A comparative repair study of thymine- and uracil-photodimers with model compounds and a photolyase repair enzyme

Cyclobutane uridine and thymidine dimers with cis-syn-structure are DNA les ions, which are efficiently repaired in many species by DNA photolyases. Th e essential step of the repair reaction is a light driven electron transfer from a reduced FAD cofactor (FADH(-)) to the dimer lesion, which splits sp ontaneously into the monomers. Repair studies with UV-light damaged DNA. re vealed significant rate differences for the various dimer lesions. In parti cular the effect of the almost eclipsed positioned methyl groups at the thy midine cyclobutane dimer moiety on the splitting rates is unknown. In order to investigate the cleavage vulnerability of thymine and uracil cyclobutan e photodimers outside the protein environment, two model compounds, contain ing a thymine or a uracil dimer and a covalently connected flavin, were pre pared and comparatively investigated. Cleavage investigations under interna l competition conditions revealed, in contrast to all previous findings, fa ster repair of the sterically less encumbered uracil dimer, Stereoelectroni c effects are offered as a possible explanation, Ab initio calculations and X-ray crystal structure data reveal a different cyclobutane ring pucker of the uracil dimer, which leads to a better overlap of the pi*-C(4)-O(4)-orb ital with the sigma*-C(5)-C(5')-orbital. Enzymatic studies with a DNA photo lyase (A, nidulans) and oligonucleotides, which contain either a uridine or a thymidine dimer analogue, showed comparable repair efficiencies for both dimer lesions. Under internal competition conditions significantly faster repair of uridine dimers is observed.