The uranyl(IV) ion (UO2 2+) binds strongly to the phosphates of DNA and, upon irradiation with long wavelength ultraviolet light, the proximal deoxyn-boses are oxidized by the photochemically excited state of the uranyl1 ion, a very strong oxidant (1). Thus the uranyl ion is an efficient DNA photocleavage reagent (2,3) that has been used to study the sequence specific interaction with the phosphates of the DNA backbone of the respective DNA recogintion sites of various proteins, such as the λ-repressor/OR1 complex (3), RNA polymerase-deoP1 promoter of Escherzchza coli (4), RNA polymerase/cyclic AMP receptor protein (CRP)-deoP2 promoter of E coli (5), CRP/CytR repressor-deoP2 promoter of E. colz (5), and transcription factor IIIA/Xenopus 5s internal control region (6).