The remarkable chloroplast genome of dinoflagellates

Dinoflagellates are an economically and ecologically important eukaryotic algal group. The organization of their chloroplast genome appears to be radically different from that in plants and other algae. The gene content has been dramatically reduced in dinoflagellates, with the large-scale transfer of genes to the nucleus. Most of the remaining genes encode subunits of Photosystems I and II, the cytochrome b(6)f complex, and ATP synthase, as well as rRNAs and a few tRNAs. Whereas conventional chloroplast genomes have all their genes physically linked on one molecule, dinoflagellate chloroplast genes are located on small plasmids, termed 'minicircles'. Each minicircle has at most a few genes, and a distinguishable 'core' region. Genes are always in the same orientation with respect to the core region. There are also non-coding minicircles, including aberrant forms of minicircles apparently derived from other minicircles by rearrangement. The evidence that the minicircles are located in the chloroplast and that there is no conventional chloroplast genome in addition to the minicircles is discussed. Transcription of minicircles is probably initiated close to the core, generating transcripts corresponding to an almost entire minicircle. The transcripts are then cleaved to molecules corresponding to individual genes. Post-transcriptional modifications include editing and addition of a polyU tail. It is discussed why these particular genes have been retained in the dinoflagellate chloroplast, together with the possibility that the chloroplast supplies fMet-tRNA to the mitochondrion.