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Biomedical subjects

B A Bridges

Publications and source records attributed to B A Bridges.

At least 145 records · Page 8Linked to original sources
PubMed1976

Mutagenic DNA repair in Escherichia coli. III. Requirement for a function of DNA polymerase III in ultraviolet-light mutagenesis.

The polC (= dnaE) temperature-sensitive DNA polymerase III mutation from Escherichia coli BT1026 has been transduced into E. coli WP2 (to give CM731) and WP2 uvr A (to give CM741). In excision-deficient CM741 UV-induced Trp+ mutations progressively lost their photoreversibility during post-irradiation incubation at 34 degrees. Immediately after transfer to 43 degrees, however, there was no further loss of reversibility although post-replication strand joining still occurred and uptake of 3H-thymidine into DNA continued for 20 to 30 min. In excision-proficient CM731, UV lesions capable of leading to Strr mutations disappeared during post-irradiation incubation at restrictive temperature and there was no increase in the number remaining after exposure to photoreversing light. In contrast, at permissive temperature, premutational lesions were not lost and became progressively converted into non-photoreverisble mutations. It is concluded that a function of the polC gene is necessary for error-prone repair to occur and that this function is defective at 43 degrees in the enzyme specified by the polC allele from BT1026. This function seems not to be essential for most post-replication or excision repair or for normal DNA replication and may be particularly involved in the insertion of incorrect bases during error-prone repair.

Coliphages↗
PubMed1976

Use of a simplified fluctuation test to detect low levels of mutagens.

As a mutagen screening procedure we have used a modification of the Luria and Delbrück fluctuation test in which the individual tubes are scored by eye for the presence or absence of a mutation. The test is simple and extremely sensitive, detecting concentrations of mutagens up to 100-fold lower than conventional tests. Measuring mutation to tryptophan independence in Escherichia coli strain WP2 we have found that methyl methanesulphonate (0.5 mug/ml), mitomycin C (0.0015 mug/ml), dichlorvos (5 mug/ml, and K2CrO4 (0.5 mug/ml) are all positively mutagenic in the test, whereas NiCl2 is negative. Chronic exposure to low levels of mutagens using this method appears to induce more mutations than might be predicted by extrapolation from short exposure experiments at higher doses. The procedure is applicable to any system which involves mutation to prototrophy from a non-leaky auxotrophic requirement and should prove valuable in detecting and investigating the effects of low doses and chronic exposures.

Bacteriological Techniques↗
PubMed1975

Mutagenic DNA repair in Escherichia coli. II. Factors affecting loss of photoreversibility of UV induced mutations.

The photoreversibility of UV-induced mutations to Trp+ in strain Escherichia coli WP2 uvrA trp (unable to excise pyrimidine dimers) was lost at different rates during incubation in different media. In Casamino acids medium after a short initial lag, photoreversibility was lost over about one generation time; in minimal medium with tryptophan, photoreversibility persisted for more than two generations; in Casamino acids medium with pantoyl lactone photoreversibility was lost extremely slowly. The rate of loss of photoreversibility was unaffected by UV dose in either Casamino acids medium or in minimal medium. The same eventual number of induced mutants was obtained when cells were incubated for two generations in any of the three media before being transferred to selective plates supplemented with Casamino acids. Thus in each the proportion of cells capable of giving rise to a mutant was the same and only the rate at which these cells did so during post-irradiation growth varied, suggesting that there might be a specific fraction of pyrimidine dimers at a given site capable of initiating a mutagenic repair event, and that the size of this fraction is dose dependent. Segregation experiments have shown that error-prone repair appears to occur once only and is not repeated in subsequent replication cycles, in contrast to (presumed error-free) recombination repair. The results are discussed in the light of current models of UV mutagenesis.

Amino Acids↗
PubMed1975

Hypothetical dose-response curves for chronic exposures to mutagens or carcinogens subject to simple enzymatic detoxification in the mammalian body.

Dose-response curves for chemical carcinogenesis and mutagenesis in the whole mammal may be influenced (a) by processes affecting the delivery of the applied dose to the DNA of the target cell or (b) by processes affecting the response of the cell to the initial DNA damage. As an example of the former, the effect of a simple enzymatic detoxification system on the delivery to the target organ of an applied chronic dose has been considered. In certain circumstances as the applied dose is lowered, the delivered dose may become proportionately less than at high doses so that a new linear relation of lower slope is established. Such a biphasic curve is termed quasi-threshold to distinguish it from a true threshold. The slope at low doses could in principle be extrapolated to very low doses in the estimation of genetic risk.

Animals↗
PubMed1975

Mutagenic DNA repair in Escherichia coli: conditions for error-free filling of daughter strand gaps.

Two situations have been observed in which daughter strand gaps in DNA synthesized after exposure of excision-deficient Escherichia coli to ultraviolet light are filled but in which no mutations are formed as judged by loss of photoreversibility: (i) during the first 20 min of growth after u.v. irradiation, and (ii) when repair is allowed to occur in buffer. We suggest as an explanation that the majority of daughter strand gap-filling is error free and that mutations arise through a minor error-prone repair pathway which is inoperative under these conditions.

Buffers↗
PubMed1974

Effect of photoreactivation on the filling of gaps in deoxyribonucleic acid synthesized after exposure of Escherichia coli to ultraviolet light.

Daughter-strand gaps in deoxyribonucleic acid (DNA) synthesized after exposure of excision-deficient Escherichia coli to ultraviolet light are filled during subsequent incubation in buffer, and the rate of filling is increased when the incubation in buffer is carried out in the presence of 360-nm light. It is concluded that daughter-strand discontinuities are prevented from being rapidly sealed in the dark not because of some structural feature of the daughter-strand but because of the presence of a pyrimidine dimer on the opposite (parental) strand. "Photoreactivation-stimulated gap filling" is dependent on the polA(+) and recA(+) but not the exrA(+) genes. It is suggested that the removal of the dimer allows gap-filling by DNA polymerase I and polynucleotide ligase. The recA(+) gene may be needed at a very early stage, possibly for gap stabilization.

Centrifugation, Density Gradient↗