A: Upper panel: Strategy for the identification of oxidation resistant Ubc9 variants. Ubc9 was mutagenized by PCR under conditions that introduced 1 - 3 random mutations. Ubc9 variants were expressed in bacteria and the bacterial lysates of individual clones tested for E2 activity in the presence or absence of 1 mM H2O2. For this purpose, we used a FRET-based in vitro SUMOylation assay with recombinant SUMO E1, YFP-SUMO and CFP-RanGAPtail and ATP. Lower panel: Ubc9 W103R is H2O2 resistant. Bacterial lysates containing Ubc9 wt (left panel) or Ubc9 W103R (right panel) were tested as described. 