Itoh R, Takahashi H, Toda K, Kuroiwa H, Kuroiwa T
Department of Biological Sciences, Graduate School of Science, University of Tokyo, Japan.
Eur J Cell Biol. 1997 Jul;73(3):252-8.
Cyanidioschyzon merolae is a unicellular alga with one chloroplast and one mitochondrion per cell. Each of these organelles has a simple nucleoid located in the central area. Prior to mitosis and cytokinesis, both the division of organelle nucleoids and organellokinesis occur simultaneously. We investigated the effects of nalidixic acid, an inhibitor of DNA gyrase, on the replication and division of organelle nucleoids. Using microfluorometry, we monitored the synthesis of organelle DNA (organelle S phase) in synchronous cultures. In both the chloroplast and the mitochondrion, DNA synthesis occurred from the start of the light period and ceased within 3 h. The addition of nalidixic acid at the start of the light period inhibited DNA synthesis in both organelles. Next, we added nalidixic acid to the synchronous culture at the beginning (just before the organelle S phase) or at the end (organelle G2 phase) of the light period and monitored the division of organelle nucleoids in the dark period. Approximately 50 to 70% of chloroplast nucleoids divided unequally in both experiments. Despite the unequal division of the chloroplast nucleoids, electron microscopy revealed that the chloroplasts were divided into equal daughter chloroplasts by the chloroplast-divided ring. During the same treatments, all of the mitochondrial nucleoids underwent normal and equal division. Our results suggest the involvement of DNA gyrase in the replication of chloroplast and mitochondrial DNA and in the division of the chloroplast nucleoid, as well as a difference in mechanism between the division of chloroplast and mitochondrial nucleoids.
梅氏蓝纤维藻是一种单细胞藻类，每个细胞含有一个叶绿体和一个线粒体。这些细胞器中的每一个都有一个位于中心区域的简单类核。在有丝分裂和胞质分裂之前，细胞器类核的分裂和细胞器分裂同时发生。我们研究了DNA回旋酶抑制剂萘啶酸对细胞器类核复制和分裂的影响。使用显微荧光测定法，我们监测了同步培养物中细胞器DNA的合成（细胞器S期）。在叶绿体和线粒体中，DNA合成从光照期开始时发生，并在3小时内停止。在光照期开始时添加萘啶酸会抑制这两种细胞器中的DNA合成。接下来，我们在光照期开始时（就在细胞器S期之前）或结束时（细胞器G2期）向同步培养物中添加萘啶酸，并在黑暗期监测细胞器类核的分裂。在两个实验中，大约50%至70%的叶绿体类核进行了不均等分裂。尽管叶绿体类核进行了不均等分裂，但电子显微镜显示叶绿体通过叶绿体分裂环被分成了相等的子叶绿体。在相同处理过程中，所有线粒体类核都进行了正常且均等的分裂。我们的结果表明DNA回旋酶参与了叶绿体和线粒体DNA的复制以及叶绿体类核的分裂，同时也表明叶绿体和线粒体类核分裂机制存在差异。
