Photochemistry and Photobiology of Spore-Forming Bacteria

Abstract

Ten years ago, Beukers et al. (1959) and Wacker et al. (1962) observed that the pyrimidine base,thymine,could form dimers when irradiated with ultraviolet light (UV) either in frozen solution or when the thymine had been incorporated into the DNA of living cells. Figure 1 shows such a dimer in one chain of the DNA helix. Dimerization occurs when the excited complex of two adjacent pyrimidines causes the formation of a stable cyclobutane ring. Fortunately, thymine can be labeled easily with radioactivity and the dimers may be quantiated by acid hydrolysis and chromatography. Using these and other photochemical and photobiological techniques, the Setlows (see Setlow, 1960, for review) showed that the dimers caused 50 to 90% of the lethal effect of UV in viruses and bacteria. This effect can be partially circumvented by two different types of enzymatic repair. Shortly before the discovery of cyclobutane dimers, Kelner (1951) found that certain bacteria exposed to a lethal dose of light of wavelengths around 260 nm could be reactivated by exposure to light in the region of 400 nm. Cook (1967) then showed that this photoreactivation occurred by the enzymatic monomerization of the dimerized pyrimidines back to the normal base in the presence of light. The second repair mechanism occurs in cells maintained in the dark and consists of the removal of a short segment of one strand of DNA containing the dimer and resynthesis of that segment using the complementary strand as template. Cells lacking this type of repair are very sensitive to UV as one might expect (Setlow and Carrier,1964; Boyce and Howard-Flanders, 1964).