In vitro sumoylation assays. The assays to test in vitro sumoylation of UL44 by SUMO-1, -2, and -3 were performed using purified wild-type and mutant 6His-UL44 or GST-UL44 fusion proteins and the SUMOlink SUMO-1 kit from Active Motif or the SUMOylation Kit from Enzo Life Science according to the manufacturer's suggestions. In some experiments, double stranded (ds) DNA (e.g., activated calf thymus DNA from Amersham Pharmacia Biotec or salmon testes DNA from Sigma) or singlestranded (ss) DNA (e.g., single-stranded calf lung DNA, from Crinos, Como, Italy) was added to the reaction mixture at a final concentration of 500 nM.