Ryall Kim, Harper James T, Keeling Patrick J
Canadian Institute for Advanced Research, Department of Botany, University of British Columbia, V6T 1Z4 Vancouver, BC, Canada.
Gene. 2003 Aug 14;313:139-48. doi: 10.1016/s0378-1119(03)00671-1.
Fatty acid biosynthesis is a critical process for living organisms, but the evolution of the enzymes involved in this pathway is poorly understood. Animals and fungi use a Type I fatty acid synthase (FAS), a large multifunctional protein found in the cytosol. Bacteria use a Type II complex, where each enzymatic domain is a discrete polypeptide. In plants, fatty acid biosynthesis takes place in the plastid, and utilises a Type II enzyme complex. Recently, the apicomplexan parasites Plasmodium and Toxoplasma have been shown to contain the plastid-targeted Type II FAS. To investigate the distribution of this pathway, we have characterised two Type II enzymes, FabD and FabI, in three other eukaryotes with plastids derived from red algal endosymbionts: cryptomonads, heterokonts, and haptophytes. Collectively, these are referred to as chromists, and are thought to be related to apicomplexa and their relatives. Phylogenies of these enzymes show that the plastid Type II FAS enzymes are found in all groups studied, which most likely means that they originated from the red algal endosymbiont at the outset of the secondary endosymbiosis of their plastids. In addition, although plastid fab D genes are clearly related to one another, they are not related to cyanobacterial homologues, as would be expected. On the other hand, the strongly supported plastid fab I clade is related to cyanobacteria, and contains genes from chlamydiales.
脂肪酸生物合成是生物体的一个关键过程，但人们对该途径中所涉及酶的进化了解甚少。动物和真菌使用I型脂肪酸合酶（FAS），这是一种存在于胞质溶胶中的大型多功能蛋白质。细菌使用II型复合体，其中每个酶结构域都是一个离散的多肽。在植物中，脂肪酸生物合成发生在质体中，并利用II型酶复合体。最近，已证明顶复门寄生虫疟原虫和弓形虫含有靶向质体的II型FAS。为了研究该途径的分布，我们在另外三种具有源自红藻内共生体的质体的真核生物中对两种II型酶FabD和FabI进行了表征：隐藻、不等鞭毛类和定鞭藻。这些生物统称为色素体生物，被认为与顶复门及其亲缘生物有关。这些酶的系统发育表明，在所有研究的类群中都发现了质体II型FAS酶，这很可能意味着它们在质体的次生内共生开始时就起源于红藻内共生体。此外，尽管质体fab D基因彼此明显相关，但它们与蓝细菌的同源物无关，而这是预期中的情况。另一方面，得到有力支持的质体fab I进化枝与蓝细菌有关，并且包含来自衣原体目（Chlamydiales）的基因。
