Fig. 5. Schematic representation of three main pathways for the repair of Top1 covalent complexes.
A. 5′-end-religation requires realignment of the 5′-hydroxyl with the end of the broken DNA bonded to Top1. This would require a “pull-back” (“regression”) of the replication or transcription complexes (see Figs. 4B and 4C). Fork regression and restart require helicase activities (in particular RecQ helicases such as Sgs-1 in budding yeast and BLM or WRN in humans) in association with Top3. B. Top1 excision by Tdp1 requires prior proteolysis of Top1 [106] or denaturation of Top1 [105] to expose the phosphotyrosyl bond to be attacked (see Fig. 6A–C). Tdp1 generates a 3′-phosphate DNA end, which needs to be hydrolyzed by polynucleotide kinase phosphatase (PNKP = hPNK). PNKP also catalyzes the phosphorylation of the 5′-end of the DNA. Tdp1 and PNKP are part of the XRCC1 complex (shown at the bottom). C. Excision of the Top1-DNA covalent complex by 3′-endonucleases. Studies in budding yeast have implicated at least 3 endonuclease families. The human orthologs are listed: Rad1/Rad10, Mus81/Eme1 and Mre11/Rad50. The resulting DNA lesion is probably processed by homologous recombination initiated by the Rad51, Rad52 complexes and by non-homologous end joining (Ku-DNA-PK pathway).