Gupta R S, Singh B
Department of Biochemistry, McMaster University, Hamilton, Ontario, Canada.
Curr Biol. 1994 Dec 1;4(12):1104-14. doi: 10.1016/s0960-9822(00)00249-9.
The evolutionary relationships between archaebacteria, eubacteria and eukaryotic cells are of central importance in biology. The current view is that each of these three groups of organisms constitutes a monophyletic domain, and that eukaryotic cells have evolved fom an archaebacterial ancestor. Recent studies on a number of highly conserved protein sequences do not, however, support this view and raise important questions concerning the evolutionary relationships between all extant organisms, particularly regarding the origin of eukaryotic cells.
We have used sequences of 70 kD heat shock protein (hsp70)--the most conserved protein found to date in all species--to examine the evolutionary relationship between various species. We have obtained two new archaebacterial hsp70 sequences from the species, Thermoplasma acidophilum and Halobacterium cutirubrum. A global comparison of hsp70 sequences, including our two new sequences, shows that all known archaebacterial homologs share a number of sequence signatures with the Gram-positive group of bacteria that are not found in any other prokaryotic or eukaryotic species. In contrast, the eukaryotic homologs are shown to share a number of unique sequence features with the Gram-negative bacteria that are not present in any archaebacteria. Detailed phylogenetic analyses of hsp70 sequences strongly support a specific evolutionary relationship between archaebacteria and Gram-positive bacteria on the one hand, and Gram-negative bacteria and eukaryotes on the other. The phylogenetic analyses also indicate a polyphyletic branching of archaebacteria within the Gram-positive species. The possibility that the observed relationships are due to horizontal gene transfers can be excluded on the basis of sequence characteristics of different groups of homologs.
Our results do not support the view that archaebacteria constitute a monophyletic domain, but instead suggest a close evolutionary linkage between archaebacteria and Gram-positive bacteria. Furthermore, in contrast to the presently accepted view, eukaryotic hsp70s show a close and specific relationship to those from Gram-negative species. To explain the phylogenies based on different gene sequences, a chimeric model for the origin of the eukaryotic cell nucleus involving fusion between an archaebacterium and a Gram-negative eubacterium is proposed. Several predictions from the chimeric model are discussed.
古细菌、真细菌和真核细胞之间的进化关系在生物学中至关重要。目前的观点认为，这三类生物各自构成一个单系域，且真核细胞是从古细菌祖先进化而来。然而，最近对一些高度保守蛋白质序列的研究并不支持这一观点，并引发了关于所有现存生物之间进化关系的重要问题，特别是关于真核细胞的起源。
我们使用了70kD热休克蛋白（hsp70）的序列——迄今为止在所有物种中发现的最保守的蛋白质——来研究不同物种之间的进化关系。我们从嗜酸嗜热放线菌和红皮盐杆菌这两个物种中获得了两条新的古细菌hsp70序列。对hsp70序列进行的全面比较，包括我们的两条新序列，表明所有已知的古细菌同源物与革兰氏阳性菌组共享许多序列特征，而这些特征在任何其他原核生物或真核生物物种中都未发现。相反，真核生物同源物显示出与革兰氏阴性菌共享许多独特的序列特征，而这些特征在任何古细菌中都不存在。对hsp70序列进行的详细系统发育分析有力地支持了一方面古细菌与革兰氏阳性菌之间、另一方面革兰氏阴性菌与真核生物之间存在特定的进化关系。系统发育分析还表明古细菌在革兰氏阳性物种中呈多系分支。基于不同同源物组的序列特征，可以排除观察到的关系是由于水平基因转移所致的可能性。
我们的结果不支持古细菌构成一个单系域的观点，而是表明古细菌与革兰氏阳性菌之间存在密切的进化联系。此外，与目前公认观点相反，真核生物hsp70与革兰氏阴性物种的hsp70显示出密切而特定的关系。为了解释基于不同基因序列的系统发育，提出了一个关于真核细胞核起源的嵌合模型，该模型涉及古细菌与革兰氏阴性真细菌之间的融合。讨论了嵌合模型的几个预测。
