Effect of 5-bromodeoxyuridine on deoxyribonucleic acid-protein adducts induced by ultraviolet light on chromatin cells.

The formation of DNA-protein adducts induced by ultraviolet irradiation (254-nm. wavelength) has been analyzed by cesium chloride equilibrium sedimentation. The formation of ultraviolet-induced DNA-protein adducts is increased in 5-bromodeoxyuridine-substituted chromatin. Cross-linking is dependent in part upon the extent of 5-bromodeoxyuridine substitution, and is detectable either in chromatin irradiated in vitro or in chromatin from cells irradiated in vivo prior to the isolation of chromatin. As much as 80\% of the DNA can be cross-linked to proteins by ultraviolet irradiation of 5-bromodeoxyuridine-substituted chromatin at a fluence of 2,928 Jm-2. In unsubstituted chromatin 11,712 Jm-2 are required to obtain to obtain the same effect. Under the same conditions of 5-bromodeoxyuridine replacement and ultraviolet irradiation only 10\% of the total chromosomal proteins can be cross-linked to DNA, whether in vivo or in vitro. Approximately two-thirds of the cross-linked proteins chromatograph as nonhistone proteins, the remaining one-third as histones.