Substrate Dependence of the Action Spectrum for Photoenzymatic Repair of Dna

The absolute action spectrum has been determined for photoenzymatic splitting of cyclobutadipyrimidines (“pyrimidine dimers”) from natural DNA, and from the synthetic polydeoxyribonucleotides poly(dA)·poly(dT) (forming only cyclobutadithymine) and poly(dG)·poly(dC) (forming only cyclobutadicytosine). These action spectra differ strikingly from each other, even when using the same enzyme preparations. On the other hand, the action spectrum for splitting cyclobutadithymine in natural DNA containing “dimers” of only this one type closely resembles the action spectrum for splitting the total mixture of “dimer” types in natural DNA, and is entirely different from the spectrum for splitting of the same photoproduct from poly(dA)·poly(dT). These results mean that the action spectrum is not simply the absorption spectrum of a chromophore carried by the photoreactivating enzyme, nor is it solely determined by the nature of the substrate photoproduct. It is at least partly determined by the over‐all polynueleotide structure (viz. exact helical dimensions, pattern of neighboring bases to the “dimers,” or both), affecting a ground state interaction between the enzyme and substrate in the enzyme‐substrate complex.