Photoprotection from Ultraviolet Killing in Escherichia coli B

Abstract

Photoprotection (PP) is a reduction in final UV damage produced by light of lower photon energy administered before the UV irradiation. Only a few cases have been reported, and the present study is an attempt to gain some knowledge of the mechanism. Cells suspended in phosphate buffer were exposed to light of 3100 to 6000 Angstroms (PP light) prior to exposure to UV of 2537 Angstroms. No PP was found with X-rays. PP was found to be a large effect under certain conditions, and to be very stable, showing only 50% decay in 28 hours at 23[degree] C. PP requires doses of PP light so high as to be slightly lethal to control cells. It does not show constant dose reduction, the effect disappearing at 10-3 survival. As a function of dose of PP light, PP is virtually independent of temperature in the range 3[degree] to 37[degree] C during illumination (Q10 of 1.05), and is independent of dose rate when this is changed by a factor of 16. It is clear that, in this system, PP differs in many respects from photoreactivation, especially in that limiting thermal reactions are not involved in the early events in PP.