Photoreactivation of DNA by an Archaeal nucleoprotein Sso7d

Sso7d is a chromosomal protein of hyperthermophilic Archaea. The crystal structure of Sso7d-d(GTAAT_IUAC)₂ has been clarified at high resolution, showing that the protein binds in the minor groove of DNA, causing a sharp kink of ≈60°. Recently, we found that photoirradiation of Sso7d and 5-iodouracil-(¹U)-containing 5′-d(GTAAT ^IUAC)-3′ efficiently induced the abstraction of hydrogen from the methyl group of T₅ at the kink. In the present study, we found that the photoreactivity of 5-bromouracil (^BrU)-containing 5′-d(GTAAT^BrUAC)-3′ was enhanced in the presence of Sso7d. Using hypoxanthine (I)-containing 5′-d(ITAAT^BrUAC)- 3′, we demonstrated that electron transfer occurs efficiently from Sso7d to DNA. Product analysis showed that Trp-24 of Sso7d, located at the surface of the DNA, is consumed to produce N′-formylkynurenine during photoirradiation, indicating that Trp-24 acts as an electron source. To explore the possibility of electron transfer between Sso7d and other DMA substrates, we examined the photochemical repair of the thymine dimer 5′- d(GTAAT<>TAC)-3′ by Sso7d. Sso7d effectively repaired 5′-d(GTAAT<>TAC)-3′ to 5′-d(GTAATTAC)-3′ under irradiation conditions. During this reaction, Trp-24 was not oxidized significantly, indicating that the anion radical of the repaired TT sequence is oxidized by the cation radical of Trp-24, and that a so-called "circular electron transfer" mechanism is operating in this system.