Cost of rNTP/dNTP pool imbalance at the replication fork.

The concentration of ribonucleoside triphosphates (rNTPs) in cells is far greater than the concentration of deoxyribonucleoside triphosphates (dNTPs), and this pool imbalance presents a challenge for DNA polymerases (Pols) to select their proper substrate. This report examines the effect of nucleotide pool imbalance on the rate and fidelity of the Escherichia coli replisome. We find that rNTPs decrease replication fork rate by competing with dNTPs at the active site of the C-family Pol III replicase at a step that does not require correct base-pairing. The effect of rNTPs on Pol rate generalizes to B-family eukaryotic replicases, Pols δ and ε. Imbalance of the dNTP pool also slows the replisome and thus is not specific to rNTPs. We observe a measurable frequency of rNMP incorporation that predicts one rNTP incorporated every 2.3 kb during chromosome replication. Given the frequency of rNMP incorporation, the repair of rNMPs is likely rapid. RNase HII nicks DNA at single rNMP residues to initiate replacement with dNMP. Considering that rNMPs will mark the new strand, RNase HII may direct strand-specificity for mismatch repair (MMR). How the newly synthesized strand is recognized for MMR is uncertain in eukaryotes and most bacteria, which lack a methyl-directed nicking system. Here we demonstrate that Bacillus subtilis incorporates rNMPs in vivo, that RNase HII plays a role in their removal, and the RNase HII gene deletion enhances mutagenesis, suggesting a possible role of incorporated rNMPs in MMR.