Evolution of the Cytosolic Iron/Sulfur cluster Assembly machinery in Blastocystis sp. and other microbial eukaryotes

The Cytosolic Iron/Sulfur cluster Assembly (CIA) machinery is responsible for the assembly of cytosolic and nuclear iron/sulfur clusters, cofactors that are vital for all living cells. This machinery is uniquely found in eukaryotes and consists of at least eight proteins in opisthokont lineages such as animals and yeast. We sought to identify and characterize homologues of the CIA system proteins in the anaerobic stramenopile parasite Blastocystis sp. NandII strain. We identified transcripts encoding six of the components - Cia1, Cia2, MMS19, Nbp35, Nar1, and a putative Tah18 - and showed that the last three of them localized to the cytoplasm of the cell using immuno-fluorescence microscopy, immuno-electron microscopy and subcellular fractionation. We then used comparative genomic and phylogenetic approaches to investigate the evolutionary history of these proteins. While most Blastocystis homologues branch with their eukaryotic counterparts, the putative Blastocystis Tah18 seems to have a separate evolutionary origin and therefore possibly a different function. Furthermore, our phylogenomic analyses revealed that all eight CIA components described in opisthokonts originated before the diversification of extant eukaryotic lineages and were likely already present in the Last Eukaryotic Common Ancestor (LECA). Nbp35, Nar1 Cia1 and Cia2 proteins have been conserved during the subsequent evolutionary diversification of eukaryotes and are present in virtually all extant lineages, whereas the other CIA proteins have patchy phylogenetic distributions. Cia2 appears to be homologous to SufT, a component of the prokaryotic SUF system, making this the first reported evolutionary link between the CIA and any other Fe/S biogenesis pathway. All of our results suggest that the CIA machinery is an ubiquitous biosynthetic pathway in eukaryotes, but its apparent plasticity in composition raises questions regarding how it functions in non-model organisms and how it interfaces with various iron/sulfur cluster systems (i.e., ISC, NIF and/or SUF) found in eukaryotic cells.