Amplification of phleomycin induced death and DNA breakdown by caffeine in Escherichia coli

Optimized vectors and selection for transformation of Neurospora crassa and Aspergillus nidulans to bleomycin and phleomycin resistance

To provide a dominant selectable marker for transformation of Neurospora crassa strains lacking specific auxotrophic mutations, we have engineered the bleomycin (Bm) resistance-encoding gene (ble) from the bacterial transposon Tn5 for expression in N. crassa. The coding region of the ble gene was fused to the promoter and terminator regions of the N. crassa am gene. In some vectors, multiple cloning sites were placed flanking the ble gene to provide a versatile ble cassette. When introduced into N. crassa, the hybrid ble gene conferred resistance to greater than 15 micrograms Bm/ml. Under optimal conditions, the levels of Bm required (2.5 micrograms/ml) make even large-scale transformation experiments very economical. Aspergillus nidulans could also be efficiently transformed to Bm resistance using the N. crassa ble gene fusion. Since the ble gene functions in both N. crassa and A. nidulans, the gene should be useful as a transformation marker for the many other filamentous fungi which are sensitive to Bm.

Inhibition and enhancement of phleomycin-induced DNA breakdown by aromatic tricyclic compounds

Cationic aromatic tricyclic compounds including triphenylmethane dyes, phenazines, phenoxazines, acridines, phenothiazines, phenanthridinium compounds, anthracenes and xanthene dyes, which amplify cell killing in phleomycin-treated Escherichia coli B cells also modified phleomycin-induced breakdown of DNA to acid-soluble fragments. A plot of DNA breakdown as a function of concentration was bell-shaped for each of the active compounds, i.e. as the concentration increased, DNA breakdown was enhanced initially, but above a certain concentration, the proportion of DNA degraded declined, often to zero. One of the compounds, acriflavine, when tested also inhibited DNA breakdown following ultraviolet irradiation. A study, by sedimentation methods, of DNA single-strand breakage in phleomycin-treated E. coli cells, using 3 representative compounds, Crystal Violet, 3,6-diaminoacridine and Methylene Blue, revealed a consistent increase in DNA strand breaks as concentration of compound increased. In similar experiments with ethidium bromide the breakage yield/concentration curve exhibited a maximum. In general, however, it seems that the inhibition of DNA-breakdown observed at higher concentrations of these amplifying compounds is not explicable by an effect on the primary breakage event, but is due to suppression of exonucleolytic activity in the cells.

Genetic toxicology of bleomycin

Bleomycin (BLM), an antibiotic obtained from Streptomyces verticillus, is of significance as an antineoplastic agent. The compound is actually the mixture of some 200 related forms which differ from each other in the amine moiety. The drug, at low concentrations, can cause elimination of bases, particularly thymine. This causes strand breakage of DNA and inhibition of cell growth. The influence of BLM on cell growth may be unrelated to the effects on DNA. In general, mitotically dividing cells show more DNA damage than non-dividing cells. G2 seems to be the most sensitive phase indicating that cell death may not be related to a direct effect of BLM on DNA replication. The antibiotic shows specific effects on chromatin and causes chromosomal damage in all sub-phases of interphase. It can affect early prophase chromosomes also. Suggestion has been made that BLM-induced breakage and cell death are similar to those induced by densely ionizing radiations. Whereas the antibiotic affects the frequency of somatic crossing over and produces micronuclei, the data on mutation induction and production of sister-chromatid exchanges do not permit classifying BLM as a potent inducer of these phenomena. The genetic effects of BLM can be modified quantitatively by thiol compounds, caffeine, hyperthermia and H2O2. It is concluded that the available data do not permit assessment of genetic damage in the offsprings of BLM-treated patients. Such studies are urgently needed, as are the studies to find out the effects of BLM on meiotic phenomena.

Caffeine

Most of the population of the world is exposed to caffeine to a greater or lesser extent since it occurs in a number of plants used in the preparation of widely consumed drinks, and has in addition a limited therapeutic use. Chromosomal abnormalities are induced by caffeine in both plant cells and in mammalian cells in culture and it also has some anti-mitotic activity. DNA-repair processes sensitive to caffeine have been demonstrated in a number of cell systems and it has been shown to affect a wide range of other cellular processes. Caffeine has potent mutagenic effects in Escherichia coli and other micro-organisms both when acting alone and in combination with other mutagens. However its mutagenic activity in Drosophila has been disputed and the available evidence suggests that it is neither mutagenic in mammals nor synergistic with other mutagens although at very high doses it appears to have some teratogenic activity in mammals.

Caffeine enhancement of digestion of DNA by nuclease S1

The activity of Aspergillus orzae nuclease S1 on DNA has been investigated under varying pH and metal ion conditions. Nuclease S1 was found to preferentially digest denatured DNA. With native DNA as substrate the enzyme could only digest the DNA when caffeine was added to the reaction mixture. The enzyme was more active in sodium acetate buffer (pH 4.5), than in either standard saline citrate (PH 7.0) or sodium phosphate buffer (pH 6.8). Caffeine was also found to affect the thermal stability of DNA, resulting in a melting profile characterized by two transitions. The first transition (poorly defined) was below the normal melting temperature of the DNA, while the next transition was at the normal melting temperature of the DNA, while the next transition was at the normal melting temperature of the DNA. The susceptibility of caffeine-treated DNA to nuclease digestion seems to be a result of the local unwinding that caffeine causes in the regions of DNA that melt in the first transition. This selective destabilization presumably sensitizes the unwound regions to nuclease hydrolysis. The hydrolysates of the DNA digested by nuclease S1 were subjected first to ion exchange chromatography followed by paper chromatography. The results from this partial characterization of the digestion products showed that they contain mononucleotides as well as oligonucleotides of varying lengths. The base composition of the mononucleotide digests suggests that caffeine has greater preference for interacting with A-T base-pairs in DNA.

Effects of coumarin, thiopurines, and pyronin Y on amplification of phleomycin-induced death and deoxyribonucleic acid breakdown in Escherichia coli

Phleomycin (</=2 mug/ml) induces neither deoxyribonucleic acid (DNA) breakdown nor cell death in stationary-phase Escherichia coli B cells, but the addition of 8 mm caffeine immediately initiates these changes in the same way as increasing the phleomycin concentration 10-fold. This phenomenon is termed "amplification" (6). Pyronin Y, a number of nontoxic thio- and mercaptopurines (of which the most active were 6,7- and 6,9-dimethyl-2-methylthiopurine), and coumarin have been found to be considerably more efficient amplifiers of phleomycin activity than caffeine. Thus 2 mm 6,7- and 6,9-dimethyl-2-methylthiopurine, 0.16 mm pyronin, and 4 mm coumarin killed 10 to 100 times more phleomycin-treated bacteria within 2 hr than 8 mm caffeine. As with caffeine, amplification of cell death by these compounds was accompanied by degradation of DNA to acid-soluble fragments. A number of compounds including 2,6-dichloropurine, 6-hydroxy-2-methylthiopurine, alpha-naphthol, beta-naphthol, naphthionic acid, and alpha-naphthol-4,8-disulphonic acid inhibited the action of phleomycin, if they were present in the cell suspension during phleomycin treatment, but some caused amplification if added subsequent to the phleomycin. Although no mutants resistant to >/=10 mug of phleomycin per ml were observed among 10(11)E. coli B cells screened, such mutants occurred with a frequency of 10(-6) to 10(-7) among cultures resistant to 1 to 2 mug of phleomycin per ml. These double mutants were cross-resistant to phleomycin plus caffeine. The amplifying compounds, though structurally dissimilar, shared the common characteristic of binding selectively to denatured DNA as measured by equilibrium dialysis methods. The implications of these observations in supporting a model of phleomycin amplification proposed previously (6) and their utility in providing a logic for developing a new class of antibiotics are discussed.