Effects of ‘Visible’ Light on 5-Bromouracil-labelled DNA

Abstract

INCORPORATION of 5-bromodeoxyuridine (BUdR) into the DNA of bacteriophages, bacteria and mammalian cells results in highly increased sensitivity not only to short-wave ultra-violet light^1–4 but also, and even more profoundly, to ‘visible’ light^2,4 or more probably to its long-wave ultra-violet component. An attempt was made to determine the photochemical mechanism of this phenomenon, as a logical extension of our earlier work on the mechanism of short-wave (2537 Å) ultra-violet light effects on BUdR-labelled cells and transforming DNA⁵. It was suspected that photochemical dehalogenation, a reaction already observed with short-wave ultra-violet light⁶, accounts for most of the lethal effects of ‘visible’ light, especially of the 3000–3600 Å component. The experiments were performed either with intact cells or with protein- and RNA-free DNA isolated from genetically transforming strains of Bacillus subtilis. In a few experiments, a thymine-requiring strain of Escherichia coli, 15 T⁻, was also used. The growth conditions resulting in massive incorporation of BUdR were described earlier⁷. 5-Bromouracil labelled with carbon-14 was used to label the cell DNA for the chromatographic experiments. A fluorescent lamp (General Electric, warm-white, 20 W) served as the source of ‘visible’ illumination. (Prolonged irradiation of frozen thymine solution with this lamp results in formation of small quantities of the thymine dimer, indicating that a minute fraction of its spectral output is a photochemically active ultra-violet component, which otherwise would scarcely be detectable by the usual spectrophotometric procedures.)