Deoxynucleoside triphosphate (dNTP) synthesis and destruction regulate the replication of both cell and virus genomes

Biochemical reactions, even those as complex as replicating the DNA genome of cells, follow the principle that the process is regulated by both the substrate concentration and by the enzymes that mediate the process. Deoxynucleoside triphosphates (dNTPs), the substrates for DNA polymerizing enzymes, have long been known to be limited in their concentration in cells because the enzyme that synthesizes deoxynucleotides from ribonucleotides, ribonucleotide reductase (RNR), is synthesized and enzymatically activated as cells enter the S phase (1, 2). RNR, discovered by Peter Reichard 52 y ago (3), converts all four ribonucleotide diphosphates (rNDPs) to the respective deoxynucleoside disposphates (dNDPs), which are then rapidly converted to dNTP. Low levels and activity of RNR provide sufficient dNTPs for mitochondrial DNA synthesis and for DNA repair in noncycling cells and during the G1 phase of the cell-division cycle in proliferating cells, but RNR levels and activity are hugely increased as cells commit to replicate DNA during the S phase of the cell-division cycle or following extensive DNA repair (4). Indeed, RNR is one of the most highly regulated enzymes known. The mammalian enzyme synthesizes all four dNDPs in a cycle, is allosterically activated by dATP, dTTP, and dGTP to balance the relative levels of the four dNTPs (dCTP, dTTP, dGTP and dATP), and is feed-back–inhibited by dATP, because dATP is the last dNTP to be made in the cycle of synthesizing all four dNTPs by a single RNR enzyme (1). Specific inhibitory proteins (in yeasts) also control RNR activity and RNR subunit levels are regulated by cell cycle-dependent transcription of the genes encoding the subunits and by subunit protein stability (4, 5). On the basis of these observations, one might expect that dNTP synthesis by RNR should be sufficient to control how and when genome DNA replication occurs because RNR is only maximally active during the S phase. However, recent studies, including those emerging from far-afield studies of how HIV replication is restricted to certain cell types (6, 7), have uncovered a new control of dNTP levels, dNTP destruction. The sterile alpha motif and HD-domain containing protein 1 (SAMHD1) protein is a deoxynucleoside triphosphohydrolase that cleaves dNTPs to the respective deoxynucleoside and a triphosphate (8). In PNAS, Franzolin et al. (9) show that dNTP destruction by SAMHD1 also contributes to dNTP concentration control during the cell-division cycle of proliferating cells, thereby affecting both DNA replication and cell-cycle progression