Prokaryotic Microbes with Eukaryote-like Genes

Since the late 1980s, all life forms have been split into three groups on the phylogenetic tree of life: bacteria, archaea, and eukaryotes. Eukaryotes and archaea have long been considered “sister groups” based on similarities in their genes and metabolic pathways. But it wasn’t clear whether eukaryotes and archaea shared a common ancestor, or if eukaryotes originated within a subset of archaea.

This is the most exciting and important answer on big questions about eukaryotic origins and the tree of life in years. University in Sweden and his colleagues identified the new group of archaea while analyzing the genomic diversity of marine sediments found near a mid-Atlantic hydrothermal vent . Initial sequencing revealed that these microbes belonged to one of the most abundant groups of marine archaea, none of which have been cultured or sequenced. this identified group of deep sea-dwelling microbes has been classified as archaea—prokaryotic, primitive microorganisms and named as Lokiarchaea,

Using deep metagenomics techniques,  assembling of  a 92 percent complete, 5.1 mega base pair composite genome sequence for Lokiarchaea. Approximately 175 predicted proteins in these microbes were most similar to eukaryotic proteins involved in phagocytosis, cell shape formation, and membrane remodeling. Moreover, phylogenetic analysis revealed that eukaryotes clustered within this archaeal group rather than as a related branch on the tree.

The genetic backbone of all the housekeeping genes, Loki [is] very much archaeal, Loki ended up together with eukaryotes, meaning that they share a common ancestor.The researchers found no signs of contamination. The eukaryote-like genes were flanked by prokaryotic sequences, Lokiarchaea carried many other archaeal features, and the sediment sample appeared free of 18s rRNA sequences—a characteristic of eukaryotes.The researchers found that the Lokiarchaea genome contains five actin homologs that are more similar to eukaryotic actins than to archaeal actin-like proteins. Nearly 70 homologs of Ras-family small GTPases were also found, accounting for approximately 2 percent of Lokiarchaea’s predicted proteins. These proteins are found at similar levels in many unicellular eukaryotes. The researchers also found sequences similar to eukaryotic ESCRT genes in the microbes; these proteins are key components of intracellular vesicle trafficking mechanisms.

These microbes harbor a suite of genes found in eukaryotes which are typically used to remodel intracellular membranes to form vesicles, or for phagocytosis. This “genomic starter kit” could have enabled ancestral forms of these microbes, named Lokiarchaea, to evolve into more complex eukaryotic cells,  The discovery supports the long-standing hypothesis that archaea are the ancestors of eukaryotes, and helps fill an evolutionary gap between the two groups.

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