Cerenkov ultraviolet radiation (137Cs gamma-rays) and direct excitation (137Cs gamma-rays and 50 kVp X-rays) produce photoreactivable damage in Escherichia coli.

The mechanism of formation of photoreactivable damage in deoxyribonucleic acid (DNA) by ionizing radiation in a dark repair deficient strain of Escherichia coli (uvrA recA) has been investigated. By altering the ratio of damage produced directly (by ionization) to that formed indirectly (by Cerenkov ultraviolet (U.V.) radiation by 137Cs gamma-rays, it has been demonstrated that the major portion of the photoreactivable damage is produced by Cerenkov U.V. radiation. The amount of photoreactivable damage produced by 50 kVp X-rays, which cannot generate Cerenkov radiation, is similar to that component of photoreactivable damage produced by 137Cs gamma-rays that is not attributed to Cerenkov radiation. It is suggested that the second mechanism of formation of photoreactivable damage in DNA by ionizing radiation is the direct excitation of DNA. The possible role of Cerenkov U.V. radiation in ionizing radiation mutagenesis is discussed.