Synthesis and Excision Repair of Site-Specific 3'-End DNA-Histone Cross-Links Derived from Abasic Sites

Histones catalyze the DNA strand incision at apurinic/apyrimidinic (AP) sites accompanied by formation of reversible but long-lived DNA-protein cross-links (DPCs) at 3'-DNA termini within single-strand breaks. These DPCs need to be removed because 3'-hydroxyl is required for gap-filling DNA repair synthesis but are challenging to study because of their reversible nature. Here we report a chemical approach to synthesize stable and site-specific 3'-histone-DPCs and their repair by three nucleases, human AP endonuclease 1, tyrosyl-DNA phosphodiesterase 1, and three-prime repair exonuclease 1. Our method employs oxime ligation to install an alkyne to 3'-DNA terminus, genetic incorporation of an azidohomoalanine to histone H4 at a defined position, and click reaction to conjugate DNA to H4 site-specifically. Using these model DPC substrates, we demonstrated that the DPC repair efficiency is highly affected by the local protein environment, and prior DPC proteolysis facilitates the repair.