GENOME STRUCTURE AND GENE CONTENT IN PROTIST MITOCHONDRIAL DNAS

Although the collection of completely sequenced mitochondrial genomes is expanding rapidly, only recently has a phylogenetically broad repre sentation of mtDNA sequences from protists (mostly unicellular eukaryo tes) become available. This review surveys the 23 complete protist mtD NA sequences that have been determined to date, commenting on such asp ects as mitochondrial genome structure, gene content, ribosomal RNA, i ntrons, transfer RNAs and the genetic code and phylogenetic implicatio ns, We also illustrate the utility of a comparative genomics approach to gene identification by providing evidence that orfB in plant and pr otist mtDNAs is the homolog of atp8, the gene in animal and fungal mtD NA that encodes subunit 8 of the F-0 portion of mitochondrial ATP synt hase, Although several protist mtDNAs, like those of animals and most fungi, are seen to be highly derived, others appear to be have retaine d a number of features of the ancestral, proto-mitochondrial genome, S ome of these ancestral features are also shared with plant mtDNA, alth ough the latter have evidently expanded considerably in size, if not i n gene content, in the course of evolution, Comparative analysis of pr otist mtDNAs is providing a new perspective on mtDNA evolution: how th e original mitochondrial genome was organized, what genes it contained , and in what ways it must have changed in different eukaryotic phyla.