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Abstract
Pyronin Y (0·06 and 0·16 mm), coumarin (2 and 4 mm), 6,9-dimethyl 2-methyl-thiopurine (2 mm) and caffeine (8 mm), strongly inhibit Uvr repair (presumed excision repair) of u.v. lesions in Escherichia coli. With 0·16 mm-pyronin Y or 4 mm-coumarin, one thymine dimer was a lethal event in the k12 strain jc2926 rec A-13 and one to three dimers in the b/r derivative wp-2 rec A-13 and the b exr A derivative bs-2. Neither these compounds nor 8 mm-caffeine had a significant effect on the survival of irradiated bacteria of the corresponding uvr strains wp-2 Hcr−, bs-12, and bs-8. I suggest that 8 mm-caffeine, 0·16 mm-pyronin Y, 4 mm-coumarin and 2 mm-6,9-dimethyl 2-methylthiopurine selectively block excision repair without substantially affecting recombination repair.
Caffeine at 12 mm only slightly depressed the viability of a u.v.-irradiated culture of wp-2 Hcr− or bs-12, but at 16 mm there was a much greater effect, particularly at low u.v. doses (< 40 ergs/mm2). Since excision repair appeared to be effectively blocked by 8 mm-caffeine, the depression of u.v. survival of the uvr mutants by 16 mm-caffeine was presumed to be due to interference with recombination repair (Witkin & Farquharson, 1969).
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