Abstract
The breakage of closed covalent λ DNA molecules in lysogenic host cells after ultravilet irradiation was investigated. In repair proficient host cells incisions are introduced immediately following irradiation. A steady-state of strand interruptions is observed within 20–50 seconds after irradiation, where the number of broken molecules is dose dependent for doses up to 600 ergs/mm2. No ultraviolet promoted strand breaks were observed in uvrA or uvrB lysogens, in accordance with previous results obtained by Shimada, Ogawa & Tomizawa [Molec. gen. Genet. 101, 245 (1968)]. In contrast, uvrC mutants have the ability to form breaks in superinfecting λ DNA molecules after ultraviolet irradiation. The ultraviolet specific endonucleolytic activity observed in uvrC host cells differs from that observed in uvr+ host cells in that, (i) the first break is introduced at least 15 times slower, (ii) for doses below 300 ergs/mm2 the number of strand breaks is higher, (iii) the dose dependence terminates at a lower dose. The possible function of the uvrC gene product in the repair is discussed.
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