Department of Medical Biochemistry and Biophysics, Umeå University, Umeå, Sweden.
PLoS Genet. 2018 Mar 30;14(3):e1007315. doi: 10.1371/journal.pgen.1007315. eCollection 2018 Mar.
Ribonucleotides (rNMPs) are frequently incorporated during replication or repair by DNA polymerases and failure to remove them leads to instability of nuclear DNA (nDNA). Conversely, rNMPs appear to be relatively well-tolerated in mitochondrial DNA (mtDNA), although the mechanisms behind the tolerance remain unclear. We here show that the human mitochondrial DNA polymerase gamma (Pol γ) bypasses single rNMPs with an unprecedentedly high fidelity and efficiency. In addition, Pol γ exhibits a strikingly low frequency of rNMP incorporation, a property, which we find is independent of its exonuclease activity. However, the physiological levels of free rNTPs partially inhibit DNA synthesis by Pol γ and render the polymerase more sensitive to imbalanced dNTP pools. The characteristics of Pol γ reported here could have implications for forms of mtDNA depletion syndrome (MDS) that are associated with imbalanced cellular dNTP pools. Our results show that at the rNTP/dNTP ratios that are expected to prevail in such disease states, Pol γ enters a polymerase/exonuclease idling mode that leads to mtDNA replication stalling. This could ultimately lead to mtDNA depletion and, consequently, to mitochondrial disease phenotypes such as those observed in MDS.
核苷酸（rNMPs）在 DNA 聚合酶复制或修复过程中经常被掺入，如果不能将其去除，就会导致核 DNA（nDNA）不稳定。相反，rNMPs 在线粒体 DNA（mtDNA）中似乎相对耐受，尽管耐受的机制尚不清楚。我们在这里表明，人类线粒体 DNA 聚合酶γ（Pol γ）以空前的高保真度和效率绕过单个 rNMP。此外，Pol γ 表现出极低的 rNMP 掺入频率，我们发现这一特性与其外切酶活性无关。然而，生理浓度的游离 rNTP 部分抑制 Pol γ 的 DNA 合成，并使聚合酶对不平衡的 dNTP 池更敏感。这里报道的 Pol γ 的特性可能对与不平衡细胞 dNTP 池相关的线粒体 DNA 耗竭综合征（MDS）形式有影响。我们的结果表明，在这种疾病状态下预计会出现的 rNTP/dNTP 比例下，Pol γ 进入聚合酶/外切酶空闲模式，导致 mtDNA 复制停滞。这可能最终导致 mtDNA 耗竭，并因此导致线粒体疾病表型，如 MDS 中观察到的那些表型。
