The ribosomal RNA gene region in Acanthamoeba castellanii mitochondrial DNA. A case of evolutionary transfer of introns between mitochondria and plastids?

Acanthamoeba castellanii, an amoeboid protozoan, occupies an intriguing position in phylogenetic trees based on nuclear rRNA sequences, branching together with or near (as an outgroup to) green algae and land plants. To gain insight into the organization, expression and evolutionary affiliations of the mtDNA of this non-photosynthetic protist, we determined the sequence of a 7778 base-pair region containing the single-copy large subunit (LSU) and small subunit (SSU) rRNA genes (rnl and rns, respectively) of the approximately 40 kilobase-pair A. castellanii mitochondrial genome. We also sequenced the 5'- and 3'-terminal portions of the corresponding LSU and SSU rRNAs. In A. castellanii mtDNA, rnl is flanked both upstream and downstream by a cluster of five tRNA genes, with rns and then cox1 (the cytochrome oxidase subunit 1 gene) following immediately further downstream. These genes are all in the same transcriptional orientation and are separated by only short non-coding spacers. Although rnl and rns are organized in a novel way in A. castellanii mtDNA, their SSU and LSU rRNA products are strikingly similar to their eubacterial homologs in primary sequence, secondary structure and post-transcriptional modification. In these characteristics, the A. castellanii mitochondrial rRNAs much more closely resemble their counterparts in land plants than do the corresponding mitochondrial rRNAs in the green alga, Chlamydomonas reinhardtii. Although no intervening sequences have so far been found in the mitochondrial rnl of angiosperms (flowering plants), A. castellanii mitochondrial rnl contains three group I introns, all located within highly conserved regions in the 3'-half of the gene and each possessing a free-standing open reading frame (ORF). The insertion site of one of these introns is identical to that of the single group I intron in the chloroplast rnl of C. reinhardtii, and sequence comparison reveals that these two introns (one mitochondrial, the other chloroplast) are structurally homologous both within the core region and within the ORFs they encode. These observations are indicative of intron movement between mitochondria and chloroplasts, either intracellularly in a photosynthetic, remote common ancestor of A. castellanii and C. reinhardtii or, more recently, as a result of an intercellular exchange of genetic information.