Postreplication Repair in an Excision‐defective Mutant of Escherichia Coli: Ultraviolet Light‐induced Incorporation of Bromodeoxyuridine Into Parental Dna*

Ultraviolet (UV) light‐induced incorporation of bromodeoxyuridine (BrdUrd) into parental DNA of an excision‐defective mutant of Escherichia coli has been observed by selective photolysis of bromouracil (BrUra)‐containing regions in the parental DNA. It appears that the BrUra‐containing regions occur only in that DNA which has served as a template for normal semiconservative replication. After an exposure at 254 nm which results in one pyrimidine dimer per 45times 106 daltons, incubation in BrdUrd resulted in BrUra–containing regions ˜ 1.5 times 104 nucleotides in length at intervals of ˜ 55 times 106 daltons in the parental DNA. Thus approximately one BrUra‐containing region has occurred for every 1.2 pyrimidine dimers in the parental DNA. The observed incorporation of BrdUrd is interpreted in terms of a proposed model for postreplication repair in which genetic exchanges produce single‐strand gaps in the parental DNA.