Regulation of Activity of DNA Polymerases β and λ by XRCC1, PARP1, and PARP2 Proteins

DNA damage repair by the base excision repair (BER) mechanism is a complex, multistage process that requires precise coordination and regulation of activities of enzymes at each step of DNA repair. The central stage in this process is DNA synthesis in the gap formed by removal of the damaged nucleotide. DNA polymerases β (Pol β) and λ (Pol λ) of the X family possess all properties necessary for the DNA repair synthesis. Pol β is the major enzyme in DNA synthesis in BER, which may be due to the influence of other BER factors regulating its activity. The scaffold protein XRCC1 and poly(ADP-ribose) polymerases 1 and 2 (PARP1 and PARP2) are essential for the formation of functional BER complexes. We investigated the effect of XRCC1, PARP1, PARP2, and poly(ADP-ribosyl)ation on the activity of Pol β and Pol λ in the single-nucleotide gap filling reaction in BER. XRCC1 stimulated the activity of Pol λ to a significantly greater extent than the Pol β activity. PARP1 and PARP2 inhibited both DNA polymerases; the inhibitory effect of PARP1 was more pronounced on DNA with a tetrahydrofuran-phosphate group at the 5' end of the gap, while PARP2 effect was more significant on DNA containing a 5' phosphate group. XRCC1 partially restored the DNA polymerase activity, especially that of Pol β. It can be assumed that the XRCC1-Pol β complex competes with PARP1/2 for DNA binding more strongly than the XRCC1-Pol λ complex. Addition of NAD⁺ to PARP2 led to a more efficient restoration of Pol β (but not Pol λ) activity. These differences may be one of the causes why Pol β is the main DNA polymerase in BER.