RôLE OF DNA REPAIR MECHANISMS IN MICROBIAL INACTIVATION AND RECOVERY PHENOMENA

Mutation induction in haploid yeast after split-dose radiation exposure. II. Combination of UV-irradiation and X-rays

Split-dose protocols can be used to investigate the kinetics of recovery from radiation damage and to elucidate the mechanisms of cell inactivation and mutation induction. In this study, a haploid strain of the yeast, Saccharomyces cerevisiae, wild-type with regard to radiation sensitivity, was irradiated with 254-nm ultraviolet (UV) light and then exposed to X-rays after incubation for 0-6 hr. The cells were incubated either on nutrient medium or salt agar between the treatments. Loss of reproductive ability and mutation to canavanine resistance were measured. When the X-ray exposure immediately followed UV-irradiation, the X-ray survival curves had the same slope irrespective of the pretreatment, while the X-ray mutation induction curves were changed from linear to linear quadratic with increasing UV fluence. Incubations up to about 3 hr on nutrient medium between the treatments led to synergism with respect to cell inactivation and antagonism with respect to mutation, but after 4-6 hr the two treatments acted independently. Incubation on salt agar did not cause any change in the survival curves, but there was a strong suppression of X-ray-induced mutation with increasing UV fluence. On the basis of these results, we suggest that mutation after combined UV and X-ray exposure is affected not only by the induction and suppression of DNA repair processes, but also by radiation-induced modifications of cell-cycle progression and changes in the expression of the mutant phenotype.

Mutation induction by different types of radiation at the Hprt locus

Mutation induction at the Hprt locus in Chinese hamster cells was studied after exposure to ultraviolet light, X-rays and alpha particles. While mutant frequency as a function of dose or fluence followed a linear-quadratic relationship with UV and X-rays, it showed a linear dependence for alpha particles. If mutant frequency is plotted vs. the logarithm of surviving fraction, a linear relationship is found in all cases although with different slopes. These are about equal with the two types of ionising radiations but about 10 times larger for UV. They can be used as a measure of mutagenic potential and are termed mutagenicity. It is shown that this parameter is correlated with the maximum of mutant yield, i.e., the number of mutants per cell at risk. It is concluded from this analysis that the maximum mutant yield is always found at doses or fluences which lead to 37% survival irrespective of the kind of radiation. If mutation induction is measured in X-irradiated cells after pre-exposure to UV, mutant frequency is higher than expected on the basis of independent action of the two radiations. Deletion spectra were determined by using multiplex polymerase chain reaction. It was found that the background of spontaneous mutants varied considerably and showed frequently repetitive patterns, presumably because of clonal expansion of pre-formed mutants. UV-induced mutants did not contain any deletions, while those with both X-rays and alpha particles the majority displayed partial and total deletions. Based on a total number of 134 X-ray- and 192 alpha-induced mutants, it is concluded that the total fraction of mutant clones without deletions (partial or total) is about 40% for X-rays and only about 20% for alpha-particles.

Bridge-building between mathematical theory and molecular biology: the REV2 gene as paradigm

The DNA damage-repair theory of R.H. Haynes anticipated the possibility of dose-dependent repair processes. The mathematical formalism developed by Haynes and coworkers on the basis of this theory provided tools to probe for the existence of inducible components of mutation or recombination by analysis of dose-response curves. Subsequently, we found that biological and molecular analysis of the Saccharomyces cerevisiae REV2 gene supported the validity of the postulates derived from the mathematical analysis. In this article, we briefly review the foregoing and summarize evidence that the REV2 gene product might function in DNA damage-inducible repair and mutation processes.

"Close-fitting sleeves"--recognition of structural defects in duplex DNA

The first step in the ubiquitous cellular process of nucleotide excision-repair must involve the recognition of a lesion or structural distortion in DNA. This is followed by incision in the strand perceived as damaged; and then coordinated steps of local degradation and re-synthesis occur to replace the defective DNA segment with a new stretch of nucleotides, making use of the intact complementary strand as template. The repair patch is ultimately ligated at its 3' end to the contiguous preexisting DNA strand to restore the integrity of the normal DNA structure. Crucial to this repair scheme is the fact that the genome consists of double-stranded DNA, so that when one strand is damaged the information for its repair can, in principle, be recovered from the other strand. We will review a bit of the early speculation about the nature of the damage recognition step and then discuss the complexity of that event as we currently understand it. An important conceptual contribution to this field resulted from my collaboration with Robert Haynes in which we suggested that "the recognition step in the repair mechanism could be formally equivalent to threading the DNA through a close-fitting 'sleeve' which gauges the closeness-of-fit to the Watson-Crick structure" (Hanawalt and Haynes, 1965).

Synergism between electricity and ionizing radiation

Weak direct electric currents which produce little (or no) lethal damage to Escherichia coli bacteria are shown to act synergistically with ionizing radiation, both electromagnetic radiation (X-ray) and charged particles (beta radiation). This synergism greatly enhances the lethal effect of ionizing radiation on bacteria. This is possibly due to increased single-strand breaks in DNA, as detected by the alkaline sucrose gradient method. It is also shown that in cells with thymidine-3H incorporated into their DNA and treated with electricity, the radioactivity is released from the acid-insoluble fraction to the acid-soluble fraction, so that the ratio of radioactivity in the soluble fraction to that in the insoluble fraction increases from 0.47 in the non-treated control cells to 3.46 in the cells treated with an electric current of 1.0 mA (3.0 V) for 30 min, which indicates extensive degradation of cellular DNA. No synergism is detected between electricity and 254 nm UV radiation nor between electricity and X-rays, when these two agents are used sequentially in any order. Electricity alone produces lesions in cell membranes, as shown by electron microscopy.

Analysis of interactions between mutagens, I. Heat and ultraviolet light in Saccharomyces cerevisiae

A new mathematical approach to the description of interaction data (Ager and Haynes, 1987) is applied here to the interaction between heat and ultraviolet light (UV) in Saccharomyces cerevisiae. A strong synergism for cell killing is found to be associated with large increases in gene conversion (of up to 8-fold), and mutation (of up to 14-fold). Analysis of the interaction data for both wild-type and repair-deficient strains indicates that the heat-UV synergism arises via the inhibition of two different repair pathways. Unambiguous conclusions regarding the molecular mechanisms by which these repair processes are inhibited cannot be drawn on the basis of dose-response data alone. However, this approach does enable one to make well defined, empirical comparisons of the nature and kinetics of such interactions.

Two-quantum UV photochemistry of nucleic acids: comparison with conventional low-intensity UV photochemistry and radiation chemistry

The action of high-intensity laser u.v. radiation on nucleic acid molecules and their constituents in vitro and in vivo is compared with the results of low-intensity u.v. photolysis and gamma-radiolysis.

Elevation of the heat resistance of Salmonella typhimurium by sublethal heat shock

The survival of Salmonella typhimurium after a standard heat challenge at 55 degrees C for 25 min increased by several orders of magnitude when cells grown at 37 degrees C were pre-incubated at 42 degrees, 45 degrees or 48 degrees C before heating at the higher temperature. Heat resistance increased rapidly after the temperature shift, reaching near maximum levels within 30 min. Elevated heat resistance persisted for at least 10 h. Pre-incubation of cells at 48 degrees C for 30 min increased their resistance to subsequent heating at 50 degrees, 52 degrees, 55 degrees, 57 degrees or 59 degrees C. Survival curves of resistant cells were curvilinear. Estimated times for a '7D' inactivation increased by 2.6- to 20-fold compared with cells not pre-incubated before heat challenge.

Repair of ultraviolet-light-induced DNA damage in vibrio cholerae

Repair of ultraviolet-light-induced DNA damage in a highly pathogenic Gram-negative bacterium, Vibrio cholerae, has been examined. All three strains of V. cholerae belonging to two serotypes, Inaba and Ogawa, are very sensitive to ultraviolet irradiation, having inactivation cross-sections ranging from 0.18 to 0.24 m2/J. Although these cells are proficient in repairing the DNA damage by a photoreactivation mechanism, they do not possess efficient dark repair systems. The mild toxinogenic strain 154 of classical Vibrios presumably lacks any excision repair mechanism and studies of irradiated cell DNA indicate that the ultraviolet-induced pyrimidine dimers may not be excised. Ultraviolet-irradiated cells after saturation of dark repair can be further photoreactivated.

Analysis of dose-response patterns in mutation research

Mutation induction data in unicellular systems can be described mathematically within the framework of single-event Poisson statistics. This formal description can be linked to various mechanistic models for mutation and killing. Such mathematical links between formalism and mechanism enable one to make use of the quantitative details of dose-response relations in drawing general inferences regarding the macromolecular processes involved in mutation and killing. Mutation yields, and in particular the position and magnitude of maximum yields, should be measured as carefully as possible as a means of verifying the apparent pattern of mutation induction kinetics suggested by double-logarithmic plots of mutation frequencies. For purely linear processes of mutation induction and exponential survival the maximum mutant yield is known to occur at the LD37 dose; however for non-linear kinetic patterns, the position and magnitude of the maximum yield shifts away from the LD37 in mathematically predictable ways. For any given pattern of killing and mutation, the ratio of the maximum mutant yields plotted over lethal hit units for two mutagens is a convenient measure of their relative mutagenic efficiencies.

Synergistic interaction between UV and ionizing radiation in wild-type Schizosaccharomyces pombe

A synergistic effect of combined UV and gamma-ray exposure was observed for inactivation of wild-type Schizosaccharomyces pombe. A recombinational repair process, known to be important in restitution of damage induced by both radiations, appears to be involved; a radiation-sensitive mutant defective in this repair pathway showed essentially no synergistic interaction between UV and gamma-rays. Recovery from the synergistic effect of pre-exposure in wild-type cells did not display the expected fast gamma-recovery and slow UV-recovery kinetics previously observed for regain of resistance to further exposure to the same radiation. Rather, UV-irradiated cells recovered quickly from synergistic inactivation on subsequent gamma-exposure, while gamma-irradiated cells recovered UV-resistance slowly. Recovery from synergism thus appears to reflect the nature of the second, and not the initial, radiation.

The sporulation system of Bacillus subtilis as the basis of a multi-gene mutagen screening test

The sporulation system of B. subtilis provides the basis of a simple and unique test for the detection of forward mutations in any of several hundreds genes in 28--45 separate operons scattered throughout the chromosome. Non-sporulating or oligosporogenous mutant colonies are easily identified by their lack of a brown pigment normally present in spore-forming colonies. N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG), ethyl methanesulfonate, acridine orange, acriflavin, nitrous acid, and UV irradiation are already known to produce sporulation mutants. This paper reports the dose dependence of sporulation mutant induction by 2-nitrosofluorene, ICR-191, nitrogen mustard, ethidium bromide and MNNG; mutagenesis is also demonstrated for aflatoxin B1 and 4-nitroquinoline-N-oxide. A mammalian liver enzyme metabolizing system was necessary for activation of aflatoxin B1. Auramine-O and 4-nitro-o-phenylenediamine failed to give a significant mutagenic effect under the conditions employed. The wide variety of mutagen classes detected indicates the general applicability of the test. This test, based on many genes throughout the chromosome, may prove less apt to exclude rare mutagenic "hot-spots" than systems based on the detection of mutations in a restricted region of the chromosome.

Ultraviolet-inactivation of conidia from heterokaryons of Neurospora crassa containing uv-sensitive mutations

The effect of three UV-sensitive mutations of Neurospora crassa, upr-I, uvs-4 and uvs-6, on the ultraviolet-inactivation of conidia from two-component heterokaryons was investigated. In two-component heterokaryons with wild-type sensitivity to radiation inactivation, all three conidial fractions exhibited similar ultraviolet-inactivation curves. Each UV-sensitive mutation studied uniquely modified the ultraviolet-inactivation curves of conidia from two-component heterokaryons. In heterokaryons heterokaryotic for upr-I, the upr-I mutation was recessive and the repair function determined by the wild type allele was functional to some degree in homokaryotic upr-I conidia. All three conidial fractions of heterokaryons containing upr-I in both components showed increased sensitivity to ultraviolet light. The uvs-4 mutation was recessive and resulted in conidia with increased UV-sensitivity only when included in both components of a heterokaryon. Homokaryotic uvs-4 conidia, which arose from heterokaryons containing both uvs-4 and wild-type components, exhibited wild-type survival. Therefore, as with upr-I, there was a carryover the repair capability to conidia which were genetically UV-sensitive. The uvs-6 mutation, when included in one component of a two-component heterokaryon, resulted in increased UV-sensitivity of both heterokaryotic and homokaryotic uvs-6 conidia. When both components contained uvs-6, the UV-sensitivity of all three conidial fractions was increased and all showed similar inactivation curves. Thus, as with upr-I and uvs-4, there was a carryover of the wild-type repair capability to genetically uvs-6 conidia. Heterokaryon tests for complementation between two non-allelic UV-sensitive mutations showed that in heterokaryotic conidia, complete complementation occurred between upr-I and uvs-4.

Changes in the ultraviolet sensitivity of Escherichia coli during growth in batch cultures

The ultraviolet (UV) sensitivity of Escherichia coli B/r harvested at various times during growth in batch cultures was measured. The results showed a period of increased UV sensitivity in late log phase, just before the cultures entered stationary phase. This increase in sensitivity was associated with a decreased shoulder in the UV survival curves. The postirradiation division delay of survivors was shortest for cells harvested during the period of maximal sensitivity. This period of increased UV sensitivity during late log phase was not found in the radiation-sensitive, repair-deficient mutant B(s-1) (a strain which is unable to excise pyrimidine dimers from UV-damaged deoxyribonucleic acid). These results suggest that the variation in UV sensitivity of E. coli B/r as a function of time of harvesting of the cells from batch cultures is related to the varying capacities of these populations to repair UV-damaged deoxyribonucleic acid. Further experiments designed to elucidate the mechanism underlying this variation in UV sensitivity indicated that it arises from the partial depletion of nutrients in the medium during late log phase. We suggest that growth in such depleted media leads to a depression in the intercellular concentration or activity of one or more of the repair enzymes concerned with the repair of damaged deoxyribonucleic acid.