Spezielle Leistungen der Plastide: RNA-Edierung in Landpflanzen, Genomreduktion und Proteinimport in Peridinin-haltigen Dinoflagellaten

Topics of this thesis are the evolution of RNA-editing in plastids of land plants and the evolutionary background of the complex plastid in peridinin-containing dinoflagellates. The plastid genome of P. endiviifolia was completely sequenced (120554 bp) and transcripts of protein-coding genes showed only C to U RNA-editing at 54 sites. The relatively low number of editing sites supports the secondary loss of RNA-editing in M. polymorpha and, furthermore, editing sites are predominantly found in a sequence context known for low re-mutation rates. For this reason RNA-editing in P. endiviifolia meets the criteria for the evolution of chloroplast RNA-editing in land plants stated by Tillich (Tillich et al., 2006). RNA-editing was established to cure DNA mutations, which occurred in the plastid genomes of the first land plants, on the level of RNA. Re-mutations, eventually fostered by times of accelerated evolution rates, can cure these mutations on DNA level and thereby lead to the editotypes we observe today or even to a complete loss of RNA-editing as in M. polymorpha. Another topic was the plastidial genome reduction and the protein import in the complex plastid of peridinin-containing dinoflagellates. The method of transposon mutagenesis was used to isolate minicircles of A. carterae, which revealed that the core region of minicircles is at least subspecies-specific. Minicircles are extraordinary variant and their possible roles, as for example gene-shuttle, are discussed. Studies of protein import showed, that, in contrast to previous in vitro data, transit peptides of nucleus-encoded plastid proteins of peridinin-containing dinoflagellates and cromists were not able to mediate import into the primary plastid of land plants in vivo. For so called class I transit peptides potential additional targeting sequences were identified, named TPL2, which contained the necessary targeting information for directing proteins towards the primary plastid of land plants as it is known from proteins of the outer envelope membrane. Furthermore, class I proteins seem to have a phylogenetic bias towards the ‘green line’ of plastids indicating a possible origin via horizontal gene transfer. Finally, electron microscopy showed the insensitivity of class I protein transport to inhibition with Brefeldin A suggesting an alternative transport route for this class of transit peptides, most probably directly from the ER to the complex plastid. Therefore, proteins with class I transit peptide show specific transport mechanism for protein import into the plastid, potentially as a result of particular evolutionary origin.