Thermal resistance of UV-mutagenesis to photoreactivation in E. coli B/r uvrA ung: estimate of activation energy and further analysis |
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Authors: | Douglas F. Fix |
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Affiliation: | (1) Department of Microbiology and Immunology, Indiana University School of Medicine, 46223 Indianapolis, IN, USA;(2) Present address: Biology Department, York University, 4700 Keele Street, M3J 1P3 North York, Ontario, Canada |
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Abstract: | Summary Ultraviolet light (UV) induced mutations in the glnU and glnVutRNA genes in Escherichia coli are thought to be targeted by UV photoproducts. In a previous study with a uracil-DNA glycosylase deficient strain, UV-induced glnUoand glnVotRNA suppressor mutations became resistant to photoreactivation (PR) following thermal treatment. It was proposed that deamination of cytosine in the cytosine-containing cyclobutyl dimers at the sites of these suppressor mutations produced uracil residues in sequence upon PR. In the absence of glycosylase, the C U conversion yielded the requisite G:C A:T transitions. In the present study, this thermal resistance of UV-mutagenesis to PR is characterized. It is dependent on the initial UV-fluence and temperature of holding but not on the UmuC+ gene product. The data obtained yield an estimate of an activation energy of 17±3 kcal/mol for the deamination of cytosines contained in dimers. This compares to 29 kcal/mol for unaffected cytosines in DNA. In addition, an estimate of the probability of cyclobutyl dimer formation at the target sites for glnUoand glnVosuppressor mutations indicate that these lesions can not entirely account for the mutation frequencies recovered in the absence of PR. This is interpreted as an indication that, in addition to thyminecytosine cyclobutyl dimers, other UV-induced lesions, possibly Thy(6-4)Cyt photoproducts, may also target glnUoand glnVosuppressor mutations. |
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Keywords: | tRNA suppressor mutations Cytosine deamination Uracil-DNA glycosylase Cyclobutyl pyrimidine dimers |
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