Chemical modification of human EndoG. EndoG (0.2 pmol) was mixed with various amounts of DPEC and incubated at 25°C for 30 min. The residual activity was analyzed by standard assay conditions (lanes 1 to 4). An assay for restoration with hydroxylamine was performed by incubating 20 mM hydroxylamine and DEPC-treated EndoG together at 4°C for 5 h. The residual activity was then analyzed (lanes 6 to 9). Lane 5 represents the reaction performed in the absence of EndoG. Arrowheads denote the different topological forms of pUC18 plasmids. OC: open circular; L: linear; SC: supercoiled.