Comparative genomic studies suggest that the cyanobacterial endosymbionts of the amoeba Paulinella chromatophora possess an import apparatus for nuclear-encoded proteins

摘要

Plastids evolved from free-living cyanobacteria through a process of primary endo-symbiosis. The most widely accepted hypothesis derives three ancient lineages ofprimary plastids, i.e. those of glaucophytes, red algae and green plants, from a sin-gle cyanobacterial endosymbiosis. This hypothesis was originally predicated on theassumption that transformations of endosymbionts into organelles must be excep-tionally rare because of the difficulty in establishing efficient protein traffickingbetween a host cell and incipient organelle. It turns out, however, that highly inte-grated endosymbiotic associations are more common than once thought. Amongthem is the amoeba Paulinella chromatophora, which harbours independentlyacquired cyanobacterial endosymbionts functioning as plastids. Sequencing of thePaulinella endosymbiont genome revealed an absence of essential genes for proteintrafficking, suggesting their residence in the host nucleus and import of proteinproducts back into the endosymbiont. To investigate this hypothesis, we searchedthe Paulinella endosymbiont genome for homologues of higher plant transloconproteins that form the import apparatus in two-membrane envelopes of primaryplastids. We found homologues of Toc12, Tic21 and Tic32, but genes for other keytranslocon proteins (e.g. Omp85/Toc75 and Tic20) were missing. We propose thatthese missing genes were transferred to the Paulinella nucleus and their productsare imported and integrated into the endosymbiont envelope membranes, therebycreating an effective protein import apparatus. We further suggest that other bacte-rial/cyanobacterial endosymbionts found in protists, plants and animals could haveevolved efficient protein import systems independently and, therefore, reached thestatus of true cellular organelles.