Plastids, which possess genomes that are distinct from the primary, nuclear genomes of cells, are thought to have arisen from so-called endosymbiosis, in which a cyanobacterium is captured by another cell, leading the conversion of the cyanobacterium to a plastid organelle that is an essential constituent of a eukaryotic cell.

By analyzing DNA sequences contained in the plastid of the thecate amoeba Paulinella, researchers have shown that it is a recent endosymbiont whose genome features are virtually unchanged from those of its cyanobacterial progenitor. The findings, which help elucidate how plastid organelles arise from endosymbiosis, are reported by Debashish Bhattacharya and colleagues at the University of Iowa and appear in the September 5th issue of Current Biology, published by Cell Press.

Recent research suggests strongly that a single primary endosymbiosis event involving the capture of a cyanobacterium occurred about 1.5 billion years ago, giving birth to the plastid that has spread to all algae and plants. The ancient nature of this endosymbiosis makes it a daunting task to understand how the relationship between the host and endosymbiont was cemented in place. Paulinella is the single known exception to this "primary event" rule because it has a cyanobacteria-like plastid of recent provenance.

In fact, Paulinella was identified over 100 years ago by the German naturalist Robert Lauterborn as potentially important for the investigation of cell symbiosis. By studying Paulinella, researchers hope to understand when its plastid originated and the types of cellular changes that occurred in the amoebal host and endosymbiont and led to the establishment of the foreign cell as a permanent photosynthetic organelle. Prior data indicated a recent plastid endosymbiosis in Paulinella, but lacked information about the properties of the plastid genome.

In the new work, Dr. Bhattacharya and colleagues generated DNA libraries of Paulinella and isolated and sequenced about 14,000 bases of the plastid genome in this species. The gene data confirm a close relationship of the Paulinella plastid to Prochlorococcus- and Synechococcus-type cyanobacteria and show that its genomic architecture across the sampled regions is nearly identical to these prokaryotes.

For example, one photosynthetic gene that has been transferred to the nucleus in all algae and plants is still found on the Paulinella plastid genome, as are genes involved in nitrogen fixation that are absent from all other known plastids.

These data suggest that major insights into plastid establishment, such as control of organelle division, will most likely come from analysis of the Paulinella nuclear genome rather than the endosymbiont genome. Looking forward, the authors point out that this makes Paulinella an ideal model for a complete genome sequencing project to identify the genetic inventions in the nucleus that underlie the cellular transitions that occur following endosymbiotic events.

For expert commentary on this paper, be sure to see the Dispatch in this issue from John Archibald, "Cyanobacterial-like genome of the Paulinella plastid endosymbiont."

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