Repair of the (6-4) Photoproduct by DNA Photolyase Requires Two Photons

摘要

Ultraviolet (UV) irradiation of DNA induces two major types of harmful cross-links between adjacent pyrimidine bases (see Scheme 1): cyclobutane pyrimidine dimers (CPD) and pyrimidine(6-4)pyrimidone photoproducts ((6-4)PPs; denoted T(6-4)T in the case of two thymines). While CPDs are formed directly by [2+2] cycloaddition, formation of the (6-4)pp proceeds through an oxetane intermediate (or azetidine in the case of cytosine as the 3' base). In many organisms, these lesions are repaired by DNA photolyases (PL)-flavoenzymes using blue or near UV light for their catalytic action. For CPD repair, it has been established that the transfer of an electron from the photo-excited fully reduced flavin cofactor (FADFT) to the lesion induces the cleavage of the intradimer bonds and the restoration of the intact pyrimidines in approximately one nanosecond, including return of the excess electron to the flavin cofactor (Scheme 1, top inset). Repair of the (6-4)PP by (6-4) photolyase is far less understood. It is chemically more challenging than repair of the CPD because of the requirement to transfer a functional group (OH in the case T(6-4)T) from the 5' to the 3' base, in addition to intradimer bond cleavage. Remarkably, the quantum yield of repair of the (6-4)PP (approximately 3-11 %) is much lower than that of the CPD (approximately 50-100 %). According to a recent study using repetitive femtosecond flash excitation, excited FADH~-transfers an electron to the (6-4)PP in 225 ps; the excess electron is either returned to the flavin in 50 ps without repair of the (6-4)PP, or repair takes place and the excess electron supposedly returns to the flavin in more than 10 ns.