Repair Replication in Escherichia coli As Measured by the Photolysis of Bromodeoxyuridine

The ability to selectively photolyze bromouracil-(BrUra-)containing repaired regions in cellular DNA has allowed us to estimate the average size of repaired regions in ultraviolet (UV) light-irradiated Escherichia coli. Cells were labeled with thymidine-³H, irradiated at 254 nm, and incubated in nonradioactive bromodeoxyuridine (BrdUrd). After incubation the cells were exposed to 10⁶ ergs·mm^-2 at 313 nm, lysed, and sedimented in alkaline sucrose gradients so as to measure the average molecular weight of single DNA strands. In strains that had excised ~45 cyclobutane pyrimidine dimers/10⁸ daltons, the 313 nm treatment resulted in ~6 single-strand breaks/10⁸ daltons. In an excisionless strain, the same treatment resulted in only 1.5 breaks/10⁸ daltons. From the determination of the sensitivities of fully substituted DNAs to 313 nm light, we calculate that the repaired regions in excising strains of E. coli contain an average of 4-6 BrUra residues. Photoreactivation experiments indicate that the excision of pyrimidine dimers in the presence of BrdUrd is the primary source of repaired regions selectively photolyzed by 313 nm radiation.