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Fig. 3 | Biology Direct

Fig. 3

From: Alpha proteobacterial ancestry of the [Fe-Fe]-hydrogenases in anaerobic eukaryotes

Fig. 3

The cartoon illustrates the possible ancestry of [FeFe]-hydrogenases in eukaryotes from a Rhodospirillales organism related to metagenomic Acetobacter CAG:977. The Fds operon for NAD-dependent formate dehydrogenase [38] (top part) constitutes the common ancestor of both the NADH-reacting module of complex I – comprising subunits NuoEFG [34] – and the HymABC operon of clostridial [FeFe]-hydrogenases [36, 37]. In this operon, the N-terminal part of the original Fdsα subunit has merged with an ancestral Fe-S protein having the H cluster to form a prototypic type M3 [FeFe]-hydrogenase (HydA), which then has been transmitted to proto-eukaryotes in parallel to type A [FeFe]-hydrogenase (central part) and also the Nuo14 operon of complex I (left part of the illustration). The cartoon thus indicates that hydrogenosomes, anaerobic and aerobic mitochondria all derived from a common α proteobacterial ancestor, followed by specific gene loss and organelle relocation in different eukaryotes. Early after the initial symbiogenic event, the genes for the NuoEF subunits have been transferred from the ancestral bacterial genome to nuclear DNA and retained also in eukaryotic organisms containing hydrogenosomes such as Trichomonas. In the hydrogenosomes of the anaerobic ciliate Nyctotherus, the original NuoEF genes have been subsequently fused at the C-terminus of the HydA gene of [FeFe]-hydrogenases, thereby reproducing in a single protein the original HymABC operon that was present in the proteobacterial ancestor. However, the HymB subunit of this operon contains an additional 2Fe2S cluster at its N-terminus which is not present in the NuoF subunit of complex I and therefore distinguishes these two proteins from each other. No gene coding for a protein equivalent to HymB has been found in anaerobic eukaryotes, thereby suggesting that only the HymC prototype of eukaryotic M3 type [FeFe]-hydrogenase has been retained in the genome of the first eukaryote. Subsequent evolutionary divergence has produced eukaryotic lineages that have only derivatives of M3 type hydrogenase, while type A hydrogenase has been retained in the cytosol of a subset of flagellate protists that have highly deranged or no MRO [18, 26]. Note that the simplest derivative of type M3, type M1 is not present in mitochondria but in plastids [17], as shown in the bottom right of the illustration. Structural models for [FeFe]-hydrogenases are as shown in Fig. 1a. Other symbols are as follows (cf. [32]): yellow pentagons, flavin cofactors; dark stars with Mo inside, Mo-binding domains; grey stars without Mo, lost Mo-binding domains; dark diamonds, 2Fe2S clusters; dark blue cubes, 4Fe4S clusters; red cubes, 4Fe4S clusters with one histidine ligand, i.e., cluster N5 in NuoG; grey cubes, additional 4Fe4S clusters in Fdsα and the NuoG subunit of green complex I [33]; yellow cubes, additional 4Fe4S clusters in Fdsα and type A [FeFe]-hydrogenase

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