Single gene circles in dinoflagellate chloroplast genomes

Photosynthetic dinoflagellates are important aquatic primary producers and notorious causes of toxic 'red tides: Typical dinoflagellate chloroplasts d iffer from all other plastids in having a combination of three envelope mem branes' and peridinin-chlorophyll ale light-harvesting pigments'. Despite e vidence of a dinoflagellete satellite DNA containing chloroplast genes(3), previous attempts to obtain chloroplast gene sequences have been uniformly unsuccessful. Here we show that the dinoflagellate chloroplast DNA genome s tructure is unique. Complete sequences of chloroplast ribosomal RNA genes a nd seven chloroplast protein genes from the dinoflagellate Heterocapsa triq uetra reveal that each is located alone on a separate minicircular chromoso me:'one gene-one circle: The genes are the most divergent known from chloro plast genomes. Each circle has an unusual tripartite non-coding region (put ative replicon origin), which is highly conserved among the nine circles th rough extensive gene conversion, but is very divergent between species. Sev eral other dinoflagellate species have minicircular chloroplast genes, indi cating that this type of genomic organization may have evolved in ancestral peridinean dinoflagellates. Phylogenetic analysis indicates that dinoflage llate chloroplasts are related to chromistan and red algal chloroplasts and supports their origin by Secondary symbiogenesis(4-6)