Sso7d is a chromosomal protein of hyperthermophilic Archaea. The crystal structure of Sso7d-d(GTAAT(Br)UAC)(2) has been clarified at high resolution, showing that the protein binds in the minor groove of DNA, causing a sharp kink of about 60 degrees . We demonstrated that electron transfer occurs efficiently from Sso7d to DNA by using inosine (I)-containing 5'-d(ITAAT(Br)UAC)-3'. Product analysis showed that Trp24 of Sso7d, located at the surface of the DNA, is consumed to produce N'-formylkynurenine during photoirradiation, indicating that Trp24 acts as an electron source. To explore the possibility of electron transfer between Sso7d and other DNA 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.
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