Massive gene transfer and extensive RNA editing of a symbiotic dinoflagellate plastid genome

Genome sequencing of Symbiodinium minutum revealed that 95 of 109 plastid-associated genes have been transferred to the nucleargenomeandsubsequently expandedbygeneduplication.Only14genes remain inplastidsandoccur asDNAminicircles. Each minicircle (1.8-3.3 kb) contains one gene and a conserved noncoding region containing putative promoters and RNA-binding sites. Nine types of RNA editing, including a novel G/U type, were discovered in minicircle transcripts but not in genes transferred to the nucleus. In contrast toDNAediting sites in dinoflagellatemitochondria,which tend to be highly conserved across all taxa, editing sites employed in DNA minicircles are highly variable from species to species. Editing is crucial for core photosystem protein function. It restores evolutionarily conserved aminoacids andincreasespeptidyl hydropathy. It alsoincreases proteinplasticitynecessary to initiate photosystem complex assembly.