Cellular repair of unique DNA base damages produced by ionizing radiation

Escherichia coli endonuclease III and exonuclease III are postulated to repair X-ray-induced DNA damages. Endo III nicks X-irradiated DNA and DNA containing pyrmidine radiolysis products. Exo III is the major AP activity in E. coli and recognizes alkali-stable urea residues. The authors found that phiX-174 transfecting DNA containing thymine glycols or urea residues is inactivated at a greater rate in hosts deficient in endo III (nth) or exo III (xth), respectively than in wild type hosts. Thymine glycols are unique in the fact that they undergo SOS reactivation without concomitant mutagenesis. They found that thymine glycol-containing DNA is inactivated at the same rate in lex(Def) mutants as in wild type, in spite of the fact that SOS genes are fully derepressed in this strain. However, reactivation of thymine glycol-containing DNA does occur when lex(Def) mutants are UV-irradiated prior to transfection or when mutants with a constitutive RecA protease are used as hosts. In addition, no reactivation occurs when SOS-induced umuC mutants are used as hosts. These results suggest a requirement for RecA protease in the reactivation of thymine glycols in addition to its role in derepressing the SOS response and that the reactivation of thymine glycols is dependent on the combined action of RecA protease and UmuD,C gene products