Role of Translesion Synthesis (TLS) polymerases in rNMPs incorporation during DNA replication

Ribonuclease H (RNase H) are evolutionary conserved enzymes that cleave the RNA moiety in RNA:DNA hybrid molecules. It has been found that replicative DNA polymerases can incorporate rNTPs in place of dNTPs during DNA replication with an unexpected high frequency (McElhinny et al., 2010a; 2010b). The high rate of rNTPs mis-incorporation observed under normal conditions suggests physiological functions for rNMPs in newly replicated DNA. I was recently demonstrated that the incorporation of rNMPs during leading strand DNA synthesis acts as a strand discrimination signal for the Mismatch DNA repair machinery (Ghodgaonkar et al., 2013; Lujan et al., 2013); moreover, rNMPs embedded in chromosomal DNA can represent an imprint, positioned in S-phase, that regulates DNA transactions (Dalgaard, 2012). RNase H enzymes are crucial for the removal of these rNMPs from genomic DNA and for the maintenance of chromosome integrity. Recently we have found that impairment of RNase H activity in yeast and human causes rNMPs accumulation in the genome and chronic activation of the post-replication repair (PRR) system, which becomes essential for cell survival (Lazzaro et al., 2012; Pizzi et al. in revision). Here, we focus on the contribution of the three different TLS Polymerases (Rev1, Pol ζ and Pol η) to genomic-rNMPs tolerance in budding yeast. We observed that TLS polymerases not only have an important role in bypassing unrepaired-rNMPs in the genome, but they can also incorporate rNMPs during DNA replication. In particular the evolutionary conserved TLS polymerase Pol-η shows elevated predisposition to mis-incorporate rNMPs when the dNTPs pools are limited, while Rev1 counteracts this function avoiding the excess incorporation of rNMPs during DNA replication, preserving genome integrity.