Recognition and repair of DNA damage by bacterial adenine glycosylases

E. coli MutY is a base excision repair (BER) glycosylase that excises adenine mispaired opposite 7,8-dihydro-8-oxo-2'-deoxyguanine. MutY has helix-hairpin-helix and Gly/Pro-Asp structural motifs characteristic of the BER glycosylase superfamily. In addition, it has an iron-sulfur cluster that is critical for substrate binding and catalytic activity. In the absence of DNA this cluster is not redox active; however, in the presence of DNA, its redox potential shifts dramatically. Here, the effects of DNA binding on the 4Fe-4S cluster of MutY are analyzed by two spectroscopic methods: Electron Paramagnetic Resonance (EPR) and X-ray Absorption Spectroscopy (XAS). By EPR, no difference in levels of cluster oxidation is observed when MutY is bound to a DNA oligonucleotide duplex containing either a G:C or G:THF central base pair, despite much tighter binding of the latter. EPR also shows that oxidation of the cluster is affected by the identity of the oxidant presented. XAS on MutY bound to DNA shows an increase in covalency of the Fe-S bonds relative to the cluster in the absence of DNA. MutY from a thermophilic bacterium B. stearothermophilus (BsMY) was expressed, purified, and characterized kinetically. Its glycosylase activity showed both temperature and salt dependence. In addition, BsMY discriminated more stringently than E. coli MutY between OG:A and G:A substrates.