The photolability of DNA containing 5-bromouracil-I. Single-strand breaks and alkali-labile bonds

Abstract— The proportions of single‐strand breaks and alkali‐labile bonds produced by UV‐light were investigated in covalently‐closed circular 5‐bromouracil (BrUra)‐containing λ‐phage DNA. When BrUra DNA was irradiated in 001 M Tris‐0–001 M EDTA (pH 8‐1) buffer, the D0 was 11‐7 J/m2 for single‐strand breaks, 2–25 J/m2 for total breaks, and 2–8 J/m2 for alkali‐labile bonds. Thus, alkali‐labile bonds were the predominant photochemical products. No double‐strand breaks were observed after exposure to 7‐7 times the D0 for neutral breakage. The photolability measured under both neutral and alkaline conditions was affected by the NaCl concentration in the irradiation solvent, with the greatest resistance to breakage exhibited at the lowest concentrations. The composition of the irradiation buffer also affected sensitivity. Exposure in 1/10 SSC yielded 4‐4 (neutral) and 5–7 (alkaline) times the breakage produced in Tris‐EDTA.