Two types of x-ray-sensitive mutants of Escherichia coli B: their phenotypic characters compared with UV-sensitive mutants

A differential DNA-repair test using mixtures of E. coli K12 strains in liquid suspension and animal-mediated assays

The feasibility of performing tests for repairable DNA damage in animal assay procedures was investigated by using repair-proficient and repair-deficient derivatives of E. coli K12 strain 343/113, including mutations in the uvrB, recA, polA and dam genes. To avoid variations in the relative recovery of viable cells from different samples, the strains were further marked with auxotrophic growth requirements, so that mixtures could be treated and the survival of each strain determined individually on media containing the corresponding growth factors. Spot tests were performed with the various strains to re-assess the necessity of using a combination of repair deficiencies, when genotoxic agents of differing mode of action are to be detected. Liquid suspension tests on mixtures of the different strains, furthermore, confirmed that the survival of the individual strains can be determined separately on selective media after treatment with methyl methanesulfonate (MMS) and methyl nitrosourea (MNU). These tests were also used to demonstrate that dimethyl nitrosamine (DMNA) is activated by Aroclor-1254-induced rat-liver S9 fractions to genotoxic products, as measured by the low survival of a recA derivative compared with the repair-proficient wild-type strain. Intrasanguineous host-mediated assays using the present derivatives of E. coli K12/343/113 showed that the various strains, injected simultaneously into mice, could be recovered in amounts sufficient for the individual determination of the relative survival in liver, spleen, lungs, kidneys, pancreas and the blood stream of the host animals. Using a mixture of the repair-proficient parent and the recA derivative inoculated into mice that were subsequently treated with MMS, NMU or DMNA, we found that these chemicals induce a larger decrease in survival in the recA strain as compared with the wild-type in cells recovered from the liver and the spleen. The order of genotoxic potency so determined was DMNA greater than MNU greater than MMS; this is similar to the ranking of the carcinogenicity of these compounds in rodents and probably also reflects the various degrees of DNA alkylation in cells of the livers of the treated animals. The general usefulness of the host-mediated differential DNA repair assay for detecting genotoxic factors in various organs of animals remains to be assessed by using chemical mutagens of different modes of action.

Effect of caffeine on DNA polymerase I from Escherichia coli: studies in vitro and in vivo

The influence of caffeine on the activity of DNA polymerase I from E. coli was investigated. Caffeine had no effect on the polymerizing activity but did inhibit both 5' leads to 3' and 3' leads to 5' nuclease activities. The highest inhibition was observed with d(A--T)n as substrate: at a concentration of caffeine of 10 mM, inhibition was about 50%. In studies in vivo with 3 isogenic strains of E. coli, carrying different mutations in the DNA polymerase I gene, the effect of caffeine on survival after ultraviolet irradiation was most marked for the wild-type, pol+, followed by those mutants defective in 3' leads to 5', polA1, and 5' leads to 3' nuclease activities, polA107. Caffeine had little influence on survival of the resA1 mutant which lacks both 5' leads to 3' and 3' leads to 5' nuclease activities. These results support the idea that the influence of caffeine on dark repair may be explained in part by its effect on the nuclease activities of DNA polymerase I.

Lethla effect of nitrous acid on Escherichia coli

The effect of nitrous acid (NA) on viability, integrity of cellular DNA and on membrane transport were studied in 5 strains of Escherichia coli. Stationary phase cells, grown on mineral salts medium, were exposed to NA. The viability of strains decreased in thefollowing order: W3110 wild-type greater than WP2 wild-type, WP2 uvrA greater than NG30 recA greater than P3478 polA. Alterations were found in the DNA sedimentation profile in alkaline sucrose gradient. Disturbance of DNA synthesis was measured by 3H-labelled thymidine ([3H]Thd) incorporation. No degradation of DNA was found after NA treatment. Low doses of NA caused significant inhibition of leucine and glucose transport into whole cells. The results are interpreted in terms of the multi-target action of NA causing the death of cells.

Endonucleolytic incision of x-irradiated deoxyribonucleic acid by extracts of Escherichia coli

An enconuclease activity that reacts with x-irradiated DNA is present in extracts of E. coli. By using centrifugal methods to monitor the conversion of the supercoiled, circular double-stranded DNA for phage phi-x-174 (replicative form) or PM2 to the relaxed circular form it was possible to quantitate the rate of radiation induced endonuclease-sensitive sites in the DNA. For every single-strand break induced by x-rays under aerobic irradiation conditions, there is approximately one induced site sensitive to this endonuclease activity. Under irradiation conditions (addition OF Potassium iodide) that dramatically reduce rates of single-strand breaks and "alkalilabile" lesions, the number of endonuclease-sensitive sites relative to single-strand breaks increase approximatley 4-fold. This nuclease is present in several strains of E. coli B and K12, including mutants deficient in DNA polymerase I, recombination gene products (rec mutants), ultraviolet light incision enzyme (uvr A mutant), and endonuclease II. It is suggested that this endonuclease may be involved in an excision repair process for damages incurred in DNA by ionizing radiation.

Action of exonuclease V (the recBC enzyme) on ultraviolet-irradiated DNA

Exonuclease V (the recBC enzyme) of Escherichia coli can release pyrimidine dimers from ultraviolet-irradiated linear duplex DNA though it acts more slowly on irradiated DNA than on non-irradiated DAN. However, close circular lambda-dv DNA or phi X174 replicative form I DNA is not attacked by exonuclease V even though the DNA has been irradiated and treated with T4 endonuclease V to produce single-stranded breaks at the 5'-side of pyrimidine dimers. When irradiated circular DNA, previously nicked by T4 endonuclease V, is briefly exposed to elevated temperature, the DAN becomes susceptible to the action of exonuclease V, and pyrimidine dimers are selectively released. The increased susceptibility to exonuclease V may be resulted from locarized denaturation, or "fraying" of the 5'-termini at the nicks. The preferential release of pyrimidine dimers was observed when irradiated DNA, treated with T4 endonuclease V, was incubated with crude extracts of Escherichia coli. The activity was found in various strains defective in exonuclease V and/or DNA polymerase I.

The role of DNA polymerase I in excision-repair

The ability of three different DNA polymerase I mutants of Escherichia coli to carry out excision-repair was examined. Strains having the same genetic origin but carrying either the polAl, polA107, resAl, or pol+ alleles were compared. The rate of ultraviolet-induced dimer excision was slightly reduced, relative to that found in Pol+ strains, in the PolAl strains; greatly reduced in the PolA107 strains; and found not to occur in the resAl strain. Ultraviolet-light-induced repair synthesis as determined by the ultraviolet-stimulated incorporation of 3H-labeled 5-bromo-2'-deoxyuridine into DNA of the parental density showed that the polAl mutation resulted in an increase in repair replication, while the presence of the polA107 allele caused a reduction in the amount of repair synthesis relative to that of the Pol+ strain. The ResAl strain, however, showed no ultraviolet stimulation of the incorporation of the density label. These observations indicate that DNA polymerase I plays a key role in the excision-repair process in E. coli.

Comparative analysis of deletion and base-change mutabilities of Escherichia coli B strains differing in DNA repair capacity (wild-type, uvrA-, polA-, recA-) by various mutagens

Dose-response curves were compared for deletions [ColBR (resistant to colicin B) mutations being more than 80% deletions] and base changes (reversion of argFam to prototrophy argplus) induced in the same set of E. coli strains (wild-type for DNA repair, uvrA-, polA- and recA-) by N-methyl-N'-nitro-N-nitrosoguanidine (NTG), ethyl methanesulfonate (EMS), hydroxylamine (HA), 4-nitroquinoline I-oxide (4NQO), mitomycin C (MTC, UV and X-rays. All these agents induced deletions as well as base changes in the wild-type strain. Thus chemical mutagenesis differed in E. coli and bacteriophages in vitro, for HA, NTG, EMS and perhaps UV produced only point mutations in phage Tr. The patterns of deletion and base-change mutability in E. coli were surprisingly similar. (I) The recombination less recA- strain was mutable by only three (NTG, EMS, HA) of the seven mutagens for either deletions or base changes. (2) The uvrA- strain, unable to excise pyrimidine dimers, was very highly mutable by 4NQO and UV but immutable by MTC for both deletions and base changes. (3) The polA- strain, defective in DNA polymerase I due to a non-suppressible mutation, was very highly mutable by HA and highly mutable by MTC and 4NQO for both deletions and base changes but was highly mutable only for deletions by UV and X-rays, remaining normally mutable by the other agents for both deletions and base changes despite its high sensitivity to their inactivating action. We conclude that errors in the recA-dependent repair of induced DNA damage (after 4NQO, MTC, UV and X-rays) or errors in replication enhanced by damage to the replication system or to the template strands (after NTG, EMS, and HA) give rise to deletions as well as to base changes. From a comparative analysis of 14 dose-response curves for deletions and base changes, we conclude that the order of mutagenic efficiency relative to killing is (EMS, NTG) greater than (UV, 4NQO) greater than HA greater than (X-rays, MTC), and that X-rays, 4NQO, HA and MTC induce more ColBR deletions than Argplus base changes, whereas UV and EMS induce ColBR deletions and Argplus base changes at nearly equal rates and the specificity of NTG is intermediate between these two types.

Host- and phage-mediated repair of radiation damage in bacteriophage T4

T4+ exhibits increased ultraviolet sensitivity on derivatives of Escherichia coli K12 or B lacking deoxyribonucleic acid (DNA) polymerase I. However, the sensitivity of T4v is not affected by the absence of host DNA polymerase. T4x and T4y also show increased sensitivity on DNA polymerase-deficient strains, but to a lesser extent than observed with wild-type T4. When T4x or T4y, but not T4+, are plated on a double mutant lacking both DNA polymerase and the uvrA gene product, a partial suppression of the polymerase effect is observed. Host ligase appears to be able to suppress to some extent the T4y phenotype but has no effect on wild-type T4 or other T4 mutants. T4xv incubated in E. coli B or B(s-1) in the presence of chloramphenicol (50 mug/ml) shows increased resistance over directly plated irradiated phage. Increased survival under the same conditions was not observed with T4+ or other T4 mutants. The repair of X-ray-damaged T4 was investigated by examining survival curves of T4+, T4x, T4y, T4ts43, and T4ts30. The repair processes were further defined by observing the effects of plating irradiated phage on various hosts including strains lacking DNA polymerase I or polynucleotide ligase. Two classes of effects were observed. Firstly, the x and y gene products seem to be involved in a repair system utilizing host ligase. Secondly, in the absence of host DNA polymerase, phage sensitivity is increased in an unknown manner which is enhanced by the presence of host uvrA gene product.

Excision repair characteristics of recB - res - and uvrC - strains of Escherichia coli

An Escherichia coli strain carrying the recB21 and res-1 mutations showed an abnormally low level of colony-forming ability although it grew essentially normally in liquid medium. The recB21 res-1 strain showed little, if any, of the ultraviolet (UV)-induced deoxyribonucleic acid (DNA) breakdown characteristic of the res-1 mutant. Nevertheless, the double mutant was far more sensitive to UV than either the res-1 or the recB21 strain. When compared with a wild-type strain, the rate of release of dimers from UV-irradiated DNA was very slow in the recB21 res-1, but normal in the res-1 recB(+) or recB21 res(+) mutants. However, the ratio of dimer-to-thymine released into the acid-soluble fraction was three times higher than the wild type in recB21 res(+) and recB21 res-1 and only one-tenth as high as the wild type in res-1 rec(+). Alkaline sucrose gradient centrifugation revealed occurrence of single-strand incision of UV-irradiated DNA and the restitution of nicked DNA at a similar rate in the recB21 res-1 and recB21 res(+) strains. Mutants uvrC(-) showed increased amounts of nicks in their DNA with increasing incubation time after UV irradiation, although no detectable amounts of dimers were excised from UV-irradiated DNA. From these results, it is concluded that the increased sensitivity of the res-1 strain to UV light is due to a reduced ability to excise dimers from UV-irradiated DNA and that the high rate of UV-induced breakdown of DNA is not the primary cause. A possible role of uvrC gene in the excision repair is discussed.

Genetic and molecular characteristics of X-ray-sensitive mutants of Escherichia coli defective in repair synthesis

Alleles responsible for X-ray-sensitive characteristics of three mutants of Escherichia coli B, which were also sensitive to ultraviolet (UV) irradiation, were mapped near metE locus, and named res-1, res-2, and res-3. All the res(-) mutants showed no host cell reactivability (Hcr(-)) for transducing deoxyribonucleic acid (DNA) of P1 phage irradiated by UV but they were Hcr(+) for infective DNA of P1 phage. Furthermore, they showed no detectable activity of DNA polymerase. Characteristics of allele res-1 were studied in detail. The mutant res-1 uvr(+) showed an extensive degradation of DNA after UV irradiation. Double mutants carrying res-1 uvrA(-), res-1 uvrB(-), and res-1 uvrC(-) showed no marked increase in UV sensitivity beyond that of the uvr(-) single mutants and only negligible UV-induced DNA degradation. The uvr(-) mutations showed no such suppressive effect on DNA degradation induced by X rays in these double mutants. It is concluded that res(-) mutants are defective in the second step (repair synthesis) of the excision repair process and that DNA polymerase is partly responsible for the assumed resynthesis step.