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Repair of Radiation Lesions in Single–stranded DNA Phages

Ramendra K. Poddar, Chanchal K. Dasgupta, Ashoke R. Thakur


ϕX174, S13, and related phages have evoked considerable interest among radiation biologists because of the single-stranded nature of their DNA. In fact, the radiobiological (32P-suicide) experiment of Tessman et al. (1957) gave the first hint that phage S13 might contain single-stranded DNA (SS DNA); it was based on the observation that the decay of a single 32P atom incorporated into S13 DNA was capable of inactivating its plaque-forming ability. It was found subsequently that the survival curves obtained by irradiating various SS DNA phages with X rays (Blok et al. 1967), γ rays (Lytle and Ginoza 1968), and UV (Yarus and Sinsheimer 1964) were all of the so-called “single-hit” type, implying that one lesion occurring somewhere in the DNA genome was sufficient to inactivate the phage. As to the nature of such lesions in SS DNA, lethality in the case of 32P-suicide was attributed solely to the scissions in the SS DNA. For γ rays, however, the frequency of single-strand scission per lethal event was only 0.26 (Lytle and Ginoza 1969). Besides single-strand scissions, thymine base damages, including formation of ring-saturated compounds such as 5,6-dihydroxy-dihydrothymine, were found to be produced upon irradiation with γ rays (Swineheart and Cerruti 1975). In the UV dose ranges used in radiobiological studies, few chain scissions are likely to be produced (Poddar and Sinsheimer 1971), and David (1964) showed that thymine dimers constituted only 0.34 of the lethal lesions (the other lesions have remained uncharacterized). In contrast, there were 4.8 thymine dimers per lethal hit...

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