A Spontaneous Ring‐Opening Reaction Leads to a Repair‐Resistant Thymine Oxidation Product in Genomic DNA

Abstract

The alphabet of modified DNA bases goes beyondt he conventional four letters, with biological roles being found for many such modifications. Herein, we describe the observation of a modified thymine base that arises from spontaneous N 1 ÀC 2 ring openingo ft he oxidation product 5-formyl uracil, after N 3 deprotonation.W ef irst observed this phenomenoni ns ilico through ab initio calculations, followed by in vitro experiments to verify its formation at am ononucleoside level and in as ynthetic DNA oligonucleotide context. We show that the new base modification (T rex , thymine ring expunged) can form under physiological conditions, and is resistant to the action of commonr epair machineries. Furthermore, we found cases of the natural existence of T rex while screening an umber of human cell types and mESC (E14), thus suggesting potential biological relevance of this modification.