Photosensitized inactivation of DNA by monochromatic 334-nm radiation in the presence of 2-thiouracil: genetic activity and backbone breaks.

Abstract

AbstractMonochromatic 334‐nm radiation delivered under aerobic conditions inactivates the genetic activity (ability to transform auxotrophic recipient cells to nutritional prototrophy) of isolated transforming Bacillus subtilis DNA. The presence of superoxide dismutase (SOD), catalase, and mannitol reduces the 334‐nm inactivation. The rate of inactivation of the genetic activity by 334‐nm radiation is enhanced fivefold by the sensitizer 2‐thiouracil (s2Ura). This enhancement is substantially reversed when the irradiations are performed in the presence of mannitol, and, to a lesser extent, SOD. Catalase slightly reduces the s2Ura enhancement of 334‐nm inactivation of transforming activity. Backbone breaks induced in the same DNA by aerobic 334‐nm radiation were also enhanced markedly by the presence of s2Ura; this enhancement was reversed by the presence of mannitol and, to a lesser extent, SOD during irradiation. Catalase had no effect upon s2Ura‐enhanced, 334‐nm‐induced SSBs. Whereas DNA breakage may be responsible for a portion of the inactivation of the DNA by the photosensitized reaction between s2‐Ura and 334‐nm radiation, it is not the only inactivating lesion, because the yield of SSBs per lethal hit per unit length of DNA is not constant for all the irradiation conditions studied. The results support a complex role for active oxygen species in inactivation of transforming activity and DNA breakage by s2Ura‐enhanced 334‐nm radiation. They are also consistent with the formation of superoxide anion, hydroxyl radical, and possibly also singlet molecular oxygen, generated from ground‐state molecular oxygen by reactive s2Ura in both Type I and II reactions.