5-Formylcytosine does not change the global structure of DNA

The mechanism by which 5-formylcytosine (fC) is recognised by enzymes involved in epigenetic modification and reading of DNA is not known, and recently an unusual DNA structure (F-DNA) was proposed as the basis for enzyme recognition of clusters of fC. We used NMR and X-ray crystallography to compare several modified DNA duplexes with the unmodified analogues and show that in the crystal state they all belong to the A-family, but in solution they are all members of the B-family. Contrary to the previous study, we find that 5-formylcytosine does not significantly affect the structure of DNA, though there are modest local differences at the modification sites. Hence, global conformation changes are unlikely to account for the recognition of this modified base, and our structural data favour a mechanism that operates at base-pair resolution for the recognition of 5-formylcytosine by epigenome-modifying enzymes