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/mSTRAC¢H~mopkilus influen~--transforming DNA, which has been inactivated by ultraviolet radiation, is reactivated by visible light in the presence of a cell-free extract of Eschcric~ia coli B.The time rate of reactivation is increased by increasing the E. coli extract concentration, the temperature, and the intensity of illumination.Only DNA containing an ultraviolet-damaged genetic marker exhibits increased transforming activity after treatment with the photoreactivatlng system.The reactivating capacity of the extract remains in the top supernatant after centrifugation at 110,000 X g for 1 hour and is not present in the pellet. This capacity is destroyed by heating to 90°C. for 1 minute.The active system of the E. cdi extract is separable into dialyzable, heat-stable and non-dialyzable, heat-labile fractions. The dialyzable fraction contains at least one component which limits the max-imum degree of recovery attained.Photoreactivation, the reversal of short wave length ultraviolet effects on organisms by subsequent treatment with visible light, was first specifically described by Keiner (1, 2) for survival of the ultraviolet-irradiated conidia of Streptomyces griseus. It has since been recognized for a variety of other ultraviolet effects in a large number of species and tissues (3), including interruption of DNA synthesis (4), production of mutations (5), induction of vegetative phage in lysogenic bacteria (6), inhibition of adaptive enzyme formation in yeast (7), spheration of nudeoli in the grasshopper neuroblast (8), delaying of cleavage in eggs (9), and delaying of division in protozoa (10). Its detailed mechanism is unknown. A possible outline of the process, however, is suggested by existing evidence. * Present address:

Previous genetic analysis indicated that at least two genes determine the ultraviolet (UV) sensitivity of Bacillus subtilis spores. The present study shows that these genes independently control two distinguishable processes for removing UV-induced spore photoproduct (5-thyminyl-5,6-dihydrothymine, or TDHT) from spore deoxyribonucleic acid. The first, is a spore repair mechanism by which TDHT is removed rapidly without appearing in acid-soluble form. This mechanism, which is demonstrated in both UV-resistant and excision-deficient strains, operates to a certain extent during germination without requiring vegetative growth. The second, demonstrated in a mutant which lacks the first mechanism, removes TDHT relatively slowly and only if germinated spores are allowed to develop toward vegetative cells. The latter mechanism appears identical to excision-resynthesis repair, since the mutation abolishing it renders the irradiated vegetative cells incapable of removing cyclobutane-type pyrimidine dimers. Blocking either one of these mechanisms only slightly affects the UV sensitivity of spores, but blocking both prevents TDHT removal and gives high UV sensitivity.

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