Isolation and characterization of mutants of Escherichia coli deficient in induction of mutations by ultraviolet light.

Summary Mutants of E. coli defective in susceptibility to UV-induction of mutations were isolated by direct screening for their UV nonmutable phenotype (Umu^–). Screening of about 30,000 mutagenized clones of a uvrB ^– derivative of AB1157 yielded six Umu^– strains. The mutants can be classified into three groups by the location of the mutations, umuA, umuB and umuC. Mutations umuA and umuB are, respectively, mapped close to lexA and recA genes and mutations at both loci partially reduce UV mutagenesis. The locus of umuC is between hemA and purB and the mutations at this new locus result in a moderate increase of UV sensitivity. The mutation diminishes UV mutagenesis and UV reactivation of phage without affecting the inducibility of phophage nor the inhibition of cell division following UV irradiation. Related properties of an isogenic strain of a recF ^– mutant are compared with those of umuC ^–.     

1-Nitropyrene: a mutagenic product induced by the action of near ultraviolet light on 1-aminopyrene

A solution of 1-aminopyrene in dimethyl sulfoxide exposed to an artificial source of near ultraviolet light (600 kJ/m2) induced significant direct-acting mutagenicity in the Ames/Salmonella plating assay utilizing strain TA98. High-performance liquid chromatography of this solution resulted in a fraction that was mutagenic on TA98 but inactive on a nitroreductase-deficient strain of Salmonella (TA98NR). This observation suggested the presence of a nitro-containing compound. Mass spectral analysis confirmed that 1-nitropyrene was the active photoproduct in this fraction. These data implicate photochemical transformation of primary aromatic amines as an alternative mechanism by which nitroaromatic compounds can be formed in the environment.     

Influence of exogenous thymidine on killing and mutation of Chinese hamster V-79 cells by X-rays, ultraviolet light and N-methyl-N'-nitro-N-nitrosoguanidine

V-79 cells when exposed to thymidine (5 micrograms/ml) in growth medium after treatment with X-rays, UV light and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), responded differently depending upon the agent. For treatment with X-rays and UV light, only induction of mutation was potentiated, but for MNNG treatment, both killing and mutation induction were potentiated. The increase in killing of MNNG exposed cells could be reversed by simultaneous addition of deoxycytidine with thymidine, but, for all the three mutagenic treatments, enhancement in mutation induction could not be suppressed by deoxycytidine.     

Action spectrum for the in vitro induction of simian virus 40 by ultraviolet radiation

A line of simian virus 40-transformed hamster kidney cells was exposed to ultraviolet radiation at eleven different wavelengths in the region 238-302 nm. An action spectrum derived from the resulting exposure-response curves for the induction of simian virus 40 from these cells exhibits a broad peak in the region 260-270 nm suggesting DNA as the major chromophore for this response. This conclusion is consistent with results obtained by other investigators who have noted viral induction by a number of DNA-damaging agents.     

Differences in the mutagenic and lethal effect of ethyl methansulphonate in the isolation of auxotrophic mutants of the strain corynebacterium sp. 9366

Folia Microbiologica -     

Tryptophan auxotrophic mutants in Aspergillus niger: inactivation of the trpC gene by cotransformation mutagenesis

Summary Aspergillus niger tryptophan auxotrophic mutants have been isolated after UV irradiation of conidiospores. The mutants belong to two different complementation groups, trpA and trpB, which complement each other in heterokaryons. Neither of the mutations could be complemented with the cloned A. niger trpC gene. To obtain A. niger trpC mutants in a direct way, gene inactivation by cotransformation was performed. For this purpose an in-frame gene fusion between the A. niger trpC and Escherichia coli lacZ genes was constructed and shown to be functionally expressed after introduction into A. niger by cotransformation with the pyrA gene as selective marker. Among the -galactosidase expressing cotransformants, obtained with either circular or linearized vectors, no trpC mutants were detected, even after enrichment. Such mutants, however, could be obtained by cotransformation of A. niger with specific fragments of the fusion gene. Biochemical analysis of the cotransformants indicated that in nearly all cases the fusion gene had replaced the wild-type trpC gene. Genetic analysis showed that the trpC mutation is not linked to any of the A. niger loci described so far. The trpC mutants can be complemented by the cloned A. niger trpC gene as well as by the A. nidulans trpC gene.     

Photosensitized inhibition of nitrate reductase induction by 4,5',8-trimethylpsoralen and near ultraviolet light

4,5' ,8-trimethylpsoralen photosensitizes the inhibition of the induction of nitrate reductase in XD cells of tobacco, by near ultraviolet light. The photosensitization depends on the concentration of 4,5' ,8-trimethylpsoralen and the near ultraviolet light dose. Concomitant with the inhibition of nitrate reductase induction there is a severe inhibition of DNA and RNA synthesis. Oxygen uptake and protein synthesis are not affected. Translation of preexisting mRNA coding for nitrate reductase is insesitive to 4,5' ,8-trimethylpsoralen and near ultraviolet light. It is concluded that the DNA of these cells is the target for the photochemical reaction of 4,5' ,8-trimethylpsoralen. The template activity of the DNA is inhibited, and the expression of the genome thereby prevented. Translation of preexisting nRNA is not affected.     

Effects of DNA-repair processes on the induction of genetic duplications in bacteria by ultraviolet light

A straightforward positive selection for genetic duplication is possible in strains of Salmonella typhimurium that carry the aroC321 allele. Strains with a single copy of this allele require phenylalanine, tyrosine and tryptophan for growth. Such strains give rise to tryptophan prototrophs, which still require phenylalanine and tyrosine, through the formation of a duplication that includes about 30% of the chromosome. We have constructed strains that permit the simultaneous study of duplications and mutations and have used these strains to explore the effects of DNA repair processes on the induction of duplications by ultraviolet light (UV). UV causes dose-dependent increases in the frequency of duplications in bacteria. The exposure required to induce duplications is much less in a delta uvrB strain than in repair-proficient strains, suggesting that duplications result from DNA lesions that are subject to excision repair. The photoreversibility of UV-induced preduplication lesions implicates pyrimidine dimers in the induction of duplications. Unlike its effect on the induction of mutations, the error-prone repair process associated with plasmid pKM101 does not enhance the induction of duplications. The prevention of duplication-formation by a recA mutation suggests that the formation of duplications involves recombinational events. Taken together, the data indicate that the same DNA lesions can be mutagenic and recombinagenic in bacteria, but that the two effects involve different pathways of processing DNA damage.