Visualization of Replisome Encounters with an Antigen Tagged Blocking Lesion

Considerable insight is present into the cellular response to double strand breaks (DSBs), induced by nucleases, radiation, and other DNA breakers. In part, this reflects the availability of methods for the identification of break sites, and characterization of factors recruited to DSBs at those sequences. However, DSBs also appear as intermediates during the processing of DNA adducts formed by compounds that do not directly cause breaks, and do not react at specific sequence sites. Consequently, for most of these agents, technologies that permit the analysis of binding interactions with response factors and repair proteins are unknown. For example, DNA interstrand crosslinks (ICLs) can provoke breaks following replication fork encounters. Although formed by drugs widely used as cancer chemotherapeutics, there has been no methodology for monitoring their interactions with replication proteins. Here, we describe our strategy for following the cellular response to fork collisions with these challenging adducts. We linked a steroid antigen to psoralen, which forms photoactivation dependent ICLs in nuclei of living cells. The ICLs were visualized by immunofluorescence against the antigen tag. The tag can also be a partner in the Proximity Ligation Assay (PLA) which reports the close association of two antigens. The PLA was exploited to distinguish proteins that were closely associated with the tagged ICLs from those that were not. It was possible to define replisome proteins that were retained after encounters with ICLs and identify others that were lost. This approach is applicable to any structure or DNA adduct that can be detected immunologically.