Roger A J, Svärd S G, Tovar J, Clark C G, Smith M W, Gillin F D, Sogin M L
The Josephine Bay-Paul Center of Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA 02543-1015, USA.
Proc Natl Acad Sci U S A. 1998 Jan 6;95(1):229-34. doi: 10.1073/pnas.95.1.229.
Diplomonads, parabasalids, as represented by trichomonads, and microsporidia are three protist lineages lacking mitochondria that branch earlier than all other eukaryotes in small subunit rRNA and elongation factor phylogenies. The absence of mitochondria and plastids in these organisms suggested that they diverged before the origin of these organelles. However, recent discoveries of mitochondrial-like heat shock protein 70 and/or chaperonin 60 (cpn60) genes in trichomonads and microsporidia imply that the ancestors of these two groups once harbored mitochondria or their endosymbiotic progenitors. In this report, we describe a mitochondrial-like cpn60 homolog from the diplomonad parasite Giardia lamblia. Northern and Western blots reveal that the expression of cpn60 is independent of cellular stress and, except during excystation, occurs throughout the G. lamblia life cycle. Phylogenetic analyses position the G. lamblia cpn60 in a clade that includes mitochondrial and hydrogenosomal cpn60 proteins. The most parsimonious interpretation of these data is that the cpn60 gene was transferred from the endosymbiotic ancestors of mitochondria to the nucleus early in eukaryotic evolution, before the divergence of the diplomonads and trichomonads from other extant eukaryotic lineages. A more complicated explanation requires that these genes originated from distinct alpha-proteobacterial endosymbioses that formed transiently within these protist lineages.
双滴虫、以毛滴虫为代表的副基体和微孢子虫是三类缺乏线粒体的原生生物谱系，在小亚基核糖体RNA和延伸因子系统发育中，它们比所有其他真核生物分支更早。这些生物中缺乏线粒体和质体，这表明它们在这些细胞器起源之前就已经分化。然而，最近在毛滴虫和微孢子虫中发现了类似线粒体的热休克蛋白70和/或伴侣蛋白60（cpn60）基因，这意味着这两组生物的祖先曾经拥有线粒体或其内共生祖先。在本报告中，我们描述了来自双滴虫寄生虫蓝氏贾第鞭毛虫的一种类似线粒体的cpn60同源物。Northern和Western印迹显示，cpn60的表达与细胞应激无关，并且除了在脱囊期间外，在蓝氏贾第鞭毛虫的整个生命周期中都有表达。系统发育分析将蓝氏贾第鞭毛虫的cpn60定位在一个包括线粒体和氢化酶体cpn60蛋白的进化枝中。对这些数据最简约的解释是，cpn60基因在真核生物进化早期，在双滴虫和毛滴虫与其他现存真核生物谱系分化之前，从线粒体的内共生祖先转移到了细胞核中。一个更复杂的解释是，这些基因起源于在这些原生生物谱系中短暂形成的不同的α-变形细菌内共生体。
