Cai Xiaomin, Fuller A Lorraine, McDougald Larry R, Zhu Guan
Department of Veterinary Pathobiology, College of Veterinary Medicine, College Station, TX 77843-4467, USA.
Gene. 2003 Dec 4;321:39-46. doi: 10.1016/j.gene.2003.08.008.
Unicellular apicomplexans possess an algal-originated plastid referred to as an apicoplast. Although apicomplexan parasites are comprised of highly diverse protists, the complete apicoplast genome sequences have only been determined from the hematozoan Plasmodium falciparum and cyst-forming coccidian Toxoplasma gondii. Here, we report the third complete sequence of apicoplast genome from the intestinal coccidian Eimeria tenella that may serve as a new drug target against coccidiosis in the livestock. The AT-rich E. tenella plastid genome is a 35-kb circular element. Its gene organization resembles more closely that of T. gondii than P. falciparum. Although the E. tenella plastid genome contains an almost identical set of genes to that found in P. falciparum and T. gondii, its encoded genes share low or moderate homologies with their counterparts in the other two apicomplexans. With the addition of this coccidian plastid genome sequence, we attempted to reexamine the apicoplast genome evolution and performed phylogenetic reconstructions using maximum likelihood and Bayesian inference (BI) methods based on a concatenated dataset of plastid-encoded rpoB, rpoC1 and rpoC2 proteins. All resulting rpo protein trees placed apicoplast as a sister to Euglena within the green lineage. On the other hand, many recent studies based on the organization of plastid genes and some nuclear-encoded plastid proteins have supported a common red algal ancestry of apicomplexan and dinoflagellate plastids. If the apicoplast indeed originated from a red ancestor, the green relationship of apicomplexan genes would probably imply that the ancestral host that gave rise to the (red) apicoplast might have already contained some primary green plastid genes.
单细胞顶复门原虫拥有一种起源于藻类的质体，称为顶质体。尽管顶复门寄生虫由高度多样化的原生生物组成，但完整的顶质体基因组序列仅从血液寄生虫恶性疟原虫和形成包囊的球虫弓形虫中确定。在此，我们报告了来自肠道球虫柔嫩艾美耳球虫的顶质体基因组的第三个完整序列，该序列可能成为针对家畜球虫病的新药物靶点。富含AT的柔嫩艾美耳球虫质体基因组是一个35 kb的环状元件。其基因组织与弓形虫的更相似，而与恶性疟原虫的差异较大。尽管柔嫩艾美耳球虫质体基因组包含与恶性疟原虫和弓形虫中发现的几乎相同的基因集，但其编码的基因与其他两种顶复门原虫中的对应基因具有低或中等同源性。随着这个球虫质体基因组序列的加入，我们试图重新审视顶质体基因组的进化，并基于质体编码的rpoB、rpoC1和rpoC2蛋白的串联数据集，使用最大似然法和贝叶斯推断（BI）方法进行系统发育重建。所有得到的rpo蛋白树都将顶质体置于绿色谱系中眼虫的姐妹位置。另一方面，最近许多基于质体基因组织和一些核编码质体蛋白的研究支持顶复门原虫和甲藻质体具有共同的红藻祖先。如果顶质体确实起源于红色祖先，那么顶复门原虫基因的绿色关系可能意味着产生（红色）顶质体的祖先宿主可能已经包含一些初级绿色质体基因。
