The Nature of Ultraviolet Light‐induced Strand Breakage in Dna Containing Bromouracil

Single‐strand breaks are produced in the phosphodiester backbone of ultraviolet‐light‐irradiated 5–bromodeoxyuridine‐containing DNA (BU‐DNA) after treatment with alkali. No radiation dependent breakage is observed in thymine‐containing DNA (thy‐DNA). The relative yields of breaks terminated by 5′‐hydroxyl and 5′‐phosphate groups was determined by measuring the rate of phosphorylation achieved with polynucleotide kinase in BU‐DNA single strands before and after treatment with alkaline phosphatase. The ratio of 5′‐phosphate to 5′ hydroxyl groups ranged from 2.3 to 2.9 in different experiments. When cysteamine was present during irradiation no new end groups were produced. In order to identify the nucleoside(s) at the 5′‐termini, phosphate groups were removed with alkaline phosphatase and the 5′‐hydroxyl groups were phosphorylated with polynucleotide kinase. Electrophoresis of enzymatic digests showed a single 32P‐labeled component migrating more rapidly than any of the four usual 5′‐mononucleotides. Upon column chromatography this component resolved into a major peak coincident with 5′‐dUMP and a lesser unidentified constituent. No 5′‐dBUM32P was observed among these nucleotides.