Brindefalk Björn, Viklund Johan, Larsson Daniel, Thollesson Mikael, Andersson Siv G E
Department of Molecular Evolution, Evolutionary Biology Center, Uppsala University, Uppsala, Sweden.
Mol Biol Evol. 2007 Mar;24(3):743-56. doi: 10.1093/molbev/msl202. Epub 2006 Dec 20.
Many theories favor a fusion of 2 prokaryotic genomes for the origin of the Eukaryotes, but there are disagreements on the origin, timing, and cellular structures of the cells involved. Equally controversial is the source of the nuclear genes for mitochondrial proteins, although the alpha-proteobacterial contribution to the mitochondrial genome is well established. Phylogenetic inferences show that the nuclearly encoded mitochondrial aminoacyl-tRNA synthetases (aaRSs) occupy a position in the tree that is not close to any of the currently sequenced alpha-proteobacterial genomes, despite cohesive and remarkably well-resolved alpha-proteobacterial clades in 12 of the 20 trees. Two or more alpha-proteobacterial clusters were observed in 8 cases, indicative of differential loss of paralogous genes or horizontal gene transfer. Replacement and retargeting events within the nuclear genomes of the Eukaryotes was indicated in 10 trees, 4 of which also show split alpha-proteobacterial groups. A majority of the mitochondrial aaRSs originate from within the bacterial domain, but none specifically from the alpha-Proteobacteria. For some aaRS, the endosymbiotic origin may have been erased by ongoing gene replacements on the bacterial as well as the eukaryotic side. For others that accurately resolve the alpha-proteobacterial divergence patterns, the lack of affiliation with mitochondria is more surprising. We hypothesize that the ancestral eukaryotic gene pool hosted primordial "bacterial-like" genes, to which a limited set of alpha-proteobacterial genes, mostly coding for components of the respiratory chain complexes, were added and selectively maintained.
许多理论支持真核生物起源于两个原核基因组的融合，但对于所涉及细胞的起源、时间以及细胞结构存在分歧。线粒体蛋白质的核基因来源同样具有争议性，尽管α-变形菌对线粒体基因组的贡献已得到充分证实。系统发育推断表明，核编码的线粒体氨酰-tRNA合成酶（aaRSs）在系统树中的位置与目前已测序的任何α-变形菌基因组都不接近，尽管在20个系统树中有12个具有紧密且分辨率极高的α-变形菌分支。在8个案例中观察到两个或更多的α-变形菌簇，这表明旁系同源基因的差异丢失或水平基因转移。在10个系统树中显示了真核生物核基因组内的替换和重新定位事件，其中4个还显示了α-变形菌群的分裂。大多数线粒体aaRSs起源于细菌域内，但没有一个特别来自α-变形菌。对于一些aaRS，内共生起源可能已被细菌和真核生物方面持续的基因替换所消除。对于其他能够准确解析α-变形菌分化模式的情况，与线粒体缺乏关联则更令人惊讶。我们假设，原始真核基因库中存在原始的“类细菌”基因，有限的一组α-变形菌基因被添加到其中并被选择性保留，这些基因大多编码呼吸链复合物的成分。
