PubMed Health⌕ Search

Biomedical subjects

B A Bridges

Publications and source records attributed to B A Bridges.

At least 91 records · Page 5Linked to original sources
PubMed1986

Mutagenic DNA repair in Escherichia coli. XII. Ultraviolet mutagenesis in excision-proficient umuC and lexA (ind-) bacteria as revealed by delayed photoreversal.

Streptomycin-resistant mutations are induced in excision-proficient umuC122::Tn5 bacteria given delayed photoreversal after u.v. light. The mutations occur after much earlier photoreversal than is found with the similar induction of tryptophan-independent mutations in excision-deficient umuC bacteria and the process is not markedly influenced by the presence of Casamino acids. The yield of mutants is between 25 and 40% of that found immediately after u.v. irradiation of isogenic umu+ bacteria. Mutagenesis is not inhibited by the presence of chloramphenicol after u.v. and before photoreversal when the bacteria have also been pretreated with chloramphenicol. Loss of photoreversibility of streptomycin-resistant mutations in umu+ excision-proficient bacteria also occurs during a similar period after u.v. and is similarly unaffected by Casamino acids and chloramphenicol. The results are interpreted on a 2-step model of error-prone repair in which a misincorporation step is followed by a lesion bypass step which requires induced levels of umuC gene product, the latter step being unnecessary when the pyrimidine dimer is removed by photoreversal after the misincorporation has taken place. Differences from the same phenomenon in excision-deficient bacteria are accounted for by the fact that in excision-proficient bacteria the mutation process is initiated by excision repair rather than during normal chromosomal replication. It is suggested that loss of photoreversibility in excision-proficient umu+ bacteria may reflect the misincorporation step only, thus explaining the apparent non-inducible nature of loss of photoreversibility previously reported. The bypass step alone requires induced levels of umuD,C gene products as is evidenced by the u.v.-induction of mutations in lexA102(ind-) bacteria after delayed photoreversal.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial Proteins↗
PubMed1986

The mutagenicity of sodium fluoride to L5178Y [wild-type and TK+/- (3.7.2c)] mouse lymphoma cells.

L5178Y wild-type and TK+/- (3.7.2c) cells were treated with sodium fluoride over a range of concentrations (10-500 micrograms ml-1) and treatment times (4, 16 and 48 h) covering less than 10-100% survival. The mutant frequency at five genetic loci (resistance to ouabain, 6-thioguanine, excess thymidine, methotrexate and 1-beta-D-arabinofuranosyl cytosine) was assayed in wild-type cells and trifluorothymidine in TK+/- cells. No significant induced mutation at any locus was observed after 4 h of treatment. Sixteen hours of treatment with high concentrations of sodium fluoride did not induce resistance to ouabain, but resulted in some significant induction of 6-thioguanine, 1-beta-D-arabinofuranosyl cytosine and methotrexate resistance, although the results were variable between experiments and no dose-response was observed. At the thymidine kinase locus, a dose-related increase in mutant frequency to excess thymidine and trifluorothymidine resistance was observed. The maximum induction was approximately eight times the control frequency after TK+/- cells were treated with the highly toxic concentration of 500 micrograms ml-1 of sodium fluoride for 16 h. These observations, and an analysis of the colony size of trifluorothymidine-resistant mutants in TK+/- cells, suggest that sodium fluoride is clastogenic to dividing cultured mammalian cells at high, toxic concentrations. Further work is desirable to investigate the mechanism by which chromosomes are damaged at high concentrations of fluoride, since without such a mechanistic understanding, extrapolation of our data to the human situation must be insecure. Nevertheless, the knowledge available at present gives no reason to expect any genotoxic effects in human tissues at levels of fluoride ions to which they are currently exposed in the general population.

Animals↗
PubMed1986

New role for photoreversible pyrimidine dimers in induction of prototrophic mutations in excision-deficient Escherichia coli by UV light.

UV mutagenesis to His+ in certain recA441 lexA51 bacteria was not photoreversible, indicating that pyrimidine dimers are not target lesions. Photoreversibility was observed in recA+ lexA51 bacteria, showing that pyrimidine dimers are needed to activate the recA+ protein (unlike the recA441 protein) to perform a function in UV mutagenesis distinct from cleavage of the lexA repressor.

DNA Repair↗
PubMed1985

Uvr-independent repair of 8-methoxypsoralen crosslinks in Escherichia coli: evidence for a recombinational process.

On the basis of survival data, repair of 8-methoxypsoralen DNA crosslinks in Escherichia coli strains lacking a functional uvrABC endonuclease, is shown to require the products of the recA, recB, recF and recN genes. Bacteria, grown under conditions where most cells contain only a single genome, show no evidence of crosslink repair. Similarly, bacteriophage lambda shows evidence of crosslink repair only in SOS-induced cells, and only at multiplicities of infection greater than 1. The requirement for rec+ genes may be partly ascribed to the need for a functional SOS response, but taken together, the results suggest a recombinational step involving a homologous region of DNA may occur during uvr-independent crosslink repair.

Bacterial Proteins↗
PubMed1985

Mutagenic repair in Escherichia coli: products of the recA gene and of the umuD and umuC genes act at different steps in UV-induced mutagenesis.

When excision-deficient Escherichia coli carrying umuC or umuD alleles were exposed to visible light several hours after ultraviolet irradiation, base-pair-substitution mutations were induced in these normally non-UV-mutable bacteria. It is argued that delayed photoreversal of pyrimidine dimers removes blocks to DNA replication and allows the "survival" and expression of misincorporated bases. A model for UV mutagenesis is proposed with two steps: (i) misincorporation opposite a photoproduct, which can be mediated directly by RecA protein, and (ii) bypass, only the latter process requiring umuD+ and umuC+ alleles. Basal levels of gene products are sufficient for at least some misincorporation events, although induced levels of umuD and umuC gene products are necessary for the bypass step. umuC bacteria containing the recA441 allele showed a greater yield of mutants, and those containing recA430 a reduced yield, following delayed photoreversal. The lexA51 allele (which results in constitutive derepression of RecA protein production) did not significantly alter the yield of mutants but caused them to appear marginally sooner in a recA441 umuC strain. These results emphasize that the nature of the RecA protein and not its concentration is paramount in determining the level of misincorporation. Experiments with recA441 umuC bacteria at 43 degrees C and 30 degrees C suggest that the misincorporation effect is unlikely to be attributable to cleavage of a DNA binding protein such as a repressor or a component of the polymerase complex. Moreover, misincorporation seems to occur without the need for induced synthesis of any other protein under recA control.

Bacterial Proteins↗
PubMed1984

Mutagenic repair in Escherichia coli. X. The umuC gene product may be required for replication past pyrimidine dimers but not for the coding error in UV-mutagenesis.

Bacteria of strain TK610 uvrA-6 his-4 umuC-36, when allowed to replicate their DNA for some hours after irradiation show induction of His+ mutations when subsequently exposed to visible light. It is suggested that base pair errors can be made opposite sites of pyrimidine dimers without involvement of umuC gene product but that the latter is required for continued replication past the dimer-mismatch region. Removal of the pyrimidine dimer by photoreversal allows replication to continue thus fixing the mismatched base as as mutation.

Bacterial Proteins↗
PubMed1983

Mutagenic repair in Escherichia coli. VIII. Effect of gyrB mutations on ultraviolet light mutagenesis.

Introduction into Escherichia coli WP2 bacteria of a mutation in the gyrB locus previously shown to reduce the degree of chromosomal superhelicity caused a small decrease in the frequency of UV-induced mutations to streptomycin resistance (but not significantly) and to tryptophan independence (mostly ochre suppressors) in excision repair-proficient bacteria. It did not influence the 'broth effect' or the rate or extent of 'mutation frequency decline' of suppressor mutations. In an excision-deficient (uvrA 155) background the yield of UV-induced streptomycin-resistant mutations was lower in gyrB bacteria at all doses; the yield of tryptophan-independent mutations was slightly lower at low doses and slightly higher at high doses. In both excision-proficient and -deficient bacteria the yield of UV-induced mutations to rifampicin resistance was apparently lower in gyrB mutants but this could be due at least in part to a hypersensitivity of some Rifr gyrB bacteria to UV. The number of spontaneous tryptophan-independent mutations was lower in gyrB bacteria but this was almost certainly due to their poorer viability on tryptophan-limiting plates and not to a lower spontaneous mutation rate. In a temperature-sensitive presumed gyrase-deficient strain a small decrease in mutant yield at low doses was observed following incubation at restrictive temperature before UV. This was ascribed to an enhancement of excision repair. Our failure to find any significant effect of gyrB mutations does not support the hypothesis that hairpin formation (which should be dependent on a high degree of superhelicity) is involved in determining the 'broth effect', 'mutation frequency decline' or the probability that a mutation will occur spontaneously. Dramatic effects of superhelicity on UV mutagenesis also seem to be unlikely.

DNA Repair↗
PubMed1983

Radiosensitivity of human T-lymphocytes proliferating in long term culture.

A method is described for determining the radiation sensitivity of dividing human T-lymphocytes in long-term culture. The results are fitted to a single-hit multi-target model. For cobalt-60 gamma radiation Do values of cells from five normal individuals range from 0.99 to 1.37 Gy with an overall Do of 1.09 Gy, and the extrapolation numbers range from 1.20 to 1.71 with an overall extrapolation number of 1.42.

Cell Division↗
PubMed1982

A new pathway for repair of cross-linkable 8-methoxypsoralen mono-adducts in Uvr strains of Escherichia coli.

UvrA-deficient Escherichia coli bacteria possess a DNA repair pathway for certain photoinduced mono-adducts of 8-methoxypsoralen. Such adducts can be converted to highly lethal cross-links by a second exposure to near-UV light in the absence of 8-methoxypsoralen. If a period of incubation is allowed to intervene between the primary and cross-linking exposure, the lethal effect is considerably diminished, indicating that some of the mono-adducts become unavailable for cross-linking. A repair pathway is postulated which is pre-replicative, unaffected by chloramphenicol, acriflavine or caffeine, and inhibited at 0 degrees C. It is unaffected by a rep mutation but is blocked by a polA mutation, suggesting that it involves DNA strand breakage and at least some exonuclease action.

DNA Repair↗