The accessory subunit of DNA polymerase γ is essential for mitochondrial DNA maintenance and development in Drosophila melanogaster

DNA polymerase γ, Pol γ, is the key replicative enzyme in animal mitochondria. The Drosophila enzyme is a heterodimer comprising catalytic and accessory subunits of 125 kDa and 35 kDa, respectively. Both subunits have been cloned and characterized in a variety of model systems, and genetic mutants of the catalytic subunit were first identified in Drosophila, as chemically induced mutations that disrupt larval behavior (tamas). Mutations in the gene encoding the accessory subunit have not yet been described in any organism. Here, we report the consequences of null mutations upon mitochondrial DNA (mtDNA) replication and morphology, cell proliferation, and organismal viability. Mutations in the accessory subunit cause lethality during early pupation, concomitant with loss of mtDNA and mitochondrial mass, and reduced cell proliferation in the central nervous system. Surprisingly, the function of the central nervous system and muscle, as assessed in a locomotion assay, are only marginally affected. This finding is in contrast to our previous findings that disruption in the function of the catalytic subunit causes severe reduction in larval locomotion. We discuss our results in the context of current hypotheses for the function of the accessory subunit in mtDNA replication.

DNA聚合酶γ（DNA polymerase γ，简称Pol γ）是动物线粒体中关键的复制酶。果蝇来源的该酶为异二聚体，分别由125 kDa的催化亚基与35 kDa的辅助亚基构成。两类亚基均已在多种模式生物体系中完成克隆与功能表征；催化亚基的遗传突变体最早在果蝇中被鉴定，其表现为化学诱导突变引发幼虫行为异常（tamas）。截至目前，尚未在任何物种中报道过编码辅助亚基的基因突变。本研究报道了辅助亚基的无效突变（null mutations）对线粒体DNA（mitochondrial DNA，简称mtDNA）复制与形态、细胞增殖以及个体存活率的影响。辅助亚基的突变会导致果蝇在蛹早期死亡，同时伴随mtDNA丢失、线粒体质量下降，以及中枢神经系统的细胞增殖能力降低。令人意外的是，通过运动能力测定评估的中枢神经系统与肌肉功能仅受到轻微影响。这一结果与我们此前的研究结论相悖——此前我们发现催化亚基功能受损会导致幼虫运动能力严重下降。我们结合当前关于辅助亚基在mtDNA复制中功能的主流假说，对本研究结果展开了讨论。