Excision of Damaged Thymine Residues from Gamma-Irradiated Poly(dA-dT) by Crude Extracts of Escherichia coli

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Abstract
Crude extracts of E. coli endo I(-) and E. coli endo I(-)uvrA6(-) possess the ability to remove thymine products of the 5,6-dihydroxy-dihydrothymine type from gamma-irradiated or osmium tetroxide-oxidized poly(dA-dT). It is shown that the uvrA-gene product, which is responsible for incision close to photodimers in prereplication ultraviolet repair in E. coli, is not required for, but may aid in, the excision of gamma-ray products of the 5,6-dihydroxy-dihydrothymine type. Ring-damaged thymine products are also removed by E. coli extracts from osmium tetroxide-oxidized poly(dA-dT), which contains only 5,6-dihydroxy-dihydrothymine but no strand breakage, indicating that product excision occurs in the absence of radiation-induced breaks. On the average, 8 to 16 nucleotides are removed from the polymer per ring-damaged thymine residue excised by extracts from both strains and for gamma-irradiated and osmium tetroxide-oxidized polymer.