Formation and repair of mismatches containing ribonucleotides and oxidized bases at repeated DNA sequences [Enzymology]

September 3rd, 2015 by Cilli, P., Minoprio, A., Bossa, C., Bignami, M., Mazzei, F.

The cellular pool of ribonucleotide triphosphates (rNTPs) is higher than that of deoxyribonucleotide triphosphates. To ensure genome stability, DNA polymerases must discriminate against rNTPs and incorporated ribonucleotides must be removed by ribonucleotide excision repair (RER). We investigated DNA polymerase beta (POL beta) capacity to incorporate ribonucleotides into trinucleotide repeated DNA sequences and the efficiency of base excision repair (BER) and RER enzymes (OGG1, MUTYH, and RNase H2) when presented with an incorrect sugar and an oxidized base. POL beta incorporated rAMP and rCMP opposite 7,8-dihydro-8-oxoguanine (8-oxodG) and extended both mispairs. In addition POL beta was able to insert and elongate an oxidized rGMP when paired with dA. We show that RNAse H2 always preserves the capacity to remove a single ribonucleotide when paired to an oxidized base or to incise an oxidized ribonucleotide in a DNA duplex. In contrast BER activity is affected by the presence of a ribonucleotide opposite an 8-oxodG. In particular MUTYH activity on 8-oxodG:rA mispairs is fully inhibited, although its binding capacity is retained. This results in the reduction of RNAse H2 incision capability of this substrate. Thus complex mispairs formed by an oxidized base and a ribonucleotide can compromise BER and RER in repeated sequences.