Letter: Rejoining of DNA single-strand breaks in mammalian cells incubated in buffer or in medium after aerobic or anaerobic x-irradiation.

1 . Introduction The rejoining of DNA single-strand breaks in mammalian cellular DNA has been varified for cells in culture (e .g ., Alexander, Dean, Lehmann, Ormerod, Feldschreiber and Serianni 1970, Donlon and Norman 1971) as well as for ascites tumour cells in mice (Matsudaira, Nakagawa and Hishizawa 1969), and mouse testis (Ono and Okada 1974) and dog neurons (Wheeler and Lett 1972) irradiated in vivo . It seems likely that the same number of DNA single-strand breaks is produced in all phases of the cell-cycle (Humphrey, Steward and Sedita 1968), although the rejoining rate may vary slightly in the different phases (Lohman 1968). The problem of X-ray-induced breaks produced in the alkali-labile regions of the DNA molecule (normally not measurable, since alkaline treatment ruptures these so-called linker sites) and the repair of such breaks, which may be S-phase dependent, has been discussed by Lett, Sun and Wheeler (1972) . We have investigated the extent to which DNA single-strand breaks produced by X-irradiation under air or N 2 are rejoined after aerobic incubation in either a phosphate (PBS) or a carbonate (CBS) buffered salt solution or complete growth-medium (MEM) in order to detect any differences in the DNA rejoining process(es) of aerobically and anaerobically irradiated mammalian cells .