Protein translocons in photosynthetic organelles of Paulinella chromatophora

The rhizarian amoeba <em>Paulinella chromatophora</em> harbors two photosynthetic cyanobacterial endosymbionts (chromatophores), acquired independently of primary plastids of glaucophytes, red algae and green plants. These endosymbionts have lost many essential genes, and transferred substantial number of genes to the host nuclear genome via endosymbiotic gene transfer (EGT), including those involved in photosynthesis. This indicates that, similar to primary plastids, <em>Paulinella</em> endosymbionts must have evolved a transport system to import their EGT-derived proteins. This system involves vesicular trafficking to the outer chromatophore membrane and presumably a simplified Tic-like complex at the inner chromatophore membrane. Since both sequenced <em>Paulinella</em> strains have been shown to undergo differential plastid gene losses, they do not have to possess the same set of Toc and Tic homologs. We searched the genome of <em>Paulinella</em> FK01 strain for potential Toc and Tic homologs, and compared the results with the data obtained for <em>Paulinella</em> CCAC 0185 strain, and 72 cyanobacteria, eight Archaeplastida as well as some other bacteria. Our studies revealed that chromatophore genomes from both <em>Paulinella</em> strains encode the same set of translocons that could potentially create a simplified but fully-functional Tic-like complex at the inner chromatophore membranes. The common maintenance of the same set of translocon proteins in two <em>Paulinella</em> strains suggests a similar import mechanism and/or supports the proposed model of protein import. Moreover, we have discovered a new putative Tic component, Tic62, a redox sensor protein not identified in previous comparative studies of <em>Paulinella</em> translocons.