MULTIPLE RECRUITMENT OF CLASS-I ALDOLASE TO CHLOROPLASTS AND EUBACTERIAL ORIGIN OF EUKARYOTIC CLASS-II ALDOLASES REVEALED BY CDNAS FROM EUGLENA-GRACILIS

The photosynthetic protist Euglena gracilis is one of few organisms kn own to possess both class-I and class-II fructose-1,6-bisphosphate ald olases (FBA). We have isolated cDNA clones encoding the precursor of c hloroplast class-I FBA and cytosolic class-II FBA from Euglena. Chloro plast class-I FBA is encoded as a single subunit rather than as a poly protein, its deduced transit peptide of 139 amino acids possesses stru ctural motifs neccessary for precursor import across Euglena's three o uter chloroplast membranes. Evolutionary analyses reveal that the clas s-I FBA of Euglena was recruited to the chloroplast independently from the chloroplast class-I FBA of chlorophytes and may derive from the c ytosolic homologue of the secondary chlorophytic endosymbiont. Two dis tinct subfamilies of class-II FBA genes are shown to exist in eubacter ia, which can be traced to an ancient gene duplication which occurred in the common ancestor of contemporary gram-positive and proteobacteri al lineages. Subsequent duplications involving eubacterial class-II FB A genes resulted in functional specialization of the encoded products for substrates other than fructose-1,6-bisphosphate, Class-II FBA gene s of Euglena and ascomycetes are shown to be of eubacterial origin, ha ving been acquired via endosymbiotic gene transfer, probably from the antecedants of mitochondria. The data provide evidence for the chimaer ic nature of eukaryotic genomes.