Abstract

In most photosynthetic organisms there is a choroplast genome of 120 kbp or more. It encodes many (but not all) of the components of the photosynthetic light reactions and a protein synthesis machinery, as well as a variable number of other functions. These genes are located on a single molecule, present in multiple copies. In dinoflagellate algae, the situation is very different. Most of the genes typically found in chloroplasts have been relocated to the nucleus. Those left behind encode a subset of the light reaction polypeptides, as well as remarkable ‘minimalist’ rRNAs, and are located on small plasmids. Most of these plasmids carry a single gene. Some carry a few, and some, rather surprisingly, seem to carry none.

Given the apparent advantages of moving genes from chloroplast to nucleus, why should chloroplasts retain a genome at all? It has been suggested that some proteins may be difficult to import into the organelle, and that expression in the organelle offers better control. More recent arguments include the difficulty of transferring genes to the nucleus in organisms with a single plastid, and – for organisms with a reduced mitochondrial genome – a role in supplying tRNAs to the mitochondrion.

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