I. Determination of the Quantum Yield of Thymidine Dinucleoside Irradiated by Ultra-Violet Light. II. Studies on the Inactivation by Ultra-Violet light of T₄D Bacteriophage Containing 5-Bromodesoxyuridine Substituted DNA

Year: 1962

DOI: 10.7907/V4NH-KB09

It has been found that when frozen thymine solutions are irradiated by ultra-violet light a photoreversible steady state is formed between thymine and a thymine diner. The suggestion has been made that such photoreversible chemical reactions might be involved in the photoreactivable mutations that occur upon irradiation of viruses with ultra-violet light. In order to give further credence to this thymine dinucleoside, TpT, was prepared and irradiated by ultra-violet light as described in Part I. A wavelength dependent photosteady state was found tobe established by this, and the quantum yield of the reaction of TpT to its photoproduct determined. The existence of this steady state and the magnitude of the quantum yield (0.002) support the supposition that similar reactions may cause the photoreversible mutations in viral DNA.

Part II describes the determination of the action spectrum of in activation of plaque forming ability of T_4 bacteriophages grown in the presence of the thymidine analogue 5-bromodesoxyuridine (5-BD). Under these conditions 5-BD is substituted intothe DNA in place of thymidine. Such substituted DNA has been found to have increased sensitivity to UV light at 254 mμ. The action spectrum was determined to more precisely define this increase in sensitivity and to obtain additional information regarding the UV inactivation of DNA. It was found that 5-BD substituted T_4 was uniformly more sensitive to UV than unsubstituted T_4; this effect was most striking in the 302 to 334 mμ region where a factor of increased sensitivity up to 500 to 1000 times was noted. Furthermore, unlike unsubstituted T_4, killing of 5-BD substituted T_4 was found to follow "one hit" kinetics. The action spectrum indicated that 5-BD was directly involved in the initial steps of inactivation and was compatible with the possibility of transfer of absorbed light energy along the polynucleotide chain.