Conformation-dependent formation of the G[8-5]U intrastrand cross-link in 5-bromouracil-containing G-quadruplex DNA induced by UVA irradiation.

G-quadruplex motifs are known to be present in telomeres of human and other organisms. Recent bioinformatic studies also revealed the widespread existence of these motifs in promoter regions of human genes. Treatment of cultured cells with 5-bromo-2'-deoxyuridine ((Br)dU) is known to result in the substitution of DNA thymidine with (Br)dU; such replacement has been shown to sensitize cells to killing induced by UV light. Our previous studies revealed that the exposure of (Br)dU-carrying duplex DNA or (Br)dU-treated MCF-7 cells to UVB light could lead to the facile formation of intrastrand cross-link products initiated from (Br)dU. Here we found that the exposure of (Br)dU-bearing G-quadruplex DNA to UVA light could also give rise to the efficient formation of the G[8-5]U intrastrand cross-link, where the C8 of guanine in the external G-tetrad is covalently linked with the C5 of its adjacent 3' uracil in the loop region. In addition, the yield for the cross-link product is dependent on the conformation of the G-quadruplex. Together, the formation of intrastrand cross-link in G-quadruplex motifs may account for the photocytotoxic effect induced by (Br)dU incorporation, and the (Br)dU-mediated photo-cross-linking may constitute a useful method for monitoring the different conformations of G-quadruplex folding.