Figure 6.

Automodification of PARP-1 on SSBs Occurs In cis

Active WT or inactive mutant (E988K) versions were made for both normal and N-terminal SUMO-tagged (SMT) variants of PARP-1 (distinguishable on SDS-PAGE). On mixing active and inactive versions with different tags in the presence of an SSB mimic (DB4; Supplemental Experimental Procedures), only the active WT is automodified (slower-migrating smeared band), indicating selective in cis modification of this type of DNA damage. In contrast, both active and inactive PARP-1 molecules are modified in the presence of a DNA duplex, which allows binding on both DNA ends. Blocking one end of the duplex with a hairpin restores selective in cis modification.