, ppGpp-induced RpoS expression and, hence, upregulation of RpoS-regulated stress-defending genes, such as dps.

Sublethal effects of ultraviolet A radiation on Enterobacter cloacae

We report the sublethal effects of ultraviolet A (UVA) on Enterobacter cloacae in comparison with those produced in Escherichia coli. UVA-induced sublethal effects were investigated in either bacterial membrane and at tRNA level. Limited dependence on oxygen concentration for photoinduced inhibition of biochemical membrane functions and low levels of oxidative damage during the irradiation period were found in En. cloacae. On the other hand, ultraviolet spectroscopy and reversed-phase HPLC analysis of hydrolysed tRNA showed that radio induced damage to tRNA is similar in En. cloacae and E. coli. Nevertheless, growth delay induced by UVA in En. cloacae was shorter than that found in E. coli submitted to the same experimental conditions. A limited post-irradiation ppGpp accumulation and the absence of any influence of the membrane damage on the growth delay extent seem to be responsible for the shortness of this effect in En. cloacae. Most of the differences between En. cloacae and E. coli could be attributed to an increased ability of En. cloacae to overcome oxidative stress during UVA exposure.

Construction and analysis of photolyase mutants of Pseudomonas aeruginosa and Pseudomonas syringae: contribution of photoreactivation, nucleotide excision repair, and mutagenic DNA repair to cell survival and mutability following exposure to UV-B radiation

Based on nucleotide sequence homology with the Escherichia coli photolyase gene (phr), the phr sequence of Pseudomonas aeruginosa PAO1 was identified from the genome sequence, amplified by PCR, cloned, and shown to complement a known phr mutation following expression in Escherichia coli SY2. Stable, insertional phr mutants containing a tetracycline resistance gene cassette were constructed in P. aeruginosa PAO1 and P. syringae pv. syringae FF5 by homologous recombination and sucrose-mediated counterselection. These mutants showed a decrease in survival compared to the wild type of as much as 19-fold after irradiation at UV-B doses of 1,000 to 1,550 J m(-2) followed by a recovery period under photoreactivating conditions. A phr uvrA mutant of P. aeruginosa PAO1 was markedly sensitive to UV-B irradiation exhibiting a decrease in survival of 6 orders of magnitude following a UV-B dose of 250 J m(-2). Complementation of the phr mutations in P. aeruginosa PAO1 and P. syringae pv. syringae FF5 using the cloned phr gene from strain PAO1 resulted in a restoration of survival following UV-B irradiation and recovery under photoreactivating conditions. The UV-B survival of the phr mutants could also be complemented by the P. syringae mutagenic DNA repair determinant rulAB. Assays for increases in the frequency of spontaneous rifampin-resistant mutants in UV-B-irradiated strains containing rulAB indicated that significant UV-B mutability (up to a 51-fold increase compared to a nonirradiated control strain) occurred even in the wild-type PAO1 background in which rulAB only enhanced the UV-B survival by 2-fold under photoreactivating conditions. The frequency of occurrence of spontaneous nalidixic acid-resistant mutants in the PAO1 uvrA and uvrA phr backgrounds complemented with rulAB were 3.8 x 10(-5) and 2.1 x 10(-3), respectively, following a UV-B dose of 1,550 J m(-2). The construction and characterization of phr mutants in the present study will facilitate the determination of the roles of light and dark repair systems in organisms exposed to solar radiation in their natural habitats.

Protective role of the Mycobacterium smegmatis IdeR against reactive oxygen species and isoniazid toxicity

OBJECTIVE

To understand the mechanism by which IdeR is necessary for maintaining wild type levels of KatG and SodA enzyme activity and normal isoniazid (INH) resistance.

DESIGN

To identify the step(s) of SodA and KatG function that were affected by the ideR mutation, quantitative western immunoassays and ribonucleic acid (RNA) hybridizations were performed. To see if the increased INH sensitivity of the ideR mutant was caused by lower SodA activity, the Mycobacterium smegmatis sod gene was inactivated.

RESULTS

The levels of KatG and SodA mRNA and protein in the M. smegmatis IdeR mutant are decreased to approximately 20-40% of those observed in the wild type parent strain. This is quantitatively similar to the decrease in KatG and SodA enzyme activities originally observed in the ideR strain. The M. smegmatis sodA mutant was slightly more sensitive to INH, compared to the wild type strain and was more resistant than the ideR mutant.

CONCLUSION

IdeR is necessary for full expression of the M. smegmatis katG and sodA genes. It is not yet known whether this protein acts directly at the gene level. The lower levels of SodA contribute slightly to the increased susceptibility to INH of the ideR mutant, but cannot explain the magnitude of the INH sensitivity observed when IdeR is not present. These data suggest that IdeR is a regulator of the cellular stress response, as it has a protective role in cells facing environmental stresses, such as increased levels of reactive oxygen species and INH toxic intermediates. These conclusions do not necessarily apply to IdeR's role in M. tuberculosis physiology, since we have not inactivated its gene in this pathogen.

Characterization of Pseudomonas aeruginosa bacteriophage UNL-1, a bacterial virus with a novel UV-A-inducible DNA damage reactivation phenotype

UNL-1, a lytic virus of Pseudomonas aeruginosa, was observed to express a novel inducible DNA damage reactivation activity in UV-A-irradiated P. aeruginosa host cells. The expression of bacteriophage reactivation was quantified in hosts exposed to either UV-C or UV-A radiation. While reactivation of UV-C-damaged UNL-1 was not inducible in UV-C-irradiated host cells, an approximately 13-fold induction was observed in UV-A-irradiated host cells. When host cells were exposed to sunlight, reactivation of damaged UNL-1 virus increased eightfold. The UV-A induction of UNL-1 DNA damage reactivation was supported in hosts lacking recA gene function. This report is the first description of a recA-independent, UV-inducible virus DNA damage repair system. Our findings suggest that a combination of both host and virus DNA repair processes contribute to the persistence and sustained replication of some bacterial viruses in aquatic environments.

Functional analysis of the Pseudomonas syringae rulAB determinant in tolerance to ultraviolet B (290-320 nm) radiation and distribution of rulAB among P. syringae pathovars

The effect of the plasmid-encoded rulAB (resistance to ultraviolet radiation) determinant on responses of Pseudomonas syringae to ultraviolet-B (UV-B) radiation and the distribution of rulAB among pathovars of P. syringae were determined. The cloned rulAB determinant and the native rulAB+ plasmid pPSR1 both conferred approximately a 10-fold increase in survival on P. syringae pv. syringae FF5 following increasing doses of UV-B radiation. rulAB+ P. syringae strains also maintained significantly larger epiphytic populations on leaf surfaces irradiated with UV-B. rulAB-insertional mutants, constructed in two native rulAB+ strains, were from 10- to 100-fold more sensitive to UV-B radiation. The UV tolerance phenotype and the rulAB genes were widely distributed among P. syringae pathovars isolated from varied plant hosts throughout the world and within a broad range of genotypic backgrounds of P. syringae pv. syringae. With one exception, the rulAB determinant was harboured on pPT23A-like plasmids; these replicons are indigenous residents of the species P. syringae and also tend to encode determinants of importance in host-pathogen interactions.

Differences in the mutational specificities of sunlight and UVB radiation suggest a role for transversion-inducing DNA damage in solar photocarcinogenesis

Mutations induced by UVB radiation and natural sunlight in a plasmid-borne yeast (Saccharomyces cerevisiae) tRNA gene (SUP4-o) were characterised by DNA sequencing. For both agents, the majority (> 90%) of the total mutations analysed were single base-pair substitutions, but tandem substitutions and single base-pair deletions also were detected. Each agent induced all six types of base-pair change but the tandem substitutions involved exclusively G.C-->A.T transitions. However, the fractions of single and tandem G.C-->A.T transitions were reduced by about 50%, and the fraction of transversions at G.C pairs was increased by 11-fold for sunlight relative to UVB. Comparisons of the site and strand specificities of the substitutions suggested that dipyrimidine adducts were responsible for the transitions, and that other lesions induced by sunlight may have given rise to the transversions. The relevance of these findings to skin cancer is discussed.

Bacterial gene products in response to near-ultraviolet radiation

Our research has focused on bacterial gene products that protect cells from damage by near-ultraviolet radiation (near-UV) including gene products involved in the subsequent recovery process. Protective gene products include such anti-oxidants as catalases, superoxide dismutases and glutathione reductase. Near-UV damage recovery products include exonuclease III and DNA-glycosylases. Perhaps more critical than the products of structural genes are certain regulatory gene products that are triggered upon excess near-UV oxidation and lead to synthesis of entire batteries of anti-oxidant enzymes, DNA repair enzymes, and DNA-integrity proteins. Our recent experiments have focused on RpoS and its interaction with OxyR, two proteins that regulate the synthesis of molecules that protect cells from near-UV and other oxidative stresses.

Near ultraviolet radiation (UVA and UVB) causes a formamidopyrimidine glycosylase-dependent increase in G to T transversions

In contrast to far-UV (< 290 nm) DNA damage, a large fraction of the DNA damage caused by near-UV is oxygen-dependent, suggesting the involvement of reactive oxygen species (ROS). The oxidized base 8-oxo-7,8-dihydroguanine (GO) is characteristic of ROS-induced DNA damage and is removed by Fapy (formamidopyrimidine) glycosylase. We have recently shown that Escherichia coli strains deficient in Fapy glycosylase (fpg) are hypersensitive to the lethal effects of UVA but not far-UV (UVC), suggesting lesions recognized by this enzyme may be important premutagenic or lethal lesions generated by near-UV radiation. In this study, we have found that while the far-UV-induced mutation rates of Fapy-deficient and wild-type strains were similar, near-UV (UVA and UVB) was hypermutagenic to a Fapy-deficient strain, causing a dose-dependent increase in induced mutation relative to wild type (up to five-fold at 200 kJ/m2). Using a plasmid back mutation assay, the predominant near-UV-induced mutations in both wild-type and Fapy-deficient strains were found to be C-->T transitions and G -->T transversions. The former is probably due to replicative bypass of pyrimidine dimers or (6-4) photoproducts that are known to be generated by near-UV, whereas the latter may be due to mispairing of GO lesions with adenine during replication. Consistent with this, the frequency of near-UV-induced G-->T transversions was 16-fold higher in a Fapy-deficient strain than a wild-type strain.

Role of rpoS regulon in resistance to oxidative stress and near-UV radiation in delta oxyR suppressor mutants of Escherichia coli

Escherichia coli delta oxyR mutants are hyper-sensitive to oxidative agents but this sensitivity is reversed to hyper-resistance in delta oxyR suppressor strains (delta oxyRsup; Greenberg, J.T. and Demple, B. 1988. EMBO J. 7:2611-2618). Also, delta oxyR mutants have increased mutation rates that are also reversed in delta oxyRsup. We now report that the rpoS regulon may have a role in determining hyper-resistance and loss of hyper-mutability of delta oxyRsup. Delta oxyRsup cells were also resistant to near-ultraviolet radiation (near-UV) and survived longer in stationary phase than delta oxyR cells. In delta oxyRsup cells elevated beta-galactosidase expression from a rpoS::lacZ promoter fusion and significant overproduction of RpoS protein was observed. These increases were accompanied by substantial elevation in transcription of rpoS-dependent genes as determined by beta-galactosidase expression from katE::lacZ, dps::lacZ, and xthA::lacZ promoters. Catalase HPI and HPII activities were also increased. When rpoS::Tn10 was transduced into delta oxyRsup, phenotypes switched back to hyper-sensitive, hyper-mutable and reduced catalases I and II. Individual delta oxyR colonies exhibited significant clonal variability in beta-galactosidase expression from rpoS::lacZ promoter. These results provide further evidence of the functional and regulatory overlap between two major anti-oxidant defense systems of bacteria.

A mutant of Escherichia coli hyper-resistant to a number of DNA damaging agents: location of the mutational site

A mutant of Escherichia coli, isolated as hyper-resistant to UVC, is found to be hyper-resistant to UVA, H2O2, low concentrations of nalidixic acid, novobiocin, UVA plus H2O2 and UVA plus 8-methoxypsoralen. A mutational site (uvh) conferring the hyper-resistance phenotype to UVC, and presumably to other DNA damaging agents, has been mapped at the 89.9 min region on the chromosome. Complementation analysis with an F-prime uvh+/uvh- diploid strain showed that the uvh+ allele is dominant over uvh- in trans. Studies with a variety of plasmids, carrying various LexA regions, introduced into the UV hyper-resistant strain show that mutation at the uvh locus may be responsible for derepression of the SOS inducible repair system. Based on the results, it is suggested that uvh is a part of the SOS inducible system. A plausible explanation for the hyper-resistance phenotypes for various DNA damaging agents and a model for the genetic control of a second set of putative SOS regulons are presented.

recA-dependence of the response of Pseudomonas aeruginosa to UVA and UVB irradiation

The responses of the autochthonous soil and aquatic organism, Pseudomonas aeruginosa to UV radiation wavelengths (UVA, 320-400 nm, and UVB, 280-320 nm) has been investigated in this study. P. aeruginosa recA mutants were found to be more sensitive to both UVA and UVB radiation than were their isogenic RecA+ parents. Introduction of a low-copy-number plasmid containing the cloned wild-type P. aeruginosa recA gene restored UVA and UVB resistance to recA mutants. The concentration of RecA protein increased twofold 120 min after exposure to either UVA or UVB radiation, suggesting induction of expression of the recA gene by these wavelengths. In this study, we found that a functional RecA protein is required for activation of D3 prophage in lysogenic cells following exposure to UVB radiation. Prophage were not induced by exposure of their hosts to UVA radiation. Induction of damage-inducible (din) genes in response to UVA or UVB irradiation was also shown to be RecA dependent. These data indicate that the recA gene plays a role in the response of P. aeruginosa to exposure to wavelengths of UV radiation found in the solar spectrum.

Role of Fapy glycosylase and UvrABC excinuclease in the repair of UVA (320-400 nm)-mediated DNA damage in Escherichia coli

In contrast to the damage caused by far-UV, the damage caused by UVA (320-400 nm) is largely oxygen dependent, suggesting near-UV-mediated DNA damage involves reactive oxygen species. The DNA repair enzymes that recognize oxidized bases may, therefore, be an important part of the cell's near-UV defense repertoire. To evaluate the relative importance of Fpg (Fapy) glycosylase (an enzyme known to remove oxidized bases) and the DNA damage-inducible UvrABC excinuclease in recovery from near-UV-induced stress, we have constructed fpg- and uvrA- derivatives of Escherichia coli and tested the response (survival) of these strains to both UVA and far-UV radiation. Relative to control strains, the fpg- derivatives were found to be consistently more sensitive to the lethal effects of UVA, but not far-UV radiation. In contrast, uvrA- mutants were more sensitive than control strains to both UVA and far-UV radiation. Thymine dimers, known to be produced by far-UV and corrected by UvrABC, were not generated by the UVA fluences used in this study, suggesting that some other UVA-induced lesion(s) is recognized and repaired by this excinuclease.

Induction of resistance to hydrogen peroxide and radiation in Deinococcus radiodurans

Though bacteria of the radiation-resistant genus Deinococcus have a high resistance to the lethal and mutagenic effects of many DNA-damaging agents, the mechanisms involved in the response of these bacteria to oxidative stress are poorly understood. To investigate antioxidant enzyme responses in Deinococcus spp., the catalase activity produced by these bacteria was measured and the sensitivity of these bacteria to hydrogen peroxide was tested. Deinococcus spp. had higher levels of catalase and were more resistant to hydrogen peroxide than Escherichia coli K12. The high levels of catalase produced by Deinococcus radiodurans were, in part, regulated by growth phase. Cultures of D. radiodurans, when pretreated with sublethal levels of hydrogen peroxide, became relatively resistant to the lethal effects of hydrogen peroxide and exhibited higher levels of catalase than untreated control cultures. These pretreated cells were also resistant to lethality mediated by ultraviolet light and gamma-rays. These results suggest that Deinococcus spp. possess inducible defense mechanism(s) against the deleterious effects of oxidants and ionizing and ultraviolet radiation.

Genetic mechanisms involved in cellular recovery from oxidative stress

Sophisticated biochemical networks allow organisms such as bacteria and insects to switch from very rapid growth and development in ideal environments to dormancy during severely unfavorable conditions. These switches may be accompanied by abrupt changes in oxidation/reduction involving reactive oxygen species (ROS). ROS have the potential of damaging nucleic acids, proteins, and membranes. In Escherichia coli, certain genetically regulated circuits (regulons) turn on synthesis of anti-oxidant enzymes to protect against distinct ROS excesses (superoxide, hydrogen peroxide, organic or lipid peroxides, etc.). As examples, the soxRS regulon controls synthesis of Mn-superoxide dismutase, oxyR controls catalase HPI, rpoS positively regulates HPII, and fur regulates several oxidative reactions that involve iron uptake. Our studies have focused on the regulatory role of rpoS, known to be a sigma factor (sigma 38) that combines with RNA polymerase and is a regulator of those gene products needed to protect cells during dormancy. Since insect cells, during both active growth and dormancy, endure severe environments, analogous protective gene products may be induced. Examples are presented of insect anti-oxidant metabolism, including those involved in the aging process. In addition, we searched several DNA and protein sequence data banks to compare resemblances between anti-oxidant gene products of bacteria and insects.

Synergistic action of near-UV and phenylalanine, tyrosine or tryptophan on the inactivation of phage T7: role of superoxide radicals and hydrogen peroxide

Near ultraviolet (NUV) light can cause a variety of damage to biological systems. The effects of NUV are significantly enhanced in the presence of sensitizers. One of the most important targets of such synergistic effects is DNA. Cellular DNA exposed to NUV plus sensitizers is damaged in a variety of ways, DNA strand breaks and interstrand cross-links being the most common effects. In this study, phenylalanine, tyrosine and tryptophan are shown to act as sensitizers for NUV action of phage T7; superoxide anions are produced. The reactive species probably interacts with phage DNA causing damage responsible for phage inactivation. Superoxide dismutase reverses the synergistic activities of phenylalanine and tyrosine on NUV-induced phage inactivation, but catalase is additionally required to reverse the effect of tryptophan. Therefore, it is probable that NUV photolysis of tryptophan causes the production of superoxide ions and hydrogen peroxide, both of which contribute to phage inactivation. The ubiquitous nature of NUV in our environment and the presence of amino acids in skin cells suggests that an important mechanism for the induction of skin cancer in humans by solar exposure is amino acid photolysis by NUV.

Role of rpoS (katF) in oxyR-independent regulation of hydroperoxidase I in Escherichia coli

We present evidence showing that rpoS (katF) is a regulator of katG gene transcription in an oxyR-independent manner. Mutation of the rpoS gene in several different Escherichia coli strains caused a significant reduction in catalase HPI activity. In rpoS-delta oxyR double mutants, the level of HPI was considerably lower compared to the delta oxyR parent strain, and was restored when transformed with an rpoS+ plasmid. Overproduction of HPI in oxyR- suppressor strains was greatly diminished after inactivation of the rpoS gene and was accompanied by a substantial increase in sensitivity to menadione. Beta-galactosidase expression from a katG::lacZ promoter was lower in rpoS strains compared to rpoS+ isogenic parents. Several delta oxyR strains had detectable levels of katG transcription that was significantly diminished after rpoS gene inactivation.

Evidence for phage-mediated gene transfer among Pseudomonas aeruginosa strains on the phylloplane

As the use of genetically engineered microorganisms for agricultural tasks becomes more frequent, the ability of bacteria to exchange genetic material in the agricultural setting must be assessed. Transduction (bacterial virus-mediated horizontal gene transfer) is a potentially important mechanism of gene transfer in natural environments. This study investigated the potential of plant leaves to act as surfaces on which transduction can take place among microorganisms. Pseudomonas aeruginosa and its generalized transducing bacteriophage F116 were used as a model system. The application of P. aeruginosa lysogens of F116 to plant leaves resulted in genetic exchange among donor and recipient organisms resident on the same plant. Transduction was also observed when these bacterial strains were inoculated onto adjacent plants and contact was made possible through high-density planting.

Biochemical studies on the lethal effects of solar and artificial ultraviolet radiation on Staphylococcus aureus

The effects of UV-B radiation generated in the laboratory and as a component of sunlight on the viability and particular biochemical activities of the bacterium Staphylococcus aureus have been examined. UV-B radiation progressively inhibits protein synthesis (assayed as 3H-alanine incorporation) and kills cells. Cell respiration, and RNA and DNA synthesis (3H-uridine and 3H-thymidine incorporation) were not greatly affected by UV-B irradiation. The OH. and 1O2-free radical scavengers protected cells against killing and inhibition of protein synthesis by UV-B, suggesting that such radicals mediate the effects of UV-B on this organism. A similar protective effect using a ferric ion chelator suggests an important role for metallic ions in UV-B lethality.

Cu(II) sensitizes pBR322 plasmid DNA to inactivation by UV-B (280-315 nm)

Copper(II), in the presence of UV-B radiation (280-315 nm), can generate single-strand breaks in the sugar-phosphate backbone of pBR322 plasmid DNA. A low level of single-strand backbone breaks occurs in the presence of Cu(II) alone, but UV-B irradiation increases the rate by the more than 100-fold. Concomitant with the damage to the DNA backbone is a loss of transforming activity. Oxygen is required for generation of the single-strand breaks but not for the loss of transforming activity. A DNA glycosylase (Fpg), which participates in the repair of certain DNA nitrogenous base damage, does not repair plasmid DNA damaged by Cu(II). The hydroxyl radical scavenging compound DMSO is only somewhat effective at protecting the physical and biological properties of the DNA. These results with Cu(II) are compared to those obtained previously with pBR322 plasmid DNA in the presence of Fe(III) and UV-A.

Escherichia coli genes involved in cell survival during dormancy: role of oxidative stress

When Escherichia coli cells reach stationary phase of growth, specific gene products are synthesized that protect cells while dormant. "Aged" cells may remain viable in cultures for years. For example, agar cultures stored for 38 years still had more than 10(5) viable cells/ml. However, when specific mutants were cultured, the population of these mutants dropped sharply after 4-10 days. This defect is termed "Stationary-Phase-Death". Each mutant strain was hypersensitive to near-ultraviolet radiation and other oxidative agents. Bovine catalase rescued many of the mutants from death in dormancy, suggesting that specific gene products protect "aged" cells against oxidative damage.

Excision repair influences the site and strand specificity of sunlight mutagenesis in yeast

A collection of 384 mutations recovered in a tRNA gene (SUP4-o) following exposure of isogenic excision-repair-proficient (RAD1) or deficient (rad1) strains of the yeast Saccharomyces cerevisiae to sunlight was characterized by DNA sequencing. In each case, greater than 90% of the mutations were single base-pair substitutions with events at G.C pairs constituting most of the changes. However, more than half of these substitutions were transversions in the RAD1 strain whereas transitions predominated in the rad1 strain. Tandem double substitutions were recovered in both strains and the individual changes were exclusively G.C----A.T transitions. The majority of single substitutions, and all tandem double changes, were at base-pairs where the pyrimidine(s) was part of a dipyrimidine sequence and the site specificities were consistent with cyclobutane dimers and/or pyrimidine (6-4) pyrimidone photoproducts contributing to sunlight mutagenesis. Yet, the data also pointed to an important role for lesions that form at G.C pairs and give rise to transversions. Analysis of the strand specificity of sunlight mutagenesis indicated that transitions or transversions at G.C pairs occurred preferentially in SUP4-o at sites where a dipyrimidine or a guanine, respectively, was on the transcribed strand. These biases required a functional excision-repair system.

Lethal and mutational effects of solar and UV radiation on Staphylococcus aureus

Strains of Staphylococcus aureus, an opportunistic pathogen commonly found on human skin, were exposed to sunlight and UV C radiation, and the lethal and mutational effects measured. Sunlight killed cells with an inactivation constant of 3 x 10(-5) per joule per square metre; UV C was much more lethal, giving an inactivation constant of approximately 0.1 per joule per square metre. Some strains tested showed a sensitivity to sunlight that was dependent on the growth phase of the cells, exponentially growing cells showing a greater sensitivity. Mutational effects of irradiation were measured by the appearance of mutants sensitive to methicillin following irradiation of a multiresistant strain. Mutants appeared at a frequency of 10(-3); this high frequency of mutation in the region of the mec gene has also been observed when multiresistant strains are subjected to nutritional or thermal stress. Mutants showed the same chromosomal alteration (seen in pulse-field gel electrophoresis of SmaI-digested DNA) whether induced by solar or UV C irradiation.

Oxidative stress responses in Escherichia coli and Salmonella typhimurium

Oxidative stress is strongly implicated in a number of diseases, such as rheumatoid arthritis, inflammatory bowel disorders, and atherosclerosis, and its emerging as one of the most important causative agents of mutagenesis, tumorigenesis, and aging. Recent progress on the genetics and molecular biology of the cellular responses to oxidative stress, primarily in Escherichia coli and Salmonella typhimurium, is summarized. Bacteria respond to oxidative stress by invoking two distinct stress responses, the peroxide stimulon and the superoxide stimulon, depending on whether the stress is mediated by peroxides or the superoxide anion. The two stimulons each contain a set of more than 30 genes. The expression of a subset of genes in each stimulon is under the control of a positive regulatory element; these genes constitute the OxyR and SoxRS regulons. The schemes of regulation of the two regulons by their respective regulators are reviewed in detail, and the overlaps of these regulons with other stress responses such as the heat shock and SOS responses are discussed. The products of Oxy-R- and SoxRS-regulated genes, such as catalases and superoxide dismutases, are involved in the prevention of oxidative damage, whereas others, such as endonuclease IV, play a role in the repair of oxidative damage. The potential roles of these and other gene products in the defense against oxidative damage in DNA, proteins, and membranes are discussed in detail. A brief discussion of the similarities and differences between oxidative stress responses in bacteria and eukaryotic organisms concludes this review.

Lethal and mutagenic action of hydrogen peroxide on Haemophilus influenzae

The lethal and mutagenic effects of H2O2 on wild-type Haemophilus influenzae Rd and on uvr1, uvr2, rec1, and rec2 mutant strains were studied. The first two mutants are sensitive to UV, and the second two are defective in recombination. Rd, urv1, and rec1 strains were more sensitive to the killing effect of H2O2 treatment than were uvr2 and rec2 strains. There were peaks of mutagenesis at two H2O2 concentrations over a range of 30 to 275 mM. Our results suggest a specific repair of H2O2 damage that is independent of the Uvr2 and Rec2 gene products. Sensitivity to the killing effect of H2O2 and to the lethal action of near-UV light were similar for Rd and uvr1 strains. This finding suggests that the mechanisms of killing by and repair of H2O2 damage may have some overlap with those of near-UV radiation.

Mutagenicity of coolwhite fluorescent light for Salmonella

The most common fluorescent lamps in use today in homes and businesses in the United States, 'coolwhite' fluorescent lamps, emit light that is mutagenic for Salmonella. Strains that carry both a uvrB mutation and plasmid pKM101 are extremely susceptible to this light-induced mutation. Both base substitution and frameshift mutations can be induced without substantial lethal effects on the bacteria. Induced mutations accumulate essentially as a linear function of the time bacteria are exposed to illumination. Of Salmonella histidine-requiring strains with known nucleotide target sequences (Hartman et al., 1986; Cebula and Koch, 1989, 1990), strains either carrying one of the base substitution mutations, hisG428 and hisG46, or one of the frameshifts, hisC3076 and hisD6610, are most highly mutagenized whereas frameshift strains with hisD6580 and hisD3052 exhibit lower rates of mutagenesis. Mutagenicity does not appear to require the presence of oxygen. A filter blocking wavelengths below 370 nm eliminates mutagenesis. Polystyrene, cellulose acetate and, especially, mylar and glass filters reduce mutagenesis, indicating that at least some of the mutagenic effects can be attributed to leakage of radiations below 290 nm (far-ultraviolet light) from 'coolwhite' lamps. The more recently introduced fluorescent 'softwhite' lamps are roughly 10-fold less mutagenic at approximately equal light intensity. Incandescent light bulbs are much less mutagenic than are these fluorescent lamps. Our mutational data correlate closely with previous results in eukaryotic cells (Jacobson and Krell, 1982). A uvrB recA Salmonella double mutant is hypersensitive to the lethal effects of coolwhite fluorescent light, even when illuminated through the lids of glass Petri dishes. Thus, appropriate Salmonella strains would appear to be simple and useful screens for both the mutagenic and the lethal activities of fluorescent lamps. These systems are amenable to classroom laboratory use as relatively safe and effective means of demonstrating environmental mutagenesis.

Influence of DNA adenine methylase on the sensitivity of Escherichia coli to near-ultraviolet radiation and hydrogen peroxide

Near-ultraviolet (NUV) radiation and hydrogen peroxide (H2O2) inactivation studies were performed on Escherichia coli K-12 DNA adenine methylation (dam) mutants and on cells that carry plasmids which overexpress Dam methylase. Lack of methylation resulted in increased sensitivity to NUV and H2O2 (a photoproduct of NUV). In a dam mutant carrying a dam plasmid, the levels of Dam enzyme and resistance to NUV and H2O2 were restored. However, using a multicopy dam+ plasmid strain, increasing the methylase above wildtype levels resulted in an increase in sensitivity of the cells rather than resistance.

Mutations by near-ultraviolet radiation in Escherichia coli strains lacking superoxide dismutase

In wild-type Escherichia coli, near-ultraviolet radiation (NUV) was only weakly mutagenic. However, in an allelic mutant strain (sodA sodB) that lacks both Mn- and Fe-superoxide dismutase (SOD) and assumed to have excess superoxide anion (O2-), NUV induced a 9-fold increase in mutation above the level that normally occurs in this double mutant. When a sodA sodB double mutant contained a plasmid carrying katG+ (excess HP-I catalase), mutation by NUV was reduced to wild-type (sodA+ sodB+) levels. Also, in the sodA sodB xthA triple mutant, which lacks exonuclease III (exoIII) in addition to SOD, the mutational frequency by NUV was reduced to wild-type levels. This synergistic action of NUV and O2- suggested that pre-mutational lesions occur, with exoIII converting these lesions to stable mutants. Exposure to H2O2 induced a 2.8-fold increase in mutations in sodA sodB double mutants, but was reduced to control levels when a plasmid carrying katG+ was introduced. These results suggest that NUV, in addition to its other effects on cells, increases mutations indirectly by increasing the flux of OH. radicals, possibly by generating excess H2O2.