Arabidopsis genes encoding components of the chloroplastic protein import apparatus

The process of protein import into plastids has been studied extensively us ing isolated pea (Pisum sativum) chloroplasts. As a consequence, virtually all of the known components of the proteinaceous apparatus that mediates im port were originally cloned from pea. With the recent completion of the Ara bidopsis genome sequencing project, it is now possible to identify putative homologs of the import components in this species. Our analysis has reveal ed that Arabidopsis homologs with high sequence similarity exist for all of the pea import complex subunits, making Arabidopsis a valid model for furt her study of this system. Multiple homologs can be identified for over one- half of the components. In all but one case it is known that more than one of the putative isoforms for a particular subunit are expressed. Thus, it i s possible that multiple types of import complexes are present within the s ame cell, each having a unique affinity for different chloroplastic precurs or proteins, depending upon the exact mix of isoforms it contains. Sequence analysis of the putative Arabidopsis homologs for the chloroplast protein import apparatus has revealed many questions concerning subunit function an d evolution. It should now be possible to use the genetic tools available i n Arabidopsis, including the generation of knockout mutants and antisense t echnology, to address these questions and learn more about the molecular fu nctions of each of the components during the import process.