Adaptive doses of MNNG efficiently induce a recA-trp gene fusion

High-resolution separation and accurate size determination in pulsed-field gel electrophoresis of DNA. 4. Influence of DNA topology

Pulsed-field gel electrophoresis is a powerful technique for the fractionation of linear DNA molecules with sizes above 50 kilobase pairs (kb). Here it is demonstrated that this technique is also effective for separating smaller DNAs including linear, circular, and supercoiled species. The mobilities of linear DNAs larger than 8 kb can be modulated by pulse times between 0.1 and 100 s. The mobility of supercoiled DNA molecules up to 16 kb is generally unaffected by these pulse times except that 10-s pulse times cause a small but distinct increase in the mobility. The general insensitivity of small supercoiled DNAs to pulse time presumably occurs because these species reorient so rapidly that they spend most of their time undergoing conventional electrophoresis. However, the mobilities of larger supercoiled DNAs are affected by pulse times of less than 1 s, and at 0.1 s the molecules are better resolved by pulsed electrophoresis than by ordinary electrophoresis. The mobility of 3-19 kb nicked and relaxed circular DNA molecules is also affected by pulse time but in a complex way.

N-methyl-N'-nitro-N-nitrosoguanidine-induced resistance to ionizing radiation

N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) pretreatments increase the resistance of Escherichia coli to gamma-radiation. The increased resistance is dependent on functional polA, recA, recB, recC, and lexA genes and is partly dependent on recN. The MNNG-induced resistance is additive to resistance induced by pretreatment with gamma-radiation but not by increases induced by hydrogen peroxide. The MNNG-induced resistance occurs in adaptive response mutants and at pretreatment levels of MNNG that do not activate cells to reactivate UV-inactivated lambda phage. The MNNG-induced resistance appears to be distinct from other inductions to gamma-radiation resistance.

Response of S. cerevisiae to N-methyl-N'-nitro-N-nitrosoguanidine: mutagenesis, survival and DDR gene expression

We have examined the effects of low concentrations of the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on survival and mutagenesis of logarithmically growing Saccharomyces cerevisiae. Pretreatment of cells with nontoxic and submutagenic concentrations of MNNG for several generations does not reduce the cytotoxic and mutagenic effects of exposure to a high concentration of drug. This lack of 'adaptation' in S. cerevisiae was further investigated biochemically and cell extracts prepared from pretreated cells were shown to be deficient in the ability to remove O6-methyl guanine from alkylated DNA. Moreover, we could not detect transfer of the methyl group from the DNA to a protein acceptor in yeast cell extracts suggesting that the level of O6-methyl guanine transferase is below 200 molecules/cell following pretreatment. Exposure of S. cerevisiae to mutagenic concentrations of MNNG stimulates transcription of at least three DNA damage responsive genes that also respond to UV-irradiation and 4-nitroquinoline-1-oxide treatment. These results support the contention that in Saccharomyces alkylation damage is processed by repair pathways that operate on a variety of lesions, and that one or more of these pathways is inducible.