Oborník Miroslav, Green Beverley R
Department of Botany, University of British Columbia, Vancouver, BC, Canada.
Mol Biol Evol. 2005 Dec;22(12):2343-53. doi: 10.1093/molbev/msi230. Epub 2005 Aug 10.
Heme biosynthesis represents one of the most essential metabolic pathways in living organisms, providing the precursors for cytochrome prosthetic groups, photosynthetic pigments, and vitamin B(12). Using genomic data, we have compared the heme pathway in the diatom Thalassiosira pseudonana and the red alga Cyanidioschyzon merolae to those of green algae and higher plants, as well as to those of heterotrophic eukaryotes (fungi, apicomplexans, and animals). Phylogenetic analyses showed the mosaic character of this pathway in photosynthetic eukaryotes. Although most of the algal and plant enzymes showed the expected plastid (cyanobacterial) origin, at least one of them (porphobilinogen deaminase) appears to have a mitochondrial (alpha-proteobacterial) origin. Another enzyme, glutamyl-tRNA synthase, obviously originated in the eukaryotic nucleus. Because all the plastid-targeted sequences consistently form a well-supported cluster, this suggests that genes were either transferred from the primary endosymbiont (cyanobacteria) to the primary host nucleus shortly after the primary endosymbiotic event or replaced with genes from other sources at an equally early time, i.e., before the formation of three primary plastid lineages. The one striking exception to this pattern is ferrochelatase, the enzyme catalyzing the first committed step to heme and bilin pigments. In this case, two red algal sequences do not cluster either with the other plastid sequences or with cyanobacterial sequences and appear to have a proteobacterial origin like that of the apicomplexan parasites Plasmodium and Toxoplasma. Although the heterokonts also acquired their plastid via secondary endosymbiosis from a red alga, the diatom has a typical plastid-cyanobacterial ferrochelatase. We have not found any remnants of the plastidlike heme pathway in the nonphotosynthetic heterokonts Phytophthora ramorum and Phytophthora sojae.
血红素生物合成是生物体中最基本的代谢途径之一，为细胞色素辅基、光合色素和维生素B12提供前体。利用基因组数据，我们比较了硅藻假微型海链藻和红藻梅氏嗜热栖热菌中的血红素途径与绿藻、高等植物以及异养真核生物（真菌、顶复门原虫和动物）中的血红素途径。系统发育分析显示了光合真核生物中该途径的镶嵌特征。尽管大多数藻类和植物酶显示出预期的质体（蓝细菌）起源，但其中至少一种酶（胆色素原脱氨酶）似乎有线粒体（α-变形菌）起源。另一种酶，谷氨酰-tRNA合成酶，显然起源于真核细胞核。由于所有靶向质体的序列始终形成一个得到充分支持的聚类，这表明这些基因要么在初级内共生事件后不久从初级内共生体（蓝细菌）转移到初级宿主细胞核，要么在同样早的时间被其他来源的基因所取代，即在三个主要质体谱系形成之前。这种模式的一个显著例外是铁螯合酶，该酶催化血红素和胆色素合成的第一步关键反应。在这种情况下，两个红藻序列既不与其他质体序列聚类，也不与蓝细菌序列聚类，似乎与顶复门寄生虫疟原虫和弓形虫一样具有变形菌起源。尽管不等鞭毛类也通过从红藻的次生内共生获得了它们的质体，但硅藻具有典型的质体蓝细菌铁螯合酶。我们在非光合不等鞭毛类的致病疫霉和大豆疫霉中未发现类质体血红素途径的任何残余。
