Multiple DNA repair activities for 3'-deoxyribose fragments inEscherichia coli

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Abstract
Eschericha coli contains multiple enzymes that hydrolyze deoxyribose fragments (phosphoglycolaldehyde, PGA) from the 3'' termini of a synthetic DNA substrate. The major such activities are the main bacterial apurinic endonucleases, exonuclease III and endonuclease IV. In a double mutant deficient in both of these oxidation repair enzymes, Mg++-dependent 3''-PGA diesterase was detected at 3% the level found in wild-type bacteria. Gel filtration fractionated this residual diesterase activity into two peaks of Mr 40,000-52,000 (Pool A) and Mr 22,00-30,00 (Pool B) with differing abilities to remove 3''-phosphates from DNA. These multiple repair activities were resolved in 3''-PGA diesterase activity gels. The exonuclease III and endonuclease IV bands were identified using the purified proteins and by their specific absence from strains defective for the respective structural genes. Gel filtration Pool B yielded two activity bands of apparent Mr 25,000 and 28,000, but Pool A did not form a new band in the activity gels. Incubation of activity gels in different transition metals or boiling of the samples before electrophoresis also served to distinguish the various activities. The possible identities of the novel E. coli 3''-PGA diesterases and the importance of multiple repair enzymes for 3'' damages are discussed.