Summary Starting with the isolation of thymine dimer derived from DNA of u.v.‐irradiated bacteria, a systematic investigation was begun from which it was concluded that the basis for the action of u.v.‐light is a photochemical dimerization. Depending on the dose of u.v.‐light and the conditions for hydrolysis of DNA and isolation procedures, other photoproducts of thymine were isolated. Experimental data for the conversion of cytosine to uracil are presented. By use of monochromatic u.v.‐light we could demonstrate that only intrastrand dimerization occurs. The radiation chemical behavior of uracil and cytosine was investigated with tritium labeled compounds. The mechanism for the various kinds of sensitization with 5‐halogen‐uracils, with dyes (thiopyronin, methylene blue, and pyronin) with carcinogenic hydrocarbons and with furocoumarins, which parallels partly the photochemical degradation of guanine is discussed. U.V.‐inactivated bacteria and polyuridylic acid are reactivated better by irradiation with visible light when hydrogen peroxide or uranyl acetate are present. The mechanism of this reaction is discussed. The absence of the α, β‐unsaturated double bond could be expected to protect the thymine molecule from dimerization; after incorporation of azathymine into the DNA, bacteria do indeed exhibit u.v.‐resistance. Moreover, after treatment of bacteria with hydrogen peroxide and visible light, by which the 5,6 double bond in the thymine and cytosine molecules is also attacked, bacteria become u.v.‐resistant. The specific alteration of uracil and cytosine by u.v.‐irradiation and of guanine through photodynamic action can be used as a simple method for sequence analysis of protein codons. It is also of use in determining the interaction between messenger‐RNA, transfer‐RNA and ribosomes during protein synthesis.

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