Accommodation of an N-(Deoxyguanosin-8-yl)-2-acetylaminofluorene adduct in the active site of human DNA polymerase hoogsteen or watson-crick base pairing?

Bypass across DNA lesions by specialized polymerases is essential for maintenance of genomic tability. Human DNA polymerase ι (polι) is a bypass polymerase of the Y family. Crystal structures of polι suggest that Hoogsteen base pairing is employed to bypass minor groove DNA lesions, placing them on the spacious major groove side of the enzyme. Primer extension studies have shown that poll is also capable of error-free nucleotide incorporation opposite the bulky major groove adduct N-(deoxyguanosin- 8-yl)-2- acetylaminofluorene (dG-AAF). We present molecular dynamics simulations and free energy calculations suggesting that Watson-Crick base pairing could be employed in poll for bypass of dG-AAF. In polι with Hoogsteen-paired dG-AAF the bulky AAF moiety would reside on the cramped minor groove side of the template. The Hoogsteen-capable conformation distorts the active site, disrupting interactions necessary for error-free incorporation of dC opposite the lesion. Watson-Crick pairing places the AAF rings on the spacious major groove side, similar to the position of minor groove adducts observed with Hoogsteen pairing. Watson-Crick-paired structures show a well-ordered active site, with a near reaction-ready ternary complex. Thus our results suggest that polι would utilize the same spacious region for lesion bypass of both major and minor groove adducts. Therefore, purine adducts with bulk on the minor groove side would use Hoogsteen pairing, while adducts with the bulky lesion on the major groove side would utilize Watson-Crick base pairing as indicated by our MD simulations for dG-AAF. This suggests the possibility of an expanded role for polι in lesion bypass.