![]() Similarly, the breviate Pygsuia biforma apparently replaced the ISC system with an archeal SUF system. ![]() The NIF system operates in the cytosol of Entamoeba histolytica or in the cytosol and hydrogenosomes of Mastigamoeba balamuthi. Archamoebae replaced the ISC pathway with two components of a nitrogen-fixing (NIF) machinery that were acquired by lateral gene transfer (LGT) from ɛ-proteobacteria. There are few known exceptions to the highly conserved setup of FeS assembly machineries, and all these exceptions concern protists adapted to anaerobic or microaerobic conditions with modified mitochondria. Phylogenetic analysis suggested that the CIA pathway was present in the last eukaryotic common ancestor (LECA) and that its components are predominantly of bacterial origin. The ISC machinery is functionally linked to the third system, the cytosolic FeS cluster assembly (CIA) machinery. The pathway in plastids is called the sulfur utilization factor (SUF) system, which is present in primary as well as secondary plastids. The mitochondrial FeS cluster assembly (ISC) machinery operates in nearly all forms of mitochondria including anaerobic hydrogenosomes and highly reduced mitosomes. The organellar pathways were acquired through endosymbiosis of α-proteobacteria and cyanobacteria that evolved into mitochondria and plastids, respectively. In eukaryotes, there are three main pathways, which are localized in distinct cellular compartments: mitochondria, plastids, and the cytosol. Iron-sulfur (FeS) cluster assembly pathways are essential for all three domains of life: Bacteria, Archaea, and Eukarya. Because plastid localization of HCF101 was only known thus far, the discovery of its mitochondrial paralog explains confusion regarding the presence of HCF101 in organisms that possibly lost secondary plastids (e.g., ciliates, Cryptosporidium) or possess reduced nonphotosynthetic plastids (apicomplexans). Our searches for Nbp35-like proteins across eukaryotic lineages revealed that SAR, Haptista, and Cryptista possess mitochondrial HCF101. Interestingly, phylogenetic analysis supports rather a lateral gene transfer of ancestral HCF101 from bacteria than its acquisition being associated with either α-proteobacterial or cyanobacterial endosymbionts. Phylogenetic analysis suggested that chHCF101 and mHCF101 evolved from a common ancestral HCF101 independently of the Nbp35/Cfd1 and Ind1 proteins. Our predictions were experimentally confirmed in selected representatives of Apicomplexa ( Toxoplasma gondii), Stramenopila ( Phaeodactylum tricornutum, Thalassiosira pseudonana), and Ciliophora ( Tetrahymena thermophila) by tagging proteins with a transgenic reporter. We also identified a few exceptions, as apicomplexans possess mHCF101 predicted to localize in the cytosol and Nbp35 in the mitochondria. Surprisingly, we found a second HCF101 paralog in all members of Cryptista, Haptista, and SAR that was predicted to predominantly target mitochondria (mHCF101), whereas Ind1 appeared to be absent in these organisms. Nbp35 and Cfd1 were predicted to reside in the cytoplasm with some exceptions of Nbp35 localization to the mitochondria Ind1was found in the mitochondria, and HCF101 was predicted to reside in plastids (chHCF101) of all photosynthetically active eukaryotes. We searched for the genes of all members of the Nbp35-like protein family and analyzed their targeting sequences. To date, the cellular distribution of these proteins is considered to be highly conserved with only a few exceptions. The AbpC protein operates in Bacteria and Archaea. In eukaryotes, Ind1 is present in mitochondria, and its function is associated with the assembly of FeS clusters in subunits of respiratory Complex I, Nbp35 and Cfd1 are the components of the cytosolic FeS assembly (CIA) pathway, and HCF101 is involved in FeS assembly of photosystem I in plastids of plants (chHCF101). Nbp35-like proteins (Nbp35, Cfd1, HCF101, Ind1, and AbpC) are P-loop NTPases that serve as components of iron-sulfur cluster (FeS) assembly machineries.
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