Effect of single mutations on the specificity of Escherichia coli FPG protein for excision of purine lesions from DNA damaged by free radicals

The formamidopyrimidine N-DNA glycosylase (Fpg protein) of Escherichia coli is a DNA repair enzyme that is specific for the removal of purine-derived lesions from DNA damaged by free radicals and other oxidative processes. We investigated the effect of single mutations on the specificity of this enz yme for three purine-derived lesions in DNA damaged by free radicals. These damaging agents generate a multiplicity of base products in DNA, with the yields depending on the damaging agent. Wild type Fpg protein (wt-Fpg) remo ves 8-hydroxyguanine (8-OH-Gua), 4,6-diamino-5-formamidopyrimidine (FapyAde ), and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) from damaged D NA with similar specificities. We generated five mutant forms of this enzym e with mutations involving Lys-57 --> Gly (FpgK57G), Lys-57 --> Arg (FpgK57 R), Lys-155 --> Ala (FpaK155A), Pro-2 --> Gly (FpgP2G), and Pro-2 --> Gln ( FpgP2E), and purified them to homogeneity. FpgK57G and FpgK57R were functio nal for removal of FapyAde and FapyGua with a reduced activity when compare d with wt-Fpa. The removal of 8-OH-Gua was different in that the specificit y of FpgK57G was significantly lower for its removal from irradiated DNA, w hereas wt-Fpg, FpgK57G, and FpgK57R excised 8-OH-Gua from H2O2/Fe(III)-EDTA /ascorbic acid-treated DNA with almost the same specificity. FpgK155A and F pgP2G had very low activity and FpgP2E exhibited no activity at all. Michae lis-Menten kinetics of excision was measured and kinetic constants were obt ained. The results indicate an important role of Lys-57 residue in the acti vity of Fpg protein for 8-OH-Gua, but a lesser significant role for formami dopyrimidines. Mutations involving Lys-155 and Pro-2 had a dramatic effect with Pro-2 --> Glu leading to complete loss of activity, indicating a signi ficant role of these residues. The results show that point mutations signif icantly change the specificity of Fpg protein and suggest that point mutati ons are also expected to change specificities of other DNA repair enzymes. (C) 2001 Elsevier Science Inc.