Matsuzaki Motomichi, Kuroiwa Haruko, Kuroiwa Tsuneyoshi, Kita Kiyoshi, Nozaki Hisayoshi
Department of Biological Sciences, Graduate School of Science, University of Tokyo, Tokyo, Japan.
Mol Biol Evol. 2008 Jun;25(6):1167-79. doi: 10.1093/molbev/msn064. Epub 2008 Mar 20.
Plastids are widespread in plant and algal lineages. They are also exploited by some nonphotosynthetic protists, including malarial parasites, to support their diverse modes of life. However, cryptic plastids may exist in other nonphotosynthetic protists, which could be important in studies on the diversity and evolution of plastids. The parasite Perkinsus marinus, which causes mass mortality in oyster farms, is a nonphotosynthetic protist that is phylogenetically related to plastid-bearing dinoflagellates and apicomplexans. In this study, we searched for P. marinus methylerythritol phosphate (MEP) pathway genes, responsible for de novo isoprenoid synthesis in plastids, and determined the full-length gene sequences for 6 of 7 of these genes. Phylogenetic analyses revealed that each P. marinus gene clusters with orthologs from plastid-bearing eukaryotes, which have MEP pathway genes with essentially the same mosaic pattern of evolutionary origin. A new analytical method called sliding-window iteration of TargetP was developed to examine the distribution of targeting preferences. This analysis revealed that the sequenced genes encode bipartite targeting peptides that are characteristic of proteins targeted to secondary plastids originating from endosymbiosis of eukaryotic algae. These results support our idea that Perkinsus is a cryptic algal group containing nonphotosynthetic secondary plastids. In fact, immunofluorescent microscopy indicated that 1 of the MEP pathway enzymes, 1-deoxy-D-xylulose 5-phosphate reductoisomerase, was localized to small compartments near mitochondrion, which are possibly plastids. This tiny organelle seems to contain very low quantities of DNA or may even lack DNA entirely. The MEP pathway genes are a useful tool for investigating plastid evolution in both of the photosynthetic and nonphotosynthetic eukaryotes and led us to propose the hypothesis that ancestral "chromalveolates" harbored plastids before a secondary endosymbiotic event.
质体广泛存在于植物和藻类谱系中。一些非光合原生生物，包括疟原虫，也利用质体来支持它们多样的生活方式。然而，其他非光合原生生物中可能存在隐匿质体，这在质体多样性和进化研究中可能很重要。引起牡蛎养殖场大量死亡的寄生虫——海派金虫，是一种非光合原生生物，在系统发育上与含有质体的甲藻和顶复门生物相关。在本研究中，我们寻找了负责质体中异戊二烯从头合成的海派金虫甲基赤藓糖醇磷酸（MEP）途径基因，并确定了其中7个基因中6个基因的全长序列。系统发育分析表明，每个海派金虫基因都与来自含有质体的真核生物的直系同源基因聚类，这些真核生物的MEP途径基因具有基本相同的镶嵌进化起源模式。我们开发了一种名为TargetP滑动窗口迭代的新分析方法来研究靶向偏好的分布。该分析表明，测序的基因编码双功能靶向肽，这是靶向源自真核藻类内共生的次生质体的蛋白质的特征。这些结果支持了我们的观点，即派金虫是一个含有非光合次生质体的隐匿藻类群。事实上，免疫荧光显微镜检查表明，MEP途径中的一种酶——1-脱氧-D-木酮糖5-磷酸还原异构酶，定位于线粒体附近的小隔室，这些小隔室可能是质体。这个微小的细胞器似乎含有极少量的DNA，甚至可能完全没有DNA。MEP途径基因是研究光合和非光合真核生物中质体进化的有用工具，并促使我们提出假说，即祖先“色藻”在次生内共生事件之前就含有质体。
