On the origin of Fe / S cluster biosynthesis in eukaryotes

Word count: 188 Iron and sulfur are indispensable elements of every living cell, but on their own these elements are toxic and require dedicated machineries for the formation of Fe/S clusters. In eukaryotes, proteins requiring Fe/S clusters (Fe/S proteins) are found in or associated with various organelles including the mitochondrion, endoplasmic reticulum, cytosol and the nucleus. These proteins are involved in several pathways indispensable for the viability of each living cell including DNA maintenance, protein translation and metabolic pathways. Thus, the formation of Fe/S clusters and their delivery to these proteins has a fundamental role in the functions and the evolution of the eukaryotic cell. Currently, most eukaryotes harbor two (located in cytosol and mitochondrion) or three (located in plastid) machineries for the assembly of Fe/S clusters, but certain anaerobic microbial eukaryotes contain Sulfur Mobilization (SUF) machineries that were previously thought to be present only in archaeal linages. These machineries could not only stipulate which pathway was present in the last eukaryotic common ancestor (LECA), but they could also provide clues regarding presence of an Fe/S cluster machinery in the proto-eukaryote and evolution of Fe/S cluster assembly machineries in all eukaryotes. Contribution to the field Iron and sulphur are indispensable elements of every living cell, but on their own are toxic and require dedicate and indispensable machineries for the formation of Fe/S clusters. In eukaryotes, proteins requiring Fe/S clusters (Fe/S proteins) are found in or associated with various organelles including the mitochondrion, endoplasmic reticulum, cytosol and the nucleus. These proteins are involved in several pathways indispensable for the viability of each living cell including metabolic pathways, DNA maintenance and protein translation. Thus, the formation and delivery of the Fe/S clusters to these proteins has fundamental role in the functions and the evolution of the eukaryotic cell. Over the last decade there have been significant discoveries in regards to the evolution of eukaryotes and the role of the Fe-S biosynthetic pathways in their adaptations to unique lifestyles. Currently, most eukaryotes harbour two (located in cytosol and mitochondrion) or three (located in plastid) machineries for the assembly of Fe/S clusters. We will present a small summary of these machineries and their roles within the eukaryotic cell. Despite this, certain anaerobic microbial eukaryotes contain machineries that were previously thought to be commonly found in archaeal linages. Which these machineries are and how have they been acquired or preserved in these various eukaryotic lineages? We will present these exemptions and then we will focus on the Sulphur Mobilization (SUF) machinery, which is commonly found in plastids, but also in the cytosol and/or mitochondria in various anaerobic/microaerophilic protists such as Blastocystis, Pygsuia and Stygiella. This machinery is considered to be the most “ancient” Fe-S cluster machinery (not only in eukaryotes). We will provide alternative theories/scenarios based on current published data regarding the presence, function and evolution of this machinery and co-evolution with other machineries in eukaryotes. The presence of the SUF machinery in various eukaryotes could not only stipulate which pathway could have been present in the last eukaryotic common ancestor, but they could also provide clues into the evolution of Fe/S cluster assembly machineries in eukaryotes. Based on current data, we will propose various scenarios on the evolution of the Fe-S cluster machineries in eukaryotes and we will suggest that a SUF-like ancient Fe/S cluster machinery could have been present in proto-eukaryotic cell or the last common eukaryotic ancestor. This is timely, due to the various recent publications on sequencing the genomes of various lineages of Asgard archaea in an attempt to identify the nature of the “founding lineage” of eukaryotes. Based on the proposed scenarios that will be discussed in this article, such a lineage, will provide us with insights on the presence and function of a fundamental biosynthetic pathway such as the Fe-S cluster biosynthesis. Such an essential pathway has yet to be discovered in these archaeal lineages; but according to Prof. Thijs Ettema (presentation in a recent conference) many more archaeal lineages are soon to be published, and thus will open a new field of explorations, while providing a hypothesis to be tested. I r i w Ethics statements Studies involving animal subjects Generated Statement: No animal studies are presented in this manuscript. Studies involving human subjects Generated Statement: No human studies are presented in this manuscript. Inclusion of identifiable human data Generated Statement: No potentially identifiable human images or data is presented in this study.