Role of the Main Cytosine Radiolytic Product in Ionizing Radiation-Induced Mutagenesis

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When ^<14>C cytosine was incorporated in M13 mp10 phage, in the phage DNA both cytosine and thymine residues were labelled. The single stranded phage DNA was isolated and irradiated in a buffer solution using 60Co gamma-rays. After the addition of TCA, the DNA precipitated was filtered. The ratio of the radioactivity of the filtrate to the total increased with increasing radiation dose. The precipitate 'was hydrolyzed with acid and the base modification was analyzed by TLC. The modification yield also increased with increasing dose. Chromatograms showed that the main product was urea. In the case of cytosine radiolysis, trans-5,6-dihydroxy-5,6-dihydrouracil was found to be the main product before acid treatment; whereas, urea was the main product after acid treatment. This fact suggests that the trans-5,6-dihydroxy-5,6-dihydrouracil retained in the irradiated DNA is responsible for the gamma-ray induced C - T transition.
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