| 其他摘要 | YidC/Oxa/Alb3 family includes a group of conserved translocases that are essential for protein insertion into inner membranes of bacteria and mitochondria, and thylakoid membranes of chloroplasts. Because mitochondria and chloroplasts are of bacterial origin, Oxa and Alb3, like many other mitochondrial/chloroplastic proteins, are hypothetically derived from the pre-existing protein (YidC) of bacterial endosymbionts. Here, we test this hypothesis and investigate the evolutionary history of the whole YidC/Oxa/Alb3 family in the three domains of life based on all available genome databases. The results are as follows:
1) In archaea, YidC homologs are only sporadically distributed in Euryarchaeota; 2) Most bacteria contain only one YidC gene copy; some species in a few taxa (Bacillus, Lactobacillales, Actinobacteria and Clostridia) have two gene copies; 3) Mitochondrial Oxa, chloroplastic Alb3, and archaeal YidC form three separate clades, respectively, while bacterial YidC alone form several other clades that are largely consistent with their source organism lineages. The mitochondrial Oxa clade does not group with proteobacterial YidC clade; The Alb3 clade is nested within the large bacterial YidC clades without obviously showing a close relationship with the cyanobacterial YidC clade. 4) Fungi, metazoan, green algae and plants all possess two Oxa (Oxa1 and Oxa2) genes; these Oxa sequences form two separate groups in phylogenetic tree; Green algae and plants commonly have tow Alb3 genes. Alb3 homologs from green algae and plants cluster together. Within this large clade, green alga Alb3 form two distinct but adjacent branches, which are designated as Alb3.1 and Alb3.2 respectively. Plant Alb3 also split into two distinct and adjacent groups that are in turn sister to green alga Alb3.1. We named the two plant Alb3 groups Alb3.a and Alb3.b respectively; Domain analysis showed that Oxa2 in green algae and plants possess an additional C-terminal Tetratricopeptide Repeat (TPR) domain;
III
The number of Oxa and Alb3 homologs and their clades in the phylogenetic tree of some protists are unusual.
Based on the above results, we conclude: YidC originated in bacteria and some Euryarchaeota; Eukaryotic Oxa and Alb3 have two separate prokaryotic origins, but they might not arise directly from the YidC of proteobacteria and cyanobacteria through the endosymbiosis origins of mitochondria and chloroplast, respectively; An ancient duplication occurred on both Oxa and Alb3 immediately after their origins, and thus most eukaryotes generally bear two Oxa and two Alb3. However, secondary loss, duplication or acquisition of new domain also occurred on the two genes in some lineages, especially in protists, resulting in a rich diversity or adaptive differentiation of the two translocases in these lineages.
Finally, based on the above results and conclusions, a model for the evolutionary history of the entire YidC/Oxa/Alb3 family in the three domains of life is proposed. |
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