Rotational and translational positions determine the structural and dynamic impact of a single ribonucleotide incorporated in the nucleosome.

Ribonucleotides misincorporated by replicative DNA polymerases are by far the most common DNA lesion. The presence of ribonucleotides in DNA is associated with genome instability, causing replication stress, chromosome fragility, gross chromosomal rearrangements, and other mutagenic events. Furthermore, nucleosome and chromatin assembly as well as nucleosome positioning are affected by the presence of ribonucleotides. Notably, nucleosome formation is significantly reduced by a single ribonucleotide. Single ribonucleotides are primarily removed from DNA by the ribonucleotide excision repair (RER) pathway via the RNase H2 enzyme, which incises the DNA backbone on the 5'-side of the ribonucleotide. While the structural implications of a single ribonucleotide in free duplex DNA have been well studied, how a single ribonucleotide embedded in nucleosomal DNA impacts nucleosome structure and dynamics, and the possible consequent impact on RER, have not been explored. We have carried out 3.5 μs molecular dynamics simulations of a single ribonucleotide incorporated at various translational and rotational positions in a nucleosome core particle. We find that the presence of the 2'-OH group on the ribose impacts the local conformation and dynamics of both the ribonucleotide and nearby DNA nucleotides as well as their interactions with histones; the nature of these disturbances depends on the rotational and translational setting, including whether the ribose faces toward or away from the histones. The ribonucleotide's preferred C3'-endo pucker is stabilized by interactions with the histones, and furthermore the ribonucleotide can cause dynamic local duplex disturbance involving an abnormal C3'-endo population of the adjacent deoxyribose pucker, minor groove opening, ruptured Watson-Crick pairing, and duplex unwinding that are governed by translation-dependent histone-nucleotide interactions. Possible effects of these disturbances on RER are considered.