Excision of 8-oxoguanine within clustered damage by the yeast OGG1 protein

Clustered damages are formed in DNA by ionising radiation and radiomimetic anticancer agents and are thought to be biologically severe. 7,8-dihydro-8- oxoguanine (8-oxoG), a major DNA damage resulting from oxidative attack, is highly mutagenic leading to a high level of G .C-->T .A transversions if n ot previously excised by OGG1 DNA glycosylase/AP lyase proteins in eukaryot es. However, 8-oxoG within clustered DNA damage may present a challenge to the repair machinery of the cell. The ability of yeast OGG1 to excise 8-oxo G was determined when another type of damage [dihydrothymine, uracil, 8-oxo G, abasic (AP) site or various types of single-strand breaks (SSBs)l] is pr esent on the complementary strand 1, 3 or 5 bases 5' or 3' opposite to 8-ox oG. Base damages have little or no influence on the excision of 8-oxoG by y east OGG1 (yOGG1) whereas an AP site has a strong inhibitory effect, Variou s types of SSBs, obtained using either oligonucleotides with 3'- and 5'-pho sphate termini around a gap or through conversion of an AP site with either endonuclease III or human AP endonuclease 1, strongly inhibit excision of 8-oxoG by yOGG1. Therefore, this large inhibitory effect of an AP site or a SSB may minimise the probability of formation of a double-strand break in the processing of 8-oxoG within clustered damages.