From bacteria to mitochondria: Aconitase yields surprises

More than a century of biochemical investigation has brought a thorough understanding of metabolism in a wide variety of organisms in the biosphere. Organisms inhabiting diverse niches ranging from anaerobic to highly aerobic environments share a surprisingly similar spectrum of enzyme activities, which are organized into pathways that provide the products for growth and development. Although this great body of information provides details of metabolic pathways, relatively little is understood regarding the forces that drove the evolution of metabolic pathways or the evolution of the specific enzymes involved. Nowhere is this more evident than in the evolution of mitochondria. Mitochondria participate in a wide range of activities in eukaryotic cells, from energy metabolism to apoptotic signaling in multicellular organisms. This is quite remarkable considering that mitochondria are believed to have evolved from a bacterial endosymbiont (1). Current views on the origins of mitochondria look to a α-proteobacterium endosymbiont as the ancestor (1). The α-proteobacteriaciae includes present-day intracellular parasites such as Rickettsia prowazekii. It was the completion of the R. prowazekii genome sequence in 1998 that revealed the close relationship of mitochondria to the α-proteobacteriaciae (2). In this issue of PNAS, Baughn and Malamy (3) report the identification and characterization of genes encoding the enzymes of the oxidative branch of the Krebs cycle in Bacteroides flagilis. Based on phylogenetic analyses, these authors suggest a common origin of the enzymes of the oxidative branch of the Krebs cycle in Bacteroides sp. and mitochondria. This surprising finding supports a polyphyletc origin of the mitochondrial Krebs cycle, and raises the question of what forces drove the evolution of the mitochondrial proteome. Perhaps a consortium of bacterial endosymbionts contributed to the evolution of mitochondria.