Ribonucleotides Misincorporated into DNA Act as Strand-Discrimination Signals in Eukaryotic Mismatch Repair

Summary

To improve replication fidelity, mismatch repair (MMR) must detect non-Watson-Crick base pairs and direct their repair to the nascent DNA strand. Eukaryotic MMR in vitro requires pre-existing strand discontinuities for initiation; consequently, it has been postulated that MMR in vivo initiates at Okazaki fragment termini in the lagging strand and at nicks generated in the leading strand by the mismatch-activated MLH1/PMS2 endonuclease. We now show that a single ribonucleotide in the vicinity of a mismatch can act as an initiation site for MMR in human cell extracts and that MMR activation in this system is dependent on RNase H2. As loss of RNase H2 in S.cerevisiae results in a mild MMR defect that is reflected in increased mutagenesis, MMR in vivo might also initiate at RNase H2-generated nicks. We therefore propose that ribonucleotides misincoporated during DNA replication serve as physiological markers of the nascent DNA strand.

Abstract

Graphical Abstract

Highlights

► Ribonucleotides incorporated into eukaryotic DNA are not mismatch repair substrates ► Intermediates of rNMP processing can act as initiation sites for mismatch repair ► RNase H2 deficiency in S. cerevisiae decreases MMR efficiency ► rNMPs can act as markers of nascent DNA strands