Protein transport into "complex" diatom plastids
occurs in at least two independent steps

Diatoms and other chromophytic algae have plastids with four surrounding envelope membranes. This feature is thought to reflect an evolution by secondary endosymbiosis, i.e. by uptake of a eukaryotic photosynthetic cell by a eukaryotic host. We have cloned and sequenced the genes of the gamma subunit of chloroplast ATPase (atpC), phosphoribulokinase (prk) and fucoxanthin chlorophyll a/c binding proteins (fcp) from the marine diatom Odontella sinensis. All of them show a bipartite presequence structure, consisting of a first domain resembling eukaryotic signal peptides and a second domain with similarity to transit peptides of higher plants. We were able to demonstrate the function of these two domains in heterologous import systems. First we succeeded in directing the precursors of gamma and FCP proteins into pancreatic microsomal vesicles, indicating that the first domain indeed functions as a signal peptide for cotranslational transport at ER membranes. Furthermore -after deletion of the signal peptide domain- the remaining transit peptide domain was able to direct the diatom gamma-precursor into isolated pea plastids. In this case the proteolytic processing of the precursor into the mature form was not correct, indicating that the sequence specificity of the stromal peptidase has evolved differently in the chromophytic and chlorophytic lineages. By using chimeric constructs of preproteins from spinach and Odontella we were able to show that the signal for misprocessing is within the first 35 amino acids of the mature diatom gamma protein. Our data clearly demonstrate that a part of the import system might be common in all different plastid types. This work was supported by a grant of the Deutsche Forschungsgemeinschaft to P.G.K (SFB189, TP B3).