Survival, DNA-breakdown and induction of prophage lambda in a Escherichia coli K 12 recA uvrB double mutant

Molecular cloning and characterization of the recA gene of Pseudomonas aeruginosa PAO

The recA gene of Pseudomonas aeruginosa PAO has been isolated and introduced into Escherichia coli K-12. Resistance to killing by UV irradiation was restored in several RecA-E. coli K-12 hosts by the P. aeruginosa gene, as was resistance to methyl methanesulfonate. Recombination proficiency was also restored, as measured by HfrH-mediated conjugation and by the ability to propagate Fec-phage lambda derivatives. The cloned P. aeruginosa recA gene restored both spontaneous and mitomycin C-stimulated induction of lambda prophage in lysogens of a recA strain of E. coli K-12.

Methodologies for the determination of various genetic effects in permeable strains of E. coli K-12 differing in DNA repair capacity. Quantification of DNA adduct formation, experiments with organ homogenates and hepatocytes, and animal-mediated assays

Derivatives of E. coli K-12 strain 343/113 differing in DNA repair capacity, in permeability to large molecules, and in some metabolizing activities (nitroreductase, glutathione), were constructed for the quantitative determination of the induction of various genetic effects, such as forward and back mutations, lysogenic induction of prophage lambda, and repairable DNA damage. These E. coli strains can be used in assay procedures which allow variation and control over several experimental conditions, such as oxygen tension, time, pH, temperature of incubation and growth phase of the indicator cells. Methods are described for the simultaneous determination of genetic effects and of DNA-adduct formation during mutagen treatment, i.e. by using radio-labeled compounds or by means of an enzyme-linked immunosorbent assay (ELISA). Mammalian biotransformation of xenobiotics can be investigated by including various fractions of mammalian organs in the system. Examples of the relative effectiveness of the activating potential of S9, S100 and isolated hepatocytes for dialkylnitrosamines and other carcinogens are presented. Host-mediated assays, finally, are described which, in addition to gene mutations, can also be used for the determination of repairable DNA damage in bacteria present in different organs, including the liver, spleen, lungs, kidneys, pancreas, and the blood stream of chemically treated mice. It is concluded that quantitative tests in vitro for assessment of induced mutagenic spectrum and genotoxic potency, combined with the host-mediated assay as a monitor, in vivo, of genotoxic factors present in various organs of animals, may become useful in the assessment of genotoxic (and possibly tumor-initiating) properties of chemicals for which long-term in-vivo mutagenicity and/or carcinogenicity data are not yet available.

Pathways for repair of DNA damaged by alkylating agent in Escherichia coli

A strain with both the polA12 and the alk-1 mutation is only slightly more sensitive to methyl methane sulfonate (MMS) than isogenic strains with only one of the mutations. On the other hand, alk-1 recA1 double mutant is much more sensitive to MMS than are strains carrying either one of alk or recA mutation. It was suggested that the alk and the polA gene products are involved in the same DNA repair process whereas the recA function is independent from the process. The yield of MMS-induced mutation (Arg- (argE) to Arg+ reversion) in alk mutant is considerably higher than that in wild type strain. Thus, the repair process in which the alk gene product is involved is relatively accurate. When MMS-treated lambda phages were plated on MMS-treated bacteria, there were considerable increases in survival of treated phage even in recA alk double mutant. It seems that a new repair pathway, which is specific for alkylating agent-induced damages and is not dependent on the RecA function, may be induced on exposure of bacteria to the alkylating agent.

Metabolic characterization of the viable, residually dividing and nondividing cell classes of recombination-deficient strains of Escherichia coli

The abilities of rec+, recB- recC-, recA-, and recA- recB- rec C- strains to support growth of bacteriophage T4, to take up oxygen, and to maintain cell integrity have been measured. (i) With respect to bacteriophage T4 growth, T4 phage is produced with identical lysis time in single -step growth curves with all strains tested. rec- strains show reduced phage production (fewer infected centers), but the average burst size per infected center is the same for all strains tested. Some rec- cells are unable to produce any phage, whereas the remainder of the rec-cells produce phage as rapidly and as efficiently as rec+ cells. (ii) With respect to oxygen consumption, rec- strains are deficient relative to the rec+ control to the same extent as the deficiency in phage production by theculture. The reduction in oxygen consumption is coordinate with reduction in rate of mass increase of the rec- culture. (iii) With respect to cell integrity, rec- cultures show increased lysis as measured by release of beta-galactosidase into the medium. The kinetics of release suggest that rec- nondividing cells all eventually lyse. These results are most consistent with the idea that rec- residually dividing cells and viable cells are metabolically normal according to the parameters we have measured, whereas nondividing cells are metabolically inactive.

DNA degradation in minicells of Escherichia coli K-12. II. Effect of recA1 and recB21 mutations on DNA degradation in minicells and detection of exonuclease V activity

The properties of minicell producing mutants of Escherichia coli deficient in gentic recombination were examined. Experiments were designed to test recombinant formation in conjugal crosses, survival following UV-irradiation in cells, and the state of DNA metabolism in minicells. The REC- phenotypes are unaffected by min+/- genotypes in whole cells. In contrast to minicells produced by rec+ parental cells, minicells from a recB21 strain have limited capacity to degrade linear, Hfr transfereed DNA. The lack of a functional recA gene product, presumably involved in inhibiting the recBC nuclease action(s), permits unrestricted Hfr DNA breakdown in minicells produced by a recA1 strain. This results in an increase in TCA soluble products and in the formation of small DNA molecules that sediment near the top of an alkaline sucrose gradient. Unlike the linear DNA, circular duplex DNA from plasmids R 64-11 or lambdadv, segregated into the minicells, is resistant to breakdown. By using in vitro criteria, and [32P]-labelled linear DNA from bacteriophage T7 for substrate, we found that the ATP-dependent exonuclease of the recBC complex (exo V) is present in rec+ and recA- minicells, and is lacking in the recB21 mutant. In fact, the absence of a functional exo V in recBC- minicells results in isolation of larger than average Hfr DNA from minicells. We suggest that recombination (REC) enzymes segregate into the polar minicells at the time of minicell biogenesis. This system should be useful for studies on DNA metabolism and functions of the recBC and recA gene products.

Analysis of the growth of recombination-deficient strains of Escherichia coli K-12

Cultures of recombination-deficient strains of Escherichia coli are composed of three classes of cells: (i) viable cells, which can undergo 20 or more generations, (ii) residually dividing cells, which can undergo fewer than 20 generations (probably an average of fewer than 6), and (iii) nondividing cells, which are incapable of a single division. The nonviable but residually dividing cells contribute to the mass increase of the culture, but not to the viability, thus accounting for the apparent dissimilarity in the effects of rec mutations on growth rates and viabilities. We have determined the frequencies of cells in each of the three classes, and, by making a simplifying assumption concerning the relative division times of viable and residually dividing cells, we have been able to describe mathematically the growth of the rec(-) cultures.

Excision repair properties of isogenic rec mutants of Escherichia coli K-12

We have examined the excision repair properties of isogenic rec(-) and uvr(-) strains of Escherichia coli K-12. A recB(-)recC(-) strain excises dimers at a rate nearly that of the rec(+) parent, reaching the same extent of excision after a 1-hr postirradiation incubation. recA(-) and recA(-)recB(-) strains excise 75 to 80% of the dimers excised by their rec(+) parent, whereas a uvrB(-) strain excises no dimers during a 1-hr incubation. The doses of ultraviolet light (254 nm) required to reduce survival to 37% of the original population are 8 ergs/mm(2) for recA or recA recB mutants, 5 ergs/mm(2) for the uvrB(-) strain, 30 ergs/mm(2) for the recB recC mutant, and 230 ergs/mm(2) for the wild-type parent. From these data one cannot account for the ultraviolet light sensitivity of rec(-) strains on the basis of their excision repair properties. We conclude that rec gene products play no significant role in the early steps of excision repair. The assay we have used for excision of thymine dimers is a modification of the Carrier-Setlow technique, and is described in detail in the Appendix to this paper. To show the properties and validity of this method, results of experiments with thymine dimers formed in vitro and in vivo in E. coli K-12 are presented. These results show our method to be reproducible and sensitive to 0.005% of the total radioactive thymine present in thymine-containing dimers.

Isolation and genetic analysis of a strain of Escherichia coli K-12 with an amber recA mutation

The isolation, properties, and genetic analysis of a strain of Escherichia coli K-12 with an amber recA mutation are described. The experiments demonstrate that the recA product is a protein that is probably not essential for growth.

Involvement of recombination genes in growth and viability of Escherichia coli K-12

We have studied the growth properties of 17 isogenic strains of Escherichia coli K-12 differing only in the recA, recB, recC, and sbcA alleles. We have observed the following. (i) All recombination deficient strains have decreased growth rates and decreased viabilities compared with recombination proficient strains. The large populations of nonviable cells in Rec(-) cultures may arise by spontaneous lethal sectoring (9). (ii) A recA mutant strain which is entirely recombination deficient and which shows high ultraviolet sensitivity and "reckless" deoxyribonucleic acid (DNA) breakdown has approximately the same growth rate and twice the viability as recB and recC mutant strains which have residual recombination proficiency, moderate ultraviolet sensitivity, and "cautious" DNA breakdown. (iii) Indirectly suppressed (sbcA(-)) recombination proficient (Rec(+)) revertants of recB and recC mutant strains have approximately normal growth rates and are three times as viable as their Rec(-) ancestors (but not as viable as rec(+) cells). We suggest the following hypothesis to account for the low viability of Rec(-)E. coli. Single-strand breaks in the DNA duplex, necessary for normal bacterial growth, may be repaired in a Rec(+) cell. Failure of Rec(-) cells to repair this normal DNA damage may lead to the observed loss of viability.