Influence of cysteamine on intramolecular energy transfer in 5-bromouracil-substituted phage-DNA

SummaryWith the aid of an analytical ultracentrifuge, single- and double-strand breakage rates of 5-bromouracil (BrU)-substituted DNA of a defective B. subtilis phage (PBSH) were determined after irradiation with U.V. (302, 313 nm) in the presence and absence of cysteamine. The presence of cysteamine reduced the appearance of single-strand breaks (10 per cent) in the BrU-containing strands of hybrid and bifilarly-substituted DNA. The primary breaks lead to secondary breaks in the complementary thymine-containing strand of hybrid DNA or in the complementary BrU-containing strand of bifilarly-substituted DNA (intra-molecular energy transfer). Cysteamine strongly reduces (< 1 per cent) these secondary breaks, indicating that radical reactions are involved in the mechanism of intramolecular energy transfer.