Initiation of base excision repair is modulated by nucleosome occupancy modifying sequences.

Nucleosome occupancy varies across the genome and plays a critical role in modulating DNA accessibility. While the effect of occupancy on gene expression has been studied, its influence on DNA repair, particularly base excision repair (BER), remains unexplored. In this work, we investigate the relationship between nucleosome occupancy and the initiation of BER by reconstituting nucleosome core particles (NCPs) using four DNA sequences known to modulate nucleosome occupancy in vivo. The results demonstrate that histone-DNA interactions differ significantly among these sequences. Moreover, uracil DNA glycosylase (UDG) activity is limited to solution-accessible uracil (U) lesion sites on NCPs containing the high occupancy sequences M4 and SB. In contrast, UDG displays high activity on NCPs containing the low occupancy sequences M2 and M3, even at less solution accessible lesion sites. In fact, for NCPs containing the sequence with the lowest occupancy, M2, UDG exhibits high activity regardless of the U lesion position. However, this high level of activity regardless of lesion position was not observed for thymine DNA glycosylase (TDG) and single-stranded monofunctional uracil DNA glycosylase 1 (SMUG1). Instead, the activity of TDG was dictated by the sequence flanking the U with a preference for 5'-UpG-3' and 5'-UpA-3' sequences, consistent with the role of TDG in epigenetic regulation. SMUG1 activity is high at many U sites but is severely hindered in the dyad region. These results highlight the interplay between nucleosome occupancy and BER, offering new insights into the dynamics of chromatin and DNA repair.