Binding and catalytic properties of Xenopus (6-4) photolyase

Xenopus (6-4) photolyase binds with high affinity to DNA bearing a (6-4) photoproduct and repairs it in a light-dependent reaction. To clarify its repair mechanism of (6-4) photolyase, we determined its binding and catalytic properties using synthetic DNA substrate which carries a photoproduct at a single location. The (6-4) photolyase binds to T[6-4]T in double-stranded DNA with high affinity (KD = 10(-9)) and to T[6-4]T in single-stranded DNA and T[Dewar]T in double- and single-stranded DNA although with slightly lower affinity (KD = approximately 2 x 10(-8)). Majority of the T[6-4]T-(6-4) photolyase complex dissociates very slowly (koff = 2.9 x 10(-5) s-1). Its absolute action spectrum without a second chromophore in the 350-600 nm region closely matches the absorption spectrum of the enzyme. The quantum yield (phi) of repair is approximately 0.11. The fully reduced form (E-FADH-) of (6-4) photolyase is catalytically active. Direct analysis of the photoreactivated product showed that (6-4) photolyase restores the original pyrimidines. These findings demonstrate that cis, syn-cyclobutane pyrimidine dimer photolyase and (6-4) photolyase are quite similar, but they are different with regard to the binding properties.