Inactivation of transfecting DNA by physical and chemical agents: influence of genotypes of phage lambda and host cells

Functional characteristics of the recA gene from Serratia marcescens strain Sb

The cloned recA gene from Serratia marcescens Sb was expressed and complemented defects in the UV repair, recombination, and SOS induction of an Escherichia coli host deleted for recA. Moreover, the Serratia gene, recA (Sm), supported the same frequency of recombination per unit length of DNA as did the homologous Escherichia coli gene, recA(Ec).

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

The role of the HCR system in the repair of lethal lesions of Bacillus subtilis phages and their transfecting DNA damaged by radiation and alkylating agents

The role of the HCR system in the repair of prelethal lesions induced by UV-light, gamma-rays and alkylating agents was studied in the Bacillus subtilis SPP1 phage, its thermosensitive mutants (N3, N73 and ts1) and corresponding infectious DNA. The survival of phages and their transfecting DNA after treatment with UV light is substantially higher in hcr+ cells than in hcr cells, the differences being more striking in intact phages than in their transfecting DNA's. Repair inhibitors reduce the survival in hcr+ cells: caffeine lowers the survival of UV-irradiated phage SPP1 in exponentially growing hcr+ cells but has no effect on its survival in competent hcr+ cells; acriflavin and ethidium bromide decrease the survival of UV-irradiated SPP1 phage in both exponentially growing and competent hcr+ cells to the level of survival observed in hcr cells; moreover, ethidium bromide lowers the number of infective centres in hcr+ cells of UV-irradiated DNA of the SPP1 phage. Repair inhibitors do not lower the survival of UV-irradiated phages or their DNA in hcr cells. The repair mechanism under study repairs also lesions induced by polyfunctional alkylating agents in transfecting DNA's of B. subtilis phages but is not functional with lesions induced by these agents in free phages and lesions caused in phages and their DNA by ethyl methanesulphonate or gamma-rays.

On inactivation of bacteriophage lambda by hydroxylamine

Hydroxylamine is a mutagen which is much more active on single-stranded DNA than on double-stranded DNA. It is shown here that the cohesive ends of lambda DNA, with 10 cytidine residues, constitute a hydroxylamine target roughly equal in magnitude to the entire duplex part of the molecule, which contains ca. 25 000 cytidine residues.

W-reactivation and W-mutagenesis of gamma-irradiated phage lambda

UV irradiation of Escherichia coli wild-type cells manifested the phenomena of W-reactivation (WR) and W-mutagenesis (WM) of phage lambda irradiated by 60Co gamma-rays in broth. WR of gamma-irradiated phage was half as efficient as that of UV-irradiated phage, although the frequency of c mutations in conditions of WR was about the same in both phages. The xthA and recBrecC sbcB mutants were practically identical with wild-type cells in respect of WR and WM of UV- and gamma-irradiated phage. As in UV-irradiated phage, WR and WM of gamma-irradiated phage were absolutely dependent on the recA+ and lexA+ genes of the host cell. WR and WM required much smaller doses of UV radiation for induction in polA1 and uvrB mutants. The lig-ts mutant, temperature sensitive in polynucleotide ligase, was deficient in WR and WM of UV- and gamma-irradiated phage at the semi-permissive temperature of 37 degrees. The uvrE502 mutant and the allelic recL152 strain were absolutely deficient in WR and WM of gamma-irradiated phage. In UV-irradiated phage WR was reduced, but not eliminated, in the uvrE mutant, and WM was entirely suppressed. This is another example of uncoupling of WR and WM which shows that several repair systems are active in WR but only some of them are mutagenic.

Physiological modification of alkylating agent induced mutagenesis. II. Influence of the numbers of chromosome replicating forks and gene copies on the frequency of mutations induced in Escherichia coli

The frequency of reversions induced in Escherichia coli K-12 trpA58 by any of five different monofunctional alkylating agents increased as the growth rate of the organism was raised prior to mutagen treatment. The increase in mutation frequency did not correlate with growth rate-dependent changes in cell area or total cellular protein and DNA. After treatment of cells with N-methyl-N-nitrosourea (MNUA), no growth rate-dependent change was observed in the total DNA alkylation or percentage of O6-methylguanine present in the DNA extracted. The frequency of reversions induced by one mutagen, methyl methanesulphonate (MMS), increased in proportion to the average number of trpA gene copies per cell, whereas the frequency of reversions induced by the other compounds was dependent on the average number of chromosome replicating forks per cell. This difference was attributed to the different ratios of DNA base alkylation products observed, formed after treatment with MMS, an SN2-type reagent, or after treatment with the SN1-type reagents ethyl methanesulphonate (EMS), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), MNUA and N-ethyl-N-nitrosourea (ENUA). Possible reasons for the dependence of mutation frequency on the number of replicating forks per cell are discussed.

On the lack of host-cell reactivation of UV-irradiated phage T5 II. Further characterization of the repair inhibition exerted by T5 infection

Experiments reported in the preceding paper [4] had shown that host-cell reactivation (HCR) of UV-irradiated phage T1 in excision-repair proficient Escherichia coli cells is inhibited by superinfection with phage T5. Theoretical considerations have led to predictions concerning the dependence of repair inhibition on the multiplicity of superinfecting T5 phage and on the UV fluence to which they were exposed. These predicitions have been supported by experimental results described in this paper. The fluence dependence permitted calculation of the relative UV sensitivity of the gene function responsible for repair inhibition; it was found to be about 2.3% that of the plaque-forming ability of phage T5. The T5-inhibitable step in excision repair occurs early in the infective cycle of T1. Furthermore, experiments involving the presence of 400 mug/ml chloramphenicol showed that HCR inhibition of T1 is caused by a protein produced after the FST segment of T5 (i.e. the first 8% of the T5 genome) has entered the host cell. A previously described minor T1 recovery process, occurring in both excision-repair-proficient and -deficient host cells, is inhibited by T5 infection due to a different substance, which is most likely associated with the "second-step-transfer" region of T5 DNA (involving the remainder of the genome). Superinfection with T4v1 phage resulted in HCR inhibition of T1, resembling that observed after T5 superinfection. The discussion of these results suggests that inhibition of the bacterial excision repair system by T5 or T4 infection occurs at the level of UV-endonucleolytic incision, and that lack of HCR both in T-even phages and in T5 can be explained in the same manner.