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The Fpg protein of Escherichia coli is a DNA repair enzyme with DNA glycosylase, abasic site nicking, and deoxyribose excising activities. Analysis of the amino acid sequence of this protein suggests that the Fpg protein is a zinc finger protein with a Cys-X2-Cys-X16-Cys-X2-Cys motif. Competition experiments show that the Fpg protein substitutes Cu(II), Cd(II), and Hg(II), metal ions classically associated with substitutions in zinc finger proteins. The Fpg protein activities are inhibited following the reaction with a Cys-specific reagent at low protein:reagent ratios, suggesting that these residues are important for the enzymatic activities. Site-directed mutagenesis was used to produce 6 mutant Fpg proteins with Cys-->Gly mutations. Substitution of the zinc in these proteins by 65Zn(II) indicates that all the proteins bind zinc, but the Zn(II) is not retained as strongly in the zinc finger mutants. The mutations in the Fpg protein outside the zinc finger consensus sequence do not eliminate the Fapy-DNA glycosylase and abasic site nicking. One of the Fpg mutant proteins outside the zinc finger has a reduced capacity to release deoxyribose from abasic sites. Cys-->Gly mutations in the zinc finger consensus sequence reduce all three aforementioned activities substantially. The purified Fpg proteins with Cys-->Gly mutations in the zinc finger consensus sequence do not incise DNA at abasic sites with the same efficiency nor mechanism as the native Fpg protein. The wild type Fpg protein and the Fpg proteins mutated outside the zinc finger sequence bind an oligonucleotide with a unique chemically reduced abasic site in a defined sequence as assayed by retention on nitrocellulose filters, whereas the mutant Fpg proteins within the zinc finger sequence do not bind to the same oligonucleotide. Therefore, the disruption of zinc coordination in the zinc finger of the Fpg protein is associated with decreased binding capacity to DNA as well as decreased enzymatic activities.

Alkylkation of the N7 of guanine residues in DNA favours the opening of the imidazole ring, yielding a formamidopyrimidine (Fapy). This Fapy residue blocks DNA replication and is actively excised by a DNA glycosylase. We have cloned and sequenced the Escherichia coli gene responsible for synthesis of the enzyme, which has also been purified to homogeneity. It was found to have associated apurinic/apyrimidinic (AP) lyase activity, nicking DNA at AP sites. Chloroethylnitrosoureas are used in cancer chemotherapy. The lesions induced in DNA by these compounds, including N7-chloro- and hydroxyethylguanine, are excised by E. coli 3-methyladenine DNA glycosylase II, and we report that the corresponding imidazole ring-opened forms are repaired by Fapy-DNA glycosylase. Human cells have the counterpart to these enzymes, which could contribute to the repair of these lesions during chemotherapy.

The repair of 2,6-diamino-4-hydroxy-5-N-methyl-formamidopyrimidine (Fapy) residues in DNA is performed by a Fapy-DNA glycosylase activity which is encoded for by the fpg gene in Escherichia coli. Besides DNA glycosylase activity, this protein, the FPG protein, is endowed with an EDTA-resistant activity nicking DNA at apurinic/apyrimidinic (AP) sites. To overproduce the FPG protein, the fpg gene was placed under the control of the tac promoter in the expression vector pKK223-3 yielding the pFPG230 plasmid. The production of the FPG protein in cells harboring the pFPG230 plasmid was 800-fold higher than that of the wild type strain after induction by isopropyl-beta-D-thio-galactopyranoside. From these cells, the FPG protein was purified to homogeneity in sufficient quantity to study its physical and catalytic properties. In its active form, the FPG protein is a globular monomer of 31 kDa and has an experimentally measured isoelectric point of 8.5. When the FPG protein is heat-denatured in the presence of EDTA the two activities are more rapidly inactivated than when heated in the absence of EDTA, suggesting that the FPG protein possesses a tightly bound metal ion. Atomic absorption spectrophotometric analysis shows that there is one zinc/FPG protein molecule. The FPG protein is different from previously described DNA glycosylases and AP-nicking enzymes in E. coli. The contribution of the AP-nicking activity associated with the FPG protein represents 10-20% of the total EDTA-resistant AP-nicking activities in E. coli.

The 2,6-diamino-4-hydroxy-5N-formamidopyrimidine (Fapy)-DNA glycosylase of Escherichia coli, which is coded for by the fpg gene, excises purine bases with ring-opened imidazoles. In addition to the DNA glycosylase activity, we report that the Fapy-DNA glycosylase of E. coli has an associated activity, resistant to EDTA, that nicks DNA at apurinic/apyrimidinic (AP) sites. The levels of Fapy-DNA glycosylase and AP-nicking activity were parallel in crude lysates of E. coli HB101 harboring different plasmids constructed from the fpg gene. The fpg gene is different from the xth, nth, and nfo genes of E. coli, whose gene products also cleave DNA at AP sites. The Fapy-DNA glycosylase was purified to electrophoretic homogeneity. During this purification, the Fapy-DNA glycosylase copurified with an AP-nicking activity using chromatographic separations based on ion-exchange, molecular weight exclusion, and hydrophobicity. The cleavage at AP sites by the Fapy-DNA glycosylase left a 5'-phosphomonoester nucleotide at one terminus. In addition, DNA containing reduced AP sites was not nicked by the Fapy-DNA glycosylase. These data suggest that the mechanism of cleavage involved beta elimination. Therefore, this activity of the Fapy-DNA glycosylase nicking DNA at AP sites should be referred to as an AP lyase. The 3' terminus did not prime nick-translation by E. coli DNA polymerase I. However, the 3' terminus becomes a substrate for nick-translation if first allowed to react with calf intestine phosphatase or the E. coli exonuclease III. These data suggest that the repair of the Fapy lesion at least to some extent results in the formation of both 5'- and 3'-phosphomonoester nucleotides and the release of the deoxyribose.

The formamidopyrimidine-DNA glycosylase (Fapy-DNA glycosylase) of Escherichia coli (E. coli) was overexpressed by cloning the fpg+ gene on a multicopy plasmid and placing this gene under the control of the lac promoter. The lac promoter contributed significantly to the overall expression of the fpg gene only after the deletion of an inverted repeat sequence located immediately upstream from the fpg promoter. The biological purpose of the inverted repeat sequence may be associated with the termination of an adjacent gene transcribed in the same direction as the fpg gene in E. coli. Cells harboring the fpg gene under the control of the lac promoter were able to produce the Fapy-DNA glycosylase as at least 17% of the total soluble proteins. Such strains allow the preparation of milligram quantities of pure protein for use in the study of its catalytic properties and three dimensional crystal structure.

An Escherichia coli genomic library composed of large DNA fragments (10-15 kb) was constructed using the plasmid pBR322 as vector. From it 700 clones were individually screened for increased excision of the ring-opened form of N7-methylguanine (2-6-diamino-4-hydroxy-5N-methyl-formamidopyrimidine) or Fapy. One clone overproduced the Fapy-DNA glycosylase activity by a factor of 10-fold as compared with the wild-type strain. The Fapy-DNA glycosylase overproducer character was associated with a 15-kb recombinant plasmid (pFPG10). After subcloning a 1.4-kb fragment which contained the Fapy-DNA glycosylase gene (fpg+) was inserted in the plasmids pUC18 and pUC19 yielding pFPG50 and pFPG60 respectively. The cells harbouring pFPG60 displayed a 50- to 100-fold increase in glycosylase activity and overexpressed a 31-kd protein. From these cells the Fapy-DNA glycosylase was purified to apparent physical homogeneity as evidenced by a single protein band at 31 kd on SDS-polyacrylamide gels. The amino acid composition of the protein and the amino acid sequence deduced from the nucleotide sequence demonstrate that the cloned fragment contains the structural gene coding for the Fapy-DNA glycosylase. The nucleotide sequence of the fpg gene is composed of 809 base pairs and codes for a protein of 269 amino acids with a calculated mol. wt of 30.2 kd.