Laboratory of Malariology, Research Institute for Microbial Diseases, Osaka University, 3-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
BMC Genomics. 2012 Nov 14;13:622. doi: 10.1186/1471-2164-13-622.
Mitochondrial (mt) genomes vary considerably in size, structure and gene content. The mt genomes of the phylum Apicomplexa, which includes important human pathogens such as the malaria parasite Plasmodium, also show marked diversity of structure. Plasmodium has a concatenated linear mt genome of the smallest size (6-kb); Babesia and Theileria have a linear monomeric mt genome (6.5-kb to 8.2-kb) with terminal inverted repeats; Eimeria, which is distantly related to Plasmodium and Babesia/Theileria, possesses a mt genome (6.2-kb) with a concatemeric form similar to that of Plasmodium; Cryptosporidium, the earliest branching lineage within the phylum Apicomplexa, has no mt genome. We are interested in the evolutionary origin of linear mt genomes of Babesia/Theileria, and have investigated mt genome structures in members of archaeopiroplasmid, a lineage branched off earlier from Babesia/Theileria.
The complete mt genomes of archaeopiroplasmid parasites, Babesia microti and Babesia rodhaini, were sequenced. The mt genomes of B. microti (11.1-kb) and B. rodhaini (6.9-kb) possess two pairs of unique inverted repeats, IR-A and IR-B. Flip-flop inversions between two IR-As and between two IR-Bs appear to generate four distinct genome structures that are present at an equi-molar ratio. An individual parasite contained multiple mt genome structures, with 20 copies and 2 - 3 copies per haploid nuclear genome in B. microti and B. rodhaini, respectively.
We found a novel linear monomeric mt genome structure of B. microti and B. rhodhaini equipped with dual flip-flop inversion system, by which four distinct genome structures are readily generated. To our knowledge, this study is the first to report the presence of two pairs of distinct IR sequences within a monomeric linear mt genome. The present finding provides insight into further understanding of evolution of mt genome structure.
线粒体（mt）基因组在大小、结构和基因含量上差异很大。顶复门（Apicomplexa）的 mt 基因组结构也表现出明显的多样性，该门包括重要的人类病原体，如疟原虫（Plasmodium）。疟原虫的 mt 基因组是最小的串联线性基因组（6kb）；巴贝斯虫（Babesia）和泰勒虫（Theileria）的 mt 基因组是线性单体基因组（6.5kb 到 8.2kb），具有末端反向重复序列；与疟原虫和巴贝斯虫/泰勒虫亲缘关系较远的艾美球虫（Eimeria），具有与疟原虫类似的串联形式的 mt 基因组（6.2kb）；在顶复门中最早分支的隐孢子虫（Cryptosporidium）则没有 mt 基因组。我们对巴贝斯虫/泰勒虫的线性 mt 基因组的进化起源感兴趣，并研究了早于巴贝斯虫/泰勒虫分支的古质体（archaeopiroplasmid）成员的 mt 基因组结构。
测序了古质体寄生虫巴贝斯虫（Babesia）微小种和巴贝斯虫（Babesia）罗得西亚种的完整 mt 基因组。B. microti（11.1kb）和 B. rodhaini（6.9kb）的 mt 基因组都有两对独特的反向重复序列（IR-A 和 IR-B）。两个 IR-A 之间以及两个 IR-B 之间的翻转倒位似乎产生了四种不同的基因组结构，它们以等摩尔比例存在。单个寄生虫含有多种 mt 基因组结构，B. microti 和 B. rodhaini 的单倍体核基因组中分别有 20 个拷贝和 2-3 个拷贝。
我们发现了一种新型的线性单体 mt 基因组结构，B. microti 和 B. rhodhaini 配备了双翻转倒位系统，通过该系统可以很容易地产生四种不同的基因组结构。据我们所知，这是首次在单体线性 mt 基因组中报道存在两对不同的 IR 序列。本研究为进一步了解 mt 基因组结构的进化提供了新的认识。
