GROWTH, RESPIRATION, AND NUCLEIC ACID SYNTHESIS IN ULTRAVIOLET-IRRADIATED AND IN PHOTOREACTIVATED ESCHERICHIA COLI

Irradiation of the cell with wavelengths (circa 2,600 A) of ultraviolet light strongly absorbed by nucleic acids can result in profound cellular changes, e.g., inhibition of growth, or of adaptive enzyme formation; in increased mutation or in death. Visible and near-visible light (from about 3,600 to 4,900 A), henceforth referred to as reactivating light, when applied to the cell after ultraviolet irradiation, will prevent death of the cell (a phenomenon called photoreactivation, Kelner, 1949a, 1949b, 1951) or reduce the frequency of induced mutants among survivors (Kelner, 1949b; Novick and Szilard, 1949; Newcombe and Whitehead, 1951; Kimball and Gaither, 1951; Brown, 1951); reduce ultravioletinduced inhibition of growth (Blum et al., 1950; Blum and Matthews, 1952; Wells and Giese, 1950); prevent the inhibition of adaptive enzyme formation (Swenson and Giese, 1950); prevent injury to cells of higher plants (Bawden and Kleezkowski, 1952); prevent changes in the morphology of the nucleoli in grasshopper neuroblats (Carlson and McMaster, 1951). The multiplicity of the biological effects of ultraviolet radiation, and the fact that most of them are reversed by reactivating light, suggests the theory that ultraviolet causes some basic change in the cell, which in turn causes the various other biological effects, usually after some delay. Reactivating light, by reversing this basic change, prevents the other effects (mutation, death, inhibition of adaptive enzyme formation, etc.). Even if this theory is not correct, it would be helpful to find some ultraviolet-induced reaction which is affected by reactivating light, and 1 Aided in part by a grant from the American Cancer Society. 2 This work was done during the tenure of a Special Public Health Service Fellowship of the National Cancer Institute. 3 Present address: Department of Biology, Brandeis University, Waltham 54, Mass. which can be more simply described than can such complex phenomena as death, or even inhibition of growth. Study of the mechanism of photoreactivation of a relatively simple reaction would help in forming testable hypotheses for the action of light on the more complex reactions. Since reactivating light is effective when applied immediately after ultraviolet, or even simultaneously with it (Kelner, unpublished experiments), we have assumed that the basic reaction under consideration occurs very soon after ultraviolet irradiation, although this is not the only possibility. Therefore, we have investigated the cellular changes occurring in bacteria immediately after ultraviolet irradiation, and in order to help distinguish the basic reaction from nonpertinent side reactions we have set up the following criteria by which to judge the significance of any reactions found. 1. The cellular change should be detectable immediately after ultraviolet irradiation. 2. It should be produced in full amount by the minimum dose of ultraviolet "killing" 80 to 99 per cent of the cells. This proviso eliminates many reactions that are produced only by massive doses of ultraviolet (such as denaturation of proteins), but which do not occur necessarily with the small doses which are biologically effective. 3. The change should be immediately and almost completely reversible by doses of reactivating light known to produce maximum reactivation, as measured by the prevention of the lethal action of ultraviolet. 4. The change preferably should be a type of reaction which can be expected to be involved in reactions associated with death or mutation. Of all the reactions studied the one best satisfying these criteria was the ultraviolet-induced, specific, and immediate inhibition of the synthesis