AN ANCESTRAL MITOCHONDRIAL-DNA RESEMBLING A EUBACTERIAL GENOME IN MINIATURE

Mitochondria, organelles specialized in energy conservation reactions in eukaryotic cells, have evolved from eubacteria-like endosymbionts(1 -3) whose closest known relatives are the rickettsial group of alpha-p roteobacteria(4,5). Because characterized mitochondrial genomes vary m arkedly in structure(3), it has been impossible to infer from them the initial form of the proto-mitochondrial genome. This would require th e identification of minimally derived mitochondrial DNAs that better r eflect the ancestral state, Here we describe such a primitive mitochon drial genome, in the freshwater protozoon Reclinomonas americana(6). T his protist displays ultrastructural characteristics that ally it with the retortamonads(7,8), a protozoan group that lacks mitochondria(8,9 ). R. americana mtDNA (69,034 base pairs) contains the largest collect ion of genes (97) so far identified in any mtDNA, including genes for 5S ribosomal RNA, the RNA component of RNase P, and at least 18 protei ns not previously known to be encoded in mitochondria, Most surprising are four genes specifying a multisubunit, eubacterial-type RNA polyme rase. Features of gene content together with eubacterial characteristi cs of genome organization and expression not found before in mitochond rial genomes indicate that R. americana mtDNA more closely resembles t he ancestral proto-mitochondrial genome than any other mtDNA investiga ted to date.