Identification of the Mitochondrial Genome in the Chrysophyte Alga Ochromonas danica

ABSTRACT. Analysis of total DNA isolated from the Chrysophyte alga Ochromonas danica revealed, in addition to nuclear DNA, two genomes present as numerous copies per cell. The larger genome (?120 kilobase pairs or kbp) is the plastid DNA, which is identified by its hybridization to plasmids containing sequences for the photosynthesis genes rbcL, psbA, and psbC. The smaller genome (40 kbp) is the mitochondrial genome as identified by its hybridization with plasmids containing gene sequences of plant cytochrome oxidase subunits I and II. Both the 120- and 40-kbp genomes contain genes for the small and large subunits of rDNA. The mitochondrial genome is linear with terminal inverted repeats of about 1.6 kbp. Two other morphologically similar species were examined, Ochromonas minuta and Poteriochromonas malhamensis. All three species have linear mitochondrial DNA of 40 kbp. Comparisons of endonuclease restriction-fragment patterns of the mitochondrial and chloroplast DNAs as well as those of their nuclear rDNA repeats failed to reveal any fragment shared by any two of the species. Likewise, no common fragment size was detected by hybridization with plasmids containing heterologous DNA or with total mitochondrial DNA of O. danica; these observations support the taxonomic assignment of these three organisms to different species. The Ochromonas mitochondrial genomes are the first identified in the chlorophyll a/c group of algae. Combining these results with electron microscopic observations of putative mitochondrial genomes reported for other chromophytes and published molecular studies of other algal groups suggests that all classes of eukaryote algae may have mitochondrial genomes < 100 kbp in size, more like other protistans than land plants.