Sequence-specific DNA cleavage by Fe2+-mediated fenton reactions has possible biological implications

Preferential cleavage sites have been determined for Fe2+/H2O2-mediated oxi dations of DNA. In 50 mM H2O2, preferential cleavages occurred at the nucle oside 5' to each of the dG moieties in the sequence RGGG, a sequence found in a majority of telomere repeats. Within a plasmid containing a (TTAGGG)(8 1) human telomere insert, 7-fold more strand breakage occurred in the restr iction fragment with the insert than in a similar-sized control fragment. T his result implies that telomeric DNA could protect coding DNA from oxidati ve damage and might also link oxidative damage and iron load to telomere sh ortening and aging. In micromolar H2O2, preferential cleavage occurred at t he thymidine within the sequence RTGR, a sequence frequently found to be re quired in promoters for normal responses of many procaryotic and eucaryotic genes to iron or oxygen stress. Computer modeling of the interaction of Fe 2+ with RTGR in B-DNA suggests that due to steric hindrance with the thymin e methyl, Fe2+ associates in a specific manner with the thymine flipped out from the base stack so as to allow an octahedrally-oriented coordination o f the Fe2+ with the three purine N-7 residues. Fe2+-dependent changes in NM R spectra of duplex oligonucleotides containing ATGA versus those containin g AUGA or A(5m)CGA were consistent with this model.