title: Origin of mitochondria by intracellular enslavement of a photosynthetic purple bacterium.
abstract: Mitochondria originated by permanent enslavement of purple non-sulphur bacteria. These endosymbionts became organelles through the origin of complex protein-import machinery and insertion into their inner membranes of protein carriers for extracting energy for the host. A chicken-and-egg problem exists: selective advantages for evolving import machinery were absent until inner membrane carriers were present, but this very machinery is now required for carrier insertion. I argue here that this problem was probably circumvented by conversion of the symbiont protein-export machinery into protein-import machinery, in three phases. I suggest that the first carrier entered the periplasmic space via pre-existing beta-barrel proteins in the bacterial outer membrane that later became Tom40, and inserted into the inner membrane probably helped by a pre-existing inner membrane protein, thereby immediately providing the protoeukaryote host with photosynthesate. This would have created a powerful selective advantage for evolving more efficient carrier import by inserting Tom70 receptors. Massive gene transfer to the nucleus inevitably occurred by mutation pressure. Finally, pressure from harmful, non-selected gene transfer to the nucleus probably caused evolution of the presequence mechanism, and photosynthesis was lost.
authors.affiliation: Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK. tom.cavalier-smith@zoo.ox.ac.uk
authors.forename: Thomas
authors.initials: T
authors.lastname: Cavalier-Smith
mesh_terms: D001426:Bacterial Proteins / Q000145:classification / Q000378:metabolism / Q000502:physiology; D005075:Biological Evolution; D005057:Eukaryotic Cells / Q000648:ultrastructure; D022761:Gene Transfer, Horizontal; D016678:Genome; D008565:Membrane Proteins / Q000145:classification / Q000378:metabolism / Q000502:physiology; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D033681:Mitochondrial Membrane Transport Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism; D051336:Mitochondrial Membranes / Q000378:metabolism / Q000648:ultrastructure; D024101:Mitochondrial Proteins / Q000145:classification / Q000378:metabolism / Q000502:physiology; D008954:Models, Biological; D010587:Phagocytosis; D010788:Photosynthesis; D020560:Proteobacteria / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D013559:Symbiosis
chemical_list: D001426:Bacterial Proteins; D008565:Membrane Proteins; D033681:Mitochondrial Membrane Transport Proteins; D024101:Mitochondrial Proteins