Escherichia coli Double-Strand Uracil-DNA Glycosylase: Involvement in Uracil-Mediated DNA Base Excision Repair and Stimulation of Activity by Endonuclease IV

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Abstract
Escherichia coli double-strand uracil-DNA glycosylase (Dug) was purified to apparent homogeneity as both a native and recombinant protein. The molecular weight of recombinant Dug was 18 670, as determined by matrix-assisted laser desorption−ionization mass spectrometry. Dug was active on duplex oligonucleotides (34-mers) that contained site-specific U·G, U·A, ethenoC·G, and ethenoC·A targets; however, activity was not detected on DNA containing a T·G mispair or single-stranded DNA containing either a site-specific uracil or ethenoC residue. One of the distinctive characteristics of Dug was that the purified enzyme excised a near stoichiometric amount of uracil from U·G-containing oligonucleotide substrate. Electrophoretic mobility shift assays revealed that the lack of turnover was the result of strong binding by Dug to the reaction product apyrimidinic-site (AP) DNA. Addition of E. coli endonuclease IV stimulated Dug activity by enhancing the rate and extent of uracil excision by promoting dissociation of Dug from the AP·G-containing 34-mer. Catalytically active endonuclease IV was apparently required to mediate Dug turnover, since the addition of 5 mM EDTA mitigated the effect. Further support for this interpretation came from the observations that Dug preferentially bound 34-mer containing an AP·G target, while binding was not observed on a substrate incised 5‘ to the AP-site. We also investigated whether Dug could initiate a uracil-mediated base excision repair pathway in E. coli NR8052 cell extracts using M13mp2op14 DNA (form I) containing a site-specific U·G mispair. Analysis of reaction products revealed a time dependent appearance of repaired form I DNA; addition of purified Dug to the cell extract stimulated the rate of repair.