Genome sequencing of the anaerobic gut microbe, Monocercomonoides sp., reveals that it does not have mitochondria and produces energy via alternate energy pathways
Eukaryotic cells differ from prokaryotic cells based on their cell components, and are characterized by the presence of a membrane-bound nucleus containing DNA and organelles like the Golgi complex, chloroplasts, and mitochondria. Mitochondria are specialized structures that have the ability to generate energy in the form of ATP and produce iron-sulphur (Fe-S) compounds important to cell processes. Mitochondria are considered essential to all eurkaryotes, ranging from microbes and fungi to plants and animals.
As reported in the journal Current Biology, genome sequencing of the anaerobic microbe Monocercomonoides sp., originally found in the gut of a chinchilla, has revealed that it is the first eukaryotic organism without mitochondria. This was demonstrated using a number of shared-ancestry search strategies that indicated the absence of mitochondrial DNA and hallmark proteins of cellular machinery required to import molecules into mitochondria. Genomic analysis of the microbe did, however, show the presence of Golgi complex proteins.
If there are no mitochondria in Monocercomonoides sp., then how does it metabolize energy? The researchers examined their genome sequencing results for proteins that were part of other energy metabolism pathways and identified proteins involved in two alternative ATP-generating pathways. A set of genes for proteins involved in glycolysis, a conserved ATP-generating pathway that uses glucose, was discovered, in addition to proteins of a pathway that breaks down the amino acid arginine to produce ATP. Furthermore, the Fe-S compound assembly system conserved in mitochondria was notably absent and was replaced by a bacterial system called sulphur assimilation, which generates Fe-S compounds in environments lacking iron and oxygen.
The loss of mitochondria in Monocercomonoides sp. is hypothesized to be due to a metabolic streamlining process where over time, the organism decreases the size of its genome to get rid of unnecessary genes. While organisms in anaerobic environments have typically smaller genomes and a reduction in mitochondrial-related organelles, they still contain some recognizable mitochondrial proteins. The discovery of a complete absence of mitochondria in Monocercomonoides sp. challenges established scientific beliefs and suggests that these organelles are not crucial for eukaryotic cell survival.
Written By: Fiona Wong, PhD