Cell cycle control of secondary endosymbionts

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Cryptomonads evolved by the engulfment of a eukaryotic algal cell by a heterotrophic eukaryote. In the course of intracellular lifestyle, the symbionts are reduced to a multi-membrane bounded plastid (complex plastid). However, in comparison to other organisms with similar phylogenetic origin and complex plastids (e.g. the Apicomplexa), cryptomonads still contain the reduced nucleus of the eukaryotic symbiont, called the nucleomorph. The division of the nucleomorph preceeds those of the cell nucleus and is under the control of the host. In the course of our international nucleomorph genome project we have scanned the nucleomorph for genes encoding cell cycle factors. Thus, we are able to foresee which cell cycle components are supplemented by the host. These components are the molecular basis for the regulation of the symbiont´s cell cycle by the host. In our project we will isolate factors involved in the S- and G2-M- phases and characterize their topogenic signals as well as their regulation. This will offer for the first time the signals which are responsible for protein transport across the first two membranes of a complex plastid. As topogenic signals are markers for protein import into complex plastids, their knowledge will be used to identify proteins localized in the apicoplast membranes by screening the Apicomplexa databases.