INVESTIGATION ON THE STRUCTURE AND FUNCTION OF PYRENOIDS IN THE CHLOROPLAST OF LOWER PLANTS

形态学研究组：最近的研究表明，蛋白核是低等植物叶绿体中Rubisco的一个聚集位点。然而，Hori证明了叶绿体DNA在某些绿色藻类的蛋白核中的定位。我们研究了单细胞绿色藻类Trebouxia在整个生活史中叶绿体DNA和蛋白核的行为。当叶绿体分裂时，含有叶绿体DNA的蛋白核也分裂。在该属的生活史中，叶绿体DNA总是与蛋白核结合。石川发现DNA与透镜状蛋白核结合，这些蛋白核散布在硅藻的叶绿体基质中。最近，Suess等（1995）报道Rubisco分子在单细胞绿色衣藻（Chlamydiumreinhardtii）的蛋白核中并不高度集中。结果不一致 ...更多信息 最近的研究表明，大多数Rubisco分子位于蛋白核中。因此，Nozaki重新检查了Rubisco在莱茵衣藻细胞中的分布，并使用冷冻固定和免疫金电子显微镜发现大多数Rubisco分子位于蛋白核核心中。在裸藻中，Rubisco的小亚基定位于细胞质中的隔室化嗜锇小体（COS）中。这一结果表明，COS要么是叶绿体蛋白质合成的细胞质位点，要么是叶绿体蛋白质从内质网运输到叶绿体（Osafune）的中间隔室。Hara利用电子显微镜和分子系统学技术重新评估了蛋白核的超微结构作为紫球藻目单细胞红藻的分类特征。这一研究表明，虽然属于这一类群的藻类在外观上相似，但紫菜目并不是一个单系类群。生理研究类群：Rubisco是蛋白核的主要成分，在某些藻类中也存在DNA，如上所述。然而，蛋白核中可能存在其他次要成分。因此，冈田开发了新的生化技术，以确定蛋白核的次要成分。在不久的将来，蛋白核的次要成分将被鉴定。当细胞外CO_2浓度从5%降低到正常空气水平时，类蛋白核体积增大1.4-1.6倍，蛋白质浓度也增加。而Rubisco基因小亚基的转录量在低CO_2浓度下减少。这些结果表明，当CO_2浓度较低时，Rubisco在蛋白核中积累，而不是合成新的分子。在大多数藻类中，类囊体膜穿过蛋白核。Nakayama指出光系统II存在于蛋白核的类囊体膜中。少

Morphological research group : Recent studies indicate that the pyrenoid is an accumulation site of Rubisco in the chloroplast of lower plants. However, Hori demonstrated the localiation of chloroplast DNA in the pyrenoid core of some green algae. We have investigated the behavior of chloroplast DNA and pyrenoids throughout the life cycle of the unicellular green alga, Trebouxia. When the chloroplast divides, the pyrenoid with chloroplast DNA also divides. The chloroplast DNAs are always associated with the pyrenoids during the life cycle of Trebouxia (Miyamura). Ishikawa found the asscoclation of DNAs with lens-shaepd pyrenoids, which are scattered throughout the chloroplast matrix of the diatom, Nitschia sigmoidea. She also found the localization of Rubisco in the pyrenoid core of N.sigmoidea Recently, Suess et al (1995) reported that Rubisco molecules are not highly concentrated in the pyrenoid core of the unicellular green alga, Chlamydomonas reinhardtii. Their results are inconsis … More tent with recent studies in which most of Rubisco molecules are localized in the pyrenoid core. Thus, Nozaki re-examined the distribution of Rubisco in the cells of C.reinhardtii, and found the localization of most of Rubisco molecules in the pyrenoid core using cryo-fixation and immunogold electron microscopy. In Euglena, small subunit of Rubisco is found to be localized in the compartment alized osmiophilic body (COS) in the cytoplasm. This results suggests that the COS is either the cytoplasmic site of chloroplast protein synthesis or an intermediate compartment in the transport of chloroplast proteins from the ER to the chloroplast (Osafune). Hara re-evaluated the ultrastructure of the pyrenoid as the taxonomic character of unicellullar red algae in Porphyridiales using electron microscopy and molecular phylogenetic techniques. This study suggests that Porphyridiales is not a monophyletic group, although the algae belonging to this group is similar in appearance.Physiological research group : Rubisco is the major component of the pyrenoid, and in some algae DNA is also present, as described above. However, it is possible that other minor components are present in the pyrenoid core. Therefore, Okada developed the new biochemical techniques in order to identify the minor components of the pyrenoid. The minor components of the pyrenoid will be identified using this technique in near future. When the extracellular CO_2 concentration is lowered from 5% to ordinary air level in the cells of C.reinhardtii, the pyrenoid has become 1.4-1.6 times larger in volume, and the protein concentration has also increased (Tsuzuki). However, the amount of transcripts of small subunit of Rubisco genes decreases under lower CO_2 concentration. These results suggests that when the pyrenoid develops under lower CO_2 concentration, the Rubisco is accumulated in the pyrenoid instead of the synthesis of new molecules. It is known that the pyrenoid core is traversed by thylakoid membranes in most of algae. Nakayama showed that photosystem II is present in the thylakoid membranes in the pyrenoid. Less