The shape of the ultraviolet inactivation curve for transforming DNA.

THE very unusual inverse square kinetics of ultraviolet inactivation of transforming DNA were first reported by Rupert and Goodgal1, and subsequently there was speculation that these kinetics result from blocks to recombination formed by ultraviolet lesions in the DNA2 (ref. 2). We show here that when the recipient cell lacks both excision arid postreplication repair, the ultraviolet inactivation of transforming DNA becomes a simple exponential rather than an inverse square relation. The data suggest that the square root kinetics observed with repair-proficient recipient cells result from repair processes themselves rather than from the recombination between irradiated transforming DNA and recipient DNA.