UL44 sumoylation is stimulated by binding to DNA. As mentioned above, UL44 possesses a structural fold similar to that of the eukaryotic processivity factor PCNA [9] . In addition, like UL44, PCNA is SUMO-conjugated and its sumoylation involves both a consensus and a non-consensus site [4] . Since it has been shown that PCNA needs DNA to be sumoylated efficiently [59] , we wished to investigate whether UL44 might behave similarly and its sumoylation could be stimulated by the presence of DNA. Thus, we performed in vitro sumoylation experiments in the absence and in the presence of DNA using as a substrate a purified 6His-UL44 fusion protein treated with polymin P to eliminate residual bacterial nucleic acids. In a western blot analysis with an anti-UL44 antibody (Fig. 4A, left panel) , only a faint band corresponding to mono-sumoylated UL44 was observed in the absence of DNA (lane 2). Upon addition of dsDNA (e.g., activated calf thymus DNA) to the reaction mixture, a marked increase of the mono-sumoylated product and the appearance of bi-and trisumoylated forms of UL44 were observed (Fig. 4A , lane 4 of left panel). A western blot analysis with an anti-SUMO-1 antibody confirmed that these bands indeed contained SUMO-1 (data not shown). Similar results were obtained when GST-UL44 was used as a substrate (Fig. S7) . Thus, like PCNA sumoylation, UL44 sumoylation is strongly stimulated by the presence of DNA.